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RFC 6157 - IPv6 Transition in the Session Initiation Protocol (S

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Internet Engineering Task Force (IETF)                      G. Camarillo
Request for Comments: 6157                                      Ericsson
Updates: 3264                                                K. El Malki
Category: Standards Track                                        Athonet
ISSN: 2070-1721                                               V. Gurbani
                                               Bell Labs, Alcatel-Lucent
                                                              April 2011

        IPv6 Transition in the Session Initiation Protocol (SIP)


   This document describes how the IPv4 Session Initiation Protocol
   (SIP) user agents can communicate with IPv6 SIP user agents (and vice
   versa) at the signaling layer as well as exchange media once the
   session has been successfully set up.  Both single- and dual-stack
   (i.e., IPv4-only and IPv4/IPv6) user agents are considered.

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

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The Signaling Layer  . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . .  4
       3.1.1.  Relaying Requests across Different Networks  . . . . .  5
     3.2.  User Agent Behavior  . . . . . . . . . . . . . . . . . . .  7
   4.  The Media Layer  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Updates to RFC 3264  . . . . . . . . . . . . . . . . . . .  9
     4.2.  Initial Offer  . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . 10
   5.  Contacting Servers: Interaction of RFC 3263 and RFC 3484 . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Sample IPv4/IPv6 DNS File . . . . . . . . . . . . . . 14

1.  Introduction

   SIP [3] is a protocol to establish and manage multimedia sessions.
   After the exchange of signaling messages, SIP endpoints generally
   exchange session or media traffic, which is not transported using SIP
   but a different protocol.  For example, audio streams are typically
   carried using the Real-Time Transport Protocol (RTP) [13].

   Consequently, a complete solution for IPv6 transition needs to handle
   both the signaling layer and the media layer.  While unextended SIP
   can handle heterogeneous IPv6/IPv4 networks at the signaling layer as
   long as proxy servers and their Domain Name System (DNS) entries are
   properly configured, user agents using different networks and address
   spaces must implement extensions in order to exchange media between

   This document addresses the system-level issues in order to make SIP
   work successfully between IPv4 and IPv6.  Sections 3 and 4 provide
   discussions on the topics that are pertinent to the signaling layer
   and media layer, respectively, to establish a successful session
   between heterogeneous IPv4/IPv6 networks.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

   IPv4-only user agent:  An IPv4-only user agent supports SIP signaling
      and media only on the IPv4 network.  It does not understand IPv6

   IPv4-only node:  A host that implements only IPv4.  An IPv4-only node
      does not understand IPv6.  The installed base of IPv4 hosts
      existing before the transition begins are IPv4-only nodes.

   IPv6-only user agent:  An IPv6-only user agent supports SIP signaling
      and media only on the IPv6 network.  It does not understand IPv4

   IPv6-only node:  A host that implements IPv6 and does not implement

   IPv4/IPv6 node:  A host that implements both IPv4 and IPv6; such
      hosts are also known as "dual-stack" hosts [17].

   IPv4/IPv6 user agent:  A user agent that supports SIP signaling and
      media on both IPv4 and IPv6 networks.

   IPv4/IPv6 proxy:  A proxy that supports SIP signaling on both IPv4
      and IPv6 networks.

3.  The Signaling Layer

   An autonomous domain sends and receives SIP traffic to and from its
   user agents as well as to and from other autonomous domains.  This
   section describes the issues related to such traffic exchanges at the
   signaling layer, i.e., the flow of SIP messages between participants
   in order to establish the session.  We assume that the network
   administrators appropriately configure their networks such that the

   SIP servers within an autonomous domain can communicate between
   themselves.  This section contains system-level issues; a companion
   document [15] addresses IPv6 parser torture tests in SIP.

3.1.  Proxy Behavior

   User agents typically send SIP traffic to an outbound proxy, which
   takes care of routing it forward.  In order to support both IPv4-only
   and IPv6-only user agents, it is RECOMMENDED that domains deploy
   dual-stack outbound proxy servers or, alternatively, deploy both
   IPv4-only and IPv6-only outbound proxies.  Furthermore, there SHOULD
   exist both IPv6 and IPv4 DNS entries for outbound proxy servers.
   This allows the user agent to query DNS and obtain an IP address most
   appropriate for its use (i.e., an IPv4-only user agent will query DNS
   for A resource records (RRs), an IPv6-only user agent will query DNS
   for AAAA RRs, and a dual-stack user agent will query DNS for all RRs
   and choose a specific network.)

   Some domains provide automatic means for user agents to discover
   their proxy servers.  It is RECOMMENDED that domains implement
   appropriate discovery mechanisms to provide user agents with the IPv4
   and IPv6 addresses of their outbound proxy servers.  For example, a
   domain may support both the DHCPv4 [11] and the DHCPv6 [10] options
   for SIP servers.

   On the receiving side, user agents inside an autonomous domain
   receive SIP traffic from sources external to their domain through an
   inbound proxy, which is sometimes co-located with the registrar of
   the domain.  As was the case previously, it is RECOMMENDED that
   domains deploy dual-stack inbound proxies or, alternatively, deploy
   both IPv4-only and IPv6-only inbound proxy servers.  This allows a
   user agent external to the autonomous domain to query DNS and receive
   an IP address of the inbound proxy most appropriate for its use
   (i.e., an IPv4-only user agent will query DNS for A RRs, an IPv6-only
   user agent will query DNS for AAAA RRs, and a dual-stack user agent
   will query DNS for all RRs and choose a specific network).  This
   strategy, i.e., deploying dual-stack proxies, also allows for an
   IPv6-only user agent in the autonomous domain to communicate with an
   IPv4-only user agent in the same autonomous domain.  Without such a
   proxy, user agents using different networks identifiers will not be
   able to successfully signal each other.

   Proxies MUST follow the recommendations in Section 5 to determine the
   order in which to contact the downstream servers when routing a

3.1.1.  Relaying Requests across Different Networks

   A SIP proxy server that receives a request using IPv6 and relays it
   to a user agent (or another downstream proxy) using IPv4, and vice
   versa, needs to remain in the path traversed by subsequent requests.
   Therefore, such a SIP proxy server MUST be configured to Record-Route
   in that situation.

      Note that while this is the recommended practice, some problems
      may still arise if an RFC 2543 [14] endpoint is involved in
      signaling.  Since the ABNF in RFC 2543 did not include production
      rules to parse IPv6 network identifiers, there is a good chance
      that an RFC 2543-only compliant endpoint is not able to parse or
      regenerate IPv6 network identifiers in headers.  Thus, despite a
      dual-stack proxy inserting itself into the session establishment,
      the endpoint itself may not succeed in the signaling establishment

      This is generally not a problem with RFC 3261 endpoints; even if
      such an endpoint runs on an IPv4-only node, it still is able to
      parse and regenerate IPv6 network identifiers.

   Relaying a request across different networks in this manner has other
   ramifications.  For one, the proxy doing the relaying must remain in
   the signaling path for the duration of the session; otherwise, the
   upstream client and the downstream server would not be able to
   communicate directly.  Second, to remain in the signaling path, the
   proxy MUST insert one or two Record-Route headers: if the proxy is
   inserting a URI that contains a Fully Qualified Domain Name (FQDN) of
   the proxy, and that name has both IPv4 and IPv6 addresses in DNS,
   then inserting one Record-Route header suffices.  But if the proxy is
   inserting an IP address in the Record-Route header, then it must
   insert two such headers; the first Record-Route header contains the
   proxy's IP address that is compatible with the network type of the
   downstream server, and the second Record-Route header contains the
   proxy's IP address that is compatible with the upstream client.

   An example helps illustrate this behavior.  In the example, we use
   only those headers pertinent to the discussion.  Other headers have
   been omitted for brevity.  In addition, only the INVITE request and
   final response (200 OK) are shown; it is not the intent of the
   example to provide a complete call flow that includes provisional
   responses and other requests.

   In this example, proxy P, responsible for the domain example.com,
   receives a request from an IPv4-only upstream client.  It proxies
   this request to an IPv6-only downstream server.  Proxy P is running
   on a dual-stack host; on the IPv4 interface, it has an address of, and on the IPv6 interface, it is configured with an
   address of 2001:db8::1 (Appendix A contains a sample DNS zone file
   entry that has been populated with both IPv4 and IPv6 addresses.)

     UAC            Proxy           UAS
    (IPv4)            P           (IPv6)
      |          (IPv4/IPv6)         |
      |               |              |
      +---F1--------->|              |
      |               +---F2-------->|
      |               |              |
      |               |<--F3---------+
      |<--F4----------+              |
     ...             ...            ...
      |               |              |
      V               V              V

   F1: INVITE sip:alice@example.com SIP/2.0

   F2: INVITE sip:alice@2001:db8::10 SIP/2.0
       Record-Route: <sip:2001:db8::1;lr>
       Record-Route: <sip:;lr>

   F3: SIP/2.0 200 OK
       Record-Route: <sip:2001:db8::1;lr>
       Record-Route: <sip:;lr>

   F4: SIP/2.0 200 OK
       Record-Route: <sip:2001:db8::1;lr>
       Record-Route: <sip:;lr>

   Figure 1: Relaying requests across different networks

   When the User Agent Server (UAS) gets an INVITE and it accepts the
   invitation, it sends a 200 OK (F3) and forms a route set.  The first
   entry in its route set corresponds to the proxy's IPv6 interface.
   Similarly, when the 200 OK reaches the User Agent Client (UAC) (F4),
   it creates a route set by following the guidelines of RFC 3261 and
   reversing the Record-Route headers.  The first entry in its route set
   corresponds to the proxy's IPv4 interface.  In this manner, both the
   UAC and the UAS will have the correct address of the proxy to which
   they can target subsequent requests.

   Alternatively, the proxy could have inserted its FQDN in the Record-
   Route URI and the result would have been the same.  This is because
   the proxy has both IPv4 and IPv6 addresses in the DNS; thus, the URI
   resolution would have yielded an IPv4 address to the UAC and an IPv6
   address to the UAS.

3.2.  User Agent Behavior

   User agent clients MUST follow the normative text specified in
   Section 4.2 to gather IP addresses pertinent to the network.  Having
   done that, clients MUST follow the recommendations in Section 5 to
   determine the order of the downstream servers to contact when routing
   a request.

   Autonomous domains SHOULD deploy dual-stack user agent servers, or
   alternatively, deploy both IPv4-only and IPv6-only servers.  In
   either case, the RR in DNS for reaching the server should be
   specified appropriately.

4.  The Media Layer

   SIP establishes media sessions using the offer/answer model [4].  One
   endpoint, the offerer, sends a session description (the offer) to the
   other endpoint, the answerer.  The offer contains all the media
   parameters needed to exchange media with the offerer: codecs,
   transport addresses, protocols to transfer media, etc.

   When the answerer receives an offer, it elaborates an answer and
   sends it back to the offerer.  The answer contains the media
   parameters that the answerer is willing to use for that particular
   session.  Offer and answer are written using a session description
   protocol.  The most widespread protocol to describe sessions at
   present is called, aptly enough, the Session Description Protocol
   (SDP) [2].

   A direct offer/answer exchange between an IPv4-only user agent and an
   IPv6-only user agent does not result in the establishment of a
   session.  The IPv6-only user agent wishes to receive media on one or
   more IPv6 addresses, but the IPv4-only user agent cannot send media
   to these addresses, and generally does not even understand their
   format.  Consequently, user agents need a means to obtain both IPv4
   and IPv6 addresses to receive media and to place them in offers and

      This IP version incompatibility problem would not exist if hosts
      implementing IPv6 also implemented IPv4, and were configured with
      both IPv4 and IPv6 addresses.  In such a case, a UA would be able

      to pick a compatible media transport address type to enable the
      hosts to communicate with each other.

   Pragmatism dictates that IPv6 user agents undertake the greater
   burden in the transition period.  Since IPv6 user agents are not
   widely deployed yet, it seems appropriate that IPv6 user agents
   obtain IPv4 addresses instead of mandating an upgrade on the
   installed IPv4 base.  Furthermore, IPv6 user agents are expected to
   be dual-stacked and thus also support IPv4, unlike the larger IPv4-
   only user agent base that does not or cannot support IPv6.

   An IPv6 node SHOULD also be able to send and receive media using IPv4
   addresses, but if it cannot, it SHOULD support Session Traversal
   Utilities for NAT (STUN) relay usage [8].  Such a relay allows the
   IPv6 node to indirectly send and receive media using IPv4.

   The advantage of this strategy is that the installed base of IPv4
   user agents continues to function unchanged, but it requires an
   operator that introduces IPv6 to provide additional servers for
   allowing IPv6 user agents to obtain IPv4 addresses.  This strategy
   may come at an additional cost to SIP operators deploying IPv6.
   However, since IPv4-only SIP operators are also likely to deploy STUN
   relays for NAT (Network Address Translator) traversal, the additional
   effort to deploy IPv6 in an IPv4 SIP network should be limited in
   this aspect.

   However, there will be deployments where an IPv4/IPv6 node is unable
   to use both interfaces natively at the same time, and instead, runs
   as an IPv6-only node.  Examples of such deployments include:

   1.  Networks where public IPv4 addresses are scarce and it is
       preferable to make large deployments only on IPv6.

   2.  Networks utilizing Layer-2's that do not support concurrent IPv4
       and IPv6 usage on the same link.

4.1.  Updates to RFC 3264

   This section provides a normative update to RFC 3264 [4] in the
   following manner:

   1.  In some cases, especially those dealing with third party call
       control (see Section 4.2 of [12]), there arises a need to specify
       the IPv6 equivalent of the IPv4 unspecified address ( in
       the SDP offer.  For this, IPv6 implementations MUST use a domain
       name within the .invalid DNS top-level domain instead of using
       the IPv6 unspecified address (i.e., ::).

   2.  Each media description in the SDP answer MUST use the same
       network type as the corresponding media description in the offer.
       Thus, if the applicable "c=" line for a media description in the
       offer contained a network type with the value "IP4", the
       applicable "c=" line for the corresponding media description in
       the answer MUST contain "IP4" as the network type.  Similarly, if
       the applicable "c=" line for a media description in the offer
       contained a network type with the value "IP6", the applicable
       "c=" line for the corresponding media description in the answer
       MUST contain "IP6" as the network type.

4.2.  Initial Offer

   We now describe how user agents can gather addresses by following the
   Interactive Connectivity Establishment (ICE) [7] procedures.  ICE is
   protocol that allows two communicating user agents to arrive at a
   pair of mutually reachable transport addresses for media
   communications in the presence of NATs.  It uses the STUN [18]
   protocol, applying its binding discovery and relay usages.

   When following the ICE procedures, in addition to local addresses,
   user agents may need to obtain addresses from relays; for example, an
   IPv6 user agent would obtain an IPv4 address from a relay.  The relay
   would forward the traffic received on this IPv4 address to the user
   agent using IPv6.  Such user agents MAY use any mechanism to obtain
   addresses in relays, but, following the recommendations in ICE, it is
   RECOMMENDED that user agents support STUN relay usage [6] [8] for
   this purpose.

   IPv4/IPv6 user agents SHOULD gather both IPv4 and IPv6 addresses
   using the ICE procedures to generate all their offers.  This way,
   both IPv4-only and IPv6-only answerers will be able to generate a
   mutually acceptable answer that establishes a session (having used
   ICE to gather both IPv4 and IPv6 addresses in the offer reduces the
   session establishment time because all answerers will find the offer

      Implementations are encouraged to use ICE; however, the normative
      strength of the text above is left at a SHOULD since in some
      managed networks (such as a closed enterprise network) it is
      possible for the administrator to have control over the IP version
      utilized in all nodes and thus deploy an IPv6-only network, for
      example.  The use of ICE can be avoided for signaling messages
      that stay within such managed networks.

4.3.  Connectivity Checks

   Once the answerer has generated an answer following the ICE
   procedures, both user agents perform the connectivity checks as
   specified by ICE.  These checks help prevent some types of flooding
   attacks and allow user agents to discover new addresses that can be
   useful in the presence of NATs.

5.  Contacting Servers: Interaction of RFC 3263 and RFC 3484

   RFC 3263 maps a SIP or SIPS URI to a set of DNS SRV records for the
   various servers that can handle the URI.  The Expected Output, given
   an Application Unique String (the URI) is one or more SRV records,
   sorted by the "priority" field, and further ordered by the "weight"
   field in each priority class.

      The terms "Expected Output" and "Application Unique String", as
      they are to be interpreted in the context of SIP, are defined in
      Section 8 of RFC 3263 [5].

   To find a particular IP address to send the request to, the client
   will eventually perform an A or AAAA DNS lookup on a target.  As
   specified in RFC 3263, this target will have been obtained through
   NAPTR and SRV lookups, or if NAPTR and SRV lookup did not return any
   records, the target will simply be the domain name of the Application
   Unique String.  In order to translate the target to the corresponding
   set of IP addresses, IPv6-only or dual-stack clients MUST use the
   newer getaddrinfo() name lookup function, instead of gethostbyname()
   [16].  The new function implements the Source and Destination Address
   Selection algorithms specified in RFC 3484 [9], which is expected to
   be supported by all IPv6 hosts.

   The advantage of the additional complexity is that this technique
   will output an ordered list of IPv6/IPv4 destination addresses based
   on the relative merits of the corresponding source/destination pairs.
   This will guarantee optimal routing.  However, the Source and
   Destination Selection algorithms of RFC3484 are dependent on broad
   operating system support and uniform implementation of the
   application programming interfaces that implement this behavior.

      Developers should carefully consider the issues described by Roy
      et al. [19] with respect to address resolution delays and address
      selection rules.  For example, implementations of getaddrinfo()
      may return address lists containing IPv6 global addresses at the
      top of the list and IPv4 addresses at the bottom, even when the
      host is only configured with an IPv6 local scope (e.g., link-
      local) and an IPv4 address.  This will, of course, introduce a
      delay in completing the connection.

6.  Security Considerations

   This document describes how IPv4 SIP user agents can communicate with
   IPv6 user agents (and vice versa).  To do this, it uses additional
   protocols (STUN relay usage [6], ICE [7], SDP [2]); the threat model
   of each such protocol is included in its respective document.  The
   procedures introduced in this document do not introduce the
   possibility of any new security threats; however, they may make hosts
   more amenable to existing threats.  Consider, for instance, a UAC
   that allocates an IPv4 and an IPv6 address locally and inserts these
   into the SDP.  Malicious user agents that may intercept the request
   can mount a denial-of-service attack targeted to the different
   network interfaces of the UAC.  In such a case, the UAC should use
   mechanisms that protect confidentiality and integrity of the
   messages, such as using the SIPS URI scheme as described in Section
   26.2.2 of RFC3261 [3], or secure MIME as described in Section 23 of
   RFC3261 [3].  If HTTP Digest is used as an authentication mechanism
   in SIP, then the UAC should ensure that the quality of protection
   also includes the SDP payload.

7.  Acknowledgments

   The authors would like to thank Mohamed Boucadair, Christine Fischer,
   Cullen Jennings, Aki Niemi, Jonathan Rosenberg, and Robert Sparks for
   discussions on the working group list that improved the quality of
   this document.

   Additionally, Francois Audet, Mary Barnes, Keith Drage, and Dale
   Worley provided invaluable comments as part of the working group Last
   Call review process.  Jari Arkko, Lars Eggert, Tobias Gondrom, Suresh
   Krishnan, and Tim Polk conducted an in-depth review of the work as
   part of the IESG and Gen-ART reviews.

8.  References

8.1.  Normative References

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

   [2]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

   [3]   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.

   [4]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         the Session Description Protocol (SDP)", RFC 3264, June 2002.

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

   [6]   Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
         Relays around NAT (TURN): Relay Extensions to Session Traversal
         Utilities for NAT (STUN)", RFC 5766, April 2010.

   [7]   Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Protocol for Network Address Translator (NAT) Traversal for
         Offer/Answer Protocols", RFC 5245, April 2010.

   [8]   Camarillo, G., Novo, O., and S. Perreault, "Traversal Using
         Relays around NAT (TURN) Extension for IPv6", RFC 6156, April

   [9]   Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

8.2.  Informative References

   [10]  Schulzrinne, H. and B. Volz, "Dynamic Host Configuration
         Protocol (DHCPv6) Options for Session Initiation Protocol (SIP)
         Servers", RFC 3319, July 2003.

   [11]  Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-
         for-IPv4) Option  for Session Initiation Protocol (SIP)
         Servers", RFC 3361, August 2002.

   [12]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
         April 2004.

   [13]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [14]  Handley, M., Schulzrinne, H., Schooler, E., and J. Rosenberg,
         "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [15]  Gurbani, V., Boulton, C., and R. Sparks, "Session Initiation
         Protocol (SIP) Torture Test Messages for Internet Protocol
         Version 6 (IPv6)", RFC 5118, February 2008.

   [16]  Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E. Castro,
         "Application Aspects of IPv6 Transition", RFC 4038, March 2005.

   [17]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
         IPv6 Hosts and Routers", RFC 4213, October 2005.

   [18]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session
         Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.

   [19]  Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor Discovery On-
         Link Assumption Considered Harmful", RFC 4943, September 2007.

Appendix A.  Sample IPv4/IPv6 DNS File

   A portion of a sample DNS zone file entry is reproduced below that
   has both IPv4 and IPv6 addresses.  This entry corresponds to a proxy
   server for the domain "example.com".  The proxy server supports the
   Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
   transport for both IPv4 and IPv6 networks.

       _sip._tcp  SRV  20 0 5060 sip1.example.com
                  SRV   0 0 5060 sip2.example.com
       _sip._udp  SRV  20 0 5060 sip1.example.com
                  SRV   0 0 5060 sip2.example.com

       sip1 IN A
       sip1 IN AAAA  2001:db8::1
       sip2 IN A
       sip2 IN AAAA  2001:db8::2

Authors' Addresses

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420

   EMail: Gonzalo.Camarillo@ericsson.com

   Karim El Malki
   AREA Science Park
   Padriciano 99
   Trieste (TS)  34149

   EMail: karim@athonet.com

   Vijay K. Gurbani
   Bell Laboratories, Alcatel-Lucent
   1960 Lucent Lane
   Rm 9C-533
   Naperville, IL  60563

   Phone: +1 630 224 0216
   EMail: vkg@bell-labs.com


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