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Internet Engineering Task Force (IETF)                      O. Johansson
Request for Comments: 7984                                     Edvina AB
Updates: 3263                                               G. Salgueiro
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                               V. Gurbani
                                               Bell Labs, Nokia Networks
                                                          D. Worley, Ed.
                                                          September 2016

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


   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 updates the DNS procedures
   described in RFC 3263 for dual-stack SIP implementations in
   preparation for forthcoming specifications for applying "Happy
   Eyeballs" principles to SIP.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

Copyright Notice

   Copyright (c) 2016 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  DNS Procedures in a Dual-Stack Network  . . . . . . . . . . .   4
     3.1.  Dual-Stack SIP UA DNS Record Lookup Procedure . . . . . .   4
     3.2.  Indicating Address Family Preference in DNS SRV Records .   5
   4.  Clarification of Interaction with RFC 6724  . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] and the additional
   documents that extended it provide support for both IPv4 and IPv6.
   However, this support does not fully extend to the highly hybridized
   environments that are characteristic of the transitional migratory
   phase from IPv4 to IPv6 networks.  During this phase, 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
   protocols and applications.

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

   This document updates the DNS lookup procedures of RFC 3263 [RFC3263]
   in preparation for the specification of the application of Happy
   Eyeballs to SIP.  Happy Eyeballs will provide enhanced performance,
   and consequently enhanced 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.

   Section 4 of this document clarifies the interaction of [RFC3263]
   with [RFC6157] and [RFC6724].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   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", and "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.

   While this document focuses on the dual-stack situation described in
   RFC 6555 and other documents, concerning the migration from an
   IPv4-only network to a network supporting both IPv4 and IPv6, the
   techniques described can be used in other situations.  Possible
   situations include when a device has multiple interfaces with
   distinct addressing characteristics and when additional IP address
   families are created in the future.  This document uses the general
   term "dual-stack" to include all situations where the client has
   access to multiple communication methods that have distinct
   addressing characteristics.

   The term "address records" means the DNS records that translate a
   domain name into addresses within the address family or families that
   the entity supports (as A records provide IPv4 addresses and AAAA
   records provide IPv6 addresses), regardless of whether the address
   family was defined before or after this document was approved.

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

3.  DNS Procedures in a Dual-Stack Network

   This specification introduces two normative DNS lookup procedures.
   These are designed to improve the performance of dual-stack clients
   in IPv4/IPv6 networks.

3.1.  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 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
   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 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] documents 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 in 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 look up address records for all
      address families that it supports for the domain name and add the
      resulting addresses to the list of IP addresses to be contacted.
      A client MUST be prepared for the existence of DNS resource
      records containing addresses in families that it does not support;
      if such records may be returned by the client's DNS queries, such
      records MUST be ignored as unusable and the supported addresses
      used as specified herein.

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3.2.  Indicating Address Family Preference in DNS SRV Records

   The Happy Eyeballs algorithm [RFC6555] is particularly effective for
   dual-stack HTTP client applications that have significant performance
   differences between their IPv4 and IPv6 network paths.  This is
   because the client can initiate two TCP connections to the server,
   one using IPv4 and one using IPv6, and then use the connection that
   completes first.  This works properly because the client can test
   each route by initiating a TCP connection, but simply opening a TCP
   connection to an HTTP server does not change the server's state; the
   client will send the HTTP request on only one connection.

   Unfortunately, in common SIP situations, it is not possible to "race"
   simultaneous request attempts using two address families.  If the SIP
   requests are transmitted as single UDP packets, sending two copies of
   the request to two different addresses risks having two copies of the
   request propagating through the SIP network at the same time.  The
   difference between SIP and HTTP is that in SIP, the sender cannot
   test a route in a non-state-changing way.

   (If two copies of the same request arrive at the destination client,
   the client SHOULD reject the second of them with a response code of
   482 [RFC3261].  To convey information on why the request was rejected
   to the originator, the client can include a descriptive reason
   phrase, for example, "Merged Request".  However, issuing the 482
   response is not sufficient to prevent user-visible differences in
   behavior.  A proxy that is upstream of the second request to arrive
   at the client may (almost immediately!) serially fork the second
   request to further destinations (e.g., the voicemail service for the
   destination client).)

   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 host names that only resolve in IPv6 and the next priority
   with host names that resolve in both address families.

   Note that host names that have addresses in only one address family
   are discouraged by [RFC6555].  Such special-purpose host names SHOULD
   be used only as described in this section, as targets of SRV records
   for an aggregate host name, where the aggregate host name ultimately
   resolves to addresses in all families supported by the client.

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

4.  Clarification of Interaction with RFC 6724

   Section 5 of [RFC6157] specifies that the addresses from the address
   records for a single target DNS name for a server's DNS name must be
   contacted in the order specified by the source and destination
   address selection algorithms defined in [RFC6724].  The set of
   addresses provided to a single invocation of the destination address
   selection algorithm MUST be the address records for the target DNS
   name in a single SRV record (or, if there are no SRV records, the DNS
   name in the URI or derived via NAPTR) -- the destination address
   selection algorithm MUST NOT reorder addresses derived from different
   SRV records.  Typically, destination address selection is done by
   using the (relatively new) getaddrinfo() function to translate the
   target DNS name into a list of IPv4 and/or IPv6 addresses in the
   order in which they are to be contacted, as that function implements

   Thus, if SRV lookup on the server's DNS name is successful, the major
   ordering of the complete list of destination addresses is determined
   by the priority and weight fields of the SRV records (as specified in
   [RFC2782]), and the (minor) ordering among the destinations derived
   from the "target" field of a single SRV record is determined by

   For example, consider a server with DNS name example.com, with TCP
   transport specified.  The relevant SRV records for example.com are:

      _sip._tcp.example.com.  300 IN SRV 10 1 5060 sip-1.example.com.
      _sip._tcp.example.com.  300 IN SRV 20 1 5060 sip-2.example.com.

   The processing of [RFC2782] results in this ordered list of target
   domain names:


   The address records for sip-1.example.com, as ordered by [RFC6724],

      sip-1.example.com.  300 IN AAAA 2001:0db8:58:c02::face
      sip-1.example.com.  300 IN AAAA 2001:0db8:c:a06::2:cafe
      sip-1.example.com.  300 IN AAAA 2001:0db8:44:204::d1ce
      sip-1.example.com.  300 IN A
      sip-1.example.com.  300 IN A
      sip-1.example.com.  300 IN A

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

   And the address records for sip-2.example.com, as ordered by
   [RFC6724], are:

      sip-2.example.com.  300 IN AAAA 2001:0db8:58:c02::dead
      sip-2.example.com.  300 IN AAAA 2001:0db8:c:a06::2:beef
      sip-2.example.com.  300 IN AAAA 2001:0db8:44:204::c0de
      sip-2.example.com.  300 IN A
      sip-2.example.com.  300 IN A
      sip-2.example.com.  300 IN A

   Thus, the complete list of destination addresses has this ordering:


   In particular, the destination addresses derived from
   sip-1.example.com and those derived from sip-2.example.com are not
   interleaved; [RFC6724] does not operate on the complete list.  This
   would be true even if the two SRV records had the same priority and
   were (randomly) ordered based on their weights, as the address
   records of two target DNS names are never interleaved.

5.  Security Considerations

   This document introduces two new normative procedures to the existing
   DNS procedures used to locate SIP servers.  A client may contact
   additional target addresses for a URI beyond those prescribed in
   [RFC3263], and/or it may contact target addresses in a different
   order than prescribed in [RFC3263].  Neither of these changes
   introduce any new security considerations because it has always been
   assumed that a client desiring to send to a URI may contact any of
   its targets that are listed in DNS.

   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.

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              DOI 10.17487/RFC3263, June 2002,

   [RFC6157]  Camarillo, G., El Malki, K., and V. Gurbani, "IPv6
              Transition in the Session Initiation Protocol (SIP)",
              RFC 6157, DOI 10.17487/RFC6157, April 2011,

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

6.2.  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,
              DOI 10.17487/RFC3261, June 2002,

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016


   The authors 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 Working Group
   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, Mary Barnes, Keith Drage, Cullen Jennings, Simon Perreault,
   Paul Kyzivat, Adam Roach, Richard Barnes, Ben Campbell, Stefan
   Winter, Spencer Dawkins, and Suresh Krishnan.  Adam Roach devised the
   example in Section 4.

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RFC 7984            Locating SIP Servers in IPv4/IPv6     September 2016

Authors' Addresses

   Olle E. Johansson
   Edvina AB
   Runbovaegen 10
   Sollentuna  SE-192 48

   Email: oej@edvina.net

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

   Email: gsalguei@cisco.com

   Vijay K. Gurbani
   Bell Labs, Nokia Networks
   1960 Lucent Lane
   Rm 9C-533
   Naperville, IL  60563
   United States of America

   Email: vkg@bell-labs.com

   Dale R. Worley (editor)
   Ariadne Internet Services
   738 Main St.
   Waltham, MA  02451
   United States of America

   Email: worley@ariadne.com

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