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.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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
   Protocol (TCP) [RFC0793] connections for different types of media.

   An example is the Open Mobile Alliance (OMA)-defined "Instant Message
   using SIMPLE" [OMA-SIMPLE], where one MSRP UA of every MSRP transport
   connection represents a media server, which is always located in the
   carrier network.  The media server has a globally reachable IP

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   address and handles application-specific policy control as well as
   NAT traversal.  The OMA IM (Instant Messenger) uses COMEDIA for NAT
   traversal, and all OMA IM MSRP clients support COMEDIA.

   This document defines how an MSRP UA uses COMEDIA in order to
   negotiate which UA will create the MSRP transport TCP connection
   towards the other UA.  The document also defines how an MSRP UA that
   uses COMEDIA can establish an MSRP transport connection with a remote
   UA that does not support COMEDIA.

2.  Terminology

   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].

3.  Applicability Statement

   Support of this specification is OPTIONAL for MSRP UAs in general.
   UAs that are likely to be deployed in networks with NATs SHOULD
   support this specification.  It will improve the odds of being able
   to make TCP connections successfully traverse NATs, since UAs behind
   NATs can be requested to initiate the establishment of the TCP
   connections.

4.  COMEDIA for MSRP

4.1.  General

   This section defines how an MSRP UA that supports this specification
   uses the COMEDIA SDP attributes defined in [RFC4145].

4.2.  a=setup

4.2.1.  General

   An MSRP UA uses the SDP a=setup attribute [RFC4145] in order to
   negotiate which endpoint will create the MSRP transport connection
   towards the other UA.

   An MSRP UA MUST always include an explicit a=setup attribute in its
   SDP offers and answers, since it might be useful for the other
   endpoint, or for entities in the network, to know whether the UA
   supports COMEDIA or not.

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   An MSRP UA MUST support the a=setup "active", "actpass", and
   "passive" attribute values.  An MSRP UA MUST NOT send the "holdconn"
   attribute value.  If an MSRP UA receives the "holdconn" attribute
   value, it MUST ignore it and process the message as if it did not
   contain an a=setup attribute.

4.2.2.  Attribute Usage

   When the a=setup attribute value is "actpass" or "passive", the IP
   address and port values in the MSRP URI of the SDP a=path attribute
   MUST contain the actual address and port values on which the UA can
   receive a TCP connection for the MSRP transport connection.

   In accordance with [RFC4145], if the a=setup attribute value is
   "active", the port number value should be 9.

   If an MSRP UA can provide a globally reachable IP address that the
   other endpoint can use as a destination for a TCP connection, the UA
   MUST use the a=setup "actpass" attribute value in SDP offers.  This
   is in order to allow the remote UA to send an SDP answer with an
   a=setup "active" attribute value if the UA is located behind a NAT,
   and in order to be compatible with UAs that do not support COMEDIA
   and thus always will act as passive endpoints.  If an MSRP UA cannot
   provide the actual transport address, the UA MUST use the a=setup
   "active" attribute value.

   The UA MUST NOT use the a=setup "passive" attribute value in an SDP
   offer.

   The MSRP UA can determine that it provides a globally reachable IP
   address in the following scenarios:

   o  the UA can determine that it is not located behind a NAT;

   o  the UA relays its MSRP transport connections via a relay (e.g., an
      MSRP relay or Traversal Using Relay NAT (TURN) server); or

   o  the UA has used Session Traversal Utilities for NAT (STUN)
      [RFC5389] signaling to retrieve the NAT address and port through
      the local port to be used for the eventual transport connection,
      while also having determined that the NAT has endpoint-independent
      mapping and filtering behavior [RFC5382], e.g., using the
      mechanism defined in [RFC5780].

   Some UAs can determine whether the SIP [RFC3261] signaling has
   traversed a NAT by inspecting the SIP Via header field in the 200
   (OK) response to the initial SIP REGISTER request, and comparing the
   IP addresses in the Via sent-by and the received header field

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   parameters.  If the IP addresses are not the same, then the UA can
   determine that there is a NAT in the path.  Even though the media
   transport might not traverse the NAT, it is safe to assume that it
   will.  This comparing mechanism does not work in all scenarios,
   though.  For example, if SIP a request crosses a SIP proxy before
   crossing a NAT, the UA will not be able to detect the NAT by
   comparing the IP addresses.

   If an SDP offer includes an a=setup "actpass" attribute value, the
   SDP answerer MAY include an a=setup "active" attribute value in the
   SDP answer, but SHOULD include an a=setup "passive" attribute value
   if it knows that it is not located behind a NAT.

   Once the active UA has established the MSRP transport connection, the
   UA must immediately send an MSRP SEND request, as defined in
   [RFC4975].

      NOTE: According to [RFC4975], the initiating UA is always active,
      but when COMEDIA is used, the a=setup attribute is used to
      negotiate which UA becomes active.

4.3.  TLS

   If an MSRP UA conformant to this document uses Transport Layer
   Security (TLS), it MUST use the TLS mechanisms defined in [RFC4975]
   and [RFC4976].

   According to [RFC4975], the connection can be established with or
   without TLS mutual authentication.  In case mutual authentication is
   not used, the listening device waits until it receives a request on
   the connection, at which time it infers the identity of the
   connecting device from the associated session description.  From the
   TLS authentication point of view, it is thus irrelevant whether an
   endpoint takes the active or passive role.

   If an MSRP UA uses a self-signed TLS certificate to authenticate
   itself to MSRP peers, it also includes its certificate fingerprint in
   the SDP.

   Note that fingerprints can only be exchanged in peer-to-peer
   communication, as MSRP relays [RFC4976] will not receive the SDP
   payloads containing the fingerprint attributes.

4.4.  a=connection

   MSRP UAs MUST NOT use the SDP a=connection attribute.  [RFC4975]
   defines connection reuse procedures for MSRP, and this document does
   not modify those procedures.

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   If an MSRP UA receives an a=connection attribute, the UA MUST
   ignore it.

4.5.  MSRP Relay Connection

   If an MSRP UA is located behind an MSRP relay [RFC4976], the UA MUST
   always initiate a transport connection towards the relay, no matter
   what value the client has provided in the a=setup attribute.

      NOTE: Even if an MSRP UA initiates the TCP connection towards its
      relay, the UA will only send a SEND request if the UA is active,
      based on the COMEDIA negotiation.

5.  Interoperability with the Connection Model Defined in RFC 4975

   An MSRP UA conformant to this document can interoperate with a UA
   that follows the connection model defined in [RFC4975].  However, if
   an MSRP UA conformant to this document is located behind a NAT, does
   not proxy its MSRP communication via an MSRP relay, and receives an
   SDP offer from a remote UA that follows the connection model defined
   in [RFC4975], then NAT traversal can only be achieved if the MSRP UA
   supports ICE [ICE-TCP] or if the network supports Session Border
   Controller (SBC)-assisted NAT traversal for TCP.

6.  Security Considerations

   According to the connection model defined in [RFC4975], the MSRP UA
   that sends the SDP offer becomes the active party, and it is
   responsible for creating the MSRP transport connection towards the
   remote UA if a new connection is required.

   When COMEDIA is used, either the sender or the receiver of the SDP
   offer can become the active party.  [RFC4975] requires that the
   active party immediately issue an MSRP SEND request once the
   connection has been established.  This allows the passive party to
   bind the inbound TCP connection to the message session identified by
   the session id part of its MSRP URI.  The use of COMEDIA does not
   change this requirement, but the sender of the SDP offer is no longer
   assumed to always become the active party.

   The active party also takes the role of the TLS client, if TLS is
   used to protect the MSRP messages.  However, there are no procedures
   in [RFC4975] that would break in case the receiver of the SDP offer
   takes the role of the TLS client, and the level of security provided
   by TLS is not affected.

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7.  Acknowledgements

   Thanks to Ben Campbell, Remi Denis-Courmont, Nancy Greene, Hadriel
   Kaplan, Adam Roach, Robert Sparks, Salvatore Loreto, and Shida
   Schubert for their guidance and input in producing this document.

8.  References

8.1.  Normative References

   [RFC0793]     Postel, J., "Transmission Control Protocol", STD 7,
                 RFC 793, September 1981.

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

   [RFC4145]     Yon, D. and G. Camarillo, "TCP-Based Media Transport in
                 the Session Description Protocol (SDP)", RFC 4145,
                 September 2005.

   [RFC4975]     Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
                 "The Message Session Relay Protocol (MSRP)", RFC 4975,
                 September 2007.

   [RFC4976]     Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
                 for the Message Sessions Relay Protocol (MSRP)",
                 RFC 4976, September 2007.

8.2.  Informative References

   [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.

   [RFC5382]     Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and
                 P. Srisuresh, "NAT Behavioral Requirements for TCP",
                 BCP 142, RFC 5382, October 2008.

   [RFC5389]     Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
                 "Session Traversal Utilities for NAT (STUN)", RFC 5389,
                 October 2008.

   [RFC5780]     MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
                 Using Session Traversal Utilities for NAT (STUN)",
                 RFC 5780, May 2010.

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   [ICE-TCP]     Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
                 "TCP Candidates with Interactive Connectivity
                 Establishment (ICE)", Work in Progress, February 2011.

   [OMA-SIMPLE]  Open Mobile Alliance, "Instant Messaging using SIMPLE",
                 OMA-TS-SIMPLE_IM-V1_0-20090901-D, September 2009.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: christer.holmberg@ericsson.com

   Staffan Blau
   Ericsson AB
   PO Box 407
   Sweden

   EMail: staffan.blau@ericsson.com

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