Internet Engineering Task Force (IETF) S. Kiesel Request for Comments: 7286 University of Stuttgart Category: Standards Track M. Stiemerling ISSN: 2070-1721 NEC Europe Ltd. N. Schwan Thales Deutschland M. Scharf Alcatel-Lucent Bell Labs H. Song Huawei November 2014
Application-Layer Traffic Optimization (ALTO) Server Discovery
Abstract
The goal of Application-Layer Traffic Optimization (ALTO) is to provide guidance to applications that have to select one or several hosts from a set of candidates capable of providing a desired resource. ALTO is realized by a client-server protocol. Before an ALTO client can ask for guidance, it needs to discover one or more ALTO servers.
This document specifies a procedure for resource-consumer-initiated ALTO server discovery, which can be used if the ALTO client is embedded in the resource consumer.
Status of This Memo
This is an Internet Standards Track document.
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 Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7286.
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Copyright Notice
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.
The goal of Application-Layer Traffic Optimization (ALTO) is to provide guidance to applications that have to select one or several hosts from a set of candidates capable of providing a desired resource [RFC5693]. ALTO is realized by a client-server protocol; see requirement AR-1 in [RFC6708]. Before an ALTO client can ask for guidance it needs to discover one or more ALTO servers that can provide guidance to this specific client.
This document specifies a procedure for resource-consumer-initiated ALTO server discovery, which can be used if the ALTO client is embedded in the resource consumer. In other words, this document meets requirement AR-32 in [RFC6708] while AR-33 is out of scope. A different approach, which tries to meet requirement AR-33, i.e., third-party ALTO server discovery, is addressed in [3PDISC].
A more detailed discussion of various options on where to place the functional entities comprising the overall ALTO architecture can be found in [ALTO-DEPLOY].
The ALTO protocol specification [RFC7285] is based on HTTP and expects the discovery procedure to yield the HTTP(S) URI of an ALTO server's Information Resource Directory (IRD). Therefore, this procedure is based on a combination of the Dynamic Host Configuration Protocol (DHCP) or local configuration and URI-enabled Naming Authority Pointer (U-NAPTR) resource records in the Domain Name System (DNS), in order to deliver such URIs.
This document makes use of the ALTO terminology defined in [RFC5693].
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 RFC 2119 [RFC2119].
The ALTO protocol specification [RFC7285] expects that the ALTO discovery procedure yields the HTTP(S) URI [RFC7230] of the ALTO server's Information Resource Directory (IRD), which gives further information about the capabilities and services provided by that ALTO server.
On hosts with more than one interface or address family (IPv4/v6), the ALTO server discovery procedure has to be run for every interface and address family. For more details see Section 4.2.
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The ALTO server discovery procedure is performed in two steps:
1. One DNS domain name is retrieved for each combination of interface and address family, either by local configuration (e.g., manual input into a menu or configuration file) or by means of DHCP.
2. These DNS domain names are used for U-NAPTR lookups yielding one or more URIs. Further DNS lookups may be necessary to determine the ALTO server's IP address(es).
The primary means for retrieving the DNS domain name is DHCP. However, there may be situations where DHCP is not available or does not return a suitable value. Furthermore, there might be situations in which the user wishes to override the value that could be retrieved from DHCP. In these situations, local configuration may be used. Consequently, the algorithm first checks for a locally configured override, before it tries to retrieve a value from DHCP.
Typically, but not necessarily, the DNS domain name is the domain name in which the client is located, i.e., a PTR lookup on the client's IP address (according to [RFC1035], Section 3.5 for IPv4 or [RFC3596], Section 2.5 for IPv6) would yield a similar name. However, due to the widespread use of Network Address Translation (NAT), trying to determine the DNS domain name through a PTR lookup on an interface's IP address is not recommended for resource consumer initiated ALTO server discovery (see also [RFC3424]).
As already outlined in Section 2, the ALTO server discovery procedure is performed for every address family on every interface the application considers for communicating with resource providers.
First, the algorithm checks for a locally configured domain name, as specified in Section 3.1.1. If no such name was configured, it tries to retrieve one from DHCP, as specified in Section 3.1.2. If still no domain name could be found, the procedure has failed and terminates with an appropriate error code.
If one or more domain names were found, they will be used as U-NAPTR/ DDDS (URI-Enabled NAPTR/Dynamic Delegation Discovery Service) [RFC4848] application-unique strings for a DNS lookup, as specified in Section 3.2.
The preferred way to acquire a domain name related to an interface's point of network attachment is the use of DHCP (see Section 3.1.2). However, in some network deployment scenarios, there is no DHCP server available. Furthermore, a user may want to use an ALTO service instance provided by an entity that is not the operator of the underlying IP network. Therefore, we allow the user to specify a DNS domain name, for example, in a configuration file option. An example domain name is:
my-alternative-alto-provider.example.org
Implementations MAY give the user the opportunity (e.g., by means of configuration file options or menu items) to specify an individual domain name for every address family on every interface. Implementations SHOULD allow the user to specify a default name that is used if no more specific name has been configured.
Network operators may provide the domain name to be used for service discovery within an access network using DHCP.
RFC 5986 [RFC5986] defines DHCP IPv4 and IPv6 access network domain name options to identify a domain name that is suitable for service discovery within the access network. RFC 2132 [RFC2132] defines the DHCP IPv4 domain name option. While this option is less suitable, it still may be useful if the RFC 5986 option is not available.
For IPv6, the ALTO server discovery procedure MUST try to retrieve DHCP option 57 (OPTION_V6_ACCESS_DOMAIN). If no such option can be retrieved the procedure fails for this interface. For IPv4, the ALTO server discovery procedure MUST try to retrieve DHCP option 213 (OPTION_V4_ACCESS_DOMAIN). If no such option can be retrieved, the procedure SHOULD try to retrieve option 15 (Domain Name). If neither option can be retrieved, the procedure fails for this interface. If a result can be retrieved, it will be used as an input for the next step (U-NAPTR resolution). One example result could be:
The first step of the ALTO server discovery procedure (see Section 3.1) retrieved one or -- in case of multiple interfaces and/ or IPv4/v6 dual-stack operation -- several domain names, which will be used as U-NAPTR/DDDS (URI-Enabled NAPTR/Dynamic Delegation Discovery Service) [RFC4848] application unique strings. An example is:
example.net
In the second step, the ALTO server discovery procedure uses a U-NAPTR [RFC4848] lookup with the "ALTO" Application Service Tag and either the "http" or the "https" Application Protocol Tag to obtain one or more URIs (indicating protocol, host, and possibly path elements) for the ALTO server's Information Resource Directory. In this document, only the HTTP and HTTPS URI schemes are defined, as the ALTO protocol specification defines the access over both protocols, but no other [RFC7285]. Note that the result can be any valid HTTP(S) URI.
If no ALTO-specific U-NAPTR records can be retrieved, the discovery procedure fails for this domain name (and the corresponding interface and IP protocol version). If further domain names retrieved by Step 1 are known, the discovery procedure may perform the corresponding U-NAPTR lookups immediately. However, before retrying a lookup that has failed, a client MUST wait a time period that is appropriate for the encountered error (NXDOMAIN, timeout, etc.).
Section 3.1.2 describes the usage of a DHCP option that provides a means for the network operator of the network in which the ALTO client is located to provide a DNS domain name. However, this assumes that this particular DHCP option is correctly passed from the DHCP server to the actual host with the ALTO client, and that the particular host understands this DHCP option. This memo assumes the client to be able to understand the proposed DHCP option; otherwise, there is no further use of the DHCP option, but the client has to use the other proposed mechanisms.
There are well-known issues with the handling of DHCP options in home gateways. One issue is that unknown DHCP options are not passed through some home gateways, effectively eliminating the DHCP option.
Another well-known issue is the use of home-gateway-specific DNS domain names that "override" the DNS domain name provided by the network operator. For instance, a host behind a home gateway may receive a DNS domain name ".local" instead of "example.net". In general, this domain name is not usable for the server discovery procedure, unless a DNS server in the home gateway resolves the corresponding NAPTR lookup correctly, e.g., by means of a DNS split horizon approach.
4.2. Issues with Multihoming, Mobility, and Changing IP Addresses
If the user decides to enter only one (default) DNS domain name in the local configuration facility (see Section 3.1.1), only one set of ALTO servers will be discovered, irrespective of multihoming and mobility. Particularly in mobile scenarios, this can lead to undesirable results.
The DHCP-based discovery method can discover different sets of ALTO servers for each interface and address family (i.e., IPv4/v6). In general, if a client wishes to communicate using one of its interfaces and using a specific IP address family, it SHOULD query the ALTO server or servers that have been discovered for this specific interface and address family. How to select an interface and IP address family as well as how to compare results returned from different ALTO servers are out of the scope of this document.
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A change of the IP address at an interface invalidates the result of the ALTO server discovery procedure. For instance, if the IP address assigned to a mobile host changes due to host mobility, it is required to re-run the ALTO server discovery procedure without relying on earlier gained information.
There are several challenges with DNS on hosts with multiple interfaces [RFC6418], which can affect the ALTO server discovery. If the DNS resolution is performed on the wrong interface, it can return an ALTO server that could provide suboptimal or wrong guidance. Finding the best ALTO server for multi-interfaced hosts is outside the scope of this document.
When using Virtual Private Network (VPN) connections, there is usually no DHCP. The user has to enter the DNS domain name in the local configuration facility. For good optimization results, a DNS domain name corresponding to the VPN concentrator, not corresponding to the user's current location, has to be entered. Similar considerations apply for Mobile IP.
IANA has registered the following U-NAPTR [RFC4848] application service tag for ALTO:
Application Service Tag: ALTO
Intended usage: see [RFC5693] or: "The goal of Application-Layer Traffic Optimization (ALTO) is to provide guidance to applications that have to select one or several hosts from a set of candidates capable of providing a desired resource."
Defining Publication: The specification contained within this document
Contact information: The authors of this document
Author/Change controller: The IESG
Interoperability considerations: No interoperability issues are known or expected. This tag is to be registered specifically for ALTO, which is a new application without any legacy deployments.
Security considerations: see Section 6 of this document.
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Related publications: This document specifies a procedure for discovering an HTTP or HTTPS URI of an ALTO server. HTTP and HTTPS are specified in [RFC7230]. The HTTP(S)-based ALTO protocol is specified in [RFC7285].
Application Protocol Tag: This document specifies how to use the application service tag "ALTO" with the application protocol tags "http" and "https", which have already been registered in the relevant IANA registry. Therefore, IANA is not requested by this document to register any new application protocol tag.
A high-level discussion of security issues related to ALTO is part of the ALTO problem statement [RFC5693]. A classification of unwanted information disclosure risks, as well as specific security-related requirements can be found in the ALTO requirements document [RFC6708].
The remainder of this section focuses on security threats and protection mechanisms for the ALTO server discovery procedure as such. Once the ALTO server's URI has been discovered and the communication between the ALTO client and the ALTO server starts, the security threats and protection mechanisms discussed in the ALTO protocol specification [RFC7285] apply.
Scenario Description An attacker could compromise the ALTO server discovery procedure or infrastructure in a way that ALTO clients would discover a "wrong" ALTO server URI.
Threat Discussion This is probably the most serious security concern related to ALTO server discovery. The discovered "wrong" ALTO server might not be able to give guidance to a given ALTO client at all, or it might give suboptimal or forged information. In the latter case, an attacker could try to use ALTO to affect the traffic distribution in the network or the performance of applications (see also Section 15.1. of [RFC7285]). Furthermore, a hostile ALTO server could threaten user privacy (see also Section 5.2.1, case (5a) in [RFC6708]).
However, it should also be noted that, if an attacker was able to compromise DHCP and/or DNS servers used for ALTO server discovery (see below), (s)he could also launch significantly more serious other attacks (e.g., redirecting various application protocols).
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Protection Strategies and Mechanisms The ALTO server discovery procedure consists of three building blocks (local configuration, DHCP, and DNS) and each of them is a possible attack vector.
The problem of users possibly following "bad advice" that tricks them into manually configuring unsuitable ALTO servers cannot be solved by technical means and is out of the scope of this document.
Due to the nature of the protocol, DHCP is rather prone to attacks. As already mentioned, an attacker that is able to inject forged DHCP replies into the network may do significantly more harm than only configuring a wrong ALTO server. Best current practices for safely operating DHCP should be followed.
A further threat is the possible alteration of the DNS records used in U-NAPTR resolution. If an attacker was able to modify or spoof any of the DNS records used in the DDDS resolution, this URI could be replaced by a forged URI. The application of DNS security (DNSSEC) [RFC4033] provides a means to limit attacks that rely on modification of the DNS records used in U-NAPTR resolution. Security considerations specific to U-NAPTR are described in more detail in [RFC4848].
A related risk is the impersonation of the ALTO server (i.e., attacks after the correct URI has been discovered). This threat and protection strategies are discussed in Section 15.1 of [RFC7285]. Note that if Transport Layer Security (TLS) is used to protect ALTO, the server certificate will contain the host name (CN). Consequently, only the host part of the HTTPS URI will be authenticated, i.e., the result of the ALTO server discovery procedure. The U-NAPTR based mapping within the ALTO server discovery procedure needs to be secured as described above, e.g., by using DNSSEC.
In addition to active protection mechanisms, users and network operators can monitor application performance and network traffic patterns for poor performance or abnormalities. If it turns out that relying on the guidance of a specific ALTO server does not result in better-than-random outcomes, the use of the ALTO server may be discontinued (see also Section 15.2 of [RFC7285]).
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6.2. Availability of the ALTO Server Discovery Procedure
Scenario Description An attacker could compromise the ALTO server discovery procedure or infrastructure in a way that ALTO clients would not be able to discover any ALTO server.
Threat Discussion If no ALTO server can be discovered (although a suitable one exists), applications have to make their decisions without ALTO guidance. As ALTO could be temporarily unavailable for many reasons, applications must be prepared to do so. However, the resulting application performance and traffic distribution will correspond to a deployment scenario without ALTO.
Protection Strategies and Mechanisms Operators should follow best current practices to secure their DHCP, DNS, and ALTO (see Section 15.5 of [RFC7285]) servers against Denial-of-Service (DoS) attacks.
Scenario Description An unauthorized party could invoke the ALTO server discovery procedure, or intercept discovery messages between an authorized ALTO client and the DHCP and DNS servers, in order to acquire knowledge of the ALTO server's URI.
Threat Discussion
In the ALTO use cases that have been described in the ALTO problem statement [RFC5693] and/or discussed in the ALTO working group, the ALTO server's URI as such has always been considered as public information that does not need protection of confidentiality.
Protection Strategies and Mechanisms No protection mechanisms for this scenario have been provided, as it has not been identified as a relevant threat. However, if a new use case is identified that requires this kind of protection, the suitability of this ALTO server discovery procedure as well as possible security extensions have to be re-evaluated thoroughly.
Scenario Description An unauthorized party could intercept discovery messages between an ALTO client and the DHCP and DNS servers, and thereby find out the fact that said ALTO client uses (or at least tries to use) the ALTO service.
Threat Discussion In the ALTO use cases that have been described in the ALTO problem statement [RFC5693] and/or discussed in the ALTO working group, this scenario has not been identified as a relevant threat.
Protection Strategies and Mechanisms No protection mechanisms for this scenario have been provided, as it has not been identified as a relevant threat. However, if a new use case is identified that requires this kind of protection, the suitability of this ALTO server discovery procedure as well as possible security extensions have to be re-evaluated thoroughly.
[RFC7285] Alimi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S., Roome, W., Shalunov, S., and R. Woundy, "Application-Layer Traffic Optimization (ALTO) Protocol", RFC 7285, September 2014, <http://www.rfc-editor.org/info/rfc7285>.
[3PDISC] Kiesel, S., Krause, K., and M. Stiemerling, "Third-Party ALTO Server Discovery (3pdisc)", Work in Progress, draft-kist-alto-3pdisc-05, January 2014.
[ALTO-DEPLOY] Stiemerling, M., Kiesel, S., Previdi, S., and M. Scharf, "ALTO Deployment Considerations", Work in Progress, draft-ietf-alto-deployments-10, July 2014.
Olafur Gudmundsson provided an excellent DNS expert review on an earlier version of this document. Thanks to Tina Tsou for an accurate security review.
Michael Scharf is supported by the German-Lab project <http://www.german-lab.de> funded by the German Federal Ministry of Education and Research (BMBF).
Martin Stiemerling is partially supported by the CHANGE project <http://www.change-project.eu>, a research project supported by the European Commission under its 7th Framework Program (contract no. 257422). The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the CHANGE project or the European Commission.
Contributors
The initial version of this document was coauthored by Marco Tomsu.
Hannes Tschofenig provided the initial input to the U-NAPTR solution part. Hannes and Martin Thomson provided excellent feedback and input to the server discovery.
The authors would also like to thank the following persons for their contribution to this document or its predecessors: Richard Alimi, David Bryan, Roni Even, Gustavo Garcia, Jay Gu, Xingfeng Jiang, Enrico Marocco, Victor Pascual, Y. Richard Yang, Yu-Shun Wang, Yunfei Zhang, Ning Zong.
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Authors' Addresses
Sebastian Kiesel University of Stuttgart Information Center Networks and Communication Systems Department Allmandring 30 Stuttgart 70550 Germany