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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 6947

Network Working Group                                       M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Informational                                 H. Kaplan
Expires: August 2, 2013                                      Acme Packet
                                                               R. Gilman
                                                             Independent
                                                         S. Veikkolainen
                                                                   Nokia
                                                        January 29, 2013


    Session Description Protocol (SDP) Alternate Connectivity (ALTC)
                               Attribute
                     draft-boucadair-mmusic-altc-09

Abstract

   This document proposes a mechanism which allows to carry multiple IP
   addresses, of different address families (e.g., IPv4, IPv6), in the
   same SDP offer.  The proposed attribute solves the backward
   compatibility problem which plagued ANAT (Alternative Network Address
   Types), due to its syntax.

   The proposed solution is applicable to scenarios where connectivity
   checks are not required.  If connectivity checks are required, ICE
   (RFC 5245) provides such a solution.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 http://datatracker.ietf.org/drafts/current/.

   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."




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   This Internet-Draft will expire on August 2, 2013.

Copyright Notice

   Copyright (c) 2013 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.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Overall Context  . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Overview of the ALTC Mechanism . . . . . . . . . . . . . . . .  7
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Rationale for the Chosen Syntax  . . . . . . . . . . . . .  8
   4.  Alternate Connectivity Attribute . . . . . . . . . . . . . . .  8
     4.1.  ALTC Syntax  . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Usage and Interaction  . . . . . . . . . . . . . . . . . . 10
       4.2.1.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.2.2.  Usage of ALTC in an SDP Answer . . . . . . . . . . . . 11
       4.2.3.  Interaction with ICE . . . . . . . . . . . . . . . . . 11
       4.2.4.  Interaction with SDP-Cap-Neg . . . . . . . . . . . . . 12
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  ALTC Use Cases  . . . . . . . . . . . . . . . . . . . 15
     A.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . 15
     A.2.  Multicast Use Case . . . . . . . . . . . . . . . . . . . . 16
     A.3.  Introducing IPv6 into SIP-based Architectures  . . . . . . 17
       A.3.1.  Avoid Crossing CGN Devices . . . . . . . . . . . . . . 17
       A.3.2.  Basic Scenario for IPv6 SIP Service Delivery . . . . . 17
       A.3.3.  Avoid IPv4/IPv6 Interworking . . . . . . . . . . . . . 18
       A.3.4.  DBE Bypass Procedure . . . . . . . . . . . . . . . . . 20
       A.3.5.  Direct Communications Between IPv6-enabled User
               Agents . . . . . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

















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1.  Introduction

1.1.  Overall Context

   Due to the IPv4 address exhaustion problem, IPv6 deployment is
   becoming an urgent need, along with the need to properly handle IPv6
   and IPv4 co-existence.  The reality of IPv4-IPv6 co-existence
   introduces heterogeneous scenarios with combinations of IPv4 and IPv6
   nodes, some of which are capable of supporting both IPv4 and IPv6
   dual-stack (DS) and some of which are capable of supporting only IPv4
   or only IPv6.  In this context, Session Initiation Protocol (SIP
   [RFC3261]) User Agents (UAs) need to be able to indicate their
   available IP capabilities in order to increase the ability to
   establish successful SIP sessions, and also to avoid invocation of
   adaptation functions such as Application Layer Gateways (ALGs) and
   IPv4-IPv6 interconnection functions (e.g., NAT64 [RFC6146]), and to
   avoid using private IPv4 addresses through consumer NATs or Carrier
   Grade NATs (CGN, [I-D.ietf-behave-lsn-requirements]).

   In the meantime, service providers are investigating scenarios to
   upgrade their service offering to be IPv6-capable.  The current
   strategies involve either offering IPv6 only, for example to mobile
   devices, or providing both IPv4 and IPv6 but with private IPv4
   addresses which are NAT'ed by CGNs.  In the latter case the end
   device may be using "normal" IPv4 and IPv6 stacks and interfaces, or
   it may tunnel the IPv4 packets though a DS-Lite stack integrated into
   the host [RFC6333]; in either case the device has both address
   families available from a SIP and media perspective.

   Regardless of the IPv6 transition strategy being used, it is obvious
   that there will be a need for dual-stack SIP devices to communicate
   with IPv4-only legacy UAs, and IPv6-only UAs, and other dual-stack
   UAs.  It may not, for example, be possible for a dual-stack UA to
   communicate with an IPv6-only UA unless the dual-stack UA had a means
   of providing the IPv6-only UA with its IPv6 local address for media,
   while clearly it needs to provide a legacy IPv4-only device its local
   IPv4 address.  The communication must be possible in a backwards-
   compatible fashion, such that IPv4-only SIP devices need not support
   the new mechanism to communicate with dual-stack UAs.

   The current means by which multiple address families can be
   communicated are through ANAT [RFC4091] or ICE [RFC5245].  ANAT has
   serious backwards-compatibility problems as described in [RFC4092],
   which effectively make it unusable, and it is deprecated by the IETF
   [RFC5245].  ICE at least allows interoperability with legacy devices,
   by not doing ICE in such cases, but it is a complicated and
   processing intensive mechanism, and has seen limited deployment and
   implementation in SIP applications.



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   ALTC has been implemented as reported in
   [I-D.boucadair-pcp-nat64-experiments]; no issue has been reported in
   that document.

1.2.  Purpose

   This document proposes a new alternative: a backwards-compatible
   syntax for indicating multiple media connection addresses and ports
   in an SDP offer, which can immediately be selected from and used in
   an SDP answer.

   The proposed mechanism is independent of the model described in
   [RFC5939] and does not require implementation of sdp-capabilities-
   negotiations (a.k.a., SDPCapNeg) to function.

   It should be noted that "backwards-compatible" in this document
   generally refers to working with legacy IPv4-only devices.  The
   choice has to be made, one way or the other, because to interoperate
   with legacy devices requires constructing SDP bodies which they would
   understand and support, such that they detect their local address
   family in the SDP connection line.  It is not possible to support
   interworking with both legacy IPv4-only and legacy IPv6-only devices
   with the same SDP offer.  Clearly, there are far more legacy IPv4-
   only devices in existence, and thus those are the ones assumed in
   this document.  However, the syntax allows for a UA to choose which
   address family to be backwards-compatible with, in case it has some
   means of determining it.

   Furthermore, even for cases where both sides support the same address
   family, there should be a means by which the "best" address family
   transport is used, based on what the UAs decide.  The address family
   which is "best" for a particular session cannot always be known a
   priori.  For example, in some cases the IPv4 transport may be better,
   even if both UAs support IPv6.

   The proposed solution provides the following benefits:

   o  Allows a UA to signal more than one IP address (type) in the same
      SDP offer/answer;
   o  Is backwards compatible.  No parsing or semantic errors will be
      experienced by a legacy UA or intermediary SIP nodes which do not
      understand this new mechanism;
   o  Is as lightweight as possible to achieve the goal, while still
      allowing and interoperating with nodes which support other similar
      or related mechanisms;
   o  Is easily deployable in managed networks;





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   o  Requires minimal increase of the length of the SDP offer (I.e.,
      minimizes fragmentation risks).

   ALTC may also be useful for the multicast context (e.g., Section 3.4
   of [I-D.venaas-behave-v4v6mc-framework] or Section 3.3 of
   [I-D.ietf-mboned-multrans-addr-acquisition]).

   More detailed information about ALTC use cases is provided in
   Appendix A.

1.3.  Scope

   This document proposes an alternative scheme, as replacement to the
   ANAT procedure [RFC4091], to carry several IP address types in the
   same SDP offer/answer while preserving backward compatibility.

   While clearly two UAs communicating directly at a SIP layer need to
   be able to support the same address family for SIP itself, current
   SIP deployments almost always have Proxy Servers or B2BUA's in the
   SIP signaling path, which can provide the necessary interworking of
   the IP address family at the SIP layer (e.g., [RFC6157]).  SIP-layer
   address family interworking is out of scope of this document.
   Instead, this document focuses on the problem of communicating media
   address family capabilities in a backwards-compatible fashion.  Since
   media can go directly between two UAs, without a priori knowledge by
   the UAC of which address family the far-end UAS supports, it has to
   offer both, in a backwards-compatible fashion.


2.  Use Cases

   The ALTC mechanism defined in this document is primary meant for
   managed networks.  In particular, the following use cases were
   explicitly considered:

   o  A dual-stack UAC initiating a SIP session without knowing the
      address family of the ultimate target UAS.
   o  A UA receiving a SIP session request with SDP offer and wishes to
      avoid using IPv4, or to avoid IPv6.
   o  An IPv6-only UA wishes to avoid using a NAT64 [RFC6146].
   o  A SIP UA behind a Dual-Stack Lite CGN [RFC6333].
   o  A SIP Service Provider or Enterprise domain of IPv4-only and/or
      IPv6-only UA, which provides interworking by invoking IPv4-IPv6
      media relays, wishes to avoid invoking such functions and let
      media go end-to-end as much as possible.
   o  A SIP Service Provider or Enterprise domain of a UA, which
      communicates with other domains and wishes to either avoid
      invoking IPv4-IPv6 interworking or let media go end-to-end as much



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      as possible.
   o  A SIP Service Provider providing transit peering services for SIP
      sessions, which may need to modify SDP in order to provide IPv4-
      IPv6 interworking, but would prefer to avoid such interworking or
      avoid relaying media in general, as much as possible.
   o  SIP sessions using the new mechanism crossing legacy SDP-aware
      middleboxes which may not understand this new mechanism.


3.  Overview of the ALTC Mechanism

3.1.  Overview

   The ALTC mechanism relies solely on the SDP offer/answer mechanism,
   with specific syntax to indicate alternative connection addresses.
   The basic concept is to use a new SDP attribute "altc", to indicate
   the IP addresses for potential alternative connection addresses.  The
   address which is most likely to get chosen for the session is in the
   normal 'c=' line.  Typically in current operational networks this
   would be an IPv4 address.  The "a=altc" lines contain the alternative
   addresses offered for this session.  This way, a dual-stack UA might
   encode its IPv4 address in the "c=" line, while possibly preferring
   to use an IPv6 address by explicitly indicating the preference order
   in the corresponding "a=altc" line.  One of the "a=altc" lines
   duplicates the address contained in the "c=" line, for reasons
   explained in Section 3.2).  The SDP answerer would indicate its
   chosen address, by simply using that address family in the "c=" line
   of its response.

   An example of an SDP offer using this mechanism is as follows when
   IPv4 is considered most likely to be used for the session, but IPv6
   is preferred:

   v=0
   o=- 25678 753849 IN IP4 192.0.2.1
   s=
   c=IN IP4 192.0.2.1
   t=0 0
   m=audio 12340 RTP/AVP 0 8
   a=altc:1 IP6 2001:db8::1 45678
   a=altc:2 IP4 192.0.2.1 12340

   If IPv6 was considered most likely to be used for the session, the
   SDP offer would be as follows:







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   v=0
   o=- 25678 753849 IN IP6 2001:db8::1
   s=
   c=IN IP6 2001:db8::1
   t=0 0
   m=audio 45678 RTP/AVP 0 8
   a=altc:1 IP6 2001:db8::1 45678
   a=altc:2 IP4 192.0.2.1 12340

   Since an alternative address is likely to require an alternative TCP/
   UDP port number as well, the new "altc" attribute includes both an IP
   address and a receive transport port number (or multiple port
   numbers).  The ALTC mechanism does not itself support offering a
   different transport type (i.e., UDP vs. TCP), codec, nor any other
   attribute.  It is only intended for offering an alternative IP
   address and port number.

3.2.  Rationale for the Chosen Syntax

   The use of an 'a=' attribute line is, according to [RFC4566], the
   primary means for extending SDP and tailoring it to particular
   applications or media.  A compliant SDP parser will ignore the
   unsupported attribute lines.

   The rationale for encoding the same address and port in the "a=altc"
   line as in the "m=" and "c=" lines is to provide detection of legacy
   SDP-changing middleboxes.  Such systems may change the connection
   address and media transport port numbers, but not support this new
   mechanism, and thus two UAs supporting this mechanism would try to
   connect to the wrong addresses.  Therefore, the rules detailed in
   this document require the SDP processor to check for matching altc
   and connection line addresses and media ports, before choosing one of
   the alternatives.


4.  Alternate Connectivity Attribute

4.1.  ALTC Syntax

   The altc attribute adheres to the [RFC4566] "attribute" production.
   The ABNF syntax [RFC5234] of altc is provided below:

   altc-attr = "altc" ":" att-value
   att-value = altc-num SP addrtype SP connection-address SP port  ["/" rtcp-port]
   altc-num  = 1*DIGIT
   rtcp-port = port





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             Figure 1: Connectivity Capability Attribute ABNF

   The meaning of the fields are listed hereafter:

   o  altc-num: digit to uniquely refer to an address alternative.  It
      must be in preference order; "1" indicates the most preferred
      address.
   o  addrtype: the addrtype field as defined in [RFC4566] for
      connection data.
   o  connection-address: a network address as defined in [RFC4566]
      corresponding to the address type specified by addrtype.
   o  port: the port number to be used, as defined in [RFC4566].
      Distinct port numbers may be used per IP address type.  If the
      specified address type does not require a port number, a value
      defined for that address type should be used.
   o  rtcp-port: Including an RTCP port is optional.  An RTCP port may
      be indicated in the alternative "c=" line when the RTCP port can
      not be derived from the RTP port.

   The "altc" attribute is only applicable in an SDP offer.  The "altc"
   attribute is a media-level-only attribute, and MUST NOT appear at the
   SDP session level (since it defines a port number, it is inherently
   tied to the media level).  There MUST NOT be more than one "altc"
   attribute per addrtype within each media description.  This
   restriction is necessary in order that the addrtype of the reply may
   be used by the offerer to determine which alternative was accepted.

   The <addrtype>'s of the altc MUST correspond to the <nettype> of the
   current connection (c=) line.

   A media description MUST contain two "altc" attributes: the
   alternative address and an alternative port as well as an address and
   port which "duplicates" the address/port information from the current
   'c=' and 'm=' lines.  Each media level MUST contain at least one such
   duplicate altc attribute, of the same IP address family, address, and
   transport port number as those in the SDP connection and media lines
   of its level.  In particular, if a 'c=' line appears within a media
   description, the addr-type and connection-address from that 'c=' line
   MUST be used in the duplicate "altc" attribute for that media
   description.  If a 'c=' line appears only at the session level and a
   given media description does not have its own connection line, then
   the duplicate "altc" attribute for that media description MUST be the
   same as the session-level address information.

   The "altc" attributes appearing within a media description MUST be
   prioritized; the explicit preference order is indicated in each line
   ("1" is used to indicate the address with the highest priority).
   Given this rule, and the requirement that the address information



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   provided in the "m=" line and "o=" line must be provided in an "altc"
   attribute as well, it is possible that the address in the "m=" line
   and "o=" line are not the preferred choice.

   If the addrtype of an "altc" attribute is not compatible with the
   transport protocol or media format specified in the media
   description, that altc attribute MUST be ignored.

   Note that "a=altc" lines describe alternative connection addresses,
   NOT addresses for parallel connections.  When several altc lines are
   present, multiple sessions establishment MUST be avoided.  Only one
   session is to be maintained with the remote party for the associated
   media description.

   If no port number is indicated for the alternative address, the same
   port number is used for all address families.

4.2.  Usage and Interaction

4.2.1.  Usage

   In an SDP offer/answer model, the SDP offer includes "altc"
   attributes to indicate alternative connection information (i.e.,
   address type, address and port number(s)), including the "duplicate"
   connection information already identified in the 'c=' and 'm=' lines.

   Additional, subsequent offers MAY include "altc" attributes again,
   and may change the IP address, port numbers, and order of preference;
   but they MUST include a duplicate "altc" attribute for the connection
   and media lines in that specific subsequent offer.  In other words,
   every offered SDP media description with an alternative address offer
   with an "altc" attribute has two of them:

      - one duplicating the 'c=' and 'm=' line information for that
      media description, and
      - one for the alternative,

   even though these need not be the same as the original SDP offer.

   The purpose of encoding a duplicate "altc" attribute is to allow
   receivers of the SDP offer to detect if a legacy SDP-changing middle
   box has modified the 'c=' and/or 'm=' line address/port information.
   If the SDP answerer does not find a duplicate "altc" attribute value
   for which the address and port match exactly those in the 'c=' line
   and 'm=' line, the SDP answerer MUST ignore the "altc" attributes and
   use the 'c=' and 'm=' offered address/ports for the entire SDP
   instead, as if no "altc" attributes were present.  The rationale for
   this is that many SDP-changing middleboxes will end the media



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   sessions if they do not detect media flowing through them; if a
   middlebox modified the SDP addresses, media MUST be sent using the
   modified information.

   Note that for RTCP, if applicable for the given media types, each
   side would act as if the chosen "altc" attribute's port number was in
   the 'm=' media line.  Typically, this would mean RTCP is sent to the
   odd +1 of the port number, unless some other attribute determines
   otherwise.  For example the RTP/RTCP multiplexing mechanism defined
   in [RFC5761] can still be used with ALTC, such that if both sides
   support multiplexing they will indicate so using the 'a=rtcp-mux'
   attribute as defined in [RFC5761]; but the IP connection address and
   port they use may be chosen using the ALTC mechanism.

   If the SDP offerer wishes to use the RTCP attribute defined in
   [RFC3605], a complication can arise since it may not be clear which
   address choice the 'a=rtcp' attribute applies to, relative the
   choices offered by ALTC.  Technically RFC 3605 allows indicating the
   address for RTCP explicitly in the 'a=rtcp' attribute itself, but
   this is optional and rarely used.  For this reason, this document
   recommends using 'a=rtcp' attribute to be for the address choice
   encoded in the "m=" line, and include an alternate RTCP port in the
   'a=altc' attribute corresponding to the alternative address.  In
   other words, if the 'a=rtcp' attribute explicitly encodes an address
   in its attribute, then that applies for ALTC as per [RFC3605]; if it
   does not, then ALTC assumes the 'a=rtcp' attribute is for the address
   in the "m=" line, and the alternative "altc" attribute include an
   RTCP alternate port number.

4.2.2.  Usage of ALTC in an SDP Answer

   The SDP answer SHOULD NOT contain "altc" attributes, as the answer's
   'c=' line implicitly and definitively chooses the address family from
   the offer and includes it in "c=" and "m=" lines of the answer.
   Furthermore, this avoids establishing several sessions simultaneously
   between the participating peers.

   Any solution requiring the use of ALTC in SDP answer SHOULD document
   its usage, in particular how sessions are established between the
   participating peers.

4.2.3.  Interaction with ICE

   Since ICE [RFC5245] also includes address and port number information
   in its candidate attributes, a potential problem arises: which one
   wins.  Since ICE also includes specific ICE attributes in the SDP
   answer, the problem is easily avoided: if the SDP offerer supports
   both ALTC and ICE, it may include both sets of attributes in the same



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   SDP offer.  A legacy ICE-only answerer will simply ignore the ALTC
   attributes, and use ICE.  An ALTC-only answerer will ignore the ICE
   attributes and reply without them.  An answerer which supports both
   MUST choose one and only one of the mechanisms to use: either ICE or
   ALTC (unless the 'm=' or 'c=' lines were changed by a middlebox, in
   which case the rules for both ALTC and ICE would make the answerer
   revert to basic SDP semantics).

4.2.4.  Interaction with SDP-Cap-Neg

   The ALTC mechanism is orthogonal to SDPCapNeg [RFC5939].  If the
   offerer supports both ALTC and SDPCapNeg, it may offer both.


5.  IANA Considerations

   This document requests the following new SDP attribute:

      SDP Attribute ("att-field"):

         Attribute name      altc
         Long form           Alternate Connectivity
         Type of name        att-field
         Type of attribute   Media level only
         Subject to charset  No
         Purpose             See Section 1.2, Section 3
         Specification       Section 4

   The contact person for this registration is Mohamed Boucadair (email:
   mohamed.boucadair@orange.com; phone: +33 2 99 12 43 71).


6.  Security Considerations

   The security implications for ALTC are effectively the same as they
   are for SDP in general [RFC4566].


7.  Acknowledgements

   Many thanks to T. Taylor, F. Andreasen and G. Camarillo for their
   review and comments.


8.  References






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8.1.  Normative References

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

   [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.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              October 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

8.2.  Informative References

   [I-D.boucadair-pcp-nat64-experiments]
              Abdesselam, M., Boucadair, M., Hasnaoui, A., and J.
              Queiroz, "PCP NAT64 Experiments",
              draft-boucadair-pcp-nat64-experiments-00 (work in
              progress), September 2012.

   [I-D.ietf-behave-lsn-requirements]
              Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common requirements for Carrier Grade NATs
              (CGNs)", draft-ietf-behave-lsn-requirements-10 (work in
              progress), December 2012.

   [I-D.ietf-mboned-64-multicast-address-format]
              Boucadair, M., Qin, J., Lee, Y., Venaas, S., Li, X., and
              M. Xu, "IPv6 Multicast Address With Embedded IPv4
              Multicast Address",
              draft-ietf-mboned-64-multicast-address-format-04 (work in
              progress), August 2012.

   [I-D.ietf-mboned-multrans-addr-acquisition]
              Tsou, T., Clauberg, A., Boucadair, M., Venaas, S., and Q.
              Sun, "Address Acquisition For Multicast Content When
              Source and Receiver Support Differing IP Versions",



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              draft-ietf-mboned-multrans-addr-acquisition-01 (work in
              progress), January 2013.

   [I-D.ietf-mboned-v4v6-mcast-ps]
              Jacquenet, C., Boucadair, M., Lee, Y., Qin, J., Tsou, T.,
              and Q. Sun, "IPv4-IPv6 Multicast: Problem Statement and
              Use Cases", draft-ietf-mboned-v4v6-mcast-ps-01 (work in
              progress), November 2012.

   [I-D.venaas-behave-v4v6mc-framework]
              Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
              Multicast Translation",
              draft-venaas-behave-v4v6mc-framework-03 (work in
              progress), June 2011.

   [RFC2871]  Rosenberg, J. and H. Schulzrinne, "A Framework for
              Telephony Routing over IP", RFC 2871, June 2000.

   [RFC4091]  Camarillo, G. and J. Rosenberg, "The Alternative Network
              Address Types (ANAT) Semantics for the Session Description
              Protocol (SDP) Grouping Framework", RFC 4091, June 2005.

   [RFC4092]  Camarillo, G. and J. Rosenberg, "Usage of the Session
              Description Protocol (SDP) Alternative Network Address
              Types (ANAT) Semantics in the Session Initiation Protocol
              (SIP)", RFC 4092, June 2005.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5853]  Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
              A., and M. Bhatia, "Requirements from Session Initiation
              Protocol (SIP) Session Border Control (SBC) Deployments",
              RFC 5853, April 2010.

   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", RFC 5939, September 2010.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

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




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   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

   [RFC6406]  Malas, D. and J. Livingood, "Session PEERing for
              Multimedia INTerconnect (SPEERMINT) Architecture",
              RFC 6406, November 2011.


Appendix A.  ALTC Use Cases

A.1.  Terminology

   The following terms are used:
   o  SBE (Signaling Path Border Element) denotes a functional element,
      located at the boundaries of an ITAD (IP Telephony Administrative
      Domain, [RFC2871]), which is responsible for intercepting
      signaling flows received from User Agents and relay them to the
      core service platform.  A SBE may be located at the access segment
      (i.e., be the service contact point for User Agents) or be located
      at the interconnection with adjacent domains ([RFC6406]).  A SBE
      controls one or more DBEs.  SBE and DBE may be located in the same
      device (e.g., SBC [RFC5853]) or be separated.
   o  DBE (Data Path Border Element) denotes a functional element,
      located at the boundaries of an ITAD, which is responsible for
      intercepting media/data flows received from User Agents and relay
      them to another DBE (or media servers, e.g., announcement server
      or IVR).  An example of DBE is a media gateway intercepting RTP
      flows.  SBE may be located at the access segment (i.e., be the
      service contact point for User Agents) or be located at the
      interconnection with adjacent domains ([RFC6406]).
   o  Core service platform is a macro functional block including
      session routing, interfaces to advanced services and access
      control.  Figure 2 provides an overview of the overall
      architecture including SBE, DBE and Core service platform.
















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                                   +----------+
                                   | Core SIP |
                        +--------->|    SPF   |<---------+
                        |  SIP     +----------+     SIP  |
                        v                                v
                  +-----------+                   +-----------+
    +-----+  SIP  |    SBE    |                   |    SBE    |  SIP
    |  S  |<----->|           |                   |           |<----->
    |  I  |       +-----------+                   +-----------+
    |  P  |              ||                              ||
    |     |       +-----------+                   +-----------+
    |  U  |  RTP  |    DBE    |       RTP         |    DBE    |   RTP
    |  A  |<----->|           |<----------------->|           | <----->
    +-----+       +-----------+                   +-----------+

    SIP UA can be embedded in the CPE or in a host behind the CPE


                 Figure 2: Service Architecture: Overview

A.2.  Multicast Use Case

   Recently, a significant effort has been undertaken within IETF to
   specify new mechanisms to interconnect IPv6-only hosts to IPv4-only
   servers (e.g., [RFC6146]).  This effort covered exclusively unicast
   transfer mode.  An ongoing initiative, called multrans, has been
   launched to cover multicast issues to be encountered during IPv6
   transition.  The overall problem statement is documented in
   [I-D.ietf-mboned-v4v6-mcast-ps].

   A particular issue encountered in the context of IPv4/IPv6 co-
   existence and IPv6 transition of multicast services is the discovery
   of multicast group and source (refer to Section 3.4 of
   [I-D.ietf-mboned-v4v6-mcast-ps]):

   1.  An IPv6-only receiver requesting multicast content generated by
       an IPv4-only source:
       (1.1)  An ALG is required to help an IPv6 receiver to select the
              appropriate IP address when only the IPv4 address is
              advertised (e.g., using SDP); otherwise the access to the
              IPv4 multicast content can not be offered to the IPv6
              receiver.  The ALG may be located downstream the receiver.
              As such, the ALG does not know in advance whether the
              receiver is dual-stack or IPv6-only.  The ALG may be tuned
              to insert both the original IPv4 address and corresponding
              IPv6 multicast address using for instance the ALTC SDP
              attribute.




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       (1.2)  In order to avoid involving an ALG in the path, an IPv4-
              only source can advertise both its IPv4 address and IPv4-
              embedded IPv6 multicast address
              [I-D.ietf-mboned-64-multicast-address-format] using for
              instance the ALTC SDP attribute.
   2.  A dual-stack source sending its multicast content over IPv4 and
       IPv6: both IPv4 and IPv6 addresses need to be inserted in the SDP
       part.  A means (e.g, ALTC) is needed for this purpose.

A.3.  Introducing IPv6 into SIP-based Architectures

A.3.1.  Avoid Crossing CGN Devices

   Some service providers are in the process of enabling DS-Lite
   [RFC6333] as a means to continue delivering IPv4 services to their
   customers.  To avoiding crossing four levels of NAT when placing a
   media session (2 NAT in DS-Lite AFTR + 2 NAT in the DBE), it is
   recommended to enable IPv6 functions in some SBEs/DBEs.  Therefore
   DS-Lite AFTRs won't be crossed for DS-Lite serviced customers if
   their UA is IPv6-enabled:

   o  For SIP UA embedded in the CPE, this is easy to implement since
      the SIP UA [RFC3261] can be tuned to behave as IPv6-only UA when
      DS-Lite is enabled.  No ALTC is required for that use case.
   o  But for SIP User Agents located behind the CPE, a solution to
      indicate both IPv4 and IPv6 (e.g., ALTC) is required in order to
      avoid crossing the DS-Lite CGN.

A.3.2.  Basic Scenario for IPv6 SIP Service Delivery

   A basic solution to deliver SIP-based services using IPv4-only core
   service platform to IPv6-enabled UA is to enabled IPv4/IPv6
   interworking function in SBE/DBE.  Signaling and media between two
   SBEs and DBEs is maintained over IPv4.  IPv6 is used between an IPv6-
   enabled UA and a SBE/DBE.

   Figure 3 shows the results of session establishment between UAs.  In
   this scenario, IPv4/IPv6 interworking function is invoked even when
   both involved UAs are IPv6-enabled.












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                                 +----------+
                                 | Core SIP |
                            +--->|SPF (IPv4)|<---+
                   IPv4 SIP |    +----------+    |IPv4 SIP
                            v                    v
                      +-----------+        +-----------+
                      |    SBE    |        |    SBE    |  SIP
             +------->|IPv4/v6 IWF|        |           |<-------+
             |IPv6    +-----------+        +-----------+    IPv4|
             | SIP                                           SIP|
      +----+ |        +-----------+        +-----------+        | +----+
      |IPv6|-+IPv6 RTP|    DBE    |IPv4 RTP|    DBE    |IPv4 RTP+-|IPv4|
      | UA |<-------->|IPv4/v6 IWF|<------>|           |<-------->| UA |
      +----+          +-----------+        +-----------+          +----+


                                 +----------+
                                 | Core SIP |
                            +--->|SPF (IPv4)|<---+
                   IPv4 SIP |    +----------+    |IPv4 SIP
                            v                    v
                      +-----------+        +-----------+
                      |    SBE    |        |    SBE    |  SIP
             +------->|IPv4/v6 IWF|        |IPv4/v6 IWF|<-------+
             |IPv6    +-----------+        +-----------+    IPv6|
             | SIP                                           SIP|
      +----+ |        +-----------+        +-----------+        | +----+
      |IPv6|-+IPv6 RTP|    DBE    |IPv4 RTP|    DBE    |IPv6 RTP+-|IPv6|
      | UA |<-------->|IPv4/v6 IWF|<------>|IPv4/v6 IWF|<-------->| UA |
      +----+          +-----------+        +-----------+          +----+



                         Figure 3: Basic scenario

   Solutions to avoid redundant IPv4/IPv6 NAT and involving several DBEs
   may be valuable to consider by service providers.

A.3.3.  Avoid IPv4/IPv6 Interworking

   For services providers wanting:
   1.  Means to promote the invocation of IPv6 transfer capabilities can
       be enabled while no parsing error is to be experienced by core
       service nodes legacy nodes
   2.  Optimize cost related to IPv4-IPv6 translation licenses
   3.  Reduce the dual-stack lifetime





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   4.  Maintain an IPv4-only core
   5.  Only a set of SBE/DBE are IPv6-enabled

   A solution to indicate both IPv4 and IPv6 addresses is required.
   This section provides an overview of this procedure:

   When a SBE receives an INVITE, it instantiates in its DBE an IPv6-
   IPv6 context and an IPv6-IPv4 context.  Both an IPv6 address and an
   IPv4 address are returned together with other information such as
   port numbers.  SBE builds an SDP offer including both IPv4 and IPv6-
   related information using ALTC attribute.  IPv6 is indicated as
   preferred connectivity type.

                     o=- 25678 753849 IN IP4 192.0.2.2
                     c=IN IP4 192.0.2.2
                     m=audio 12340 RTP/AVP 0 8
                     a=altc:1 IP6 2001:db8::2 6000
                     a=altc:2 IP4 192.0.2.2 12340

                  Figure 4: SDP offer updated by the SBE

   The request is then forwarded to the core SPF which in its turn
   forwards the requests to the terminating SBE.

   o  If this SBE is a legacy one, then it will ignore ALTC attributes
      and use "c" line.
   o  If the terminating SBE is IPv6-enabled:
      *  If the called UA is IPv4-only, then an IPv6-IPv4 context is
         created in the corresponding DBE.
      *  If the called UA is IPv6-enabled, then an IPv6-IPv6 context is
         created in the corresponding DBE.

   Figure 5 shows the result of the procedure when placing a session
   between an IPv4 and IPv6 UAs while Figure 6 shows the results of
   establishing a session between two IPv6-enabled UAs.  The result is
   still not optima since redundant NAT66 is required (Appendix A.3.4).















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                                 +----------+
                                 | Core SIP |
                            +--->|SPF (IPv4)|<---+
                   IPv4 SIP |    +----------+    |IPv4 SIP
                            v                    v
                      +-----------+        +-----------+
                      |    SBE    |        |    SBE    |  SIP
             +------->|IPv4/v6 IWF|        |IPv4/v6 IWF|<-------+
             |IPv6    +-----------+        +-----------+    IPv4|
             | SIP                                           SIP|
      +----+ |        +-----------+        +-----------+        | +----+
      |IPv6|-+IPv6 RTP|    DBE    |IPv6 RTP|    DBE    |IPv4 RTP+-|IPv4|
      | UA |<-------->|   NAT66   |<------>|IPv4/v6 IWF|<-------->| UA |
      +----+          +-----------+        +-----------+          +----+
                       2001:db8::2

        Figure 5: Session establishement between IPv4 and IPv6 UAs

                                 +----------+
                                 | Core SIP |
                            +--->|SPF (IPv4)|<---+
                   IPv4 SIP |    +----------+    |IPv4 SIP
                            v                    v
                      +-----------+        +-----------+
                      |    SBE    |        |    SBE    |  SIP
             +------->|IPv4/v6 IWF|        |IPv4/v6 IWF|<-------+
             |IPv6    +-----------+        +-----------+    IPv6|
             | SIP                                           SIP|
      +----+ |        +-----------+        +-----------+        | +----+
      |IPv6|-+IPv6 RTP|    DBE    |IPv6 RTP|    DBE    |IPv6 RTP+-|IPv6|
      | UA |<-------->|   NAT66   |<------>|   NAT66   |<-------->| UA |
      +----+          +-----------+        +-----------+          +----+
                       2001:db8::2

             Figure 6: Session establishement between IPv6 UAs

A.3.4.  DBE Bypass Procedure

   For service providers wanting to involve only one DBE in the media
   path, when not all SBE/DBE and UAs are IPv6-enabled, a means to
   indicate both IPv4 and IPv6 addresses without inducing session
   failures is required.  Below is proposed an example of a proposed
   procedure using ALTC attribute.

   When the originating SBE receives an INVITE from an IPv6-enabled UA,
   it instantiates in its DBE an IPv6-IPv6 context and an IPv6-IPv4
   context.  Both an IPv6 address and an IPv4 address are returned
   together with other information such as port numbers.  SBE builds an



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   SDP offer including both IPv4 and IPv6-related information using ALTC
   attribute (Figure 7).  IPv6 is indicated as preferred connectivity
   type.

                     o=- 25678 753849 IN IP4 192.0.2.2
                     c=IN IP4 192.0.2.2
                     m=audio 12340 RTP/AVP 0 8
                     a=altc:1 IP6 2001:db8::2 6000
                     a=altc:2 IP4 192.0.2.2 12340

                  Figure 7: SDP offer updated by the SBE

   The request is then forwarded to the core SPF which in its turn
   forwards the requests to the terminating SBE:
   o  If the destination UA is IPv6 or reachable with a public IPv4
      address, the SBEs only forwards the request without altering the
      SDP offer.  No parsing error is experienced by core service nodes
      since ALTC is backward compatible.
   o  If the terminating SBE does not support ALTC, it will ignore this
      attribute an uses the legacy procedure.

   As a consequence, only one DBE is maintained in the path when one of
   the involved parties is IPv6-enabled.  Figure 8 shows the overall
   procedure when involved UAs are IPv6-enabled.

                                 +----------+
                                 | Core SIP |
                            +--->|SPF (IPv4)|<---+
                   IPv4 SIP |    +----------+    |IPv4 SIP
                            v                    v
                      +-----------+        +-----------+
                      |    SBE    |        |    SBE    |  SIP
             +------->|IPv4/v6 IWF|        |IPv4/v6 IWF|<-------+
             |IPv6    +-----------+        +-----------+    IPv6|
             | SIP                                           SIP|
      +----+ |        +-----------+                             | +----+
      |IPv6|-+IPv6 RTP|    DBE    |          IPv6 RTP           +-|IPv6|
      | UA |<-------->|   NAT66   |<----------------------------->| UA |
      +----+          +-----------+                               +----+
   2001:db8::1        2001:db8::2

                       Figure 8: DBE Bypass Overview

   The main advantages of such solutions are as follows:

   o  DBE resources are optimized





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   o  No redundant NAT is maintained in the path when IPv6-enabled UAs
      are involved
   o  End-to-end delay is optimized
   o  The robustness of the service is optimized since the delivery of
      the service relies on fewer nodes
   o  The signaling path is also optimized since no communication
      between the SBE (through SPDF in TISPAN/IMS context) and DBE at
      the terminating side is required for some sessions.

A.3.5.  Direct Communications Between IPv6-enabled User Agents

   For service providers wanting to allow direct IPv6 communications
   between IPv6-enabled UAs, when not all SBE/DBE and UA are IPv6-
   enabled, a means to indicate both IPv4 and IPv6 addresses without
   inducing session failures is required.  Below is proposed an example
   of a proposed procedure using ALTC attribute.

   At the SBE originating side, when the SBE receives an INVITE from the
   calling IPv6 UA (Figure 9), it uses ALTC to indicate two IP
   addresses:
   1.  An IPv4 address belonging to its controlled DBE
   2.  The same IPv6 address and port as received in the initial offer
       made by the calling IPv6

   Figure 10 shows an excerpt example of the SDP offer generated by the
   originating SBE.

                    o=- 25678 753849 IN IP6 2001:db8::1
                    c=IN IP6 2001:db8::1
                    m=audio 6000 RTP/AVP 0 8

                   Figure 9: SDP offer of the calling UA

                     o=- 25678 753849 IN IP4 192.0.2.2
                     c=IN IP4 192.0.2.2
                     m=audio 12340 RTP/AVP 0 8
                     a=altc:1 IP6 2001:db8::1 6000
                     a=altc:2 IP4 192.0.2.2 12340

                  Figure 10: SDP offer updated by the SBE

   The INVITE message will be routed appropriately to the destination
   SBE:
   1.  If the SBE is a legacy device (i.e., IPv4-only); it will ignore
       IPv6 addresses and contacts its DBE to instantiate an IPv4-IPv4
       context.





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   2.  If the SBE is IPv6-enabled, it will only forwards the INVITE to
       the address of contact of the called party:
       A.  If the called party is IPv6-enabled, the communication will
           be placed using IPv6.  As such no DBE is involved in the data
           path as illustrated in Figure 11.
       B.  If not, IPv4 will be used between the originating DBE and
           called UA.


                                 +----------+
                                 | Core SIP |
                            +--->|SPF (IPv4)|<---+
                   IPv4 SIP |    +----------+    |IPv4 SIP
                            v                    v
                      +-----------+        +-----------+
                      |    SBE    |        |    SBE    |  SIP
             +------->|IPv4/v6 IWF|        |IPv4/v6 IWF|<-------+
             |IPv6    +-----------+        +-----------+    IPv6|
             | SIP                                           SIP|
      +----+ |                                                  | +----+
      |IPv6|-+                         IPv6 RTP                 +-|IPv6|
      | UA |<---------------------------------------------------->| UA |
      +----+                                                      +----+
      2001:db8::1


                   Figure 11: Direct IPv6 communication


Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


   Hadriel Kaplan
   Acme Packet
   71 Third Ave.
   Burlington, MA  01803
   USA

   Email: hkaplan@acmepacket.com





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   Robert R Gilman
   Independent

   Email: bob_gilman@comcast.net
   URI:


   Simo Veikkolainen
   Nokia

   Email: Simo.Veikkolainen@nokia.com
   URI:







































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