





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



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   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



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   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.







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      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"
   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.







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   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



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   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



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   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



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   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.



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   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.



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   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




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   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



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   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.



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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.



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   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.



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   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.




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      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.






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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.






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   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



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   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






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   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



















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   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









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   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.




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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.



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   [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







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17.  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.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

Acknowledgement

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



















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