MMUSIC Working Group A. Johnston Internet-Draft WorldCom Expires: April 27, 2003 R. Sparks dynamicsoft October 27, 2002 Session Description Protocol Offer Answer Examples draft-johnston-mmusic-offer-answer-examples-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 April 27, 2003. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document gives examples of Session Description Protocol (SDP) offer answer exchanges. Examples include codec negotiation and selection, hold and resume, and addition and deletion of media streams. The examples show multiple media types, bidirectional, unidirectional, inactive streams and dynamic payload types. Common Third Party Call Control (3pcc) examples are also given. Johnston & Sparks Expires April 27, 2003 [Page 1] Internet-Draft SDP Offer Answer Examples October 2002 Table of Contents 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Codec Negotiation and Selection . . . . . . . . . . . . . . . 3 2.1 Audio: 3 offered, 1 accepted, Video: 2 offered, 1 accepted . . 3 2.2 Audio: 3 offered, 2 accepted, Video declined, second exchange Audio: 1 offered, 1 accepted . . . . . . . . . . . . 4 2.3 Audio: 3 offered, 1 accepted with different dynamic payload, Video: 2 offered, 1 accepted . . . . . . . . . . . . 6 2.4 Audio: sendrecv and sendonly established . . . . . . . . . . . 7 2.5 Audio and Video established in first exchange, second exchange changes address and port . . . . . . . . . . . . . . 7 2.6 Two audio streams offered, one accepted, one declined . . . . 9 2.7 Audio and Video session established, second video codec added in second exchange . . . . . . . . . . . . . . . . . . . 9 2.8 Audio: 3 offered, 1 accepted, Video: 2 offered, 1 accepted with different address . . . . . . . . . . . . . . . . . . . . 11 3. Hold and Resume Scenarios . . . . . . . . . . . . . . . . . . 12 3.1 Audio: offered, placed on hold in answer, counter offer taken off hold . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2 Two Audio Streams established in initial exchange - one is put on hold in second exchange . . . . . . . . . . . . . . . . 13 4. Addition and Deletion of Media Streams . . . . . . . . . . . . 14 4.1 Audio: 1 stream established in first exchange, 2nd exchange establishes 2nd one-way stream . . . . . . . . . . . 14 4.2 Audio session established, video added after second exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.3 Two media streams established in the first exchange, one is deleted in the second exchange . . . . . . . . . . . . . . 17 5. Third Party Call Control (3pcc) . . . . . . . . . . . . . . . 18 5.1 No media offered and accepted, media added in second exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2 Audio: 0.0.0.0 in offer, accepted, second exchange, address changed and stream established . . . . . . . . . . . . 19 5.3 Audio: offered, accepted with address 0.0.0.0, second exchange, address changed and stream established . . . . . . . 20 6. Security Considerations . . . . . . . . . . . . . . . . . . . 22 References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 23 Johnston & Sparks Expires April 27, 2003 [Page 2] Internet-Draft SDP Offer Answer Examples October 2002 1. Overview This document describes offer answer examples of Session Description Protocol (SDP) based on RFC 3264 [1]. The SDP in these examples are defined by RFC 2327 [2]. The offers and answers are assumed to be transported using a protocol such as Session Initiation Protocol (SIP) [3]. Examples include codec negotiation and selection, hold and resume, and addition and deletion of media streams. The examples show multiple media types, bidirectional, unidirectional, inactive streams and dynamic payload types. Common Third Party Call Control (3pcc) [5] examples are also given. The following sections contain examples in which two parties, Alice and Bob, exchange SDP offers, answers, and, in some cases, counter (additional) offers and answers. 2. Codec Negotiation and Selection 2.1 Audio: 3 offered, 1 accepted, Video: 2 offered, 1 accepted This common scenario shows a video and audio session in which multiple codecs are offered but only one is accepted. As a result of the exchange shown below, Alice may send only PCMU audio and MPV video, but Bob may send either PCMU, PCMA, or iLBC audio and either H261 or MPV video. Note: Dynamic payload type 97 is used for iLBC codec. Johnston & Sparks Expires April 27, 2003 [Page 3] Internet-Draft SDP Offer Answer Examples October 2002 [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 8 97 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 32 a=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49174 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 49170 RTP/AVP 32 a=rtpmap:32 MPV/90000 2.2 Audio: 3 offered, 2 accepted, Video declined, second exchange Audio: 1 offered, 1 accepted Alice can support PCMU, PCMA, and iLBC codecs, but not more than one at the same time. Alice offers all three to maximize chances of a successful exchange and Bob accepts two of them. Audio only session is established in initial exchange between Alice and Bob using either PCMU or PCMA codecs (payload type in RTP packet tells which is being used). Since Alice only supports one audio codec at a time, a counter offer is made with just that one codec to limit the codec choice to just one. Note: the version number is incremented in both SDP messages in the counter exchange. Now only PCMU codec may be used for media session between Alice and Bob. Note: The declined video stream still present in the second exchange of SDP with ports set to zero. Johnston & Sparks Expires April 27, 2003 [Page 4] Internet-Draft SDP Offer Answer Examples October 2002 [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 8 97 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 32 a=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 0 8 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 m=video 0 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Offer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 51372 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 0 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Answer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 0 Johnston & Sparks Expires April 27, 2003 [Page 5] Internet-Draft SDP Offer Answer Examples October 2002 a=rtpmap:0 PCMU/8000 m=video 0 RTP/AVP 31 a=rtpmap:31 H261/90000 2.3 Audio: 3 offered, 1 accepted with different dynamic payload, Video: 2 offered, 1 accepted As a result of this exchange, Bob can send with either PCMU, PCMA, or iLBC for audio and H261 or MPV for video. Alice can send with iLBC and H261. Note: change of dynamic payload type from 97 to 99 between the offer and the answer is OK since it references same codec. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 8 97 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 32 a=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 99 a=rtpmap:99 iLBC m=video 51374 RTP/AVP 31 a=rtpmap:31 H261/90000 Johnston & Sparks Expires April 27, 2003 [Page 6] Internet-Draft SDP Offer Answer Examples October 2002 2.4 Audio: sendrecv and sendonly established Alice sends but can not receive RFC 2833 tones [4] in a separate audio stream. Bob accepts both audio streams. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 97 a=rtpmap:0 PCMU/8000 a=rtpmap:97 iLBC m=audio 49172 RTP/AVP 98 a=rtpmap:98 telephone-event a=sendonly [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 97 a=rtpmap:97 iLBC m=audio 49174 RTP/AVP 98 a=rtpmap:98 telephone-event a=recvonly 2.5 Audio and Video established in first exchange, second exchange changes address and port Alice and Bob establish an audio and video session. In a second exchange, Bob changes his address for media and Alice accepts with the same SDP as the initial exchange (and does not increment the version number). [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= Johnston & Sparks Expires April 27, 2003 [Page 7] Internet-Draft SDP Offer Answer Examples October 2002 c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 a=rtpmap:31 H261/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49174 RTP/AVP 97 a=rtpmap:0 PCMU/8000 m=video 49170 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Offer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 newhost.there.com t=0 0 m=audio 49178 RTP/AVP 97 a=rtpmap:0 PCMU/8000 m=video 49188 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Answer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 a=rtpmap:31 H261/90000 Johnston & Sparks Expires April 27, 2003 [Page 8] Internet-Draft SDP Offer Answer Examples October 2002 2.6 Two audio streams offered, one accepted, one declined Alice wishes to establish an audio session with Bob using either PCMU codec or iLBC codec with RFC2833 tones, but not both at the same time. The offer contains these two media streams. Bob declines the first one and accepts the second one. If both media streams had been accepted, Alice would have counter offered declining one of the streams, as shown in Section 4.3. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=audio 51372 RTP/AVP 97 101 a=rtpmap:97 iLBC a=rtpmap:101 telephone-events [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 0 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=audio 49170 RTP/AVP 97 101 a=rtpmap:97 iLBC a=rtpmap:101 telephone-events 2.7 Audio and Video session established, second video codec added in second exchange Alice and Bob establish an audio and video session in the first exchange. In the second exchange, Alice adds a second video codec which Bob accepts. Johnston & Sparks Expires April 27, 2003 [Page 9] Internet-Draft SDP Offer Answer Examples October 2002 [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 99 a=rtpmap:99 iLBC m=video 51372 RTP/AVP 31 a=rtpmap:31 H261/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 99 a=rtpmap:99 iLBC m=video 51374 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Offer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 99 a=rtpmap:99 iLBC m=video 51372 RTP/AVP 31 32 a=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 [Counter-Answer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 99 a=rtpmap:99 iLBC m=video 51374 RTP/AVP 31 32 a=rtpmap:31 H261/90000 Johnston & Sparks Expires April 27, 2003 [Page 10] Internet-Draft SDP Offer Answer Examples October 2002 a=rtpmap:32 MPV/90000 2.8 Audio: 3 offered, 1 accepted, Video: 2 offered, 1 accepted with different address This scenario shows an audio and video offer that is accepted, but the answerer wants the video sent to a different address than the audio. This is a common scenario in conferencing where the video and audio mixing utilizes different servers. In this example, Alice offers audio and video and Bob accepts. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 8 97 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 32 a=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49174 RTP/AVP 0 a=rtpmap:0 PCMU/8000 c=IN IP4 otherhost.there.com m=video 49172 RTP/AVP 32 a=rtpmap:32 MPV/90000 Johnston & Sparks Expires April 27, 2003 [Page 11] Internet-Draft SDP Offer Answer Examples October 2002 3. Hold and Resume Scenarios 3.1 Audio: offered, placed on hold in answer, counter offer taken off hold Alice calls Bob, but Bob answers placing Alice on hold. Bob then takes Alice off hold in the counter offer. Alice changes port number in the second exchange. The media session between Alice and Bob is now active after Alice's counter answer. Note that a=sendrecv could be present in both counter-offer and counter-exchange. This is a common flow in 3pcc [5] scenarios. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 97 a=rtpmap:0 PCMU/8000 a=rtpmap:97 iLBC [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 placeholder.there.com t=0 0 m=audio 49172 RTP/AVP 97 a=rtpmap:97 iLBC a=sendonly [Counter-Offer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Answer] v=0 Johnston & Sparks Expires April 27, 2003 [Page 12] Internet-Draft SDP Offer Answer Examples October 2002 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49178 RTP/AVP 97 a=rtpmap:97 iLBC 3.2 Two Audio Streams established in initial exchange - one is put on hold in second exchange Alice sends but can not receive RFC2833 tones in a separate audio stream. Bob accepts both audio streams. Bob then puts Alice's audio stream on hold but not the tone stream. Alice responds with identical SDP to the initial offer. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 97 a=rtpmap:0 PCMU/8000 a=rtpmap:97 iLBC m=audio 49172 RTP/AVP 98 a=rtpmap:98 telephone-event a=sendonly [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 97 a=rtpmap:97 iLBC m=audio 49174 RTP/AVP 98 a=rtpmap:98 telephone-event a=recvonly [Counter-Offer] v=0 Johnston & Sparks Expires April 27, 2003 [Page 13] Internet-Draft SDP Offer Answer Examples October 2002 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 97 a=rtpmap:97 iLBC a=sendonly m=audio 49174 RTP/AVP 98 a=rtpmap:98 telephone-event a=recvonly [Counter-Answer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 97 a=rtpmap:0 PCMU/8000 a=rtpmap:97 iLBC m=audio 49172 RTP/AVP 98 a=rtpmap:98 telephone-event a=sendonly 4. Addition and Deletion of Media Streams This section shows addition and deletion of media streams. 4.1 Audio: 1 stream established in first exchange, 2nd exchange establishes 2nd one-way stream The second stream is added by Bob's media server (different connection address) to receive RFC 2833 telephone-events (DTMF digits, typically) from Alice. Alice accepts. Even though the 2nd stream is unidirectional, Alice receives RTCP packets on port 49173 from the media server. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 Johnston & Sparks Expires April 27, 2003 [Page 14] Internet-Draft SDP Offer Answer Examples October 2002 m=audio 49170 RTP/AVP 0 97 a=rtpmap:0 PCMU/8000 a=rtpmap:97 iLBC [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Offer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC c=IN IP4 mediaserver.there.com m=audio 48282 RTP/AVP 98 a=rtpmap:98 telephone-events a=recvonly [Counter-Answer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC c=IN IP4 host.anywhere.com m=audio 49172 RTP/AVP 98 a=rtpmap:98 telephone-events a=sendonly 4.2 Audio session established, video added after second exchange Audio only session is established in initial exchange between Alice and Bob using PCMU codec. Alice adds a video stream which is Johnston & Sparks Expires April 27, 2003 [Page 15] Internet-Draft SDP Offer Answer Examples October 2002 accepted by Bob. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/800 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000 [Counter-Offer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 49172 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Answer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49170 RTP/AVP 0 a=rtpmap:0 PCMU/8000 m=video 49168 RTP/AVP 31 a=rtpmap:31 H261/90000 Johnston & Sparks Expires April 27, 2003 [Page 16] Internet-Draft SDP Offer Answer Examples October 2002 4.3 Two media streams established in the first exchange, one is deleted in the second exchange Alice and Bob establish an audio and video session. In a second exchange, Bob deletes the video session resulting in an audio only session. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC m=video 51372 RTP/AVP 31 a=rtpmap:31 H261/90000 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49174 RTP/AVP 97 a=rtpmap:0 PCMU/8000 m=video 49170 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Offer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49174 RTP/AVP 97 a=rtpmap:0 PCMU/8000 m=video 0 RTP/AVP 31 a=rtpmap:31 H261/90000 [Counter-Answer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= Johnston & Sparks Expires April 27, 2003 [Page 17] Internet-Draft SDP Offer Answer Examples October 2002 c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC m=video 0 RTP/AVP 31 a=rtpmap:31 H261/90000 5. Third Party Call Control (3pcc) This section shows examples common in Third Party Call Control (3pcc) flows [5]. Call hold and resume flows are also common in 3pcc. 5.1 No media offered and accepted, media added in second exchange The first offer from Alice contains no media lines, so Bob accepts with no media lines. In the second exchange, Alice adds an audio stream which Bob accepts. Johnston & Sparks Expires April 27, 2003 [Page 18] Internet-Draft SDP Offer Answer Examples October 2002 [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 [Counter-Offer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Answer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 97 a=rtpmap:97 iLBC 5.2 Audio: 0.0.0.0 in offer, accepted, second exchange, address changed and stream established The first offer from Alice contains the connection address 0.0.0.0 and a random port number, which means that Bob can not send media to Alice (the media stream is "black holed" or "bh"). Bob accepts with normal SDP. In the second exchange, Alice changes the connection address, Bob accepts, and a media session is established. Johnston & Sparks Expires April 27, 2003 [Page 19] Internet-Draft SDP Offer Answer Examples October 2002 [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 0.0.0.0 t=0 0 m=audio 23442 RTP/AVP 97 a=rtpmap:97 iLBC [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Offer] v=0 o=alice 2890844526 2890844527 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC 5.3 Audio: offered, accepted with address 0.0.0.0, second exchange, address changed and stream established The first offer from Alice contains an audio stream, but the answer from Bob contains the connection address 0.0.0.0 and a random port Johnston & Sparks Expires April 27, 2003 [Page 20] Internet-Draft SDP Offer Answer Examples October 2002 number, which means that Alice can not send media to Bob (the media stream is "black holed" or "bh"). In the second exchange, Bob changes the connection address, Alice accepts, and a media session is established. [Offer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC [Answer] v=0 o=bob 2808844564 2808844564 IN IP4 host.there.com s= c=IN IP4 0.0.0.0 t=0 0 m=audio 9322 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Offer] v=0 o=bob 2808844564 2808844565 IN IP4 host.there.com s= c=IN IP4 host.there.com t=0 0 m=audio 49172 RTP/AVP 97 a=rtpmap:97 iLBC [Counter-Answer] v=0 o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4 host.anywhere.com t=0 0 m=audio 49170 RTP/AVP 97 a=rtpmap:97 iLBC Johnston & Sparks Expires April 27, 2003 [Page 21] Internet-Draft SDP Offer Answer Examples October 2002 6. Security Considerations SDP offer and answer messages can contain private information about addresses and sessions to be established between parties. If this information needs to be kept private, some security mechanism in the protocol used to carry the offers and answers must be used. For SIP, this means using TLS transport and/or S/MIME encryption of the SDP message body. References [1] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [3] 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. [4] Schulzrinne, H. and S. Petrak, "RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals", RFC 2833, May 2000. [5] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, "Best Current Practices for Third Party Call Control in the Session Initiation Protocol", draft-ietf-sipping-3pcc-02 (work in progress), June 2002. Authors' Addresses Alan Johnston WorldCom 100 South 4th Street St. Louis, MO 63104 EMail: alan.johnston@wcom.com Robert J. Sparks dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 EMail: rsparks@dynamicsoft.com Johnston & Sparks Expires April 27, 2003 [Page 22] Internet-Draft SDP Offer Answer Examples October 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Johnston & Sparks Expires April 27, 2003 [Page 23]