    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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

    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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

    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.

    Rosenberg & Schulzrinne     Standards Track                    [Page 17]

    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

       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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

       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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

       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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

       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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

    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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

       [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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    RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002

    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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