SIMPLE Working Group                                         C. Holmberg
Internet-Draft                                                  Ericsson
Intended status: Standards Track                                 S. Blau
Expires: October 22, 2010                                    Ericsson AB
                                                          April 20, 2010

 An Alternative Connection Model for the Message Session Relay Protocol


   This document defines an alternative connection model for Message
   Session Relay Protocol (MSRP) User Agents (UAs), which 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 TCP connection, in order for MSRP messages to
   traverse the NAT.

Status of this Memo

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

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   This Internet-Draft will expire on October 22, 2010.

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Applicability statement . . . . . . . . . . . . . . . . . . . . 3
   4.  COMEDIA for MSRP  . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.2.  a=setup . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.3.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     4.4.  a=connection  . . . . . . . . . . . . . . . . . . . . . . . 6
     4.5.  MSRP relay connection . . . . . . . . . . . . . . . . . . . 6
   5.  Interoperability with connection model defined in RFC 4975  . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     10.1. Normative References  . . . . . . . . . . . . . . . . . . . 8
     10.2. Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   The Message Session Relay Protocol (MSRP) core specification
   [RFC4975] defines that the MSRP UA which sends the 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-oriended media mechanism, COMEDIA,
   that endpoints can use to negotiate the endpoint which initiates the
   creation of media transport connection.

   COMEDIA is especially useful when an endpoint is located behind a
   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.

   An example is the Open Mobile Alliance (OMA) defined "Instant Message
   using SIMPLE" [OMA-TS-SIMPLE_IM-V1_0-20090901-D], 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
   global IP 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 which
   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Applicability statement

   Support of this specification is optional for MSRP user agents in
   general.  User Agents that are likely to be deployed in networks
   where User Agents need to establish the TCP connections in order to
   traverse NATs SHOULD support this specification in order to improve
   the odds of being able to communicate across NATs.

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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

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

   The a=setup attribute is particularly useful when one MSRP UA
   represents a network media server, or any other entity that is not
   located behind a NAT.  The a=setup attribute allows the media server
   to ensure that MSRP UAs create the MSRP transport connections towards
   the server, so that NATs at user's premises will not interfere with
   the connection creation.

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

   An MSRP UA MUST support the a=setup "active", "actpass" and "passive"
   attribute values.

   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 Open request 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 global IP address that the other endpoint
   can use as destination for a TCP Open request, 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 NAT, and in order to be
   compatible with MSRP 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

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

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

   - the UA relays its MSRP transport connections via a relay (e.g.
   MSRP relay or TURN server); or

   - the UA has used Session Traversal Utilities for NAT (STUN)
   [RFC5389] signaling to retrieve NAT address and port through the
   local port to be used for the eventual transport connection, while
   also having determined that the NAT is not address restricted.

   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
   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, and set the a=setup attribute accordingly.  This comparing
   mechanism does not work in all scenarios, though.  For example, if
   SIP a requests 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 answer MAY include an a=setup "active" attribute value, but
   SHOULD include a=setup "passive" attribute value if the SDP answerer
   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

   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 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 TLS

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   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.

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

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 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 NAT, and
   does not proxy its MSRP communication via an MSRP relay, and the UA
   receives an SDP offer from a remote UA that follows the connection
   model defined in [RFC4975], NAT traversal can only be achieved if the
   MSRP UA supports ICE [I.D.ietf-mmusic-ice-tcp] and the network
   provides TURN servers, or if the network supports 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

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   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 issues 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 as 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 as TLS client, and the level of security provided by
   TLS is not affected.

7.  IANA Considerations

   This document has no actions for IANA.

8.  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 order to produce this

9.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing]

   Changes from draft-ietf-simple-msrp-acm-07
   o  WGLC editorial changes.

   Changes from draft-ietf-simple-msrp-acm-06
   o  WGLC changes.

   Changes from draft-ietf-simple-msrp-acm-05
   o  TLS section modified.

   Changes from draft-ietf-simple-msrp-acm-04

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   o  TLS section modified.
   o  Security considerations section modified.

   Changes from draft-ietf-simple-msrp-acm-03
   o  Changes based on WGLC comments from Adam Roach and Ben Campbell.
   o  New section added related to interoperability with connection
      model defined in RFC 4975.
   o  Text related to a=setup "holdconn" attribute value removed.
   o  NAT keepalive section removed.
   o  Usage of COMEDIA-TLS removed.

   Changes from draft-ietf-simple-mscp-acm-02
   o  Changes based on WGLC comments from Salvatore Loreto and Shida

   Changes from draft-ietf-simple-msrp-acm-01
   o  Procedures for using SDP c/m for routing of MSRP messages removed.
   o  Procedures related to modification of MSRP address information by
      intermediates moved to separate document.
   o  Solution to open issue on usage of the SDP a=connection

10.  References

10.1.  Normative References

   [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., Mahy, R., and C. Jennings, "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.

10.2.  Informative References

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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,

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              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

Authors' Addresses

   Christer Holmberg
   Hirsalantie 11
   Jorvas  02420


   Staffan Blau
   Ericsson AB
   P.O Box 407


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