draft-ietf-sipping-sip-offeranswer-12.txt   draft-ietf-sipping-sip-offeranswer-13.txt 
Sipping P. Kyzivat Sipping S. Okumura
Internet-Draft Cisco Systems, Inc. Internet-Draft Softfront
Intended status: Informational T. Sawada Intended status: Informational P. Kyzivat
Expires: September 9, 2010 KDDI Corporation Expires: November 11, 2010 Cisco Systems, Inc.
March 8, 2010 May 10, 2010
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-12 draft-ietf-sipping-sip-offeranswer-13
Abstract Abstract
The Session Initiation Protocol (SIP) utilizes the offer/answer model The Session Initiation Protocol (SIP) utilizes the offer/answer 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.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at This Internet-Draft will expire on November 11, 2010.
http://www.ietf.org/ietf/1id-abstracts.txt.
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This Internet-Draft will expire on September 9, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Summary of SIP usage of the Offer/Answer Model . . . . . . . . 3 2. Summary of SIP usage of the Offer/Answer Model . . . . . . . . 3
2.1. Offer/Answer Exchange Pairs in SIP Messages . . . . . . . 4 2.1. Offer/Answer Exchange Pairs in SIP Messages . . . . . . . 4
2.2. Rejection of an Offer . . . . . . . . . . . . . . . . . . 5 2.2. Rejection of an Offer . . . . . . . . . . . . . . . . . . 5
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 3.1. Offer/Answer for the INVITE method with 100rel
extension . . . . . . . . . . . . . . . . . . . . . . . . 7 extension . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. INVITE Request with SDP . . . . . . . . . . . . . . . 8 3.1.1. INVITE Request with SDP . . . . . . . . . . . . . . . 7
3.1.2. INVITE request without SDP . . . . . . . . . . . . . . 9 3.1.2. INVITE request without SDP . . . . . . . . . . . . . . 10
3.2. Offer/Answer Exchange in Early Dialog . . . . . . . . . . 10 3.2. Offer/Answer Exchange in Early Dialog . . . . . . . . . . 11
3.3. Offer/Answer Exchange in an Established Dialog . . . . . . 11 3.3. Offer/Answer Exchange in an Established Dialog . . . . . . 12
3.4. Recovering From a Failed ReINVITE . . . . . . . . . . . . 11 3.4. Recovering From a Failed ReINVITE . . . . . . . . . . . . 12
4. Exceptional Case Handling . . . . . . . . . . . . . . . . . . 12 4. Exceptional Case Handling . . . . . . . . . . . . . . . . . . 13
4.1. Message Crossing Case Handling . . . . . . . . . . . . . . 12 4.1. Message Crossing Case Handling . . . . . . . . . . . . . . 13
4.2. Glare Case Handling . . . . . . . . . . . . . . . . . . . 14 4.2. Glare Case Handling . . . . . . . . . . . . . . . . . . . 18
5. Content of Offers and Answers . . . . . . . . . . . . . . . . 15 4.3. Interworking of UPDATE and reINVITE . . . . . . . . . . . 21
5.1. General Principle for Constructing Offers and Answers . . 16 5. Content of Offers and Answers . . . . . . . . . . . . . . . . 24
5.1. General Principle for Constructing Offers and Answers . . 25
5.2. Choice of Media Types and Formats to Include and 5.2. Choice of Media Types and Formats to Include and
Exclude . . . . . . . . . . . . . . . . . . . . . . . . . 16 Exclude . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.2.1. Sending an Initial INVITE with Offer . . . . . . . . . 16 5.2.1. Sending an Initial INVITE with Offer . . . . . . . . . 25
5.2.2. Responding with an Offer when the Initial INVITE 5.2.2. Responding with an Offer when the Initial INVITE
has no Offer . . . . . . . . . . . . . . . . . . . . . 17 has no Offer . . . . . . . . . . . . . . . . . . . . . 26
5.2.3. Answering an Initial INVITE with Offer . . . . . . . . 17 5.2.3. Answering an Initial INVITE with Offer . . . . . . . . 26
5.2.4. Answering when the Initial INVITE had no Offer . . . . 18 5.2.4. Answering when the Initial INVITE had no Offer . . . . 27
5.2.5. Subsequent Offers and Answers . . . . . . . . . . . . 18 5.2.5. Subsequent Offers and Answers . . . . . . . . . . . . 27
5.3. Hold and Resume of media . . . . . . . . . . . . . . . . . 19 5.3. Hold and Resume of media . . . . . . . . . . . . . . . . . 28
5.4. Behavior on receiving SDP with c=0.0.0.0 . . . . . . . . . 20 5.4. Behavior on receiving SDP with c=0.0.0.0 . . . . . . . . . 30
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21 7. Security Considerations . . . . . . . . . . . . . . . . . . . 30
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 21 8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 30
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21 9.1. Normative References . . . . . . . . . . . . . . . . . . . 30
9.2. Informative References . . . . . . . . . . . . . . . . . . 22 9.2. Informative References . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction 1. Introduction
SIP utilizes the offer/answer model to establish and update sessions. SIP utilizes the offer/answer model to establish and update sessions.
The rules to govern the offer/answer behaviors in SIP are described The rules to govern the offer/answer behaviors in SIP are described
in the several RFCs. ([RFC3261], [RFC3262], [RFC3264], [RFC3311], in the several RFCs. ([RFC3261], [RFC3262], [RFC3264], [RFC3311],
and [I-D.camarillo-sipcore-reinvite].) and [I-D.camarillo-sipcore-reinvite].)
The primary purpose of this document is to describe all forms of SIP 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 readers usage of the offer/answer model in one document to help the readers
skipping to change at page 4, line 17 skipping to change at page 4, line 17
Currently, the rules on the offer/answer model are defined in Currently, the rules on the offer/answer model are defined in
[RFC3261], [RFC3262], [RFC3264], [RFC3311] and [RFC3261], [RFC3262], [RFC3264], [RFC3311] and
[I-D.camarillo-sipcore-reinvite]. In these RFCs, only the six [I-D.camarillo-sipcore-reinvite]. In these RFCs, only the six
patterns shown in Table 1 are defined for exchanging an offer and an patterns shown in Table 1 are defined for exchanging an offer and an
answer with SIP messages. 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 which if there is no offer in the INVITE request. This means that UA which
receives the INVITE request without an offer must include an offer in receives the INVITE request without an offer must include an offer in
the first reliable response with 100rel extension. If no reliable the first reliable response with 100rel extension. If no reliable
provisional response has been sent, the UAS must include an offer provisional response has been sent, the UAS must include an offer
when sending 2xx response. when sending 2xx response.
In pattern 3, the first reliable provisional response may or may not In Pattern 3, the first reliable provisional response may or may 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 offer/ NOTE: It is legal to have UPDATE/2xx exchanges without offer/
answer exchanges (pattern 6). However when re-INVITEs are sent answer exchanges (Pattern 6). However when re-INVITEs are sent
for non-offer/answer purposes, an offer/answer exchange is for non-offer/answer purposes, an offer/answer exchange is
required. In that case the prior SDP will typically be repeated. required. In that case the prior SDP will 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 ensure single dialog at any point in time. Section 4 explains how to ensure
this. When an INVITE results in multiple dialogs each has a separate this. When an INVITE results in multiple dialogs each has a 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 using a There may be a second offer/answer negotiation in progress using a
skipping to change at page 5, line 14 skipping to change at page 5, line 14
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 [RFC3262]. 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 the not. Only the initial INVITE transaction can be used to exchange the
offer/answer to establish a multimedia session. offer/answer to establish a multimedia session.
skipping to change at page 5, line 48 skipping to change at page 5, line 48
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 response field indicating the reason of the rejection, unless another response
code is more appropriate to reject it. (Pattern 1 and Pattern 3.) code is more appropriate to reject it. (Pattern 1 and Pattern 3.)
If this is a reINVITE extra care must be taken, as detailed in If this is a reINVITE extra care must be taken, as detailed in
[I-D.camarillo-sipcore-reinvite]. Specifically, if the offer [I-D.camarillo-sipcore-reinvite]. Specifically, if the offer
contains any changes or additions to media stream properties, and contains any changes or additions to media stream properties, and
those have already been used to transmit/receive media before the those have already been used to transmit/receive media before the
final response is sent, then a 2xx response should be sent, with a final response is sent, then a 2xx response should be sent, with a
syntactically correct response. This may optionally be followed by syntactically correct session description. This may optionally be
an UPDATE request to rearrange the session parameters if both ends followed by an UPDATE request to rearrange the session parameters if
support the UPDATE method. Alternatively the UA may terminate the both ends support the UPDATE method. Alternatively the UA may send
dialog and send an error response to the INVITE request. an error response to the (re)INVITE request to terminate the dialog
or to roll back the offer-answer status before sending reINVITE
request. In this case UAS should not continue to retransmit the
unacknowledged reliable provisional response, UAC should not continue
to retransmit a PRACK request.
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 Warning accept, it should respond with a 488 response preferably with Warning
header field indicating the reason of the rejection, unless another header field indicating the reason of the rejection, unless 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. (Pattern 5) dialog and send an error response to the INVITE request. (Pattern 5)
(While it may be tempting to respond with a 488 response in this The 488 response is another proposed solution, UAS may respond with a
case, that is not recommended, because it does not acknowledge the 488 response and then UAC should send again a PRACK request without
response.) an offer.
NOTE: In [RFC3262], the following restriction is defined with
regard to responding to a PRACK request.
"If the PRACK does match an unacknowledged reliable provisional
response, it MUST be responded to with a 2xx response."
This description is not completely correct. There are cases where
it is unacceptable to send a 2xx response. For example, 401
response can not be avoided.
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 and should respond to the offer with the correct session description and
rearrange the session parameters by initiating a new offer/answer rearrange the session parameters by initiating a new offer/answer
exchange, or alternatively terminate the session. (Pattern 2 and exchange, or alternatively terminate the session. (Pattern 2 and
Pattern 4) When initiating a new offer/answer, a UA should take care Pattern 4) When initiating a new offer/answer, a UA 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
(*) If this was a reINVITE, a failure response should not be sent if (*) If this was a reINVITE, a failure response should not be sent if
media has already been exchanged using the new offer. media has already been exchanged using the new 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 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 offer/answer session description to describe capabilities apart from offer/answer
exchange. Examples of this are 200 OK responses for OPTIONS and 488 exchange. Examples of this are 200 OK responses for 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 [RFC3261]. 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, 4 and 5 are added and this complicates the
the rules. An INVITE request may cause multiple responses. Note rules. An INVITE request may cause multiple responses. Note that
that even if both UAs support the 100rel extension, not all the 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.
reliable provisional response is allowed without a session
description if the UAS does not wish to send the answer yet. An 3.1.1. INVITE Request with SDP
unreliable provisional response may include a session description in
the body if the UAS has not sent a reliable response, but its session When a UAC includes an SDP body in the INVITE request as an offer,
description is neither an offer nor an answer. All the session only the first SDP in a reliable non-failure response to the INVITE
descriptions in the unreliable responses to the INVITE request must request is the real answer. No other offer/answer exchanges can
be identical to the answer which is included in the reliable occur within the messages (other responses and ACK) of the INVITE
response. A session description in an unreliable response that transaction.
precedes a reliable response can be considered a "preview" of the
answer that will be coming, and hence may be treated like an answer In [RFC3261] there are some descriptions about an offer/answer
until the actual one arrives. exchange, but those cause a little confusion. We interpret those
descriptions as follows,
UAC behavior
1. If the first SDP that UAC received is included in an
unreliable provisional response to the INVITE request, UAC
MUST treat it as an answer. But the SDP is not a real answer,
therefore the offer/answer exchange is not yet completed.
2. After UAC has received the answer in a reliable provisional
response to the INVITE, any SDP in subsequent responses to the
INVITE MUST be ignored.
3. If the second and subsequent SDP(including a real answer) is
different from the first SDP, UAC should consider that the SDP
is equal to the first SDP. Therefore, UAC should not switch
to the new SDP.
UAS behavior
1. All SDP in the responses to the INVITE request MUST be the
same exactly.
2. After UAS has sent the answer in a reliable provisional
response to the INVITE, UAS should not include any SDP in
subsequent responses to the INVITE.
3. UAS MAY send any provisional response without a SDP regardless
of the transmission of the answer.
A session description in an unreliable response that precedes a
reliable response can be considered a "preview" of the answer that
will be coming.
NOTE: This "preview" session description rule applies to a single NOTE: This "preview" session description rule applies to a single
offer/answer exchange. In parallel offer/answer exchanges (caused offer/answer exchange. In parallel offer/answer exchanges (caused
by forking) a UA may obviously receive a different "preview" of an by forking) a UA may obviously receive a different "preview" of an
answer in each dialog. UAs are expected to deal with this. answer in each dialog. UAs are expected to deal with this.
Although RFC 3261 says a UA should accept media once an INVITE with Although [RFC3261] 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 Agents o To avoid receiving media from undesired sources, some User Agents
assume symmetric RTP will be used, ignore all incoming media assume symmetric RTP will be used, ignore all incoming media
packets until an address/port has been received from the other packets until an address/port has been received from the other
end, and then use that address/port to filter incoming media end, and then use that address/port to filter incoming 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 the stream before it can flow and requires a confirming answer to 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 a 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 it support 100rel, the UAS should send a preview of the answer before it
starts sending media. starts sending media.
3.1.1. INVITE Request with SDP
When a UAC includes an SDP body in the INVITE request as an offer, it
expects the answer to be received with one of the reliable responses.
Other than that, no offer/answer exchanges can occur in the messages
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) | |
|-------------------->| | UAC must not send a new offer. |-------------------->| | UAC must not send a new offer.
skipping to change at page 8, line 49 skipping to change at page 9, line 46
| F10 PRACK | has been completed. | F10 PRACK | has been completed.
|-------------------->| |-------------------->|
| F11 2xx PRA | | F11 2xx PRA |
|<--------------------| |<--------------------|
| | | |
| F12 2xx INV | <- SDP should not be included in the | F12 2xx INV | <- SDP should not be included in the
|<--------------------| final response once offer/answer has |<--------------------| final response once offer/answer has
| F13 ACK | been completed. | F13 ACK | been completed.
|-------------------->| |-------------------->|
Figure 1 Example of Offer/Answer with 100rel Extension (1) Figure 1: Example of Offer/Answer with 100rel Extension (1)
For example, in Figure 1, only the SDP in F6 is the answer. The SDP For example, in Figure 1, only the SDP in F6 is the answer. The SDP
in the non-reliable response (F2) is the preview of the answer and in the non-reliable response (F2) is the preview of the answer and
must be the same as the answer in F6. Receiving F2, the UAC should must be the same as the answer in F6. Receiving F2, the UAC should
act as if it receives the answer. However, offer/answer exchange is act as if it receives the answer. However, offer/answer exchange is
not completed yet and the UAC must not send a new offer until it not completed yet and the UAC must not send a new offer until it
receives the same SDP in the first reliable response, which is the receives the same SDP in a reliable non-failure response, which is
real answer. After sending the SDP in F6, the UAS must prepare to the real answer. After sending the SDP in F6, the UAS must prepare
receive a new offer from the UAC with an UPDATE request or a PRACK to receive a new offer from the UAC in a PRACK request, or in an
request. UPDATE request if the UAS supports UPDATE.
The UAS does not include SDP in responses F9 and F12. However, the The UAS does not include SDP in responses F9 and F12. However, the
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 a SDP body in the INVITE request, it
expects the offer to be received with the first reliable response. expects an offer to be received with the first reliable non-failure
The UAC will send the answer in the request to acknowledge the response. And a UAS MUST include an offer in the first reliable non-
response, i.e. PRACK or ACK request of the reliable response. Other failure response and should not include any SDP in the other
than that, no offer/answer exchanges can occur in the messages within responses to the INVITE request. The UAC will send the answer in the
the INVITE transaction. request to acknowledge the response, i.e. PRACK or ACK request of
the reliable response. Other than that, no offer/answer exchanges
can occur in the messages within the INVITE transaction.
NOTE: The UAS should not include SDP in the responses F6 and F9. NOTE: The UAS should not include SDP in the responses F6 and F9.
However, the UAC should prepare to receive SDP bodies in F6 and/or However, the UAC should prepare to receive SDP bodies in F6 and/or
F9, and just ignore them to handle a peer that does not conform to F9, and just ignore them to handle a peer that does 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 |
skipping to change at page 10, line 30 skipping to change at page 11, line 30
| F7 PRACK | | | F7 PRACK | |
|-------------------->| | UAC can send a new offer in an UPDATE |-------------------->| | UAC can send a new offer in an UPDATE
| 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 accept Note that in the case that the UAC needs to prompt the user to accept
or reject the offer, the reliable provisional response with SDP as an or reject the offer, the reliable provisional response with SDP as an
offer (pattern 4) can result in the retransmission until the PRACK offer (Pattern 4) can result in the retransmission until the PRACK
request can be sent. The UAC should take care to avoid this request can be sent. The UAC should take care to avoid 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 has session in the early dialog once the first offer/answer exchange has
been completed. been completed.
From a UA sending an INVITE request: From a UA sending an INVITE request:
skipping to change at page 12, line 46 skipping to change at page 13, line 46
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 exchanges (summarized in Table 1) there are a limited number of valid exchanges
of messages that may lead to this message crossing case. These are of messages that may lead to this message crossing case. These are
enumerated in Table 3. (This table only shows messages containing enumerated in Table 3. (This table only shows messages containing
offers or answers. There could be other messages, without session offers or answers. There could be other messages, without session
descriptions, which are not shown.) descriptions, which are not shown.)
There are variants, shown in Figures 4a and 4b, which are dependent When a response to UPDATE request crosses a reliable response to
on an INVITE (Mx) that contains no offer. These are also included in INVITE request, there are variants shown in Figures 4 and 5, which
Table 3. are dependent on an INVITE (Mx) that contains no offer. These are
also included in 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
|SDP-3 /\ (1xx-rel/2xx)| | |SDP-3 /\ (1xx-rel/2xx)| |
M3 |<===========/ \===============| <-+ M3 |<===========/ \===============| <-+
|SDP-4 answer2 (PRACK/ACK)| |SDP-4 answer2 (PRACK/ACK)|
My |------------------------------>| My |------------------------------>|
| | | |
Figure 4a Avoidable message crossing cases Figure 4: Avoidable message crossing cases
A B To avoid this message crossing condition shown in Figure 4, UA A
| | should not send this reINVITE request at this point. If UA B
|re-INV (no offer) | encounters this message crossing condition, it should reject this
Mx |------------------------------>| --+ reINVITE request with a 500 response.
|SDP-1 offer1(UPD) | |
M1 |==============================>| |
|SDP-2 answer1 (2xx-UPD)| |
M2 |<===========\ /===============| | first reliable
| \/ offer2| | response
|SDP-3 /\ (1xx-rel/2xx)| |
M3 |<===========/ \===============| <-+
|SDP-4 answer2 (PRACK/ACK)|
My |------------------------------>|
| |
Figure 4b Avoidable message crossing cases A B
| |
|re-INV (no offer)|
Mx |------------------------------>| --+
|SDP-1 offer1(UPD)| |
M1 |==============================>| |
|SDP-2 answer1 (2xx-UPD)| |
M2 |<===========\ /===============| | first reliable
| \/ offer2| | response
|SDP-3 /\ (1xx-rel/2xx)| |
M3 |<===========/ \===============| <-+
|SDP-4 answer2 (PRACK/ACK)|
My |------------------------------>|
| |
| M1 | M3 | M2 | Action Figure 5: Avoidable message crossing cases
+--------+----------+---------+---------
| UPDATE | 2xx-UPD | UPDATE | (1)
| | +---------|---------
| | | INVITE | (1)
| | +---------+---------
| | | 1xx-INV | (2)
| | +---------+---------
| | | 2xx-INV | (2)
+--------+----------+---------+---------
| PRACK | 2xx-PRA | UPDATE | (1)
+--------+----------+---------+---------
| 2xx-INV| ACK | UPDATE | (1)
| | +---------+---------
| | | INVITE | (1)
+--------+----------+---------+---------
| INVITE | 1xx-rel | ??? | (3)
| |----------+---------+---------
| | 2xx-INV | ??? | (3)
+--------+----------+---------+---------
| 1xx-rel| PRACK | ??? | (3)
+--------+----------+---------+---------
Table 3. Offer / Answer Crossing Message Sequences To avoid this message crossing condition shown in Figure 5, UA A
should not send this UPDATE request at this point. If UA B
encounters this message crossing condition, it should reject this
UPDATE request with a 500 response.
(1) This is indistinguishable from true glare. UA A should respond The situation when PRACK request crosses UPDATE request is shown in
to M2 with a 491 response. Figure 6.
(2) This can only occur in situations depicted in figures 4a and 4b. A B
It is easier for UA A to avoid these situations than to recover | |
from them. The situation in Figure 4a can be avoided by | re-INV (no offer)|
refraining from sending a re-INVITE without offer when an 1st reliable+-- |<------------------------------|
unanswered offer is outstanding. The situation in Figure 4b can response | M1|1xx-rel (offer1) |
be avoided by refraining from sending any message containing an +-> |==============================>| --+
offer while an INVITE without offer is outstanding. | answer1(PRA)| M3| Acknowledge
|<===========\ /===============| <-+
| \/ |
| /\ offer2(UPD)|
|<===========/ \===============| M2
|500 (UPD) |
|------------------------------>|
|2xx-PRA |
|------------------------------>|
| |
(3) There are no valid sequences that result in these cases. Figure 6: Avoidable message crossing cases
Summarizing, a UA that has an outstanding unanswered offer should: To avoid the message crossing condition shown in Figure 6, UA B
o refrain from sending a re-INVITE without an offer; should not send this UPDATE request at this point. If UA A
o reject (491) an INVITE or UPDATE containing an offer. encounters this message crossing condition, it should reject this
UPDATE request with a 500 response.
The situation when a reliable provisional response to INVITE request
crosses UPDATE request is shown in Figure 7.
A B
| |
|re-INV (offer1) |
M1 |==============================>|
| answer1 (1xx-rel)|
|<===========\ /===============| M3
| \/ |
| /\ offer1(UPD)|
+-- |<===========/ \===============| M2
| |491 (UPD) |
Acknowledge | |------------------------------>|
| |PRACK |
+-> |------------------------------>|
| |
Figure 7: Avoidable message crossing cases
To avoid the message crossing condition shown in Figure 7, UA B
should not send this UPDATE request at this point. If UA A
encounters this message crossing condition, it should reject this
UPDATE request with a 491 response.
The situation when a 2xx response to INVITE request crosses UPDATE
request is shown in Figure 8.
A B
| |
|re-INV (offer1) |
|==============================>|
| answer1 (2xx)|
|<===========\ /===============|
| \/ |
| /\ offer1(UPD)|
+-- |<===========/ \===============|
| |491 (UPD) |
Acknowledge | |------------------------------>|
| |ACK |
+-> |------------------------------>|
| |
Figure 8: Avoidable message crossing cases
To avoid the message crossing condition shown in Figure 8, UA B
should not send this UPDATE request at this point. If UA A
encounters this message crossing condition, it should reject this
UPDATE request with a 491 response.
The situation when a response to UPDATE request crosses PRACK request
is shown in Figure 9.
A B
| |
| re-INV (offer0)|
|<------------------------------|
|1xx-rel (answer0) |
|------------------------------>| --+
|offer1(UPD) | |
M1 |==============================>| |
| answer1 (2xx-UPD)| | Acknowledge
|<===========\ /===============| M3|
| \/ | |
| /\ offer2(PRA)| M2|
|<===========/ \===============| <-+
| |
Figure 9: Avoidable message crossing case
To avoid the message crossing condition shown in Figure 9, UA A
should not send this UPDATE request at this point. If UA B
encounters this message crossing condition, it should reject this
UPDATE request with a 491 response.
Table 3 summarize this section. Each action is described in
Section 4.3.
| M1 | M3 | M2 |Action |Action |Figure|
|(offer1)|(answer1) |(offer2) | of A | of B | |
+--------+----------+-----------+-------+-------+------+
| UPDATE | 2xx-UPD | UPDATE |UAS-UcU| | |
| | +-----------+ | - | |
| | | INVITE |UAS-UcI| | |
| | +-----------+-------+-------+------+
| | | 1xx-INV | | | |
| | +-----------+UAC-UI,|UAS-UsI| 4,5 |
| | | 2xx-INV |UAC-IU |UAS-IsU| |
| | +-----------+-------+-------+------+
| | | PRACK (*)|UAC-IU |UAS-IcU| 9 |
+--------+----------+-----------+-------+-------+------+
| PRACK | 2xx-PRA | UPDATE |UAS-IcU| | |
+--------+----------+-----------+ | | |
| 2xx-INV| ACK | UPDATE |UAS-IsU| - | |
| | +-----------+ | | |
| | | INVITE |UAS-IsI| | |
+--------+----------+-----------+ +-------+------+
| 1xx-rel| PRACK | UPDATE |UAS-IsU|UAC-IU | 6 |
+--------+----------+-----------+ | +------+
| INVITE | 1xx-rel | UPDATE (*)|UAS-IcU|UAC-IU | 7 |
| +----------+-----------+ | +------+
| | 2xx-INV | UPDATE (*)|UAS-IcU|UAC-IU | 8 |
+--------+----------+-----------+-------+-------+------+
(*) invalid sequences if INVITE request is an initial one
Table 3: Offer / Answer Crossing Message Sequences
4.2. Glare Case Handling 4.2. Glare Case Handling
When both ends in a dialog send a new offer at nearly the same time, When both ends in a dialog send a new offer at nearly the same time,
as described in Figure 5, a UA may receive a new offer before it as described in Figure 10, a UA may receive a new offer before it
receives the answer to the offer it sent. This case is usually receives the answer to the offer it sent. This case is usually
called a 'glare' case. called a 'glare' case.
A B A B
|offer1 offer2| |offer1 offer2|
|-------\ /-------| M1 |-------\ /-------| M2
| \/ | | \/ |
| /\ | | /\ |
|<------/ \------>| |<------/ \------>|
Figure 5 Glare Case Figure 10: 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 or 500 response.
When offer2 is in a PRACK request (within the current rules, only There is a variant of Figure 7. When offer2 is in a PRACK request
possible if offer1 is in an UPDATE request), UA A has a dilemma: all (within the current rules, only possible if offer1 is in an UPDATE
PRACKs are supposed to be accepted with 200 response, yet there is no request), as shown in Figure 11, UA A has a dilemma.
way to indicate the problem with a 200 response. At best it could
proceed on the assumption that its INVITE will be rejected with a
491. To avoid this glare condition, UA A should not send an offer if
it has already sent a reliable provisional response containing an
answer to a previous offer and has not received the corresponding
PRACK request.
Glare can also occur when offer2 is in a 1xx or 2xx response. To A B
avoid this situation, when UA A has sent a (re)INVITE request without | |
session description, it should not send an offer until it has | re-INV (offer0)|
received an offer in a reliable response to the (re)INVITE, and sent |<------------------------------|
an answer to that offer. |1xx-rel (answer0) |
|------------------------------>| --+
|offer1(UPD) offer2(PRA)| M2| Acknowledge
M1 |============\ /===============| <-+
| \/ |
| /\ |
|<===========/ \==============>|
| 491 (UPD)|
|<------------------------------|
| |
Figure 11: Avoidable glare case
All PRACKs are supposed to be accepted with 200 response, yet there
is no way to indicate the problem with a 200 response. At best it
could proceed on the assumption that its INVITE will be rejected with
a 491. To avoid this glare condition shown in Figure 11, UA A should
not send this UPDATE request at this point. If UA B encounters this
glare condition it should reject this UPDATE request with a 491
response.
Glare can also occur when offer2 is in a 1xx or 2xx response. This
is a variant of Figure 5, as shown in Figure 12.
A B
| |
|re-INV (no offer) |
|------------------------------>| --+ 1st reliable
|offer1(UPD) offer2| M2| response
M1 |============\ /===============| <-+
| \/ (1xx-rel/2xx)|
| /\ |
|<===========/ \==============>|
| 500 (UPD)|
|<------------------------------|
| |
Figure 12: Avoidable glare case
To avoid this glare condition shown in Figure 12, UA A should not
send this UPDATE request at this point. If UA B encounters this
glare condition it should reject this UPDATE request with a 500
response.
There is a variant of Figure 4, as shown in Figure 13.
A B
| |
|offer1(UPD) |
|==========\ |
|re-INV \ |
|------------\----------------->| --+
|(no offer) \ | |1st reliable
| \ offer2| | response
|<==============\===============| <-+
| \ (1xx-rel/2xx)|
| \ |
| \===========>|
| 500 (UPD)|
|<------------------------------|
| |
Figure 13: Avoidable glare case
To avoid this glare condition shown in Figure 13, UA A should not
send this reINVITE request at this point. If UA B encounters this
glare condition it should reject this UPDATE request with a 500
response.
Table 4 summarize this section. Each action is described in
Section 4.3.
| offer1 | offer2 |Action |Action |Figure|
| M1 | M2 | of A | of B | |
+----------+----------+-------+-------+------+
| | reINVITE |UAS-IcI|UAS-IcI| |
| reINVITE +----------+ | | |
| | UPDATE |UAS-IcU|UAS-UcI| |
+----------+----------+ | | |
| | UPDATE |UAS-UcU|UAS-UcU| |
| +----------+-------+ +------+
| | 1xx-rel | | | |
| UPDATE +----------+UAC-IU,|UAS-IsU|12,13 |
| | 2xx-INV |UAC-UI | | |
| +----------+-------+ +------+
| | PRACK (*)|UAC-IU |UAS-IcU| 11 |
+----------+----------+-------+-------+------+
(*) invalid sequences if INVITE request is an initial one
Table 4: Offer / Answer Glare Message Sequences
4.3. Interworking of UPDATE and reINVITE
Almost all exceptional cases are caused by an interworking of UPDATE
and reINVITE. The interworking is described in Section 5 of
[RFC3311]. And UAC Behavior sending an UPDATE is described in
Section 5.1 of [RFC3311]. There are two concerns in this section,
1. It seems to describe different rules for each of initial INVITE
and reINVITE. But there is no particular reason why the rules
are separated. The lack of restrictions for sending a reINVITE
request cause a lot of the problems shown in Section 4.1.
2. It seems to describe that a UA may send an UPDATE request after
sending or receiving a PRACK request. But it should be "after
PRACK transaction is completed by 2xx response", because it
causes the message-crossing case shown in Figure 6
Since it is assumed that the language in this section itself is non-
normative and is justified as a corollary of 3261, we interpret it as
follows,
UAC-II: While an INVITE transaction is incomplete, a UA must not
send an another INVITE request.
UAC-UU: While an UPDATE transaction is incomplete, a UA must not
send an another UPDATE request.
UAC-UI: While an UPDATE transaction is incomplete, a UA should not
send a reINVITE request.
UAC-IU: While an INVITE transaction is incomplete and a PRACK
transaction associated with offer-answer is incomplete, a
UA should not send an UPDATE request.
UAS Behavior receiving an UPDATE is described in Section 5.2 of
[RFC3311]. There are two concerns in this section,
1. There are no description about the interworking of an UPDATE
request and an INVITE request without an offer.
2. There are no description about the interworking of an UPDATE
request and reliable response to INVITE with an offer.
We interpret this section as follows,
UAS-IcI: While an INVITE client transaction is incomplete, a UA must
reject an another INVITE request with a 491 response.
UAS-IsI: While an INVITE server transaction is incomplete, a UA must
reject an another INVITE request with a 500 response.
UAS-UcU: While an UPDATE client transaction is incomplete, a UA must
reject an another UPDATE request with a 491 response.
UAS-UsU: While an UPDATE server transaction is incomplete, a UA must
reject an another UPDATE request with a 500 response.
UAS-UcI: While an UPDATE client transaction is incomplete, a UA
should reject a reINVITE request with a 491 response.
UAS-UsI: While an UPDATE server transaction is incomplete, a UA
should reject a reINVITE request with a 500 response.
UAS-IcU: While an INVITE client transaction is incomplete, and a
PRACK transaction associated with offer-answer is
incomplete, a UA should reject an UPDATE request with a 491
response.
UAS-IsU: While an INVITE server transaction is incomplete, and a
PRACK transaction associated with offer-answer is
incomplete, a UA should reject an UPDATE request with a 500
response.
A B
| |
| UPDATE|
|<------------------------------|
|UPDATE |
|==============================>|
| 491|
|<==============================|
| |
Figure 14: Example of UAC-UU and UAS-UcU
A B
| |
|UPDATE CSeq:m |
|------------------------------>|
|UPDATE CSeq:n(>m) |
|==============================>|
| 500 (UPDATE CSeq:n)|
|<==============================|
| |
Figure 15: Example of UAC-UU and UAS-UsU
A B
| |
| UPDATE(offer1)|
|<------------------------------|
|reINVITE(no offer) |
|==============================>|
| 491 (INVITE)|
|<==============================|
| |
Figure 16: Example of UAC-UI and UAS-UcI
A B
| |
|UPDATE(offer1) |
|------------------------------>|
|reINVITE(no offer) |
|==============================>|
| 500 (INVITE)|
|<==============================|
| |
Figure 17: Example of UAC-UU and UAS-UsI
A B
| |
| reINVITE(no offer)|
|<------------------------------|
|1xx-rel(offer0) |
|------------------------------>|
|UPDATE(offer1) |
|==============================>|
| 491 (UPDATE)|
|<==============================|
| |
Figure 18: Example of UAC-IU and UAS-IcU
A B
| |
|reINVITE(no offer) |
|------------------------------>|
| 1xx-rel(offer0)|
|<------------------------------|
|UPDATE(offer1) |
|==============================>|
| 500 (UPDATE)|
|<==============================|
| |
Figure 19: Example of UAC-IU and UAS-IsU
In addition, it is assumed that the UPDATE request in this section
include a offer. The interworking of a reINVITE and an UPDATE
without an offer is out of scope for this document.
5. Content of Offers and Answers 5. Content of Offers and Answers
While [RFC3264] and [RFC3312] 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 is about exactly what should be included in an offer or answer. This 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 User Details of behavior depend on the capabilities and state of the User
Agent. The kinds of recommendations that can be made are limited by Agent. The kinds of recommendations that can be made are limited by
skipping to change at page 16, line 18 skipping to change at page 25, line 22
o grouping of media streams o grouping of media streams
o preconditions o 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 the INVITE that contains no offer. (However in the case of re-INVITE the
constraints of RFCs 3261 and 3264 must be observed.) constraints of [RFC3261] and [RFC3264] must be observed.)
A UA should send an answer that includes as close an approximation to 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, while what the UA and its user are interested in doing at that time, while
remaining consistent with the offer/answer rules of [RFC3264] and remaining consistent with the offer/answer rules of [RFC3264] and
other RFCs. other RFCs.
NOTE: "at that time" is important. The device may permit the user NOTE: "at that time" is important. The device may permit the user
to configure which supported media are to be used by default. to configure which supported media are to be used by 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
skipping to change at page 17, line 41 skipping to change at page 26, line 44
include the minimum set of media the user is able to select. include the 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 lines When a UAS receives an initial INVITE with an offer, what media lines
the answer may contain is constrained by [RFC3264]. The answer must the answer may contain is constrained by [RFC3264]. The answer must
contain the same number of m-lines as the offer, and they must contain the same number of m-lines as the offer, and they must
contain the same media types. Each media line may be accepted, by contain the same media types. Each media line may be accepted, by
including a non-zero port number, or rejected by including a zero including a non-zero port number, or rejected by including a zero
port number in the answer. The media lines that are accepted should port number in the answer. The media lines that are accepted should
typically be those that would have been offered had the INVITE not typically be those with types and formats the UAS would have included
contained an offer, excluding those not offered. if it were the offerer.
The media formats the answer may contain are constrained by The media formats the answer may contain are constrained by
[RFC3264]. 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 the 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 to offer. The UAS may also include other media formats it is able to
support at this time. However there is little benefit to including support at this time. Doing so establishes an asymmetric media
added types. format situation, where these "other" media formats may only be sent
from the offerer to the answerer. This asymmetric media situation is
also limited because it cannot be sustained if there is a subsequent
offer/answer exchange in the opposite direction. Also, there is
limited value in including these other media formats because there is
no assurance that the offerer will be able to use them.
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.
When the UAS wishes to reject all of the media lines in the offer, it
may send a 488 failure response. Alternatively it may send a
reliable non-failure response including all media lines with port
numbers set 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.
Because the offer arrives in a response to the INVITE, the UAC cannot
reject the message containing the offer. If the UAC wishes to reject
the entire offer, it must send a PRACK or ACK request including all
the media lines with ports set to zero. Then, if it does not wish to
continue the session it may send a CANCEL or BYE request to terminate
the dialog.
5.2.5. Subsequent Offers and Answers 5.2.5. Subsequent Offers and Answers
The guidelines above (Section 5.1 and Section 5.2.1 through The guidelines above (Section 5.1 and Section 5.2.1 through
Section 5.2.4) apply, but constraints in [RFC3264] must also be Section 5.2.4) apply, but constraints in [RFC3264] must also be
followed. The following are of particular note because they have followed. The following are of particular note because they have
proven 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. (Section 6 of [RFC3264].) 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
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state of the recipient. However, a UA that has been "placed on hold" state of the recipient. However, a UA that has been "placed on hold"
may itself desire to initiate its own hold status, based on local may itself desire to initiate its own hold status, based on 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 will offer containing "a=sendonly" to UA1. Upon receipt of that, UA1 will
answer with "a=inactive" because that is the only valid answer that answer with "a=inactive" because that is the only valid answer that
reflects its desire not to receive media. reflects its desire not to receive media.
NOTE: Section 8.4 of RFC3264 contains a conflicting recommendation NOTE: Section 8.4 of [RFC3264] contains a conflicting
that the offer contain "a=inactive" in this case. We interpret recommendation that the offer contain "a=inactive" in this case.
that recommendation to be non-normative. The use of "a=sendonly" We interpret that recommendation to be non-normative. The use of
in this case will never produce a worse outcome, and can produce a "a=sendonly" in this case will never produce a worse outcome, and
better outcome in useful cases. can produce a better outcome in useful cases.
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 with will then send an offer with "a=sendrecv". The UA2 will respond 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 its the offer with "a=sendrecv", if it has decided it wants to reset 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
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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
[RFC3264] specifies that an agent MUST be capable of receiving SDP [RFC3264] specifies that an agent MUST be capable of receiving SDP
with a connection address of 0.0.0.0, in which case it means that 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. 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, the If a UA generates an answer to the offer received with "c=IN IP4
direction attribute of the accepted media stream in the answer must 0.0.0.0", the direction attribute of the accepted media stream in the
be based on direction attribute of the offered stream and rules answer must still be based on direction attribute of the offered
specified in RFC 3264 to form the a-line in the answer. c=0.0.0.0 has stream and rules specified in [RFC3264] to form the direction a-line
no special meaning for the direction attribute of the accepted stream in the answer. There is no clear rule about the use of "c=IN IP4
in the answer. 0.0.0.0" in the answer - it may be used or c-line with a valid IP
address may be used. RTP/RTCP will not be sent toward an address of
0.0.0.0 because it is an invalid address.
6. IANA Considerations 6. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
7. Security Considerations 7. Security Considerations
There are not any security issues beyond the referenced RFCs. There are not any security issues beyond the referenced RFCs.
8. Acknowledgement 8. 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, Jonathan Rosenberg, Gonzalo Camarillo and Nataraju A B, Byron Campen, Jonathan Rosenberg, Gonzalo Camarillo and
Shinji Okumura for their thorough reviews and comments. Many of Yang Gao for their thorough reviews and comments. Many of their
their suggestions and ideas have been incorporated in this document. suggestions and ideas have been incorporated in this document. Also,
a big thank you to Sawada Takuya, who was first author of this
document.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] 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.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
skipping to change at page 22, line 19 skipping to change at page 32, line 7
December 2004. December 2004.
[I-D.ietf-sipping-config-framework] [I-D.ietf-sipping-config-framework]
Channabasappa, S., "A Framework for Session Initiation Channabasappa, S., "A Framework for Session Initiation
Protocol User Agent Profile Delivery", Protocol User Agent Profile Delivery",
draft-ietf-sipping-config-framework-17 (work in progress), draft-ietf-sipping-config-framework-17 (work in progress),
February 2010. February 2010.
Authors' Addresses Authors' Addresses
OKUMURA Shinji
Softfront
28-196, Noth9, West15, Chuo-ku
Sapporo, Hokkaido 060-0009
Japan
Email: shinji.okumura@softfront.jp
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
Takuya Sawada
KDDI Corporation
3-10-10, Iidabashi, Chiyoda-ku
Tokyo
Japan
Email: tu-sawada@kddi.com
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