SIPCORE O. Johansson
Internet-Draft Edvina AB
Intended status: Standards Track G. Salgueiro
Expires: October 27, 2014 Cisco Systems
April 25, 2014
Locating Session Initiation Protocol (SIP) Servers in a Dual-Stack IP
Network
draft-ietf-sipcore-dns-dual-stack-00
Abstract
RFC 3263 defines how a Session Initiation Protocol (SIP)
implementation, given a SIP Uniform Resource Identifier (URI), should
locate the next hop SIP server using Domain Name System (DNS)
procedures. As SIP networks increasingly transition from IPv4-only
to dual-stack, a quality user experience must be ensured for dual-
stack SIP implementations. This document supplements the DNS
procedures described in RFC 3263 for dual-stack SIP implementations
and ensures that they properly align to the optimizations detailed by
Happy Eyeballs.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 27, 2014.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DNS Procedures in a Dual-Stack Network . . . . . . . . . . . 4
4.1. Dual-Stack SIP UA DNS Record Lookup Procedure . . . . . . 4
4.2. Indicating Address Family Preference in DNS SRV Records . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The core SIP [RFC3261] RFCs were written with support for both IPv4
and IPv6 in mind, but they were not fully equipped to handle highly
hybridized environments during this transitional phase of migration
from IPv4 to IPv6 networks, where many server and client
implementations run on dual-stack hosts. In such environments, a
dual-stack host will likely suffer greater connection delay, and by
extension an inferior user experience, than an IPv4-only host. The
need to remedy this diminished performance of dual-stack hosts led to
the development of the Happy Eyeballs [RFC6555] algorithm, which has
since been implemented in many applications.
RFC 6157[RFC6157] focuses on handling media in a dual-stack network
path between two SIP user agents (UAs). This doesn't solve the
signalling issues that can occur when trying to find the best network
path to the next hop SIP server.
[[TODO: Sync with Vijay Gurbani on impacts of this draft to RFC 6157,
especially relative to the additional requirement that DNS be
populated such that a certain address family is preferred.]]
This document aims to provide a more holistic design solution by
clarifying the DNS lookup procedures of RFC 3263[RFC3263] to ensure
enhanced performance, and consequently user experience, in highly
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hybridized dual-stack SIP networks. The procedures described herein
are such that a dual-stack client SHOULD look up both A and AAAA
records in DNS and then select the best way to set up a network flow.
The details of how the latter is done is considered out of scope for
this document. See the Happy Eyeballs algorithm and implementation
and design considerations in RFC 6555 [RFC6555] for more information
about issues with setting up dual-stack network flows.
2. Notational Conventions
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].
3. Terminology
RFC 3261 [RFC3261] defines additional terms used in this document
that are specific to the SIP domain such as "proxy"; "registrar";
"redirect server"; "user agent server" or "UAS"; "user agent client"
or "UAC"; "back-to-back user agent" or "B2BUA"; "dialog";
"transaction"; "server transaction".
This document uses the term "SIP Server" that is defined to include
the following SIP entities: user agent server, registrar, redirect
server, a SIP proxy in the role of user agent server, and a B2BUA in
the role of a user agent server.
This document also uses the following terminology to make clear
distinction between SIP entities supporting only IPv4, only IPv6 or
supporting both IPv4 and IPv6.
IPv4-only UA/UAC/UAS: An IPv4-only UA/UAC/UAS supports SIP signaling
and media only on the IPv4 network. It does not understand IPv6
addresses.
IPv6-only UA/UAC/UAS: An IPv6-only UA/UAC/UAS supports SIP signaling
and media only on the IPv6 network. It does not understand IPv4
addresses.
IPv4/IPv6 UA/UAC/UAS: A UA/UAC/UAS that supports SIP signaling and
media on both IPv4 and IPv6 networks; such a UA/UAC/UAS is known
(and will be referred to in this document) as a "dual-stack"
[RFC4213] UA/UAC/UAS.
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4. DNS Procedures in a Dual-Stack Network
This specification introduces two normative DNS lookup procedures.
These are designed to improve the performace of dual-stack clients in
IPv4/IPv6 networks.
4.1. Dual-Stack SIP UA DNS Record Lookup Procedure
Once the transport protocol has been determined, the procedure for
discovering an ip address if the TARGET is not a numeric IP address
but the port is explicitly stated in the URI, is detailed in
Section 4.2 of RFC 3263[RFC3263]. The piece relevant to to this
discussion is:
"If the TARGET was not a numeric IP address, but a port is present
in the URI, the client performs an A or AAAA record lookup of the
domain name. The result will be a list of IP addresses, each of
which can be contacted at the specific port from the URI and
transport protocol determined previously."
Section 4.2 of RFC 3263 [RFC3263] also goes on to describe the
complete procedure for discovering an ip address if the TARGET is not
a numeric IP address, and no port is present in the URI. The piece
relevant to to this discussion is:
"If no SRV records were found, the client performs an A or AAAA
record lookup of the domain name. The result will be a list of IP
addresses, each of which can be contacted using the transport
protocol determined previously, at the default port for that
transport. Processing then proceeds as described above for an
explicit port once the A or AAAA records have been looked up."
Happy Eyeballs [RFC6555] has proven that looking up the "A or AAAA
record" is not an effective practice for dual-stack clients and that
it can add significant connection delay and greatly degrade user
experience. Therefore, this document makes the following normative
addendum to the DNS lookup procedures of Section 4.2 of RFC 3263
[RFC3263] for IPv4/IPv6 hybrid SIP networks and recommends it as a
best practice for such dual-stack networks:
The dual-stack client SHOULD perform an A and AAAA record lookup
of the domain name and add the respective IPv4/IPv6 addresses to
the list of IP addresses to be contacted.
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4.2. Indicating Address Family Preference in DNS SRV Records
The Happy Eyeballs algorithm [RFC6555] is particularly effective when
dual-stack client applications have significant performance
differences in their IPv4 or IPv6 network paths. In this common
scenario it is often necessary for a dual-stack client to indicate a
preference for either IPv4 or IPv6. A service may use DNS SRV
records to indicate such a preference for an address family. This
way, a server with a high-latency and/or low-capacity IPv4 tunnel may
indicate a preference for being contacted using IPv6. A server that
wishes to do this can use the lowest SRV priority to publish
hostnames that only resolve in IPv6 and the next priority with host
names that resolve in both address families.
When indicating address family preference through SRV, IPv4-only and/
or IPv6-only clients should be prepared to handle SRV record sets
that don't resolve into an ip address in the address family used by
that client. In such a case, the client should simply proceed to the
next priority and try the hostnames in the alternate address family.
5. Security Considerations
This document introduces two new normative procedures to the existing
DNS procedures used to locate SIP servers. While both of these
procedures are optimizations designed to improve the performance of
dual-stack clients, neither introduces any new security
considerations.
The specific security vulnerabilities, attacks and threat models of
the various protocols discussed in this document (SIP, DNS, SRV
records, Happy Eyeballs requirements and algorithm, etc.) are well
documented in their respective specifications.
6. IANA Considerations
This document does not require any actions by IANA.
7. Acknowledgments
The author would like to acknowledge the support and contribution of
the SIP Forum IPv6 Working Group. This document is based on a lot of
tests and discussions at SIPit events, organized by the SIP Forum.
This document has benefited from the expertise and review feedback of
many participants of the IETF DISPATCH and SIPCORE WG mailing lists
as well as those on the SIP Forum IPv6 Task Group mailing list. The
authors wish to specifically call out the efforts and express their
gratitude for the detailed and thoughtful comments and corrections of
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Dan Wing, Brett Tate, Rifaat Shekh-Yusef, Carl Klatsky, Dale Worley,
Mary Barnes, Keith Drage, Vijay Gurbani and Cullen Jennings.
The authors also thank the SIPCORE WG chairs, Paul Kyzivat and Adam
Roach, and assigned Area Director, Richard Barnes, for their support
and thorough evaluation of this work.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002.
8.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC6157] Camarillo, G., El Malki, K., and V. Gurbani, "IPv6
Transition in the Session Initiation Protocol (SIP)", RFC
6157, April 2011.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
Authors' Addresses
Olle E. Johansson
Edvina AB
Runbovaegen 10
Sollentuna SE-192 48
SE
Email: oej@edvina.net
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Gonzalo Salgueiro
Cisco Systems
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: gsalguei@cisco.com
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