MMUSIC Working Group D. Yon Internet-Draft Dialout.Net, Inc Expires:
November 12,December 10, 2004 G. Camarillo Ericsson May 14,June 11, 2004 Connection-Oriented Media Transport in the Session Description Protocol (SDP) draft-ietf-mmusic-sdp-comedia-06.txtdraft-ietf-mmusic-sdp-comedia-07.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I 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. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 12,December 10, 2004. 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 two new protocol identifiers:the SDP TCP and TCP/TLS. It also definesprotocol identifier, the SDP setup attribute, which describes the connection setup procedure, and the SDP reconnect attribute.connid attribute, which provides a connection identifier. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 3 3.1 TCP . . . . . .Identifier . . . . . . . . . . . . . . . . . . . . . 3 3.2 TCP/TLS . .4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4 4.4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4 5. The Connid Attribute . . . . . . . . . . . . . . . . . 4 4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4 4.2 Multiple-Connection Avoidance when Using Actpass .. . . . 5 5. The Reconnect Attribute5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6 6. Connection Lifetime5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 6 6. Connection Management . . 7 6.1 Session Renegotiation. . . . . . . . . . . . . . . . . . 7 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8 7.2 Passive/Active with Reconnect . . . . . . . .Connection Reestablishment . . . . . . 98 7.3 Actpass . . . .Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 109 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 1110 11.1 Normative References . . . . . . . . . . . . . . . . . . . . 1110 11.2 Informational References . . . . . . . . . . . . . . . . . . 1110 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 1211 Intellectual Property and Copyright Statements . . . . . . . . 1312 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 , 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 transportstransports, such as TCPTCP, are more appropriate. We define twoa new protocol identifiers:identifier, TCP, to describe TCP and TCP/TLS. Both represent connection-oriented reliable transports.connetions in SDP. Connection-oriented protocols introduce atwo new factor when describing a session: how and when should end points perform the connection setup procedure. We define two new attributes to describe connection setup:setups: setup and reconnect.connid. 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 IdentifiersIdentifier 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 We define twoa new values for the proto field: TCP and TCP/TLS. 3.1 TCPTCP. 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 lines with the TCP protocol identifier are carried using TCP . 3.2 TCP/TLS The TCP/TLSIt is RECOMMENDED that documents defining new SDP protocol identifier specifiesidentifiers that involve extra protocol layers between TCP and the session willmedia itself (e.g., TLS  over TCP) start with the string "TCP/" (e.g., TCP/TLS). The following sections define the setup and the connid attributes. While they are applicable to m= lines that use the Transport Layer Security (TLS) protocol  on top on aTCP  connection. Anprotocol identifier, they are not limited to them. These attributes SHOULD be used in any m= line that containwhich uses a connection-oriented transport protocol, even if the TCP/TLSprotocol identifier MUST further qualifyof the protocol using a fmt identifier.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" Active: The endpoint will initiate an outgoing connection. Passive: The endpoint will accept an incoming connection. ActPass: The endpoint will bothis willing to accept an incoming connection and willor to initiate an outgoing connection. 4.1 The default value of the setup attribute is actpass. That is, an m= line without an associated setup line is considered to be actpass. 4.1 The Setup Attribute inSetup 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 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 passive active actpass active / passive / actpassThe value active indicates that the 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, asexcept to denote an m= line that carries other semantics in SDP.has been or is being refused. The value passive indicates that the endpoint SHOULD be ready to accept a connection on the port number specified in the m= line. The value actpass indicates that the endpoint SHOULDofferer can either initiate a connection to the port number on the m= line of the other endpoint and thatin the endpoint SHOULD be ready toanswer or accept a connection on the port number specified in the m= line. It is RECOMMENDED that, if possible, endpoints set the port number on their m=line to the source port number which they will use to establish the connection towards the remote endpoint. This way, the transport-layer protocol (e.g., TCP) can take care of simultaneous opens. Endpoints typically usein the actpass value foroffer. That is, the following reasons: 1. Theofferer has no preference as to whether it accepts or initiates the connection and, so, is letting the answerer choose. 2.The endpoints intend to use a single connection to transport the media, but it is not known whether NAT (Network Address Translator) issues will prevent either endpoint from initiating or acceptingdefault value of the connection. So, both endpoints will attempt to initiate a connection hoping that at least one will succeed. 4.2 Multiple-Connection Avoidance when Using Actpass Whensetup attribute in an offer/answer exchange resultsis active in actpass, each endpoint attempts to establish a transport connection towardsthe other endpoint. If only oneoffer and passive in the answer. 5. The Connid Attribute The preceding description of the connections succeeds, this connection is used to transfer media. Nevertheless, if both connections succeed, onesetup attribute has been in the context of them needsusing SDP to be terminated so that both endpoints exchange data overinitiate a single connection. In this section, we provide rules to choose which of the two connections shouldsession. Still, SDP may be terminated (or not even initiated). First of all, if theexchanged between endpoints follow the recommendationat various stages of setting the port number in their m= linea session to the source port number which they will use to establish the connection towards the remote endpoint, the transport layer should take care of simultaneous opens (at least if TCP is the transport protocol). If, for some reason, any of the endpoints does not follow this recommendation, both endpoints should follow the rules below. If an endpoint is notified about a connection establishment attempt from the other endpoint before performing its own connection attempt, it SHOULD behave as a passive endpoint and SHOULD NOT attempt to establish any other connection. In case two connections are established, if an endpoint receives data (i.e., media) over one of the connections before having sent any data on any of the connections, the endpoint SHOULD terminate the connection that has not carried any data. When two connections are established and both endpoints start sending data before receiving anything from the other endpoint, it may happen that each of the endpoints choose a different connection to send data. If the answerer receives data over a connection after having sent data on the other connection, it SHOULD continue sending data on the other connection until an application-layer data boundary. At that point, the answerer SHOULD terminate this connection and start using the connection on which the offerer was sending data. Note that different applications may define application-layer boundaries in different ways. A typical suitable point for the answerer to change connections is the end of an application-layer message and the beginning of the next one. 5. The Reconnect 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 mediaaccomplish 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 reconnectmedia-level connid attribute, which is charset-independent and can be a session-level or a media-level attribute,charset-independent, is used to disambiguate these two scenarios. The following is the ABNF of the reconnectconnid attribute: reconnect-attrconnid = "a=reconnect" On reception of"a=connid:" connection-identifier connection-identifier = 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. 5.1 Offerer Behaviour Offerers and answerers use the connid 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. When an offerer generates an m= line which uses a connection-oriented transport, it SHOULD provide such an m= line with a reconnectconnection identifier using a connid attribute, unless the endpoints SHOULD closeapplication using the existing connection,m= line has other means to deal with connection reestablishment. The connid attribute in casean initial offer (i.e., no transport connection has been established yet) can take any value. This value identifies the initial connection that the endpoints will attempt to establish. 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 connid value for the m= line. If, on the other hand, the offerer wants to establish a new transport-layer connection for the m= line, it was still up,MUST use a new connid value. This new connid value MUST be different from the current connid value in use and SHOULD establishbe 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 connid value different than the one in use is enough to trigger the establishment of a new connection accordingconnection. Still, we recommend to use a value different than all the setup attributeprevious ones used in the m= line. Eitherline to make debugging easier. Note that, according to the offerer orrules in this section, an offer that changes the answerer can includetransport address (IP address plus port number) of an m= line will have a reconnect attributenew connid value for this m=line. 5.2 Answerer Behaviour The connid value for an m= line is negotiated using the offer/answer model. The resulting connid value after an offer/answer exchange is the connid value in the answer. For an m= line. In any event,line, if the offer contained this attribute,contains a new connid value (i.e., different from the answerone in use) the answerer MUST contain it as well. 6. Connection Lifetime An endpoint that intends to initiateuse this value in the connection SHOULD initiateanswer. If the connection immediately after it has sufficient information to do so, even if it does not intend to immediately begin sending media tooffer contains the connid value in use and the remote endpoint. This allows mediaanswerer wishes to flow fromcontinue using the remote endpoint. An endpoint SHOULD NOT closeexisting transport-layer connection, the connection untilanswerer MUST use this connid value in the session has expired, been explicitly terminated, oranswer. If the media stream is redirectedoffer contains the connid value in use but the answerer wishes to establish a different address or port. Ifnew transport-layer connection, the endpoint determines thatanswerer MUST use a new connid value in the connection has been closed, it MAY attempt to re-establishanswer. If the connection. The decision to do so is application and context dependant. 6.1 Session Renegotiation There are scenarios where SDP is sent byconnid value for an endpoint in order to renegotiatem= line resulting from an existing session. These include muting/unmuting a session, renegotiatingoffer/answer exchange is different than the attributes ofconnid in use so far, the media usedendpoints SHOULD establish a new transport-layer connection as indicated by the session, or extending the length ofsetup attribute. If a session about to expire. Connection-oriented media introduces some ambiguities into session renegotiation as to whenprevious connection is still up, the direction attribute must be obeyed and whenendpoint responsible for establishing the new connection performing the active role SHOULD close it as soon as the offer/answer exchange is ignored. The scenario of extendingcompleted. If the duration ofconnid value for an existing sessionm= line resulting from an offer/answer exchange is a good example:the same as the connid in order to extend an existing session,use so far, the endpoints will typically resendSHOULD continue using the original SDP with updated time information.existing connection. In connectionless mediathe result is no changepast, it was proposed to use the existing media streams. The problem with connection oriented media is that the original SDP will containpresence of a setupmedia-level SDP attribute which can be consideredas a requestflag to createindicate that a new connection, as opposedconnection needed to be established. We chose not to follow the flag approach because an offerer whose intent was to signal "no changes" in a session would need to issue a requestdifferent offer than the previous one (i.e., it would need to maintain steady state. The following rule help avoid this ambiguity: Ifremove the flag from the transport section (the c= andm= lines) ofline). By using the connid attribute instead, an SDP description describesofferer signals "no changes" in a session by issuing an existingidentical offer to the one in use. 6. Connection Management An endpoint that according to an offer/answer exchange is supposed to initiate a new connection between two endpoints andSHOULD initiate it as soon as the m= lineoffer/ answer exchange is completed, even if the endpoint does not contain a reconnect attribute,intend to immediately begin sending media to the remote endpoint. This allows media to flow from the remote endpoint if needed. Typically, endpoints SHOULD use thatdo not close the connection to carryuntil the media described insession has expired, been explicitly terminated, or a new connid value has been provided for the m= line. Additionaly, specific applications can describe further scenarios where an end-point may close a given connection. In case the remaindersession is explicitly terminated by one of the message. Theendpoints SHOULD NOT attempt to set up(e.g., the endpoint sends a new connection, regardless of whatSIP  BYE), the end point terminating the session is specified inresponsible for closing the setup attribute. Notetransport-connection. If an endpoint determines that if the port number inthe transport-connection for an m= line changes, there is no need to usehas been closed and it should be reestablished, it SHOULD perform a new offer/answer exchange using a new connid value for this m= line. Note that the reconnectSDP direction attribute because(e.g., a=sendonly) deals with the new port will triggermedia sent over the establishment of a new connection anyway.transport-connection, but has no impact on the transport-connection itself. 7. Examples What follows are a number ofThe following examples thatshow 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 examplesexamples, which only show m= lines and is assumed to be the following: v=0 o=me 2890844526 2890842807 IN IP4 10.1.1.2 s=Call me using TCP t=3034423619 3042462419their 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: c=IN IP4 192.0.2.2m=image 54111 TCP t38 c=IN IP4 192.0.2.2 a=setup:passive a=connid:1 An answerer at 192.0.2.1 receiving this offer responds with the following answer: c=IN IP4 192.0.2.1 m=image 9 TCP t38 a=setup:active a=connid:1 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 ReconnectConnection Reestablishment Continuing the preceding example, consider the scenario where the TCP connection fails and the endpoints wish to reestablish the connection for the session. The endpoint at 192.0.2.2 signals this intent with the following SDP: c=IN IP4 192.0.2.2m=image 54111 TCP t38 c=IN IP4 192.0.2.2 a=setup:passive a=reconnecta=connid:2 The reconnect attributenew connid value informs the endpoint at 192.0.2.1 that this SDP represents the intent to establish a new connection for media transport, rather than continuing with the original connection. Because the endpoint at 192.0.2.1 may not yet be aware that the TCP connection has failed, this eliminates any ambiguity.If 192.0.2.1 agrees to continue the session using a new connection, it responds with: c=IN IP4 192.0.2.1m=image 9 TCP t38 a=setup:active INc=IN IP4 a=reconnect192.0.2.1 a=setup:active a=connid:2 7.3 Actpass AnActpass/Passive In another example, an offerer at 192.0.2.2 signals its availability for a T.38 fax session at TCP port 54111. Additionally, this offerer is also willing to set up the media stream by initiating the TCP connection: c=IN IP4 192.0.2.2m=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 with the following description: c=IN IP4 192.0.2.1m=image 54321 TCP t38 a=setup:actpassc=IN IP4 192.0.2.1 a=setup:passive a=connid:3 This will cause the offerer (at 192.0.2.2) to initiate a connection to port 54321 at 192.0.2.1 and the answerer (at 192.0.2.1) to initiate a connection to port 54111 at 192.0.2.2. Ideally, the offerer would use 192.0.2.2:5411 as the source of its connection attempt and the answerer would use 192.0.2.1:54321 as its.192.0.2.1. 8. Security Considerations See RFC 2327  for security and other considerations specific to the Session Description Protocol in general. An attacker may attempt to substitute TCP/TLS with only TCP in a session description.modify the values of the connid attributes to have endpoints reestablish connections unnecesaryly. 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. This document touches upon NAT traversal. Implementers should be aware of some issues that relate to the use of private IP addresses within the offer/answer model (i.e., they are not specific to this document). When an endpoint receives a session description with a private IP address that belongs to a different address space, in most of the cases, the endpoint will not be able to reach such an address. Nevertheless, if this particular address also exists in the endpoint's address space, the endpoint may end up reaching a different peer than the one that generated the session description. It is RECOMMENDED that endpoints authenticate their peer somehow (e.g., using TLS ) or that they encrypt their media.9. IANA Considerations This document defines two session and media level SDP attributes: setup and reconnect.connid. 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 twoa proto values: TCP and TCP/TLS. Their formats areTCP. Its format is defined in Section 3.1 and Section 3.2 respectively. These two3. This proto valuesvalue should be registered by the IANA on http:// www.iana.org/assignments/sdp-parametershttp://www.iana.org/assignments/sdp-parameters under "proto". 10. Acknowledgements The authors would like to thankJonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert Fairlie-Cuninghame, andColin Perkins for theirPerkins, 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.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. 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.  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. 11.2 Informational 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. Authors' Addresses David Yon Dialout.Net, Inc One Indian Head Plaza Nashua, NH 03060 USA EMail: firstname.lastname@example.org 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's procedures with respect to rights in IETF Documents can be found in BCP 78 and BCP 79. 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