Internet Engineering Task Force (IETF) M. Shore Request for Comments: 7279 No Mountain Software BCP: 189 C. Pignataro Category: Best Current Practice Cisco Systems, Inc. ISSN: 2070-1721 May 2014
An Acceptable Use Policy for New ICMP Types and Codes
In this document we provide a basic description of ICMP's role in the IP stack and some guidelines for future use.
This document is motivated by concerns about lack of clarity concerning when to add new Internet Control Message Protocol (ICMP) types and/or codes. These concerns have highlighted a need to describe policies for when adding new features to ICMP is desirable and when it is not.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
There has been some recent concern expressed about a lack of clarity around when new message types and codes should be added to ICMP (including ICMPv4 [RFC0792] and ICMPv6 [RFC4443]). We lay out a policy regarding when (and when not) to move functionality into ICMP.
This document is the result of discussions among ICMP experts within the Operations and Management (OPS) area's IP Diagnostics Technical Interest Group [DIAGNOSTICS] and concerns expressed by the OPS area leadership.
Note that this document does not supercede the "IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers" [RFC2780], which specifies best practices and processes for the allocation of values in the IANA registries but does not describe the policies to be applied in the standards process.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
In this document, we describe an acceptable use policy for new ICMP message types and codes, and provide some background about the policy.
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In summary, any future message types added to ICMP should be limited to two broad categories:
1. to inform a datagram's originator that a forwarding plane anomaly has been encountered downstream. The datagram originator must be able to determine whether or not the datagram was discarded by examining the ICMP message.
2. to discover and convey dynamic information about a node (other than information usually carried in routing protocols), to discover and convey network-specific parameters, and to discover on-link routers and hosts.
Normally, ICMP SHOULD NOT be used to implement a general-purpose routing or network management protocol. However, ICMP does have a role to play in conveying dynamic information about a network, which would belong in category 2 above.
As mentioned in Section 2, using ICMP as a general-purpose routing or network management protocol is not advisable and SHOULD NOT be used that way.
ICMP has a role in the Internet as an integral part of the IP layer; it is not as a routing protocol or as a transport protocol for other layers including routing information. From a more pragmatic perspective, some of the key characteristics of ICMP make it a less- than-ideal choice for a routing protocol. These key characteristics include that ICMP is frequently filtered, is not authenticated, and is easily spoofed. In addition, specialist hardware processing of ICMP would disrupt the deployment of an ICMP-based routing or management protocol.
RPL, the IPv6 routing protocol for low-power and lossy networks (see [RFC6550]) uses ICMP as a transport. In this regard, it is an exception among the ICMP message types. Note that, although RPL is an IP routing protocol, it is not deployed on the general Internet; it is limited to specific, contained networks.
This should be considered anomalous and is not a model for future ICMP message types. That is, ICMP is not intended as a transport for other protocols and SHOULD NOT be used in that way in future specifications. In particular, while it is adequate to use ICMP as a discovery protocol, it does not extend to full routing capabilities.
Some applications make use of ICMP error notifications, or even deliberately create anomalous conditions in order to elicit ICMP messages. These ICMP messages are then used to generate feedback to the higher layer. Some of these applications include some of the most widespread examples, such as PING, TRACEROUTE, and Path MTU Discovery (PMTUD). These uses are considered acceptable because they use existing ICMP message types and do not change ICMP functionality.
ICMP multi-part messages are specified in [RFC4884] by defining an extension mechanism for selected ICMP messages. This mechanism addresses a fundamental problem in ICMP extensibility. An ICMP multi-part message carries all of the information that ICMP messages carried previously, as well as additional information that applications may require.
Some currently defined ICMP extensions include ICMP extensions for Multiprotocol Label Switching [RFC4950] and ICMP extensions for interface and next-hop identification [RFC5837].
Extensions to ICMP SHOULD follow the requirements provided in [RFC4884].
Because ICMPv6 is used for IPv6 Neighbor Discovery, deployed IPv6 routers, IPv6-capable security gateways, and IPv6-capable firewalls normally support administrator configuration of how specific ICMPv6 message types are handled. By contrast, deployed IPv4 routers, IPv4-capable security gateways, and IPv4-capable firewalls are less likely to allow an administrator to configure how specific ICMPv4 message types are handled. So, at present, ICMPv6 messages usually have a higher probability of travelling end-to-end than ICMPv4 messages.
ICMP was originally intended to be a mechanism for gateways or destination hosts to report error conditions back to source hosts in ICMPv4 [RFC0792]; ICMPv6 [RFC4443] is modeled after it. ICMP is also used to perform IP-layer functions, such as diagnostics (e.g., PING).
ICMP is defined to be an integral part of IP and must be implemented by every IP module. This is true for ICMPv4 as an integral part of IPv4 (see the Introduction of [RFC0792]), and for ICMPv6 as an
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integral part of IPv6 (see Section 2 of [RFC4443]). When first defined, ICMP messages were thought of as IP messages that didn't carry any higher-layer data. It could be conjectured that the term "control" was used because ICMP messages were not "data" messages.
The word "control" in the protocol name did not describe ICMP's function (i.e., it did not "control" the Internet); rather, it was used to communicate about the control functions in the Internet. For example, even though ICMP included a redirect message type that affects routing behavior in the context of a LAN segment, it was not and is not used as a generic routing protocol.
4. Security Considerations
This document describes a high-level policy for adding ICMP types and codes. While special attention must be paid to the security implications of any particular new ICMP type or code, this recommendation presents no new security considerations.
From a security perspective, ICMP plays a part in the Photuris protocol [RFC2521]. But more generally, ICMP is not a secure protocol and does not include features to be used to discover network security parameters or to report on network security anomalies in the forwarding plane.
Additionally, new ICMP functionality (e.g., ICMP extensions, or new ICMP types or codes) needs to consider potential ways that ICMP can be abused (e.g., Smurf IP DoS [CA-1998-01]).
This document was originally proposed by, and received substantial review and suggestions from, Ron Bonica. Discussions with Pascal Thubert helped clarify the history of RPL's use of ICMP. We are very grateful for the review, feedback, and comments from Ran Atkinson, Tim Chown, Joe Clarke, Adrian Farrel, Ray Hunter, Hilarie Orman, Eric Rosen, JINMEI Tatuya, and Wen Zhang, which resulted in a much improved document.
[RFC2521] Karn, P. and W. Simpson, "ICMP Security Failures Messages", RFC 2521, March 1999.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers", BCP 37, RFC 2780, March 2000.
[RFC4950] Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP Extensions for Multiprotocol Label Switching", RFC 4950, August 2007.
[RFC5837] Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR. Rivers, "Extending ICMP for Interface and Next-Hop Identification", RFC 5837, April 2010.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP., and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, March 2012.
[RFC6918] Gont, F. and C. Pignataro, "Formally Deprecating Some ICMPv4 Message Types", RFC 6918, April 2013.
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Melinda Shore No Mountain Software PO Box 16271 Two Rivers, AK 99716 US