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Internet Engineering Task Force (IETF)                      G. CamarilloRequest for Comments: 6157                                      EricssonUpdates:3264                                                K. El MalkiCategory: Standards Track                                        AthonetISSN: 2070-1721                                               V. Gurbani                                               Bell Labs, Alcatel-Lucent                                                              April 2011IPv6 Transition in the Session Initiation Protocol (SIP)Abstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6157.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Camarillo, et al.            Standards Track                    [Page 1]

RFC 6157                 IPv6 Transition in SIP               April 2011   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .33.  The Signaling Layer  . . . . . . . . . . . . . . . . . . . . .43.1.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . .43.1.1.  Relaying Requests across Different Networks  . . . . .53.2.  User Agent Behavior  . . . . . . . . . . . . . . . . . . .74.  The Media Layer  . . . . . . . . . . . . . . . . . . . . . . .74.1.  Updates toRFC 3264  . . . . . . . . . . . . . . . . . . .94.2.  Initial Offer  . . . . . . . . . . . . . . . . . . . . . .94.3.  Connectivity Checks  . . . . . . . . . . . . . . . . . . .105.  Contacting Servers: Interaction ofRFC 3263 andRFC 3484 . . .106.  Security Considerations  . . . . . . . . . . . . . . . . . . .117.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .118.  References . . . . . . . . . . . . . . . . . . . . . . . . . .128.1.  Normative References . . . . . . . . . . . . . . . . . . .128.2.  Informative References . . . . . . . . . . . . . . . . . .12Appendix A.  Sample IPv4/IPv6 DNS File . . . . . . . . . . . . . .141.  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   them.Camarillo, et al.            Standards Track                    [Page 2]

RFC 6157                 IPv6 Transition in SIP               April 2011   This document addresses the system-level issues in order to make SIP   work successfully between IPv4 and IPv6.  Sections3 and4 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",   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as   described inBCP 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      addresses.   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      addresses.   IPv6-only node:  A host that implements IPv6 and does not implement      IPv4.   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 theCamarillo, et al.            Standards Track                    [Page 3]

RFC 6157                 IPv6 Transition in SIP               April 2011   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 inSection 5 to determine the   order in which to contact the downstream servers when routing a   request.Camarillo, et al.            Standards Track                    [Page 4]

RFC 6157                 IPv6 Transition in SIP               April 20113.1.1.  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 anRFC 2543 [14] endpoint is involved in      signaling.  Since the ABNF inRFC 2543 did not include production      rules to parse IPv6 network identifiers, there is a good chance      that anRFC 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      phase.      This is generally not a problem withRFC 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 ofCamarillo, et al.            Standards Track                    [Page 5]

RFC 6157                 IPv6 Transition in SIP               April 2011   192.0.2.1, 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:192.0.2.1;lr>       ...   F3: SIP/2.0 200 OK       Record-Route: <sip:2001:db8::1;lr>       Record-Route: <sip:192.0.2.1;lr>       ...   F4: SIP/2.0 200 OK       Record-Route: <sip:2001:db8::1;lr>       Record-Route: <sip:192.0.2.1;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 ofRFC 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.Camarillo, et al.            Standards Track                    [Page 6]

RFC 6157                 IPv6 Transition in SIP               April 2011   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 inSection 4.2 to gather IP addresses pertinent to the network.  Having   done that, clients MUST follow the recommendations inSection 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   answers.      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 ableCamarillo, et al.            Standards Track                    [Page 7]

RFC 6157                 IPv6 Transition in SIP               April 2011      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 toRFC 3264   This section provides a normative update toRFC 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 (0.0.0.0) 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., ::).Camarillo, et al.            Standards Track                    [Page 8]

RFC 6157                 IPv6 Transition in SIP               April 2011   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   valid.)      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.Camarillo, et al.            Standards Track                    [Page 9]

RFC 6157                 IPv6 Transition in SIP               April 20114.3.  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 ofRFC 3263 andRFC 3484RFC 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 inSection 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 inRFC 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 inRFC 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 ofRFC3484 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.Camarillo, et al.            Standards Track                   [Page 10]

RFC 6157                 IPv6 Transition in SIP               April 20116.  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 inSection26.2.2 of RFC3261 [3], or secure MIME as described inSection 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.  References8.1.  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.Camarillo, et al.            Standards Track                   [Page 11]

RFC 6157                 IPv6 Transition in SIP               April 2011   [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         2011.   [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.Camarillo, et al.            Standards Track                   [Page 12]

RFC 6157                 IPv6 Transition in SIP               April 2011   [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.Camarillo, et al.            Standards Track                   [Page 13]

RFC 6157                 IPv6 Transition in SIP               April 2011Appendix 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     192.0.2.1       sip1 IN AAAA  2001:db8::1       sip2 IN A     192.0.2.2       sip2 IN AAAA  2001:db8::2       ...Camarillo, et al.            Standards Track                   [Page 14]

RFC 6157                 IPv6 Transition in SIP               April 2011Authors' Addresses   Gonzalo Camarillo   Ericsson   Hirsalantie 11   Jorvas  02420   Finland   EMail: Gonzalo.Camarillo@ericsson.com   Karim El Malki   Athonet   AREA Science Park   Padriciano 99   Trieste (TS)  34149   Italy   EMail: karim@athonet.com   Vijay K. Gurbani   Bell Laboratories, Alcatel-Lucent   1960 Lucent Lane   Rm 9C-533   Naperville, IL  60563   USA   Phone: +1 630 224 0216   EMail: vkg@bell-labs.comCamarillo, et al.            Standards Track                   [Page 15]