draft-ietf-sipping-sip-offeranswer-07.txt   draft-ietf-sipping-sip-offeranswer-08.txt 
SIPPING Working Group T. Sawada SIPPING Working Group T. Sawada
Internet Draft KDDI Corporation Internet Draft KDDI Corporation
Intended status: Informational P. Kyzivat Intended status: Informational P. Kyzivat
Expires: September 2008 Cisco Systems, Inc. Expires: October 2008 Cisco Systems, Inc.
March 31, 2008 April 25, 2008
SIP (Session Initiation Protocol) Usage of the Offer/Answer Model SIP (Session Initiation Protocol) Usage of the Offer/Answer Model
draft-ietf-sipping-sip-offeranswer-07.txt draft-ietf-sipping-sip-offeranswer-08.txt
Status of this Memo Status of this Memo
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Abstract Abstract
The Session Initiation Protocol (SIP) utilizes the offer/answer The Session Initiation Protocol (SIP) utilizes the offer/answer
model to establish and update multimedia sessions using the Session model to establish and update multimedia sessions using the Session
Description Protocol (SDP). The description of the offer/answer Description Protocol (SDP). The description of the offer/answer
model in SIP is dispersed across multiple RFCs. This document model in SIP is dispersed across multiple RFCs. This document
summarizes all the current usages of the offer/answer model in SIP summarizes all the current usages of the offer/answer model in SIP
communication. communication.
Table of Contents Table of Contents
1. Introduction................................................... 3 1. Introduction.................................................. 3
2. Summary of SIP usage of the Offer/Answer Model................. 3 1.1. Terminology.............................................. 3
2.1. Offer/Answer Exchange Pairs in SIP Messages............... 3 2. Summary of SIP usage of the Offer/Answer Model................ 3
2.2. Rejection of an Offer..................................... 5 2.1. Offer/Answer Exchange Pairs in SIP Messages.............. 4
2.3. Session Description which is not Offer nor Answer......... 6 2.2. Rejection of an Offer.................................... 5
3. Detailed Discussion of the Offer/Answer Model for SIP.......... 6 2.3. Session Description which is not Offer nor Answer........ 6
3.1. Offer/Answer for the INVITE method with 100rel extension.. 6 3. Detailed Discussion of the Offer/Answer Model for SIP......... 7
3.1.1. INVITE Request with SDP.............................. 7 3.1. Offer/Answer for the INVITE method with 100rel extension. 7
3.1.2. INVITE request without SDP........................... 9 3.1.1. INVITE Request with SDP............................. 7
3.2. Offer/Answer Exchange in Early Dialog.................... 10 3.1.2. INVITE request without SDP.......................... 9
3.3. Offer/Answer Exchange in an Established Dialog........... 10 3.2. Offer/Answer Exchange in Early Dialog................... 10
4. Exceptional Case Handling..................................... 11 3.3. Offer/Answer Exchange in an Established Dialog.......... 10
4.1. Message Crossing Case Handling........................... 11 4. Exceptional Case Handling.................................... 11
4.2. Glare Case Handling...................................... 13 4.1. Message Crossing Case Handling.......................... 11
5. Content of Offers and Answers................................. 14 4.2. Glare Case Handling..................................... 13
5.1. General Principle for Constructing Offers and Answers.... 14 5. Content of Offers and Answers................................ 14
5.2. Choice of Media Types and Formats to Include and Exclude. 15 5.1. General Principle for Constructing Offers and Answers... 14
5.2.1. Sending an Initial INVITE with Offer................ 15 5.2. Choice of Media Types and Formats to Include and Exclude 15
5.2.1. Sending an Initial INVITE with Offer............... 15
5.2.2. Responding with an Offer when the Initial INVITE has 5.2.2. Responding with an Offer when the Initial INVITE has
no Offer................................................... 15 no Offer.................................................. 15
5.2.3. Answering an Initial INVITE with Offer.............. 16 5.2.3. Answering an Initial INVITE with Offer............. 16
5.2.4. Answering when the Initial INVITE had no Offer...... 17 5.2.4. Answering when the Initial INVITE had no Offer..... 17
5.2.5. Subsequent Offers and Answers....................... 17 5.2.5. Subsequent Offers and Answers...................... 17
5.3. Hold and Resume of media................................. 18 5.3. Hold and Resume of media................................ 18
5.4. Behavior on receiving SDP with c=0.0.0.0................. 19 5.4. Behavior on receiving SDP with c=0.0.0.0................ 19
6. Remaining Issues or Best Practices on Offer/Answer............ 19 6. Remaining Issues or Best Practices on Offer/Answer........... 19
6.1. Rejecting PRACK Offer.................................... 20 6.1. Rejecting PRACK Offer................................... 20
6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
Transaction................................................... 21 Transaction.................................................. 21
6.3. Offer in a Reliable Response............................. 22 6.3. Offer in a Reliable Response............................ 22
6.4. Requesting Hold while already on Hold.................... 23 6.4. Requesting Hold while already on Hold................... 23
7. Add New Offer/Answer Usage in SIP............................. 23 7. Add New Offer/Answer Usage in SIP............................ 23
7.1. Explicit Usage........................................... 23 7.1. Explicit Usage.......................................... 23
7.2. Rejection of an Offer.................................... 23 7.2. Rejection of an Offer................................... 23
7.3. Backward Compatibility................................... 23 7.3. Backward Compatibility.................................. 23
7.4. Exceptional Case Handling................................ 23 7.4. Exceptional Case Handling............................... 23
8. IANA Considerations........................................... 24 8. IANA Considerations.......................................... 24
9. Security Considerations....................................... 24 9. Security Considerations...................................... 24
10. References................................................... 24 10. Acknowledgement............................................. 24
10.1. Normative References.................................... 24 11. References.................................................. 24
10.2. Informative References.................................. 24 11.1. Normative References................................... 24
Author's Addresses............................................... 25 11.2. Informative References................................. 24
Full Copyright Statement......................................... 25 Author's Addresses.............................................. 25
Intellectual Property Statement.................................. 25 Full Copyright Statement........................................ 25
Acknowledgment................................................... 26 Intellectual Property Statement................................. 25
Acknowledgment.................................................. 26
1. Introduction 1. Introduction
SIP utilizes the offer/answer model to establish and update the SIP utilizes the offer/answer model to establish and update the
session. The rules to govern the offer/answer behaviors in SIP are session. The rules to govern the offer/answer behaviors in SIP are
described in the several RFCs. described in the several RFCs.
The primary purpose of this document is to describe all forms of The primary purpose of this document is to describe all forms of
SIP usage of the offer/answer model in one document to help the SIP usage of the offer/answer model in one document to help the
readers to fully understand it. Also, this document tries to readers to fully understand it. Also, this document tries to
incorporate the results of the discussions on the controversial incorporate the results of the discussions on the controversial
issues to avoid repeating the same discussions later. issues to avoid repeating the same discussions later.
This document is not intended to make normative changes. Rather, it This document is not intended to make normative changes. Rather, it
makes the remaining open issues clear and leaves them for further makes the remaining open issues clear and leaves them for further
study. study.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 [1].
This document only uses these key words when referencing normative
statements in existing RFCs.
2. Summary of SIP usage of the Offer/Answer Model 2. Summary of SIP usage of the Offer/Answer Model
The offer/answer model itself is independent from the higher layer The offer/answer model itself is independent from the higher layer
application protocols which utilize it. SIP is one of the application protocols which utilize it. SIP is one of the
applications using the offer/answer model. RFC 3264 [3] defines the applications using the offer/answer model. RFC 3264 [4] defines the
offer/answer model, but does not specify which SIP messages should offer/answer model, but does not specify which SIP messages should
convey an offer or an answer. This should be defined in the SIP convey an offer or an answer. This should be defined in the SIP
core and extensions RFCs. core and extensions RFCs.
In theory, any SIP message can include a session description in its In theory, any SIP message can include a session description in its
body. But a session description in a SIP message is not necessarily body. But a session description in a SIP message is not necessarily
an offer or an answer. Only certain session description usages that an offer or an answer. Only certain session description usages that
conform to the rules described in standards-track RFCs can be conform to the rules described in standards-track RFCs can be
interpreted as an offer or an answer. The rules for how to handle interpreted as an offer or an answer. The rules for how to handle
the offer/answer model are currently defined in several RFCs. the offer/answer model are currently defined in several RFCs.
The offer/answer model defines a mechanism for update of sessions. The offer/answer model defines a mechanism for update of sessions.
In SIP, a dialog is used to associate an offer/answer exchange with In SIP, a dialog is used to associate an offer/answer exchange with
the session which it is to update. In other words, only the the session which it is to update. In other words, only the
offer/answer exchange in the SIP dialog can update the session offer/answer exchange in the SIP dialog can update the session
which is managed by that dialog. which is managed by that dialog.
2.1. Offer/Answer Exchange Pairs in SIP Messages 2.1. Offer/Answer Exchange Pairs in SIP Messages
Currently, the rules on the offer/answer model are defined in RFC Currently, the rules on the offer/answer model are defined in RFC
3261 [1], RFC 3262 [2] and RFC 3311 [4]. In these RFCs, only the 3261 [2], RFC 3262 [3] and RFC 3311 [5]. In these RFCs, only the
six patterns shown in Table 1 are defined for exchanging an offer six patterns shown in Table 1 are defined for exchanging an offer
and an answer with SIP messages. and an answer with SIP messages.
Note that an offer/answer exchange initiated by an INVITE request Note that an offer/answer exchange initiated by an INVITE request
must follow exactly one of the patterns 1, 2, 3, 4. When an initial must follow exactly one of the patterns 1, 2, 3, 4. When an initial
INVITE causes multiple dialogs due to forking, an offer/answer INVITE causes multiple dialogs due to forking, an offer/answer
exchange is carried out independently in each distinct dialog. When exchange is carried out independently in each distinct dialog. When
an INVITE request contains no offer, only pattern 2 or pattern 4 an INVITE request contains no offer, only pattern 2 or pattern 4
apply. 'The first reliable non-failure message' must have an offer apply. 'The first reliable non-failure message' must have an offer
if there is no offer in the INVITE request. This means that UA if there is no offer in the INVITE request. This means that UA
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exchange is required. In that case the prior SDP will exchange is required. In that case the prior SDP will
typically be repeated. typically be repeated.
There may be ONLY ONE offer/answer negotiation in progress for a There may be ONLY ONE offer/answer negotiation in progress for a
single dialog at any point in time. Section 4 explains how to single dialog at any point in time. Section 4 explains how to
ensure this. When an INVITE results in multiple dialogs each has a ensure this. When an INVITE results in multiple dialogs each has a
separate offer/answer negotiation. separate offer/answer negotiation.
NOTE: This is when using a Content-Disposition of "session". NOTE: This is when using a Content-Disposition of "session".
There may be a second offer/answer negotiation in progress There may be a second offer/answer negotiation in progress
using a Content-Disposition of "early-session" [6]. That is using a Content-Disposition of "early-session" [7]. That is
not addressed by this draft. not addressed by this draft.
Offer Answer RFC Ini Est Early Offer Answer RFC Ini Est Early
------------------------------------------------------------------- -------------------------------------------------------------------
1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N 1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N
2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N 2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N
3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N 3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N
4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N 4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N
5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y 5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y
6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y 6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y
Table 1. Summary of SIP Usage of the Offer/Answer Model Table 1. Summary of SIP Usage of the Offer/Answer Model
In Table 1, '1xx-rel' corresponds to the reliable provisional In Table 1, '1xx-rel' corresponds to the reliable provisional
response which contains the 100rel option defined in RFC 3262 [2]. response which contains the 100rel option defined in RFC 3262 [3].
The 'Ini' column shows the ability to exchange the offer/answer to The 'Ini' column shows the ability to exchange the offer/answer to
initiate the session. 'Y' indicates that the pattern can be used in initiate the session. 'Y' indicates that the pattern can be used in
the initial offer/answer exchange, while 'N' indicates that it can the initial offer/answer exchange, while 'N' indicates that it can
not. Only the initial INVITE transaction can be used to exchange not. Only the initial INVITE transaction can be used to exchange
the offer/answer to establish a multimedia session. the offer/answer to establish a multimedia session.
The 'Est' column shows the ability to update the established The 'Est' column shows the ability to update the established
session. session.
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When a UA receives an INVITE request with an unacceptable offer, it When a UA receives an INVITE request with an unacceptable offer, it
should respond with a 488 response, preferably with Warning header should respond with a 488 response, preferably with Warning header
field indicating the reason of the rejection, unless another field indicating the reason of the rejection, unless another
response code is more appropriate to reject it. (Pattern 1 and response code is more appropriate to reject it. (Pattern 1 and
Pattern 3) Pattern 3)
When a UA receives an UPDATE request with an offer which it can not When a UA receives an UPDATE request with an offer which it can not
accept, it should respond with a 488 response preferably with accept, it should respond with a 488 response preferably with
Warning header field indicating the reason of the rejection, unless Warning header field indicating the reason of the rejection, unless
another response code is more appropriate to reject it. (Pattern 6) another response code is more appropriate to reject it. (Pattern 6)
When a UA receives a PRACK request with an offer which it can not When a UA receives a PRACK request with an offer which it can not
accept, it may respond with a 200 response with a syntactically accept, it may respond with a 200 response with a syntactically
correct session description. This may optionally be followed by an correct session description. This may optionally be followed by an
UPDATE request to rearrange the session parameters if both ends UPDATE request to rearrange the session parameters if both ends
support the UPDATE method. Alternatively the UA may terminate the support the UPDATE method. Alternatively the UA may terminate the
dialog and send an error response to the INVITE request. The dialog and send an error response to the INVITE request. The
validity and consequences of a 488 response to PRACK is an open validity and consequences of a 488 response to PRACK is an open
issue which is discussed within a sequent section. (Pattern 5) issue which is discussed within a subsequent section (Section
6.1. ). (Pattern 5)
When a UA receives a response with an offer which it can not accept, When a UA receives a response with an offer which it can not accept,
the UA does not have a way to reject it explicitly. Therefore, a UA the UA does not have a way to reject it explicitly. Therefore, a UA
should respond to the offer with the correct session description should respond to the offer with the correct session description
and rearrange the session parameters by initiating a new and rearrange the session parameters by initiating a new
offer/answer exchange, or alternatively terminate the session. offer/answer exchange, or alternatively terminate the session.
(Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA (Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA
should take care not to cause an infinite offer/answer loop. should take care not to cause an infinite offer/answer loop.
Offer Rejection Offer Rejection
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session description to describe capabilities apart from session description to describe capabilities apart from
offer/answer exchange. Examples of this are 200 OK responses for offer/answer exchange. Examples of this are 200 OK responses for
OPTIONS and 488 responses for INVITE. OPTIONS and 488 responses for INVITE.
3. Detailed Discussion of the Offer/Answer Model for SIP 3. Detailed Discussion of the Offer/Answer Model for SIP
3.1. Offer/Answer for the INVITE method with 100rel extension 3.1. Offer/Answer for the INVITE method with 100rel extension
The INVITE method provides the basic procedure for offer/answer The INVITE method provides the basic procedure for offer/answer
exchange in SIP. Without the 100rel option, the rules are simple as exchange in SIP. Without the 100rel option, the rules are simple as
described in RFC 3261 [1]. If an INVITE request includes a session described in RFC 3261 [2]. If an INVITE request includes a session
description, pattern 1 is applied and if an INVITE request does not description, pattern 1 is applied and if an INVITE request does not
include a session description, pattern 2 is applied. include a session description, pattern 2 is applied.
With 100rel, pattern 3 and pattern 4 are added and this complicates With 100rel, pattern 3 and pattern 4 are added and this complicates
the rules. An INVITE request may cause multiple responses. Note the rules. An INVITE request may cause multiple responses. Note
that even if both UAs support the 100rel extension, not all the that even if both UAs support the 100rel extension, not all the
provisional responses may be sent reliably. Note also that a provisional responses may be sent reliably. Note also that a
reliable provisional response is allowed without a session reliable provisional response is allowed without a session
description if the UAS does not wish to send the answer yet. An description if the UAS does not wish to send the answer yet. An
unreliable provisional response may include a session description unreliable provisional response may include a session description
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retransmission of the UPDATE request. retransmission of the UPDATE request.
A UA can send a PRACK request with a new offer only when A UA can send a PRACK request with a new offer only when
acknowledging the reliable provisional response carrying the answer acknowledging the reliable provisional response carrying the answer
to an offer in the INVITE request. Compared to using the UPDATE to an offer in the INVITE request. Compared to using the UPDATE
method, using PRACK can reduce the number of messages exchanged method, using PRACK can reduce the number of messages exchanged
between the UAs. However, to avoid problems or delays caused by between the UAs. However, to avoid problems or delays caused by
PRACK offer rejection, the UA is recommended to send a PRACK PRACK offer rejection, the UA is recommended to send a PRACK
request only when it has strong reasons to expect the receiver will request only when it has strong reasons to expect the receiver will
accept it. For example, the procedure used in precondition accept it. For example, the procedure used in precondition
extension [5] is a case where a PRACK request should be used for extension [6] is a case where a PRACK request should be used for
updating the session status in an early dialog. Note also that if a updating the session status in an early dialog. Note also that if a
UAS needs to prompt the user to accept or reject the offer, the UAS needs to prompt the user to accept or reject the offer, the
delay can result in retransmission of the PRACK request. delay can result in retransmission of the PRACK request.
From a UA receiving an INVITE request: From a UA receiving an INVITE request:
A UA can send an UPDATE request with a new offer if both ends A UA can send an UPDATE request with a new offer if both ends
support the UPDATE method. A UAS can not send a new offer in the support the UPDATE method. A UAS can not send a new offer in the
reliable provisional response, so the UPDATE method is the only reliable provisional response, so the UPDATE method is the only
method for a UAS to update an early session. method for a UAS to update an early session.
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extensions are needed. Additionally, the INVITE method allows the extensions are needed. Additionally, the INVITE method allows the
peer to take time to decide whether it will accept a session update peer to take time to decide whether it will accept a session update
or not by sending provisional responses. That is, re-INVITE allows or not by sending provisional responses. That is, re-INVITE allows
the UAS to interact with the user at the peer, while UPDATE needs the UAS to interact with the user at the peer, while UPDATE needs
to be answered automatically by the UAS. It is noted that re-INVITE to be answered automatically by the UAS. It is noted that re-INVITE
should be answered immediately unless such a user interaction is should be answered immediately unless such a user interaction is
needed. Otherwise, some 3pcc flows will break. needed. Otherwise, some 3pcc flows will break.
4. Exceptional Case Handling 4. Exceptional Case Handling
In RFC 3264 [3], the following restrictions are defined with regard In RFC 3264 [4], the following restrictions are defined with regard
to sending a new offer. to sending a new offer.
"It MUST NOT generate a new offer if it has received an offer "It MUST NOT generate a new offer if it has received an offer
which it has not yet answered or rejected. It MUST NOT generate which it has not yet answered or rejected. It MUST NOT generate
a new offer if it has generated a prior offer for which it has a new offer if it has generated a prior offer for which it has
not yet received an answer or a rejection." not yet received an answer or a rejection."
Assuming that the above rules are guaranteed, there seem to be two Assuming that the above rules are guaranteed, there seem to be two
possible 'exceptional' cases to be considered in SIP offer/answer possible 'exceptional' cases to be considered in SIP offer/answer
usage: the 'message crossing' case, and the 'glare' case. One of usage: the 'message crossing' case, and the 'glare' case. One of
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offer and has not received the corresponding PRACK request. offer and has not received the corresponding PRACK request.
To avoid a glare condition involving an offer in a response, when To avoid a glare condition involving an offer in a response, when
UA A has sent a (re)INVITE request without session description, it UA A has sent a (re)INVITE request without session description, it
should not send an offer until it has received an offer in a should not send an offer until it has received an offer in a
reliable response to the (re)INVITE, and sent an answer to that reliable response to the (re)INVITE, and sent an answer to that
offer. offer.
5. Content of Offers and Answers 5. Content of Offers and Answers
While RFCs 3264[3] and 3312[5] give some guidance, questions remain While RFCs 3264 [4] and 3312[6] give some guidance, questions remain
about exactly what should be included in an offer or answer. This about exactly what should be included in an offer or answer. This
is especially a problem when the common "hold" feature has been is especially a problem when the common "hold" feature has been
activated, and when there is the potential for a multimedia call. activated, and when there is the potential for a multimedia call.
Details of behavior depend on the capabilities and state of the Details of behavior depend on the capabilities and state of the
User Agent. The kinds of recommendations that can be made are User Agent. The kinds of recommendations that can be made are
limited by the model of device capabilities and state that is limited by the model of device capabilities and state that is
presumed to exist. presumed to exist.
This section focuses on a few key aspects of offers and answers This section focuses on a few key aspects of offers and answers
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A UA should send an offer that indicates what it, and its user, are A UA should send an offer that indicates what it, and its user, are
interested in using/doing at that time, without regard for what the interested in using/doing at that time, without regard for what the
other party in the call may have indicated previously. This is the other party in the call may have indicated previously. This is the
case even when the offer is sent in response to an INVITE or re- case even when the offer is sent in response to an INVITE or re-
INVITE that contains no offer. (However in the case of re-INVITE INVITE that contains no offer. (However in the case of re-INVITE
the constraints of RFCs 3261 and 3264 must be observed.) the constraints of RFCs 3261 and 3264 must be observed.)
A UA should send an answer that includes as close an approximation A UA should send an answer that includes as close an approximation
to what the UA and its user are interested in doing at that time, to what the UA and its user are interested in doing at that time,
while remaining consistent with the offer/answer rules of RFC while remaining consistent with the offer/answer rules of RFC
3264[3] and other RFCs. 3264 [4] and other RFCs.
NOTE: "at that time" is important. The device may permit the NOTE: "at that time" is important. The device may permit the
user to configure which supported media are to be used by user to configure which supported media are to be used by
default. default.
In some cases a UA may not have direct knowledge of what it is In some cases a UA may not have direct knowledge of what it is
interested in doing at a particular time. If it is an intermediary interested in doing at a particular time. If it is an intermediary
it may be able to delegate the decision. In the worst case it may it may be able to delegate the decision. In the worst case it may
apply a default, such as assuming it wants to use all of its apply a default, such as assuming it wants to use all of its
capabilities. capabilities.
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5.2. Choice of Media Types and Formats to Include and Exclude 5.2. Choice of Media Types and Formats to Include and Exclude
5.2.1. Sending an Initial INVITE with Offer 5.2.1. Sending an Initial INVITE with Offer
When a UAC sends an initial INVITE with an offer, it has complete When a UAC sends an initial INVITE with an offer, it has complete
freedom to choose which media type(s) and media format(s) (payload freedom to choose which media type(s) and media format(s) (payload
types in the case of RTP) it should include in the offer. types in the case of RTP) it should include in the offer.
The media types may be all or a subset of the media the UAC is The media types may be all or a subset of the media the UAC is
capable of supporting, with the particular subset being determined capable of supporting, with the particular subset being determined
by the design and configuration [6] of the UAC combined with input by the design and configuration [7] of the UAC combined with input
from the user interface of the UAC. from the user interface of the UAC.
The media formats may be all or a subset of the media formats the The media formats may be all or a subset of the media formats the
UAC is capable of supporting for the corresponding media type, with UAC is capable of supporting for the corresponding media type, with
the particular subset being determined by the design and the particular subset being determined by the design and
configuration [6] of the UAC combined with input from the user configuration [7] of the UAC combined with input from the user
interface of the UAC. interface of the UAC.
Including all supported media formats will maximize the possibility Including all supported media formats will maximize the possibility
that the other party will have a supported format in common. But that the other party will have a supported format in common. But
including many can result in an unacceptably large SDP body. including many can result in an unacceptably large SDP body.
5.2.2. Responding with an Offer when the Initial INVITE has no Offer 5.2.2. Responding with an Offer when the Initial INVITE has no Offer
When a UAS has received an initial INVITE without an offer, it must When a UAS has received an initial INVITE without an offer, it must
include an offer in the first reliable response to the INVITE. It include an offer in the first reliable response to the INVITE. It
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sent reliably, and changed with an UPDATE if the user desires sent reliably, and changed with an UPDATE if the user desires
a change. If PRACK and UPDATE are not supported then the a change. If PRACK and UPDATE are not supported then the
initial offer cannot be changed until the call is fully initial offer cannot be changed until the call is fully
established. In that case either the offer should be delayed established. In that case either the offer should be delayed
until the 200 is sent, or else the offer should include the until the 200 is sent, or else the offer should include the
minimum set of media the user is able to select. minimum set of media the user is able to select.
5.2.3. Answering an Initial INVITE with Offer 5.2.3. Answering an Initial INVITE with Offer
When a UAS receives an initial INVITE with an offer, what media When a UAS receives an initial INVITE with an offer, what media
lines the answer may contain is constrained by RFC 3264.[3] The lines the answer may contain is constrained by RFC 3264 [4]. The
answer must contain the same number of m-lines as the offer, and answer must contain the same number of m-lines as the offer, and
they must contain the same media types. Each media line may be they must contain the same media types. Each media line may be
accepted, by including a non-zero port number, or rejected by accepted, by including a non-zero port number, or rejected by
including a zero port number in the answer. The media lines that including a zero port number in the answer. The media lines that
are accepted should typically be those that would have been offered are accepted should typically be those that would have been offered
had the INVITE not contained an offer, excluding those not offered. had the INVITE not contained an offer, excluding those not offered.
The media formats the answer may contain are constrained by RFC The media formats the answer may contain are constrained by RFC
3264 [3]. For each accepted m-line in the answer, there must be at 3264 [4]. For each accepted m-line in the answer, there must be at
least one media format in common with the corresponding m-line of least one media format in common with the corresponding m-line of
the offer. The UAS may also include other media formats it is able the offer. The UAS may also include other media formats it is able
to support at this time. However there is little benefit to to support at this time. However there is little benefit to
including added types. including added types.
If the UAS does not wish to indicate support for any of the media If the UAS does not wish to indicate support for any of the media
types in a particular media line of the offer it must reject the types in a particular media line of the offer it must reject the
corresponding media line, by setting the port number to zero. corresponding media line, by setting the port number to zero.
5.2.4. Answering when the Initial INVITE had no Offer 5.2.4. Answering when the Initial INVITE had no Offer
When a UAC has sent an initial INVITE without an offer, and then When a UAC has sent an initial INVITE without an offer, and then
receives a response with the first offer, it should answer in the receives a response with the first offer, it should answer in the
same way as a UAS receiving an initial INVITE with an offer. same way as a UAS receiving an initial INVITE with an offer.
5.2.5. Subsequent Offers and Answers 5.2.5. Subsequent Offers and Answers
The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. ) The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. )
apply, but constraints in RFC 3264 [3] must also be followed. The apply, but constraints in RFC 3264 [4] must also be followed. The
following are of particular note because they have proven following are of particular note because they have proven
troublesome: troublesome:
o The number of m-lines may not be reduced in a subsequent offer. o The number of m-lines may not be reduced in a subsequent offer.
Previously rejected media streams must remain, or be reused to Previously rejected media streams must remain, or be reused to
offer the same or a different stream. (RFC 3264[3] section 6.) offer the same or a different stream. (RFC 3264 [4] section 6.)
o In the o-line, only the version number may change, and if it o In the o-line, only the version number may change, and if it
changes it must increment by one from the one previously sent as changes it must increment by one from the one previously sent as
an offer or answer. (RFC 3264[3] section 8.) If it doesn't an offer or answer. (RFC 3264 [4] section 8.) If it doesn't
change then the entire SDP body must be identical to what was change then the entire SDP body must be identical to what was
previously sent as an offer or answer. Changing the o-line, previously sent as an offer or answer. Changing the o-line,
except version number value, during the session is an error case. except version number value, during the session is an error case.
The behavior when receiving such a non-compliant offer/answer The behavior when receiving such a non-compliant offer/answer
SDP body is implementation dependent. If a UA needs to negotiate SDP body is implementation dependent. If a UA needs to negotiate
a 'new' SDP session, it should use the INVITE/Replaces method. a 'new' SDP session, it should use the INVITE/Replaces method.
o In the case of RTP, the mapping from a particular dynamic o In the case of RTP, the mapping from a particular dynamic
payload type number to a particular codec within that media payload type number to a particular codec within that media
stream (m-line) must not change for the duration of the session. stream (m-line) must not change for the duration of the session.
(RFC 3264[3] section 8.3.2.) (RFC 3264 [4] section 8.3.2.)
NOTE: This may be impossible for a B2BUA to follow in some NOTE: This may be impossible for a B2BUA to follow in some
cases (e.g. 3pcc transfer) if it does not terminate media. cases (e.g. 3pcc transfer) if it does not terminate media.
When the new offer is sent in response to an offerless (re)INVITE, When the new offer is sent in response to an offerless (re)INVITE,
all codecs supported by the UA are to be included, not just the all codecs supported by the UA are to be included, not just the
ones that were negotiated by previous offer/answer exchanges. The ones that were negotiated by previous offer/answer exchanges. The
same is true for media types - so if UA A initially offered audio same is true for media types - so if UA A initially offered audio
and video to UA B, and they end up with only audio, and UA B sends and video to UA B, and they end up with only audio, and UA B sends
an offerless (re)INVITE to UA A, A's resulting offer should re- an offerless (re)INVITE to UA A, A's resulting offer should re-
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NOTE: The behavior above is recommended, but it is not always NOTE: The behavior above is recommended, but it is not always
achievable - for example in some interworking scenarios. Or, achievable - for example in some interworking scenarios. Or,
the offerer may simply not have enough resources to offer the offerer may simply not have enough resources to offer
"everything" at that point. Even if the UAS is not able to "everything" at that point. Even if the UAS is not able to
offer any other SDP that the one currently being used, it offer any other SDP that the one currently being used, it
should not reject the re-INVITE. Instead, it should generate should not reject the re-INVITE. Instead, it should generate
an offer with the currently used SDP with o- line unchanged. an offer with the currently used SDP with o- line unchanged.
5.3. Hold and Resume of media 5.3. Hold and Resume of media
RFC 3264 [3] specifies (non-normatively) that "hold" should be RFC 3264 [4] specifies (non-normatively) that "hold" should be
indicated in an established session by sending a new offer indicated in an established session by sending a new offer
containing "a=sendonly" for each media stream to be held. An containing "a=sendonly" for each media stream to be held. An
answerer is then to respond with "a=recvonly" to acknowledge that answerer is then to respond with "a=recvonly" to acknowledge that
the hold request has been understood. the hold request has been understood.
Note that the use of sendonly/recvonly is not limited to hold. Note that the use of sendonly/recvonly is not limited to hold.
These may be used for other reasons, such as devices that are only These may be used for other reasons, such as devices that are only
capable of sending or receiving. So receiving an offer with capable of sending or receiving. So receiving an offer with
"a=sendonly" must not be treated as a certain indication that the "a=sendonly" must not be treated as a certain indication that the
offerer has placed the media stream on hold. offerer has placed the media stream on hold.
This model is based on an assumption that the UA initiating the This model is based on an assumption that the UA initiating the
hold will want to play Music on Hold, which is not always the case. hold will want to play Music on Hold, which is not always the case.
A UA may, if desired, initiate hold by offering "a=inactive" if it A UA may, if desired, initiate hold by offering "a=inactive" if it
does not intend to transmit any media while in hold status. does not intend to transmit any media while in hold status.
The rules of RFC 3264 [3] constrain what may be in an answer when The rules of RFC 3264 [4] constrain what may be in an answer when
the offer contains "sendonly", "recvonly", or "inactive" in an a= the offer contains "sendonly", "recvonly", or "inactive" in an a=
line. But they do not constrain what must be in a subsequent offer. line. But they do not constrain what must be in a subsequent offer.
The General Principle for Constructing Offers and Answers (section The General Principle for Constructing Offers and Answers (section
5.1. ) is important here. The initiation of "hold" is a local 5.1. ) is important here. The initiation of "hold" is a local
action. It should reflect the desired state of the UA. It then action. It should reflect the desired state of the UA. It then
affects what the UA includes in offers and answers until the local affects what the UA includes in offers and answers until the local
state is reset. state is reset.
The receipt of an offer containing "a=sendonly" or "a=inactive" and The receipt of an offer containing "a=sendonly" or "a=inactive" and
the sending of a compatible answer should not change the desired the sending of a compatible answer should not change the desired
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Constructing Offers and Answers (section 5.1. ). If it previously Constructing Offers and Answers (section 5.1. ). If it previously
initiated a "hold" by sending "a=sendonly" or "a=inactive" then it initiated a "hold" by sending "a=sendonly" or "a=inactive" then it
should offer that again. If it had not previously initiated "hold" should offer that again. If it had not previously initiated "hold"
then it should offer "a=sendrecv", even if it had previously been then it should offer "a=sendrecv", even if it had previously been
forced to answer something else. Without this behavior it is forced to answer something else. Without this behavior it is
possible to get "stuck on hold" in some cases, especially when a possible to get "stuck on hold" in some cases, especially when a
third-party call controller is involved. third-party call controller is involved.
5.4. Behavior on receiving SDP with c=0.0.0.0 5.4. Behavior on receiving SDP with c=0.0.0.0
RFC 3264[3] specifies that An agent MUST be capable of receiving RFC 3264 [4] specifies that An agent MUST be capable of receiving
SDP with a connection address of 0.0.0.0, in which case it means 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. that neither RTP nor RTCP should be sent to the peer.
If a UA generates an answer to the offer received with c=0.0.0.0, If a UA generates an answer to the offer received with c=0.0.0.0,
the direction attribute of the accepted media stream in the answer the direction attribute of the accepted media stream in the answer
must be based on direction attribute of the offered stream and must be based on direction attribute of the offered stream and
rules specified in RFC 3264 to form the a-line in the answer. rules specified in RFC 3264 to form the a-line in the answer.
c=0.0.0.0 has no special meaning for the direction attribute of the c=0.0.0.0 has no special meaning for the direction attribute of the
accepted stream in the answer. accepted stream in the answer.
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removed or modified based upon the received 488 response. If the removed or modified based upon the received 488 response. If the
488 response is sent by UAS (open issue), it cannot assume that the 488 response is sent by UAS (open issue), it cannot assume that the
UAC thinks that the reliable transaction has been adequately UAC thinks that the reliable transaction has been adequately
acknowledged even though the UAS may treat otherwise (open issue). acknowledged even though the UAS may treat otherwise (open issue).
If a 488 response is sent by UAS, the UAC should accommodate If a 488 response is sent by UAS, the UAC should accommodate
receiving the altered PRACK with higher CSeq without expecting it receiving the altered PRACK with higher CSeq without expecting it
to trigger a 481 response (open issue). to trigger a 481 response (open issue).
NOTE: Deprecation of the usage of offer in PRACK may be NOTE: Deprecation of the usage of offer in PRACK may be
another solution. As the precondition mechanism specification another solution. As the precondition mechanism specification
[2] explicitly shows a usage of sending offer in PRACK, its [3] explicitly shows a usage of sending offer in PRACK, its
deprecation could cause backward compatibility issues. deprecation could cause backward compatibility issues.
6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE
Transaction Transaction
When a re-INVITE transaction fails, the dialog remains with the When a re-INVITE transaction fails, the dialog remains with the
session bound to it. The issue here is: what is the session status session bound to it. The issue here is: what is the session status
if an offer/answer exchange has been completed (if a session if an offer/answer exchange has been completed (if a session
description has been sent in a reliable provisional response to the description has been sent in a reliable provisional response to the
re-INVITE request), or if subsequent offer/answer exchanges have re-INVITE request), or if subsequent offer/answer exchanges have
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it up is raised in the future. it up is raised in the future.
6.4. Requesting Hold while already on Hold 6.4. Requesting Hold while already on Hold
RFC 3264, section 8.4, contains procedures for putting a unicast RFC 3264, section 8.4, contains procedures for putting a unicast
media stream on hold. Of particular note, it states: media stream on hold. Of particular note, it states:
"If the stream to be placed on hold was previously a recvonly "If the stream to be placed on hold was previously a recvonly
media stream, it is placed on hold by marking it inactive." media stream, it is placed on hold by marking it inactive."
Section 5.3. of the current document makes a best practice Section 5.3. of the current document makes a recommendation for
recommentation for this case which conflicts with that, and this case which conflicts with that, and explains why. Some
explains why. Some concerns have been raised that such a concerns have been raised that such a recommendation is invalid
recommendation is invalid because RFC 3264 is normative on this because RFC 3264 is normative on this subject.
subject.
This document takes the position that Section 8.4 of RFC 3264 is This document takes the position that Section 8.4 of RFC 3264 is
non-normative, and so may be overridden. It is further recommended non-normative, and so may be overridden. It is further recommended
that RFC 3264 be revised to avoid the confusion. that RFC 3264 be revised to avoid the confusion.
7. Add New Offer/Answer Usage in SIP 7. Add New Offer/Answer Usage in SIP
This document recommends against the addition of new offer/answer This document recommends against the addition of new offer/answer
methods using SIP. However, it may be necessary to define new methods using SIP. However, it may be necessary to define new
offer/answer exchange methods as SIP extensions evolve. This offer/answer exchange methods as SIP extensions evolve. This
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The authors would like to thank Christer Holmberg, Rajeev Seth, The authors would like to thank Christer Holmberg, Rajeev Seth,
Nataraju A B, Byron Campen and Jonathan Rosenberg for their Nataraju A B, Byron Campen and Jonathan Rosenberg for their
thorough reviews and comments. Many of their suggestions and ideas thorough reviews and comments. Many of their suggestions and ideas
are incorporated to complete this document. are incorporated to complete this document.
11. References 11. References
11.1. Normative References 11.1. Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[2] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional [3] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in the Session Initiation Protocol (SIP)", RFC 3262, Responses in the Session Initiation Protocol (SIP)", RFC 3262,
June 2002. June 2002.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
SDP", RFC 3264, June 2002. SDP", RFC 3264, June 2002.
[4] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE [5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, September 2002. Method", RFC 3311, September 2002.
[5] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration [6] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration
of Resource Management and Session Initiation Protocol (SIP)", of Resource Management and Session Initiation Protocol (SIP)",
RFC 3312, October 2002. RFC 3312, October 2002.
11.2. Informative References 11.2. Informative References
[6] G. Camarillo, "The Early Session Disposition Type for the [7] G. Camarillo, "The Early Session Disposition Type for the
Session Initiation Protocol (SIP)", RFC 3959, December 2004. Session Initiation Protocol (SIP)", RFC 3959, December 2004.
[7] Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent
Profile Data Set for Media Policy", draft-ietf-sipping-media-
policy-dataset-05 (work in progress), November 2007.
Author's Addresses Author's Addresses
Takuya Sawada Takuya Sawada
KDDI Corporation KDDI Corporation
3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan 3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan
Email: tu-sawada@kddi.com Email: tu-sawada@kddi.com
Paul H. Kyzivat Paul H. Kyzivat
Cisco Systems, Inc. Cisco Systems, Inc.
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