An Alternative Connection Model for the Message Session Relay Protocol (MSRP)
RFC 6135

Internet Engineering Task Force (IETF)                       C. Holmberg
Request for Comments: 6135                                      Ericsson
Category: Standards Track                                        S. Blau
ISSN: 2070-1721                                              Ericsson AB
                                                           February 2011

                An Alternative Connection Model for the
                 Message Session Relay Protocol (MSRP)

Abstract

   This document defines an alternative connection model for Message
   Session Relay Protocol (MSRP) User Agents (UAs); this model uses the
   connection-oriented media (COMEDIA) mechanism in order to create the
   MSRP transport connection.  The model allows MSRP UAs behind Network
   Address Translators (NATs) to negotiate which endpoint initiates the
   establishment of the Transmission Control Protocol (TCP) connection,
   in order for MSRP messages to traverse the NAT.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6135.

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   document authors.  All rights reserved.

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Holmberg & Blau              Standards Track                    [Page 1]
RFC 6135                          MSRP                     February 2011

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Applicability Statement .........................................3
   4. COMEDIA for MSRP ................................................3
      4.1. General ....................................................3
      4.2. a=setup ....................................................3
           4.2.1. General .............................................3
           4.2.2. Attribute Usage .....................................4
      4.3. TLS ........................................................5
      4.4. a=connection ...............................................5
      4.5. MSRP Relay Connection ......................................6
   5. Interoperability with the Connection Model Defined in RFC 4975 ..6
   6. Security Considerations .........................................6
   7. Acknowledgements ................................................7
   8. References ......................................................7
      8.1. Normative References .......................................7
      8.2. Informative References .....................................7

1.  Introduction

   The Message Session Relay Protocol (MSRP) core specification
   [RFC4975] states that the MSRP User Agent (UA) that sends the Session
   Description Protocol (SDP) offer is "active", and it is responsible
   for creating the MSRP transport connection towards the remote UA if a
   new connection is required.  The core specification also allows, but
   does not define, alternate mechanisms for MSRP UAs to create MSRP
   transport connections.

   [RFC4145] defines a connection-oriented media (COMEDIA) mechanism,
   which endpoints can use to negotiate the endpoint that initiates the
   creation of media transport connection.

   COMEDIA is especially useful when one of the endpoints is located
   behind a Network Address Translator (NAT).  The endpoint can use the
   mechanism to indicate that it will create the media transport
   connection, in order for the media to traverse the NAT without the
   usage of relays and without being required to support more complex
   mechanisms (e.g., "TCP Candidates with Interactive Connectivity
   Establishment (ICE)" [ICE-TCP]).  In addition, COMEDIA allows the
   usage of identical procedures in establishing Transmission Control