Network Working Group                                       M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Standards Track                               H. Kaplan
Expires: September 8, 2011                                   Acme Packet
                                                               R. Gilman
                                                         S. Veikkolainen
                                                           March 7, 2011

    Session Description Protocol (SDP) Alternate Connectivity (ALTC)


   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, due to its syntax.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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
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   This Internet-Draft will expire on September 8, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

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   This document may contain material from IETF Documents or IETF
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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 . . . . . . . . . . . . . 11
   5.  The ALTC Option Tag  . . . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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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)
   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
   [I-D.ietf-behave-v6v4-xlate-stateful]), and to avoid using private
   IPv4 addresses through consumer NATs or Carrier Grade NATs (CGN).

   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; 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
   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

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   deployment models, ICE is not usable at all.

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
   [I-D.ietf-mmusic-sdp-capability-negotiation] and does not require
   implementation of sdp-capabilities-negotiations (a.k.a., sdp-cap-neg)
   to function.  When sdp-cap-neg is supported, the CCAP attribute
   defined in [I-D.garcia-mmusic-sdp-misc-cap] should be 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.

   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 nodes (e.g., Proxy
      Servers, Registrar Servers, etc.) which do not understand this new

   o  Is as lightweight as possible to achieve the goal, while still
      allowing and interoperating with nodes which support other similar
      or related mechanisms.

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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 ALTC 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 UA behind a Dual-Stack Lite CGN

   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
      as possible.

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   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, in preference
   order, 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 indicating this by the
   "a=altc" attribute line ordering.  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:

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

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

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   o=- 25678 753849 IN IP6 2001:db8::1
   c=IN IP6 2001:db8::1
   t=0 0
   m=audio 12340 RTP/AVP 0 8
   a=altc IP6 2001:db8::1 45678
   a=altc IP4 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 any
   session description that contains attribute lines it does not

   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 RFC 4566 "attribute" production.
   The ABNF syntax of altc is provided below:

      altc-attr     = "altc" att-value
      att-value     = addrtype SP connection-address SP port ["/" integer] ;defined in [RFC4566]

             Figure 1: Connectivity Capability Attribute ABNF

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   The meaning of the fields are listed hereafter:

   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.

   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 at least two "altc" attributes: the
   alternative address and 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 in order of appearance, with the first altc given highest
   priority and the following altc attributes prioritized in decending
   order.  Given this rule, and the requirement that the address
   information 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

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   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.

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 at least two of them:

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

      - one for each of the alternatives,

   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
   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

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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 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 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 sdp-cap-neg.  If the offerer
   supports both ALTC and sdp-cap-neg, it may offer both.

   A method based on sdp-cap-neg is described in
   [I-D.garcia-mmusic-sdp-misc-cap] and may be used to specify different
   connectivity for alternative configurations.

5.  The ALTC Option Tag

   This document defines a new SIP option-tag for use in the "Supported"
   SIP header field called "altc".  This option-tag is for the purpose
   of indicating that a UA supports the ALTC 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 ALTC.

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   A UA MUST NOT include this option tag unless it both (1) supports the
   ALTC mechanism AND (2) has both an IPv4 and IPv6 address available
   for media use.  The reason it only includes the "altc" option-tag if
   it actually has both addresses, is that having only a single address
   family available implies the UA cannot truly perform ALTC 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 "altc"
   attribute in SDP unless one has both address families available)

   A UA SHOULD include the ALTC 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 ALTC mechanism.  A UA MUST NOT include the ALTC option-
   tag in the "Require" or "Proxy-Require" SIP header fields under any

6.  IANA Considerations

   If this document moves forward, it requests a new SDP attribute name
   "altc", as defined earlier; and a new SIP option-tag be reserved,
   named "altc", for the purposes described earlier.

7.  Security Considerations

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

8.  References

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.

   [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

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              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.

8.2.  Informative References

              Boucadair, M., Noisette, Y., and A. Allen, "The atypes
              media feature tag for Session Initiation Protocol (SIP)",
              draft-boucadair-sipping-ipv6-atypes-02 (work in progress),
              July 2009.

              Garcia, M., Veikkolainen, S., and R. Gilman,
              "Miscellaneous Capabilities Negotiation in the Session
              Description Protocol (SDP)",
              draft-garcia-mmusic-sdp-misc-cap-01 (work in progress),
              July 2009.

              Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers",
              draft-ietf-behave-v6v4-xlate-stateful-12 (work in
              progress), July 2010.

              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.

              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-13 (work in
              progress), March 2010.

              Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-

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              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", draft-ietf-softwire-dual-stack-lite-07 (work
              in progress), March 2011.

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   3, Av Francois Chateau
   Rennes  35000


   Hadriel Kaplan
   Acme Packet
   71 Third Ave.
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