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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (July 16, 2004) is 7225 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 793 (ref. '1') (Obsoleted by RFC 9293) ** Obsolete normative reference: RFC 2327 (ref. '3') (Obsoleted by RFC 4566) -- Obsolete informational reference (is this intentional?): RFC 2326 (ref. '5') (Obsoleted by RFC 7826) -- Obsolete informational reference (is this intentional?): RFC 2246 (ref. '6') (Obsoleted by RFC 4346) -- Obsolete informational reference (is this intentional?): RFC 2616 (ref. '7') (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) Summary: 9 errors (**), 0 flaws (~~), 6 warnings (==), 10 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 MMUSIC Working Group D. Yon 2 Internet-Draft Dialout.Net, Inc 3 Expires: January 14, 2005 G. Camarillo 4 Ericsson 5 July 16, 2004 7 Connection-Oriented Media Transport in the Session Description 8 Protocol (SDP) 9 draft-ietf-mmusic-sdp-comedia-08.txt 11 Status of this Memo 13 By submitting this Internet-Draft, I certify that any applicable 14 patent or other IPR claims of which I am aware have been disclosed, 15 and any of which I become aware will be disclosed, in accordance with 16 RFC 3668. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that other 20 groups may also distribute working documents as Internet-Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at http:// 28 www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on January 14, 2005. 35 Copyright Notice 37 Copyright (C) The Internet Society (2004). All Rights Reserved. 39 Abstract 41 This document describes how to express media transport over 42 connection-oriented protocols using the Session Description Protocol 43 (SDP). It defines the SDP TCP protocol identifier, the SDP setup 44 attribute, which describes the connection setup procedure, and the 45 SDP connid attribute, which provides a connection identifier. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 50 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3 52 4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4 53 4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4 54 5. The Connid Attribute . . . . . . . . . . . . . . . . . . . . . 5 55 5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6 56 5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7 57 6. Connection Management . . . . . . . . . . . . . . . . . . . . 7 58 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 59 7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8 60 7.2 Passive/Active with Connection Reestablishment . . . . . . 9 61 7.3 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9 62 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 63 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 64 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 65 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 66 11.1 Normative References . . . . . . . . . . . . . . . . . . . . 10 67 11.2 Informative References . . . . . . . . . . . . . . . . . . . 11 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 69 Intellectual Property and Copyright Statements . . . . . . . . 13 71 1. Introduction 73 The Session Description Protocol [3] provides a general-purpose 74 format for describing multimedia sessions in announcements or 75 invitations. SDP uses an entirely textual data format (the US-ASCII 76 subset of UTF-8 [10]) to maximize portability among transports. SDP 77 does not define a protocol, but only the syntax to describe a 78 multimedia session with sufficient information to participate in that 79 session. Session descriptions may be sent using arbitrary existing 80 application protocols for transport (e.g., SAP [8], SIP [9], RTSP 81 [5], email, HTTP [7], etc.). 83 SDP [3] defines two protocol identifiers: RTP/AVP and UDP, both of 84 which represent unreliable connectionless protocols. While these 85 transports are appropriate choices for multimedia streams, there are 86 applications for which connection-oriented transports, such as TCP, 87 are more appropriate. This document defines a new protocol 88 identifier, TCP, to describe TCP connetions in SDP. 90 Connection-oriented protocols introduce two new factor when 91 describing a session: how and when should end points perform the 92 connection setup procedure. This document defines two new attributes 93 to describe connection setups: setup and connid. 95 2. Terminology 97 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 98 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT 99 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as 100 described in BCP 14, RFC 2119 [2] and indicate requirement levels for 101 compliant implementations. 103 3. Protocol Identifier 105 The following is the ABNF for an m= line, as specified by RFC 2327 106 [3]. 108 media-field = "m=" media space port ["/" integer] 109 space proto 1*(space fmt) CRLF 111 This document defines a new value for the proto field: TCP. 113 The TCP protocol identifier is similar to the UDP protocol identifier 114 in that it only describes the transport protocol, and not the 115 upper-layer protocol. An m= line that specifies "TCP" MUST further 116 qualify the application-layer protocol using an fmt identifier. Media 117 lines with the TCP protocol identifier are carried using TCP [1]. 119 It is RECOMMENDED that documents defining new SDP protocol 120 identifiers that involve extra protocol layers between TCP and the 121 media itself (e.g., TLS [6] over TCP) start with the string "TCP/" 122 (e.g., TCP/TLS). 124 The following sections define the setup and the connid attributes. 125 While both attributes are applicable to m= lines that use the TCP 126 protocol identifier, they are not limited to them. These attributes 127 MAY be used in any m= line which uses a connection-oriented transport 128 protocol, even if the protocol identifier of the m= line is not TCP. 130 4. Setup Attribute 132 The setup attribute indicates which of the end points should initiate 133 the connection establishment (e.g., send the initial TCP SYN). The 134 setup attribute is charset-independent and can be a session-level or 135 a media-level attribute. The following is the ABNF of the setup 136 attribute: 138 setup-attr = "a=setup:" role 139 role = "active" / "passive" / "actpass" 140 / "holdconn" 142 Active: The endpoint will initiate an outgoing connection. 144 Passive: The endpoint will accept an incoming connection. 146 ActPass: The endpoint is willing to accept an incoming connection 147 or to initiate an outgoing connection. 149 Holdconn: The endpoint does not want the connection to be 150 established for the time being. 152 4.1 The Setup Attribute in the Offer/answer Model 154 The offer/answer model, defined in RFC 3264 [4], provides endpoints 155 with a means to obtain shared view of a session. Some session 156 parameters are negotiated (e.g., codecs to use), while others are 157 simply communicated from one endpoint to the other (e.g., IP 158 addresses). The value of the setup attribute falls into the first 159 category. That is, both endpoints negotiate its value using the 160 offer/answer model. 162 The negotiation of the value of the setup attribute takes places as 163 follows. The offerer states which role or roles it is willing to 164 perform and the answerer, taking the offerer's willingness into 165 consideration, chooses which roles both endpoints will actually 166 perform during connection establishment. The following are the values 167 that the setup attribute can take in an offer/answer exchange: 169 Offer Answer 170 ________________ 171 active passive / holdconn 172 passive active / holdconn 173 actpass active / passive / holdconn 174 holdconn holdconn 176 The active endpoint SHOULD initiate a connection to the port number 177 on the m= line of the other endpoint. The port number on its own m= 178 line is irrelevant, and the opposite endpoint MUST NOT attempt to 179 initiate a connection to the port number specified there. 180 Nevertheless, since the m= line must contain a valid port number, the 181 endpoint specifying using the value active SHOULD specify a port 182 number of 9 (the discard port) on its m= line. The endpoint MUST NOT 183 specify a port number of zero, except to denote an m= line that has 184 been or is being refused. 186 The passive endpoint SHOULD be ready to accept a connection on the 187 port number specified in the m= line. 189 A value of actpass indicates that the offerer can either initiate a 190 connection to the port number on the m= line in the answer or accept 191 a connection on the port number specified in the m= line in the 192 offer. That is, the offerer has no preference as to whether it 193 accepts or initiates the connection and, so, is letting the answerer 194 choose. 196 A value of holdconn indicates that the connection should not be 197 established for the time being. 199 The default value of the setup attribute in an offer/answer exchange 200 is active in the offer and passive in the answer. 202 5. The Connid Attribute 204 The preceding description of the setup attribute has been in the 205 context of using SDP to initiate a session. Still, SDP may be 206 exchanged between endpoints at various stages of a session to 207 accomplish tasks such as terminating a session, redirecting media to 208 a new endpoint, or renegotiating the media parameters for a session. 209 After the initial session has been established, it may be ambiguous 210 as to whether subsequent SDP exchange represents a confirmation that 211 the endpoint is to continue using the current media connection 212 unchanged, or is a request to make a new media connection. The 213 media-level connid attribute, which is charset-independent, is used 214 to disambiguate these two scenarios. The following is the ABNF of the 215 connid attribute: 217 connid = "a=connid:" connection-identifier 218 connection-identifier = token 220 The connid attribute provides an identifier for the transport-layer 221 connection used by the m= line. Connid values are meaningful in the 222 context of a particular m= line. So, different m= lines in the same 223 session description MAY have the same connid value. 225 5.1 Offerer Behaviour 227 Offerers and answerers use the connid attribute to decide whether a 228 new transport connection needs to be established or, on the other 229 hand, the existing transport connection should still be used. 231 When an offerer generates an m= line which uses a connection-oriented 232 transport, it SHOULD provide such an m= line with a connection 233 identifier using a connid attribute, unless the application using the 234 m= line has other means to deal with connection reestablishment. The 235 connid attribute in an initial offer (i.e., no transport connection 236 has been established yet) can take any value. This value identifies 237 the initial connection that the endpoints will attempt to establish. 239 After the initial offer/answer exchange, any of the endpoints can 240 generate a new offer to change some characteristics of the session 241 (e.g., the direction attribute). If such an offerer wants to continue 242 using the previously-established transport-layer connection for the 243 m= line, the offerer MUST use the same connid value for the m= line. 244 If, on the other hand, the offerer wants to establish a new 245 transport-layer connection for the m= line, it MUST use a new connid 246 value. This new connid value MUST be different from the current 247 connid value in use and SHOULD be different than any connid value 248 used previously in the same m= line. 250 The connid value in an offer is only compared with the connid 251 value currently in use. So, having a connid value different than 252 the one in use is enough to trigger the establishment of a new 253 connection. Still, it is recommended to use a value different than 254 all the previous ones used in the m= line to make debugging 255 easier. 256 Note that, according to the rules in this section, an offer that 257 changes the transport address (IP address plus port number) of an 258 m= line will have a new connid value for this m= line. 260 5.2 Answerer Behaviour 262 The connid value for an m= line is negotiated using the offer/answer 263 model. The resulting connid value after an offer/answer exchange is 264 the connid value in the answer. 266 For an m= line, if the offer contains a new connid value (i.e., 267 different from the one in use) the answerer MUST use this value in 268 the answer. If the offer contains the connid value in use and the 269 answerer wishes to continue using the existing transport-layer 270 connection, the answerer MUST use this connid value in the answer. If 271 the offer contains the connid value in use but the answerer wishes to 272 establish a new transport-layer connection, the answerer MUST use a 273 new connid value in the answer. 275 If the connid value for an m= line resulting from an offer/answer 276 exchange is different than the connid in use so far, the endpoints 277 SHOULD establish a new transport-layer connection as indicated by the 278 setup attribute. If a previous connection is still up, the endpoint 279 responsible for establishing the new connection performing the active 280 role SHOULD close it as soon as the offer/answer exchange is 281 completed. It is up to the application to ensure proper data 282 synchornization between the two connections. 284 If the connid value for an m= line resulting from an offer/answer 285 exchange is the same as the connid in use so far, the endpoints 286 SHOULD continue using the existing connection. 288 In the past, it was proposed to use the presence of a media-level 289 SDP attribute as a flag to indicate that a new connection needed 290 to be established. It was decided not to follow the flag approach 291 because an offerer whose intent was to signal "no changes" in a 292 session would need to issue a different offer than the previous 293 one (i.e., it would need to remove the flag from the m= line). By 294 using the connid attribute instead, an offerer signals "no 295 changes" in a session by issuing an identical offer to the one in 296 use. 298 6. Connection Management 300 An endpoint that according to an offer/answer exchange is supposed to 301 initiate a new connection SHOULD initiate it as soon as the offer/ 302 answer exchange is completed, even if the endpoint does not intend to 303 immediately begin sending media to the remote endpoint. This allows 304 media to flow from the remote endpoint if needed. 306 Typically, endpoints do not close the connection until the session 307 has expired, been explicitly terminated, or a new connid value has 308 been provided for the m= line. Additionaly, specific applications can 309 describe further scenarios where an end-point may close a given 310 connection. In case the session is explicitly terminated by one of 311 the endpoints (e.g., the endpoint sends a SIP [9] BYE), the end point 312 terminating the session is responsible for closing the 313 transport-connection. 315 If an endpoint determines that the transport-connection for an m= 316 line has been closed and it should be reestablished, it SHOULD 317 perform a new offer/answer exchange using a new connid value for this 318 m= line. 320 Note that the SDP direction attribute (e.g., a=sendonly) deals 321 with the media sent over the transport-connection, but has no 322 impact on the transport-connection itself. 324 7. Examples 326 The following examples show the most common usage of the setup 327 attribute combined with TCP-based media descriptions. For the purpose 328 of brevity, the main portion of the session description is omitted in 329 the examples, which only show m= lines and their attributes 330 (including c= lines). 332 7.1 Passive/Active 334 An offerer at 192.0.2.2 signals its availability for a T.38 fax 335 session at port 54111: 337 m=image 54111 TCP t38 338 c=IN IP4 192.0.2.2 339 a=setup:passive 340 a=connid:1 342 An answerer at 192.0.2.1 receiving this offer responds with the 343 following answer: 345 c=IN IP4 192.0.2.1 346 m=image 9 TCP t38 347 a=setup:active 348 a=connid:1 350 The endpoint at 192.0.2.1 then initiates the TCP connection to port 351 54111 at 192.0.2.2. 353 7.2 Passive/Active with Connection Reestablishment 355 Continuing the preceding example, consider the scenario where the TCP 356 connection fails and the endpoints wish to reestablish the connection 357 for the session. The endpoint at 192.0.2.2 signals this intent with 358 the following SDP: 360 m=image 54111 TCP t38 361 c=IN IP4 192.0.2.2 362 a=setup:passive 363 a=connid:2 365 The new connid value informs the endpoint at 192.0.2.1 that this SDP 366 represents the intent to establish a new connection for media 367 transport, rather than continuing with the original connection. If 368 192.0.2.1 agrees to continue the session using a new connection, it 369 responds with: 371 m=image 9 TCP t38 372 c=IN IP4 192.0.2.1 373 a=setup:active 374 a=connid:2 376 7.3 Actpass/Passive 378 In another example, an offerer at 192.0.2.2 signals its availability 379 for a T.38 fax session at TCP port 54111. Additionally, this offerer 380 is also willing to set up the media stream by initiating the TCP 381 connection: 383 m=image 54111 TCP t38 384 c=IN IP4 192.0.2.2 385 a=setup:actpass 386 a=connid:3 388 The endpoint at 192.0.2.1 responds with the following description: 390 m=image 54321 TCP t38 391 c=IN IP4 192.0.2.1 392 a=setup:passive 393 a=connid:3 395 This will cause the offerer (at 192.0.2.2) to initiate a connection 396 to port 54321 at 192.0.2.1. 398 8. Security Considerations 400 See RFC 2327 [3] for security and other considerations specific to 401 the Session Description Protocol in general. 403 An attacker may attempt to modify the values of the connid and setup 404 attributes to have endpoints reestablish connections unnecesaryly or 405 to keep them from establishing a connection. So, it is STRONGLY 406 RECOMMENDED that integrity protection be applied to the SDP session 407 descriptions. For session descriptions carried in SIP [9], S/MIME is 408 the natural choice to provide such end-to-end integrity protection, 409 as described in RFC 3261 [9]. Other applications MAY use a different 410 form of integrity protection. 412 9. IANA Considerations 414 This document defines two session and media level SDP attributes: 415 setup and connid. Their formats are defined in Section 4 and Section 416 5 respectively. These two attributes should be registered by the IANA 417 on 419 http://www.iana.org/assignments/sdp-parameters 421 under "att-field (both session and media level)". 423 This document defines a proto values: TCP. Its format is defined in 424 Section 3. This proto value should be registered by the IANA on 426 http://www.iana.org/assignments/sdp-parameters 428 under "proto". 430 10. Acknowledgements 432 Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul 433 Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer 434 Holmberg provided valuable insights and contributions. 436 11. References 438 11.1 Normative References 440 [1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, 441 September 1981. 443 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement 444 Levels", BCP 14, RFC 2119, March 1997. 446 [3] Handley, M. and V. Jacobson, "SDP: Session Description 447 Protocol", RFC 2327, April 1998. 449 [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 450 Session Description Protocol (SDP)", RFC 3264, June 2002. 452 11.2 Informative References 454 [5] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming 455 Protocol (RTSP)", RFC 2326, April 1998. 457 [6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 458 2246, January 1999. 460 [7] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., 461 Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- 462 HTTP/1.1", RFC 2616, June 1999. 464 [8] Handley, M., Perkins, C. and E. Whelan, "Session Announcement 465 Protocol", RFC 2974, October 2000. 467 [9] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 468 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 469 Session Initiation Protocol", RFC 3261, June 2002. 471 [10] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 472 63, RFC 3629, November 2003. 474 Authors' Addresses 476 David Yon 477 Dialout.Net, Inc 478 One Indian Head Plaza 479 Nashua, NH 03060 480 USA 482 EMail: yon@dialout.net 483 Gonzalo Camarillo 484 Ericsson 485 Hirsalantie 11 486 Jorvas 02420 487 Finland 489 EMail: Gonzalo.Camarillo@ericsson.com 491 Intellectual Property Statement 493 The IETF takes no position regarding the validity or scope of any 494 Intellectual Property Rights or other rights that might be claimed to 495 pertain to the implementation or use of the technology described in 496 this document or the extent to which any license under such rights 497 might or might not be available; nor does it represent that it has 498 made any independent effort to identify any such rights. 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Please address the information to the IETF at 513 ietf-ipr@ietf.org. 515 Disclaimer of Validity 517 This document and the information contained herein are provided on an 518 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 519 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 520 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 521 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 522 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 523 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 525 Copyright Statement 527 Copyright (C) The Internet Society (2004). This document is subject 528 to the rights, licenses and restrictions contained in BCP 78, and 529 except as set forth therein, the authors retain all their rights. 531 Acknowledgment 533 Funding for the RFC Editor function is currently provided by the 534 Internet Society.