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rfc:rfc3264

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

                                                             June 2002
 An Offer/Answer Model with the Session Description Protocol (SDP)

Status of this Memo

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

Copyright Notice

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

Abstract

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

Table of Contents

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

Rosenberg & Schulzrinne Standards Track [Page 1] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 2] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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.

Rosenberg & Schulzrinne Standards Track [Page 3] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

    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.

Rosenberg & Schulzrinne Standards Track [Page 4] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 5] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 6] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 7] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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.

Rosenberg & Schulzrinne Standards Track [Page 8] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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.

Rosenberg & Schulzrinne Standards Track [Page 9] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 10] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 11] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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.

Rosenberg & Schulzrinne Standards Track [Page 12] 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.

Rosenberg & Schulzrinne Standards Track [Page 13] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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.

Rosenberg & Schulzrinne Standards Track [Page 14] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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.

Rosenberg & Schulzrinne Standards Track [Page 15] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

    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.

Rosenberg & Schulzrinne Standards Track [Page 16] 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

Rosenberg & Schulzrinne Standards Track [Page 18] 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

Rosenberg & Schulzrinne Standards Track [Page 19] RFC 3264 An Offer/Answer Model Session Description Protocol June 2002

 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

Rosenberg & Schulzrinne Standards Track [Page 20] 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

Rosenberg & Schulzrinne Standards Track [Page 21] 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.

Rosenberg & Schulzrinne Standards Track [Page 22] 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.

Rosenberg & Schulzrinne Standards Track [Page 23] 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

Rosenberg & Schulzrinne Standards Track [Page 24] 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.

Rosenberg & Schulzrinne Standards Track [Page 25]

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