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Camarillo 3 Internet-Draft Ericsson 4 Expires: April 15, 2004 October 16, 2003 6 The Early Session Disposition Type for the Session Initiation 7 Protocol (SIP) 8 draft-camarillo-sipping-early-disposition-00.txt 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that other 17 groups may also distribute working documents as Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six months 20 and may be updated, replaced, or obsoleted by other documents at any 21 time. It is inappropriate to use Internet-Drafts as reference 22 material or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at http:// 25 www.ietf.org/ietf/1id-abstracts.txt. 27 The list of Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html. 30 This Internet-Draft will expire on April 15, 2004. 32 Copyright Notice 34 Copyright (C) The Internet Society (2003). All Rights Reserved. 36 Abstract 38 This document defines a new disposition type (early-session) for the 39 Content-Disposition header field in SIP. The treatment of 40 "early-session" bodies is similar to the treatment of "session" 41 bodies. That is, they follow the offer/answer model. Their only 42 difference is that session descriptions whose disposition type is 43 "early-session" are used to establish early media sessions within 44 early dialogs, as opposed to regular sessions within regular dialogs. 46 Table of Contents 48 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 49 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 50 3. Issues Related to Early Media Session Establishment . . . . . . 3 51 4. The Early Session Disposition Type . . . . . . . . . . . . . . . 4 52 5. Option tag . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 53 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 54 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 55 Normative References . . . . . . . . . . . . . . . . . . . . . . 7 56 Informational References . . . . . . . . . . . . . . . . . . . . 7 57 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 58 Intellectual Property and Copyright Statements . . . . . . . . . 9 60 1. Introduction 62 A SIP [2] user agent establishing an INVITE dialog may need to 63 exchange media with the destination user agent (or user agents if the 64 INVITE forks) or with application servers in the path before the 65 dialog is established. Media exchanged this way is referred to as 66 early media. 68 Section 3 describes the current approach to establish early media 69 sessions in SIP are discusses its problems. Section 4 defines the 70 "early-session" disposition type to resolve those problems. 72 2. Terminology 74 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 75 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT 76 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as 77 described in BCP 14, RFC 2119 [1] and indicate requirement levels for 78 compliant implementations. 80 3. Issues Related to Early Media Session Establishment 82 Traditionally, early media sessions have been established in the same 83 way as regular sessions. That is, using an offer/answer exchange 84 where the disposition type of the session descriptions is "session". 85 Application servers perform an offer/answer exchange with the UAC to 86 exchange early media exclusively, while UASs use the same offer/ 87 answer exchange, first to exchange early media, and once the regular 88 dialog is established, to exchange regular media. There are issues 89 related to both, application servers and UASs using this mechanism. 91 Application servers may not be able to generate an answer for an 92 offer received in the INVITE. The UAC created the offer for the UAS, 93 and so, it may have applied end-to-end encryption or have included 94 information (e.g., related to key management) that the application 95 server is not supposed to use. Therefore, application servers need a 96 means to perform an offer/answer exchange with the UAC which is 97 independent from the offer/answer exchange between both UAs. 99 UASs using the offer/answer exchange that will carry regular media to 100 send and receive early media can cause media clipping, as described 101 in Section 2.1.1 of [5]. Some UACs cannot receive early media from 102 different UASs at the same time. So, when an INVITE forks and several 103 UASs start sending early media, the UAC mutes all the UASs but one 104 (which is usually randomly chosen). If the UAS that accepts the 105 INVITE (i.e., sends a 200 OK) was muted, a new offer/answer exchange 106 is needed to unmute it. This usually causes media clipping. 107 Therefore, UASs need a means to perform an offer/answer exchange with 108 the UAC to exchange early media which is independent from the offer/ 109 answer exchanged used to exchange regular media. 111 A potential solution to this need would be to establish a different 112 dialog using a globally routable URI to perform an independent offer/ 113 answer exchange. This dialog would be labelled as a dialog for early 114 media and would be related to the original dialog somehow at the UAC. 115 However, performing all the offer/answer exchanges within the 116 original dialog has many advantages: 118 It is simpler. 120 It does not have synchronization problems, because all the early 121 dialogs are terminated when the session is accepted. 123 It does not require globally routable URIs. 125 It does not introduce service interaction issues related to 126 services that may be wrongly applied to the new dialog. 128 It makes firewall management easier. 130 4. The Early Session Disposition Type 132 We define a new disposition type for the Content-Disposition header 133 field: early-session. User agents MUST use early-session bodies to 134 establish early media sessions in the same way as they use session 135 bodies to establish regular sessions, as described in RFC 3261 [2] 136 and in RFC 3264 [3]. Particularly, early-session bodies MUST follow 137 the offer/answer model and MAY appear in the same messages as session 138 bodies do with the exceptions of 2xx responses for an INVITE and 139 ACKs. 141 If a UA needs to refuse an early-session offer, it MUST to so by 142 refusing all the media streams in it. When SDP [4] is used, this is 143 done by setting the port number of all the media streams to zero. 145 This is the same mechanism that UACs use to refuse regular offers 146 that arrive in a response to an empty INVITE. 148 An early media session established using early-session bodies MUST be 149 terminated when its corresponding early dialog is terminated or it 150 transitions to a regular dialog. 152 5. Option tag 154 We define an option tag to be used in Require and Supported header 155 fields. Its name is early-session. A UA adding the early-session 156 option tag to a message indicates that it understands the 157 early-session disposition type. 159 6. Example 161 Figure 1 shows the message flow between two UAs. INVITE (1) has an 162 early-session option tag in its Supported header field and the body 163 shown in Figure 2. The UAS sends back a response with two body parts 164 ,as shown in Figure 3; one of disposition type session and the other 165 early-session. The session body part is the answer to the offer in 166 the INVITE. The early-session body part is an offer to establish an 167 early media session. When the UAC receives the 183 (Session Progress) 168 response, it sends the answer to the early-session offer in a PRACK, 169 as shown in Figure 4. This early media session is terminated when the 170 early dialog transitions to a regular dialog. That is, when the UAS 171 sends the (5) 200 (OK) response for the INVITE. 173 A B 174 | | 175 |--------(1) INVITE-------->| 176 | session | 177 | | 178 |<--(2) Session Progress----| 179 | early-session | 180 | session | 181 | | 182 |---------(3) PRACK-------->| 183 | early-session | 184 | | 185 |<--------(4) 200 OK--------| 186 | | 187 | * * | 188 | ************************* | 189 |* Early Media *| 190 | ************************* | 191 | * * | 192 | | 193 |---------(5) 200 OK------->| 194 | | 195 |<---------(6) ACK----------| 196 | | 198 Figure 1 200 Content-Type: application/sdp 201 Content-Disposition: session 203 v=0 204 o=alice 2890844730 2890844731 IN IP4 host.example.com 205 s= 206 c=IN IP4 192.0.0.1 207 t=0 0 208 m=audio 20000 RTP/AVP 0 210 Figure 2 212 Content-Type: multipart/mixed; boundary="--boundary1" 213 Content-Length: 399 215 --boundary1 216 Content-Type: application/sdp 217 Content-Disposition: session 219 v=0 220 o=Bob 2890844725 2890844725 IN IP4 host.example2.com 221 s= 222 c=IN IP4 192.0.0.2 223 t=0 0 224 m=audio 30000 RTP/AVP 0 225 --boundary1 226 Content-Type: application/sdp 227 Content-Disposition: early-session 229 v=0 230 o=Bob 2890844714 2890844714 IN IP4 host.example2.com 231 s= 232 c=IN IP4 192.0.0.2 233 t=0 0 234 m=audio 30002 RTP/AVP 0 235 --boundary1 237 Figure 3 239 Content-Type: application/sdp 240 Content-Disposition: early-session 242 v=0 243 o=alice 2890844717 2890844717 IN IP4 host.example.com 244 s= 245 c=IN IP4 192.0.0.1 246 t=0 0 247 m=audio 20002 RTP/AVP 0 249 Figure 4 251 7. IANA Considerations 253 This document defines a new Content-Disposition header field 254 disposition type (early-session) in Section 4. This value should be 255 registered in the IANA registry for Content-Dispositions with the 256 following description: 258 early-session the body describes an early communications 259 session, for example, an RFC 2327 SDP body 261 This document defines a SIP option tag (early-session) in Section 5. 262 It should be registered in the SIP parameters registry (http:// 263 www.iana.org/assignments/sip-parameters) under "Option Tags", with 264 the following description. 266 A UA adding the early-session option tag to a message indicates 267 that it understands the early-session content disposition. 269 Normative References 271 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 272 Levels", BCP 14, RFC 2119, March 1997. 274 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 275 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 276 Session Initiation Protocol", RFC 3261, June 2002. 278 [3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 279 Session Description Protocol (SDP)", RFC 3264, June 2002. 281 Informational References 283 [4] Handley, M. and V. Jacobson, "SDP: Session Description 284 Protocol", RFC 2327, April 1998. 286 [5] Camarillo, G. and H. Schulzrinne, "Early Media and Ringback Tone 287 Generation in the Session Initiation Protocol", 288 draft-camarillo-sipping-early-media-02 (work in progress), July 289 2003. 291 Author's Address 293 Gonzalo Camarillo 294 Ericsson 295 Hirsalantie 11 296 Jorvas 02420 297 Finland 299 EMail: Gonzalo.Camarillo@ericsson.com 301 Intellectual Property Statement 303 The IETF takes no position regarding the validity or scope of any 304 intellectual property or other rights that might be claimed to 305 pertain to the implementation or use of the technology described in 306 this document or the extent to which any license under such rights 307 might or might not be available; neither does it represent that it 308 has made any effort to identify any such rights. Information on the 309 IETF's procedures with respect to rights in standards-track and 310 standards-related documentation can be found in BCP-11. 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