SIMPLE Working Group C. Holmberg Internet-Draft Ericsson Expires: June 20, 2010 S. Blau Ericsson AB December 17, 2009 An Alternative Connection Model for the Message Session Relay Protocol (MSRP) draft-ietf-simple-msrp-acm-03.txt Abstract This document defines an alternative connection model for MSRP UAs, which uses COMEDIA in order to create the MSRP transport connection. The model allows MSRP UAs behind NATs to negotiate which UA will initiate the establishment of the TCP connection, in order for MSRP messages to traverse the NAT. The document also defines how an MSRP UA which is located behind a NAT keeps the NAT binding alive. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 20, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. Holmberg & Blau Expires June 20, 2010 [Page 1] Internet-Draft MRSP December 2009 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect 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 BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 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. NAT keepalive . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Holmberg & Blau Expires June 20, 2010 [Page 2] Internet-Draft MRSP December 2009 1. Introduction [RFC4975] defines that the MSRP UA which sends the SDP offer is "active", and it is responsible to create the MSRP transport connection towards the remote UA if a new connection is required. [RFC4975] allows other mechanisms to be used to create MSRP transport connection, but does not define any other mechanisms. [RFC4145] defines a mechanism, COMEDIA, which 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 OMA (Open Mobile Alliance) 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 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. Conventions 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 BCP 14, RFC 2119 [RFC2119]. 3. Applicability statement An MSRP UA conformant to this document can interop with an [RFC4975] conformant MSRP UA. However, if an MSRP UA conformant to this document is located behind NAT, and does not proxy its MSRP communication via an MSRP-Relay node, and the UA receives an SDP offer from an [RFC4975] conformant remote UA, NAT traversal can only be achieved if the MSRP UA supports ICE and the network provides TURN Holmberg & Blau Expires June 20, 2010 [Page 3] Internet-Draft MRSP December 2009 servers, or if the network supports SBC assisted NAT traversal for TCP. 4. COMEDIA for MSRP 4.1. General This section defines how an MSRP UA uses the COMEDIA SDP attributes defined in [RFC4145], how an MSRP UA can use the COMEDIA TLS extensions [RFC4572], and how an MSRP UA keeps a NAT binding alive. 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 which 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. An MSRP UA MUST NOT use the a=setup "holdcon" attribute value. When the a=setup attribute value is "actpass" or "passive", the IP address:port value in the MSRP URI of the SDP a=path attribute MUST contain the actual address:port on which the UA can receive a TCP Open request for the MSRP transport connection. If the a=setup attribute value is "active", the port number value MUST either be the actual port number that the MSRP UA will use for the TCP endpoint, or the port value 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 Holmberg & Blau Expires June 20, 2010 [Page 4] Internet-Draft MRSP December 2009 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 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 STUN signalling to retrieve NAT address:port through the local port to be used for the the eventual transport connection, while also having determined that the NAT is not address restricted. If an MSRP UA is located behind a NAT, both SIP signaling and MSRP media will pass trough the NAT, and a UA can determine whether the media transport will be NATed by inspecting the SIP Via header in the 200 (OK) response if the initial REGISTER request, by comparing the IP addresses in the Via sent-by and received parameters. If these are not the same then the UA can determine that there is a NAT in the path. 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 [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 MSRP UAs negotiate a TLS transport connection for MSRP, the role of TLS client and TLS server MUST negotiate the relevant parameter as defined in COMEDIA-TLS [RFC4572], and in accordance with [RFC4975] the MSRP URI scheme SHOULD be msrps and the m-line protocol indicator SHOULD be TCP/TLS/MSRP. Holmberg & Blau Expires June 20, 2010 [Page 5] Internet-Draft MRSP December 2009 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 it. NOTE: If an MSRP UA uses ICE [I.D.ietf-mmusic-ice] when it establishes the MSRP transport connection, the UA needs to perform the normal MSRP checks for possible reuse of an existing transport connection before it sends any TCP Open requests. 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. NAT keepalive If an MSRP UA is located behind a NAT the UA MUST keep the NAT binding alive, in accordance with section 10 of [I.D.ietf-mmusic- ice]. The UA SHOULD use the character string CR-LF as NAT keepalive. However, if the NAT traversal method selected by ICE was based on successful connectivity checks, using STUN, then the UA MAY alternatively use STUN as defined in [I.D-ietf-mmusic-ice]. 6. Security Considerations The procedures define in this document do not impact the security characteristics defined in [RFC4975]. 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 order to produce this document. Holmberg & Blau Expires June 20, 2010 [Page 6] Internet-Draft MRSP December 2009 8. Change Log [RFC EDITOR NOTE: Please remove this section when publishing] Changes from draft-ietf-simple-mscp-acm-02 o Changes based on WGLC comments from Salvatore Loreto and Shida Schubert. 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 implemented. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS Names", BCP 32, RFC 2606, June 1999. [RFC3323] Peterson, J., "A Privacy Mechanism for the Session Initiation Protocol (SIP)", RFC 3323, November 2002. [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the Session Description Protocol (SDP)", RFC 4145, September 2005. [RFC4572] Lennox, J., "Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP)", RFC 4572, July 2006. [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. [I-D.ietf-mmusic-ice] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", Holmberg & Blau Expires June 20, 2010 [Page 7] Internet-Draft MRSP December 2009 draft-ietf-mmusic-ice-19 (work in progress), October 2007. 9.2. Informative References Authors' Addresses Christer Holmberg Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: christer.holmberg@ericsson.com Staffan Blau Ericsson AB P.O Box 407 Sweden Email: staffan.blau@ericsson.com Holmberg & Blau Expires June 20, 2010 [Page 8]