





Network Working Group                                       G. Camarillo
Request for Comments: 4092                                      Ericsson
Category: Standards Track                                   J. Rosenberg
                                                           Cisco Systems
                                                               June 2005


            Usage of the Session Description Protocol (SDP)
          Alternative Network Address Types (ANAT) Semantics
                in the Session Initiation Protocol (SIP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes how to use the Alternative Network Address
   Types (ANAT) semantics of the Session Description Protocol (SDP)
   grouping framework in SIP.  In particular, we define the sdp-anat SIP
   option-tag.  This SIP option-tag ensures that SDP session
   descriptions that use ANAT are only handled by SIP entities with ANAT
   support.  To justify the need for such a SIP option-tag, we describe
   what could possibly happen if an ANAT-unaware SIP entity tried to
   handle media lines grouped with ANAT.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  The sdp-anat Option-Tag . . . . . . . . . . . . . . . . . . . . 2
   4.  Backward Compatibility . . . . . . . . . . . . . . . . . . . .  3
       4.1.  Answerer Supports All the Network Types Offered  . . . .  3
       4.2.  Answerer Does Not Support All the Network Types Offered.  3
       4.3.  OPTIONS Requests . . . . . . . . . . . . . . . . . . . .  4
   5.  Option-Tag Usage . . . . . . . . . . . . . . . . . . . . . . .  4
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  4
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  4
   8.  Normative References . . . . . . . . . . . . . . . . . . . . .  5




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1.  Introduction

   SIP [3] UAs (User Agents) often support different network address
   types.  For example, a UA may have an IPv6 address and an IPv4
   address.  Such a UA will typically be willing to use any of its
   addresses to establish a media session with a remote UA.  If the
   remote UA only supports IPv6, for instance, both UAs will use IPv6 to
   send and receive media.

   The Alternative Network Address Types (ANAT) semantics [7] of the SDP
   [2] grouping framework [5] allow UAs to offer [4] alternative
   addresses of different types in an SDP session description.  The
   IPv4/IPv6 dual-stack SIP UA of our previous example would generate an
   offer grouping an IPv6 media line and an IPv4 media line using ANAT.
   Upon receipt of this offer, the answerer [4] would accept one media
   line and reject the other.

   If the recipient of an offer that uses ANAT supports the ANAT
   semantics, everything works as described in the ANAT specification
   [7].  Nevertheless, the recipient of such an offer (i.e., the
   answerer) may not support ANAT.  In this case, different
   implementations of the answerer would react in different ways.  This
   document discusses the answerer's behaviors that are most likely to
   be found and describes their consequences.  To avoid these
   consequences, we define the sdp-anat SIP option-tag.

   The sdp-anat option-tag can be used to ensure that an offer using
   ANAT is not processed by answerers without support for ANAT.  This
   option-tag can also be used to explicitly discover the capabilities
   of a UA (i.e., whether it supports ANAT).

2.  Terminology

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

3.  The sdp-anat Option-Tag

   We define the option-tag sdp-anat for use in the Require and
   Supported SIP [3] header fields.  SIP user agents that place this
   option-tag in a Supported header field understand the ANAT semantics
   as defined in [7].






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4.  Backward Compatibility

   Answerers without support for ANAT will react in different ways upon
   receipt of an offer using ANAT.  We expect that, even under the same
   circumstances, different implementations will behave in different
   ways.  In this section, we analyze these behaviors (i.e., the
   following subsections assume that the answerer does not support
   ANAT).

4.1.  Answerer Supports All the Network Types Offered

   If the answerer supports all the network types in the offer, it may
   accept the offer and establish all the media streams in it.  This
   behavior is not what the offerer expects because it results in too
   many media streams being established.  If the answerer starts sending
   media over all of them, the result may be a high bandwidth usage.

   The answerer may also reject the offer, because although it supports
   all the network types in it, the answerer may not support them
   simultaneously.  The error response sent by the answerer will most
   likely not be explicit enough about the situation.  So, the offerer
   will not understand what went wrong.

   In the previous scenarios, the sdp-anat option-tag would avoid the
   establishment of too many media streams and would allow the answerer
   to explicitly inform the offerer that the answerer did not support
   ANAT.

4.2.  Answerer Does Not Support All the Network Types Offered

   If the answerer does not support all the network types in the offer,
   it may only establish the media streams whose address types it
   understands and reject the rest.  This would be an acceptable
   behavior from the offerer's point of view.

   On the other hand, the answerer may also reject the offer because it
   contains unknown address types.  The error response sent by the
   answerer will most likely not be explicit enough about the situation.
   So, the offerer will not understand what went wrong.

   In the previous scenario, the sdp-anat option-tag would allow the
   answerer to explicitly inform the offerer that the answerer did not
   support ANAT.








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4.3.  OPTIONS Requests

   Although RFC 3388 [5] provides servers with a means to indicate
   support for ANAT in an SDP description, many servers do not include
   an SDP description in their responses to OPTIONS requests.  The
   sdp-anat option-tag makes it possible to discover if any server
   supports ANAT, since they would include this option-tag in a
   Supported header field in their responses.

5.  Option-Tag Usage

   As discussed in the previous section, the use of the sdp-anat
   option-tag makes SIP messages more explicit about ANAT support.  So,
   SIP entities generating an offer that uses the ANAT semantics SHOULD
   place the sdp-anat option-tag in a Require header field.  SIP
   entities that support the ANAT semantics MUST understand the sdp-anat
   option-tag.

6.  Security Considerations

   An attacker may attempt to add the sdp-anat option tag to the Require
   header field of a message to perform a DoS attack.  If the UAS does
   not support ANAT, it will return an error response instead of
   processing the message.

   An attacker may attempt to remove the sdp-anat option-tag from the
   Require header field of a message.  This may result in the
   establishment of too many media streams.

   To avoid the previous attacks, integrity protection of the Require
   header field is RECOMMENDED.  The natural choice to integrity protect
   header fields in SIP is S/MIME [6].

7.  IANA Considerations

   This document defines a SIP option-tag (sdp-anat) in Section 3.  It
   has been registered by the IANA in the SIP parameter registry.

   SIP user agents that place the sdp-anat option-tag in a Supported
   header field understand the ANAT semantics.











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8.  Normative References

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

   [2]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [3]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

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

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

   [6]  Peterson, J., "S/MIME Advanced Encryption Standard (AES)
        Requirement for the Session Initiation Protocol (SIP)", RFC
        3853, July 2004.

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

Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com


   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   EMail: jdrosen@cisco.com






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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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