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Network Working Group                                       M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Standards Track                               H. Kaplan
Expires: January 7, 2010                                     Acme Packet
                                                           July 06, 2009


   Session Description Protocol (SDP) Connectivity Capability (CCAP)
                               Attribute
                   draft-boucadair-mmusic-ccap-00.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 7, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This memo proposes a mechanism which allows to carry multiple IP



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   addresses, of different address families (e.g., IPv4, IPv6), in the
   same SDP offer/answer.  The proposed attribute solves the backward
   compatibility problem which plagued ANAT, due to its syntax.

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].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Overall Context  . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview of the CCAP Mechanism . . . . . . . . . . . . . . . .  6
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Connectivity Capability Attribute  . . . . . . . . . . . . . .  7
     4.1.  CCAP Syntax  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Usage and Interaction  . . . . . . . . . . . . . . . . . .  8
       4.2.1.  Usage  . . . . . . . . . . . . . . . . . . . . . . . .  8
       4.2.2.  Interaction with ICE . . . . . . . . . . . . . . . . .  9
       4.2.3.  Interaction with SDP-Cap-Neg . . . . . . . . . . . . . 10
   5.  The CCAP Option Tag  . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  ANAT and ICE  . . . . . . . . . . . . . . . . . . . . 12
     A.1.  ANAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     A.2.  ICE  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14













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1.  Introduction

   [Editorial Note: this section is lengthy/verbose, and is here simply
   to provide some initial background.  This section, as well as
   Appendix-A, will be removed or at least reduced after initial draft
   versions.]

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, SIP 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 ALGs and NAT64, and to
   avoid using private IPv4 addresses through consumer NATs or Carrier-
   Grade NATs (CG-NAT).

   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 CG-NATs.  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; 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 [I-D.ietf-mmusic-ice].
   ANAT has serious backwards-compatibility problems as described in
   [RFC4092], which effectively make it unusable, and it is planned to
   be deprecated by the IETF.  ICE at least allows interoperability with



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   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.  In some
   deployment models, ICE is not usable at all.  Further details of why
   neither model is appropriate are described in Appendix A.

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 (called ccap) follows the model described in
   [I-D.ietf-mmusic-sdp-capability-negotiation] in syntax, but does not
   propose a full implementation of sdp-capabilities-negotiations
   (a.k.a., sdp-cap-neg) to function.  If the full model is implemented,
   the mechanism proposed in this memo works with it as well, but
   orthogonally.  The mechanism is an alternative to ICE, such that both
   mechanisms may be offered, but only one is chosen and used.

   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.  [Note: though this may be considered odd,
   it is technically and practically possible for certain devices to
   know such a thing in real-world deployments]

   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.  Which address
   family is "best" for a particular session is not defineable 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;




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   o  Is backwards compatible.  No parsing or semantic errors will be
      experienced by a legacy UA or intermediary nodes (e.g., Proxy
      Servers, Registrar Servers, etc.) 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.

1.3.  Scope

   This document proposes an alternative scheme, as replacement to the
   ANAT procedure, 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.  SIP-layer address family
   interworking is out of scope of this document (see
   [I-D.boucadair-sipping-ipv6-atypes] for a solution candidate).
   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

   Although the CCAP mechanism defined in this document is meant for
   general use, 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.

   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.





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   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
      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 CCAP Mechanism

3.1.  Overview

   The CCAP mechanism relies solely on the SDP offer/answer mechanism,
   with specific syntax to indicate capabilities.  Following the sdp-
   cap-neg model, the basic concept is to use a new SDP attribute
   "ccap", to indicate the IP addresses for potential alternative
   connection addresses, while using the most likely-to-succeed address
   in the normal 'c=' connection line.  Typically this would be an IPv4
   address, however the new attribute also indicate if another address
   is more preferred.  For example, a dual-stack UA might encode its
   IPv4 in the connection line, while possibly preferring to use an IPv6
   address by indicating such in the attributes (though, it actually
   encodes both addresses in the attributes, for reasons explained
   later).  The SDP answerer would indicate its chosen address, by
   simply using that address family in the SDP connection line of its
   response.

   An example of SDP offer using this mechanism is as follows:

   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=ccap:1 IP6 2001:db8::1 45678
   a=ccap: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 attribute includes both an IP
   address and a receive transport port number.  The CCAP mechanism does
   not itself support offering a different transport type (i.e., UDP vs.



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   TCP), codec, nor any other attribute.  It is only intended for
   offering an alternative IP address and port number.  The syntax of
   the attributes follows sdp-cap-neg and ICE in some regards, but does
   not require support for either of them.  Other mechanisms, such as
   sdp-cap-neg, may be used at the same time to offer other alternative
   semantics, but they are orthogonal to the address and port
   alternatives in this memo.

3.2.  Rationale

   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.  An SDP parser will ignore any session
   description that contains attribute lines it does not support.
   [Note: of course some devices in the wild may not ignore unknown
   attributes, but then it is not compliant with SDP rules, and nothing
   will help it]

   The rationale for encoding the same address/port as in the media and
   connection 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 ccap and connection
   line addresses and media ports, before choosing one of the
   alternatives.


4.  Connectivity Capability Attribute

4.1.  CCAP Syntax

   The ccap attribute adheres to the RFC 4566 "attribute" production.
   The ABNF syntax of ccap is provided below:

      ccap-attr     = "ccap" ":" att-value
      att-value     = addr-cap-num SP addrtype SP connection-address SP port ;defined in [RFC4566]
      addr-cap-num  = 1*DIGIT ;defined in [RFC5234]

             Figure 1: Connectivity Capability Attribute ABNF

   Note that white space is not permitted before the addr-cap-num.

   The meaning of the fields are listed hereafter:






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   o  addr-cap-num: digit to uniquely refer to an address alternative.
      It must be in preference order (1=most-preferred).

   o  addrtype: the addrtype field as defined in [RFC4566] for
      connection data.

   o  connection-address: an IPv4 or IPv6 address as defined in
      [RFC4566].

   o  port: the port number to be used, as defined in [RFC4566].
      Distinct port numbers may be used per IP address type.

   The ccap attribute is only applicable in an SDP offer.  The ccap
   attribute 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 ccap attribute per IP Address family, per media
   level.  Each and every media level MUST contain exactly two ccap
   attributes: one for one address family, and a second for the other.

   This document's mechanism requires a "duplicate" ccap attribute to be
   included, with the same address/port information as in the RFC 4566
   base SDP 'c=' connection and 'm=' media lines.  Each media level MUST
   contain at least one such duplicate ccap attribute, of the same IP
   address family, address, and transport port number as those in the
   SDP connection and media lines of its level.

   If a 'c=' connection line appears at the media level, the same
   address as that 'c=' line MUST be used in the duplicate ccap
   attribute for that media level.

   If a 'c=' connection line appears only at the session level and a
   given media line does not have its own connection line, then the
   duplicate ccap attribute for that media line MUST be the same as the
   session-level address information.

   When several ccap lines are present, multiple sessions establishment
   MUST be avoided.  Only one session is to be maintained with remote
   party.

4.2.  Usage and Interaction

4.2.1.  Usage

   In an SDP offer/answer model, the SDP offer would include ccap
   attributes to indicate alternative connection information (i.e.,
   address family, address and port number), as well as the "duplicate"
   connection information already identified in the 'c=' connection and
   'm=' media lines.  The SDP answer MUST NOT contain ccap attributes,



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   as the answer's 'c=' line implicitly and definitively "chooses" the
   address family from the offer.

   Additional, subsequent offers MAY include ccap attributes again, and
   change the IP address, ports, and order of preference; but they MUST
   include a duplicate ccap attribute of the connection and media lines
   in that specific subsequent offer.  In other words, every SDP offer
   with a ccap attribute has two of them:

      - one duplicating the 'c=' and 'm=' line information in that SDP
      offer, and

      - one for the alternative, even though both of those need not be
      the same as the original SDP offer.

   The purpose of encoding a "duplicate" ccap 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 ccap attribute value
   for which the address and port match exactly those in the 'c=' line
   and 'm=' line, the SDP answerer MUST ignore the ccap attributes and
   use the 'c=' and 'm=' offered address/ports for the entire SDP
   instead, as if no ccap attributes were present.  The rationale for
   this is that many SDP-changing middleboxes will end the SIP session
   if they do not detect media flowing through them; if a middlebox
   modified the SDP, 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 ccap 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.

4.2.2.  Interaction with ICE

   Since ICE 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 CCAP
   and ICE, it may include both sets of attributes in the same SDP
   offer.  A legacy ICE-only answerer will simply ignore the CCAP
   attributes, and use ICE.  A CCAP-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
   CCAP (unless the 'm=' or 'c=' lines were changed by a middlebox, in
   which case the rules for both CCAP and ICE would make the answerer
   revert to basic SDP semantics).




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4.2.3.  Interaction with SDP-Cap-Neg

   The CCAP mechanism is orthogonal to sdp-cap-neg.  If the offerer
   supports both ccap and sdp-cap-neg, it may offer both.  At this time,
   sdp-cap-neg does not provide a means of offering alternative
   addresses/ports, other than through ICE, for which the behavior was
   described previously.  Therefore, there is no conflicting
   interaction.  CCAP capabilities are not negotiated as part of the
   potential and actual configuration attribute syntax and semantics
   defined in [I-D.ietf-mmusic-sdp-capability-negotiation].

   [Note: it was tempting to in fact make CCAP be yet another set of
   alternative capabilities in an sdp-cap-neg, but the complexities of
   sdp-cap-neg, and the subtleties of potentially tying address/port
   options with media capabilities do not seem to be worth the effort
   for this case]


5.  The CCAP Option Tag

   This document defines a new SIP option-tag for use in the "Supported"
   SIP header field called "ccap".  This option-tag is for the purpose
   of indicating that a UA supports the CCAP mechanism defined in this
   document AND actually has multiple address family addresses
   available, in order to improve troubleshooting, and in some cases
   provide a hint to other nodes that the UA is capable of both IPv4 and
   IPv6 and CCAP.

   A UA MUST NOT include this option tag unless it both (1) supports the
   CCAP mechanism AND (2) has *both* an IPv4 and IPv6 address available
   for media use.  The reason it only includes the ccap option-tag if it
   actually has both addresses, is that having only a single address
   family available implies the UA cannot truly perform CCAP in an
   offer; it may have the necessary logic to, but it does not have the
   addresses to do so. (remember one does not include the ccap attribute
   in SDP unless one has both address families available)

   A UA SHOULD include the CCAP option-tag in a "Supported" SIP header
   field in SIP REGISTER, OPTIONS, and INVITE requests and related
   responses, if it has both address-family addresses available and
   supports the CCAP mechanism.  A UA MUST NOT include the CCAP option-
   tag in the "Require" or "Proxy-Require" SIP header fields under any
   conditions.


6.  IANA Considerations

   If this document moves forward, it requests a new SDP attribute name



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   "ccap", as defined earlier; and a new SIP option-tag be reserved,
   named "ccap", for the purposes described earlier.


7.  Security Considerations

   The security implications for CCAP are effectively the same as they
   are for SDP in general.


8.  Acknowledgements

   TBC


9.  References

9.1.  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.

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.

   [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.

   [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.






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9.2.  Informative References

   [I-D.boucadair-sipping-ipv6-atypes]
              Boucadair, M., Noisette, Y., and A. Allen, "The atypes
              media feature tag for Session Initiation Protocol (SIP)",
              draft-boucadair-sipping-ipv6-atypes-01 (work in progress),
              March 2009.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [I-D.ietf-mmusic-sdp-capability-negotiation]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-10 (work in
              progress), May 2009.


Appendix A.  ANAT and ICE

A.1.  ANAT

   [RFC4091] describes a mechanism allowing multiple alternative network
   addresses to be enclosed in a single SDP offer/answer.  This proposal
   consists at introducing a new attribute called ANAT (Alternative
   Network Address Types).  ANAT is based on media grouping [RFC3388].
   ANAT specification lists IP address as an example of Network Address
   (without providing other examples).

   This attribute allows inserting multiple media/connection lines in
   the same SDP offer (or SDP answer).  [RFC4092] defines how SIP can
   exploit the ANAT semantic by introducing a new option tag called
   "sdp-anat".  This tag can be useful for SIP UAs to be aware of the
   capabilities of each other and then select from the supported media/
   network description lines the ones that are suitable for setting up
   the SIP communication (and also according to local preferences).  A
   use case for illustrating the usage of this tag is a Dual Stack SIP
   UA which can communicate either using its IPv6 or its IPv4
   connectivity.  This type of SIP UAs can also set a preference
   associated with each type of enclosed connectivity type.

   [RFC4092] states that answerers without support for ANAT will react
   in different ways upon receipt of an offer using ANAT and different
   implementations will behave in different ways.  This issue is a real
   problem in current operational SIP-based service offerings.  Indeed,
   in order to support IPv6 is SIP-based architectures, several



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   scenarios may be envisaged.  The most pragmatic one is to update the
   access segment to support IPv6.  The support of ANAT is such
   situations would encounter backward compatibility issues since core
   service nodes are not ANAT-compliant.  This limitation may be a
   hurdle for the use of IPv6 and particularly to activate policies to
   encourage the usage of IPv6 and to guarantee successful
   communications involving heterogeneous (i.e.  IPv4 and IPv6) parties.

   Unfortunately, even with the sdp-anat option tag addition, ANAT is
   not truly usable in modern SIP usage.  In most SIP usage today, the
   SIP UAC generates the SDP offer in its initial INVITE request.  Since
   it does not know the capabilities of the ultimate far-end UAS, it
   cannot include ANAT syntax in its SDP due to the backwards-
   compatibility problem.  Inserting an sdp-anat option tag in the
   Require header would lead to numerous failed INVITE attempts.
   Inserting it in the Supported header would only allow it to re-
   negotiate SDP using ANAT afterwards, which would lead to failed
   initial INVITE requests if it chose to offer an address family
   initially that the far-end could not support .  Neither case is
   attractive, and in particular failed INVITE attempts are highly
   undesirable if not outright unacceptable, leaving the UAC with no
   choice but to either send an offer-less INVITE, or simply assume
   IPv4.  Assuming IPv4 does not solve the address transition problem,
   as it would require all devices to continue to use IPv4 indefinitely,
   and offer-less INVITEs have well-known interoperability problems in
   practice.

   Note that ANAT specification is to be deprecated by ICE
   [I-D.ietf-mmusic-ice].

A.2.  ICE

   ICE solves the IPv4-v6 SDP offer problem by having the UAC offer both
   addresses as alternative candidates in the SDP offer.  If the far-end
   UAS supports ICE, it can choose among them; else it will simply use
   the one offered in the normal SDP connection line.  If ICE is
   supported, STUN connectivity checks are performed, in a controlled
   fashion, along with an additional round of SDP negotiation for the
   final chosen connection path.

   This solves the problem of backwards-compatibility, but at a heavy
   price: both sides must implement ICE.  While ICE provides other
   benefits, specifically basic NAT traversal without the aid of
   middleboxes other than TURN servers, it is complicated and difficult
   to troubleshoot.  It is a very high bar to place on SIP UAs, just to
   achieve IP address family negotiation.  What's needed is as simple a
   mechanism as possible to achieve the goals, in order to provide
   reasonable chance of widespread adoption and deployment.



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   Furthermore, ICE does not work in some scenarios.  In particular, it
   does not work when the address(es) are determined based on where the
   SIP session "goes".  ICE assumes there may be many layers of NAT, but
   that they all cascade from a private to public side, towards the
   public Internet, and assume that both SIP UAs (ICE clients) can
   obtain addresses in the public Internet (e.g., through TURN servers),
   which can be used as a last resort point of media packet rendezvous.
   Such is not always the case.  For example, in common SIP Provider
   peering arrangements, a SIP UAC is in a private network of one
   Provider and the UAS in a private network of the other Provider, and
   media communication does not and cannot cross the public Internet.
   Multi-hop transit peering cases exacerbate this issue even further.
   The only devices with knowledge of the correct addresses to use in
   such scenarios are middleboxes, and they do not know the addresses
   until the SIP session is initially signaled (and in fact the
   addresses may change during the session's lifetime).

   For the sole purpose of negotiating IP address families, therefore,
   ICE is neither necessary nor sufficient.


Authors' Addresses

   Mohamed Boucadair
   France Telecom
   3, Av Francois Chateau
   Rennes  35000
   France

   Email: mohamed.boucadair@orange-ftgroup.com


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

   Email: hkaplan@acmepacket.com












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