MMUSIC Working Group D. Yon Internet-Draft
Dialout.Net, IncTactical Software, LLC Expires: January 14,March 30, 2005 G. Camarillo Ericsson July 16,September 29, 2004 Connection-Oriented Media Transport in the Session Description Protocol (SDP) draft-ietf-mmusic-sdp-comedia-08.txtdraft-ietf-mmusic-sdp-comedia-09.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, I certifyeach author represents that any applicable patent or other IPR claims of which I amhe or she is aware have been or will be disclosed, and any of which Ihe or she become aware will be disclosed, in accordance with RFC 3668. 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.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 January 14,March 30, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved.Abstract This document describes how to express media transport over connection-oriented protocols using the Session Description Protocol (SDP). It defines the SDP TCP protocol identifier, the SDP setup attribute, which describes the connection setup procedure, and the SDP connidconnection attribute, which provides ahandles connection identifier.reestablishment. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3 4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4 4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4 5. The ConnidConnection Attribute . . . . . . . . . . . . . . . . . . . . .5 5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6 5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7 6. Connection Management . . . . . . . . . . . . . . . . . . . . 7 6.1 Connection Establishment . . . . . . . . . . . . . . . . . 7 6.2 Connection Reestablishment . . . . . . . . . . . . . . . . 8 6.3 Connection Termination . . . . . . . . . . . . . . . . . . 8 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8 7.2 Passive/Active with Connection ReestablishmentActpass/Passive . . . . . . . . . . . . . . . . . . . . . 9 7.3 Actpass/PassiveExisting Connection Reuse . . . . . . . . . . . . . . . . 9 7.4 Existing Connection Refusal . . . . . 9. . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 1011 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 1011 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 1012 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 1012 11.1 Normative References . . . . . . . . . . . . . . . . . . . . 1012 11.2 Informative References . . . . . . . . . . . . . . . . . . . 1112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 1113 Intellectual Property and Copyright Statements . . . . . . . . 1314 1. Introduction The Session Description Protocol  provides a general-purpose format for describing multimedia sessions in announcements or invitations. SDP uses an entirely textual data format (the US-ASCII subset of UTF-8 )) to maximize portability among transports. SDP does not define a protocol, but only the syntax to describe a multimedia session with sufficient information to participate in that session. Session descriptions may be sent using arbitrary existing application protocols for transport (e.g., SAP , SIP, SIP , RTSP ,, email, HTTP ,, etc.). SDP  defines two protocol identifiers: RTP/AVP and UDP, both of which represent unreliable connectionless protocols. While these transports are appropriate choices for multimedia streams, there are applications for which connection-oriented transports, such as TCP, are more appropriate. This document defines a new protocol identifier, TCP, to describe TCP connetions in SDP. Connection-oriented protocols introduce two new factor when describing a session: how and when should end pointsendpoints perform the connection setup procedure. This document defines two new attributes to describe connection setups: setup and connid.connection. 2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119  and indicate requirement levels for compliant implementations. 3. Protocol Identifier The following is the ABNF for an m= line, as specified by RFC 2327 .. media-field = "m=" media space port ["/" integer] space proto 1*(space fmt) CRLF This document defines a new value for the proto field: TCP. The TCP protocol identifier is similar to the UDP protocol identifier in that it only describes the transport protocol, and not the upper-layer protocol. An m= line that specifies "TCP" MUST further qualify the application-layer protocol using an fmt identifier. Media described using an m= lines withcontaining the TCP protocol identifier are carried using TCP . It is RECOMMENDED that documents defining new SDP protocol identifiers that involve extra protocol layers between TCP and the media itself (e.g., TLS  over TCP) start with the string "TCP/" (e.g., TCP/TLS). The following sections define the setup and the connidconnection attributes. While both attributes are applicable to m= lines that use the TCP protocol identifier, they are not limited to them. These attributes MAY be used in conjunction with any m= line which uses a connection-orientedconnection- oriented transport protocol, even if the protocol identifier of the m= line is not TCP. 4. Setup Attribute The setup attribute indicates which of the end points should initiate the connection establishment (e.g., send the initial TCP SYN). The setup attribute is charset-independent and can be a session-level or a media-level attribute. The following is the ABNF of the setup attribute: setup-attr = "a=setup:" role role = "active" / "passive" / "actpass" / "holdconn" Active: The endpoint will initiate an outgoing connection. Passive: The endpoint will accept an incoming connection. ActPass: The endpoint is willing to accept an incoming connection or to initiate an outgoing connection. Holdconn: The endpoint does not want the connection to be established for the time being. 4.1 The Setup Attribute in the Offer/answer Model The offer/answer model, defined in RFC 3264 ,, provides endpoints with a means to obtain shared view of a session. Some session parameters are negotiated (e.g., codecs to use), while others are simply communicated from one endpoint to the other (e.g., IP addresses). The value of the setup attribute falls into the first category. That is, both endpoints negotiate its value using the offer/answer model. The negotiation of the value of the setup attribute takes places as follows. The offerer states which role or roles it is willing to perform and the answerer, taking the offerer's willingness into consideration, chooses which roles both endpoints will actually perform during connection establishment. The following are the values that the setup attribute can take in an offer/answer exchange: Offer Answer ________________ active passive / holdconn passive active / holdconn actpass active / passive / holdconn holdconn holdconn The active endpoint SHOULD initiate a connection to the port number on the m= line of the other endpoint. The port number on its own m= line is irrelevant, and the opposite endpoint MUST NOT attempt to initiate a connection to the port number specified there. Nevertheless, since the m= line must contain a valid port number, the endpoint specifying using the value active SHOULD specify a port number of 9 (the discard port) on its m= line. The endpoint MUST NOT specify a port number of zero, except to denote an m= line that has been or is being refused. The passive endpoint SHOULD be ready to accept a connection on the port number specified in the m= line. A value of actpass indicates that the offerer can either initiate a connection to the port number on the m= line in the answer or accept a connection on the port number specified in the m= line in the offer. That is, the offerer has no preference as to whether it accepts or initiates the connection and, so, is letting the answerer choose. A value of holdconn indicates that the connection should not be established for the time being. The default value of the setup attribute in an offer/answer exchange is active in the offer and passive in the answer. 5. The ConnidConnection Attribute The preceding description of the setup attribute has been in the context of using SDP to initiate a session. Still, SDP may be exchanged between endpoints at various stages of a session to accomplish tasks such as terminating a session, redirecting media to a new endpoint, or renegotiating the media parameters for a session. After the initial session has been established, it may be ambiguous as to whether subsequent SDP exchange represents a confirmation that the endpoint is to continue using the current media connection unchanged, or is a request to make a new media connection. The media-level connidconnection attribute, which is charset-independent, is used to disambiguate these two scenarios. The following is the ABNF of the connidconnection attribute: connidconnection-attr = "a=connid:" connection-identifier connection-identifier"a=connection:" conn-value conn-value = token The connid attribute provides an identifier for the transport-layer connection used by the m= line. Connid values are meaningful in the context of a particular m= line. So, different m= lines in the same session description MAY have the same connid value."new" / "existing" 5.1 Offerer Behaviour Offerers and answerers use the connidconnection attribute to decide whether a new transport connection needs to be established or, on the other hand, the existing transport connection should still be used. The connection value resulting from an offer/answer exchange is the connection value in the answer. If the connection value in the answer is "new", the end-points SHOULD establish a new connection. If the connection value in the answer is "existing", the end-points SHOULD continue using the exiting connection. When an offerer generates an m= line which uses a connection-oriented transport, it SHOULD provide such an m= line witha connection identifier using a connid attribute,attribute for the m= line unless the application using the m= line has other means to deal with connection reestablishment. The connidconnection attribute in an initial offer (i.e., no transport connection has been established yet) can take any value. This value identifies the initial connection thattakes the endpoints will attempt to establish.value of "new". After the initial offer/answer exchange, any of the endpoints can generate a new offer to change some characteristics of the session (e.g., the direction attribute). If such an offerer wants to continue using the previously-established transport-layer connection for the m= line, the offerer MUST use the same conniduse a connection value of "existing" for the m= line. If, on the other hand, the offerer wants to establish a new transport-layer connection for the m= line, it MUST use a new connid value. This new connid value MUST be different from the current connid value in use and SHOULD be different than any connid value used previously in the same m= line. The connid value in an offer is only compared with the connid value currently in use. So, having a connidconnection value different than the one in use is enough to trigger the establishmentof a new connection. Still, it is recommended to use a value different than all the previous ones used in the m= line to make debugging easier."new". Note that, according to the rules in this section, an offer that changes the transport address (IP address plusor port number) of an m= line will have a new connidconnection value for this m= line.of "new". The default value of the connection attribute in an offer/answer exchange is "new". 5.2 Answerer Behaviour The connidconnection value for an m= line is negotiated using the offer/answeroffer/ answer model. The resulting connidconnection value after an offer/answer exchange is the connidconnection value in the answer. For an m= line, ifIf the offer contains a new connidconnection value (i.e., different from the onein use)the offer is "new", the answerer MUST also use thisa value of "new" in the answer. If the connection value in the offer containsis "existing", the connidanswerer uses a value of "existing" in use andthe answereranswer if it wishes to continue using the existing transport-layer connection, the answerer MUST use this connid value in the answer. If the offer contains the connidconnection and a value in use but the answerer wishes to establishof "new" if it wants a new transport-layer connection,connection to be established. In some scenarios where third party call control  is used, an endpoint may receive an initial offer with a connection value of "existing". Following the previous rules, such an answerer MUSTwould use a new connidconnection value of "new" in the answer. If the connidconnection value for an m= line resulting from an offer/answer exchange is different than the connid in use so far,"new", the endpoints SHOULD establish a new transport-layer connection as indicated by the setup attribute. If a previous connection is still up, the endpoint responsible for establishing the new connection performing the active roleendpoints SHOULD close it as soon as the offer/answer exchange is completed. It is up to the application to ensure proper data synchornization between the two connections. If the connidconnection value for an m= line resulting from an offer/answer exchange is the same as the connid in use so far,"existing", the endpoints SHOULD continue using the existing connection. In the past, it was proposed to use the presence of a media-level SDP attribute as a flag to indicate that a new connection needed to be established. It was decided not to follow the flag approach because an offerer whose intent was to signal "no changes" in a session would need to issue a different offer than the previous one (i.e., it would need to remove the flag from the m= line). By using the connid attribute instead, an offerer signals "no changes" in a session by issuing an identical offer to the one in use.6. Connection Management This section addresses connection establishment, connection reestablishment, and connection termination. 6.1 Connection Establishment An endpoint that according to an offer/answer exchange is supposed to initiate a new connection SHOULD initiate it as soon as the offer/ answer exchangeit is completed,able to, even if the endpoint does not intend to immediately begin sending media to the remote endpoint. This allows media to flow from the remote endpoint if needed. Typically,Note that some endpoints do not close the connection until the session has expired, been explicitly terminated, or a new connid value has been providedneed to wait for some event to happen before being able to establish the m= line. Additionaly, specific applications can describe further scenarios where an end-pointconnection. For example, a wireless terminal may closeneed to set up a given connection. In case the session is explicitly terminated by one of the endpoints (e.g., the endpoint sendsradio bearer before being able to initiate a SIP  BYE), the end point terminating the session is responsible for closing the transport-connection.connection. 6.2 Connection Reestablishment If an endpoint determines that the transport-connection for an m= line has been closed and it should be reestablished, it SHOULD perform a new offer/answer exchange using a new connidconnection value of "new" for this m= line. Note that the SDP direction attribute (e.g., a=sendonly) deals with the media sent over the transport-connection, but has no impact on the transport-connection itself. 6.3 Connection Termination Typically, endpoints do not close the connection until the session has expired, been explicitly terminated, or a new connection value has been provided for the m= line. Additionaly, specific applications can describe further scenarios where an end-point may close a given connection. As soon as an end-point notices that it needs to terminate a connection, it SHOULD do so. While in TCP both end-points need to close a connection, other connection-oriented transport protocols may not have the concept of half-close connections. In this case, a connection would be terminated as soon as one of the end-points closed it, making it unnecessary for the other end-point to perform any further action to terminate the connection. In any case, individual applications may provide further considerations on how to achieve a graceful connection termination. For example, a file application using TCP receiving a FIN from the remote endpoint may need to finish the ongoing transmission of a file before sending its own FIN. 7. Examples The following examples show the most common usage of the setup attribute combined with TCP-based media descriptions. For the purpose of brevity, the main portion of the session description is omitted in the examples, which only show m= lines and their attributes (including c= lines). 7.1 Passive/Active An offerer at 192.0.2.2 signals its availability for a T.38 fax session at port 54111: m=image 54111 TCP t38 c=IN IP4 192.0.2.2 a=setup:passive a=connid:1a=connection:new An answerer at 192.0.2.1 receiving this offer responds with the following answer: c=IN IP4 192.0.2.1m=image 9 TCP t38 c=IN IP4 192.0.2.1 a=setup:active a=connid:1a=connection:new The endpoint at 192.0.2.1 then initiates the TCP connection to port 54111 at 192.0.2.2. 7.2 Passive/Active with Connection Reestablishment Continuing the precedingActpass/Passive In another example, consider the scenario where the TCP connection fails and the endpoints wish to reestablish the connection for the session. The endpointan offerer at 192.0.2.2 signals its availability for a T.38 fax session at TCP port 54111. Additionally, this intent withofferer is also willing to set up the following SDP:media stream by initiating the TCP connection: m=image 54111 TCP t38 c=IN IP4 192.0.2.2 a=setup:passive a=connid:2a=setup:actpass a=connection:new The new connid value informs theendpoint at 192.0.2.1 that this SDP representsresponds with the following description: m=image 54321 TCP t38 c=IN IP4 192.0.2.1 a=setup:passive a=connection:new This will cause the intentofferer (at 192.0.2.2) to establishinitiate a newconnection for media transport, rather than continuing withto port 54321 at 192.0.2.1. 7.3 Existing Connection Reuse Subsequent to the original connection. Ifexchange in Section 7.2, another offer/answer exchange is initiated in the opposite direction. The endpoint at 192.0.2.1 agreeswishes to continue the sessionusing a new connection, itthe existing connection: m=image 54321 TCP t38 c=IN IP4 192.0.2.1 a=setup:passive a=connection:existing The endpoint at 192.0.2.2 also wishes to use the existing connection and responds with:with the following description: m=image 9 TCP t38 c=IN IP4 192.0.2.119220.127.116.11 a=setup:active a=connid:2 7.3 Actpass/Passive In another example, an offerer ata=connection:existing The existing connection from 192.0.2.2 signals its availability for a T.38 fax sessionto 192.0.2.1 will be reused. Note that the endpoint at TCP192.0.2.2 uses setup:active in response to the offer of setup:passive, and uses port 54111. Additionally, this offerer9 because it is also willingactive. 7.4 Existing Connection Refusal Subsequent to set upthe media streamexchange in Section 7.3, another offer/answer exchange is initiated by initiatingthe TCPendpoint at 192.0.2.2, again wishing to reuse the existing connection: m=image 54111 TCP t38 c=IN IP4 192.0.2.2 a=setup:actpass a=connid:3 The endpoint at 192.0.2.1 responds witha=connection:existing However, this time the following description:answerer is unaware of the old connection and so wishes to establish a new one. (This could be the result of a transfer via 3pcc.) It is unable to act in the passive mode so responds as active: m=image 543219 TCP t38 c=IN IP4 192.0.2.1 a=setup:passive a=connid:3 This will cause192.0.2.3 a=setup:active a=connection:new The endpoint at 192.0.2.3 then initiates the offerer (at 192.0.2.2) to initiate aTCP connection to port 5432154111 at 192.0.2.1.192.0.2.2, and the endpoint at 192.0.2.2 closes the old connection. Note that the endpoint at 192.0.2.2, while specifying connection: existing has reverted to setup:actpass and its real port number, rather than repeating setup:active and port 9 from the previous cycle. Had it not done this, this negotiation would have failed. 8. Security Considerations See RFC 2327  for security and other considerations specific to the Session Description Protocol in general. An attacker may attempt to modify the values of the connidconnection and setup attributes to have endpoints reestablish connections unnecesaryly or to keep them from establishing a connection. So, it is STRONGLY RECOMMENDED that integrity protection be applied to the SDP session descriptions. For session descriptions carried in SIP ,, S/MIME is the natural choice to provide such end-to-end integrity protection, as described in RFC 3261 .. Other applications MAY use a different form of integrity protection. 9. IANA Considerations This document defines two session and media level SDP attributes: setup and connid.connection. Their formats are defined in Section 4 and Section 5 respectively. These two attributes should be registered by the IANA on http://www.iana.org/assignments/sdp-parameters under "att-field (both session and media level)". This document defines a proto values:value: TCP. Its format is defined in Section 3. This proto value should be registered by the IANA on http://www.iana.org/assignments/sdp-parameters under "proto". Specifications defining new proto values, like this one, must define the rules by which their media format (fmt) namespace is managed. For the TCP protocol, new formats SHOULD have an associated MIME registration. Use of an existing MIME subtype for the format is encouraged. If no MIME subtype exists, it is RECOMMENDED that a suitable one is registered through the IETF process  by production of, or reference to, a standards-track RFC that defines the transport protocol for the format. 10. Acknowledgements Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer Holmberg provided valuable insights and contributions. 11. References 11.1 Normative References  Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.  Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", BCP 13, RFC 2048, November 1996.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. 11.2 Informative References  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998.  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.  Handley, M., Perkins, C. and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000.  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.  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, "Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April 2004. Authors' Addresses David Yon Dialout.Net, Inc One Indian Head Plaza Nashua,Tactical Software, LLC 670 N Commercial St Manchester, NH 0306003101 USA EMail: email@example.com@rfdsoftware.com Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland EMail: Gonzalo.Camarillo@ericsson.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the IETF'sprocedures with respect to rights in IETF DocumentsRFC documents can be found in BCP 78 and BCP 79. 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