RFC 4317 INFORMATIONAL SDP · SIP · EXAMPLES
Session Description Protocol (SDP) Offer/Answer Examples
Worked examples of two parties using SDP offers and answers to agree on codecs, hold calls and add or drop streams.
RFC EDITOR
Johnston, A. and R. Sparks, "Session Description Protocol (SDP) Offer/Answer Examples", RFC 4317, DOI 10.17487/RFC4317, December 2005, <https://www.rfc-editor.org/info/rfc4317>.
BIBTEX
@misc{rfc4317,
series = {Request for Comments},
number = 4317,
howpublished = {RFC 4317},
publisher = {RFC Editor},
doi = {10.17487/RFC4317},
url = {https://www.rfc-editor.org/info/rfc4317},
author = {A. Johnston and R. Sparks},
title = {{Session Description Protocol (SDP) Offer/Answer Examples}},
pagetotal = 24,
year = 2005,
month = dec,
} RFC text
Contents
Network Working Group A. Johnston
Request for Comments: 4317 Tello Corporation
Category: Informational R. Sparks
Estacado Systems
December 2005
Session Description Protocol (SDP)
Offer/Answer Examples
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
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.
Table of Contents
1. Overview ........................................................3
2. Codec Negotiation and Selection .................................3
2.1. Audio and Video 1 ..........................................3
2.2. Audio and Video 2 ..........................................4
2.3. Audio and Video 3 ..........................................5
2.4. Two Audio Streams ..........................................6
2.5. Audio and Video 4 ..........................................7
2.6. Audio Only 1 ...............................................8
2.7. Audio and Video 5 ..........................................9
2.8. Audio and Video 6 .........................................10
3. Hold and Resume Scenarios ......................................12
3.1. Hold and Unhold 1 .........................................12
3.2. Hold with Two Streams .....................................13
4. Addition and Deletion of Media Streams .........................15
4.1. Second Audio Stream Added .................................15
4.2. Audio, then Video Added ...................................16
4.3. Audio and Video, Then Video Deleted .......................17
5. Third Party Call Control (3pcc) ................................19
5.1. No Media, Then Audio Added ................................19
5.2. Hold and Unhold 2 .........................................20
5.3. Hold and Unhold 3 .........................................21
6. Security Considerations ........................................22
7. Informative References .........................................22
1. Overview
This document describes offer/answer examples of Session Description
Protocol (SDP) based on RFC 3264 [1]. The SDP in these examples is
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, additional
offers and answers. Note that the subject line (s=) contains a
single space character.
2. Codec Negotiation and Selection
2.1. Audio and Video 1
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 and Bob may send only PCMU audio and
MPV video. Note: Dynamic payload type 97 is used for iLBC codec [6].
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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/8000
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.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.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 and Video 2
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. An audio-only
session is established in the 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 second 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
second exchange. After this exchange, only the 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.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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/8000
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.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.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
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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
[Second-Answer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31
a=rtpmap:31 H261/90000
2.3. Audio and Video 3
Alice offers three audio and two video codecs, while Bob accepts with
a single audio and video codec. As a result of this exchange, Bob
and Alice use iLBC for audio and H261 for video.
Note: change of dynamic payload type from 97 to 99 between the offer
and the answer is OK since the same codec is referenced.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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/8000
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.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 99
a=rtpmap:99 iLBC/8000
m=video 51374 RTP/AVP 31
a=rtpmap:31 H261/90000
2.4. Two Audio Streams
In this example, Alice wishes to establish separate audio streams,
one for normal audio and the other for telephone-events. Alice
offers two separate streams, one audio with two codecs and the other
with RFC 2833 [4] tones (for DTMF). Bob accepts both audio streams
choosing the iLBC codec and telephone-events.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 97
a=rtpmap:0 PCMU/8000
a=rtpmap:97 iLBC/8000
m=audio 49172 RTP/AVP 98
a=rtpmap:98 telephone-event/8000
a=sendonly
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=audio 49174 RTP/AVP 98
a=rtpmap:98 telephone-event/8000
a=recvonly
2.5. Audio and Video 4
Alice and Bob establish an audio and video session with a single
audio and video codec. In a second exchange, Bob changes his address
for media and Alice accepts with the same SDP as the initial exchange
(and as a result does not increment the version number).
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49174 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 49170 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 newhost.biloxi.example.com
t=0 0
m=audio 49178 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 49188 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Answer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000
2.6. Audio Only 1
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 sent a second declining one of the
streams, as shown in Section 4.3.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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/8000
a=rtpmap:101 telephone-event/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.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/8000
a=rtpmap:101 telephone-event/8000
2.7. Audio and Video 5
Alice and Bob establish an audio and video session in the first
exchange with a single audio and video codec. In the second
exchange, Alice adds a second video codec, which Bob accepts. This
allows Alice and Bob to switch between the two video codecs without
another offer/answer exchange.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 99
a=rtpmap:99 iLBC/8000
m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 99
a=rtpmap:99 iLBC/8000
m=video 51374 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 99
a=rtpmap:99 iLBC/8000
m=video 51372 RTP/AVP 31 32
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
[Second-Answer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 99
a=rtpmap:99 iLBC/8000
m=video 51374 RTP/AVP 31 32
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
2.8. Audio and Video 6
This example shows an audio and video offer that is accepted, but the
answerer wants the video sent to a different address than that of 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.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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/8000
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.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49174 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 49172 RTP/AVP 32
c=IN IP4 otherhost.biloxi.example.com
a=rtpmap:32 MPV/90000
3. Hold and Resume Scenarios
3.1. Hold and Unhold 1
Alice calls Bob, but when Bob answers he places Alice on hold. Bob
then takes Alice off hold in the second offer. Alice changes port
number in the second exchange. The media session between Alice and
Bob is now active after Alice's second answer. Note that a=sendrecv
could be present in both second offer and answer exchange. This is a
common flow in 3pcc [5] scenarios.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 97
a=rtpmap:0 PCMU/8000
a=rtpmap:97 iLBC/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 placeholder.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
a=sendonly
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49178 RTP/AVP 97
a=rtpmap:97 iLBC/8000
3.2. Hold with Two Streams
In this example, two audio streams have been established in the first
offer/answer exchange. In this second offer/answer exchange, one of
the audio streams is placed on hold. Alice offers two media streams,
a bidirectional audio stream and a send-only telephone event stream.
Bob accepts both 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.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 97
a=rtpmap:0 PCMU/8000
a=rtpmap:97 iLBC/8000
m=audio 49172 RTP/AVP 98
a=rtpmap:98 telephone-event/8000
a=sendonly
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=audio 49174 RTP/AVP 98
a=rtpmap:98 telephone-event/8000
a=recvonly
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
a=sendonly
m=audio 49174 RTP/AVP 98
a=rtpmap:98 telephone-event/8000
a=recvonly
[Second-Answer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 97
a=rtpmap:0 PCMU/8000
a=rtpmap:97 iLBC/8000
m=audio 49172 RTP/AVP 98
a=rtpmap:98 telephone-event/8000
a=sendonly
4. Addition and Deletion of Media Streams
This section shows addition and deletion of media streams.
4.1. Second Audio Stream Added
In this example, the first offer/answer exchange establishes a single
audio stream with a single codec. The second offer/answer exchange
adds a second audio stream for telephone events. 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 second stream is
unidirectional, Alice receives RTCP packets on port 49173 from the
media server.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 97
a=rtpmap:0 PCMU/8000
a=rtpmap:97 iLBC/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=audio 48282 RTP/AVP 98
c=IN IP4 mediaserver.biloxi.example.com
a=rtpmap:98 telephone-event/8000
a=recvonly
[Second-Answer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=audio 49172 RTP/AVP 98
c=IN IP4 host.atlanta.example.com
a=rtpmap:98 telephone-event/8000
a=sendonly
4.2. Audio, then Video Added
An audio-only session is established in the initial exchange between
Alice and Bob using PCMU codec. Alice adds a video stream that is
accepted by Bob.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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
[Second-Answer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 49168 RTP/AVP 31
a=rtpmap:31 H261/90000
4.3. Audio and Video, Then Video Deleted
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.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49174 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 49170 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49174 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 0 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Answer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
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, Then Audio Added
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 that Bob accepts.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
5.2. Hold and Unhold 2
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.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 0.0.0.0
t=0 0
m=audio 23442 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
5.3. Hold and Unhold 3
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
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.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 0.0.0.0
t=0 0
m=audio 9322 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Offer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 97
a=rtpmap:97 iLBC/8000
[Second-Answer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 97
a=rtpmap:97 iLBC/8000
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.
It is important that SDP offer and answer messages be properly
authenticated and authorized before they are used to establish a
media session. Examples of SIP mechanisms include SIP Digest, certs,
and cryptographically-verified SIP identity.
7. Informative 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. Petrack, "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 (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
April 2004.
[6] Duric, A. and S. Andersen, "Real-time Transport Protocol (RTP)
Payload Format for internet Low Bit Rate Codec (iLBC) Speech",
RFC 3952, December 2004.
Authors' Addresses
Alan Johnston
Tello Corporation
999 Baker Way, Suite 250
San Mateo, CA 94404
EMail: ajohnston@tello.com
Robert J. Sparks
Estacado Systems
EMail: rjsparks@estacado.net
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
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 procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
PROBLEMThe SDP offer/answer model can be hard to apply correctly, and implementers need concrete cases showing what each side should send.
MECHANISMThe RFC walks through exchanges between two parties, Alice and Bob, with full offers and answers: codec negotiation, hold and resume, adding and deleting media streams and third party call control flows.
STATUS TODAYIt is an Informational set of examples that defines no new protocol. It builds on the offer/answer model of RFC 3264 and the SDP of RFC 2327.
§1What the Examples Cover
The document gives offer/answer examples of the Session Description Protocol (SDP), based on the offer/answer model of RFC 3264. The SDP itself is as defined in RFC 2327. The offers and answers are assumed to be carried by a protocol such as the Session Initiation Protocol (RFC 3261).
In every example two parties, Alice and Bob, exchange an offer and an answer, and in some cases a second offer and answer. The examples show several media types, bidirectional, unidirectional and inactive streams, and dynamic payload types. The RFC notes that the subject line (s=) in each example contains a single space character.
Alice and Bob are two callers who trade little notes saying what they can do. The RFC shows many sample note swaps.
§2Codec Negotiation and Selection
In the first example Alice offers audio codecs PCMU, PCMA and iLBC (payload types 0, 8 and 97) and video codecs H261 and MPV. Bob answers with one of each, so the session uses only PCMU audio and MPV video. Payload type 97 is a dynamic payload type for iLBC.
Other examples show how the answer shapes the call. If Alice can support only one audio codec at a time, she makes a second offer with just that one codec. The version number in the o= line is incremented in both messages of that second exchange. A declined stream stays in the SDP with its port set to zero. A dynamic payload type may change between offer and answer (97 to 99 in one example) because the same codec is referenced.
Further cases: separate audio streams for normal audio and for telephone-events (RFC 2833 tones, for DTMF); a stream with a different connection address for video than for audio, common in conferencing where mixing uses different servers; and adding a second video codec so that the parties can switch without another exchange. When Bob changes his media address and Alice accepts with the same SDP as before, she does not increment the version number.
Alice lists all the ways she can talk. Bob picks the ones he also knows. If she wants to be sure of one way, she asks again with only that one.
Alice's offer
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.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/8000
m=video 51372 RTP/AVP 31 32
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000§3Hold and Resume
Hold is expressed with the direction attributes sendonly and recvonly. In one example Alice calls Bob, but when Bob answers he places Alice on hold: his answer marks the audio stream a=sendonly and uses a placeholder connection address. In his second offer, Bob takes Alice off hold. Alice changes her port number in her second answer, and the media session is then active. The RFC notes a=sendrecv could be present in the second exchange, and that this is a common flow in third party call control.
A second example has two audio streams, a bidirectional audio stream and a send-only telephone event stream. Bob puts only the audio stream on hold, not the tone stream, and Alice answers with SDP identical to her initial offer.
To pause a call, one side says it will only send, not listen. To resume, it sends a fresh note with the normal settings.
AliceBob
- OfferAudio with PCMU and iLBC.
- Answer (sendonly)Bob places Alice on hold.
- Second offerBob takes Alice off hold.
- Second answerAlice uses a new port; media is active.
§4Adding and Deleting Streams
A second exchange can add a stream. In one example the first exchange sets up a single audio stream and the second adds an audio stream for telephone events, supplied by Bob's media server (a different connection address) to receive RFC 2833 telephone-events, typically DTMF digits, from Alice. In another, an audio-only session using PCMU gets a video stream added by Alice and accepted by Bob.
A stream is deleted by setting its port to zero. When Bob deletes the video stream from an audio and video session, the media line stays in the SDP with port 0 in both his second offer and Alice's second answer, leaving an audio-only session. The RFC also notes that if both streams of an offer had been accepted, Alice would have sent a second offer declining one of them.
To add video, you add a new line to the note. To drop video, you keep the line but write zero as its port, which means no thanks.
§5Third Party Call Control
Third Party Call Control (3pcc, RFC 3725) flows are common, and hold and resume flows are common in them. Three examples are given.
- An offer with no media lines is answered with no media lines. In the second exchange Alice adds an audio stream that Bob accepts.
- Alice's first offer has connection address
0.0.0.0and a random port number, so Bob cannot send media to her (the stream is "black holed"). In the second exchange she changes the address and a media session is established. - The same idea in reverse: Bob's answer carries
0.0.0.0, so Alice cannot send media to him, until Bob changes the address in a second offer.
Sometimes a helper sets up the call between two people. The helper can start with an address that means nowhere, then swap in the real address when everyone is ready.
§6Security Considerations
Offers and answers can contain private information about addresses and sessions. If that must stay private, the protocol carrying them must provide security. For SIP, this means TLS transport and/or S/MIME encryption of the SDP body.
Offers and answers also need to be properly authenticated and authorized before they are used to set up a media session. The RFC names SIP Digest, certificates and cryptographically-verified SIP identity as examples of SIP mechanisms.
The notes say who is calling from where, so they should be kept private, and you should check who sent them.
§7References
[1]RFC 3264Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.July 2002[2]RFC 2327Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.April 1998[3]RFC 3261Rosenberg, J., et al., "SIP: Session Initiation Protocol", RFC 3261, June 2002.July 2002[4]RFC 2833Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals", RFC 2833, May 2000.May 2000[5]RFC 3725Rosenberg, 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.April 2004[6]RFC 3952Duric, A. and S. Andersen, "Real-time Transport Protocol (RTP) Payload Format for internet Low Bit Rate Codec (iLBC) Speech", RFC 3952, December 2004.December 2004