draft-ietf-sipping-sip-offeranswer-09.txt   draft-ietf-sipping-sip-offeranswer-10.txt 
SIPPING Working Group T. Sawada Sipping T. Sawada
Internet Draft KDDI Corporation Internet-Draft KDDI Corporation
Intended status: Informational P. Kyzivat Intended status: Informational P. Kyzivat
Expires: April 2009 Cisco Systems, Inc. Expires: July 5, 2009 Cisco Systems, Inc.
November 3, 2008 January 1, 2009
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-09.txt draft-ietf-sipping-sip-offeranswer-10
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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
model to establish and update multimedia sessions using the Session 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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology............................................3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Summary of SIP usage of the Offer/Answer Model...............3 2. Summary of SIP usage of the Offer/Answer Model . . . . . . . . 4
2.1. Offer/Answer Exchange Pairs in SIP Messages.............4 2.1. Offer/Answer Exchange Pairs in SIP Messages . . . . . . . 5
2.2. Rejection of an Offer...................................5 2.2. Rejection of an Offer . . . . . . . . . . . . . . . . . . 6
2.3. Session Description which is not Offer nor Answer.......6 2.3. Session Description which is not Offer nor Answer . . . . 7
3. Detailed Discussion of the Offer/Answer Model for SIP........7 3. Detailed Discussion of the Offer/Answer Model for SIP . . . . 7
3.1. Offer/Answer for the INVITE method with 100rel extension.7 3.1. Offer/Answer for the INVITE method with 100rel
3.1.1. INVITE Request with SDP............................8 extension . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.2. INVITE request without SDP.........................9 3.1.1. INVITE Request with SDP . . . . . . . . . . . . . . . 8
3.2. Offer/Answer Exchange in Early Dialog..................10 3.1.2. INVITE request without SDP . . . . . . . . . . . . . . 10
3.3. Offer/Answer Exchange in an Established Dialog.........11 3.2. Offer/Answer Exchange in Early Dialog . . . . . . . . . . 11
4. Exceptional Case Handling...................................11 3.3. Offer/Answer Exchange in an Established Dialog . . . . . . 11
4.1. Message Crossing Case Handling.........................12 4. Exceptional Case Handling . . . . . . . . . . . . . . . . . . 12
4.2. Glare Case Handling...................................14 4.1. Message Crossing Case Handling . . . . . . . . . . . . . . 12
5. Content of Offers and Answers...............................15 4.2. Glare Case Handling . . . . . . . . . . . . . . . . . . . 14
5.1. General Principle for Constructing Offers and Answers...16 5. Content of Offers and Answers . . . . . . . . . . . . . . . . 15
5.2. Choice of Media Types and Formats to Include and Exclude16 5.1. General Principle for Constructing Offers and Answers . . 16
5.2.1. Sending an Initial INVITE with Offer..............16 5.2. Choice of Media Types and Formats to Include and
5.2.2. Responding with an Offer when the Initial INVITE has Exclude . . . . . . . . . . . . . . . . . . . . . . . . . 16
no Offer................................................17 5.2.1. Sending an Initial INVITE with Offer . . . . . . . . . 16
5.2.3. Answering an Initial INVITE with Offer............17 5.2.2. Responding with an Offer when the Initial INVITE
5.2.4. Answering when the Initial INVITE had no Offer.....18 has no Offer . . . . . . . . . . . . . . . . . . . . . 17
5.2.5. Subsequent Offers and Answers.....................18 5.2.3. Answering an Initial INVITE with Offer . . . . . . . . 17
5.3. Hold and Resume of media...............................19 5.2.4. Answering when the Initial INVITE had no Offer . . . . 18
5.4. Behavior on receiving SDP with c=0.0.0.0...............20 5.2.5. Subsequent Offers and Answers . . . . . . . . . . . . 18
6. Remaining Issues or Best Practices on Offer/Answer..........21 5.3. Hold and Resume of media . . . . . . . . . . . . . . . . . 19
6.1. Rejecting PRACK Offer..................................21 5.4. Behavior on receiving SDP with c=0.0.0.0 . . . . . . . . . 20
6.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE 6. Remaining Issues or Best Practices on Offer/Answer . . . . . . 20
Transaction................................................22 6.1. Rejecting PRACK Offer . . . . . . . . . . . . . . . . . . 21
6.3. Offer in a Reliable Response...........................24 6.2. Commit/Rollback of Offer/Answer on Unsuccessful
6.4. Requesting Hold while already on Hold..................24 re-INVITE Transaction . . . . . . . . . . . . . . . . . . 21
7. Add New Offer/Answer Usage in SIP...........................25 6.3. Offer in a Reliable Response . . . . . . . . . . . . . . . 23
7.1. Explicit Usage........................................25 6.4. Requesting Hold while already on Hold . . . . . . . . . . 23
7.2. Rejection of an Offer..................................25 7. Add New Offer/Answer Usage in SIP . . . . . . . . . . . . . . 24
7.3. Backward Compatibility.................................25 7.1. Explicit Usage . . . . . . . . . . . . . . . . . . . . . . 24
7.4. Exceptional Case Handling..............................25 7.2. Rejection of an Offer . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations........................................25 7.3. Backward Compatibility . . . . . . . . . . . . . . . . . . 24
9. Security Considerations....................................25 7.4. Exceptional Case Handling . . . . . . . . . . . . . . . . 24
10. Acknowledgement...........................................25 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
11. References................................................26 9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References..................................26 10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 25
11.2. Informative References................................26 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Author's Addresses............................................26 11.1. Normative References . . . . . . . . . . . . . . . . . . . 25
Full Copyright Statement......................................27 11.2. Informative References . . . . . . . . . . . . . . . . . . 25
Intellectual Property Statement................................27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Acknowledgment................................................27
1. Introduction 1. Introduction
SIP utilizes the offer/answer model to establish and update SIP utilizes the offer/answer model to establish and update sessions.
sessions. The rules to govern the offer/answer behaviors in SIP are The rules to govern the offer/answer behaviors in SIP are described
described in the several RFCs. (RFC 3261 [2], RFC 3262 [3], RFC in the several RFCs. ([RFC3261], [RFC3262], [RFC3264], and
3264 [4], and RFC 3311 [5].) [RFC3311].)
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
SIP usage of the offer/answer model in one document to help the usage of the offer/answer model in one document to help the readers
readers to fully understand it. Also, this document tries to to fully understand it. Also, this document tries to incorporate the
incorporate the results of the discussions on the controversial results of the discussions on the controversial issues to avoid
issues to avoid repeating the same discussions later. 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 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
this document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [RFC2119].
This document only uses these key words when referencing normative This document only uses these key words when referencing normative
statements in existing RFCs. 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 [4] defines the applications using the offer/answer model. [RFC3264] 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
core and extensions RFCs. 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/
offer/answer exchange in the SIP dialog can update the session answer exchange in the SIP dialog can update the session which is
which is managed by that dialog. 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
3261 [1], RFC 3262 [3], RFC 3264 [4], and RFC 3311 [5]. In these [RFC3261], [RFC3262], [RFC3264], and [RFC3311]. In these RFCs, only
RFCs, only the six patterns shown in Table 1 are defined for the six patterns shown in Table 1 are defined for exchanging an offer
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 which
which receives the INVITE request without an offer must include an receives the INVITE request without an offer must include an offer in
offer in the first reliable response with 100rel extension. If no the first reliable response with 100rel extension. If no reliable
reliable provisional response has been sent, the UAS must include provisional response has been sent, the UAS must include an offer
an offer when sending 2xx response. when sending 2xx response.
In pattern 3, the first reliable provisional response may or may In pattern 3, the first reliable provisional response may or may not
not have an answer. When a reliable provisional response contains a have an answer. When a reliable provisional response contains a
session description, and is the first to do so, then that session session description, and is the first to do so, then that session
description is the answer to the offer in the INVITE request. The description is the answer to the offer in the INVITE request. The
answer can not be updated, and a new offer can not be sent in a answer can not be updated, and a new offer can not be sent in a
subsequent reliable response for the same INVITE transaction. subsequent reliable response for the same INVITE transaction.
In pattern 5, a PRACK request can contain an offer only if the In pattern 5, a PRACK request can contain an offer only if the
reliable response which it acknowledges contains an answer to the reliable response which it acknowledges contains an answer to the
previous offer/answer exchange. previous offer/answer exchange.
NOTE: It is legal to have UPDATE/2xx exchanges without NOTE: It is legal to have UPDATE/2xx exchanges without offer/
offer/answer exchanges (pattern 6). However when re-INVITEs answer exchanges (pattern 6). However when re-INVITEs are sent
are sent for non-offer/answer purposes, an offer/answer for non-offer/answer purposes, an offer/answer exchange is
exchange is required. In that case the prior SDP will 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
ensure this. When an INVITE results in multiple dialogs each has a this. When an INVITE results in multiple dialogs each has a separate
separate offer/answer negotiation. 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
using a Content-Disposition of "early-session" [7]. That is Content-Disposition of "early-session" [RFC3959]. That is not
not addressed by this draft. 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 [3]. response which contains the 100rel option defined in [RFC3262].
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
the offer/answer to establish a multimedia session. 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.
The 'Early' column indicates which patterns may be used to modify The 'Early' column indicates which patterns may be used to modify the
the established session in an early dialog. There are two ways to established session in an early dialog. There are two ways to
exchange a subsequent offer/answer in an early dialog. exchange a subsequent offer/answer in an early dialog.
2.2. Rejection of an Offer 2.2. Rejection of an Offer
It is not entirely clear how to reject an offer when it is It is not entirely clear how to reject an offer when it is
unacceptable, and some methods do not allow explicit rejection of unacceptable, and some methods do not allow explicit rejection of an
an offer. For each of the patterns in Table 1, Table 2 shows how to offer. For each of the patterns in Table 1, Table 2 shows how to
reject an offer. reject an offer.
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
response code is more appropriate to reject it. (Pattern 1 and 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
Warning header field indicating the reason of the rejection, unless header field indicating the reason of the rejection, unless another
another response code is more appropriate to reject it. (Pattern 6) 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
issue which is discussed within a subsequent section (Section which is discussed in Section 6.1. (Pattern 5)
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
and rearrange the session parameters by initiating a new rearrange the session parameters by initiating a new offer/answer
offer/answer exchange, or alternatively terminate the session. exchange, or alternatively terminate the session. (Pattern 2 and
(Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA Pattern 4) When initiating a new offer/answer, a UA should take care
should take care not to cause an infinite offer/answer loop. not to cause an infinite offer/answer loop.
Offer Rejection Offer Rejection
----------------------------------------------------- -----------------------------------------------------
1. INVITE Req. 488 INVITE Response 1. INVITE Req. 488 INVITE Response
2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer 2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer
OR termination of dialog OR termination of dialog
3. INVITE Req. 488 INVITE Response (same as Pattern 1.) 3. INVITE Req. 488 INVITE Response (same as Pattern 1.)
4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer 4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer
5. PRACK Req. (*) 200 PRACK Resp. followed by new offer 5. PRACK Req. (*) 200 PRACK Resp. followed by new offer
OR termination of dialog OR termination of dialog
6. UPDATE Req. 488 UPDATE Response 6. UPDATE Req. 488 UPDATE Response
Table 2. Rejection of an Offer
(*) A UA should only use PRACK to send an offer when it has strong (*) A UA should only use PRACK to send an offer when it has strong
reasons to expect the receiver will accept the offer. reasons to expect the receiver will accept the offer.
Table 2. Rejection of an Offer
2.3. Session Description which is not Offer nor Answer 2.3. Session Description which is not Offer nor Answer
As previously stated, a session description in a SIP message is not As previously stated, a session description in a SIP message is not
necessarily an offer or an answer. For example, SIP can use a necessarily an offer or an answer. For example, SIP can use a
session description to describe capabilities apart from session description to describe capabilities apart from offer/answer
offer/answer exchange. Examples of this are 200 OK responses for exchange. Examples of this are 200 OK responses for OPTIONS and 488
OPTIONS and 488 responses for INVITE. 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 [RFC3261]. 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 in
in the body if the UAS has not sent a reliable response, but its the body if the UAS has not sent a reliable response, but its session
session description is neither an offer nor an answer. All the description is neither an offer nor an answer. All the session
session descriptions in the unreliable responses to the INVITE descriptions in the unreliable responses to the INVITE request must
request must be identical to the answer which is included in the be identical to the answer which is included in the reliable
reliable response. A session description in an unreliable response response. A session description in an unreliable response that
that precedes a reliable response can be considered a "preview" of precedes a reliable response can be considered a "preview" of the
the answer that will be coming, and hence may be treated like an answer that will be coming, and hence may be treated like an answer
answer until the actual one arrives. until the actual one arrives.
NOTE: This "preview" session description rule applies to a NOTE: This "preview" session description rule applies to a single
single offer/answer exchange. In parallel offer/answer offer/answer exchange. In parallel offer/answer exchanges (caused
exchanges (caused by forking) a UA may obviously receive a by forking) a UA may obviously receive a different "preview" of an
different "preview" of an answer in each dialog. UAs are answer in each dialog. UAs are expected to deal with this.
expected to deal with this.
Although RFC 3261 says a UA should accept media once an INVITE with Although RFC 3261 says a UA should accept media once an INVITE with
an offer has been sent, in many cases, an answer (or, at least a an offer has been sent, in many cases, an answer (or, at least a
preview of it) is required in order for media to be accepted. Two preview of it) is required in order for media to be accepted. Two
examples of why this might be required are: examples of why this might be required are:
o To avoid receiving media from undesired sources, some User o To avoid receiving media from undesired sources, some User Agents
Agents assume symmetric RTP will be used, ignore all incoming assume symmetric RTP will be used, ignore all incoming media
media packets until an address/port has been received from the packets until an address/port has been received from the other
other end, and then use that address/port to filter incoming end, and then use that address/port to filter incoming media
media packets. packets.
o In some networks, an intermediate node must authorize a media o In some networks, an intermediate node must authorize a media
stream before it can flow and requires a confirming answer to stream before it can flow and requires a confirming answer to the
the offer before doing so. offer before doing so.
Therefore, a UAS should send an SDP answer reliably (if possible) Therefore, a UAS should send an SDP answer reliably (if possible)
before it starts sending media. And, if neither the UAC nor the UAS before it starts sending media. And, if neither the UAC nor the UAS
support 100rel, the UAS should send a preview of the answer before support 100rel, the UAS should send a preview of the answer before it
it starts sending media. starts sending media.
3.1.1. INVITE Request with SDP 3.1.1. INVITE Request with SDP
When a UAC includes an SDP body in the INVITE request as an offer, When a UAC includes an SDP body in the INVITE request as an offer, it
it expects the answer to be received with one of the reliable expects the answer to be received with one of the reliable responses.
responses. Other than that, no offer/answer exchanges can occur in Other than that, no offer/answer exchanges can occur in the messages
the messages within the INVITE transaction. within the INVITE transaction.
UAC UAS UAC UAS
| F1 INVITE (SDP) | <- The offer in the offer/answer model | F1 INVITE (SDP) | <- The offer in the offer/answer model
|-------------------->| |-------------------->|
| F2 1xx (SDP) | <- The offer/answer exchange is not | F2 1xx (SDP) | <- The offer/answer exchange is not
|<--------------------| closed yet, but UAC acts as if it |<--------------------| closed yet, but UAC acts as if it
| | ^ receives the answer. | | ^ receives the answer.
| F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer | F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer
|<--------------------| | SDP. |<--------------------| | SDP.
| F4 PRACK (no SDP) | | | F4 PRACK (no SDP) | |
skipping to change at page 9, line 25 skipping to change at page 10, line 11
UAC should prepare to receive SDP bodies in F9 and/or F12, and just UAC should prepare to receive SDP bodies in F9 and/or F12, and just
ignore them, to handle a peer that does not conform to the ignore them, to handle a peer that does not conform to the
recommended implementation. recommended implementation.
3.1.2. INVITE request without SDP 3.1.2. INVITE request without SDP
When a UAC does not include an SDP body in the INVITE request, it When a UAC does not include an SDP body in the INVITE request, it
expects the offer to be received with the first reliable response. expects the offer to be received with the first reliable response.
The UAC will send the answer in the request to acknowledge the The UAC will send the answer in the request to acknowledge the
response, i.e. PRACK or ACK request of the reliable response. Other response, i.e. PRACK or ACK request of the reliable response. Other
than that, no offer/answer exchanges can occur in the messages than that, no offer/answer exchanges can occur in the messages within
within the INVITE transaction. the INVITE transaction.
NOTE: The UAS should not include SDP in the responses F6 and NOTE: The UAS should not include SDP in the responses F6 and F9.
F9. However, the UAC should prepare to receive SDP bodies in However, the UAC should prepare to receive SDP bodies in F6 and/or
F6 and/or F9, and just ignore them to handle a peer that does F9, and just ignore them to handle a peer that does not conform to
not conform to the recommended implementation. the recommended implementation.
UAC UAS UAC UAS
| F1 INVITE (no SDP) | | F1 INVITE (no SDP) |
|-------------------->| |-------------------->|
| F2 1xx | | F2 1xx |
|<--------------------| |<--------------------|
| | | |
| F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP | F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP
|<--------------------| as the offer. |<--------------------| as the offer.
| F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel | F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel
skipping to change at page 10, line 32 skipping to change at page 10, line 46
| F8 2xx PRA | | request after F4. | F8 2xx PRA | | request after F4.
|<--------------------| v |<--------------------| v
| | | |
| F9 2xx INV (no SDP) | <- The final response should not | F9 2xx INV (no SDP) | <- The final response should not
|<--------------------| contain SDP. |<--------------------| contain SDP.
| F10 ACK | | F10 ACK |
|-------------------->| |-------------------->|
Figure 2 Example of Offer/Answer with 100rel Extension (2) Figure 2 Example of Offer/Answer with 100rel Extension (2)
Note that in the case that the UAC needs to prompt the user to Note that in the case that the UAC needs to prompt the user to accept
accept or reject the offer, the reliable provisional response with or reject the offer, the reliable provisional response with SDP as an
SDP as an offer (pattern 4) can result in the retransmission until offer (pattern 4) can result in the retransmission until the PRACK
the PRACK request can be sent. The UAC should take care to avoid request can be sent. The UAC should take care to avoid this
this situation when it sends the INVITE request without SDP. situation when it sends the INVITE request without SDP.
3.2. Offer/Answer Exchange in Early Dialog 3.2. Offer/Answer Exchange in Early Dialog
When both UAs support the 100rel extension, they can update the When both UAs support the 100rel extension, they can update the
session in the early dialog once the first offer/answer exchange session in the early dialog once the first offer/answer exchange has
has been completed. been completed.
From a UA sending an INVITE request: From a UA sending 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
support the UPDATE method. Note that if the UAS needs to prompt the the UPDATE method. Note that if the UAS needs to prompt the user to
user to accept or reject the offer, the delay can result in accept or reject the offer, the delay can result in retransmission of
retransmission of the UPDATE request. 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
request only when it has strong reasons to expect the receiver will only when it has strong reasons to expect the receiver will accept
accept it. For example, the procedure used in precondition it. For example, the procedure used in precondition extension
extension [6] is a case where a PRACK request should be used for [RFC3312] is a case where a PRACK request should be used for updating
updating the session status in an early dialog. Note also that if a the session status in an early dialog. Note also that if a UAS needs
UAS needs to prompt the user to accept or reject the offer, the to prompt the user to accept or reject the offer, the delay can
delay can result in retransmission of the PRACK request. 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
support the UPDATE method. A UAS can not send a new offer in the the UPDATE method. A UAS can not send a new offer in the reliable
reliable provisional response, so the UPDATE method is the only provisional response, so the UPDATE method is the only method for a
method for a UAS to update an early session. UAS to update an early session.
3.3. Offer/Answer Exchange in an Established Dialog 3.3. Offer/Answer Exchange in an Established Dialog
Both the re-INVITE and UPDATE methods can be used in an established Both the re-INVITE and UPDATE methods can be used in an established
dialog to update the session. dialog to update the session.
The UPDATE method is simpler and can save at least one message The UPDATE method is simpler and can save at least one message
compared with the INVITE method. But both ends must support the compared with the INVITE method. But both ends must support the
UPDATE method for it to be used. UPDATE method for it to be used.
The INVITE method needs at least three messages to complete but no The INVITE method needs at least three messages to complete but no
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
to be answered automatically by the UAS. It is noted that re-INVITE 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 [4], the following restrictions are defined with regard In [RFC3264], the following restrictions are defined with regard to
to sending a new offer. sending a new offer.
"At any time, either agent MAY generate a new offer that updates "At any time, either agent MAY generate a new offer that updates
the session. However, it MUST NOT generate a new offer if it the session. However, it MUST NOT generate a new offer if it has
has received an offer which it has not yet answered or rejected. received an offer which it has not yet answered or rejected. It
It MUST NOT generate a new offer if it has generated a prior MUST NOT generate a new offer if it has generated a prior offer
offer for which it has not yet received an answer or a for which it has not yet received an answer or a rejection."
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 the
the reasons why the usage of SIP methods to exchange offer/answer reasons why the usage of SIP methods to exchange offer/answer needs
needs to be carefully restricted in the RFCs is to ensure that the to be carefully restricted in the RFCs is to ensure that the UA can
UA can detect and handle appropriately the 'exceptional' cases to detect and handle appropriately the 'exceptional' cases to avoid
avoid incompatible behavior. incompatible behavior.
4.1. Message Crossing Case Handling 4.1. Message Crossing Case Handling
When message packets cross in the transport network, an offer may When message packets cross in the transport network, an offer may be
be received before the answer for the previous offer/answer received before the answer for the previous offer/answer exchange, as
exchange, as shown in Figure 3. In such a case, UA A must detect shown in Figure 3. In such a case, UA A must detect that the session
that the session description SDP-2 is not the answer to offer1. description SDP-2 is not the answer to offer1.
A B A B
|SDP-1 (offer1)| |SDP-1 (offer1)|
M1 |----------------->| M1 |----------------->|
|SDP-2 (answer1)| |SDP-2 (answer1)|
M2 |<------\ /-------| M2 |<------\ /-------|
| \/ | | \/ |
|SDP-3 /\(offer2)| |SDP-3 /\(offer2)|
M3 |<------/ \-------| M3 |<------/ \-------|
Figure 3 Message Crossing Case Figure 3 Message Crossing Case
Because of the restrictions on placement of offers and answers Because of the restrictions on placement of offers and answers
(summarized in Table 1) there are a limited number of valid (summarized in Table 1) there are a limited number of valid exchanges
exchanges of messages that may lead to this message crossing case. of messages that may lead to this message crossing case. These are
These are enumerated in Table 3. (This table only shows messages enumerated in Table 3. (This table only shows messages containing
containing offers or answers. There could be other messages, offers or answers. There could be other messages, without session
without session descriptions, which are not shown.) descriptions, which are not shown.)
There is a variant, shown in Figure 4, which is dependent on an There is a variant, shown in Figure 4, which is dependent on an
INVITE (Mx) that contains no offer. This case should be extremely INVITE (Mx) that contains no offer. This case should be extremely
rare - it is easily avoided by delaying Mx until answer1 is rare - it is easily avoided by delaying Mx until answer1 is received.
received. It adds another possibility to Table 3. It adds another possibility to Table 3.
A B A B
| | | |
|SDP-1 offer1(UPD) | |SDP-1 offer1(UPD) |
M1 |==============================>| M1 |==============================>|
|re-INV (no offer) | |re-INV (no offer) |
Mx |------------------------------>| --+ Mx |------------------------------>| --+
|SDP-2 answer1 (2xx-UPD)| | |SDP-2 answer1 (2xx-UPD)| |
M2 |<===========\ /===============| | first reliable M2 |<===========\ /===============| | first reliable
| \/ offer2| | response | \/ offer2| | response
skipping to change at page 13, line 37 skipping to change at page 13, line 41
| PRACK | 200-PRA | UPDATE | | PRACK | 200-PRA | UPDATE |
|--------+----------+---------| |--------+----------+---------|
| UPDATE | 200-UPD | UPDATE | | UPDATE | 200-UPD | UPDATE |
| | |---------| | | |---------|
| | | INVITE | (no INV in progress) | | | INVITE | (no INV in progress)
| | |---------| | | |---------|
| | | 2xx-INV | (INV in progress) | | | 2xx-INV | (INV in progress)
| | |---------| | | |---------|
| | | 1xx-rel | (from Figure 4) | | | 1xx-rel | (from Figure 4)
|-----------------------------| |-----------------------------|
Table 3. Offer / Answer Crossing Message Sequences Table 3. Offer / Answer Crossing Message Sequences
Table 3 shows that there are only two ambiguous cases when an Table 3 shows that there are only two ambiguous cases when an answer
answer is expected and an arriving message M2 containing SDP could is expected and an arriving message M2 containing SDP could be either
be either the expected answer or an offer. These are a reliable 1xx the expected answer or an offer. These are a reliable 1xx response
response to an INVITE, or an UPDATE. to an INVITE, or an UPDATE.
When message M2 is an UPDATE request or a (re)INVITE request, then When message M2 is an UPDATE request or a (re)INVITE request, then
message M1 must also have been an UPDATE or INVITE. There may have message M1 must also have been an UPDATE or INVITE. There may have
been message crossing, or not. If not then it is a glare case. been message crossing, or not. If not then it is a glare case.
Either way, the remedy is for UA A to reject message M2 with a 491 Either way, the remedy is for UA A to reject message M2 with a 491
response with Retry-After header field. response with Retry-After header field.
When M2 is a reliable provisional response or a successful final When M2 is a reliable provisional response or a successful final
response, and M1 was an UPDATE, then SDP-2 cannot be the expected response, and M1 was an UPDATE, then SDP-2 cannot be the expected
answer1. In this case, since UA A can not reject offer2 in reliable answer1. In this case, since UA A can not reject offer2 in reliable
skipping to change at page 14, line 46 skipping to change at page 15, line 4
called a 'glare' case. called a 'glare' case.
A B A B
|offer1 offer2| |offer1 offer2|
|-------\ /-------| |-------\ /-------|
| \/ | | \/ |
| /\ | | /\ |
|<------/ \------>| |<------/ \------>|
Figure 5 Glare Case Figure 5 Glare Case
When offer2 is in an UPDATE request or (re-)INVITE request, it must When offer2 is in an UPDATE request or (re-)INVITE request, it must
be rejected with a 491 response. be rejected with a 491 response.
When offer2 is in a PRACK request (within the current rules, only When offer2 is in a PRACK request (within the current rules, only
possible if offer1 is in an UPDATE request), the PRACK may be possible if offer1 is in an UPDATE request), the PRACK may be
accepted with 200 or may be rejected with a 491 response. A 491 accepted with 200 or may be rejected with a 491 response. A 491
response is valid to satisfy the offer/answer model but it may response is valid to satisfy the offer/answer model but it may delay
delay the completion of the reliable response transfer mechanism or, the completion of the reliable response transfer mechanism or, in
in worst case, may result in the failure to complete the SIP worst case, may result in the failure to complete the SIP transaction
transaction because there is no clear retry rule when a PRACK because there is no clear retry rule when a PRACK request is rejected
request is rejected with a 491 response. To avoid this glare with a 491 response. To avoid this glare condition, UA A should not
condition, UA A should not send an offer if it has already sent a send an offer if it has already sent a reliable provisional response
reliable provisional response containing an answer to a previous containing an answer to a previous offer and has not received the
offer and has not received the corresponding PRACK request. 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
UA A has sent a (re)INVITE request without session description, it 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
reliable response to the (re)INVITE, and sent an answer to that 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[4] and 3312[6] give some guidance, questions remain While [RFC3264] and [RFC3312] 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
is especially a problem when the common "hold" feature has been 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
User Agent. The kinds of recommendations that can be made are Agent. The kinds of recommendations that can be made are limited by
limited by the model of device capabilities and state that is 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
that have been identified as troublesome, and will consider other
aspects to be out of scope. This section considers:
- choice of supported media types and formats to include and
exclude
- hold and resume of media This section focuses on a few key aspects of offers and answers that
have been identified as troublesome, and will consider other aspects
to be out of scope. This section considers:
o choice of supported media types and formats to include and exclude
o hold and resume of media
The following are out of scope for this document: The following are out of scope for this document:
o NAT traversal and ICE
- NAT traversal and ICE o specific codecs and their parameters
o the negotiation of secure media streams
- specific codecs and their parameters o grouping of media streams
o preconditions
- the negotiation of secure media streams
- grouping of media streams
- preconditions
5.1. General Principle for Constructing Offers and Answers 5.1. General Principle for Constructing Offers and Answers
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
the constraints of RFCs 3261 and 3264 must be observed.) 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
to what the UA and its user are interested in doing at that time, what the UA and its user are interested in doing at that time, while
while remaining consistent with the offer/answer rules of RFC remaining consistent with the offer/answer rules of [RFC3264] and
3264[4] and other RFCs. 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
user to configure which supported media are to be used by 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.
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
by the design and configuration [7] of the UAC combined with input the design and configuration (e.g., via
[I-D.ietf-sipping-config-framework]) 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
UAC is capable of supporting for the corresponding media type, with is capable of supporting for the corresponding media type, with the
the particular subset being determined by the design and particular subset being determined by the design and configuration of
configuration [7] of the UAC combined with input from the user 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
has largely the same options as when sending an initial INVITE with has largely the same options as when sending an initial INVITE with
an offer, but there are some differences. The choice may be an offer, but there are some differences. The choice may be governed
governed by both static (default) selections of media types as well by both static (default) selections of media types as well as dynamic
as dynamic selections made by a user via interaction with the selections made by a user via interaction with the device while it is
device while it is alerting. alerting.
NOTE: The offer may be sent in a provisional response, before NOTE: The offer may be sent in a provisional response, before the
the user of the device has been alerted and had an opportunity user of the device has been alerted and had an opportunity to
to select media options for the call. In this case the UAS select media options for the call. In this case the UAS cannot
cannot include any call-specific options from the user of the include any call-specific options from the user of the device. If
device. If there is a possibility that the user of the device there is a possibility that the user of the device will wish to
will wish to change what is offered before answering the call, change what is offered before answering the call, then special
then special care should be taken. If PRACK and UPDATE are care should be taken. If PRACK and UPDATE are supported by caller
supported by caller and callee then an initial offer can be and callee then an initial offer can be sent reliably, and changed
sent reliably, and changed with an UPDATE if the user desires with an UPDATE if the user desires a change. If PRACK and UPDATE
a change. If PRACK and UPDATE are not supported then the are not supported then the initial offer cannot be changed until
initial offer cannot be changed until the call is fully the call is fully established. In that case either the offer
established. In that case either the offer should be delayed should be delayed until the 200 is sent, or else the offer should
until the 200 is sent, or else the offer should include the 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
lines the answer may contain is constrained by RFC 3264.[4] The the answer may contain is constrained by [RFC3264]. The answer must
answer must contain the same number of m-lines as the offer, and contain the same number of m-lines as the offer, and they must
they must contain the same media types. Each media line may be contain the same media types. Each media line may be accepted, by
accepted, by including a non-zero port number, or rejected by including a non-zero port number, or rejected by including a zero
including a zero port number in the answer. The media lines that port number in the answer. The media lines that are accepted should
are accepted should typically be those that would have been offered typically be those that would have been offered had the INVITE not
had the INVITE not contained an offer, excluding those not offered. 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
3264 [4]. For each accepted m-line in the answer, there must be at [RFC3264]. 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
the offer. The UAS may also include other media formats it is able offer. The UAS may also include other media formats it is able to
to support at this time. However there is little benefit to support at this time. However there is little benefit to including
including added types. 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 (Section 5.1 and Section 5.2.1 through
apply, but constraints in RFC 3264 [4] must also be followed. The Section 5.2.4) apply, but constraints in [RFC3264] must also be
following are of particular note because they have proven followed. The following are of particular note because they have
troublesome: proven 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[4] section 6.) offer the same or a different stream. (Section 6 of [RFC3264].)
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[4] section 8.) If it doesn't an offer or answer. (Section 8 of [RFC3264].) 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
SDP body is implementation dependent. If a UA needs to negotiate body is implementation dependent. If a UA needs to negotiate a
a 'new' SDP session, it should use the INVITE/Replaces method. 'new' SDP session, it should use the INVITE/Replaces method.
o In the case of RTP, the mapping from a particular dynamic payload
o In the case of RTP, the mapping from a particular dynamic type number to a particular codec within that media stream
payload type number to a particular codec within that media (m-line) must not change for the duration of the session.
stream (m-line) must not change for the duration of the session. (Section 8.3.2 of [RFC3264].)
(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
ones that were negotiated by previous offer/answer exchanges. The that were negotiated by previous offer/answer exchanges. The same is
same is true for media types - so if UA A initially offered audio true for media types - so if UA A initially offered audio and video
and video to UA B, and they end up with only audio, and UA B sends to UA B, and they end up with only audio, and UA B sends an offerless
an offerless (re)INVITE to UA A, A's resulting offer should re- (re)INVITE to UA A, A's resulting offer should re- attempt video, by
attempt video, by reusing the zeroed m-line used previously. reusing the zeroed m-line used previously.
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
the offerer may simply not have enough resources to offer offerer may simply not have enough resources to offer "everything"
"everything" at that point. Even if the UAS is not able to at that point. Even if the UAS is not able to offer any other SDP
offer any other SDP that the one currently being used, it that the one currently being used, it should not reject the re-
should not reject the re-INVITE. Instead, it should generate INVITE. Instead, it should generate an offer with the currently
an offer with the currently used SDP with o- line unchanged. used SDP with o- line unchanged.
5.3. Hold and Resume of media 5.3. Hold and Resume of media
RFC 3264 [4] specifies (non-normatively) that "hold" should be [RFC3264] specifies (non-normatively) that "hold" should be indicated
indicated in an established session by sending a new offer in an established session by sending a new offer containing
containing "a=sendonly" for each media stream to be held. An "a=sendonly" for each media stream to be held. An answerer is then
answerer is then to respond with "a=recvonly" to acknowledge that to respond with "a=recvonly" to acknowledge that the hold request has
the hold request has been understood. 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
These may be used for other reasons, such as devices that are only may be used for other reasons, such as devices that are only capable
capable of sending or receiving. So receiving an offer with of sending or receiving. So receiving an offer with "a=sendonly"
"a=sendonly" must not be treated as a certain indication that the must not be treated as a certain indication that the offerer has
offerer has placed the media stream on hold. 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
hold will want to play Music on Hold, which is not always the case. will want to play Music on Hold, which is not always the case. A UA
A UA may, if desired, initiate hold by offering "a=inactive" if it may, if desired, initiate hold by offering "a=inactive" if it does
does not intend to transmit any media while in hold status. not intend to transmit any media while in hold status.
The rules of RFC 3264 [4] constrain what may be in an answer when The rules of [RFC3264] constrain what may be in an answer when the
the offer contains "sendonly", "recvonly", or "inactive" in an a= offer contains "sendonly", "recvonly", or "inactive" in an a= line.
line. But they do not constrain what must be in a subsequent offer. But they do not constrain what must be in a subsequent offer. The
The General Principle for Constructing Offers and Answers (section General Principle for Constructing Offers and Answers (Section 5.1)
5.1. ) is important here. The initiation of "hold" is a local is important here. The initiation of "hold" is a local action. It
action. It should reflect the desired state of the UA. It then should reflect the desired state of the UA. It then affects what the
affects what the UA includes in offers and answers until the local 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
state of the recipient. However, a UA that has been "placed on state of the recipient. However, a UA that has been "placed on hold"
hold" may itself desire to initiate its own hold status, based on may itself desire to initiate its own hold status, based on local
local input. input.
If UA2 has previously been "placed on hold" by UA1, via receipt of If UA2 has previously been "placed on hold" by UA1, via receipt of
"a=sendonly", then it may initiate its own hold by sending a new "a=sendonly", then it may initiate its own hold by sending a new
offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will
will answer with "a=inactive" because that is the only valid answer answer with "a=inactive" because that is the only valid answer that
that reflects its desire not to receive media. reflects its desire not to receive media.
Once in this state, to resume a two way exchange of media each side Once in this state, to resume a two way exchange of media each side
must reset its local hold status. If UA1 is first to go off hold it must reset its local hold status. If UA1 is first to go off hold it
will then send an offer with "a=sendrecv". The UA2 will respond will then send an offer with "a=sendrecv". The UA2 will respond with
with its desired state of "a=sendonly" because that is a permitted its desired state of "a=sendonly" because that is a permitted
response. When UA2 desires to also resume, it will send an offer response. When UA2 desires to also resume, it will send an offer
with "a=sendrecv". In this case, because UA1 has the same desire it with "a=sendrecv". In this case, because UA1 has the same desire it
will respond with "a=sendrecv". In the same case, when UA2 receives will respond with "a=sendrecv". In the same case, when UA2 receives
the offer with "a=sendrecv", if it has decided it wants to reset the offer with "a=sendrecv", if it has decided it wants to reset its
its local hold but has not yet signaled the intent, it may send local hold but has not yet signaled the intent, it may send
"a=sendrecv" in the answer. "a=sendrecv" in the answer.
If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive", If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive",
and subsequently wants to initiate its own hold, also using and subsequently wants to initiate its own hold, also using
"a=inactive", it need not send a new offer, since the only valid "a=inactive", it need not send a new offer, since the only valid
response is "a=inactive" and that is already in effect. However, response is "a=inactive" and that is already in effect. However, its
its local desired state will now be either "inactive" or local desired state will now be either "inactive" or "a=sendonly".
"a=sendonly". This affects what it will send in future offers and This affects what it will send in future offers and answers.
answers.
If a UA has occasion to send another offer in the session, without If a UA has occasion to send another offer in the session, without
any desire to change the hold status (e.g. in response to a re- any desire to change the hold status (e.g. in response to a re-
INVITE without an offer, or when sending a re-INVITE to refresh the INVITE without an offer, or when sending a re-INVITE to refresh the
session timer) it should follow the General Principle for session timer) it should follow the General Principle for
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[4] specifies that An agent MUST be capable of receiving [RFC3264] specifies that an agent MUST be capable of receiving SDP
SDP with a connection address of 0.0.0.0, in which case it means with a connection address of 0.0.0.0, in which case it means that
that neither RTP nor RTCP should be sent to the peer. 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
the direction attribute of the accepted media stream in the answer direction attribute of the accepted media stream in the answer must
must be based on direction attribute of the offered stream and be based on direction attribute of the offered stream and rules
rules specified in RFC 3264 to form the a-line in the answer. specified in RFC 3264 to form the a-line in the answer. c=0.0.0.0 has
c=0.0.0.0 has no special meaning for the direction attribute of the no special meaning for the direction attribute of the accepted stream
accepted stream in the answer. in the answer.
6. Remaining Issues or Best Practices on Offer/Answer 6. Remaining Issues or Best Practices on Offer/Answer
This document clarifies the offer/answer usage in SIP and This document clarifies the offer/answer usage in SIP and summarizes
summarizes the correct or recommended behaviors along with the the correct or recommended behaviors along with the existing RFCs.
existing RFCs. To create any new normative behaviors beyond these To create any new normative behaviors beyond these RFCs is not the
RFCs is not the intent of this document. intent of this document.
However, through the scrutiny of the offer/answer model in SIP, However, through the scrutiny of the offer/answer model in SIP, some
some issues are found to be unresolved within the current set of issues are found to be unresolved within the current set of RFCs.
RFCs. Those remaining issues are described in this section mainly Those remaining issues are described in this section mainly for
for further study. further study.
6.1. Rejecting PRACK Offer 6.1. Rejecting PRACK Offer
As stated in section 2.2. and 3.2. , it is recommended that an As stated in Section 2.2 and Section 3.2, it is recommended that an
offer not be sent in a PRACK request unless UAC has strong reasons offer not be sent in a PRACK request unless UAC has strong reasons to
to assume the receiver will accept it. Even so, there may be cases assume the receiver will accept it. Even so, there may be cases when
when the UAS has to reject the offer for some reason. The current the UAS has to reject the offer for some reason. The current RFCs do
RFCs do not provide a way to reject the offer and at the same time not provide a way to reject the offer and at the same time to
to indicate that the PRACK adequately acknowledged the reliable indicate that the PRACK adequately acknowledged the reliable
response. It is unclear whether a non-200 response can still response. It is unclear whether a non-200 response can still
indicate an acknowledgement of the reliable response. indicate an acknowledgement of the reliable response.
Several ideas were presented to resolve this issue, such as sending Several ideas were presented to resolve this issue, such as sending
2xx PRACK response without SDP to reject the offer, or sending SDP 2xx PRACK response without SDP to reject the offer, or sending SDP
with a decreased version value in the o-line. Some of the with a decreased version value in the o-line. Some of the candidates
candidates may also be adapted as a way to reject an unacceptable may also be adapted as a way to reject an unacceptable offer in a
offer in a response. Anyway, those proposals violate the current response. Anyway, those proposals violate the current rules and lose
rules and lose backward compatibility to some extent (e.g. section backward compatibility to some extent (e.g. section 5 of [RFC3262]).
5 of RFC 3262). It is beyond the scope of this document and remains It is beyond the scope of this document and remains for further
for further study. study.
The 488 response is another proposed solution; however the validity The 488 response is another proposed solution; however the validity
and consequences of a 488 response to PRACK is an open issue. and consequences of a 488 response to PRACK is an open issue.
Because the 488 response may be sent by a proxy, the UAC cannot Because the 488 response may be sent by a proxy, the UAC cannot
assume the reliable transaction has been adequately acknowledged. assume the reliable transaction has been adequately acknowledged. If
If a 488 response is received, the UAC should ensure acknowledgment a 488 response is received, the UAC should ensure acknowledgment of
of the reliable response by sending a new PRACK with the offer the reliable response by sending a new PRACK with the offer removed
removed or modified based upon the received 488 response. If the or modified based upon the received 488 response. If the 488
488 response is sent by UAS (open issue), it cannot assume that the response is sent by UAS (open issue), it cannot assume that the UAC
UAC thinks that the reliable transaction has been adequately thinks that the reliable transaction has been adequately acknowledged
acknowledged even though the UAS may treat otherwise (open issue). even though the UAS may treat otherwise (open issue). If a 488
If a 488 response is sent by UAS, the UAC should accommodate response is sent by UAS, the UAC should accommodate receiving the
receiving the altered PRACK with higher CSeq without expecting it altered PRACK with higher CSeq without expecting it to trigger a 481
to trigger a 481 response (open issue). 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
another solution. As the precondition mechanism specification solution. As the precondition mechanism specification [RFC3262]
[3] explicitly shows a usage of sending offer in PRACK, its 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
taken place (using UPDATE or PRACK transactions), before the re- taken place (using UPDATE or PRACK transactions), before the re-
INVITE transaction is terminated with a final error response INVITE transaction is terminated with a final error response (Figure
(Figure 6). One option is to take those offer/answer exchanges not 6). One option is to take those offer/answer exchanges not committed
committed yet and to make the session status rollback to the one yet and to make the session status rollback to the one before re-
before re-INVITE transaction was initiated. Another option is to INVITE transaction was initiated. Another option is to take those
take those exchanges committed and to keep the session status as it exchanges committed and to keep the session status as it is even
is even after re-INVITE fails. There is no clear consensus on which after re-INVITE fails. There is no clear consensus on which one is
one is the correct behavior. the correct behavior.
There are some cases where it is useful to exchange There are some cases where it is useful to exchange offer(s)/
offer(s)/answer(s) even before re-INVITE completes. The case of answer(s) even before re-INVITE completes. The case of adding a new
adding a new media (like adding video to audio only session) which media (like adding video to audio only session) which requires
requires permission from the peer through some user interaction is permission from the peer through some user interaction is one
one example. Precondition procedures can be another case which may example. Precondition procedures can be another case which may
require several offer/answer exchanges in one re-INVITE transaction. require several offer/answer exchanges in one re-INVITE transaction.
UAC UAS UAC UAS
| session established | | session established |
|<===================>| |<===================>|
| | | |
| F1 re-INVITE (SDP) | | F1 re-INVITE (SDP) |
|-------------------->| |-------------------->|
| F2 1xx-rel (SDP) | | F2 1xx-rel (SDP) |
|<--------------------| |<--------------------|
skipping to change at page 23, line 29 skipping to change at page 22, line 43
| F5 4xx/5xx/6xx INV | response to re-INVITE request (F1). | F5 4xx/5xx/6xx INV | response to re-INVITE request (F1).
|<--------------------| |<--------------------|
| F6 ACK | | F6 ACK |
|-------------------->| Issue: What is the correct session |-------------------->| Issue: What is the correct session
| | status after re-INVITE transaction. | | status after re-INVITE transaction.
Figure 6 Commit/Rollback Issue with re-INVITE transaction Figure 6 Commit/Rollback Issue with re-INVITE transaction
To make bad things worse, if a new offer from UAC and the final To make bad things worse, if a new offer from UAC and the final
response to re-INVITE are sent at nearly the same time, the UAS can response to re-INVITE are sent at nearly the same time, the UAS can
not know whether this new offer was sent before or after UAC not know whether this new offer was sent before or after UAC received
received the final failure response (Figure 7). Note that the ACK the final failure response (Figure 7). Note that the ACK request to
request to the failure response is sent hop-by-hop basis, therefore the failure response is sent hop-by-hop basis, therefore even after
even after receiving the ACK request, UAS can not make sure that receiving the ACK request, UAS can not make sure that UPDATE request
UPDATE request was sent after the final response had been reached was sent after the final response had been reached to the other end.
to the other end.
Sending a new UPDATE request from UAC to synchronize the status Sending a new UPDATE request from UAC to synchronize the status
anytime after the re-INVITE fails may be a good option. This anytime after the re-INVITE fails may be a good option. This
solution, however, requires that the UPDATE method be supported by solution, however, requires that the UPDATE method be supported by
both ends and needs care to avoid flapping when each end tries to both ends and needs care to avoid flapping when each end tries to
advertise their different views of the session status. advertise their different views of the session status.
The proper handling of this issue is undefined by existing The proper handling of this issue is undefined by existing standards.
standards. Resolution is beyond the scope of this document, and Resolution is beyond the scope of this document, and will require a
will require a new normative document. new normative document.
UAC UAS UAC UAS
| session established | | session established |
|<===================>| |<===================>|
| | | |
| F1 re-INVITE (SDP) | | F1 re-INVITE (SDP) |
|-------------------->| |-------------------->|
| F2 1xx-rel (SDP) | | F2 1xx-rel (SDP) |
|<--------------------| |<--------------------|
| F3 PRACK | | F3 PRACK |
skipping to change at page 24, line 33 skipping to change at page 23, line 39
| | | |
Figure 7 Commit/Rollback Issue with Race Condition Figure 7 Commit/Rollback Issue with Race Condition
6.3. Offer in a Reliable Response 6.3. Offer in a Reliable Response
In RFC 3261, it is stated that when an INVITE is sent without an In RFC 3261, it is stated that when an INVITE is sent without an
offer, the first reliable response MUST contain an offer. There was offer, the first reliable response MUST contain an offer. There was
discussion on whether this rule can be loosened up. There is no discussion on whether this rule can be loosened up. There is no
clear explanation why this restriction is defined. However, this clear explanation why this restriction is defined. However, this
rule will be left as it is, unless the strong necessity to loosen rule will be left as it is, unless the strong necessity to loosen it
it up is raised in the future. 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 recommendation for Section 5.3 of the current document makes a recommendation for this
this case which conflicts with that, and explains why. Some case which conflicts with that, and explains why. Some concerns have
concerns have been raised that such a recommendation is invalid been raised that such a recommendation is invalid because RFC 3264 is
because RFC 3264 is normative on this subject. normative on this 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-
non-normative, and so may be overridden. It is further recommended normative, and so may be overridden. It is further recommended that
that RFC 3264 be revised to avoid the confusion. 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/
offer/answer exchange methods as SIP extensions evolve. This answer exchange methods as SIP extensions evolve. This section
section recommends some things that should be taken into recommends some things that should be taken into considerations in
considerations in that case. that case.
7.1. Explicit Usage 7.1. Explicit Usage
New method definitions should define offer/answer usage explicitly New method definitions should define offer/answer usage explicitly
without any ambiguity. without any ambiguity.
7.2. Rejection of an Offer 7.2. Rejection of an Offer
New method definitions should define how to reject an offer where New method definitions should define how to reject an offer where
possible. possible.
skipping to change at page 25, line 43 skipping to change at page 25, line 8
This document has no actions for IANA. This document has no actions for IANA.
9. Security Considerations 9. Security Considerations
There are not any security issues beyond the referenced RFCs. There are not any security issues beyond the referenced RFCs.
10. Acknowledgement 10. Acknowledgement
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
thorough reviews and comments. Many of their suggestions and ideas reviews and comments. Many of their suggestions and ideas are
are incorporated to complete this document. incorporated to complete this document.
11. References 11. References
11.1. Normative References 11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: A., Peterson, J., Sparks, R., Handley, M., and E.
Session Initiation Protocol", RFC 3261, June 2002. Schooler, "SIP: Session Initiation Protocol", RFC 3261,
[3] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in the Session Initiation Protocol (SIP)", RFC 3262,
June 2002. June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
SDP", RFC 3264, June 2002. Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
Method", RFC 3311, September 2002. with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[6] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration [RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
of Resource Management and Session Initiation Protocol (SIP)", UPDATE Method", RFC 3311, October 2002.
RFC 3312, October 2002.
[RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
11.2. Informative References 11.2. Informative References
[7] G. Camarillo, "The Early Session Disposition Type for the [RFC3959] Camarillo, G., "The Early Session Disposition Type for the
Session Initiation Protocol (SIP)", RFC 3959, December 2004. Session Initiation Protocol (SIP)", RFC 3959,
December 2004.
Author's Addresses [I-D.ietf-sipping-config-framework]
Channabasappa, S., "A Framework for Session Initiation
Protocol User Agent Profile Delivery",
draft-ietf-sipping-config-framework-15 (work in progress),
February 2008.
Authors' 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.
1414 Massachusetts Avenue 1414 Massachusetts Avenue
Boxborough, MA 01719 Boxborough, MA 01719
USA USA
Email: pkyzivat@cisco.com Email: pkyzivat@cisco.com
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