SIPCORE                                                     O. Johansson
Internet-Draft                                                 Edvina AB
Intended status: Standards Track                            G. Salgueiro
Expires: October 27, 2014 April 29, 2015                                    Cisco Systems
                                                          April 25,
                                                        October 26, 2014

 Locating Session Initiation Protocol (SIP) Servers in a Dual-Stack IP


   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 27, 2014. April 29, 2015.

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
   ( 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.

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
   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",
   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

   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

   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.

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.

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

   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
   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

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

   Gonzalo Salgueiro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709