draft-ietf-mmusic-sdp-offer-answer-00.txt   draft-ietf-mmusic-sdp-offer-answer-01.txt 
Internet Engineering Task Force MMUSIC WG Internet Engineering Task Force MMUSIC WG
Internet Draft J.Rosenberg,H.Schulzrinne Internet Draft J.Rosenberg
draft-ietf-mmusic-sdp-offer-answer-00.txt dynamicsoft,Columbia U. dynamicsoft
January 31, 2002 H.Schulzrinne
Expires: July 2002 Columbia U.
draft-ietf-mmusic-sdp-offer-answer-01.txt
February 21, 2002
Expires: August 2002
An Offer/Answer Model with SDP An Offer/Answer Model with SDP
STATUS OF THIS MEMO STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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This document defines a mechanism by which two entities can make use This document defines a mechanism by which two entities can make use
of SDP to arrive at a common view of a multimedia session between of SDP to arrive at a common view of a multimedia session between
them. In the model, one participant offers the other a description of them. In the model, one participant offers the other a description of
the desired session from their perspective, and the other participant the desired session from their perspective, and the other participant
answers with the desired session from their perspective. This answers with the desired session from their perspective. This
offer/answer model is most useful in unicast sessions where offer/answer model is most useful in unicast sessions where
information from both participants is needed for the complete view of information from both participants is needed for the complete view of
the session. The offer/answer model is used by protocols like the the session. The offer/answer model is used by protocols like the
Session Initiation Protocol (SIP). Session Initiation Protocol (SIP).
J.Rosenberg,H.Schulzrinne [Page a]
Table of Contents Table of Contents
1 Introduction ........................................ 2 1 Introduction ........................................ 3
2 Terminology ......................................... 3 2 Terminology ......................................... 4
3 Definitions ......................................... 3 3 Definitions ......................................... 4
4 Protocol Operation .................................. 3 4 Protocol Operation .................................. 4
5 Generating the initial offer ........................ 4 5 Generating the initial offer ........................ 5
5.1 Unicast Streams ..................................... 5 5.1 Unicast Streams ..................................... 6
5.2 Multicast Streams ................................... 8 5.2 Multicast Streams ................................... 9
6 Generating the answer ............................... 8 6 Generating the answer ............................... 9
6.1 Unicast Streams ..................................... 8 6.1 Unicast Streams ..................................... 10
6.2 Multicast Streams ................................... 11 6.2 Multicast Streams ................................... 12
7 Offerer Processing of the Answer .................... 11 7 Offerer Processing of the Answer .................... 13
8 Modifying the session ............................... 11 8 Modifying the session ............................... 13
8.1 Adding a media stream ............................... 12 8.1 Adding a Media Stream ............................... 14
8.2 Removing a media stream ............................. 13 8.2 Removing a Media Stream ............................. 14
8.3 Modifying a Media Stream ............................ 13 8.3 Modifying a Media Stream ............................ 15
8.3.1 Modifying Address, Port or Transport ................ 13 8.3.1 Modifying Address, Port or Transport ................ 15
8.3.2 Changing the Set of Media Formats ................... 14 8.3.2 Changing the Set of Media Formats ................... 16
8.3.3 Changing Media Types ................................ 15 8.3.3 Changing Media Types ................................ 17
8.3.4 Changing Attributes ................................. 15 8.3.4 Changing Attributes ................................. 17
8.4 Putting a media stream on hold ...................... 16 8.4 Putting a Unicast Media Stream on Hold .............. 17
9 Indicating Capabilities ............................. 16 9 Indicating Capabilities ............................. 18
10 Example Offer/Answer Exchanges ...................... 17 10 Example Offer/Answer Exchanges ...................... 19
10.1 Basic Exchange ...................................... 17 10.1 Basic Exchange ...................................... 19
10.2 One of N Codec Selection ............................ 19 10.2 One of N Codec Selection ............................ 21
11 Changes since draft-rosenberg-mmusic-sdp-offer- 11 Security Considerations ............................. 23
answer-00 ...................................................... 21 12 IANA Considerations ................................. 23
12 Author's Addresses .................................. 22 13 Acknowledgements .................................... 24
13 Bibliography ........................................ 23 14 Author's Addresses .................................. 24
15 Normative References ................................ 24
16 Non-Normative References ............................ 25
1 Introduction 1 Introduction
The Session Description Protocol (SDP) [1] was originally conceived The Session Description Protocol (SDP) [1] was originally conceived
as a way to describe multicast sessions carried on the Mbone. The as a way to describe multicast sessions carried on the Mbone. The
Session Announcement Protocol (SAP) [2] was devised as a multicast Session Announcement Protocol (SAP) [6] was devised as a multicast
mechanism to carry SDP messages. Although the SDP specification mechanism to carry SDP messages. Although the SDP specification
allows for unicast operation, it is not complete. Unlike multicast, allows for unicast operation, it is not complete. Unlike multicast,
where there is a global view of the session that is used by all where there is a global view of the session that is used by all
participants, unicast sessions involve two participants, and a participants, unicast sessions involve two participants, and a
complete view of the session requires information from both complete view of the session requires information from both
participants, and agreement on parameters between them. participants, and agreement on parameters between them.
As an example, a multicast session requires conveying a single As an example, a multicast session requires conveying a single
multicast address for a particular media stream. However, for a multicast address for a particular media stream. However, for a
unicast session, two addresses are needed - one for each participant. unicast session, two addresses are needed - one for each participant.
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As a result, even though SDP has the expressiveness to describe As a result, even though SDP has the expressiveness to describe
unicast sessions, it is missing the semantics and operational details unicast sessions, it is missing the semantics and operational details
of how it is actually done. In this document, we remedy that by 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 defining a simple offer/answer model based on SDP. In this model, one
participant in the session generates an SDP that constitutes the participant in the session generates an SDP that constitutes the
offer - the set of media streams and codecs the offerer wishes to offer - the set of media streams and codecs the offerer wishes to
use, along with the IP addresses and ports the offer would like to use, along with the IP addresses and ports the offer would like to
use to receive the media. The offer is conveyed to the other use to receive the media. The offer is conveyed to the other
participant, called the answerer. The answerer generates an answer, participant, called the answerer. The answerer generates an answer,
which is an SDP that responds to the offer provided it. The answer which is an SDP that responds to the offer provided it. The answer
has a matching media stream for each one in the offer, indicating has a matching media stream for each stream in the offer, indicating
whether the stream is accepted or not, along with the codecs that 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 will be used and the IP addresses and ports that the answerer wants
to use to receive media. to use to receive media.
It is also possible for a multicast session to work similarly to a It is also possible for a multicast session to work similarly to a
unicast one; its parameters are negotiation between a pair of users unicast one; its parameters are negotiated between a pair of users as
as in the unicast case, but both sides send packets to the same in the unicast case, but both sides send packets to the same
multicast address, rather than unicast ones. This document also multicast address, rather than unicast ones. This document also
discusses the application of the offer/answer model to multicast discusses the application of the offer/answer model to multicast
streams. streams.
We also define guidelines for how the offer/answer model is used to We also define guidelines for how the offer/answer model is used to
update a session once it has begun. update a session after an initial offer/answer exchange.
The means by which the offers and answers are conveyed are outside 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 scope of this document. The offer/answer model defined here is
the mandatory baseline mechanism used by the Session Initiation the mandatory baseline mechanism used by the Session Initiation
Protocol (SIP) [3]. Protocol (SIP) [7].
2 Terminology 2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant implementations. indicate requirement levels for compliant implementations.
3 Definitions 3 Definitions
The following terms are used throughout this document: The following terms are used throughout this document:
Agent: An agent is the protocol implementation involved in the Agent: An agent is the protocol implementation involved in the
offer/answer exchange. There are two agents involved in an offer/answer exchange. There are two agents involved in an
offer/answer exchange. offer/answer exchange.
Answer: An SDP message sent by an answerer in response to an Answer: An SDP message sent by an answerer in response to an
offer received from an offerer. offer received from an offerer.
Answerer: An agent capable of sending and/or receiving media, Answerer: An agent which receives a session description from
which receives a session description from another agent another agent describing aspects of desired media
describing aspects of desired media communication, and then communication, and then responds to that with its own
responds to that with its own session description. session description.
Media Stream: From RTSP [5], a media stream is a single media Media Stream: From RTSP [8], a media stream is a single media
instance, e.g., an audio stream or a video stream as well instance, e.g., an audio stream or a video stream as well
as a single whiteboard or shared application group. In SDP, as a single whiteboard or shared application group. In SDP,
a media stream is described by an m-line and its associated a media stream is described by an "m=" line and its
attributes. associated attributes.
Offer: An SDP message sent by an offerer. Offer: An SDP message sent by an offerer.
Offerer: An agent capable of sending and/or receiving media, Offerer: An agent which generates a session description in order
which generates a session description in order to create or to create or modify a session.
modify a session.
4 Protocol Operation 4 Protocol Operation
The offer/answer exchange assumes the existence of a higher layer The offer/answer exchange assumes the existence of a higher layer
protocol (such as SIP) which is capable of exchanging SDP for the protocol (such as SIP) which is capable of exchanging SDP for the
purposes of communication establishment between agents. purposes of session establishment between agents.
Protocol operation begins when one agent sends an initial offer to Protocol operation begins when one agent sends an initial offer to
another agent. An offer is initial if it is outside of any context another agent. An offer is initial if it is outside of any context
that may have already been established through the higher layer that may have already been established through the higher layer
protocol. It is assumed that the higher layer protocol provides protocol. It is assumed that the higher layer protocol provides
maintenance of some kind of context which allows the various SDP maintenance of some kind of context which allows the various SDP
exchanges to be associated together. exchanges to be associated together.
The agent receiving the offer MAY generate an answer, or it MAY The agent receiving the offer MAY generate an answer, or it MAY
reject the offer. The means for rejecting an offer are dependent on reject the offer. The means for rejecting an offer are dependent on
the higher layer protocol. The offer/answer exchange is atomic; if the higher layer protocol. The offer/answer exchange is atomic; if
the answer is rejected, the session reverts to the state prior to the the answer is rejected, the session reverts to the state prior to the
offer (which may be absence of a session). offer (which may be absence of a session).
At any time, either agent MAY generate a new offer that updates the At any time, either agent MAY generate a new offer that updates the
communications session. However, it MUST NOT generate a new offer if session. However, it MUST NOT generate a new offer if it has received
it has received an offer which it has not yet answered or reject. an offer which it has not yet answered or rejected. Furthermore, it
Furthermore, it MUST NOT generate a new offer if it has generated a MUST NOT generate a new offer if it has generated a prior offer for
prior offer for which it has not yet received an answer or a which it has not yet received an answer or a rejection. If an agent
rejection. The higher layer protocol will need to provide a means for receives an offer after having sent one, but before receiving an
ordering of messages in each direction. 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.
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.
5 Generating the initial offer 5 Generating the initial offer
The offer (and answer) MUST be a valid SDP, as defined by RFC 2327 The offer (and answer) MUST be a valid SDP, as defined by RFC 2327
[1], with one exception. RFC2327 mandates that either an e or a p [1], with one exception. RFC2327 mandates that either an e or a p
line is present in the SDP. This specification relaxes that line is present in the SDP. This specification relaxes that
constraint; an SDP formulated for an offer/answer application MAY 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 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 version in the o line MUST be representable with a 64 bit signed
integer. Although the SDP specification allows for multiple session integer. Although the SDP specification allows for multiple session
descriptions to be concatenated together into a large SDP message, an descriptions to be concatenated together into a large SDP message, an
SDP message used in the offer/answer model MUST contain only a single SDP message used in the offer/answer model MUST contain exactly one
session description. session description.
The SDP "s=" line conveys the subject of the media stream, which is The SDP "s=" line conveys the subject of the media stream, which is
reasonably defined for multicast, but ill defined for unicast. For reasonably defined for multicast, but ill defined for unicast. For
unicast streams, it is RECOMMENDED that it consist of a single space unicast streams, it is RECOMMENDED that it consist of a single space
character (0x20). character (0x20).
Unfortunately, SDP does not allow the "s=" line to be Unfortunately, SDP does not allow the "s=" line to be
empty. empty.
The SDP "t=" line conveys the time of the session. Generally, streams The SDP "t=" line conveys the time of the session. Generally, streams
for unicast sessions are created and destroyed through external for unicast sessions are created and destroyed through external
signaling means, such as SIP. In that case, the "t=" line SHOULD have signaling means, such as SIP. In that case, the "t=" line SHOULD have
a value of "0 0". a value of "0 0".
The offer MAY contain zero or more media streams (each media stream The offer will contain zero or more media streams (each media stream
is described by an m line and its associated attributes). Zero media is described by an "m=" line and its associated attributes). Zero
streams implies that the offerer wishes to communicate, but that the media streams implies that the offerer wishes to communicate, but
streams for the session will be added at a later time through a that the streams for the session will be added at a later time
modified offer. The streams MAY be for a mix unicast and multicast; through a modified offer. The streams MAY be for a mix of unicast and
the latter obviously implies a multicast address in the c line. 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 Construction of each offered stream depends on whether the stream is
multicast or unicast. multicast or unicast.
5.1 Unicast Streams 5.1 Unicast Streams
If the offerer wishes to only send media on a stream to its peer, it If the offerer wishes to only send media on a stream to its peer, it
MUST include an a=sendonly attribute as part of the media MUST mark the stream as sendonly with the "a=sendonly" attribute. We
description. If the offerer wishes to only receive media from its refer to a stream as being marked with a certain direction if a
peer, it MUST include an a=recvonly attribute as part of the media direction attribute was present as either a media stream attribute or
description. If the offerer wishes to communicate, but wishes to a session attribute. If the offerer wishes to only receive media from
neither send nor receive media at this time, it MUST include an its peer, it MUST mark the stream as recvonly. If the offerer wishes
a=inactive attribute as part of the media description. The inactive to communicate, but wishes to neither send nor receive media at this
direction attribute is specified in RFC 3108 [6]. Note that in the time, it MUST mark the stream with an "a=inactive" attribute. The
case of the Real Time Transport Protocol (RTP) [7], RTCP is still inactive direction attribute is specified in RFC 3108 [3]. Note that
sent and received for sendonly, recvonly and inactive streams. That in the case of the Real Time Transport Protocol (RTP) [4], RTCP is
is, the directionality of the media stream has no impact on the RTCP still sent and received for sendonly, recvonly and inactive streams.
usage. If the offerer wishes to both send and receive media with its That is, the directionality of the media stream has no impact on the
peer, it MAY include an a=sendrecv attribute, or MAY omit it, since RTCP usage. If the offerer wishes to both send and receive media with
it is the default. 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 For recvonly and sendrecv streams, the port number and address in the
offer indicate where the offer would like to receive the media offer indicate where the offerer would like to receive the media
stream. For sendonly RTP streams, the address and port number stream. For sendonly RTP streams, the address and port number
indirectly indicate where RTCP reports are to be sent to. indirectly indicate where the offerer wants to receive RTCP reports.
Specifically, RTCP reports are sent to the port number one higher Unless there is an explicit indication otherwise, reports are sent to
than the number indicated. The IP address and port present in the the port number one higher than the number indicated. The IP address
offer indicate nothing about the source IP address and source port of and port present in the offer indicate nothing about the source IP
RTP and RTCP packets that will be sent by the offerer. A port number address and source port of RTP and RTCP packets that will be sent by
of zero in the offer indicates that the stream is offered but should the offerer. A port number of zero in the offer indicates that the
never be used. This has no useful semantics in an initial offer, but stream is offered but MUST NOT be used. This has no useful semantics
is allowed for reasons of completeness, since the response can in an initial offer, but is allowed for reasons of completeness,
contain a zero port indicating a rejected stream (Section 6). since the answer can contain a zero port indicating a rejected stream
Furthermore, existing streams can be terminated by setting the port (Section 6). Furthermore, existing streams can be terminated by
to zero (Section 8). In general, a port number of zero indicates that setting the port to zero (Section 8). In general, a port number of
the media stream is not wanted. zero indicates that the media stream is not wanted.
The list of media formats for each media stream conveys two pieces of The list of media formats for each media stream conveys two pieces of
information, namely the set of formats (codecs and any parameters information, namely the set of formats (codecs and any parameters
associated with the codec, in the case of RTP) that the offerer is associated with the codec, in the case of RTP) that the offerer is
capable of sending and/or receiving (depending on the direction capable of sending and/or receiving (depending on the direction
attributes), and, in the case of RTP, the RTP payload type numbers attributes), and, in the case of RTP, the RTP payload type numbers
used to identify those formats. If multiple formats are listed, it used to identify those formats. If multiple formats are listed, it
means that the offerer is capable of making use of any of those 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 during the session. In other words, the answerer MAY change
formats in the middle of the session, without sending a new offer, to formats in the middle of the session, without sending a new offer, to
make use of any of those listed. For a sendonly stream, the offer make use of any of the formats listed. For a sendonly stream, the
SHOULD indicate those formats the offerer is willing to send for this offer SHOULD indicate those formats the offerer is willing to send
stream. For a recvonly stream, the offer SHOULD indicate those for this stream. For a recvonly stream, the offer SHOULD indicate
formats the offerer is willing to receive for this stream. For a those formats the offerer is willing to receive for this stream. For
sendrecv stream, the offer SHOULD indicate those codecs that the a sendrecv stream, the offer SHOULD indicate those codecs that the
offerer is willing to send and receive with. offerer is willing to send and receive with.
For recvonly RTP streams, the payload type numbers indicate the value For recvonly RTP streams, the payload type numbers indicate the value
of the payload type field in RTP packets the offerer is expecting to of the payload type field in RTP packets the offerer is expecting to
receive for that codec. For sendonly RTP streams, the payload type receive for that codec. For sendonly RTP streams, the payload type
numbers indicate the value of the payload type field in RTP packets numbers indicate the value of the payload type field in RTP packets
the offerer is planning to send for that codec type. For sendrecv RTP the offerer is planning to send for that codec. For sendrecv RTP
streams, the payload type numbers indicate the value of the payload streams, the payload type numbers indicate the value of the payload
type field the offerer expects to receive, and would prefer to send. type field the offerer expects to receive, and would prefer to send.
However, for sendonly and sendrecv streams, the answer might indicate However, for sendonly and sendrecv streams, the answer might indicate
different payload type numbers for the same codecs, in which case, different payload type numbers for the same codecs, in which case,
the offerer MUST send with the payload type numbers from the answer. the offerer MUST send with the payload type numbers from the answer.
Different payload type numbers may be needed in each Different payload type numbers may be needed in each
direction because of interoperability concerns with H.323. direction because of interoperability concerns with H.323.
As per RFC 2327, fmtp parameters MAY be present to provide additional As per RFC 2327, fmtp parameters MAY be present to provide additional
parameters of the media format. parameters of the media format.
In the case of RTP streams, all media descriptions SHOULD contain In the case of RTP streams, all media descriptions SHOULD contain
"a=rtpmap" mappings from RTP payload types to encodings. If there is "a=rtpmap" mappings from RTP payload types to encodings. If there is
no "a=rtpmap", the static payload type table from RFC 1890 [8] is to no "a=rtpmap", the default payload type mapping, as defined by the
be used. current profile in use (for example, RFC 1890 [5]) is to be used.
This allows easier migration away from static payload This allows easier migration away from static payload
types. types.
In all cases, the formats in the m line MUST be listed in order of 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 preference, with the first format listed being preferred. In this
case, preferred means that the recipient of the offer SHOULD use the case, preferred means that the recipient of the offer SHOULD use the
format with the highest preference that is acceptable to it. format with the highest preference that is acceptable to it.
If the ptime attribute is present for a stream, it indicates the If the ptime attribute is present for a stream, it indicates the
desired packetization interval that the offerer would like to desired packetization interval that the offerer would like to
receive. receive. The ptime attribute MUST be greater than zero.
If the bandwidth attribute is present for a stream, it indicates the If the bandwidth attribute is present for a stream, it indicates the
desired bandwidth that the offerer would like to receive. A value of desired bandwidth that the offerer would like to receive. A value of
zero is allowed, but discouraged. It indicates that no media should zero is allowed, but discouraged. It indicates that no media should
be sent. In the case of RTP, it would also disable all RTCP. be sent. In the case of RTP, it would also disable all RTCP.
If multiple media streams of different types are present, it means If multiple media streams of different types are present, it means
that the offerer wishes to use those streams at the same time. A that the offerer wishes to use those streams at the same time. A
typical case is an audio and video stream as part of a typical case is an audio and a video stream as part of a
videoconference. videoconference.
If multiple media streams of the same type are present in an offer, 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 it means that the offerer wishes to send (and/or receive) multiple
streams of that type at the same time. When sending multiple streams 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 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 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 of video) is mapped to each stream. When a user has a single source
for a particular media type, only one policy makes sense - that for a particular media type, only one policy makes sense: the source
source is sent to each stream of the same type. Each stream MAY use is sent to each stream of the same type. Each stream MAY use
different encodings. When receiving multiple streams of the same different encodings. When receiving multiple streams of the same
type, it is a matter of local policy as to how each stream is mapped 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, 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 speakers or a recording device in the case of audio). There are a few
constraints on the policies, however. First, when receiving multiple constraints on the policies, however. First, when receiving multiple
streams of the same type, each stream MUST be mapped to at least one 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 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 intent of receiving multiple streams of the same type is that they
should all be presented in parallel, rather than choosing just one. should all be presented in parallel, rather than choosing just one.
Another constraint is that when multiple streams are received and Another constraint is that when multiple streams are received and
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combining operation would be to present all of them to the user combining operation would be to present all of them to the user
interface. The third constraint is that if multiple sources are interface. The third constraint is that if multiple sources are
mapped to the same stream, those sources MUST be combined in some mapped to the same stream, those sources MUST be combined in some
media specific way before they are sent on the stream. Although media specific way before they are sent on the stream. Although
policies beyond these constraints are flexible, an agent won't policies beyond these constraints are flexible, an agent won't
generally want a policy that will copy media from its sinks to its 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 sources unless it is a conference server (i.e., don't copy received
media on one stream to another stream). media on one stream to another stream).
A typical usage example for multiple media streams of the same type A typical usage example for multiple media streams of the same type
is a pre-paid calling card application, where the user can enter in a is a pre-paid calling card application, where the user can press and
"long pound" at any time during a call to hangup and make a new call hold the pound ("#") key at any time during a call to hangup and make
on the same card. This requires media from the user to two a new call on the same card. This requires media from the user to two
destinations - the remote gateway, and to DTMF processing application destinations - the remote gateway, and the DTMF processing
which looks for the long pound. This would be accomplished with two application which looks for the pound. This could be accomplished
media streams, one sendrecv to the gateway, and the other sendonly with two media streams, one sendrecv to the gateway, and the other
(from the perspective of the user) to the DTMF application. sendonly (from the perspective of the user) to the DTMF application.
Once the offerer has sent the offer, it MUST be prepared to receive 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 media for any recvonly streams described by that offer. It MUST be
prepared to send and receive media for any sendrecv streams in the prepared to send and receive media for any sendrecv streams in the
offer (of course, it cannot actually send until the peer provides an offer, and send media for any sendonly streams in the offer (of
answer with the needed address and port information). It MUST be course, it cannot actually send until the peer provides an answer
prepared to receive media for recvonly or sendrecv streams using any with the needed address and port information). In the case of RTP,
media formats listed for those streams in the offer. In the case of even though it may receive media before the answer arrives, it will
RTP, even though it may receive media before the answer arrives, it not be able to send RTCP receiver reports until the answer arrives.
will not be able to send RTCP receiver reports until the answer
arrives.
5.2 Multicast Streams 5.2 Multicast Streams
If a session description contains a multicast media stream which is If a session description contains a multicast media stream which is
listed as send (receive) only, it means that the answerer can only listed as receive (send) only, it means that the participants,
send (receive) on that stream. The reversal of semantics for including the offerer and answerer, can only receive (send) on that
multicast is an artifact of the strong multicast bias of RFC 2327. stream. This differs from the unicast view, where the directionality
refers to the flow of media between offerer and answerer.
Beyond that clarification, the semantics of an offered multicast Beyond that clarification, the semantics of an offered multicast
stream are exactly as described in RFC 2327 [1]. stream are exactly as described in RFC 2327 [1].
6 Generating the answer 6 Generating the answer
The answer to an offered SDP is based on the offered SDP. If the The answer to an offered SDP is based on the offered SDP. If the
answer is different from the offer in any way (different IP answer is different from the offer in any way (different IP
addresses, ports, etc.), the origin line MUST be different in the addresses, ports, etc.), the origin line MUST be different in the
answer, since the answer is generated by a different entity. In that 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 case, the version number in the "o=" line of the answer is unrelated
the version number in the o line of the offer. 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 For each "m=" line in the offer, there MUST be a corresponding "m="
the answer. The answer MUST contain exactly the same number of m line in the answer. The answer MUST contain exactly the same number
lines as the offer. This allows for streams to be matched up based on of "m=" lines as the offer. This allows for streams to be matched up
their order. This implies that if the offer contained zero m lines, based on their order. This implies that if the offer contained zero
the answer MUST contain zero m lines. "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 "t=" line in the answer MUST equal that of the offer. The time of
the session cannot be negotiated. the session cannot be negotiated.
An offered stream MAY be rejected in the answer, for any reason. If a 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 stream is rejected, the offerer and answerer MUST NOT generate media
(or RTCP packets) for that stream. To reject an offered stream, the (or RTCP packets) for that stream. To reject an offered stream, the
port number in the corresponding stream in the answer is set to zero. port number in the corresponding stream in the answer MUST be set to
Any media formats listed are ignored. At least one MUST be present, zero. Any media formats listed are ignored. At least one MUST be
as specified by SDP. present, as specified by SDP.
Constructing an answer for each offered stream differs for unicast Constructing an answer for each offered stream differs for unicast
and multicast. and multicast.
6.1 Unicast Streams 6.1 Unicast Streams
If a stream is offered with a unicast address, the answer MUST If a stream is offered with a unicast address, the answer for that
contain a unicast address. The media type of the stream in the answer stream MUST contain a unicast address. The media type of the stream
MUST match that of the offer. in the answer MUST match that of the offer.
If a stream is offered as sendonly, the corresponding stream MUST be 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 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 listed as recvonly in the offer, the answer MUST be marked as
sendonly or inactive in the answer. If an offered media stream is sendonly or inactive in the answer. If an offered media stream is
listed as sendrecv (or contains no direction attribute, in which case listed as sendrecv (or if there is no direction attribute at the
it is sendrecv by default), the corresponding stream in the answer media or session level, in which case the stream is sendrecv by
MAY be marked as sendonly, recvonly, sendrecv, or inactive in the default), the corresponding stream in the answer MAY be marked as
answer. If an offered media stream is listed as inactive, it MUST be sendonly, recvonly, sendrecv, or inactive in the answer. If an
marked as inactive in the answer. offered media stream is listed as inactive, it MUST be marked as
inactive in the answer.
For streams marked as recvonly in the answer, the m line MUST contain For streams marked as recvonly in the answer, the "m=" line MUST
at least one media format the answerer is willing to receive with contain at least one media format the answerer is willing to receive
from amongst those listed in the offer. The stream MAY indicate with from amongst those listed in the offer. The stream MAY indicate
additional media formats, not listed in the corresponding stream in additional media formats, not listed in the corresponding stream in
the offer, that the answerer is willing to receive with. For streams the offer, that the answerer is willing to receive with. For streams
marked as sendonly in the answer, the m line MUST contain at least marked as sendonly in the answer, the "m=" line MUST contain at least
one media format the answerer is willing to send with from amongst one media format the answerer is willing to send with from amongst
those listed in the offer. For streams marked as sendrecv in the 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 answer, the "m=" line MUST contain at least one codec the answerer is
willing to both send and receive with, from amongst those listed in willing to both send and receive with, from amongst those listed in
the offer. For streams marked as inactive in the answer, the list of the offer. The stream MAY indicate additional media formats, not
media formats is constructed based on the offer. If the offer was listed in the corresponding stream in the offer, that the answerer is
sendonly, the list is constructed as if the answer were recvonly. willing to send or receive with (of course, it not be able to send
Similarly, if the offer was recvonly, the list is constructed as if with them at this time, since it was not listed in the offer). For
the answer were sendonly, and if the offer was sendrecv, the list is streams marked as inactive in the answer, the list of media formats
constructed as if the answer were sendrecv. If the offer was is constructed based on the offer. If the offer was sendonly, the
inactive, the list is constructed as if the offer were actually list is constructed as if the answer were recvonly. Similarly, if the
sendrecv and the answer were sendrecv. 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 The connection address and port in the answer indicate the address
where the answerer wishes to receive media (in the case of RTP, RTCP where the answerer wishes to receive media (in the case of RTP, RTCP
will be received on the port which is one higher). This address and will be received on the port which is one higher unless there is an
port MUST be present even for sendonly streams; in the case of RTP, explicit indication otherwise). This address and port MUST be present
the port one higher is still used to receive RTCP. 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 In the case of RTP, if a particular codec was referenced with a
specific payload type number in the offer, that same payload type 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 number SHOULD be used for that codec in the answer. Even if the same
payload type number is used, the answer MUST contain rtpmap payload type number is used, the answer MUST contain rtpmap
attributes to define the payload type mappings for dynamic types, and attributes to define the payload type mappings for dynamic payload
SHOULD contain mappings for static payload types. The media formats types, and SHOULD contain mappings for static payload types. The
in the m line MUST be listed in order of preference, with the first media formats in the "m=" line MUST be listed in order of preference,
format listed being preferred. In this case, preferred means that the with the first format listed being preferred. In this case, preferred
offerer SHOULD use the format with the highest preference from the means that the offerer SHOULD use the format with the highest
answer. preference from the answer.
Although the answerer MAY list the formats in their desired order of Although the answerer MAY list the formats in their desired order of
preference, it is RECOMMENDED that unless there is a specific reason, preference, it is RECOMMENDED that unless there is a specific reason,
the answer list formats in the same relative order they were present the answerer list formats in the same relative order they were
in the offer. In other words, if a stream in the offer lists audio present in the offer. In other words, if a stream in the offer lists
codecs 8, 22 and 48, in that order, and the answerer only supports audio codecs 8, 22 and 48, in that order, and the answerer only
codecs 8 and 48, it is RECOMMENDED that, if the answerer has no supports codecs 8 and 48, it is RECOMMENDED that, if the answerer has
reason to change it, the ordering of codecs in the answer be 8, 48, no reason to change it, the ordering of codecs in the answer be 8,
and not 48, 8. This helps assure that the same codec is used in both 48, and not 48, 8. This helps assure that the same codec is used in
directions. both directions.
The interpretation of fmtp parameters in an offer depends on the The interpretation of fmtp parameters in an offer depends on the
parameters. In many cases, those parameters describe specific parameters. In many cases, those parameters describe specific
configurations of the media format, and should therefore be processed configurations of the media format, and should therefore be processed
as the media format value itself would be. This means that the same 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 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 the media format they describe is present in the answer. Other fmtp
parameters are more like parameters, for which is is perfectly parameters are more like parameters, for which it is perfectly
acceptable for each agent to use different values. In that case, the acceptable for each agent to use different values. In that case, the
answer MAY contain fmtp parameters, and those MAY have the same answer MAY contain fmtp parameters, and those MAY have the same
values as those in the offer, or MAY be different. 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 ptime attribute for any media stream; this The answerer MAY include a non-zero ptime attribute for any media
indicates the packetization interval that the answerer would like to stream; this indicates the packetization interval that the answerer
receive. There is no requirement that the packetization interval be would like to receive. There is no requirement that the packetization
the same in each direction for a particular stream. interval be the same in each direction for a particular stream.
The answerer MAY include a bandwidth attribute for any media stream; The answerer MAY include a bandwidth attribute for any media stream;
this indicates the banwdith that the answerer would like the offerer this indicates the banwdith that the answerer would like the offerer
to use when sending media. The value of zero is allowed, interpreted to use when sending media. The value of zero is allowed, interpreted
as described in Section 5. as described in Section 5.
If the answerer has no media formats in common for a particular If the answerer has no media formats in common for a particular
offered stream, the answerer MUST reject that media stream by setting offered stream, the answerer MUST reject that media stream by setting
the port to zero. the port to zero.
If there are no media formats in common for all streams, the entire If there are no media formats in common for all streams, the entire
offered session is rejected. offered session is rejected.
Once the answerer has sent the answer, it MUST be prepared to receive 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 media for any recvonly streams described by that answer. It MUST be
prepared to send and receive media for any sendrecv streams in the prepared to send and receive media for any sendrecv streams in the
answer, and MAY send media immediately. It MUST be prepared to answer, and it MAY send media immediately. The answerer MUST be
receive media for recvonly or sendrecv streams using any media prepared to receive media for recvonly or sendrecv streams using any
formats listed for those streams in the answer, and MAY send media media formats listed for those streams in the answer, and it MAY send
immediately. When sending media, it SHOULD use a packetization media immediately. When sending media, it SHOULD use a packetization
interval equal to the valueof the ptime attribute in the offer, if interval equal to the valueof the ptime attribute in the offer, if
any was present. It SHOULD send media using a banwdith no higher than any was present. It SHOULD send media using a bandwidth no higher
the value of the bandwidth attribute in the offer, if any was than the value of the bandwidth attribute in the offer, if any was
present. The answerer SHOULD send using the most preferred codec in present. The answerer SHOULD send using the most preferred media
the offer supported by the answerer. format in the offer that is also listed in the answer. In the case of
RTP, it MUST use the payload type numbers from the offer, even if
they differ from those in the answer.
6.2 Multicast Streams 6.2 Multicast Streams
Unlike unicast, where there is a two-sided view of the stream, there 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, is only a single view of the stream for multicast. As such,
generating an answer to a multicast offer generally involves generating an answer to a multicast offer generally involves
modifying a limited set of aspects of the stream. modifying a limited set of aspects of the stream.
If a multicast stream is accepted, the address and port information If a multicast stream is accepted, the address and port information
in the answer MUST match that of the offer. Similarly, the in the answer MUST match that of the offer. Similarly, the
directionality information in the answer (sendonly, recvonly, or directionality information in the answer (sendonly, recvonly, or
sendrecv) MUST equal that of the offer. 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, and underlying assumption of the
multicast bias of RFC 2327.
The set of media formats in the answer MUST be equal to or be a 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 subset of those in the offer. Removing a format is a way for the
answerer to indicate that the format is not supported. answerer to indicate that the format is not supported.
The ptime and bandwidth attributes in the answer MUST equal the ones The ptime and bandwidth attributes in the answer MUST equal the ones
in the offer, if present. If not present, one MAY be added to the in the offer, if present. If not present, a non-zero ptime MAY be
answer. added to the answer.
7 Offerer Processing of the Answer 7 Offerer Processing of the Answer
When the offerer receives the answerer, it MAY send media on that When the offerer receives the answer, it MAY send media on the
stream (assuming it is listed as sendrecv or recvonly in the answer). accepted stream(s) (assuming it is listed as sendrecv or recvonly in
It SHOULD use the first media format listed in the answer when it 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. does send.
The reason this is a SHOULD, and not a MUST, is because The reason this is a SHOULD, and not a MUST, is because
there will oftentimes be a need to change codecs on the there will oftentimes be a need to change codecs on the
fly. For example, during silence periods, an agent might fly. For example, during silence periods, an agent might
like to switch to a comfort noise codec. Or, if the user like to switch to a comfort noise codec. Or, if the user
presses a number on the keypad, the agent might like to presses a number on the keypad, the agent might like to
send that using RFC 2833 [9]. Congestion control might send that using RFC 2833 [9]. Congestion control might
necessitate changing to a lower rate codec based on necessitate changing to a lower rate codec based on
feedback. feedback.
The offerer SHOULD send media according to the value of any ptime and The offerer SHOULD send media according to the value of any ptime and
bandwidth attribute in the answer. bandwidth attribute in the answer.
The offerer MAY immediately cease listening for media formats that
were listed in the initial offer, but not present in the answer.
8 Modifying the session 8 Modifying the session
At any point during the session, either participant MAY issue a new At any point during the session, either participant MAY issue a new
offer to modify characteristics of the session. It is fundamental to offer to modify characteristics of the session. It is fundamental to
the operation of the offer/answer model that the exact same the operation of the offer/answer model that the exact same
offer/answer procedure defined above is used for modifying parameters offer/answer procedure defined above is used for modifying parameters
of an existing session. of an existing session.
The offer MAY be identical to the last SDP provided to the other 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 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 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 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 previous SDP from the answerer, or it MAY be different. If the
offered SDP is different from the previous SDP, some constraints are offered SDP is different from the previous SDP, some constraints are
placed on its construction, discussed below. placed on its construction, discussed below.
Nearly all aspects of the session can be modified. New streams can be Nearly all aspects of the session can be modified. New streams can be
added, existing streams can be deleted, and parameters of existing added, existing streams can be deleted, and parameters of existing
streams can change. When issuing an offer that modifies the session, 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 the "o=" line of the new SDP MUST be identical to that in the
SDP, except that the version in the origin field MUST increment from previous SDP, except that the version in the origin field MUST
the previous SDP by one. If the version in the origin line does not increment from the previous SDP by one. If the version in the origin
increment, the SDP MUST be identical to the SDP with that version line does not increment, the SDP MUST be identical to the SDP with
number. The answerer MUST be prepared to receive an offer that that version number. The answerer MUST be prepared to receive an
contains SDP with a version that has not changed; this is effectively offer that contains SDP with a version that has not changed; this is
a no-op. However, the answerer MUST generate a valid answer (which effectively a no-op. However, the answerer MUST generate a valid
MAY be the same as the previous SDP from the answerer, or MAY be answer (which MAY be the same as the previous SDP from the answerer,
different), according to the procedures defined in Section 6. 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 If an SDP is offered, which is different from the previous SDP, the
new SDP MUST have a matching media section for each media section in 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 media the previous SDP. In other words, if the previous SDP had N "m="
lines, the new SDP MUST have at least N media lines. The ith media 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 ith 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 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 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 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 requirements, the number of "m=" lines in a stream never decreases,
only increases. Deleted media streams from a previous SDP MUST NOT be but either stays the same or increases. Deleted media streams from a
removed from a new SDP. 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 8.1 Adding a Media Stream
New media streams are created by new additional media descriptions New media streams are created by new additional media descriptions
below the existing ones, or by reusing the "slot" used by an old 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. New media stream which had been disabled by setting its port to zero. New
media descriptions MUST appear below any existing media sections. The media descriptions MUST appear below any existing media sections. The
rules for formatting this media section are identical to those rules for formatting these media descriptions are identical to those
described in Section 5. described in Section 5.
When the answerer receives an SDP with more media descriptions than 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 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 stream in a slot where the port was previously zero, the answerer
knows that new media streams are being added. These can be rejected knows that new media streams are being added. These can be rejected
or accepted by placing a matching media description in the answer. or accepted by placing a matching media description in the answer.
The procedures for constructing the new media description in the The procedures for constructing the new media description in the
answer are described in Section 6. answer are described in Section 6.
8.2 Removing a media stream 8.2 Removing a Media Stream
Existing media streams are removed by creating a new SDP with the Existing media streams are removed by creating a new SDP with the
port number for that stream set to zero. Otherwise, the media port number for that stream set to zero. The stream description MAY
description SHOULD be formatted identically to the corresponding omit all attributes present previously, and MAY list just a single
stream in the previous SDP. media format.
A stream that is offered with a port of zero MUST be marked with port A stream that is offered with a port of zero MUST be marked with port
zero in the answer. Otherwise, the media description for the removed zero in the answer. Like the offer, the answer MAY omit all
stream SHOULD be formatted identically to the corresponding stream in attributes present previously, and MAY list just a single media
the previous SDP. format from amongst those in the offer.
Removal of a media stream implies that media is no longer sent for Removal of a media stream implies that media is no longer sent for
that stream. Any resources associated with it can be released. The that stream, and any media that is received is discarded. In the case
user interface might indicate that the stream has terminated, by of RTP, RTCP transmission also ceases, as does processing of any
closing the associated window on a PC, for example. 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 8.3 Modifying a Media Stream
Nearly all characteristics of a media stream can be modified. Nearly all characteristics of a media stream can be modified.
8.3.1 Modifying Address, Port or Transport 8.3.1 Modifying Address, Port or Transport
The port number for a stream MAY be changed. To do this, the offerer 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 creates a new media description, with the port number in the m line
different from the corresponding stream in the previous SDP. If only different from the corresponding stream in the previous SDP. If only
the port number is to be changed, the rest of the media stream the port number is to be changed, the rest of the media stream
description SHOULD remain unchanged. The offerer MUST be prepared to 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 receive media on both the old and new ports as soon as the offer is
sent. The offerer MUST NOT cease listening for media on the old port sent. The offerer SHOULD NOT cease listening for media on the old
until the answer is received and media arrives on the new port. port until the answer is received and media arrives on the new port.
Received, in this case, means that the media is passed to an audio Doing so could result in loss of media during the transition.
sink. This means that if there is an audio playout buffer, the agent Received, in this case, means that the media is passed to a media
would continue to listen on the old port until the media on the new sink. This means that if there is a playout buffer, the agent would
port reached the top of the playout buffer. At that time, it MAY continue to listen on the old port until the media on the new port
cease listening for media on the old 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 The corresponding media stream in the answer MAY be the same as the
stream in the previous SDP from the answerer, or MAY be different. stream in the previous SDP from the answerer, or it MAY be different.
If the updated stream is accepted by the answerer, the answerer If the updated stream is accepted by the answerer, the answerer
SHOULD begin sending traffic for that stream to the new port SHOULD begin sending traffic for that stream to the new port
immediately. If the answerer changes the port from the previous SDP, 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 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 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 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. 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 offer can cease 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 being prepared to receive using the new port as soon as the rejection
is received. is received.
To change the IP address where media is sent to, the same procedure 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 is followed for changing the port number. The only difference is that
the connection line is updated, not the port number. the connection line is updated, not the port number.
The transport for a stream MAY be changed. The process for doing this The transport for a stream MAY be changed. The process for doing this
is identical to changing the port, excepting the transport is is identical to changing the port, except the transport is updated,
updated, not the port. not the port.
8.3.2 Changing the Set of Media Formats 8.3.2 Changing the Set of Media Formats
The list of media formats used in the session MAY be changed. To do 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 this, the offerer creates a new media description, with the list of
media formats in the m line different from the corresponding stream media formats in the "m=" line different from the corresponding media
in the previous SDP. This list MAY include new formats, and MAY stream in the previous SDP. This list MAY include new formats, and
remove formats present from the previous SDP. However, in the case of MAY remove formats present from the previous SDP. However, in the
RTP, the mapping from a particular dynamic payload type number to a case of RTP, the mapping from a particular dynamic payload type
particular codec MUST NOT change for the duration of a session. For number to a particular codec within that media stream MUST NOT change
example, if A generates an offer with G.711 assigned to dynamic for the duration of a session. For example, if A generates an offer
payload type number 46, payload type number 46 MUST refer to G.711 with G.711 assigned to dynamic payload type number 46, payload type
from that point forward in any offers or answers for that session. number 46 MUST refer to G.711 from that point forward in any offers
However, it is acceptable for multiple payload type numbers to be or answers for that media stream within the session. However, it is
mapped to the same codec, so that an updated offer could also use acceptable for multiple payload type numbers to be mapped to the same
payload type number 72 for G.711. The mappings need to remain fixed codec, so that an updated offer could also use payload type number 72
for the duration of the session because of the loose synchronization for G.711.
between signaling exchanges of SDP and the media stream.
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 The corresponding media stream in the answer is formulated as
described in Section 6, and may result in a change in media formats 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 as well. Similarly, as described in Section 6, as soon as it sends
its answer, the answerer MAY begin sending media using any new codecs its answer, the answerer MAY begin sending media using any new codecs
in the offer (assuming the stream allows for sending), and MUST NOT in the offer (assuming the stream allows for sending), and MUST NOT
send using any formats that are not in the offer, even if they were 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 present in a previous SDP from the peer. Similarly, when the offerer
receives the answer, it MAY begin sending media using any new codecs receives the answer, it MAY begin sending media using any new codecs
in the answer (assuming the stream allows for sending), and MUST NOT in the answer (assuming the stream allows for sending), and MUST NOT
skipping to change at page 16, line 5 skipping to change at page 17, line 45
answerer SHOULD begin sending with the new media type and codecs as answerer SHOULD begin sending with the new media type and codecs as
soon as it receives the offer. soon as it receives the offer.
8.3.4 Changing Attributes 8.3.4 Changing Attributes
Any other attributes in a media description MAY be updated in an Any other attributes in a media description MAY be updated in an
offer or answer. Generally, an agent MUST send media (if the offer or answer. Generally, an agent MUST send media (if the
directionality of the stream allows) using the new parameters once directionality of the stream allows) using the new parameters once
the SDP with the change is received. the SDP with the change is received.
8.4 Putting a media stream on hold 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., 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 media streams, request that it temporarily stops sending one or more unicast media
a party offers the other an updated SDP. streams, a party offers the other an updated SDP.
If the stream to be placed on hold was previously a sendrecv media 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 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 to be placed on hold was previously a recvonly media stream, it is
placed on hold by marking it inactive. placed on hold by marking it inactive.
This means that a stream is placed "on hold" separately in each This means that a stream is placed "on hold" separately in each
direction. Each stream is placed "on hold" independently. The direction. Each stream is placed "on hold" independently. The
recipient of an offer for a stream on-hold SHOULD NOT automatically recipient of an offer for a stream on-hold SHOULD NOT automatically
return an answer with the corresponding stream on hold. An SDP with return an answer with the corresponding stream on hold. An SDP with
all streams "on hold" is referred to as held SDP all streams "on hold" is referred to as held SDP
Certain third party call control scenarios do not work when Certain third party call control scenarios do not work when
a UA responds to held SDP with held SDP. an answerer responds to held SDP with held SDP.
Typically, when a user "presses" hold, the UA will generate an offer Typically, when a user "presses" hold, the agent will generate an
with all streams in the SDP indicating a direction of sendonly, and offer with all streams in the SDP indicating a direction of sendonly,
it will also locally mute, so that no media is sent to the far end, and it will also locally mute, so that no media is sent to the far
and no media is played out. end, and no media is played out.
RFC 2543 specified that placing a user on hold was accomplished by RFC 2543 [10] specified that placing a user on hold was accomplished
setting the connection address to 0.0.0.0. This has been deprecated, by setting the connection address to 0.0.0.0. Its usage for putting a
since it doesn't allow for RTCP to be used with held streams, and call on hold is no longer recommended, since it doesn't allow for
breaks with connection oriented media. However, a UA MUST be capable RTCP to be used with held streams, doesn't work with IPv6, and breaks
of receiving SDP with a connection address of 0.0.0.0, in which case with connection oriented media. However, it can be useful in an
it means that neither RTP nor RTCP should be sent to the peer. 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. 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 9 Indicating Capabilities
Before an agent sends an offer, it is helpful to know if the media 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 formats in that offer would be acceptable to the answerer. Certain
protocols, like SIP, provide a means to query for such capabilities. protocols, like SIP, provide a means to query for such capabilities.
SDP can be used in responses to such queries to indicate SDP can be used in responses to such queries to indicate
capabilities. This section describes how such an SDP message is capabilities. This section describes how such an SDP message is
formatted. The ability of baseline SDP to indicate capabilities is formatted. Since SDP has no way to indicate that the message is for
very limited. It cannot express allowed parameter ranges or values, the purpose of capability indication, this is determined from the
and can not be done in parallel with an offer/answer itself. context of the higher layer protocol. The ability of baseline SDP to
Extensions might address such limitations in the future. 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 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 follows. It MUST be a valid SDP, except that it MAY omit both "e="
lines. The t line MUST be equal to "0 0". For each media type and "p=" lines. The "t=" line MUST be equal to "0 0". For each media
supported by the agent, there must be a corresponding media type supported by the agent, there MUST be a corresponding media
description of that type. The port and connection address have no description of that type. The session ID in the origin field MUST be
meaning, and their values are arbitrary. The transport component of unique for each SDP constructed to indicate media capabilities. The
the m line indicates the preferred transport for that media type. For port MUST be set to zero, but the connection address is arbitrary.
each media format of that type supported by the agent, there SHOULD The usage of port zero makes sure that an SDP formatted for
be a media format listed in the m line. In the case of RTP, if capabilities does not cause media streams to be established if it is
dynamic payload types are used, an rtpmap attribute MUST be present interpreted as an offer or answer.
to bind the type to a specific format. There is no way to indicate
constraints, such as how many simultaneous streams can be supported The transport component of the "m=" line indicates the transport for
for a particular codec, and so on. that media type. For each media format of that type supported by the
agent, there SHOULD be a media format listed in the "m=" line. In the
case of RTP, if dynamic payload types are used, an rtpmap attribute
MUST be present to bind the type to a specific format. There is no
way to indicate constraints, such as how many simultaneous streams
can be supported for a particular codec, and so on.
v=0 v=0
o=carol 28908764872 28908764872 IN IP4 100.3.6.6 o=carol 28908764872 28908764872 IN IP4 100.3.6.6
s=- s=-
t=0 0 t=0 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
m=audio 0 RTP/AVP 0 1 3 m=audio 0 RTP/AVP 0 1 3
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rtpmap:1 1016/8000 a=rtpmap:1 1016/8000
a=rtpmap:3 GSM/8000 a=rtpmap:3 GSM/8000
skipping to change at page 18, line 15 skipping to change at page 20, line 18
c=IN IP4 host.anywhere.com c=IN IP4 host.anywhere.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVP 31 m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
m=video 53000 RTP/AVP 32 m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000 a=rtpmap:32 MPV/90000
The callee, Bob, does not want to receive or send the first video The callee, Bob, does not want to receive or send the first video
stream, so it returns the media description below as the answer: stream, so he returns the SDP below as the answer:
v=0 v=0
o=bob 2890844730 2890844730 IN IP4 host.example.com o=bob 2890844730 2890844730 IN IP4 host.example.com
s= s=
c=IN IP4 host.example.com c=IN IP4 host.example.com
t=0 0 t=0 0
m=audio 47920 RTP/AVP 0 m=audio 49920 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31 m=video 0 RTP/AVP 31
m=video 53000 RTP/AVP 32 m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000 a=rtpmap:32 MPV/90000
At some point later, Bob decides to change the port where he will At some point later, Bob decides to change the port where he will
receive the audio stream (from 47920 to 6400), and at the same time, 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 add an additional audio stream as receive only, using the RTP payload
format for events [9]. Bob offers the following SDP in the offer: format for events [9]. Bob offers the following SDP in the offer:
v=0 v=0
o=bob 2890844730 2890844731 IN IP4 host.example.com o=bob 2890844730 2890844731 IN IP4 host.example.com
s= s=
c=IN IP4 host.example.com c=IN IP4 host.example.com
t=0 0 t=0 0
m=audio 6400 RTP/AVP 0 m=audio 65422 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31 m=video 0 RTP/AVP 31
m=video 53000 RTP/AVP 32 m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000 a=rtpmap:32 MPV/90000
m=audio 8864 RTP/AVP 110 m=audio 51434 RTP/AVP 110
a=rtpmap:110 telephone-events a=rtpmap:110 telephone-events/8000
a=recvonly a=recvonly
Alice accepts the additional media stream, and so generates the Alice accepts the additional media stream, and so generates the
following answer: following answer:
v=0 v=0
o=alice 2890844526 2890844527 IN IP4 host.anywhere.com o=alice 2890844526 2890844527 IN IP4 host.anywhere.com
s= s=
c=IN IP4 host.anywhere.com c=IN IP4 host.anywhere.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVP 31 m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000 a=rtpmap:31 H261/90000
m=video 53000 RTP/AVP 32 m=video 53000 RTP/AVP 32
a=rtpmap:32 MPV/90000 a=rtpmap:32 MPV/90000
m=audio 4520 RTP/AVP 110 m=audio 53122 RTP/AVP 110
a=rtpmap:110 telephone-events a=rtpmap:110 telephone-events/8000
a=sendonly a=sendonly
10.2 One of N Codec Selection 10.2 One of N Codec Selection
A common occurence in embedded phones is that the DSP used for A common occurence in embedded phones is that the Digital Signal
compression can support multiple codecs at a time, but once that Processor (DSP) used for compression can support multiple codecs at a
codec is selected, it cannot be readily changed on the fly. This time, but once that codec is selected, it cannot be readily changed
example shows how a session can be set up using an initial on the fly. This example shows how a session can be set up using an
offer/answer exchange, followed immediately by a second one to lock initial offer/answer exchange, followed immediately by a second one
down the set of codecs. to lock down the set of codecs.
The initial offer from Alice to Bob indicates a single audio stream 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 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 marked as inactive, since media cannot be received until a codec
is locked down: is locked down:
v=0 v=0
o=alice 2890844526 2890844526 IN IP4 host.anywhere.com o=alice 2890844526 2890844526 IN IP4 host.anywhere.com
s= s=
c=IN IP4 host.anywhere.com c=IN IP4 host.anywhere.com
t=0 0 t=0 0
m=audio 6400 RTP/AVP 0 4 18 m=audio 62986 RTP/AVP 0 4 18
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rtpmap:4 G723/8000 a=rtpmap:4 G723/8000
a=rtpmap:18 G729/8000 a=rtpmap:18 G729/8000
a=inactive a=inactive
Bob can support PCMU and G.723 simultaneously. So, he sends the Bob can dynamic switching between PCMU and G.723. So, he sends the
following answer: following answer:
v=0 v=0
o=bob 2890844730 2890844731 IN IP4 host.example.com o=bob 2890844730 2890844731 IN IP4 host.example.com
s= s=
c=IN IP4 host.example.com c=IN IP4 host.example.com
t=0 0 t=0 0
m=audio 6400 RTP/AVP 0 4 m=audio 54344 RTP/AVP 0 4
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=rtpmap:4 G723/8000 a=rtpmap:4 G723/8000
a=inactive a=inactive
Alice can then select any one of these two codecs. So, she sends an Alice can then select any one of these two codecs. So, she sends an
updated offer with a sendrecv stream: updated offer with a sendrecv stream:
v=0 v=0
o=alice 2890844526 2890844527 IN IP4 host.anywhere.com o=alice 2890844526 2890844527 IN IP4 host.anywhere.com
s= s=
c=IN IP4 host.example.com c=IN IP4 host.anywhere.com
t=0 0 t=0 0
m=audio 6400 RTP/AVP 4 m=audio 62986 RTP/AVP 4
a=rtpmap:4 G723/8000 a=rtpmap:4 G723/8000
a=sendrecv a=sendrecv
Bob accepts the single codec: Bob accepts the single codec:
v=0 v=0
o=bob 2890844730 2890844732 IN IP4 host.example.com o=bob 2890844730 2890844732 IN IP4 host.example.com
s= s=
c=IN IP4 host.example.com c=IN IP4 host.example.com
t=0 0 t=0 0
m=audio 6400 RTP/AVP 4 m=audio 54344 RTP/AVP 4
a=rtpmap:4 G723/8000 a=rtpmap:4 G723/8000
a=sendrecv a=sendrecv
As an alternative to using a=inactive in the first exchange, Alice 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, 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 generate an updated offer locking down the codec to the one just
recevied. recevied. There is a potential race condition, however. If both Bob
and Alice do this (Alice in her offer, Bob in his answer), both may
11 Changes since draft-rosenberg-mmusic-sdp-offer-answer-00 attempt to issue updated offers at the same time. The protocol which
carries offers and answers has to provide a means to resolve these
o Examples had the wrong ordering of c and t lines glare conditions, so that only one offer will be used.
o Eliminated repitions of rfc2327 requirements in section 2.
o Changed e/p line handling, so that the MAY be omitted from an
SDP used for offer/answer applications. Removed from the
examples.
o When modifying the ports/addresses for a media stream, you
have to listen on the old port until media arrives at the new
port, AND the answer arrives.
o Clarified how transitioning of codecs works when an offer
updates the codec set.
o Added text defining how the transitioning of codecs/ports
reverts when the offered stream is rejected.
o Finished extraction of SIP specific text to make this
orthogonal to SIP.
o Generalized the text in 2.1 on sending streams of the same
media type.
o Made multicast/unicast treatment at the stream level, rather
than the SDP level, to handle cases of SDP with mixed unicast
and multicast streams.
o Removed text on rfc2833 hack of adding receive only codecs to
the answer for a sendrecv stream.
o Added a terminology section.
o Added discussion of t line usage.
o Added text on the source/sink/stream mapping policies agreed
at IETF 52.
o Added text allowing for reuse of media slots previously set to 11 Security Considerations
zero.
o Added text describing the three possible approaches for There are numerous attacks possible if an attacker can modify offers
synchronizing changes in media formats. 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.
o Allow for the dynamic payload type numbers to change in each Offer/answer relies an transport within an application signaling
direction, but its a SHOULD to use the same ones. Explicitly protocol, such as SIP. It also relies on that protocol for security
called out the H.323 interop issue. 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.
o Added capabilities format. 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.
o Added example showing 1 of N codec selection. 12 IANA Considerations
o Clarified that transitions occur not when new media is There are no IANA considerations with this specification.
received, but when it is played out.
o Added protocol operation section. 13 Acknowledgements
o Added discussion on usage of the bandwidth modifier. The authors would like to thank Allison Mankin, Rohan Mahy, Joerg
Ott, and Flemming Andreasen for their detailed comments.
12 Author's Addresses 14 Author's Addresses
Jonathan Rosenberg Jonathan Rosenberg
dynamicsoft dynamicsoft
72 Eagle Rock Avenue 72 Eagle Rock Avenue
First Floor First Floor
East Hanover, NJ 07936 East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com email: jdrosen@dynamicsoft.com
Henning Schulzrinne Henning Schulzrinne
Dept. of Computer Science Dept. of Computer Science
Columbia University Columbia University
1214 Amsterdam Avenue 1214 Amsterdam Avenue
New York, NY 10027 New York, NY 10027
USA USA
email: schulzrinne@cs.columbia.edu email: schulzrinne@cs.columbia.edu
13 Bibliography 15 Normative References
[1] M. Handley and V. Jacobson, "SDP: session description protocol," [1] M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments 2327, Internet Engineering Task Force, Apr. Request for Comments 2327, Internet Engineering Task Force, Apr.
1998. 1998.
[2] M. Handley, C. Perkins, and E. Whelan, "Session announcement [2] S. Bradner, "Key words for use in RFCs to indicate requirement
protocol," Request for Comments 2974, Internet Engineering Task
Force, Oct. 2000.
[3] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation
protocol," Internet Draft, Internet Engineering Task Force, Oct.
2001. Work in progress.
[4] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments 2119, Internet Engineering Task Force, levels," Request for Comments 2119, Internet Engineering Task Force,
Mar. 1997. Mar. 1997.
[5] H. Schulzrinne, A. Rao, and R. Lanphier, "Real time streaming [3] R. Kumar and M. Mostafa, "Conventions for the use of the session
protocol (RTSP)," Request for Comments 2326, Internet Engineering
Task Force, Apr. 1998.
[6] R. Kumar and M. Mostafa, "Conventions for the use of the session
description protocol (SDP) for ATM bearer connections," Request for description protocol (SDP) for ATM bearer connections," Request for
Comments 3108, Internet Engineering Task Force, May 2001. Comments 3108, Internet Engineering Task Force, May 2001.
[7] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a [4] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
transport protocol for real-time applications," Request for Comments transport protocol for real-time applications," Request for Comments
1889, Internet Engineering Task Force, Jan. 1996. 1889, Internet Engineering Task Force, Jan. 1996.
[8] H. Schulzrinne, "RTP profile for audio and video conferences with [5] H. Schulzrinne, "RTP profile for audio and video conferences with
minimal control," Request for Comments 1890, Internet Engineering minimal control," Request for Comments 1890, Internet Engineering
Task Force, Jan. 1996. Task Force, Jan. 1996.
16 Non-Normative References
[6] M. Handley, C. Perkins, and E. Whelan, "Session announcement
protocol," Request for Comments 2974, Internet Engineering Task
Force, Oct. 2000.
[7] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation
protocol," Internet Draft, Internet Engineering Task Force, Oct.
2001. Work in progress.
[8] H. Schulzrinne, A. Rao, and R. Lanphier, "Real time streaming
protocol (RTSP)," Request for Comments 2326, Internet Engineering
Task Force, Apr. 1998.
[9] H. Schulzrinne and S. Petrack, "RTP payload for DTMF digits, [9] H. Schulzrinne and S. Petrack, "RTP payload for DTMF digits,
telephony tones and telephony signals," Request for Comments 2833, telephony tones and telephony signals," Request for Comments 2833,
Internet Engineering Task Force, May 2000. Internet Engineering Task Force, May 2000.
[10] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
Full Copyright Statement Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved. Copyright (c) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
 End of changes. 

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