SIMPLE Working Group C. Holmberg Internet-Draft Ericsson Intended status: Standards Track S. Blau Expires: October 10, 2010 Ericsson AB April 8, 2010 An Alternative Connection Model for the Message Session Relay Protocol (MSRP) draft-ietf-simple-msrp-acm-07.txt Abstract This document defines an alternative connection model for Message Session Relay Protocol (MSRP) User Agents (UAs), which uses the connection-oriended media (COMEDIA) mechanism in order to create the MSRP transport connection. The model allows MSRP UAs behind Network Address Translators (NATs) to negotiate which UA will initiate 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on October 10, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. 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 Holmberg & Blau Expires October 10, 2010 [Page 1] Internet-Draft MRSP April 2010 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 . . . . . . . . . . . . . . . . . . . . 7 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 Holmberg & Blau Expires October 10, 2010 [Page 2] Internet-Draft MRSP April 2010 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 specifcation 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", "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 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. Holmberg & Blau Expires October 10, 2010 [Page 3] Internet-Draft MRSP April 2010 4. COMEDIA for MSRP 4.1. General This section defines how an MSRP UA 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: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 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 Holmberg & Blau Expires October 10, 2010 [Page 4] Internet-Draft MRSP April 2010 offer. 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 Simple Traversal of UDP Through NATs (STUN) [RFC5389] 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. Some UAs can determinte 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 the NAT contains a SIP proxy, the UA will not be able to detect the NAT by comparing the IP addresses." NOTE: If the NAT contains a SIP proxy, 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 [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 TLS, it MUST use the TLS mechanisms defined in [RFC4975] and [RFC4976]. Holmberg & Blau Expires October 10, 2010 [Page 5] Internet-Draft MRSP April 2010 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 authentication point of view it is thus irrelevant whether an endpoint takes the active or passive role. In accordance with [RFC4975], if an MSRP UA has a TLS certificate, it always sends the certificate to the other endpoint during the TLS establishment. 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 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 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 Holmberg & Blau Expires October 10, 2010 [Page 6] Internet-Draft MRSP April 2010 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 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 document. 9. Change Log [RFC EDITOR NOTE: Please remove this section when publishing] Changes from draft-ietf-simple-msrp-acm-05 o TLS section modified. Changes from draft-ietf-simple-msrp-acm-04 Holmberg & Blau Expires October 10, 2010 [Page 7] Internet-Draft MRSP April 2010 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 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. 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, Holmberg & Blau Expires October 10, 2010 [Page 8] Internet-Draft MRSP April 2010 A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. 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 October 10, 2010 [Page 9]