Network Working Group F. Baker, Editor Request for Comments: 1812 Cisco Systems Obsoletes: 1716, 1009 June 1995 Category: Standards Track
Requirements for IP Version 4 Routers
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
PREFACE
This document is an updated version of RFC 1716, the historical Router Requirements document. That RFC preserved the significant work that went into the working group, but failed to adequately describe current technology for the IESG to consider it a current standard.
The current editor had been asked to bring the document up to date, so that it is useful as a procurement specification and a guide to implementors. In this, he stands squarely on the shoulders of those who have gone before him, and depends largely on expert contributors for text. Any credit is theirs; the errors are his.
The content and form of this document are due, in large part, to the working group's chair, and document's original editor and author: Philip Almquist. It is also largely due to the efforts of its previous editor, Frank Kastenholz. Without their efforts, this document would not exist.
Table of Contents
1. INTRODUCTION ........................................ 6 1.1 Reading this Document .............................. 8 1.1.1 Organization ..................................... 8 1.1.2 Requirements ..................................... 9 1.1.3 Compliance ....................................... 10 1.2 Relationships to Other Standards ................... 11 1.3 General Considerations ............................. 12 1.3.1 Continuing Internet Evolution .................... 12 1.3.2 Robustness Principle ............................. 13 1.3.3 Error Logging .................................... 14
Baker Standards Track [Page 1]
RFC 1812 Requirements for IP Version 4 Routers June 1995
This memo replaces for RFC 1716, "Requirements for Internet Gateways" ([INTRO:1]).
This memo defines and discusses requirements for devices that perform the network layer forwarding function of the Internet protocol suite. The Internet community usually refers to such devices as IP routers or simply routers; The OSI community refers to such devices as intermediate systems. Many older Internet documents refer to these devices as gateways, a name which more recently has largely passed out of favor to avoid confusion with application gateways.
An IP router can be distinguished from other sorts of packet switching devices in that a router examines the IP protocol header as part of the switching process. It generally removes the Link Layer header a message was received with, modifies the IP header, and replaces the Link Layer header for retransmission.
Baker Standards Track [Page 6]
RFC 1812 Requirements for IP Version 4 Routers June 1995
The authors of this memo recognize, as should its readers, that many routers support more than one protocol. Support for multiple protocol suites will be required in increasingly large parts of the Internet in the future. This memo, however, does not attempt to specify Internet requirements for protocol suites other than TCP/IP.
This document enumerates standard protocols that a router connected to the Internet must use, and it incorporates by reference the RFCs and other documents describing the current specifications for these protocols. It corrects errors in the referenced documents and adds additional discussion and guidance for an implementor.
For each protocol, this memo also contains an explicit set of requirements, recommendations, and options. The reader must understand that the list of requirements in this memo is incomplete by itself. The complete set of requirements for an Internet protocol router is primarily defined in the standard protocol specification documents, with the corrections, amendments, and supplements contained in this memo.
This memo should be read in conjunction with the Requirements for Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and routers must both be capable of originating IP datagrams and receiving IP datagrams destined for them. The major distinction between Internet hosts and routers is that routers implement forwarding algorithms, while Internet hosts do not require forwarding capabilities. Any Internet host acting as a router must adhere to the requirements contained in this memo.
The goal of open system interconnection dictates that routers must function correctly as Internet hosts when necessary. To achieve this, this memo provides guidelines for such instances. For simplification and ease of document updates, this memo tries to avoid overlapping discussions of host requirements with [INTRO:2] and [INTRO:3] and incorporates the relevant requirements of those documents by reference. In some cases the requirements stated in [INTRO:2] and [INTRO:3] are superseded by this document.
A good-faith implementation of the protocols produced after careful reading of the RFCs should differ from the requirements of this memo in only minor ways. Producing such an implementation often requires some interaction with the Internet technical community, and must follow good communications software engineering practices. In many cases, the requirements in this document are already stated or implied in the standard protocol documents, so that their inclusion here is, in a sense, redundant. They were included because some past implementation has made the wrong choice, causing problems of interoperability, performance, and/or robustness.
Baker Standards Track [Page 7]
RFC 1812 Requirements for IP Version 4 Routers June 1995
This memo includes discussion and explanation of many of the requirements and recommendations. A simple list of requirements would be dangerous, because:
o Some required features are more important than others, and some features are optional.
o Some features are critical in some applications of routers but irrelevant in others.
o There may be valid reasons why particular vendor products that are designed for restricted contexts might choose to use different specifications.
However, the specifications of this memo must be followed to meet the general goal of arbitrary router interoperation across the diversity and complexity of the Internet. Although most current implementations fail to meet these requirements in various ways, some minor and some major, this specification is the ideal towards which we need to move.
These requirements are based on the current level of Internet architecture. This memo will be updated as required to provide additional clarifications or to include additional information in those areas in which specifications are still evolving.
This memo emulates the layered organization used by [INTRO:2] and [INTRO:3]. Thus, Chapter 2 describes the layers found in the Internet architecture. Chapter 3 covers the Link Layer. Chapters 4 and 5 are concerned with the Internet Layer protocols and forwarding algorithms. Chapter 6 covers the Transport Layer. Upper layer protocols are divided among Chapters 7, 8, and 9. Chapter 7 discusses the protocols which routers use to exchange routing information with each other. Chapter 8 discusses network management. Chapter 9 discusses other upper layer protocols. The final chapter covers operations and maintenance features. This organization was chosen for simplicity, clarity, and consistency with the Host Requirements RFCs. Appendices to this memo include a bibliography, a glossary, and some conjectures about future directions of router standards.
In describing the requirements, we assume that an implementation strictly mirrors the layering of the protocols. However, strict layering is an imperfect model, both for the protocol suite and for recommended implementation approaches. Protocols in different layers interact in complex and sometimes subtle ways, and particular
Baker Standards Track [Page 8]
RFC 1812 Requirements for IP Version 4 Routers June 1995
functions often involve multiple layers. There are many design choices in an implementation, many of which involve creative breaking of strict layering. Every implementor is urged to read [INTRO:4] and [INTRO:5].
Each major section of this memo is organized into the following subsections:
(1) Introduction
(2) Protocol Walk-Through - considers the protocol specification documents section-by-section, correcting errors, stating requirements that may be ambiguous or ill-defined, and providing further clarification or explanation.
(3) Specific Issues - discusses protocol design and implementation issues that were not included in the walk-through.
Under many of the individual topics in this memo, there is parenthetical material labeled DISCUSSION or IMPLEMENTATION. This material is intended to give a justification, clarification or explanation to the preceding requirements text. The implementation material contains suggested approaches that an implementor may want to consider. The DISCUSSION and IMPLEMENTATION sections are not part of the standard.
In this memo, the words that are used to define the significance of each particular requirement are capitalized. These words are:
o MUST This word means that the item is an absolute requirement of the specification. Violation of such a requirement is a fundamental error; there is no case where it is justified.
o MUST IMPLEMENT This phrase means that this specification requires that the item be implemented, but does not require that it be enabled by default.
o MUST NOT This phrase means that the item is an absolute prohibition of the specification.
o SHOULD This word means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a
Baker Standards Track [Page 9]
RFC 1812 Requirements for IP Version 4 Routers June 1995
different course.
o SHOULD IMPLEMENT This phrase is similar in meaning to SHOULD, but is used when we recommend that a particular feature be provided but does not necessarily recommend that it be enabled by default.
o SHOULD NOT This phrase means that there may exist valid reasons in particular circumstances when the described behavior is acceptable or even useful. Even so, the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
o MAY This word means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.
Some requirements are applicable to all routers. Other requirements are applicable only to those which implement particular features or protocols. In the following paragraphs, relevant refers to the union of the requirements applicable to all routers and the set of requirements applicable to a particular router because of the set of features and protocols it has implemented.
Note that not all Relevant requirements are stated directly in this memo. Various parts of this memo incorporate by reference sections of the Host Requirements specification, [INTRO:2] and [INTRO:3]. For purposes of determining compliance with this memo, it does not matter whether a Relevant requirement is stated directly in this memo or merely incorporated by reference from one of those documents.
An implementation is said to be conditionally compliant if it satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT requirements. An implementation is said to be unconditionally compliant if it is conditionally compliant and also satisfies all the Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements. An implementation is not compliant if it is not conditionally compliant (i.e., it fails to satisfy one or more of the Relevant MUST, MUST IMPLEMENT, or MUST NOT requirements).
This specification occasionally indicates that an implementation SHOULD implement a management variable, and that it SHOULD have a certain default value. An unconditionally compliant implementation
Baker Standards Track [Page 10]
RFC 1812 Requirements for IP Version 4 Routers June 1995
implements the default behavior, and if there are other implemented behaviors implements the variable. A conditionally compliant implementation clearly documents what the default setting of the variable is or, in the absence of the implementation of a variable, may be construed to be. An implementation that both fails to implement the variable and chooses a different behavior is not compliant.
For any of the SHOULD and SHOULD NOT requirements, a router may provide a configuration option that will cause the router to act other than as specified by the requirement. Having such a configuration option does not void a router's claim to unconditional compliance if the option has a default setting, and that setting causes the router to operate in the required manner.
Likewise, routers may provide, except where explicitly prohibited by this memo, options which cause them to violate MUST or MUST NOT requirements. A router that provides such options is compliant (either fully or conditionally) if and only if each such option has a default setting that causes the router to conform to the requirements of this memo. Please note that the authors of this memo, although aware of market realities, strongly recommend against provision of such options. Requirements are labeled MUST or MUST NOT because experts in the field have judged them to be particularly important to interoperability or proper functioning in the Internet. Vendors should weigh carefully the customer support costs of providing options that violate those rules.
Of course, this memo is not a complete specification of an IP router, but rather is closer to what in the OSI world is called a profile. For example, this memo requires that a number of protocols be implemented. Although most of the contents of their protocol specifications are not repeated in this memo, implementors are nonetheless required to implement the protocols according to those specifications.
There are several reference documents of interest in checking the status of protocol specifications and standardization:
o INTERNET OFFICIAL PROTOCOL STANDARDS This document describes the Internet standards process and lists the standards status of the protocols. As of this writing, the current version of this document is STD 1, RFC 1780, [ARCH:7]. This document is periodically re-issued. You should always consult an RFC repository and use the latest version of this document.
Baker Standards Track [Page 11]
RFC 1812 Requirements for IP Version 4 Routers June 1995
o Assigned Numbers This document lists the assigned values of the parameters used in the various protocols. For example, it lists IP protocol codes, TCP port numbers, Telnet Option Codes, ARP hardware types, and Terminal Type names. As of this writing, the current version of this document is STD 2, RFC 1700, [INTRO:7]. This document is periodically re-issued. You should always consult an RFC repository and use the latest version of this document.
o Host Requirements This pair of documents reviews the specifications that apply to hosts and supplies guidance and clarification for any ambiguities. Note that these requirements also apply to routers, except where otherwise specified in this memo. As of this writing, the current versions of these documents are RFC 1122 and RFC 1123 (STD 3), [INTRO:2] and [INTRO:3].
o Router Requirements (formerly Gateway Requirements) This memo.
Note that these documents are revised and updated at different times; in case of differences between these documents, the most recent must prevail.
These and other Internet protocol documents may be obtained from the:
The InterNIC DS.INTERNIC.NET InterNIC Directory and Database Service info@internic.net +1-908-668-6587 URL: http://ds.internic.net/
The enormous growth of the Internet has revealed problems of management and scaling in a large datagram based packet communication system. These problems are being addressed, and as a result there will be continuing evolution of the specifications described in this memo. New routing protocols, algorithms, and architectures are constantly being developed. New internet layer protocols, and modifications to existing protocols, are also constantly being devised. Routers play a crucial role in the Internet, and the number
Baker Standards Track [Page 12]
RFC 1812 Requirements for IP Version 4 Routers June 1995
of routers deployed in the Internet is much smaller than the number of hosts. Vendors should therefore expect that router standards will continue to evolve much more quickly than host standards. These changes will be carefully planned and controlled since there is extensive participation in this planning by the vendors and by the organizations responsible for operation of the networks.
Development, evolution, and revision are characteristic of computer network protocols today, and this situation will persist for some years. A vendor who develops computer communications software for the Internet protocol suite (or any other protocol suite!) and then fails to maintain and update that software for changing specifications is going to leave a trail of unhappy customers. The Internet is a large communication network, and the users are in constant contact through it. Experience has shown that knowledge of deficiencies in vendor software propagates quickly through the Internet technical community.
At every layer of the protocols, there is a general rule (from [TRANS:2] by Jon Postel) whose application can lead to enormous benefits in robustness and interoperability:
Be conservative in what you do, be liberal in what you accept from others.
Software should be written to deal with every conceivable error, no matter how unlikely. Eventually a packet will come in with that particular combination of errors and attributes, and unless the software is prepared, chaos can ensue. It is best to assume that the network is filled with malevolent entities that will send packets designed to have the worst possible effect. This assumption will lead to suitably protective design. The most serious problems in the Internet have been caused by unforeseen mechanisms triggered by low probability events; mere human malice would never have taken so devious a course!
Adaptability to change must be designed into all levels of router software. As a simple example, consider a protocol specification that contains an enumeration of values for a particular header field - e.g., a type field, a port number, or an error code; this enumeration must be assumed to be incomplete. If the protocol specification defines four possible error codes, the software must not break when a fifth code is defined. An undefined code might be logged, but it must not cause a failure.
Baker Standards Track [Page 13]
RFC 1812 Requirements for IP Version 4 Routers June 1995
The second part of the principal is almost as important: software on hosts or other routers may contain deficiencies that make it unwise to exploit legal but obscure protocol features. It is unwise to stray far from the obvious and simple, lest untoward effects result elsewhere. A corollary of this is watch out for misbehaving hosts; router software should be prepared to survive in the presence of misbehaving hosts. An important function of routers in the Internet is to limit the amount of disruption such hosts can inflict on the shared communication facility.
The Internet includes a great variety of systems, each implementing many protocols and protocol layers, and some of these contain bugs and misguided features in their Internet protocol software. As a result of complexity, diversity, and distribution of function, the diagnosis of problems is often very difficult.
Problem diagnosis will be aided if routers include a carefully designed facility for logging erroneous or strange events. It is important to include as much diagnostic information as possible when an error is logged. In particular, it is often useful to record the header(s) of a packet that caused an error. However, care must be taken to ensure that error logging does not consume prohibitive amounts of resources or otherwise interfere with the operation of the router.
There is a tendency for abnormal but harmless protocol events to overflow error logging files; this can be avoided by using a circular log, or by enabling logging only while diagnosing a known failure. It may be useful to filter and count duplicate successive messages. One strategy that seems to work well is to both:
o Always count abnormalities and make such counts accessible through the management protocol (see Chapter 8); and o Allow the logging of a great variety of events to be selectively enabled. For example, it might useful to be able to log everything or to log everything for host X.
In an ideal world, routers would be easy to configure, and perhaps even entirely self-configuring. However, practical experience in the real world suggests that this is an impossible goal, and that many attempts by vendors to make configuration easy actually cause customers more grief than they prevent. As an extreme example, a
Baker Standards Track [Page 14]
RFC 1812 Requirements for IP Version 4 Routers June 1995
router designed to come up and start routing packets without requiring any configuration information at all would almost certainly choose some incorrect parameter, possibly causing serious problems on any networks unfortunate enough to be connected to it.
Often this memo requires that a parameter be a configurable option. There are several reasons for this. In a few cases there currently is some uncertainty or disagreement about the best value and it may be necessary to update the recommended value in the future. In other cases, the value really depends on external factors - e.g., the distribution of its communication load, or the speeds and topology of nearby networks - and self-tuning algorithms are unavailable and may be insufficient. In some cases, configurability is needed because of administrative requirements.
Finally, some configuration options are required to communicate with obsolete or incorrect implementations of the protocols, distributed without sources, that persist in many parts of the Internet. To make correct systems coexist with these faulty systems, administrators must occasionally misconfigure the correct systems. This problem will correct itself gradually as the faulty systems are retired, but cannot be ignored by vendors.
When we say that a parameter must be configurable, we do not intend to require that its value be explicitly read from a configuration file at every boot time. For many parameters, there is one value that is appropriate for all but the most unusual situations. In such cases, it is quite reasonable that the parameter default to that value if not explicitly set.
This memo requires a particular value for such defaults in some cases. The choice of default is a sensitive issue when the configuration item controls accommodation of existing, faulty, systems. If the Internet is to converge successfully to complete interoperability, the default values built into implementations must implement the official protocol, not misconfigurations to accommodate faulty implementations. Although marketing considerations have led some vendors to choose misconfiguration defaults, we urge vendors to choose defaults that will conform to the standard.
Finally, we note that a vendor needs to provide adequate documentation on all configuration parameters, their limits and effects.
Baker Standards Track [Page 15]
RFC 1812 Requirements for IP Version 4 Routers June 1995
In several places in this memo, specific algorithms that a router ought to follow are specified. These algorithms are not, per se, required of the router. A router need not implement each algorithm as it is written in this document. Rather, an implementation must present a behavior to the external world that is the same as a strict, literal, implementation of the specified algorithm.
Algorithms are described in a manner that differs from the way a good implementor would implement them. For expository purposes, a style that emphasizes conciseness, clarity, and independence from implementation details has been chosen. A good implementor will choose algorithms and implementation methods that produce the same results as these algorithms, but may be more efficient or less general.
We note that the art of efficient router implementation is outside the scope of this memo.
This chapter does not contain any requirements. However, it does contain useful background information on the general architecture of the Internet and of routers.
General background and discussion on the Internet architecture and supporting protocol suite can be found in the DDN Protocol Handbook [ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and [ARCH:4]. The Internet architecture and protocols are also covered in an ever-growing number of textbooks, such as [ARCH:5] and [ARCH:6].
The Internet system consists of a number of interconnected packet networks supporting communication among host computers using the Internet protocols. These protocols include the Internet Protocol (IP), the Internet Control Message Protocol (ICMP), the Internet Group Management Protocol (IGMP), and a variety transport and application protocols that depend upon them. As was described in Section [1.2], the Internet Engineering Steering Group periodically releases an Official Protocols memo listing all the Internet protocols.
All Internet protocols use IP as the basic data transport mechanism. IP is a datagram, or connectionless, internetwork service and includes provision for addressing, type-of-service specification,
Baker Standards Track [Page 16]
RFC 1812 Requirements for IP Version 4 Routers June 1995
fragmentation and reassembly, and security. ICMP and IGMP are considered integral parts of IP, although they are architecturally layered upon IP. ICMP provides error reporting, flow control, first-hop router redirection, and other maintenance and control functions. IGMP provides the mechanisms by which hosts and routers can join and leave IP multicast groups.
Reliable data delivery is provided in the Internet protocol suite by Transport Layer protocols such as the Transmission Control Protocol (TCP), which provides end-end retransmission, resequencing and connection control. Transport Layer connectionless service is provided by the User Datagram Protocol (UDP).
To communicate using the Internet system, a host must implement the layered set of protocols comprising the Internet protocol suite. A host typically must implement at least one protocol from each layer.
The protocol layers used in the Internet architecture are as follows [ARCH:7]:
o Application Layer The Application Layer is the top layer of the Internet protocol suite. The Internet suite does not further subdivide the Application Layer, although some application layer protocols do contain some internal sub-layering. The application layer of the Internet suite essentially combines the functions of the top two layers - Presentation and Application - of the OSI Reference Model [ARCH:8]. The Application Layer in the Internet protocol suite also includes some of the function relegated to the Session Layer in the OSI Reference Model.
We distinguish two categories of application layer protocols: user protocols that provide service directly to users, and support protocols that provide common system functions. The most common Internet user protocols are:
There are a number of other standardized user protocols and many private user protocols.
Baker Standards Track [Page 17]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Support protocols, used for host name mapping, booting, and management include SNMP, BOOTP, TFTP, the Domain Name System (DNS) protocol, and a variety of routing protocols.
Application Layer protocols relevant to routers are discussed in chapters 7, 8, and 9 of this memo.
o Transport Layer The Transport Layer provides end-to-end communication services. This layer is roughly equivalent to the Transport Layer in the OSI Reference Model, except that it also incorporates some of OSI's Session Layer establishment and destruction functions.
There are two primary Transport Layer protocols at present:
- Transmission Control Protocol (TCP) - User Datagram Protocol (UDP)
TCP is a reliable connection-oriented transport service that provides end-to-end reliability, resequencing, and flow control. UDP is a connectionless (datagram) transport service. Other transport protocols have been developed by the research community, and the set of official Internet transport protocols may be expanded in the future.
Transport Layer protocols relevant to routers are discussed in Chapter 6.
o Internet Layer All Internet transport protocols use the Internet Protocol (IP) to carry data from source host to destination host. IP is a connectionless or datagram internetwork service, providing no end-to-end delivery guarantees. IP datagrams may arrive at the destination host damaged, duplicated, out of order, or not at all. The layers above IP are responsible for reliable delivery service when it is required. The IP protocol includes provision for addressing, type-of-service specification, fragmentation and reassembly, and security.
The datagram or connectionless nature of IP is a fundamental and characteristic feature of the Internet architecture.
The Internet Control Message Protocol (ICMP) is a control protocol that is considered to be an integral part of IP, although it is architecturally layered upon IP - it uses IP to carry its data end-to-end. ICMP provides error reporting, congestion reporting, and first-hop router redirection.
Baker Standards Track [Page 18]
RFC 1812 Requirements for IP Version 4 Routers June 1995
The Internet Group Management Protocol (IGMP) is an Internet layer protocol used for establishing dynamic host groups for IP multicasting.
The Internet layer protocols IP, ICMP, and IGMP are discussed in chapter 4.
o Link Layer To communicate on a directly connected network, a host must implement the communication protocol used to interface to that network. We call this a Link Layer protocol.
Some older Internet documents refer to this layer as the Network Layer, but it is not the same as the Network Layer in the OSI Reference Model.
This layer contains everything below the Internet Layer and above the Physical Layer (which is the media connectivity, normally electrical or optical, which encodes and transports messages). Its responsibility is the correct delivery of messages, among which it does not differentiate.
Protocols in this Layer are generally outside the scope of Internet standardization; the Internet (intentionally) uses existing standards whenever possible. Thus, Internet Link Layer standards usually address only address resolution and rules for transmitting IP packets over specific Link Layer protocols. Internet Link Layer standards are discussed in chapter 3.
The constituent networks of the Internet system are required to provide only packet (connectionless) transport. According to the IP service specification, datagrams can be delivered out of order, be lost or duplicated, and/or contain errors.
For reasonable performance of the protocols that use IP (e.g., TCP), the loss rate of the network should be very low. In networks providing connection-oriented service, the extra reliability provided by virtual circuits enhances the end-end robustness of the system, but is not necessary for Internet operation.
Constituent networks may generally be divided into two classes:
o Local-Area Networks (LANs) LANs may have a variety of designs. LANs normally cover a small geographical area (e.g., a single building or plant site) and provide high bandwidth with low delays. LANs may be passive
Baker Standards Track [Page 19]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(similar to Ethernet) or they may be active (such as ATM).
o Wide-Area Networks (WANs) Geographically dispersed hosts and LANs are interconnected by wide-area networks, also called long-haul networks. These networks may have a complex internal structure of lines and packet-switches, or they may be as simple as point-to-point lines.
In the Internet model, constituent networks are connected together by IP datagram forwarders which are called routers or IP routers. In this document, every use of the term router is equivalent to IP router. Many older Internet documents refer to routers as gateways.
Historically, routers have been realized with packet-switching software executing on a general-purpose CPU. However, as custom hardware development becomes cheaper and as higher throughput is required, special purpose hardware is becoming increasingly common. This specification applies to routers regardless of how they are implemented.
A router connects to two or more logical interfaces, represented by IP subnets or unnumbered point to point lines (discussed in section [2.2.7]). Thus, it has at least one physical interface. Forwarding an IP datagram generally requires the router to choose the address and relevant interface of the next-hop router or (for the final hop) the destination host. This choice, called relaying or forwarding depends upon a route database within the router. The route database is also called a routing table or forwarding table. The term "router" derives from the process of building this route database; routing protocols and configuration interact in a process called routing.
The routing database should be maintained dynamically to reflect the current topology of the Internet system. A router normally accomplishes this by participating in distributed routing and reachability algorithms with other routers.
Routers provide datagram transport only, and they seek to minimize the state information necessary to sustain this service in the interest of routing flexibility and robustness.
Packet switching devices may also operate at the Link Layer; such devices are usually called bridges. Network segments that are connected by bridges share the same IP network prefix forming a single IP subnet. These other devices are outside the scope of this
Baker Standards Track [Page 20]
RFC 1812 Requirements for IP Version 4 Routers June 1995
An Autonomous System (AS) is a connected segment of a network topology that consists of a collection of subnetworks (with hosts attached) interconnected by a set of routes. The subnetworks and the routers are expected to be under the control of a single operations and maintenance (O&M) organization. Within an AS routers may use one or more interior routing protocols, and sometimes several sets of metrics. An AS is expected to present to other ASs an appearence of a coherent interior routing plan, and a consistent picture of the destinations reachable through the AS. An AS is identified by an Autonomous System number.
The concept of an AS plays an important role in the Internet routing (see Section 7.1).
An IP datagram carries 32-bit source and destination addresses, each of which is partitioned into two parts - a constituent network prefix and a host number on that network. Symbolically:
To finally deliver the datagram, the last router in its path must map the Host-number (or rest) part of an IP address to the host's Link Layer address.
Although well documented elsewhere [INTERNET:2], it is useful to describe the historical use of the network prefix. The language developed to describe it is used in this and other documents and permeates the thinking behind many protocols.
The simplest classical network prefix is the Class A, B, C, D, or E network prefix. These address ranges are discriminated by observing the values of the most significant bits of the address, and break the address into simple prefix and host number fields. This is described in [INTERNET:18]. In short, the classification is:
0xxx - Class A - general purpose unicast addresses with standard 8 bit prefix 10xx - Class B - general purpose unicast addresses with standard 16 bit prefix
Baker Standards Track [Page 21]
RFC 1812 Requirements for IP Version 4 Routers June 1995
110x - Class C - general purpose unicast addresses with standard 24 bit prefix 1110 - Class D - IP Multicast Addresses - 28 bit prefix, non- aggregatable 1111 - Class E - reserved for experimental use
This simple notion has been extended by the concept of subnets. These were introduced to allow arbitrary complexity of interconnected LAN structures within an organization, while insulating the Internet system against explosive growth in assigned network prefixes and routing complexity. Subnets provide a multi-level hierarchical routing structure for the Internet system. The subnet extension, described in [INTERNET:2], is a required part of the Internet architecture. The basic idea is to partition the <Host-number> field into two parts: a subnet number, and a true host number on that subnet:
The interconnected physical networks within an organization use the same network prefix but different subnet numbers. The distinction between the subnets of such a subnetted network is not normally visible outside of that network. Thus, routing in the rest of the Internet uses only the <Network-prefix> part of the IP destination address. Routers outside the network treat <Network-prefix> and <Host-number> together as an uninterpreted rest part of the 32-bit IP address. Within the subnetted network, the routers use the extended network prefix:
{ <Network-number>, <Subnet-number> }
The bit positions containing this extended network number have historically been indicated by a 32-bit mask called the subnet mask. The <Subnet-number> bits SHOULD be contiguous and fall between the <Network-number> and the <Host-number> fields. More up to date protocols do not refer to a subnet mask, but to a prefix length; the "prefix" portion of an address is that which would be selected by a subnet mask whose most significant bits are all ones and the rest are zeroes. The length of the prefix equals the number of ones in the subnet mask. This document assumes that all subnet masks are expressible as prefix lengths.
The inventors of the subnet mechanism presumed that each piece of an organization's network would have only a single subnet number. In practice, it has often proven necessary or useful to have several subnets share a single physical cable. For this reason, routers should be capable of configuring multiple subnets on the same
Baker Standards Track [Page 22]
RFC 1812 Requirements for IP Version 4 Routers June 1995
physical interfaces, and treat them (from a routing or forwarding perspective) as though they were distinct physical interfaces.
The explosive growth of the Internet has forced a review of address assignment policies. The traditional uses of general purpose (Class A, B, and C) networks have been modified to achieve better use of IP's 32-bit address space. Classless Inter Domain Routing (CIDR) [INTERNET:15] is a method currently being deployed in the Internet backbones to achieve this added efficiency. CIDR depends on deploying and routing to arbitrarily sized networks. In this model, hosts and routers make no assumptions about the use of addressing in the internet. The Class D (IP Multicast) and Class E (Experimental) address spaces are preserved, although this is primarily an assignment policy.
By definition, CIDR comprises three elements:
o topologically significant address assignment, o routing protocols that are capable of aggregating network layer reachability information, and o consistent forwarding algorithm ("longest match").
The use of networks and subnets is now historical, although the language used to describe them remains in current use. They have been replaced by the more tractable concept of a network prefix. A network prefix is, by definition, a contiguous set of bits at the more significant end of the address that defines a set of systems; host numbers select among those systems. There is no requirement that all the internet use network prefixes uniformly. To collapse routing information, it is useful to divide the internet into addressing domains. Within such a domain, detailed information is available about constituent networks; outside it, only the common network prefix is advertised.
The classical IP addressing architecture used addresses and subnet masks to discriminate the host number from the network prefix. With network prefixes, it is sufficient to indicate the number of bits in the prefix. Both representations are in common use. Architecturally correct subnet masks are capable of being represented using the prefix length description. They comprise that subset of all possible bits patterns that have
o a contiguous string of ones at the more significant end, o a contiguous string of zeros at the less significant end, and o no intervening bits.
Baker Standards Track [Page 23]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Routers SHOULD always treat a route as a network prefix, and SHOULD reject configuration and routing information inconsistent with that model.
An effect of the use of CIDR is that the set of destinations associated with address prefixes in the routing table may exhibit subset relationship. A route describing a smaller set of destinations (a longer prefix) is said to be more specific than a route describing a larger set of destinations (a shorter prefix); similarly, a route describing a larger set of destinations (a shorter prefix) is said to be less specific than a route describing a smaller set of destinations (a longer prefix). Routers must use the most specific matching route (the longest matching network prefix) when forwarding traffic.
IP multicasting is an extension of Link Layer multicast to IP internets. Using IP multicasts, a single datagram can be addressed to multiple hosts without sending it to all. In the extended case, these hosts may reside in different address domains. This collection of hosts is called a multicast group. Each multicast group is represented as a Class D IP address. An IP datagram sent to the group is to be delivered to each group member with the same best- effort delivery as that provided for unicast IP traffic. The sender of the datagram does not itself need to be a member of the destination group.
The semantics of IP multicast group membership are defined in [INTERNET:4]. That document describes how hosts and routers join and leave multicast groups. It also defines a protocol, the Internet Group Management Protocol (IGMP), that monitors IP multicast group membership.
Forwarding of IP multicast datagrams is accomplished either through static routing information or via a multicast routing protocol. Devices that forward IP multicast datagrams are called multicast routers. They may or may not also forward IP unicasts. Multicast datagrams are forwarded on the basis of both their source and destination addresses. Forwarding of IP multicast packets is described in more detail in Section [5.2.1]. Appendix D discusses multicast routing protocols.
Baker Standards Track [Page 24]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Traditionally, each network interface on an IP host or router has its own IP address. This can cause inefficient use of the scarce IP address space, since it forces allocation of an IP network prefix to every point-to-point link.
To solve this problem, a number of people have proposed and implemented the concept of unnumbered point to point lines. An unnumbered point to point line does not have any network prefix associated with it. As a consequence, the network interfaces connected to an unnumbered point to point line do not have IP addresses.
Because the IP architecture has traditionally assumed that all interfaces had IP addresses, these unnumbered interfaces cause some interesting dilemmas. For example, some IP options (e.g., Record Route) specify that a router must insert the interface address into the option, but an unnumbered interface has no IP address. Even more fundamental (as we shall see in chapter 5) is that routes contain the IP address of the next hop router. A router expects that this IP address will be on an IP (sub)net to which the router is connected. That assumption is of course violated if the only connection is an unnumbered point to point line.
To get around these difficulties, two schemes have been conceived. The first scheme says that two routers connected by an unnumbered point to point line are not really two routers at all, but rather two half-routers that together make up a single virtual router. The unnumbered point to point line is essentially considered to be an internal bus in the virtual router. The two halves of the virtual router must coordinate their activities in such a way that they act exactly like a single router.
This scheme fits in well with the IP architecture, but suffers from two important drawbacks. The first is that, although it handles the common case of a single unnumbered point to point line, it is not readily extensible to handle the case of a mesh of routers and unnumbered point to point lines. The second drawback is that the interactions between the half routers are necessarily complex and are not standardized, effectively precluding the connection of equipment from different vendors using unnumbered point to point lines.
Because of these drawbacks, this memo has adopted an alternate scheme, which has been invented multiple times but which is probably originally attributable to Phil Karn. In this scheme, a router that has unnumbered point to point lines also has a special IP address, called a router-id in this memo. The router-id is one of the
Baker Standards Track [Page 25]
RFC 1812 Requirements for IP Version 4 Routers June 1995
router's IP addresses (a router is required to have at least one IP address). This router-id is used as if it is the IP address of all unnumbered interfaces.
A router may be a stand-alone computer system, dedicated to its IP router functions. Alternatively, it is possible to embed router functions within a host operating system that supports connections to two or more networks. The best-known example of an operating system with embedded router code is the Berkeley BSD system. The embedded router feature seems to make building a network easy, but it has a number of hidden pitfalls:
(1) If a host has only a single constituent-network interface, it should not act as a router.
For example, hosts with embedded router code that gratuitously forward broadcast packets or datagrams on the same net often cause packet avalanches.
(2) If a (multihomed) host acts as a router, it is subject to the requirements for routers contained in this document.
For example, the routing protocol issues and the router control and monitoring problems are as hard and important for embedded routers as for stand-alone routers.
Internet router requirements and specifications may change independently of operating system changes. An administration that operates an embedded router in the Internet is strongly advised to maintain and update the router code. This might require router source code.
(3) When a host executes embedded router code, it becomes part of the Internet infrastructure. Thus, errors in software or configuration can hinder communication between other hosts. As a consequence, the host administrator must lose some autonomy.
In many circumstances, a host administrator will need to disable router code embedded in the operating system. For this reason, it should be straightforward to disable embedded router functionality.
Baker Standards Track [Page 26]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(4) When a host running embedded router code is concurrently used for other services, the Operation and Maintenance requirements for the two modes of use may conflict.
For example, router O&M will in many cases be performed remotely by an operations center; this may require privileged system access that the host administrator would not normally want to distribute.
There are two basic models for interconnecting local-area networks and wide-area (or long-haul) networks in the Internet. In the first, the local-area network is assigned a network prefix and all routers in the Internet must know how to route to that network. In the second, the local-area network shares (a small part of) the address space of the wide-area network. Routers that support this second model are called address sharing routers or transparent routers. The focus of this memo is on routers that support the first model, but this is not intended to exclude the use of transparent routers.
The basic idea of a transparent router is that the hosts on the local-area network behind such a router share the address space of the wide-area network in front of the router. In certain situations this is a very useful approach and the limitations do not present significant drawbacks.
The words in front and behind indicate one of the limitations of this approach: this model of interconnection is suitable only for a geographically (and topologically) limited stub environment. It requires that there be some form of logical addressing in the network level addressing of the wide-area network. IP addresses in the local environment map to a few (usually one) physical address in the wide- area network. This mapping occurs in a way consistent with the { IP address <-> network address } mapping used throughout the wide-area network.
Multihoming is possible on one wide-area network, but may present routing problems if the interfaces are geographically or topologically separated. Multihoming on two (or more) wide-area networks is a problem due to the confusion of addresses.
The behavior that hosts see from other hosts in what is apparently the same network may differ if the transparent router cannot fully emulate the normal wide-area network service. For example, the ARPANET used a Link Layer protocol that provided a Destination Dead indication in response to an attempt to send to a host that was off- line. However, if there were a transparent router between the
Baker Standards Track [Page 27]
RFC 1812 Requirements for IP Version 4 Routers June 1995
ARPANET and an Ethernet, a host on the ARPANET would not receive a Destination Dead indication for Ethernet hosts.
An Internet router performs the following functions:
(1) Conforms to specific Internet protocols specified in this document, including the Internet Protocol (IP), Internet Control Message Protocol (ICMP), and others as necessary.
(2) Interfaces to two or more packet networks. For each connected network the router must implement the functions required by that network. These functions typically include:
o Encapsulating and decapsulating the IP datagrams with the connected network framing (e.g., an Ethernet header and checksum),
o Sending and receiving IP datagrams up to the maximum size supported by that network, this size is the network's Maximum Transmission Unit or MTU,
o Translating the IP destination address into an appropriate network-level address for the connected network (e.g., an Ethernet hardware address), if needed, and
o Responding to network flow control and error indications, if any.
See chapter 3 (Link Layer).
(3) Receives and forwards Internet datagrams. Important issues in this process are buffer management, congestion control, and fairness.
o Recognizes error conditions and generates ICMP error and information messages as required.
o Drops datagrams whose time-to-live fields have reached zero.
o Fragments datagrams when necessary to fit into the MTU of the next network.
See chapter 4 (Internet Layer - Protocols) and chapter 5 (Internet Layer - Forwarding) for more information.
Baker Standards Track [Page 28]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(4) Chooses a next-hop destination for each IP datagram, based on the information in its routing database. See chapter 5 (Internet Layer - Forwarding) for more information.
(5) (Usually) supports an interior gateway protocol (IGP) to carry out distributed routing and reachability algorithms with the other routers in the same autonomous system. In addition, some routers will need to support an exterior gateway protocol (EGP) to exchange topological information with other autonomous systems. See chapter 7 (Application Layer - Routing Protocols) for more information.
(6) Provides network management and system support facilities, including loading, debugging, status reporting, exception reporting and control. See chapter 8 (Application Layer - Network Management Protocols) and chapter 10 (Operation and Maintenance) for more information.
A router vendor will have many choices on power, complexity, and features for a particular router product. It may be helpful to observe that the Internet system is neither homogeneous nor fully connected. For reasons of technology and geography it is growing into a global interconnect system plus a fringe of LANs around the edge. More and more these fringe LANs are becoming richly interconnected, thus making them less out on the fringe and more demanding on router requirements.
o The global interconnect system is composed of a number of wide-area networks to which are attached routers of several Autonomous Systems (AS); there are relatively few hosts connected directly to the system.
o Most hosts are connected to LANs. Many organizations have clusters of LANs interconnected by local routers. Each such cluster is connected by routers at one or more points into the global interconnect system. If it is connected at only one point, a LAN is known as a stub network.
Routers in the global interconnect system generally require:
o Advanced Routing and Forwarding Algorithms
These routers need routing algorithms that are highly dynamic, impose minimal processing and communication burdens, and offer type-of-service routing. Congestion is still not a completely resolved issue (see Section [5.3.6]). Improvements in these areas are expected, as the research community is actively working on these issues.
Baker Standards Track [Page 29]
RFC 1812 Requirements for IP Version 4 Routers June 1995
o High Availability
These routers need to be highly reliable, providing 24 hours a day, 7 days a week service. Equipment and software faults can have a wide-spread (sometimes global) effect. In case of failure, they must recover quickly. In any environment, a router must be highly robust and able to operate, possibly in a degraded state, under conditions of extreme congestion or failure of network resources.
o Advanced O&M Features
Internet routers normally operate in an unattended mode. They will typically be operated remotely from a centralized monitoring center. They need to provide sophisticated means for monitoring and measuring traffic and other events and for diagnosing faults.
o High Performance
Long-haul lines in the Internet today are most frequently full duplex 56 KBPS, DS1 (1.544 Mbps), or DS3 (45 Mbps) speeds. LANs, which are half duplex multiaccess media, are typically Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps). However, network media technology is constantly advancing and higher speeds are likely in the future.
The requirements for routers used in the LAN fringe (e.g., campus networks) depend greatly on the demands of the local networks. These may be high or medium-performance devices, probably competitively procured from several different vendors and operated by an internal organization (e.g., a campus computing center). The design of these routers should emphasize low average latency and good burst performance, together with delay and type-of-service sensitive resource management. In this environment there may be less formal O&M but it will not be less important. The need for the routing mechanism to be highly dynamic will become more important as networks become more complex and interconnected. Users will demand more out of their local connections because of the speed of the global interconnects.
As networks have grown, and as more networks have become old enough that they are phasing out older equipment, it has become increasingly imperative that routers interoperate with routers from other vendors.
Even though the Internet system is not fully interconnected, many parts of the system need to have redundant connectivity. Rich connectivity allows reliable service despite failures of communication lines and routers, and it can also improve service by
Baker Standards Track [Page 30]
RFC 1812 Requirements for IP Version 4 Routers June 1995
shortening Internet paths and by providing additional capacity. Unfortunately, this richer topology can make it much more difficult to choose the best path to a particular destination.
The current Internet architecture is based on a set of assumptions about the communication system. The assumptions most relevant to routers are as follows:
o The Internet is a network of networks.
Each host is directly connected to some particular network(s); its connection to the Internet is only conceptual. Two hosts on the same network communicate with each other using the same set of protocols that they would use to communicate with hosts on distant networks.
o Routers do not keep connection state information.
To improve the robustness of the communication system, routers are designed to be stateless, forwarding each IP packet independently of other packets. As a result, redundant paths can be exploited to provide robust service in spite of failures of intervening routers and networks.
All state information required for end-to-end flow control and reliability is implemented in the hosts, in the transport layer or in application programs. All connection control information is thus co-located with the end points of the communication, so it will be lost only if an end point fails. Routers control message flow only indirectly, by dropping packets or increasing network delay.
Note that future protocol developments may well end up putting some more state into routers. This is especially likely for multicast routing, resource reservation, and flow based forwarding.
o Routing complexity should be in the routers.
Routing is a complex and difficult problem, and ought to be performed by the routers, not the hosts. An important objective is to insulate host software from changes caused by the inevitable evolution of the Internet routing architecture.
Baker Standards Track [Page 31]
RFC 1812 Requirements for IP Version 4 Routers June 1995
o The system must tolerate wide network variation.
A basic objective of the Internet design is to tolerate a wide range of network characteristics - e.g., bandwidth, delay, packet loss, packet reordering, and maximum packet size. Another objective is robustness against failure of individual networks, routers, and hosts, using whatever bandwidth is still available. Finally, the goal is full open system interconnection: an Internet router must be able to interoperate robustly and effectively with any other router or Internet host, across diverse Internet paths.
Sometimes implementors have designed for less ambitious goals. For example, the LAN environment is typically much more benign than the Internet as a whole; LANs have low packet loss and delay and do not reorder packets. Some vendors have fielded implementations that are adequate for a simple LAN environment, but work badly for general interoperation. The vendor justifies such a product as being economical within the restricted LAN market. However, isolated LANs seldom stay isolated for long. They are soon connected to each other, to organization-wide internets, and eventually to the global Internet system. In the end, neither the customer nor the vendor is served by incomplete or substandard routers.
The requirements in this document are designed for a full-function router. It is intended that fully compliant routers will be usable in almost any part of the Internet.
Although [INTRO:1] covers Link Layer standards (IP over various link layers, ARP, etc.), this document anticipates that Link-Layer material will be covered in a separate Link Layer Requirements document. A Link-Layer Requirements document would be applicable to both hosts and routers. Thus, this document will not obsolete the parts of [INTRO:1] that deal with link-layer issues.
Routers have essentially the same Link Layer protocol requirements as other sorts of Internet systems. These requirements are given in chapter 3 of Requirements for Internet Gateways [INTRO:1]. A router MUST comply with its requirements and SHOULD comply with its recommendations. Since some of the material in that document has become somewhat dated, some additional requirements and explanations are included below.
Baker Standards Track [Page 32]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION It is expected that the Internet community will produce a Requirements for Internet Link Layer standard which will supersede both this chapter and the chapter entitled "INTERNET LAYER PROTOCOLS" in [INTRO:1].
This document does not attempt to specify the interface between the Link Layer and the upper layers. However, note well that other parts of this document, particularly chapter 5, require various sorts of information to be passed across this layer boundary.
This section uses the following definitions:
o Source physical address
The source physical address is the Link Layer address of the host or router from which the packet was received.
o Destination physical address
The destination physical address is the Link Layer address to which the packet was sent.
The information that must pass from the Link Layer to the Internetwork Layer for each received packet is:
(1) The IP packet [5.2.2],
(2) The length of the data portion (i.e., not including the Link- Layer framing) of the Link Layer frame [5.2.2],
(3) The identity of the physical interface from which the IP packet was received [5.2.3], and
(4) The classification of the packet's destination physical address as a Link Layer unicast, broadcast, or multicast [4.3.2], [5.3.4].
In addition, the Link Layer also should provide:
(5) The source physical address.
The information that must pass from the Internetwork Layer to the Link Layer for each transmitted packet is:
Baker Standards Track [Page 33]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(1) The IP packet [5.2.1]
(2) The length of the IP packet [5.2.1]
(3) The destination physical interface [5.2.1]
(4) The next hop IP address [5.2.1]
In addition, the Internetwork Layer also should provide:
(5) The Link Layer priority value [5.3.3.2]
The Link Layer must also notify the Internetwork Layer if the packet to be transmitted causes a Link Layer precedence-related error [5.3.3.3].
Routers that can connect to ten megabit Ethernets MAY be able to receive and forward Ethernet packets encapsulated using the trailer encapsulation described in [LINK:1]. However, a router SHOULD NOT originate trailer encapsulated packets. A router MUST NOT originate trailer encapsulated packets without first verifying, using the mechanism described in [INTRO:2], that the immediate destination of the packet is willing and able to accept trailer-encapsulated packets. A router SHOULD NOT agree (using these mechanisms) to accept trailer-encapsulated packets.
Routers that implement ARP MUST be compliant and SHOULD be unconditionally compliant with the requirements in [INTRO:2].
The link layer MUST NOT report a Destination Unreachable error to IP solely because there is no ARP cache entry for a destination; it SHOULD queue up to a small number of datagrams breifly while performing the ARP request/reply sequence, and reply that the destination is unreachable to one of the queued datagrams only when this proves fruitless.
A router MUST not believe any ARP reply that claims that the Link Layer address of another host or router is a broadcast or multicast address.
Baker Standards Track [Page 34]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Routers that can connect to ten megabit Ethernets MUST be compliant and SHOULD be unconditionally compliant with the Ethernet requirements of [INTRO:2].
The MTU of each logical interface MUST be configurable within the range of legal MTUs for the interface.
Many Link Layer protocols define a maximum frame size that may be sent. In such cases, a router MUST NOT allow an MTU to be set which would allow sending of frames larger than those allowed by the Link Layer protocol. However, a router SHOULD be willing to receive a packet as large as the maximum frame size even if that is larger than the MTU.
DISCUSSION Note that this is a stricter requirement than imposed on hosts by [INTRO:2], which requires that the MTU of each physical interface be configurable.
If a network is using an MTU smaller than the maximum frame size for the Link Layer, a router may receive packets larger than the MTU from misconfigured and incompletely initialized hosts. The Robustness Principle indicates that the router should successfully receive these packets if possible.
Contrary to [INTRO:1], the Internet does have a standard point to point line protocol: the Point-to-Point Protocol (PPP), defined in [LINK:2], [LINK:3], [LINK:4], and [LINK:5].
A point to point interface is any interface that is designed to send data over a point to point line. Such interfaces include telephone, leased, dedicated or direct lines (either 2 or 4 wire), and may use point to point channels or virtual circuits of multiplexed interfaces such as ISDN. They normally use a standardized modem or bit serial interface (such as RS-232, RS-449 or V.35), using either synchronous or asynchronous clocking. Multiplexed interfaces often have special physical interfaces.
A general purpose serial interface uses the same physical media as a point to point line, but supports the use of link layer networks as well as point to point connectivity. Link layer networks (such as X.25 or Frame Relay) use an alternative IP link layer specification.
Baker Standards Track [Page 35]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Routers that implement point to point or general purpose serial interfaces MUST IMPLEMENT PPP.
PPP MUST be supported on all general purpose serial interfaces on a router. The router MAY allow the line to be configured to use point to point line protocols other than PPP. Point to point interfaces SHOULD either default to using PPP when enabled or require configuration of the link layer protocol before being enabled. General purpose serial interfaces SHOULD require configuration of the link layer protocol before being enabled.
This section provides guidelines to router implementors so that they can ensure interoperability with other routers using PPP over either synchronous or asynchronous links.
It is critical that an implementor understand the semantics of the option negotiation mechanism. Options are a means for a local device to indicate to a remote peer what the local device will accept from the remote peer, not what it wishes to send. It is up to the remote peer to decide what is most convenient to send within the confines of the set of options that the local device has stated that it can accept. Therefore it is perfectly acceptable and normal for a remote peer to ACK all the options indicated in an LCP Configuration Request (CR) even if the remote peer does not support any of those options. Again, the options are simply a mechanism for either device to indicate to its peer what it will accept, not necessarily what it will send.
The PPP Link Control Protocol (LCP) offers a number of options that may be negotiated. These options include (among others) address and control field compression, protocol field compression, asynchronous character map, Maximum Receive Unit (MRU), Link Quality Monitoring (LQM), magic number (for loopback detection), Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), and the 32-bit Frame Check Sequence (FCS).
A router MAY use address/control field compression on either synchronous or asynchronous links. A router MAY use protocol field compression on either synchronous or asynchronous links. A router that indicates that it can accept these compressions MUST be able to accept uncompressed PPP header information also.
Baker Standards Track [Page 36]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION These options control the appearance of the PPP header. Normally the PPP header consists of the address, the control field, and the protocol field. The address, on a point to point line, is 0xFF, indicating "broadcast". The control field is 0x03, indicating "Unnumbered Information." The Protocol Identifier is a two byte value indicating the contents of the data area of the frame. If a system negotiates address and control field compression it indicates to its peer that it will accept PPP frames that have or do not have these fields at the front of the header. It does not indicate that it will be sending frames with these fields removed.
Protocol field compression, when negotiated, indicates that the system is willing to receive protocol fields compressed to one byte when this is legal. There is no requirement that the sender do so.
Use of address/control field compression is inconsistent with the use of numbered mode (reliable) PPP.
IMPLEMENTATION Some hardware does not deal well with variable length header information. In those cases it makes most sense for the remote peer to send the full PPP header. Implementations may ensure this by not sending the address/control field and protocol field compression options to the remote peer. Even if the remote peer has indicated an ability to receive compressed headers there is no requirement for the local router to send compressed headers.
A router MUST negotiate the Asynchronous Control Character Map (ACCM) for asynchronous PPP links, but SHOULD NOT negotiate the ACCM for synchronous links. If a router receives an attempt to negotiate the ACCM over a synchronous link, it MUST ACKnowledge the option and then ignore it.
DISCUSSION There are implementations that offer both synchronous and asynchronous modes of operation and may use the same code to implement the option negotiation. In this situation it is possible that one end or the other may send the ACCM option on a synchronous link.
A router SHOULD properly negotiate the maximum receive unit (MRU). Even if a system negotiates an MRU smaller than 1,500 bytes, it MUST be able to receive a 1,500 byte frame.
A router SHOULD negotiate and enable the link quality monitoring (LQM) option.
Baker Standards Track [Page 37]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION This memo does not specify a policy for deciding whether the link's quality is adequate. However, it is important (see Section [3.3.6]) that a router disable failed links.
A router SHOULD implement and negotiate the magic number option for loopback detection.
A router MAY support the authentication options (PAP - Password Authentication Protocol, and/or CHAP - Challenge Handshake Authentication Protocol).
A router MUST support 16-bit CRC frame check sequence (FCS) and MAY support the 32-bit CRC.
A router MAY offer to perform IP address negotiation. A router MUST accept a refusal (REJect) to perform IP address negotiation from the peer.
Routers operating at link speeds of 19,200 BPS or less SHOULD implement and offer to perform Van Jacobson header compression. Routers that implement VJ compression SHOULD implement an administrative control enabling or disabling it.
A router MUST have a mechanism to allow routing software to determine whether a physical interface is available to send packets or not; on multiplexed interfaces where permanent virtual circuits are opened for limited sets of neighbors, the router must also be able to determine whether the virtual circuits are viable. A router SHOULD have a mechanism to allow routing software to judge the quality of a physical interface. A router MUST have a mechanism for informing the routing software when a physical interface becomes available or unavailable to send packets because of administrative action. A router MUST have a mechanism for informing the routing software when it detects a Link level interface has become available or unavailable, for any reason.
DISCUSSION It is crucial that routers have workable mechanisms for determining that their network connections are functioning properly. Failure to detect link loss, or failure to take the proper actions when a problem is detected, can lead to black holes.
Baker Standards Track [Page 38]
RFC 1812 Requirements for IP Version 4 Routers June 1995
The mechanisms available for detecting problems with network connections vary considerably, depending on the Link Layer protocols in use and the interface hardware. The intent is to maximize the capability to detect failures within the Link-Layer constraints.
This chapter and chapter 5 discuss the protocols used at the Internet Layer: IP, ICMP, and IGMP. Since forwarding is obviously a crucial topic in a document discussing routers, chapter 5 limits itself to the aspects of the protocols that directly relate to forwarding. The current chapter contains the remainder of the discussion of the Internet Layer protocols.
Routers MUST implement the IP protocol, as defined by [INTERNET:1]. They MUST also implement its mandatory extensions: subnets (defined in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).
Router implementors need not consider compliance with the section of [INTRO:2] entitled "Internet Protocol -- IP," as that section is entirely duplicated or superseded in this document. A router MUST be compliant, and SHOULD be unconditionally compliant, with the requirements of the section entitled "SPECIFIC ISSUES" relating to IP in [INTRO:2].
In the following, the action specified in certain cases is to silently discard a received datagram. This means that the datagram will be discarded without further processing and that the router will not send any ICMP error message (see Section [4.3]) as a result. However, for diagnosis of problems a router SHOULD provide the capability of logging the error (see Section [1.3.3]), including the contents of the silently discarded datagram, and SHOULD count datagrams discarded.
Baker Standards Track [Page 39]
RFC 1812 Requirements for IP Version 4 Routers June 1995
In datagrams received by the router itself, the IP layer MUST interpret IP options that it understands and preserve the rest unchanged for use by higher layer protocols.
Higher layer protocols may require the ability to set IP options in datagrams they send or examine IP options in datagrams they receive. Later sections of this document discuss specific IP option support required by higher layer protocols.
DISCUSSION Neither this memo nor [INTRO:2] define the order in which a receiver must process multiple options in the same IP header. Hosts and routers originating datagrams containing multiple options must be aware that this introduces an ambiguity in the meaning of certain options when combined with a source-route option.
Here are the requirements for specific IP options:
(a) Security Option
Some environments require the Security option in every packet originated or received. Routers SHOULD IMPLEMENT the revised security option described in [INTERNET:5].
DISCUSSION Note that the security options described in [INTERNET:1] and RFC 1038 ([INTERNET:16]) are obsolete.
(b) Stream Identifier Option
This option is obsolete; routers SHOULD NOT place this option in a datagram that the router originates. This option MUST be ignored in datagrams received by the router.
(c) Source Route Options
A router MUST be able to act as the final destination of a source route. If a router receives a packet containing a completed source route, the packet has reached its final destination. In such an option, the pointer points beyond the last field and the destination address in the IP header
Baker Standards Track [Page 40]
RFC 1812 Requirements for IP Version 4 Routers June 1995
addresses the router. The option as received (the recorded route) MUST be passed up to the transport layer (or to ICMP message processing).
In the general case, a correct response to a source-routed datagram traverses the same route. A router MUST provide a means whereby transport protocols and applications can reverse the source route in a received datagram. This reversed source route MUST be inserted into datagrams they originate (see [INTRO:2] for details) when the router is unaware of policy constraints. However, if the router is policy aware, it MAY select another path.
Some applications in the router MAY require that the user be able to enter a source route.
A router MUST NOT originate a datagram containing multiple source route options. What a router should do if asked to forward a packet containing multiple source route options is described in Section [5.2.4.1].
When a source route option is created (which would happen when the router is originating a source routed datagram or is inserting a source route option as a result of a special filter), it MUST be correctly formed even if it is being created by reversing a recorded route that erroneously includes the source host (see case (B) in the discussion below).
DISCUSSION Suppose a source routed datagram is to be routed from source _S to destination D via routers G1, G2, Gn. Source S constructs a datagram with G1's IP address as its destination address, and a source route option to get the datagram the rest of the way to its destination. However, there is an ambiguity in the specification over whether the source route option in a datagram sent out by S should be (A) or (B):
(A): {>>G2, G3, ... Gn, D} <--- CORRECT
(B): {S, >>G2, G3, ... Gn, D} <---- WRONG
(where >> represents the pointer). If (A) is sent, the datagram received at D will contain the option: {G1, G2, ... Gn >>}, with S and D as the IP source and destination addresses. If (B) were sent, the datagram received at D would again contain S and D as the same IP source and destination addresses, but the option would be: {S, G1, ...Gn >>}; i.e., the originating host would be the first hop in the route.
Baker Standards Track [Page 41]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(d) Record Route Option
Routers MAY support the Record Route option in datagrams originated by the router.
(e) Timestamp Option
Routers MAY support the timestamp option in datagrams originated by the router. The following rules apply:
o When originating a datagram containing a Timestamp Option, a router MUST record a timestamp in the option if
- Its Internet address fields are not pre-specified or - Its first pre-specified address is the IP address of the logical interface over which the datagram is being sent (or the router's router-id if the datagram is being sent over an unnumbered interface).
o If the router itself receives a datagram containing a Timestamp Option, the router MUST insert the current time into the Timestamp Option (if there is space in the option to do so) before passing the option to the transport layer or to ICMP for processing. If space is not present, the router MUST increment the Overflow Count in the option.
o A timestamp value MUST follow the rules defined in [INTRO:2].
IMPLEMENTATION To maximize the utility of the timestamps contained in the timestamp option, the timestamp inserted should be, as nearly as practical, the time at which the packet arrived at the router. For datagrams originated by the router, the timestamp inserted should be, as nearly as practical, the time at which the datagram was passed to the Link Layer for transmission.
The timestamp option permits the use of a non-standard time clock, but the use of a non-synchronized clock limits the utility of the time stamp. Therefore, routers are well advised to implement the Network Time Protocol for the purpose of synchronizing their clocks.
Routers are called upon to insert their address into Record Route, Strict Source and Record Route, Loose Source and Record Route, or Timestamp Options. When a router inserts its address into such an option, it MUST use the IP address of the logical interface on which
Baker Standards Track [Page 42]
RFC 1812 Requirements for IP Version 4 Routers June 1995
the packet is being sent. Where this rule cannot be obeyed because the output interface has no IP address (i.e., is an unnumbered interface), the router MUST instead insert its router-id. The router's router-id is one of the router's IP addresses. The Router ID may be specified on a system basis or on a per-link basis. Which of the router's addresses is used as the router-id MUST NOT change (even across reboots) unless changed by the network manager. Relevant management changes include reconfiguration of the router such that the IP address used as the router-id ceases to be one of the router's IP addresses. Routers with multiple unnumbered interfaces MAY have multiple router-id's. Each unnumbered interface MUST be associated with a particular router-id. This association MUST NOT change (even across reboots) without reconfiguration of the router.
DISCUSSION This specification does not allow for routers that do not have at least one IP address. We do not view this as a serious limitation, since a router needs an IP address to meet the manageability requirements of Chapter [8] even if the router is connected only to point-to-point links.
IMPLEMENTATION
One possible method of choosing the router-id that fulfills this requirement is to use the numerically smallest (or greatest) IP address (treating the address as a 32-bit integer) that is assigned to the router.
The IP header contains two reserved bits: one in the Type of Service byte and the other in the Flags field. A router MUST NOT set either of these bits to one in datagrams originated by the router. A router MUST NOT drop (refuse to receive or forward) a packet merely because one or more of these reserved bits has a non-zero value; i.e., the router MUST NOT check the values of thes bits.
DISCUSSION Future revisions to the IP protocol may make use of these unused bits. These rules are intended to ensure that these revisions can be deployed without having to simultaneously upgrade all routers in the Internet.
Baker Standards Track [Page 43]
RFC 1812 Requirements for IP Version 4 Routers June 1995
The Type-of-Service byte in the IP header is divided into three sections: the Precedence field (high-order 3 bits), a field that is customarily called Type of Service or TOS (next 4 bits), and a reserved bit (the low order bit).
Rules governing the reserved bit were described in Section [4.2.2.3].
A more extensive discussion of the TOS field and its use can be found in [ROUTE:11].
The description of the IP Precedence field is superseded by Section [5.3.3]. RFC 795, Service Mappings, is obsolete and SHOULD NOT be implemented.
As stated in Section [5.2.2], a router MUST verify the IP checksum of any packet that is received, and MUST discard messages containing invalid checksums. The router MUST NOT provide a means to disable this checksum verification.
A router MAY use incremental IP header checksum updating when the only change to the IP header is the time to live. This will reduce the possibility of undetected corruption of the IP header by the router. See [INTERNET:6] for a discussion of incrementally updating the checksum.
IMPLEMENTATION A more extensive description of the IP checksum, including extensive implementation hints, can be found in [INTERNET:6] and [INTERNET:7].
A router MUST ignore IP options which it does not recognize. A corollary of this requirement is that a router MUST implement the End of Option List option and the No Operation option, since neither contains an explicit length.
DISCUSSION All future IP options will include an explicit length.
Baker Standards Track [Page 44]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Fragmentation, as described in [INTERNET:1], MUST be supported by a router.
When a router fragments an IP datagram, it SHOULD minimize the number of fragments. When a router fragments an IP datagram, it SHOULD send the fragments in order. A fragmentation method that may generate one IP fragment that is significantly smaller than the other MAY cause the first IP fragment to be the smaller one.
DISCUSSION There are several fragmentation techniques in common use in the Internet. One involves splitting the IP datagram into IP fragments with the first being MTU sized, and the others being approximately the same size, smaller than the MTU. The reason for this is twofold. The first IP fragment in the sequence will be the effective MTU of the current path between the hosts, and the following IP fragments are sized to minimize the further fragmentation of the IP datagram. Another technique is to split the IP datagram into MTU sized IP fragments, with the last fragment being the only one smaller, as described in [INTERNET:1].
A common trick used by some implementations of TCP/IP is to fragment an IP datagram into IP fragments that are no larger than 576 bytes when the IP datagram is to travel through a router. This is intended to allow the resulting IP fragments to pass the rest of the path without further fragmentation. This would, though, create more of a load on the destination host, since it would have a larger number of IP fragments to reassemble into one IP datagram. It would also not be efficient on networks where the MTU only changes once and stays much larger than 576 bytes. Examples include LAN networks such as an IEEE 802.5 network with a MTU of 2048 or an Ethernet network with an MTU of 1500).
One other fragmentation technique discussed was splitting the IP datagram into approximately equal sized IP fragments, with the size less than or equal to the next hop network's MTU. This is intended to minimize the number of fragments that would result from additional fragmentation further down the path, and assure equal delay for each fragment.
Routers SHOULD generate the least possible number of IP fragments.
Work with slow machines leads us to believe that if it is necessary to fragment messages, sending the small IP fragment first maximizes the chance of a host with a slow interface of receiving all the fragments.
Baker Standards Track [Page 45]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Time to Live (TTL) handling for packets originated or received by the router is governed by [INTRO:2]; this section changes none of its stipulations. However, since the remainder of the IP Protocol section of [INTRO:2] is rewritten, this section is as well.
Note in particular that a router MUST NOT check the TTL of a packet except when forwarding it.
A router MUST NOT originate or forward a datagram with a Time-to-Live (TTL) value of zero.
A router MUST NOT discard a datagram just because it was received with TTL equal to zero or one; if it is to the router and otherwise valid, the router MUST attempt to receive it.
On messages the router originates, the IP layer MUST provide a means for the transport layer to set the TTL field of every datagram that is sent. When a fixed TTL value is used, it MUST be configurable. The number SHOULD exceed the typical internet diameter, and current wisdom suggests that it should exceed twice the internet diameter to allow for growth. Current suggested values are normally posted in the Assigned Numbers RFC. The TTL field has two functions: limit the lifetime of TCP segments (see RFC 793 [TCP:1], p. 28), and terminate Internet routing loops. Although TTL is a time in seconds, it also has some attributes of a hop-count, since each router is required to reduce the TTL field by at least one.
TTL expiration is intended to cause datagrams to be discarded by routers, but not by the destination host. Hosts that act as routers by forwarding datagrams must therefore follow the router's rules for TTL.
A higher-layer protocol may want to set the TTL in order to implement an "expanding scope" search for some Internet resource. This is used by some diagnostic tools, and is expected to be useful for locating the "nearest" server of a given class using IP multicasting, for example. A particular transport protocol may also want to specify its own TTL bound on maximum datagram lifetime.
A fixed default value must be at least big enough for the Internet "diameter," i.e., the longest possible path. A reasonable value is
Baker Standards Track [Page 46]
RFC 1812 Requirements for IP Version 4 Routers June 1995
about twice the diameter, to allow for continued Internet growth. As of this writing, messages crossing the United States frequently traverse 15 to 20 routers; this argues for a default TTL value in excess of 40, and 64 is a common value.
As noted in 2.2.5.1, there are now five classes of IP addresses: Class A through Class E. Class D addresses are used for IP multicasting [INTERNET:4], while Class E addresses are reserved for experimental use. The distinction between Class A, B, and C addresses is no longer important; they are used as generalized unicast network prefixes with only historical interest in their class.
An IP multicast address is a 28-bit logical address that stands for a group of hosts, and may be either permanent or transient. Permanent multicast addresses are allocated by the Internet Assigned Number Authority [INTRO:7], while transient addresses may be allocated dynamically to transient groups. Group membership is determined dynamically using IGMP [INTERNET:4].
We now summarize the important special cases for general purpose unicast IP addresses, using the following notation for an IP address:
{ <Network-prefix>, <Host-number> }
and the notation -1 for a field that contains all 1 bits and the notation 0 for a field that contains all 0 bits.
(a) { 0, 0 }
This host on this network. It MUST NOT be used as a source address by routers, except the router MAY use this as a source address as part of an initialization procedure (e.g., if the router is using BOOTP to load its configuration information).
Incoming datagrams with a source address of { 0, 0 } which are received for local delivery (see Section [5.2.3]), MUST be accepted if the router implements the associated protocol and that protocol clearly defines appropriate action to be taken. Otherwise, a router MUST silently discard any locally-delivered datagram whose source address is { 0, 0 }.
Baker Standards Track [Page 47]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION Some protocols define specific actions to take in response to a received datagram whose source address is { 0, 0 }. Two examples are BOOTP and ICMP Mask Request. The proper operation of these protocols often depends on the ability to receive datagrams whose source address is { 0, 0 }. For most protocols, however, it is best to ignore datagrams having a source address of { 0, 0 } since they were probably generated by a misconfigured host or router. Thus, if a router knows how to deal with a given datagram having a { 0, 0 } source address, the router MUST accept it. Otherwise, the router MUST discard it.
See also Section [4.2.3.1] for a non-standard use of { 0, 0 }.
(b) { 0, <Host-number> }
Specified host on this network. It MUST NOT be sent by routers except that the router MAY use this as a source address as part of an initialization procedure by which the it learns its own IP address.
(c) { -1, -1 }
Limited broadcast. It MUST NOT be used as a source address.
A datagram with this destination address will be received by every host and router on the connected physical network, but will not be forwarded outside that network.
(d) { <Network-prefix>, -1 }
Directed Broadcast - a broadcast directed to the specified network prefix. It MUST NOT be used as a source address. A router MAY originate Network Directed Broadcast packets. A router MUST receive Network Directed Broadcast packets; however a router MAY have a configuration option to prevent reception of these packets. Such an option MUST default to allowing reception.
(e) { 127, <any> }
Internal host loopback address. Addresses of this form MUST NOT appear outside a host.
The <Network-prefix> is administratively assigned so that its value will be unique in the routing domain to which the device is connected.
Baker Standards Track [Page 48]
RFC 1812 Requirements for IP Version 4 Routers June 1995
IP addresses are not permitted to have the value 0 or -1 for the <Host-number> or <Network-prefix> fields except in the special cases listed above. This implies that each of these fields will be at least two bits long.
DISCUSSION Previous versions of this document also noted that subnet numbers must be neither 0 nor -1, and must be at least two bits in length. In a CIDR world, the subnet number is clearly an extension of the network prefix and cannot be interpreted without the remainder of the prefix. This restriction of subnet numbers is therefore meaningless in view of CIDR and may be safely ignored.
For further discussion of broadcast addresses, see Section [4.2.3.1].
When a router originates any datagram, the IP source address MUST be one of its own IP addresses (but not a broadcast or multicast address). The only exception is during initialization.
For most purposes, a datagram addressed to a broadcast or multicast destination is processed as if it had been addressed to one of the router's IP addresses; that is to say:
o A router MUST receive and process normally any packets with a broadcast destination address.
o A router MUST receive and process normally any packets sent to a multicast destination address that the router has asked to receive.
The term specific-destination address means the equivalent local IP address of the host. The specific-destination address is defined to be the destination address in the IP header unless the header contains a broadcast or multicast address, in which case the specific-destination is an IP address assigned to the physical interface on which the datagram arrived.
A router MUST silently discard any received datagram containing an IP source address that is invalid by the rules of this section. This validation could be done either by the IP layer or (when appropriate) by each protocol in the transport layer. As with any datagram a router discards, the datagram discard SHOULD be counted.
DISCUSSION A misaddressed datagram might be caused by a Link Layer broadcast of a unicast datagram or by another router or host that is confused or misconfigured.
Baker Standards Track [Page 49]
RFC 1812 Requirements for IP Version 4 Routers June 1995
For historical reasons, there are a number of IP addresses (some standard and some not) which are used to indicate that an IP packet is an IP broadcast. A router
(1) MUST treat as IP broadcasts packets addressed to 255.255.255.255 or { <Network-prefix>, -1 }.
(2) SHOULD silently discard on receipt (i.e., do not even deliver to applications in the router) any packet addressed to 0.0.0.0 or { <Network-prefix>, 0 }. If these packets are not silently discarded, they MUST be treated as IP broadcasts (see Section [5.3.5]). There MAY be a configuration option to allow receipt of these packets. This option SHOULD default to discarding them.
(3) SHOULD (by default) use the limited broadcast address (255.255.255.255) when originating an IP broadcast destined for a connected (sub)network (except when sending an ICMP Address Mask Reply, as discussed in Section [4.3.3.9]). A router MUST receive limited broadcasts.
(4) SHOULD NOT originate datagrams addressed to 0.0.0.0 or { <Network-prefix>, 0 }. There MAY be a configuration option to allow generation of these packets (instead of using the relevant 1s format broadcast). This option SHOULD default to not generating them.
DISCUSSION In the second bullet, the router obviously cannot recognize addresses of the form { <Network-prefix>, 0 } if the router has no interface to that network prefix. In that case, the rules of the second bullet do not apply because, from the point of view of the router, the packet is not an IP broadcast packet.
An IP router SHOULD satisfy the Host Requirements with respect to IP multicasting, as specified in [INTRO:2]. An IP router SHOULD support local IP multicasting on all connected networks. When a mapping from IP multicast addresses to link-layer addresses has been specified (see the various IP-over-xxx specifications), it SHOULD use that mapping, and MAY be configurable to use the link layer broadcast instead. On point-to-point links and all other interfaces, multicasts are encapsulated as link layer broadcasts. Support for
Baker Standards Track [Page 50]
RFC 1812 Requirements for IP Version 4 Routers June 1995
local IP multicasting includes originating multicast datagrams, joining multicast groups and receiving multicast datagrams, and leaving multicast groups. This implies support for all of [INTERNET:4] including IGMP (see Section [4.4]).
DISCUSSION Although [INTERNET:4] is entitled Host Extensions for IP Multicasting, it applies to all IP systems, both hosts and routers. In particular, since routers may join multicast groups, it is correct for them to perform the host part of IGMP, reporting their group memberships to any multicast routers that may be present on their attached networks (whether or not they themselves are multicast routers).
Some router protocols may specifically require support for IP multicasting (e.g., OSPF [ROUTE:1]), or may recommend it (e.g., ICMP Router Discovery [INTERNET:13]).
To eliminate fragmentation or minimize it, it is desirable to know what is the path MTU along the path from the source to destination. The path MTU is the minimum of the MTUs of each hop in the path. [INTERNET:14] describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. For a path that passes through a router that does not support [INTERNET:14], this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by older techniques or the current practice.
When a router is originating an IP datagram, it SHOULD use the scheme described in [INTERNET:14] to limit the datagram's size. If the router's route to the datagram's destination was learned from a routing protocol that provides Path MTU information, the scheme described in [INTERNET:14] is still used, but the Path MTU information from the routing protocol SHOULD be used as the initial guess as to the Path MTU and also as an upper bound on the Path MTU.
Under certain circumstances, it may be desirable to support subnets of a particular network being interconnected only through a path that is not part of the subnetted network. This is known as discontiguous subnetwork support.
Routers MUST support discontiguous subnetworks.
Baker Standards Track [Page 51]
RFC 1812 Requirements for IP Version 4 Routers June 1995
IMPLEMENTATION In classical IP networks, this was very difficult to achieve; in CIDR networks, it is a natural by-product. Therefore, a router SHOULD NOT make assumptions about subnet architecture, but SHOULD treat each route as a generalized network prefix.
DISCUSSION The Internet has been growing at a tremendous rate of late. This has been placing severe strains on the IP addressing technology. A major factor in this strain is the strict IP Address class boundaries. These make it difficult to efficiently size network prefixes to their networks and aggregate several network prefixes into a single route advertisement. By eliminating the strict class boundaries of the IP address and treating each route as a generalized network prefix, these strains may be greatly reduced.
The technology for currently doing this is Classless Inter Domain Routing (CIDR) [INTERNET:15].
For similar reasons, an address block associated with a given network prefix could be subdivided into subblocks of different sizes, so that the network prefixes associated with the subblocks would have different length. For example, within a block whose network prefix is 8 bits long, one subblock may have a 16 bit network prefix, another may have an 18 bit network prefix, and a third a 14 bit network prefix.
Routers MUST support variable length network prefixes in both their interface configurations and their routing databases.
ICMP is an auxiliary protocol, which provides routing, diagnostic and error functionality for IP. It is described in [INTERNET:8]. A router MUST support ICMP.
ICMP messages are grouped in two classes that are discussed in the following sections:
If an ICMP message of unknown type is received, it MUST be passed to the ICMP user interface (if the router has one) or silently discarded (if the router does not have one).
When originating an ICMP message, the router MUST initialize the TTL. The TTL for ICMP responses must not be taken from the packet that triggered the response.
Historically, every ICMP error message has included the Internet header and at least the first 8 data bytes of the datagram that triggered the error. This is no longer adequate, due to the use of IP-in-IP tunneling and other technologies. Therefore, the ICMP datagram SHOULD contain as much of the original datagram as possible without the length of the ICMP datagram exceeding 576 bytes. The returned IP header (and user data) MUST be identical to that which was received, except that the router is not required to undo any modifications to the IP header that are normally performed in forwarding that were performed before the error was detected (e.g., decrementing the TTL, or updating options). Note that the requirements of Section [4.3.3.5] supersede this requirement in some cases (i.e., for a Parameter Problem message, if the problem is in a modified field, the router must undo the modification). See Section [4.3.3.5]).
Except where this document specifies otherwise, the IP source address in an ICMP message originated by the router MUST be one of the IP addresses associated with the physical interface over which the ICMP message is transmitted. If the interface has no IP addresses
Baker Standards Track [Page 53]
RFC 1812 Requirements for IP Version 4 Routers June 1995
associated with it, the router's router-id (see Section [5.2.5]) is used instead.
ICMP error messages SHOULD have their TOS bits set to the same value as the TOS bits in the packet that provoked the sending of the ICMP error message, unless setting them to that value would cause the ICMP error message to be immediately discarded because it could not be routed to its destination. Otherwise, ICMP error messages MUST be sent with a normal (i.e., zero) TOS. An ICMP reply message SHOULD have its TOS bits set to the same value as the TOS bits in the ICMP request that provoked the reply.
ICMP Source Quench error messages, if sent at all, MUST have their IP Precedence field set to the same value as the IP Precedence field in the packet that provoked the sending of the ICMP Source Quench message. All other ICMP error messages (Destination Unreachable, Redirect, Time Exceeded, and Parameter Problem) SHOULD have their precedence value set to 6 (INTERNETWORK CONTROL) or 7 (NETWORK CONTROL). The IP Precedence value for these error messages MAY be settable.
An ICMP reply message MUST have its IP Precedence field set to the same value as the IP Precedence field in the ICMP request that provoked the reply.
If the packet which provokes the sending of an ICMP error message contains a source route option, the ICMP error message SHOULD also contain a source route option of the same type (strict or loose), created by reversing the portion before the pointer of the route recorded in the source route option of the original packet UNLESS the ICMP error message is an ICMP Parameter Problem complaining about a source route option in the original packet, or unless the router is aware of policy that would prevent the delivery of the ICMP error message.
DISCUSSION In environments which use the U.S. Department of Defense security option (defined in [INTERNET:5]), ICMP messages may need to include a security option. Detailed information on this topic should be available from the Defense Communications Agency.
Baker Standards Track [Page 54]
RFC 1812 Requirements for IP Version 4 Routers June 1995
An ICMP error message MUST NOT be sent as the result of receiving:
o An ICMP error message, or
o A packet which fails the IP header validation tests described in Section [5.2.2] (except where that section specifically permits the sending of an ICMP error message), or
o A packet destined to an IP broadcast or IP multicast address, or
o A packet sent as a Link Layer broadcast or multicast, or
o A packet whose source address has a network prefix of zero or is an invalid source address (as defined in Section [5.3.7]), or
o Any fragment of a datagram other then the first fragment (i.e., a packet for which the fragment offset in the IP header is nonzero).
Furthermore, an ICMP error message MUST NOT be sent in any case where this memo states that a packet is to be silently discarded.
NOTE: THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.
DISCUSSION These rules aim to prevent the broadcast storms that have resulted from routers or hosts returning ICMP error messages in response to broadcast packets. For example, a broadcast UDP packet to a non- existent port could trigger a flood of ICMP Destination Unreachable datagrams from all devices that do not have a client for that destination port. On a large Ethernet, the resulting collisions can render the network useless for a second or more.
Every packet that is broadcast on the connected network should have a valid IP broadcast address as its IP destination (see Section [5.3.4] and [INTRO:2]). However, some devices violate this rule. To be certain to detect broadcast packets, therefore, routers are required to check for a link-layer broadcast as well as an IP-layer address.
IMPLEMENTATION+ This requires that the link layer inform the IP layer when a link-layer broadcast packet has been received; see Section [3.1].
Baker Standards Track [Page 55]
RFC 1812 Requirements for IP Version 4 Routers June 1995
A router which sends ICMP Source Quench messages MUST be able to limit the rate at which the messages can be generated. A router SHOULD also be able to limit the rate at which it sends other sorts of ICMP error messages (Destination Unreachable, Redirect, Time Exceeded, Parameter Problem). The rate limit parameters SHOULD be settable as part of the configuration of the router. How the limits are applied (e.g., per router or per interface) is left to the implementor's discretion.
DISCUSSION Two problems for a router sending ICMP error message are: (1) The consumption of bandwidth on the reverse path, and (2) The use of router resources (e.g., memory, CPU time)
To help solve these problems a router can limit the frequency with which it generates ICMP error messages. For similar reasons, a router may limit the frequency at which some other sorts of messages, such as ICMP Echo Replies, are generated.
IMPLEMENTATION Various mechanisms have been used or proposed for limiting the rate at which ICMP messages are sent:
(1) Count-based - for example, send an ICMP error message for every N dropped packets overall or per given source host. This mechanism might be appropriate for ICMP Source Quench, if used, but probably not for other types of ICMP messages.
(2) Timer-based - for example, send an ICMP error message to a given source host or overall at most once per T milliseconds.
(3) Bandwidth-based - for example, limit the rate at which ICMP messages are sent over a particular interface to some fraction of the attached network's bandwidth.
If a router cannot forward a packet because it has no routes at all (including no default route) to the destination specified in the packet, then the router MUST generate a Destination Unreachable, Code 0 (Network Unreachable) ICMP message. If the router does have routes to the destination network specified in the packet but the TOS specified for the routes is neither the default TOS (0000) nor the TOS of the packet that the router is attempting to route, then the
Baker Standards Track [Page 56]
RFC 1812 Requirements for IP Version 4 Routers June 1995
router MUST generate a Destination Unreachable, Code 11 (Network Unreachable for TOS) ICMP message.
If a packet is to be forwarded to a host on a network that is directly connected to the router (i.e., the router is the last-hop router) and the router has ascertained that there is no path to the destination host then the router MUST generate a Destination Unreachable, Code 1 (Host Unreachable) ICMP message. If a packet is to be forwarded to a host that is on a network that is directly connected to the router and the router cannot forward the packet because no route to the destination has a TOS that is either equal to the TOS requested in the packet or is the default TOS (0000) then the router MUST generate a Destination Unreachable, Code 12 (Host Unreachable for TOS) ICMP message.
DISCUSSION The intent is that a router generates the "generic" host/network unreachable if it has no path at all (including default routes) to the destination. If the router has one or more paths to the destination, but none of those paths have an acceptable TOS, then the router generates the "unreachable for TOS" message.
The ICMP Redirect message is generated to inform a local host that it should use a different next hop router for certain traffic.
Contrary to [INTRO:2], a router MAY ignore ICMP Redirects when choosing a path for a packet originated by the router if the router is running a routing protocol or if forwarding is enabled on the router and on the interface over which the packet is being sent.
A router SHOULD NOT originate ICMP Source Quench messages. As specified in Section [4.3.2], a router that does originate Source Quench messages MUST be able to limit the rate at which they are generated.
DISCUSSION Research seems to suggest that Source Quench consumes network bandwidth but is an ineffective (and unfair) antidote to congestion. See, for example, [INTERNET:9] and [INTERNET:10]. Section [5.3.6] discusses the current thinking on how routers ought to deal with overload and network congestion.
A router MAY ignore any ICMP Source Quench messages it receives.
Baker Standards Track [Page 57]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION A router itself may receive a Source Quench as the result of originating a packet sent to another router or host. Such datagrams might be, e.g., an EGP update sent to another router, or a telnet stream sent to a host. A mechanism has been proposed ([INTERNET:11], [INTERNET:12]) to make the IP layer respond directly to Source Quench by controlling the rate at which packets are sent, however, this proposal is currently experimental and not currently recommended.
When a router is forwarding a packet and the TTL field of the packet is reduced to 0, the requirements of section [5.2.3.8] apply.
When the router is reassembling a packet that is destined for the router, it is acting as an Internet host. [INTRO:2]'s reassembly requirements therefore apply.
When the router receives (i.e., is destined for the router) a Time Exceeded message, it MUST comply with [INTRO:2].
A router MUST generate a Parameter Problem message for any error not specifically covered by another ICMP message. The IP header field or IP option including the byte indicated by the pointer field MUST be included unchanged in the IP header returned with this ICMP message. Section [4.3.2] defines an exception to this requirement.
A new variant of the Parameter Problem message was defined in [INTRO:2]: Code 1 = required option is missing.
DISCUSSION This variant is currently in use in the military community for a missing security option.
A router MUST implement an ICMP Echo server function that receives Echo Requests sent to the router, and sends corresponding Echo Replies. A router MUST be prepared to receive, reassemble and echo an ICMP Echo Request datagram at least as the maximum of 576 and the MTUs of all the connected networks.
The Echo server function MAY choose not to respond to ICMP echo requests addressed to IP broadcast or IP multicast addresses.
Baker Standards Track [Page 58]
RFC 1812 Requirements for IP Version 4 Routers June 1995
A router SHOULD have a configuration option that, if enabled, causes the router to silently ignore all ICMP echo requests; if provided, this option MUST default to allowing responses.
DISCUSSION The neutral provision about responding to broadcast and multicast Echo Requests derives from [INTRO:2]'s "Echo Request/Reply" section.
As stated in Section [10.3.3], a router MUST also implement a user/application-layer interface for sending an Echo Request and receiving an Echo Reply, for diagnostic purposes. All ICMP Echo Reply messages MUST be passed to this interface.
The IP source address in an ICMP Echo Reply MUST be the same as the specific-destination address of the corresponding ICMP Echo Request message.
Data received in an ICMP Echo Request MUST be entirely included in the resulting Echo Reply.
If a Record Route and/or Timestamp option is received in an ICMP Echo Request, this option (these options) SHOULD be updated to include the current router and included in the IP header of the Echo Reply message, without truncation. Thus, the recorded route will be for the entire round trip.
If a Source Route option is received in an ICMP Echo Request, the return route MUST be reversed and used as a Source Route option for the Echo Reply message, unless the router is aware of policy that would prevent the delivery of the message.
A router SHOULD NOT originate or respond to these messages.
DISCUSSION The Information Request/Reply pair was intended to support self- configuring systems such as diskless workstations, to allow them to discover their IP network prefixes at boot time. However, these messages are now obsolete. The RARP and BOOTP protocols provide better mechanisms for a host to discover its own IP address.
A router MAY implement Timestamp and Timestamp Reply. If they are implemented then:
Baker Standards Track [Page 59]
RFC 1812 Requirements for IP Version 4 Routers June 1995
o The ICMP Timestamp server function MUST return a Timestamp Reply to every Timestamp message that is received. It SHOULD be designed for minimum variability in delay.
o An ICMP Timestamp Request message to an IP broadcast or IP multicast address MAY be silently discarded.
o The IP source address in an ICMP Timestamp Reply MUST be the same as the specific-destination address of the corresponding Timestamp Request message.
o If a Source Route option is received in an ICMP Timestamp Request, the return route MUST be reversed and used as a Source Route option for the Timestamp Reply message, unless the router is aware of policy that would prevent the delivery of the message.
o If a Record Route and/or Timestamp option is received in a Timestamp Request, this (these) option(s) SHOULD be updated to include the current router and included in the IP header of the Timestamp Reply message.
o If the router provides an application-layer interface for sending Timestamp Request messages then incoming Timestamp Reply messages MUST be passed up to the ICMP user interface.
The preferred form for a timestamp value (the standard value) is milliseconds since midnight, Universal Time. However, it may be difficult to provide this value with millisecond resolution. For example, many systems use clocks that update only at line frequency, 50 or 60 times per second. Therefore, some latitude is allowed in a standard value:
(a) A standard value MUST be updated at least 16 times per second (i.e., at most the six low-order bits of the value may be undefined).
(b) The accuracy of a standard value MUST approximate that of operator-set CPU clocks, i.e., correct within a few minutes.
IMPLEMENTATION To meet the second condition, a router may need to query some time server when the router is booted or restarted. It is recommended that the UDP Time Server Protocol be used for this purpose. A more advanced implementation would use the Network Time Protocol (NTP) to achieve nearly millisecond clock synchronization; however, this is not required.
Baker Standards Track [Page 60]
RFC 1812 Requirements for IP Version 4 Routers June 1995
A router MUST implement support for receiving ICMP Address Mask Request messages and responding with ICMP Address Mask Reply messages. These messages are defined in [INTERNET:2].
A router SHOULD have a configuration option for each logical interface specifying whether the router is allowed to answer Address Mask Requests for that interface; this option MUST default to allowing responses. A router MUST NOT respond to an Address Mask Request before the router knows the correct address mask.
A router MUST NOT respond to an Address Mask Request that has a source address of 0.0.0.0 and which arrives on a physical interface that has associated with it multiple logical interfaces and the address masks for those interfaces are not all the same.
A router SHOULD examine all ICMP Address Mask Replies that it receives to determine whether the information it contains matches the router's knowledge of the address mask. If the ICMP Address Mask Reply appears to be in error, the router SHOULD log the address mask and the sender's IP address. A router MUST NOT use the contents of an ICMP Address Mask Reply to determine the correct address mask.
Because hosts may not be able to learn the address mask if a router is down when the host boots up, a router MAY broadcast a gratuitous ICMP Address Mask Reply on each of its logical interfaces after it has configured its own address masks. However, this feature can be dangerous in environments that use variable length address masks. Therefore, if this feature is implemented, gratuitous Address Mask Replies MUST NOT be broadcast over any logical interface(s) which either:
o Are not configured to send gratuitous Address Mask Replies. Each logical interface MUST have a configuration parameter controlling this, and that parameter MUST default to not sending the gratuitous Address Mask Replies.
o Share subsuming (but not identical) network prefixes and physical interface.
The { <Network-prefix>, -1 } form of the IP broadcast address MUST be used for broadcast Address Mask Replies.
DISCUSSION The ability to disable sending Address Mask Replies by routers is required at a few sites that intentionally lie to their hosts about the address mask. The need for this is expected to go away
Baker Standards Track [Page 61]
RFC 1812 Requirements for IP Version 4 Routers June 1995
as more and more hosts become compliant with the Host Requirements standards.
The reason for both the second bullet above and the requirement about which IP broadcast address to use is to prevent problems when multiple IP network prefixes are in use on the same physical network.
An IP router MUST support the router part of the ICMP Router Discovery Protocol [INTERNET:13] on all connected networks on which the router supports either IP multicast or IP broadcast addressing. The implementation MUST include all the configuration variables specified for routers, with the specified defaults.
DISCUSSION Routers are not required to implement the host part of the ICMP Router Discovery Protocol, but might find it useful for operation while IP forwarding is disabled (i.e., when operating as a host).
DISCUSSION We note that it is quite common for hosts to use RIP Version 1 as the router discovery protocol. Such hosts listen to RIP traffic and use and use information extracted from that traffic to discover routers and to make decisions as to which router to use as a first-hop router for a given destination. While this behavior is discouraged, it is still common and implementors should be aware of it.
IGMP [INTERNET:4] is a protocol used between hosts and multicast routers on a single physical network to establish hosts' membership in particular multicast groups. Multicast routers use this information, in conjunction with a multicast routing protocol, to support IP multicast forwarding across the Internet.
A router SHOULD implement the host part of IGMP.
Baker Standards Track [Page 62]
RFC 1812 Requirements for IP Version 4 Routers June 1995
There is no separate specification of the forwarding function in IP. Instead, forwarding is covered by the protocol specifications for the internet layer protocols ([INTERNET:1], [INTERNET:2], [INTERNET:3], [INTERNET:8], and [ROUTE:11]).
Since none of the primary protocol documents describe the forwarding algorithm in any detail, we present it here. This is just a general outline, and omits important details, such as handling of congestion, that are dealt with in later sections.
It is not required that an implementation follow exactly the algorithms given in sections [5.2.1.1], [5.2.1.2], and [5.2.1.3]. Much of the challenge of writing router software is to maximize the rate at which the router can forward packets while still achieving the same effect of the algorithm. Details of how to do that are beyond the scope of this document, in part because they are heavily dependent on the architecture of the router. Instead, we merely point out the order dependencies among the steps:
(1) A router MUST verify the IP header, as described in section [5.2.2], before performing any actions based on the contents of the header. This allows the router to detect and discard bad packets before the expenditure of other resources.
(2) Processing of certain IP options requires that the router insert its IP address into the option. As noted in Section [5.2.4], the address inserted MUST be the address of the logical interface on which the packet is sent or the router's router-id if the packet is sent over an unnumbered interface. Thus, processing of these options cannot be completed until after the output interface is chosen.
(3) The router cannot check and decrement the TTL before checking whether the packet should be delivered to the router itself, for reasons mentioned in Section [4.2.2.9].
(4) More generally, when a packet is delivered locally to the router, its IP header MUST NOT be modified in any way (except that a
Baker Standards Track [Page 63]
RFC 1812 Requirements for IP Version 4 Routers June 1995
router may be required to insert a timestamp into any Timestamp options in the IP header). Thus, before the router determines whether the packet is to be delivered locally to the router, it cannot update the IP header in any way that it is not prepared to undo.
This section covers the general forwarding algorithm. This algorithm applies to all forms of packets to be forwarded: unicast, multicast, and broadcast.
(1) The router receives the IP packet (plus additional information about it, as described in Section [3.1]) from the Link Layer.
(2) The router validates the IP header, as described in Section [5.2.2]. Note that IP reassembly is not done, except on IP fragments to be queued for local delivery in step (4).
(3) The router performs most of the processing of any IP options. As described in Section [5.2.4], some IP options require additional processing after the routing decision has been made.
(4) The router examines the destination IP address of the IP datagram, as described in Section [5.2.3], to determine how it should continue to process the IP datagram. There are three possibilities:
o The IP datagram is destined for the router, and should be queued for local delivery, doing reassembly if needed.
o The IP datagram is not destined for the router, and should be queued for forwarding.
o The IP datagram should be queued for forwarding, but (a copy) must also be queued for local delivery.
Since the local delivery case is well covered by [INTRO:2], the following assumes that the IP datagram was queued for forwarding. If the destination is an IP unicast address:
(5) The forwarder determines the next hop IP address for the packet, usually by looking up the packet's destination in the router's routing table. This procedure is described in more detail in Section [5.2.4]. This procedure also decides which network
Baker Standards Track [Page 64]
RFC 1812 Requirements for IP Version 4 Routers June 1995
interface should be used to send the packet.
(6) The forwarder verifies that forwarding the packet is permitted. The source and destination addresses should be valid, as described in Section [5.3.7] and Section [5.3.4] If the router supports administrative constraints on forwarding, such as those described in Section [5.3.9], those constraints must be satisfied.
(7) The forwarder decrements (by at least one) and checks the packet's TTL, as described in Section [5.3.1].
(8) The forwarder performs any IP option processing that could not be completed in step 3.
(9) The forwarder performs any necessary IP fragmentation, as described in Section [4.2.2.7]. Since this step occurs after outbound interface selection (step 5), all fragments of the same datagram will be transmitted out the same interface.
(10) The forwarder determines the Link Layer address of the packet's next hop. The mechanisms for doing this are Link Layer- dependent (see chapter 3).
(11) The forwarder encapsulates the IP datagram (or each of the fragments thereof) in an appropriate Link Layer frame and queues it for output on the interface selected in step 5.
(12) The forwarder sends an ICMP redirect if necessary, as described in Section [4.3.3.2].
If the destination is an IP multicast, the following steps are taken.
Note that the main differences between the forwarding of IP unicasts and the forwarding of IP multicasts are
o IP multicasts are usually forwarded based on both the datagram's source and destination IP addresses,
o IP multicast uses an expanding ring search,
o IP multicasts are forwarded as Link Level multicasts, and
o ICMP errors are never sent in response to IP multicast datagrams.
Baker Standards Track [Page 65]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Note that the forwarding of IP multicasts is still somewhat experimental. As a result, the algorithm presented below is not mandatory, and is provided as an example only.
(5a) Based on the IP source and destination addresses found in the datagram header, the router determines whether the datagram has been received on the proper interface for forwarding. If not, the datagram is dropped silently. The method for determining the proper receiving interface depends on the multicast routing algorithm(s) in use. In one of the simplest algorithms, reverse path forwarding (RPF), the proper interface is the one that would be used to forward unicasts back to the datagram source.
(6a) Based on the IP source and destination addresses found in the datagram header, the router determines the datagram's outgoing interfaces. To implement IP multicast's expanding ring search (see [INTERNET:4]) a minimum TTL value is specified for each outgoing interface. A copy of the multicast datagram is forwarded out each outgoing interface whose minimum TTL value is less than or equal to the TTL value in the datagram header, by separately applying the remaining steps on each such interface.
(7a) The router decrements the packet's TTL by one.
(8a) The forwarder performs any IP option processing that could not be completed in step (3).
(9a) The forwarder performs any necessary IP fragmentation, as described in Section [4.2.2.7].
(10a) The forwarder determines the Link Layer address to use in the Link Level encapsulation. The mechanisms for doing this are Link Layer-dependent. On LANs a Link Level multicast or broadcast is selected, as an algorithmic translation of the datagrams' IP multicast address. See the various IP-over-xxx specifications for more details.
(11a) The forwarder encapsulates the packet (or each of the fragments thereof) in an appropriate Link Layer frame and queues it for output on the appropriate interface.
Baker Standards Track [Page 66]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Before a router can process any IP packet, it MUST perform a the following basic validity checks on the packet's IP header to ensure that the header is meaningful. If the packet fails any of the following tests, it MUST be silently discarded, and the error SHOULD be logged.
(1) The packet length reported by the Link Layer must be large enough to hold the minimum length legal IP datagram (20 bytes).
(2) The IP checksum must be correct.
(3) The IP version number must be 4. If the version number is not 4 then the packet may be another version of IP, such as IPng or ST-II.
(4) The IP header length field must be large enough to hold the minimum length legal IP datagram (20 bytes = 5 words).
(5) The IP total length field must be large enough to hold the IP datagram header, whose length is specified in the IP header length field.
A router MUST NOT have a configuration option that allows disabling any of these tests.
If the packet passes the second and third tests, the IP header length field is at least 4, and both the IP total length field and the packet length reported by the Link Layer are at least 16 then, despite the above rule, the router MAY respond with an ICMP Parameter Problem message, whose pointer points at the IP header length field (if it failed the fourth test) or the IP total length field (if it failed the fifth test). However, it still MUST discard the packet and still SHOULD log the error.
These rules (and this entire document) apply only to version 4 of the Internet Protocol. These rules should not be construed as prohibiting routers from supporting other versions of IP. Furthermore, if a router can truly classify a packet as being some other version of IP then it ought not treat that packet as an error packet within the context of this memo.
IMPLEMENTATION It is desirable for purposes of error reporting, though not always entirely possible, to determine why a header was invalid. There are four possible reasons:
Baker Standards Track [Page 67]
RFC 1812 Requirements for IP Version 4 Routers June 1995
o The Link Layer truncated the IP header
o The datagram is using a version of IP other than the standard one (version 4).
o The IP header has been corrupted in transit.
o The sender generated an illegal IP header.
It is probably desirable to perform the checks in the order listed, since we believe that this ordering is most likely to correctly categorize the cause of the error. For purposes of error reporting, it may also be desirable to check if a packet that fails these tests has an IP version number indicating IPng or ST-II; these should be handled according to their respective specifications.
Additionally, the router SHOULD verify that the packet length reported by the Link Layer is at least as large as the IP total length recorded in the packet's IP header. If it appears that the packet has been truncated, the packet MUST be discarded, the error SHOULD be logged, and the router SHOULD respond with an ICMP Parameter Problem message whose pointer points at the IP total length field.
DISCUSSION Because any higher layer protocol that concerns itself with data corruption will detect truncation of the packet data when it reaches its final destination, it is not absolutely necessary for routers to perform the check suggested above to maintain protocol correctness. However, by making this check a router can simplify considerably the task of determining which hop in the path is truncating the packets. It will also reduce the expenditure of resources down-stream from the router in that down-stream systems will not need to deal with the packet.
Finally, if the destination address in the IP header is not one of the addresses of the router, the router SHOULD verify that the packet does not contain a Strict Source and Record Route option. If a packet fails this test (if it contains a strict source route option), the router SHOULD log the error and SHOULD respond with an ICMP Parameter Problem error with the pointer pointing at the offending packet's IP destination address.
DISCUSSION Some people might suggest that the router should respond with a Bad Source Route message instead of a Parameter Problem message. However, when a packet fails this test, it usually indicates a
Baker Standards Track [Page 68]
RFC 1812 Requirements for IP Version 4 Routers June 1995
protocol error by the previous hop router, whereas Bad Source Route would suggest that the source host had requested a nonexistent or broken path through the network.
When a router receives an IP packet, it must decide whether the packet is addressed to the router (and should be delivered locally) or the packet is addressed to another system (and should be handled by the forwarder). There is also a hybrid case, where certain IP broadcasts and IP multicasts are both delivered locally and forwarded. A router MUST determine which of the these three cases applies using the following rules.
o An unexpired source route option is one whose pointer value does not point past the last entry in the source route. If the packet contains an unexpired source route option, the pointer in the option is advanced until either the pointer does point past the last address in the option or else the next address is not one of the router's own addresses. In the latter (normal) case, the packet is forwarded (and not delivered locally) regardless of the rules below.
o The packet is delivered locally and not considered for forwarding in the following cases:
- The packet's destination address exactly matches one of the router's IP addresses,
- The packet's destination address is a limited broadcast address ({-1, -1}), or
- The packet's destination is an IP multicast address which is never forwarded (such as 224.0.0.1 or 224.0.0.2) and (at least) one of the logical interfaces associated with the physical interface on which the packet arrived is a member of the destination multicast group.
o The packet is passed to the forwarder AND delivered locally in the following cases:
- The packet's destination address is an IP broadcast address that addresses at least one of the router's logical interfaces but does not address any of the logical interfaces associated with the physical interface on which the packet arrived
Baker Standards Track [Page 69]
RFC 1812 Requirements for IP Version 4 Routers June 1995
- The packet's destination is an IP multicast address which is permitted to be forwarded (unlike 224.0.0.1 and 224.0.0.2) and (at least) one of the logical interfaces associated with the physical interface on which the packet arrived is a member of the destination multicast group.
o The packet is delivered locally if the packet's destination address is an IP broadcast address (other than a limited broadcast address) that addresses at least one of the logical interfaces associated with the physical interface on which the packet arrived. The packet is ALSO passed to the forwarder unless the link on which the packet arrived uses an IP encapsulation that does not encapsulate broadcasts differently than unicasts (e.g., by using different Link Layer destination addresses).
o The packet is passed to the forwarder in all other cases.
DISCUSSION The purpose of the requirement in the last sentence of the fourth bullet is to deal with a directed broadcast to another network prefix on the same physical cable. Normally, this works as expected: the sender sends the broadcast to the router as a Link Layer unicast. The router notes that it arrived as a unicast, and therefore must be destined for a different network prefix than the sender sent it on. Therefore, the router can safely send it as a Link Layer broadcast out the same (physical) interface over which it arrived. However, if the router can't tell whether the packet was received as a Link Layer unicast, the sentence ensures that the router does the safe but wrong thing rather than the unsafe but right thing.
IMPLEMENTATION As described in Section [5.3.4], packets received as Link Layer broadcasts are generally not forwarded. It may be advantageous to avoid passing to the forwarder packets it would later discard because of the rules in that section.
Some Link Layers (either because of the hardware or because of special code in the drivers) can deliver to the router copies of all Link Layer broadcasts and multicasts it transmits. Use of this feature can simplify the implementation of cases where a packet has to both be passed to the forwarder and delivered locally, since forwarding the packet will automatically cause the router to receive a copy of the packet that it can then deliver locally. One must use care in these circumstances to prevent treating a received loop-back packet as a normal packet that was received (and then being subject to the rules of forwarding, etc.).
Baker Standards Track [Page 70]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Even without such a Link Layer, it is of course hardly necessary to make a copy of an entire packet to queue it both for forwarding and for local delivery, though care must be taken with fragments, since reassembly is performed on locally delivered packets but not on forwarded packets. One simple scheme is to associate a flag with each packet on the router's output queue that indicates whether it should be queued for local delivery after it has been sent.
When a router is going to forward a packet, it must determine whether it can send it directly to its destination, or whether it needs to pass it through another router. If the latter, it needs to determine which router to use. This section explains how these determinations are made.
This section makes use of the following definitions:
o LSRR - IP Loose Source and Record Route option
o SSRR - IP Strict Source and Record Route option
o Source Route Option - an LSRR or an SSRR
o Ultimate Destination Address - where the packet is being sent to: the last address in the source route of a source-routed packet, or the destination address in the IP header of a non-source-routed packet
o Adjacent - reachable without going through any IP routers
o Next Hop Address - the IP address of the adjacent host or router to which the packet should be sent next
o IP Destination Address - the ultimate destination address, except in source routed packets, where it is the next address specified in the source route
o Immediate Destination - the node, System, router, end-system, or whatever that is addressed by the IP Destination Address.
Baker Standards Track [Page 71]
RFC 1812 Requirements for IP Version 4 Routers June 1995
o the destination address in the IP header is one of the addresses of the router,
o the packet contains a Source Route Option, and
o the pointer in the Source Route Option does not point past the end of the option,
then the next IP Destination Address is the address pointed at by the pointer in that option. If:
o the destination address in the IP header is one of the addresses of the router,
o the packet contains a Source Route Option, and
o the pointer in the Source Route Option points past the end of the option,
then the message is addressed to the system analyzing the message.
A router MUST use the IP Destination Address, not the Ultimate Destination Address (the last address in the source route option), when determining how to handle a packet.
It is an error for more than one source route option to appear in a datagram. If it receives such a datagram, it SHOULD discard the packet and reply with an ICMP Parameter Problem message whose pointer points at the beginning of the second source route option.
After it has been determined that the IP packet needs to be forwarded according to the rules specified in Section [5.2.3], the following algorithm MUST be used to determine if the Immediate Destination is directly accessible (see [INTERNET:2]).
(1) For each network interface that has not been assigned any IP address (the unnumbered lines as described in Section [2.2.7]), compare the router-id of the other end of the line to the IP Destination Address. If they are exactly equal, the packet can be transmitted through this interface.
Baker Standards Track [Page 72]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION In other words, the router or host at the remote end of the line is the destination of the packet or is the next step in the source route of a source routed packet.
(2) If no network interface has been selected in the first step, for each IP address assigned to the router:
(a) isolate the network prefix used by the interface.
IMPLEMENTATION The result of this operation will usually have been computed and saved during initialization.
(b) Isolate the corresponding set of bits from the IP Destination Address of the packet.
(c) Compare the resulting network prefixes. If they are equal to each other, the packet can be transmitted through the corresponding network interface.
(3) If the destination was neither the router-id of a neighbor on an unnumbered interface nor a member of a directly connected network prefix, the IP Destination is accessible only through some other router. The selection of the router and the next hop IP address is described in Section [5.2.4.3]. In the case of a host that is not also a router, this may be the configured default router.
Ongoing work in the IETF [ARCH:9, NRHP] considers some cases such as when multiple IP (sub)networks are overlaid on the same link layer network. Barring policy restrictions, hosts and routers using a common link layer network can directly communicate even if they are not in the same IP (sub)network, if there is adequate information present. The Next Hop Routing Protocol (NHRP) enables IP entities to determine the "optimal" link layer address to be used to traverse such a link layer network towards a remote destination.
(4) If the selected "next hop" is reachable through an interface configured to use NHRP, then the following additional steps apply:
(a) Compare the IP Destination Address to the destination addresses in the NHRP cache. If the address is in the cache, then send the datagram to the corresponding cached link layer address. (b) If the address is not in the cache, then construct an NHRP request packet containing the IP Destination Address. This message is sent to the NHRP server configured for that interface. This may be a logically separate process or entity in the router itself.
Baker Standards Track [Page 73]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(c) The NHRP server will respond with the proper link layer address to use to transmit the datagram and subsequent datagrams to the same destination. The system MAY transmit the datagram(s) to the traditional "next hop" router while awaiting the NHRP reply.
EDITORS+COMMENTS The router applies the algorithm in the previous section to determine if the IP Destination Address is adjacent. If so, the next hop address is the same as the IP Destination Address. Otherwise, the packet must be forwarded through another router to reach its Immediate Destination. The selection of this router is the topic of this section.
If the packet contains an SSRR, the router MUST discard the packet and reply with an ICMP Bad Source Route error. Otherwise, the router looks up the IP Destination Address in its routing table to determine an appropriate next hop address.
DISCUSSION Per the IP specification, a Strict Source Route must specify a sequence of nodes through which the packet must traverse; the packet must go from one node of the source route to the next, traversing intermediate networks only. Thus, if the router is not adjacent to the next step of the source route, the source route can not be fulfilled. Therefore, the router rejects such with an ICMP Bad Source Route error.
The goal of the next-hop selection process is to examine the entries in the router's Forwarding Information Base (FIB) and select the best route (if there is one) for the packet from those available in the FIB.
Conceptually, any route lookup algorithm starts out with a set of candidate routes that consists of the entire contents of the FIB. The algorithm consists of a series of steps that discard routes from the set. These steps are referred to as Pruning Rules. Normally, when the algorithm terminates there is exactly one route remaining in the set. If the set ever becomes empty, the packet is discarded because the destination is unreachable. It is also possible for the algorithm to terminate when more than one route remains in the set. In this case, the router may arbitrarily discard all but one of them, or may perform "load-splitting" by choosing whichever of the routes has been least recently used.
With the exception of rule 3 (Weak TOS), a router MUST use the following Pruning Rules when selecting a next hop for a packet. If a
Baker Standards Track [Page 74]
RFC 1812 Requirements for IP Version 4 Routers June 1995
router does consider TOS when making next-hop decisions, the Rule 3 must be applied in the order indicated below. These rules MUST be (conceptually) applied to the FIB in the order that they are presented. (For some historical perspective, additional pruning rules, and other common algorithms in use, see Appendix E.)
DISCUSSION Rule 3 is optional in that Section [5.3.2] says that a router only SHOULD consider TOS when making forwarding decisions.
(1) Basic Match This rule discards any routes to destinations other than the IP Destination Address of the packet. For example, if a packet's IP Destination Address is 10.144.2.5, this step would discard a route to net 128.12.0.0/16 but would retain any routes to the network prefixes 10.0.0.0/8 and 10.144.0.0/16, and any default routes.
More precisely, we assume that each route has a destination attribute, called route.dest and a corresponding prefix length, called route.length, to specify which bits of route.dest are significant. The IP Destination Address of the packet being forwarded is ip.dest. This rule discards all routes from the set of candidates except those for which the most significant route.length bits of route.dest and ip.dest are equal.
For example, if a packet's IP Destination Address is 10.144.2.5 and there are network prefixes 10.144.1.0/24, 10.144.2.0/24, and 10.144.3.0/24, this rule would keep only 10.144.2.0/24; it is the only route whose prefix has the same value as the corresponding bits in the IP Destination Address of the packet.
(2) Longest Match Longest Match is a refinement of Basic Match, described above. After performing Basic Match pruning, the algorithm examines the remaining routes to determine which among them have the largest route.length values. All except these are discarded.
For example, if a packet's IP Destination Address is 10.144.2.5 and there are network prefixes 10.144.2.0/24, 10.144.0.0/16, and 10.0.0.0/8, then this rule would keep only the first (10.144.2.0/24) because its prefix length is longest.
Baker Standards Track [Page 75]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(3) Weak TOS Each route has a type of service attribute, called route.tos, whose possible values are assumed to be identical to those used in the TOS field of the IP header. Routing protocols that distribute TOS information fill in route.tos appropriately in routes they add to the FIB; routes from other routing protocols are treated as if they have the default TOS (0000). The TOS field in the IP header of the packet being routed is called ip.tos.
The set of candidate routes is examined to determine if it contains any routes for which route.tos = ip.tos. If so, all routes except those for which route.tos = ip.tos are discarded. If not, all routes except those for which route.tos = 0000 are discarded from the set of candidate routes.
Additional discussion of routing based on Weak TOS may be found in [ROUTE:11].
DISCUSSION The effect of this rule is to select only those routes that have a TOS that matches the TOS requested in the packet. If no such routes exist then routes with the default TOS are considered. Routes with a non-default TOS that is not the TOS requested in the packet are never used, even if such routes are the only available routes that go to the packet's destination.
(4) Best Metric Each route has a metric attribute, called route.metric, and a routing domain identifier, called route.domain. Each member of the set of candidate routes is compared with each other member of the set. If route.domain is equal for the two routes and route.metric is strictly inferior for one when compared with the other, then the one with the inferior metric is discarded from the set. The determination of inferior is usually by a simple arithmetic comparison, though some protocols may have structured metrics requiring more complex comparisons.
(5) Vendor Policy Vendor Policy is sort of a catch-all to make up for the fact that the previously listed rules are often inadequate to choose from the possible routes. Vendor Policy pruning rules are extremely vendor-specific. See section [5.2.4.4].
This algorithm has two distinct disadvantages. Presumably, a router implementor might develop techniques to deal with these
Baker Standards Track [Page 76]
RFC 1812 Requirements for IP Version 4 Routers June 1995
disadvantages and make them a part of the Vendor Policy pruning rule.
(1) IS-IS and OSPF route classes are not directly handled.
(2) Path properties other than type of service (e.g., MTU) are ignored.
It is also worth noting a deficiency in the way that TOS is supported: routing protocols that support TOS are implicitly preferred when forwarding packets that have non-zero TOS values.
The Basic Match and Longest Match pruning rules generalize the treatment of a number of particular types of routes. These routes are selected in the following, decreasing, order of preference:
(1) Host Route: This is a route to a specific end system.
(2) Hierarchical Network Prefix Routes: This is a route to a particular network prefix. Note that the FIB may contain several routes to network prefixes that subsume each other (one prefix is the other prefix with additional bits). These are selected in order of decreasing prefix length.
(5) Default Route: This is a route to all networks for which there are no explicit routes. It is by definition the route whose prefix length is zero.
If, after application of the pruning rules, the set of routes is empty (i.e., no routes were found), the packet MUST be discarded and an appropriate ICMP error generated (ICMP Bad Source Route if the IP Destination Address came from a source route option; otherwise, whichever of ICMP Destination Host Unreachable or Destination Network Unreachable is appropriate, as described in Section [4.3.3.1]).
One suggested mechanism for the Vendor Policy Pruning Rule is to use administrative preference, which is a simple prioritization algorithm. The idea is to manually prioritize the routes that one might need to select among.
Each route has associated with it a preference value, based on various attributes of the route (specific mechanisms for assignment of preference values are suggested below). This preference value is an integer in the range [0..255], with zero being the most preferred and 254 being the least preferred. 255 is a special
Baker Standards Track [Page 77]
RFC 1812 Requirements for IP Version 4 Routers June 1995
value that means that the route should never be used. The first step in the Vendor Policy pruning rule discards all but the most preferable routes (and always discards routes whose preference value is 255).
This policy is not safe in that it can easily be misused to create routing loops. Since no protocol ensures that the preferences configured for a router is consistent with the preferences configured in its neighbors, network managers must exercise care in configuring preferences.
o Address Match It is useful to be able to assign a single preference value to all routes (learned from the same routing domain) to any of a specified set of destinations, where the set of destinations is all destinations that match a specified network prefix.
o Route Class For routing protocols which maintain the distinction, it is useful to be able to assign a single preference value to all routes (learned from the same routing domain) which have a particular route class (intra-area, inter-area, external with internal metrics, or external with external metrics).
o Interface It is useful to be able to assign a single preference value to all routes (learned from a particular routing domain) that would cause packets to be routed out a particular logical interface on the router (logical interfaces generally map one-to-one onto the router's network interfaces, except that any network interface that has multiple IP addresses will have multiple logical interfaces associated with it).
o Source router It is useful to be able to assign a single preference value to all routes (learned from the same routing domain) that were learned from any of a set of routers, where the set of routers are those whose updates have a source address that match a specified network prefix.
o Originating AS For routing protocols which provide the information, it is useful to be able to assign a single preference value to all routes (learned from a particular routing domain) which originated in another particular routing domain. For BGP routes, the originating AS is the first AS listed in the route's AS_PATH attribute. For OSPF external routes, the originating AS may be considered to be the low order 16 bits of the route's
Baker Standards Track [Page 78]
RFC 1812 Requirements for IP Version 4 Routers June 1995
external route tag if the tag's Automatic bit is set and the tag's Path Length is not equal to 3.
o External route tag It is useful to be able to assign a single preference value to all OSPF external routes (learned from the same routing domain) whose external route tags match any of a list of specified values. Because the external route tag may contain a structured value, it may be useful to provide the ability to match particular subfields of the tag.
o AS path It may be useful to be able to assign a single preference value to all BGP routes (learned from the same routing domain) whose AS path "matches" any of a set of specified values. It is not yet clear exactly what kinds of matches are most useful. A simple option would be to allow matching of all routes for which a particular AS number appears (or alternatively, does not appear) anywhere in the route's AS_PATH attribute. A more general but somewhat more difficult alternative would be to allow matching all routes for which the AS path matches a specified regular expression.
At the end of the Next-hop selection process, multiple routes may still remain. A router has several options when this occurs. It may arbitrarily discard some of the routes. It may reduce the number of candidate routes by comparing metrics of routes from routing domains that are not considered equivalent. It may retain more than one route and employ a load-splitting mechanism to divide traffic among them. Perhaps the only thing that can be said about the relative merits of the options is that load-splitting is useful in some situations but not in others, so a wise implementor who implements load-splitting will also provide a way for the network manager to disable it.
The IP header contains several reserved bits, in the Type of Service field and in the Flags field. Routers MUST NOT drop packets merely because one or more of these reserved bits has a non-zero value.
Routers MUST ignore and MUST pass through unchanged the values of these reserved bits. If a router fragments a packet, it MUST copy these bits into each fragment.
Baker Standards Track [Page 79]
RFC 1812 Requirements for IP Version 4 Routers June 1995
DISCUSSION Future revisions to the IP protocol may make use of these unused bits. These rules are intended to ensure that these revisions can be deployed without having to simultaneously upgrade all routers in the Internet.
As was discussed in Section [4.2.2.7], a router MUST support IP fragmentation.
A router MUST NOT reassemble any datagram before forwarding it.
DISCUSSION A few people have suggested that there might be some topologies where reassembly of transit datagrams by routers might improve performance. The fact that fragments may take different paths to the destination precludes safe use of such a feature.
Nothing in this section should be construed to control or limit fragmentation or reassembly performed as a link layer function by the router.
Similarly, if an IP datagram is encapsulated in another IP datagram (e.g., it is tunnelled), that datagram is in turn fragmented, the fragments must be reassembled in order to forward the original datagram. This section does not preclude this.
The ICMP Destination Unreachable message is sent by a router in response to a packet which it cannot forward because the destination (or next hop) is unreachable or a service is unavailable. Examples of such cases include a message addressed to a host which is not there and therefore does not respond to ARP requests, and messages addressed to network prefixes for which the router has no valid route.
A router MUST be able to generate ICMP Destination Unreachable messages and SHOULD choose a response code that most closely matches the reason the message is being generated.
The following codes are defined in [INTERNET:8] and [INTRO:2]:
Baker Standards Track [Page 80]
RFC 1812 Requirements for IP Version 4 Routers June 1995
0 = Network Unreachable - generated by a router if a forwarding path (route) to the destination network is not available;
1 = Host Unreachable - generated by a router if a forwarding path (route) to the destination host on a directly connected network is not available (does not respond to ARP);
2 = Protocol Unreachable - generated if the transport protocol designated in a datagram is not supported in the transport layer of the final destination;
3 = Port Unreachable - generated if the designated transport protocol (e.g., UDP) is unable to demultiplex the datagram in the transport layer of the final destination but has no protocol mechanism to inform the sender;
4 = Fragmentation Needed and DF Set - generated if a router needs to fragment a datagram but cannot since the DF flag is set;
5 = Source Route Failed - generated if a router cannot forward a packet to the next hop in a source route option;
6 = Destination Network Unknown - This code SHOULD NOT be generated since it would imply on the part of the router that the destination network does not exist (net unreachable code 0 SHOULD be used in place of code 6);
7 = Destination Host Unknown - generated only when a router can determine (from link layer advice) that the destination host does not exist;
11 = Network Unreachable For Type Of Service - generated by a router if a forwarding path (route) to the destination network with the requested or default TOS is not available;
12 = Host Unreachable For Type Of Service - generated if a router cannot forward a packet because its route(s) to the destination do not match either the TOS requested in the datagram or the default TOS (0).
The following additional codes are hereby defined:
13 = Communication Administratively Prohibited - generated if a router cannot forward a packet due to administrative filtering;
14 = Host Precedence Violation. Sent by the first hop router to a host to indicate that a requested precedence is not permitted for the particular combination of source/destination host or
Baker Standards Track [Page 81]
RFC 1812 Requirements for IP Version 4 Routers June 1995
network, upper layer protocol, and source/destination port;
15 = Precedence cutoff in effect. The network operators have imposed a minimum level of precedence required for operation, the datagram was sent with a precedence below this level;
NOTE: [INTRO:2] defined Code 8 for source host isolated. Routers SHOULD NOT generate Code 8; whichever of Codes 0 (Network Unreachable) and 1 (Host Unreachable) is appropriate SHOULD be used instead. [INTRO:2] also defined Code 9 for communication with destination network administratively prohibited and Code 10 for communication with destination host administratively prohibited. These codes were intended for use by end-to-end encryption devices used by U.S military agencies. Routers SHOULD use the newly defined Code 13 (Communication Administratively Prohibited) if they administratively filter packets.
Routers MAY have a configuration option that causes Code 13 (Communication Administratively Prohibited) messages not to be generated. When this option is enabled, no ICMP error message is sent in response to a packet that is dropped because its forwarding is administratively prohibited.
Similarly, routers MAY have a configuration option that causes Code 14 (Host Precedence Violation) and Code 15 (Precedence Cutoff in Effect) messages not to be generated. When this option is enabled, no ICMP error message is sent in response to a packet that is dropped because of a precedence violation.
Routers MUST use Host Unreachable or Destination Host Unknown codes whenever other hosts on the same destination network might be reachable; otherwise, the source host may erroneously conclude that all hosts on the network are unreachable, and that may not be the case.
[INTERNET:14] describes a slight modification the form of Destination Unreachable messages containing Code 4 (Fragmentation needed and DF set). A router MUST use this modified form when originating Code 4 Destination Unreachable messages.
The ICMP Redirect message is generated to inform a local host the it should use a different next hop router for a certain class of traffic.
Routers MUST NOT generate the Redirect for Network or Redirect for Network and Type of Service messages (Codes 0 and 2) specified in
Baker Standards Track [Page 82]
RFC 1812 Requirements for IP Version 4 Routers June 1995
[INTERNET:8]. Routers MUST be able to generate the Redirect for Host message (Code 1) and SHOULD be able to generate the Redirect for Type of Service and Host message (Code 3) specified in [INTERNET:8].
DISCUSSION If the directly connected network is not subnetted (in the classical sense), a router can normally generate a network Redirect that applies to all hosts on a specified remote network. Using a network rather than a host Redirect may economize slightly on network traffic and on host routing table storage. However, the savings are not significant, and subnets create an ambiguity about the subnet mask to be used to interpret a network Redirect. In a CIDR environment, it is difficult to specify precisely the cases in which network Redirects can be used. Therefore, routers must send only host (or host and type of service) Redirects.
A Code 3 (Redirect for Host and Type of Service) message is generated when the packet provoking the redirect has a destination for which the path chosen by the router would depend (in part) on the TOS requested.
Routers that can generate Code 3 redirects (Host and Type of Service) MUST have a configuration option (which defaults to on) to enable Code 1 (Host) redirects to be substituted for Code 3 redirects. A router MUST send a Code 1 Redirect in place of a Code 3 Redirect if it has been configured to do so.
If a router is not able to generate Code 3 Redirects then it MUST generate Code 1 Redirects in situations where a Code 3 Redirect is called for.
Routers MUST NOT generate a Redirect Message unless all the following conditions are met:
o The packet is being forwarded out the same physical interface that it was received from,
o The IP source address in the packet is on the same Logical IP (sub)network as the next-hop IP address, and
o The packet does not contain an IP source route option.
The source address used in the ICMP Redirect MUST belong to the same logical (sub)net as the destination address.
A router using a routing protocol (other than static routes) MUST NOT consider paths learned from ICMP Redirects when forwarding a packet. If a router is not using a routing protocol, a router MAY have a
Baker Standards Track [Page 83]
RFC 1812 Requirements for IP Version 4 Routers June 1995
configuration that, if set, allows the router to consider routes learned through ICMP Redirects when forwarding packets.
DISCUSSION ICMP Redirect is a mechanism for routers to convey routing information to hosts. Routers use other mechanisms to learn routing information, and therefore have no reason to obey redirects. Believing a redirect which contradicted the router's other information would likely create routing loops.
On the other hand, when a router is not acting as a router, it MUST comply with the behavior required of a host.
A router MUST generate a Time Exceeded message Code 0 (In Transit) when it discards a packet due to an expired TTL field. A router MAY have a per-interface option to disable origination of these messages on that interface, but that option MUST default to allowing the messages to be originated.
IGMP [INTERNET:4] is a protocol used between hosts and multicast routers on a single physical network to establish hosts' membership in particular multicast groups. Multicast routers use this information, in conjunction with a multicast routing protocol, to support IP multicast forwarding across the Internet.
A router SHOULD implement the multicast router part of IGMP.
Baker Standards Track [Page 84]
RFC 1812 Requirements for IP Version 4 Routers June 1995
The Time-to-Live (TTL) field of the IP header is defined to be a timer limiting the lifetime of a datagram. It is an 8-bit field and the units are seconds. Each router (or other module) that handles a packet MUST decrement the TTL by at least one, even if the elapsed time was much less than a second. Since this is very often the case, the TTL is effectively a hop count limit on how far a datagram can propagate through the Internet.
When a router forwards a packet, it MUST reduce the TTL by at least one. If it holds a packet for more than one second, it MAY decrement the TTL by one for each second.
If the TTL is reduced to zero (or less), the packet MUST be discarded, and if the destination is not a multicast address the router MUST send an ICMP Time Exceeded message, Code 0 (TTL Exceeded in Transit) message to the source. Note that a router MUST NOT discard an IP unicast or broadcast packet with a non-zero TTL merely because it can predict that another router on the path to the packet's final destination will decrement the TTL to zero. However, a router MAY do so for IP multicasts, in order to more efficiently implement IP multicast's expanding ring search algorithm (see [INTERNET:4]).
DISCUSSION The IP TTL is used, somewhat schizophrenically, as both a hop count limit and a time limit. Its hop count function is critical to ensuring that routing problems can't melt down the network by causing packets to loop infinitely in the network. The time limit function is used by transport protocols such as TCP to ensure reliable data transfer. Many current implementations treat TTL as a pure hop count, and in parts of the Internet community there is a strong sentiment that the time limit function should instead be performed by the transport protocols that need it.
In this specification, we have reluctantly decided to follow the strong belief among the router vendors that the time limit function should be optional. They argued that implementation of the time limit function is difficult enough that it is currently not generally done. They further pointed to the lack of documented cases where this shortcut has caused TCP to corrupt data (of course, we would expect the problems created to be rare and difficult to reproduce, so the lack of documented cases provides little reassurance that there haven't been a number of undocumented cases).
Baker Standards Track [Page 85]
RFC 1812 Requirements for IP Version 4 Routers June 1995
IP multicast notions such as the expanding ring search may not work as expected unless the TTL is treated as a pure hop count. The same thing is somewhat true of traceroute.
ICMP Time Exceeded messages are required because the traceroute diagnostic tool depends on them.
Thus, the tradeoff is between severely crippling, if not eliminating, two very useful tools and avoiding a very rare and transient data transport problem that may not occur at all. We have chosen to preserve the tools.
The Type-of-Service byte in the IP header is divided into three sections: the Precedence field (high-order 3 bits), a field that is customarily called Type of Service or "TOS (next 4 bits), and a reserved bit (the low order bit). Rules governing the reserved bit were described in Section [4.2.2.3]. The Precedence field will be discussed in Section [5.3.3]. A more extensive discussion of the TOS field and its use can be found in [ROUTE:11].
A router SHOULD consider the TOS field in a packet's IP header when deciding how to forward it. The remainder of this section describes the rules that apply to routers that conform to this requirement.
A router MUST maintain a TOS value for each route in its routing table. Routes learned through a routing protocol that does not support TOS MUST be assigned a TOS of zero (the default TOS).
To choose a route to a destination, a router MUST use an algorithm equivalent to the following:
(1) The router locates in its routing table all available routes to the destination (see Section [5.2.4]).
(2) If there are none, the router drops the packet because the destination is unreachable. See section [5.2.4].
(3) If one or more of those routes have a TOS that exactly matches the TOS specified in the packet, the router chooses the route with the best metric.
(4) Otherwise, the router repeats the above step, except looking at routes whose TOS is zero.
Baker Standards Track [Page 86]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(5) If no route was chosen above, the router drops the packet because the destination is unreachable. The router returns an ICMP Destination Unreachable error specifying the appropriate code: either Network Unreachable with Type of Service (code 11) or Host Unreachable with Type of Service (code 12).
DISCUSSION Although TOS has been little used in the past, its use by hosts is now mandated by the Requirements for Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Support for TOS in routers may become a MUST in the future, but is a SHOULD for now until we get more experience with it and can better judge both its benefits and its costs.
Various people have proposed that TOS should affect other aspects of the forwarding function. For example:
(1) A router could place packets that have the Low Delay bit set ahead of other packets in its output queues.
(2) a router is forced to discard packets, it could try to avoid discarding those which have the High Reliability bit set.
These ideas have been explored in more detail in [INTERNET:17] but we don't yet have enough experience with such schemes to make requirements in this area.
This section specifies requirements and guidelines for appropriate processing of the IP Precedence field in routers. Precedence is a scheme for allocating resources in the network based on the relative importance of different traffic flows. The IP specification defines specific values to be used in this field for various types of traffic.
The basic mechanisms for precedence processing in a router are preferential resource allocation, including both precedence- ordered queue service and precedence-based congestion control, and selection of Link Layer priority features. The router also selects the IP precedence for routing, management and control traffic it originates. For a more extensive discussion of IP Precedence and its implementation see [FORWARD:6].
Precedence-ordered queue service, as discussed in this section, includes but is not limited to the queue for the forwarding process and queues for outgoing links. It is intended that a
Baker Standards Track [Page 87]
RFC 1812 Requirements for IP Version 4 Routers June 1995
router supporting precedence should also use the precedence indication at whatever points in its processing are concerned with allocation of finite resources, such as packet buffers or Link Layer connections. The set of such points is implementation- dependent.
DISCUSSION Although the Precedence field was originally provided for use in DOD systems where large traffic surges or major damage to the network are viewed as inherent threats, it has useful applications for many non-military IP networks. Although the traffic handling capacity of networks has grown greatly in recent years, the traffic generating ability of the users has also grown, and network overload conditions still occur at times. Since IP-based routing and management protocols have become more critical to the successful operation of the Internet, overloads present two additional risks to the network:
(1) High delays may result in routing protocol packets being lost. This may cause the routing protocol to falsely deduce a topology change and propagate this false information to other routers. Not only can this cause routes to oscillate, but an extra processing burden may be placed on other routers.
(2) High delays may interfere with the use of network management tools to analyze and perhaps correct or relieve the problem in the network that caused the overload condition to occur.
Implementation and appropriate use of the Precedence mechanism alleviates both of these problems.
Routers SHOULD implement precedence-ordered queue service. Precedence-ordered queue service means that when a packet is selected for output on a (logical) link, the packet of highest precedence that has been queued for that link is sent. Routers that implement precedence-ordered queue service MUST also have a configuration option to suppress precedence-ordered queue service in the Internet Layer.
Any router MAY implement other policy-based throughput management procedures that result in other than strict precedence ordering, but it MUST be configurable to suppress them (i.e., use strict ordering).
As detailed in Section [5.3.6], routers that implement precedence- ordered queue service discard low precedence packets before discarding high precedence packets for congestion control purposes.
Baker Standards Track [Page 88]
RFC 1812 Requirements for IP Version 4 Routers June 1995
Preemption (interruption of processing or transmission of a packet) is not envisioned as a function of the Internet Layer. Some protocols at other layers may provide preemption features.
Routers that implement precedence-ordered queuing MUST IMPLEMENT, and other routers SHOULD IMPLEMENT, Lower Layer Precedence Mapping.
A router that implements Lower Layer Precedence Mapping:
o MUST be able to map IP Precedence to Link Layer priority mechanisms for link layers that have such a feature defined.
o MUST have a configuration option to select the Link Layer's default priority treatment for all IP traffic
o SHOULD be able to configure specific nonstandard mappings of IP precedence values to Link Layer priority values for each interface.
DISCUSSION Some research questions the workability of the priority features of some Link Layer protocols, and some networks may have faulty implementations of the link layer priority mechanism. It seems prudent to provide an escape mechanism in case such problems show up in a network.
On the other hand, there are proposals to use novel queuing strategies to implement special services such as multimedia bandwidth reservation or low-delay service. Special services and queuing strategies to support them are current research subjects and are in the process of standardization.
Implementors may wish to consider that correct link layer mapping of IP precedence is required by DOD policy for TCP/IP systems used on DOD networks. Since these requirements are intended to encourage (but not force) the use of precedence features in the hope of providing better Internet service to all users, routers supporting precedence-ordered queue service should default to maintaining strict precedence ordering regardless of the type of service requested.
Baker Standards Track [Page 89]
RFC 1812 Requirements for IP Version 4 Routers June 1995
A router (whether or not it employs precedence-ordered queue service):
(1) MUST accept and process incoming traffic of all precedence levels normally, unless it has been administratively configured to do otherwise.
(2) MAY implement a validation filter to administratively restrict the use of precedence levels by particular traffic sources. If provided, this filter MUST NOT filter out or cut off the following sorts of ICMP error messages: Destination Unreachable, Redirect, Time Exceeded, and Parameter Problem. If this filter is provided, the procedures required for packet filtering by addresses are required for this filter also.
DISCUSSION Precedence filtering should be applicable to specific source/destination IP Address pairs, specific protocols, specific ports, and so on.
An ICMP Destination Unreachable message with code 14 SHOULD be sent when a packet is dropped by the validation filter, unless this has been suppressed by configuration choice.
(3) MAY implement a cutoff function that allows the router to be set to refuse or drop traffic with precedence below a specified level. This function may be activated by management actions or by some implementation dependent heuristics, but there MUST be a configuration option to disable any heuristic mechanism that operates without human intervention. An ICMP Destination Unreachable message with code 15 SHOULD be sent when a packet is dropped by the cutoff function, unless this has been suppressed by configuration choice.
A router MUST NOT refuse to forward datagrams with IP precedence of 6 (Internetwork Control) or 7 (Network Control) solely due to precedence cutoff. However, other criteria may be used in conjunction with precedence cutoff to filter high precedence traffic.
DISCUSSION Unrestricted precedence cutoff could result in an unintentional cutoff of routing and control traffic. In the general case, host traffic should be restricted to a value of 5 (CRITIC/ECP) or below; this is not a requirement and may not be correct in certain systems.
Baker Standards Track [Page 90]
RFC 1812 Requirements for IP Version 4 Routers June 1995
(4) MUST NOT change precedence settings on packets it did not originate.
(5) SHOULD be able to configure distinct precedence values to be used for each routing or management protocol supported (except for those protocols, such as OSPF, which specify which precedence value must be used).
(6) MAY be able to configure routing or management traffic precedence values independently for each peer address.
(7) MUST respond appropriately to Link Layer precedence-related error indications where provided. An ICMP Destination Unreachable message with code 15 SHOULD be sent when a packet is dropped because a link cannot accept it due to a precedence-related condition, unless this has been suppressed by configuration choice.
DISCUSSION The precedence cutoff mechanism described in (3) is somewhat controversial. Depending on the topological location of the area affected by the cutoff, transit traffic may be directed by routing protocols into the area of the cutoff, where it will be dropped. This is only a problem if another path that is unaffected by the cutoff exists between the communicating points. Proposed ways of avoiding this problem include providing some minimum bandwidth to all precedence levels even under overload conditions, or propagating cutoff information in routing protocols. In the absence of a widely accepted (and implemented) solution to this problem, great caution is recommended in activating cutoff mechanisms in transit networks.
A transport layer relay could legitimately provide the function prohibited by (4) above. Changing precedence levels may cause subtle interactions with TCP and perhaps other protocols; a correct design is a non-trivial task.
The intent of (5) and (6) (and the discussion of IP Precedence in ICMP messages in Section [4.3.2]) is that the IP precedence bits should be appropriately set, whether or not this router acts upon those bits in any other way. We expect that in the future specifications for routing protocols and network management protocols will specify how the IP Precedence should be set for messages sent by those protocols.
The appropriate response for (7) depends on the link layer protocol in use. Typically, the router should stop trying to send offensive traffic to that destination for some period of time, and
Baker Standards Track [Page 91]
RFC 1812 Requirements for IP Version 4 Routers June 1995
should return an ICMP Destination Unreachable message with code 15 (service not available for precedence requested) to the traffic source. It also should not try to reestablish a preempted Link Layer connection for some time.
The encapsulation of IP packets in most Link Layer protocols (except PPP) allows a receiver to distinguish broadcasts and multicasts from unicasts simply by examining the Link Layer protocol headers (most commonly, the Link Layer destination address). The rules in this section that refer to Link Layer broadcasts apply only to Link Layer protocols that allow broadcasts to be distinguished; likewise, the rules that refer to Link Layer multicasts apply only to Link Layer protocols that allow multicasts to be distinguished.
A router MUST NOT forward any packet that the router received as a Link Layer broadcast, unless it is directed to an IP Multicast address. In this latter case, one would presume that link layer broadcast was used due to the lack of an effective multicast service.
A router MUST NOT forward any packet which the router received as a Link Layer multicast unless the packet's destination address is an IP multicast address.
A router SHOULD silently discard a packet that is received via a Link Layer broadcast but does not specify an IP multicast or IP broadcast destination address.
When a router sends a packet as a Link Layer broadcast, the IP destination address MUST be a legal IP broadcast or IP multicast address.
There are two major types of IP broadcast addresses; limited broadcast and directed broadcast. In addition, there are three subtypes of directed broadcast: a broadcast directed to a specified network prefix, a broadcast directed to a specified subnetwork, and a broadcast directed to all subnets of a specified network. Classification by a router of a broadcast into one of these categories depends on the broadcast address and on the router's understanding (if any) of the subnet structure of the destination network. The same broadcast will be classified differently by different routers.
A limited IP broadcast address is defined to be all-ones: { -1, -1 } or 255.255.255.255.
Baker Standards Track [Page 92]
RFC 1812 Requirements for IP Version 4 Routers June 1995
A network-prefix-directed broadcast is composed of the network prefix of the IP address with a local part of all-ones or { <Network- prefix>, -1 }. For example, a Class A net broadcast address is net.255.255.255, a Class B net broadcast address is net.net.255.255 and a Class C net broadcast address is net.net.net.255 where net is a byte of the network address.
The all-subnets-directed-broadcast is not well defined in a CIDR environment, and was deprecated in version 1 of this memo.
As was described in Section [4.2.3.1], a router may encounter certain non-standard IP broadcast addresses:
o 0.0.0.0 is an obsolete form of the limited broadcast address
o { <Network-prefix>, 0 } is an obsolete form of a network-prefix- directed broadcast address.
As was described in that section, packets addressed to any of these addresses SHOULD be silently discarded, but if they are not, they MUST be treated according to the same rules that apply to packets addressed to the non-obsolete forms of the broadcast addresses described above. These rules are described in the next few sections.
Limited broadcasts MUST NOT be forwarded. Limited broadcasts MUST NOT be discarded. Limited broadcasts MAY be sent and SHOULD be sent instead of directed broadcasts where limited broadcasts will suffice.
DISCUSSION Some routers contain UDP servers which function by resending the requests (as unicasts or directed broadcasts) to other servers. This requirement should not be interpreted as prohibiting such servers. Note, however, that such servers can easily cause packet looping if misconfigured. Thus, providers of such servers would probably be well advised to document their setup carefully and to consider carefully the TTL on packets that are sent.
A router MUST classify as network-prefix-directed broadcasts all valid, directed broadcasts destined for a remote network or an attached nonsubnetted network. Note that in view of CIDR, such appear to be host addresses within the network prefix; we preclude inspection of the host part of such network prefixes. Given a route and no overriding policy, then, a router MUST forward network- prefix-directed broadcasts. Network-Prefix-Directed broadcasts MAY
Baker Standards Track [Page 93]
RFC 1812 Requirements for IP Version 4 Routers June 1995
be sent.
A router MAY have an option to disable receiving network-prefix- directed broadcasts on an interface and MUST have an option to disable forwarding network-prefix-directed broadcasts. These options MUST default to permit receiving and forwarding network-prefix- directed broadcasts.
DISCUSSION There has been some debate about forwarding or not forwarding directed broadcasts. In this memo we have made the forwarding decision depend on the router's knowledge of the destination network prefix. Routers cannot determine that a message is unicast or directed broadcast apart from this knowledge. The decision to forward or not forward the message is by definition only possible in the last hop router.
The first version of this memo described an algorithm for distributing a directed broadcast to all the subnets of a classical network number. This algorithm was stated to be "broken," and certain failure cases were specified.
In a CIDR routing domain, wherein classical IP network numbers are meaningless, the concept of an all-subnets-directed-broadcast is also meaningless. To the knowledge of the working group, the facility was never implemented or deployed, and is now relegated to the dustbin of history.
The first version of this memo spelled out procedures for dealing with subnet-directed-broadcasts. In a CIDR routing domain, these are indistinguishable from net-drected-broadcasts. The two are therefore treated together in section [5.3.5.2 Directed Broadcasts], and should be viewed as network-prefix directed broadcasts.
Congestion in a network is loosely defined as a condition where demand for resources (usually bandwidth or CPU time) exceeds capacity. Congestion avoidance tries to prevent demand from exceeding capacity, while congestion recovery tries to restore an operative state. It is possible for a router to contribute to both of these mechanisms. A great deal of effort has been spent studying the problem. The reader is encouraged to read [FORWARD:2] for a survey of the work. Important papers on the subject include
Baker Standards Track [Page 94]
RFC 1812 Requirements for IP Version 4 Routers June 1995
[FORWARD:3], [FORWARD:4], [FORWARD:5], [FORWARD:10], [FORWARD:11], [FORWARD:12], [FORWARD:13], [FORWARD:14], and [INTERNET:10], among others.
The amount of storage that router should have available to handle peak instantaneous demand when hosts use reasonable congestion policies, such as described in [FORWARD:5], is a function of the product of the bandwidth of the link times the path delay of the flows using the link, and therefore storage should increase as this Bandwidth*Delay product increases. The exact function relating storage capacity to probability of discard is not known.
When a router receives a packet beyond its storage capacity it must (by definition, not by decree) discard it or some other packet or packets. Which packet to discard is the subject of much study but, unfortunately, little agreement so far. The best wisdom to date suggests discarding a packet from the data stream most heavily using the link. However, a number of additional factors may be relevant, including the precedence of the traffic, active bandwidth reservation, and the complexity associated with selecting that packet.
A router MAY discard the packet it has just received; this is the simplest but not the best policy. Ideally, the router should select a packet from one of the sessions most heavily abusing the link, given that the applicable Quality of Service policy permits this. A recommended policy in datagram environments using FIFO queues is to discard a packet randomly selected from the queue (see [FORWARD:5]). An equivalent algorithm in routers using fair queues is to discard from the longest queue or that using the greatest virtual time (see [FORWARD:13]). A router MAY use these algorithms to determine which packet to discard.
If a router implements a discard policy (such as Random Drop) under which it chooses a packet to discard from a pool of eligible packets:
o If precedence-ordered queue service (described in Section [5.3.3.1]) is implemented and enabled, the router MUST NOT discard a packet whose IP precedence is higher than that of a packet that is not discarded.
o A router MAY protect packets whose IP headers request the maximize reliability TOS, except where doing so would be in violation of the previous rule.
o A router MAY protect fragmented IP packets, on the theory that dropping a fragment of a datagram may increase congestion by causing all fragments of the datagram to be retransmitted by the
Baker Standards Track [Page 95]
RFC 1812 Requirements for IP Version 4 Routers June 1995
source.
o To help prevent routing perturbations or disruption of management functions, the router MAY protect packets used for routing control, link control, or network management from being discarded. Dedicated routers (i.e., routers that are not also general purpose hosts, terminal servers, etc.) can ac