Offer_answer_with_SDP_rfc3264

Network Working Group J. Rosenberg
Request for Comments: 3264 dynamicsoft
Obsoletes: 2543 H. Schulzrinne
Category: Standards Track Columbia U.
June 2002

An Offer/Answer Model with the Session Description Protocol (SDP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

This document defines a mechanism by which two entities can make use
of the Session Description Protocol (SDP) to arrive at a common view
of a multimedia session between them. In the model, one participant
offers the other a description of the desired session from their
perspective, and the other participant answers with the desired
session from their perspective. This offer/answer model is most
useful in unicast sessions where information from both participants
is needed for the complete view of the session. The offer/answer
model is used by protocols like the Session Initiation Protocol
(SIP).

Table of Contents

1 Introduction ........................................ 2
2 Terminology ......................................... 3
3 Definitions ......................................... 3
4 Protocol Operation .................................. 4
5 Generating the Initial Offer ........................ 5
5.1 Unicast Streams ..................................... 5
5.2 Multicast Streams ................................... 8
6 Generating the Answer ............................... 9
6.1 Unicast Streams ..................................... 9
6.2 Multicast Streams ................................... 12
7 Offerer Processing of the Answer .................... 12
8 Modifying the Session ............................... 13

8.1 Adding a Media Stream ............................... 13
8.2 Removing a Media Stream ............................. 14
8.3 Modifying a Media Stream ............................ 14
8.3.1 Modifying Address, Port or Transport ................ 14
8.3.2 Changing the Set of Media Formats ................... 15
8.3.3 Changing Media Types ................................ 17
8.3.4 Changing Attributes ................................. 17
8.4 Putting a Unicast Media Stream on Hold .............. 17
9 Indicating Capabilities ............................. 18
10 Example Offer/Answer Exchanges ...................... 19
10.1 Basic Exchange ...................................... 19
10.2 One of N Codec Selection ............................ 21
11 Security Considerations ............................. 23
12 IANA Considerations ................................. 23
13 Acknowledgements .................................... 23
14 Normative References ................................ 23
15 Informative References .............................. 24
16 Authors' Addresses .................................. 24
17 Full Copyright Statement............................. 25

1 Introduction

The Session Description Protocol (SDP) [1] was originally conceived
as a way to describe multicast sessions carried on the Mbone. The
Session Announcement Protocol (SAP) [6] was devised as a multicast
mechanism to carry SDP messages. Although the SDP specification
allows for unicast operation, it is not complete. Unlike multicast,
where there is a global view of the session that is used by all
participants, unicast sessions involve two participants, and a
complete view of the session requires information from both
participants, and agreement on parameters between them.

As an example, a multicast session requires conveying a single
multicast address for a particular media stream. However, for a
unicast session, two addresses are needed - one for each participant.
As another example, a multicast session requires an indication of
which codecs will be used in the session. However, for unicast, the
set of codecs needs to be determined by finding an overlap in the set
supported by each participant.

As a result, even though SDP has the expressiveness to describe
unicast sessions, it is missing the semantics and operational details
of how it is actually done. In this document, we remedy that by
defining a simple offer/answer model based on SDP. In this model,
one participant in the session generates an SDP message that
constitutes the offer - the set of media streams and codecs the
offerer wishes to use, along with the IP addresses and ports the
offerer would like to use to receive the media. The offer is

conveyed to the other participant, called the answerer. The answerer
generates an answer, which is an SDP message that responds to the
offer provided by the offerer. The answer has a matching media
stream for each stream in the offer, indicating whether the stream is
accepted or not, along with the codecs that will be used and the IP
addresses and ports that the answerer wants to use to receive media.

It is also possible for a multicast session to work similar to a
unicast one; its parameters are negotiated between a pair of users as
in the unicast case, but both sides send packets to the same
multicast address, rather than unicast ones. This document also
discusses the application of the offer/answer model to multicast
streams.

We also define guidelines for how the offer/answer model is used to
update a session after an initial offer/answer exchange.

The means by which the offers and answers are conveyed are outside
the scope of this document. The offer/answer model defined here is
the mandatory baseline mechanism used by the Session Initiation
Protocol (SIP) [7].

2 Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant implementations.

3 Definitions

The following terms are used throughout this document:

Agent: An agent is the protocol implementation involved in the
offer/answer exchange. There are two agents involved in an
offer/answer exchange.

Answer: An SDP message sent by an answerer in response to an offer
received from an offerer.

Answerer: An agent which receives a session description from
another agent describing aspects of desired media
communication, and then responds to that with its own session
description.

Media Stream: From RTSP [8], a media stream is a single media
instance, e.g., an audio stream or a video stream as well as a
single whiteboard or shared application group. In SDP, a media
stream is described by an "m=" line and its associated
attributes.

Offer: An SDP message sent by an offerer.

Offerer: An agent which generates a session description in order
to create or modify a session.

4 Protocol Operation

The offer/answer exchange assumes the existence of a higher layer
protocol (such as SIP) which is capable of exchanging SDP for the
purposes of session establishment between agents.

Protocol operation begins when one agent sends an initial offer to
another agent. An offer is initial if it is outside of any context
that may have already been established through the higher layer
protocol. It is assumed that the higher layer protocol provides
maintenance of some kind of context which allows the various SDP
exchanges to be associated together.

The agent receiving the offer MAY generate an answer, or it MAY
reject the offer. The means for rejecting an offer are dependent on
the higher layer protocol. The offer/answer exchange is atomic; if
the answer is rejected, the session reverts to the state prior to the
offer (which may be absence of a session).

At any time, either agent MAY generate a new offer that updates the
session. However, it MUST NOT generate a new offer if it has
received an offer which it has not yet answered or rejected.
Furthermore, it MUST NOT generate a new offer if it has generated a
prior offer for which it has not yet received an answer or a
rejection. If an agent receives an offer after having sent one, but
before receiving an answer to it, this is considered a "glare" glare：
condition.

The term glare was originally used in circuit switched
telecommunications networks to describe the condition where two
switches both attempt to seize the same available circuit on the
same trunk at the same time. Here, it means both agents have
attempted to send an updated offer at the same time.

The higher layer protocol needs to provide a means for resolving such
conditions. The higher layer protocol will need to provide a means
for ordering of messages in each direction. SIP meets these
requirements [7].

5 Generating the Initial Offer

The offer (and answer) MUST be a valid SDP message, as defined by RFC
2327 [1], with one exception. RFC 2327 mandates that either an e or
a p line is present in the SDP message. This specification relaxes
that constraint; an SDP formulated for an offer/answer application
MAY omit both the e and p lines. The numeric value of the session id
and version in the o line MUST be representable with a 64 bit signed
integer. The initial value of the version MUST be less than
(2**62)-1, to avoid rollovers. Although the SDP specification allows
for multiple session descriptions to be concatenated together into a
large SDP message, an SDP message used in the offer/answer model MUST
contain exactly one session description.

The SDP "s=" line conveys the subject of the session, which is
reasonably defined for multicast, but ill defined for unicast. For
unicast sessions, it is RECOMMENDED that it consist of a single space
character (0x20) or a dash (-).

Unfortunately, SDP does not allow the "s=" line to be empty.

The SDP "t=" line conveys the time of the session. Generally,
streams for unicast sessions are created and destroyed through
external signaling means, such as SIP. In that case, the "t=" line
SHOULD have a value of "0 0".

The offer will contain zero or more media streams (each media stream
is described by an "m=" line and its associated attributes). Zero
media streams implies that the offerer wishes to communicate, but
that the streams for the session will be added at a later time
through a modified offer. The streams MAY be for a mix of unicast
and multicast; the latter obviously implies a multicast address in
the relevant "c=" line(s).

Construction of each offered stream depends on whether the stream is
multicast or unicast.

5.1 Unicast Streams

If the offerer wishes to only send media on a stream to its peer, it
MUST mark the stream as sendonly with the "a=sendonly" attribute. We
refer to a stream as being marked with a certain direction if a
direction attribute was present as either a media stream attribute or

a session attribute. If the offerer wishes to only receive media
from its peer, it MUST mark the stream as recvonly. If the offerer
wishes to communicate, but wishes to neither send nor receive media
at this time, it MUST mark the stream with an "a=inactive" attribute.
The inactive direction attribute is specified in RFC 3108 [3]. Note
that in the case of the Real Time Transport Protocol (RTP) [4], RTCP
is still sent and received for sendonly, recvonly, and inactive
streams. That is, the directionality of the media stream has no
impact on the RTCP usage. If the offerer wishes to both send and
receive media with its peer, it MAY include an "a=sendrecv"
attribute, or it MAY omit it, since sendrecv is the default.

For recvonly and sendrecv streams, the port number and address in the
offer indicate where the offerer would like to receive the media
stream. For sendonly RTP streams, the address and port number
indirectly indicate where the offerer wants to receive RTCP reports.
Unless there is an explicit indication otherwise, reports are sent to
the port number one higher than the number indicated. The IP address
and port present in the offer indicate nothing about the source IP
address and source port of RTP and RTCP packets that will be sent by
the offerer. A port number of zero in the offer indicates that the
stream is offered but MUST NOT be used. This has no useful semantics
in an initial offer, but is allowed for reasons of completeness,
since the answer can contain a zero port indicating a rejected stream
(Section 6). Furthermore, existing streams can be terminated by
setting the port to zero (Section 8). In general, a port number of
zero indicates that the media stream is not wanted.

The list of media formats for each media stream conveys two pieces of
information, namely the set of formats (codecs and any parameters
associated with the codec, in the case of RTP) that the offerer is
capable of sending and/or receiving (depending on the direction
attributes), and, in the case of RTP, the RTP payload type numbers
used to identify those formats. If multiple formats are listed, it
means that the offerer is capable of making use of any of those
formats during the session. In other words, the answerer MAY change
formats in the middle of the session, making use of any of the
formats listed, without sending a new offer. For a sendonly stream,
the offer SHOULD indicate those formats the offerer is willing to
send for this stream. For a recvonly stream, the offer SHOULD
indicate those formats the offerer is willing to receive for this
stream. For a sendrecv stream, the offer SHOULD indicate those
codecs that the offerer is willing to send and receive with.

For recvonly RTP streams, the payload type numbers indicate the value
of the payload type field in RTP packets the offerer is expecting to
receive for that codec. For sendonly RTP streams, the payload type
numbers indicate the value of the payload type field in RTP packets

the offerer is planning to send for that codec. For sendrecv RTP
streams, the payload type numbers indicate the value of the payload
type field the offerer expects to receive, and would prefer to send.
However, for sendonly and sendrecv streams, the answer might indicate
different payload type numbers for the same codecs, in which case,
the offerer MUST send with the payload type numbers from the answer.

Different payload type numbers may be needed in each direction
because of interoperability concerns with H.323.

As per RFC 2327, fmtp parameters MAY be present to provide additional
parameters of the media format.

In the case of RTP streams, all media descriptions SHOULD contain
"a=rtpmap" mappings from RTP payload types to encodings. If there is
no "a=rtpmap", the default payload type mapping, as defined by the
current profile in use (for example, RFC 1890 [5]) is to be used.

This allows easier migration away from static payload types.

In all cases, the formats in the "m=" line MUST be listed in order of
preference, with the first format listed being preferred. In this
case, preferred means that the recipient of the offer SHOULD use the
format with the highest preference that is acceptable to it.

If the ptime attribute is present for a stream, it indicates the
desired packetization interval that the offerer would like to
receive. The ptime attribute MUST be greater than zero.

If the bandwidth attribute is present for a stream, it indicates the
desired bandwidth that the offerer would like to receive. A value of
zero is allowed, but discouraged. It indicates that no media should
be sent. In the case of RTP, it would also disable all RTCP.

If multiple media streams of different types are present, it means
that the offerer wishes to use those streams at the same time. A
typical case is an audio and a video stream as part of a
videoconference.

If multiple media streams of the same type are present in an offer,
it means that the offerer wishes to send (and/or receive) multiple
streams of that type at the same time. When sending multiple streams
of the same type, it is a matter of local policy as to how each media
source of that type (for example, a video camera and VCR in the case
of video) is mapped to each stream. When a user has a single source
for a particular media type, only one policy makes sense: the source
is sent to each stream of the same type. Each stream MAY use
different encodings. When receiving multiple streams of the same

type, it is a matter of local policy as to how each stream is mapped
to the various media sinks for that particular type (for example,
speakers or a recording device in the case of audio). There are a
few constraints on the policies, however. First, when receiving
multiple streams of the same type, each stream MUST be mapped to at
least one sink for the purpose of presentation to the user. In other
words, the intent of receiving multiple streams of the same type is
that they should all be presented in parallel, rather than choosing
just one. Another constraint is that when multiple streams are
received and sent to the same sink, they MUST be combined in some
media specific way. For example, in the case of two audio streams,
the received media from each might be mapped to the speakers. In
that case, the combining operation would be to mix them. In the case
of multiple instant messaging streams, where the sink is the screen,
the combining operation would be to present all of them to the user
interface. The third constraint is that if multiple sources are
mapped to the same stream, those sources MUST be combined in some
media specific way before they are sent on the stream. Although
policies beyond these constraints are flexible, an agent won't
generally want a policy that will copy media from its sinks to its
sources unless it is a conference server (i.e., don't copy received
media on one stream to another stream).

A typical usage example for multiple media streams of the same type
is a pre-paid calling card application, where the user can press and
hold the pound ("#") key at any time during a call to hangup and make
a new call on the same card. This requires media from the user to
two destinations - the remote gateway, and the DTMF processing
application which looks for the pound. This could be accomplished
with two media streams, one sendrecv to the gateway, and the other
sendonly (from the perspective of the user) to the DTMF application.

Once the offerer has sent the offer, it MUST be prepared to receive
media for any recvonly streams described by that offer. It MUST be
prepared to send and receive media for any sendrecv streams in the
offer, and send media for any sendonly streams in the offer (of
course, it cannot actually send until the peer provides an answer
with the needed address and port information). In the case of RTP,
even though it may receive media before the answer arrives, it will
not be able to send RTCP receiver reports until the answer arrives.

5.2 Multicast Streams

If a session description contains a multicast media stream which is
listed as receive (send) only, it means that the participants,
including the offerer and answerer, can only receive (send) on that
stream. This differs from the unicast view, where the directionality
refers to the flow of media between offerer and answerer.

Beyond that clarification, the semantics of an offered multicast
stream are exactly as described in RFC 2327 [1].

6 Generating the Answer

The answer to an offered session description is based on the offered
session description. If the answer is different from the offer in
any way (different IP addresses, ports, etc.), the origin line MUST
be different in the answer, since the answer is generated by a
different entity. In that case, the version number in the "o=" line
of the answer is unrelated to the version number in the o line of the
offer.

For each "m=" line in the offer, there MUST be a corresponding "m="
line in the answer. The answer MUST contain exactly the same number
of "m=" lines as the offer. This allows for streams to be matched up
based on their order. This implies that if the offer contained zero
"m=" lines, the answer MUST contain zero "m=" lines.

The "t=" line in the answer MUST equal that of the offer. The time
of the session cannot be negotiated.

An offered stream MAY be rejected in the answer, for any reason. If
a stream is rejected, the offerer and answerer MUST NOT generate
media (or RTCP packets) for that stream. To reject an offered
stream, the port number in the corresponding stream in the answer
MUST be set to zero. Any media formats listed are ignored. At least
one MUST be present, as specified by SDP.

Constructing an answer for each offered stream differs for unicast
and multicast.

6.1 Unicast Streams

If a stream is offered with a unicast address, the answer for that
stream MUST contain a unicast address. The media type of the stream
in the answer MUST match that of the offer.

If a stream is offered as sendonly, the corresponding stream MUST be
marked as recvonly or inactive in the answer. If a media stream is
listed as recvonly in the offer, the answer MUST be marked as
sendonly or inactive in the answer. If an offered media stream is
listed as sendrecv (or if there is no direction attribute at the
media or session level, in which case the stream is sendrecv by
default), the corresponding stream in the answer MAY be marked as
sendonly, recvonly, sendrecv, or inactive. If an offered media
stream is listed as inactive, it MUST be marked as inactive in the
answer.

For streams marked as recvonly in the answer, the "m=" line MUST
contain at least one media format the answerer is willing to receive
with from amongst those listed in the offer. The stream MAY indicate
additional media formats, not listed in the corresponding stream in
the offer, that the answerer is willing to receive. For streams
marked as sendonly in the answer, the "m=" line MUST contain at least
one media format the answerer is willing to send from amongst those
listed in the offer. For streams marked as sendrecv in the answer,
the "m=" line MUST contain at least one codec the answerer is willing
to both send and receive, from amongst those listed in the offer.
The stream MAY indicate additional media formats, not listed in the
corresponding stream in the offer, that the answerer is willing to
send or receive (of course, it will not be able to send them at this
time, since it was not listed in the offer). For streams marked as
inactive in the answer, the list of media formats is constructed
based on the offer. If the offer was sendonly, the list is
constructed as if the answer were recvonly. Similarly, if the offer
was recvonly, the list is constructed as if the answer were sendonly,
and if the offer was sendrecv, the list is constructed as if the
answer were sendrecv. If the offer was inactive, the list is
constructed as if the offer were actually sendrecv and the answer
were sendrecv.

The connection address and port in the answer indicate the address
where the answerer wishes to receive media (in the case of RTP, RTCP
will be received on the port which is one higher unless there is an
explicit indication otherwise). This address and port MUST be
present even for sendonly streams; in the case of RTP, the port one
higher is still used to receive RTCP.

In the case of RTP, if a particular codec was referenced with a
specific payload type number in the offer, that same payload type
number SHOULD be used for that codec in the answer. Even if the same
payload type number is used, the answer MUST contain rtpmap
attributes to define the payload type mappings for dynamic payload
types, and SHOULD contain mappings for static payload types. The
media formats in the "m=" line MUST be listed in order of preference,
with the first format listed being preferred. In this case,
preferred means that the offerer SHOULD use the format with the
highest preference from the answer.

Although the answerer MAY list the formats in their desired order of
preference, it is RECOMMENDED that unless there is a specific reason,
the answerer list formats in the same relative order they were
present in the offer. In other words, if a stream in the offer lists
audio codecs 8, 22 and 48, in that order, and the answerer only
supports codecs 8 and 48, it is RECOMMENDED that, if the answerer has

no reason to change it, the ordering of codecs in the answer be 8,
48, and not 48, 8. This helps assure that the same codec is used in
both directions.

The interpretation of fmtp parameters in an offer depends on the
parameters. In many cases, those parameters describe specific
configurations of the media format, and should therefore be processed
as the media format value itself would be. This means that the same
fmtp parameters with the same values MUST be present in the answer if
the media format they describe is present in the answer. Other fmtp
parameters are more like parameters, for which it is perfectly
acceptable for each agent to use different values. In that case, the
answer MAY contain fmtp parameters, and those MAY have the same
values as those in the offer, or they MAY be different. SDP
extensions that define new parameters SHOULD specify the proper
interpretation in offer/answer.

The answerer MAY include a non-zero ptime attribute for any media
stream; this indicates the packetization interval that the answerer
would like to receive. There is no requirement that the
packetization interval be the same in each direction for a particular
stream.

The answerer MAY include a bandwidth attribute for any media stream;
this indicates the bandwidth that the answerer would like the offerer
to use when sending media. The value of zero is allowed, interpreted
as described in Section 5.

If the answerer has no media formats in common for a particular
offered stream, the answerer MUST reject that media stream by setting
the port to zero.

If there are no media formats in common for all streams, the entire
offered session is rejected.

Once the answerer has sent the answer, it MUST be prepared to receive
media for any recvonly streams described by that answer. It MUST be
prepared to send and receive media for any sendrecv streams in the
answer, and it MAY send media immediately. The answerer MUST be
prepared to receive media for recvonly or sendrecv streams using any
media formats listed for those streams in the answer, and it MAY send
media immediately. When sending media, it SHOULD use a packetization
interval equal to the value of the ptime attribute in the offer, if
any was present. It SHOULD send media using a bandwidth no higher
than the value of the bandwidth attribute in the offer, if any was
present. The answerer MUST send using a media format in the offer
that is also listed in the answer, and SHOULD send using the most
preferred media format in the offer that is also listed in the

answer. In the case of RTP, it MUST use the payload type numbers
from the offer, even if they differ from those in the answer.

6.2 Multicast Streams

Unlike unicast, where there is a two-sided view of the stream, there
is only a single view of the stream for multicast. As such,
generating an answer to a multicast offer generally involves
modifying a limited set of aspects of the stream.

If a multicast stream is accepted, the address and port information
in the answer MUST match that of the offer. Similarly, the
directionality information in the answer (sendonly, recvonly, or
sendrecv) MUST equal that of the offer. This is because all
participants in a multicast session need to have equivalent views of
the parameters of the session, an underlying assumption of the
multicast bias of RFC 2327.

The set of media formats in the answer MUST be equal to or be a
subset of those in the offer. Removing a format is a way for the
answerer to indicate that the format is not supported.

The ptime and bandwidth attributes in the answer MUST equal the ones
in the offer, if present. If not present, a non-zero ptime MAY be
added to the answer.

7 Offerer Processing of the Answer

When the offerer receives the answer, it MAY send media on the
accepted stream(s) (assuming it is listed as sendrecv or recvonly in
the answer). It MUST send using a media format listed in the answer,
and it SHOULD use the first media format listed in the answer when it
does send.

The reason this is a SHOULD, and not a MUST (its also a SHOULD,
and not a MUST, for the answerer), is because there will
oftentimes be a need to change codecs on the fly. For example,
during silence periods, an agent might like to switch to a comfort
noise codec. Or, if the user presses a number on the keypad, the
agent might like to send that using RFC 2833 [9]. Congestion
control might necessitate changing to a lower rate codec based on
feedback.

The offerer SHOULD send media according to the value of any ptime and
bandwidth attribute in the answer.

The offerer MAY immediately cease listening for media formats that
were listed in the initial offer, but not present in the answer.

8 Modifying the Session

At any point during the session, either participant MAY issue a new
offer to modify characteristics of the session. It is fundamental to
the operation of the offer/answer model that the exact same
offer/answer procedure defined above is used for modifying parameters
of an existing session.

The offer MAY be identical to the last SDP provided to the other
party (which may have been provided in an offer or an answer), or it
MAY be different. We refer to the last SDP provided as the "previous
SDP". If the offer is the same, the answer MAY be the same as the
previous SDP from the answerer, or it MAY be different. If the
offered SDP is different from the previous SDP, some constraints are
placed on its construction, discussed below.

Nearly all aspects of the session can be modified. New streams can
be added, existing streams can be deleted, and parameters of existing
streams can change. When issuing an offer that modifies the session,
the "o=" line of the new SDP MUST be identical to that in the
previous SDP, except that the version in the origin field MUST
increment by one from the previous SDP. If the version in the origin
line does not increment, the SDP MUST be identical to the SDP with
that version number. The answerer MUST be prepared to receive an
offer that contains SDP with a version that has not changed; this is
effectively a no-op. However, the answerer MUST generate a valid
answer (which MAY be the same as the previous SDP from the answerer,
or MAY be different), according to the procedures defined in Section
6.

If an SDP is offered, which is different from the previous SDP, the
new SDP MUST have a matching media stream for each media stream in
the previous SDP. In other words, if the previous SDP had N "m="
lines, the new SDP MUST have at least N "m=" lines. The i-th media
stream in the previous SDP, counting from the top, matches the i-th
media stream in the new SDP, counting from the top. This matching is
necessary in order for the answerer to determine which stream in the
new SDP corresponds to a stream in the previous SDP. Because of
these requirements, the number of "m=" lines in a stream never
decreases, but either stays the same or increases. Deleted media
streams from a previous SDP MUST NOT be removed in a new SDP;
however, attributes for these streams need not be present.

8.1 Adding a Media Stream

New media streams are created by new additional media descriptions
below the existing ones, or by reusing the "slot" used by an old
media stream which had been disabled by setting its port to zero.

Reusing its slot means that the new media description replaces the
old one, but retains its positioning relative to other media
descriptions in the SDP. New media descriptions MUST appear below
any existing media sections. The rules for formatting these media
descriptions are identical to those described in Section 5.

When the answerer receives an SDP with more media descriptions than
the previous SDP from the offerer, or it receives an SDP with a media
stream in a slot where the port was previously zero, the answerer
knows that new media streams are being added. These can be rejected
or accepted by placing an appropriately structured media description
in the answer. The procedures for constructing the new media
description in the answer are described in Section 6.

8.2 Removing a Media Stream

Existing media streams are removed by creating a new SDP with the
port number for that stream set to zero. The stream description MAY
omit all attributes present previously, and MAY list just a single
media format.

A stream that is offered with a port of zero MUST be marked with port
zero in the answer. Like the offer, the answer MAY omit all
attributes present previously, and MAY list just a single media
format from amongst those in the offer.

Removal of a media stream implies that media is no longer sent for
that stream, and any media that is received is discarded. In the
case of RTP, RTCP transmission also ceases, as does processing of any
received RTCP packets. Any resources associated with it can be
released. The user interface might indicate that the stream has
terminated, by closing the associated window on a PC, for example.

8.3 Modifying a Media Stream

Nearly all characteristics of a media stream can be modified.

8.3.1 Modifying Address, Port or Transport

The port number for a stream MAY be changed. To do this, the offerer
creates a new media description, with the port number in the m line
different from the corresponding stream in the previous SDP. If only
the port number is to be changed, the rest of the media stream
description SHOULD remain unchanged. The offerer MUST be prepared to
receive media on both the old and new ports as soon as the offer is
sent. The offerer SHOULD NOT cease listening for media on the old
port until the answer is received and media arrives on the new port.
Doing so could result in loss of media during the transition.

Received, in this case, means that the media is passed to a media
sink. This means that if there is a playout buffer, the agent would
continue to listen on the old port until the media on the new port
reached the top of the playout buffer. At that time, it MAY cease
listening for media on the old port.

The corresponding media stream in the answer MAY be the same as the
stream in the previous SDP from the answerer, or it MAY be different.
If the updated stream is accepted by the answerer, the answerer
SHOULD begin sending traffic for that stream to the new port
immediately. If the answerer changes the port from the previous SDP,
it MUST be prepared to receive media on both the old and new ports as
soon as the answer is sent. The answerer MUST NOT cease listening
for media on the old port until media arrives on the new port. At
that time, it MAY cease listening for media on the old port. The
same is true for an offerer that sends an updated offer with a new
port; it MUST NOT cease listening for media on the old port until
media arrives on the new port.

Of course, if the offered stream is rejected, the offerer can cease
being prepared to receive using the new port as soon as the rejection
is received.

To change the IP address where media is sent to, the same procedure
is followed for changing the port number. The only difference is
that the connection line is updated, not the port number.

The transport for a stream MAY be changed. The process for doing
this is identical to changing the port, except the transport is
updated, not the port.

8.3.2 Changing the Set of Media Formats

The list of media formats used in the session MAY be changed. To do
this, the offerer creates a new media description, with the list of
media formats in the "m=" line different from the corresponding media
stream in the previous SDP. This list MAY include new formats, and
MAY remove formats present from the previous SDP. However, in the
case of RTP, the mapping from a particular dynamic payload type
number to a particular codec within that media stream MUST NOT change
for the duration of a session. For example, if A generates an offer
with G.711 assigned to dynamic payload type number 46, payload type
number 46 MUST refer to G.711 from that point forward in any offers
or answers for that media stream within the session. However, it is
acceptable for multiple payload type numbers to be mapped to the same
codec, so that an updated offer could also use payload type number 72
for G.711.

The mappings need to remain fixed for the duration of the session
because of the loose synchronization between signaling exchanges
of SDP and the media stream.

The corresponding media stream in the answer is formulated as
described in Section 6, and may result in a change in media formats
as well. Similarly, as described in Section 6, as soon as it sends
its answer, the answerer MUST begin sending media using any formats
in the offer that were also present in the answer, and SHOULD use the
most preferred format in the offer that was also listed in the answer
(assuming the stream allows for sending), and MUST NOT send using any
formats that are not in the offer, even if they were present in a
previous SDP from the peer. Similarly, when the offerer receives the
answer, it MUST begin sending media using any formats in the answer,
and SHOULD use the most preferred one (assuming the stream allows for
sending), and MUST NOT send using any formats that are not in the
answer, even if they were present in a previous SDP from the peer.

When an agent ceases using a media format (by not listing that format
in an offer or answer, even though it was in a previous SDP) the
agent will still need to be prepared to receive media with that
format for a brief time. How does it know when it can be prepared to
stop receiving with that format? If it needs to know, there are three
techniques that can be applied. First, the agent can change ports in
addition to changing formats. When media arrives on the new port, it
knows that the peer has ceased sending with the old format, and it
can cease being prepared to receive with it. This approach has the
benefit of being media format independent. However, changes in ports
may require changes in resource reservation or rekeying of security
protocols. The second approach is to use a totally new set of
dynamic payload types for all codecs when one is discarded. When
media is received with one of the new payload types, the agent knows
that the peer has ceased sending with the old format. This approach
doesn't affect reservations or security contexts, but it is RTP
specific and wasteful of a very small payload type space. A third
approach is to use a timer. When the SDP from the peer is received,
the timer is set. When it fires, the agent can cease being prepared
to receive with the old format. A value of one minute would
typically be more than sufficient. In some cases, an agent may not
care, and thus continually be prepared to receive with the old
formats. Nothing need be done in this case.

Of course, if the offered stream is rejected, the offer can cease
being prepared to receive using any new formats as soon as the
rejection is received.

8.3.3 Changing Media Types

The media type (audio, video, etc.) for a stream MAY be changed. It
is RECOMMENDED that the media type be changed (as opposed to adding a
new stream), when the same logical data is being conveyed, but just
in a different media format. This is particularly useful for
changing between voiceband fax and fax in a single stream, which are
both separate media types. To do this, the offerer creates a new
media description, with a new media type, in place of the description
in the previous SDP which is to be changed.

The corresponding media stream in the answer is formulated as
described in Section 6. Assuming the stream is acceptable, the
answerer SHOULD begin sending with the new media type and formats as
soon as it receives the offer. The offerer MUST be prepared to
receive media with both the old and new types until the answer is
received, and media with the new type is received and reaches the top
of the playout buffer.

8.3.4 Changing Attributes

Any other attributes in a media description MAY be updated in an
offer or answer. Generally, an agent MUST send media (if the
directionality of the stream allows) using the new parameters once
the SDP with the change is received.

8.4 Putting a Unicast Media Stream on Hold

If a party in a call wants to put the other party "on hold", i.e.,
request that it temporarily stops sending one or more unicast media
streams, a party offers the other an updated SDP.

If the stream to be placed on hold was previously a sendrecv media
stream, it is placed on hold by marking it as sendonly. If the
stream to be placed on hold was previously a recvonly media stream,
it is placed on hold by marking it inactive.

This means that a stream is placed "on hold" separately in each
direction. Each stream is placed "on hold" independently. The
recipient of an offer for a stream on-hold SHOULD NOT automatically
return an answer with the corresponding stream on hold. An SDP with
all streams "on hold" is referred to as held SDP.

Certain third party call control scenarios do not work when an
answerer responds to held SDP with held SDP.

Typically, when a user "presses" hold, the agent will generate an
offer with all streams in the SDP indicating a direction of sendonly,
and it will also locally mute, so that no media is sent to the far
end, and no media is played out.

RFC 2543 [10] specified that placing a user on hold was accomplished
by setting the connection address to 0.0.0.0. Its usage for putting
a call on hold is no longer recommended, since it doesn't allow for
RTCP to be used with held streams, doesn't work with IPv6, and breaks
with connection oriented media. However, it can be useful in an
initial offer when the offerer knows it wants to use a particular set
of media streams and formats, but doesn't know the addresses and
ports at the time of the offer. Of course, when used, the port
number MUST NOT be zero, which would specify that the stream has been
disabled. An agent MUST be capable of receiving SDP with a
connection address of 0.0.0.0, in which case it means that neither
RTP nor RTCP should be sent to the peer.

9 Indicating Capabilities

Before an agent sends an offer, it is helpful to know if the media
formats in that offer would be acceptable to the answerer. Certain
protocols, like SIP, provide a means to query for such capabilities.
SDP can be used in responses to such queries to indicate
capabilities. This section describes how such an SDP message is
formatted. Since SDP has no way to indicate that the message is for
the purpose of capability indication, this is determined from the
context of the higher layer protocol. The ability of baseline SDP to
indicate capabilities is very limited. It cannot express allowed
parameter ranges or values, and can not be done in parallel with an
offer/answer itself. Extensions might address such limitations in
the future.

An SDP constructed to indicate media capabilities is structured as
follows. It MUST be a valid SDP, except that it MAY omit both "e="
and "p=" lines. The "t=" line MUST be equal to "0 0". For each
media type supported by the agent, there MUST be a corresponding
media description of that type. The session ID in the origin field
MUST be unique for each SDP constructed to indicate media
capabilities. The port MUST be set to zero, but the connection
address is arbitrary. The usage of port zero makes sure that an SDP
formatted for capabilities does not cause media streams to be
established if it is interpreted as an offer or answer.

The transport component of the "m=" line indicates the transport for
that media type. For each media format of that type supported by the
agent, there SHOULD be a media format listed in the "m=" line. In
the case of RTP, if dynamic payload types are used, an rtpmap

attribute MUST be present to bind the type to a specific format.
There is no way to indicate constraints, such as how many
simultaneous streams can be supported for a particular codec, and so
on.

v=0
o=carol 28908764872 28908764872 IN IP4 100.3.6.6
s=-
t=0 0
c=IN IP4 192.0.2.4
m=audio 0 RTP/AVP 0 1 3
a=rtpmap:0 PCMU/8000
a=rtpmap:1 1016/8000
a=rtpmap:3 GSM/8000
m=video 0 RTP/AVP 31 34
a=rtpmap:31 H261/90000
a=rtpmap:34 H263/90000

Figure 1: SDP Indicating Capabilities

The SDP of Figure 1 indicates that the agent can support three audio
codecs (PCMU, 1016, and GSM) and two video codecs (H.261 and H.263).

10 Example Offer/Answer Exchanges

This section provides example offer/answer exchanges.

10.1 Basic Exchange

Assume that the caller, Alice, has included the following description
in her offer. It includes a bidirectional audio stream and two
bidirectional video streams, using H.261 (payload type 31) and MPEG
(payload type 32). The offered SDP is:

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=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000
m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000

The callee, Bob, does not want to receive or send the first video
stream, so he returns the SDP below as the answer:

v=0
o=bob 2890844730 2890844730 IN IP4 host.example.com
s=
c=IN IP4 host.example.com
t=0 0
m=audio 49920 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31
m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000

At some point later, Bob decides to change the port where he will
receive the audio stream (from 49920 to 65422), and at the same time,
add an additional audio stream as receive only, using the RTP payload
format for events [9]. Bob offers the following SDP in the offer:

v=0
o=bob 2890844730 2890844731 IN IP4 host.example.com
s=
c=IN IP4 host.example.com
t=0 0
m=audio 65422 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31
m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000
m=audio 51434 RTP/AVP 110
a=rtpmap:110 telephone-events/8000
a=recvonly

Alice accepts the additional media stream, and so generates the
following 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
a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31
a=rtpmap:31 H261/90000
m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000
m=audio 53122 RTP/AVP 110
a=rtpmap:110 telephone-events/8000
a=sendonly

10.2 One of N Codec Selection

A common occurrence in embedded phones is that the Digital Signal
Processor (DSP) used for compression can support multiple codecs at a
time, but once that codec is selected, it cannot be readily changed
on the fly. This example shows how a session can be set up using an
initial offer/answer exchange, followed immediately by a second one
to lock down the set of codecs.

The initial offer from Alice to Bob indicates a single audio stream
with the three audio codecs that are available in the DSP. The
stream is marked as inactive, since media cannot be received until a
codec is locked down:

v=0
o=alice 2890844526 2890844526 IN IP4 host.anywhere.com
s=
c=IN IP4 host.anywhere.com
t=0 0
m=audio 62986 RTP/AVP 0 4 18
a=rtpmap:0 PCMU/8000
a=rtpmap:4 G723/8000
a=rtpmap:18 G729/8000
a=inactive

Bob can support dynamic switching between PCMU and G.723. So, he
sends the following answer:

v=0
o=bob 2890844730 2890844731 IN IP4 host.example.com
s=
c=IN IP4 host.example.com
t=0 0
m=audio 54344 RTP/AVP 0 4
a=rtpmap:0 PCMU/8000
a=rtpmap:4 G723/8000
a=inactive

Alice can then select any one of these two codecs. So, she sends an
updated offer with a sendrecv stream:

v=0
o=alice 2890844526 2890844527 IN IP4 host.anywhere.com
s=
c=IN IP4 host.anywhere.com
t=0 0
m=audio 62986 RTP/AVP 4
a=rtpmap:4 G723/8000
a=sendrecv

Bob accepts the single codec:

v=0
o=bob 2890844730 2890844732 IN IP4 host.example.com
s=
c=IN IP4 host.example.com
t=0 0
m=audio 54344 RTP/AVP 4
a=rtpmap:4 G723/8000
a=sendrecv

If the answerer (Bob), was only capable of supporting one-of-N
codecs, Bob would select one of the codecs from the offer, and place
that in his answer. In this case, Alice would do a re-INVITE to
activate that stream with that codec.

As an alternative to using "a=inactive" in the first exchange, Alice
can list all codecs, and as soon as she receives media from Bob,
generate an updated offer locking down the codec to the one just
received. Of course, if Bob only supports one-of-N codecs, there
would only be one codec in his answer, and in this case, there is no
need for a re-INVITE to lock down to a single codec.

11 Security Considerations

There are numerous attacks possible if an attacker can modify offers
or answers in transit. Generally, these include diversion of media
streams (enabling eavesdropping), disabling of calls, and injection
of unwanted media streams. If a passive listener can construct fake
offers, and inject those into an exchange, similar attacks are
possible. Even if an attacker can simply observe offers and answers,
they can inject media streams into an existing conversation.

Offer/answer relies on transport within an application signaling
protocol, such as SIP. It also relies on that protocol for security
capabilities. Because of the attacks described above, that protocol
MUST provide a means for end-to-end authentication and integrity
protection of offers and answers. It SHOULD offer encryption of
bodies to prevent eavesdropping. However, media injection attacks
can alternatively be resolved through authenticated media exchange,
and therefore the encryption requirement is a SHOULD instead of a
MUST.

Replay attacks are also problematic. An attacker can replay an old
offer, perhaps one that had put media on hold, and thus disable media
streams in a conversation. Therefore, the application protocol MUST
provide a secure way to sequence offers and answers, and to detect
and reject old offers or answers.

SIP [7] meets all of these requirements.

12 IANA Considerations

There are no IANA considerations with this specification.

13 Acknowledgements

The authors would like to thank Allison Mankin, Rohan Mahy, Joerg
Ott, and Flemming Andreasen for their detailed comments.

14 Normative References

[1] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.

[2] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[3] Kumar, R. and M. Mostafa, "Conventions For the Use of The
Session Description Protocol (SDP) for ATM Bearer Connections",
RFC 3108, May 2001.

[4] Schulzrinne, H., Casner, S, Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.

[5] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996.

15 Informative References

[6] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.

[7] 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.

[8] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.

[9] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", RFC 2833, May 2000.

[10] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.

16 Authors' Addresses

Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936

EMail: jdrosen@dynamicsoft.com

Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA

EMail: schulzrinne@cs.columbia.edu

17. Full Copyright Statement

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The limited permissions granted above are perpetual and will not be
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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