Internet Engineering Task Force (IETF) S. Okumura
Request for Comments: 6337 Softfront
Category: Informational T. Sawada
ISSN: 2070-1721 KDDI Corporation
P. Kyzivat
August 2011
Session Initiation Protocol (SIP) Usage of the Offer/Answer Model
Abstract
The Session Initiation Protocol (SIP) utilizes the offer/answer model
to establish and update multimedia sessions using the Session
Description Protocol (SDP). The description of the offer/answer
model in SIP is dispersed across multiple RFCs. This document
summarizes all the current usages of the offer/answer model in SIP
communication.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6337.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Okumura, et al. Informational [Page 1]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Summary of SIP Usage of the Offer/Answer Model ..................3
2.1. Terminology ................................................3
2.2. Offer/Answer Exchange Pairs in SIP Messages ................4
2.3. Rejection of an Offer ......................................5
2.4. Session Description That Is Not an Offer or an Answer ......7
3. Detailed Discussion of the Offer/Answer Model for SIP ...........8
3.1. Offer/Answer for the INVITE method with 100rel Extension ...8
3.1.1. INVITE Request with SDP .............................8
3.1.2. INVITE Request without SDP .........................11
3.2. Offer/Answer Exchange in Early Dialog .....................12
3.3. Offer/Answer Exchange in an Established Dialog ............12
3.4. Recovering from a Failed Re-INVITE ........................13
4. Exceptional Case Handling ......................................13
4.1. Message Crossing Case Handling ............................13
4.2. Glare Case Handling .......................................18
4.3. Interworking of UPDATE and Re-INVITE ......................21
5. Content of Offers and Answers ..................................25
5.1. General Principle for Constructing Offers and Answers .....26
5.2. Choice of Media Types and Formats to Include and Exclude ..26
5.2.1. Sending an Initial INVITE with Offer ...............26
5.2.2. Responding with an Offer When the Initial
INVITE Has No Offer ................................27
5.2.3. Answering an Initial INVITE with Offer .............27
5.2.4. Answering When the Initial INVITE Had No Offer .....28
5.2.5. Subsequent Offers and Answers ......................28
5.3. Hold and Resume of Media ..................................29
5.4. Behavior on Receiving SDP with c=0.0.0.0 ..................31
6. Security Considerations ........................................31
7. Acknowledgements ...............................................31
8. References .....................................................32
8.1. Normative References ......................................32
8.2. Informative References ....................................33
Okumura, et al. Informational [Page 2]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
1. Introduction
SIP utilizes the offer/answer model to establish and update sessions.
The rules that govern the offer/answer behaviors in SIP are described
in several RFCs: [RFC3261], [RFC3262], [RFC3264], [RFC3311], and
[RFC6141].
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
to fully understand it. Also, this document tries to incorporate the
results of the discussions on the controversial issues to avoid
repeating the same discussions later.
This document describes ambiguities in the current specifications and
the authors' understanding of the correct interpretation of these
specifications. This document is not intended to make any changes to
those specifications, but rather is intended to provide a reference
for future standards development work on the SIP offer/answer model
and to developers looking for advice on how to implement in
compliance with the standards.
2. Summary of SIP Usage of the Offer/Answer Model
The offer/answer model itself is independent from the higher layer
application protocols that utilize it. SIP is one of the
applications using the offer/answer model. [RFC3264] defines the
offer/answer model, but does not specify which SIP messages should
convey an offer or an answer. This should be defined in the SIP core
and extension RFCs.
In theory, any SIP message can include a session description in its
body. But a session description in a SIP message is not necessarily
an offer or an answer. Only certain session description usages that
conform to the rules described in Standards-Track RFCs can be
interpreted as an offer or an answer. The rules for how to handle
the offer/answer model are defined in several RFCs.
The offer/answer model defines a mechanism for update of sessions.
In SIP, a dialog is used to associate an offer/answer exchange with
the session that it is to update. In other words, only the offer/
answer exchange in the SIP dialog can update the session that is
managed by that dialog.
2.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Okumura, et al. Informational [Page 3]
RFC 6337 SIP Usage of the Offer/Answer Model August 2011
The following abbreviations are used in this document.
UA: User Agent.
UAC: User Agent Client.
UAS: User Agent Server.
SDP: Session Description Protocol [RFC4566].
2.2. Offer/Answer Exchange Pairs in SIP Messages
Currently, the rules on the offer/answer model are defined in
[RFC3261], [RFC3262], [RFC3264], [RFC3311], and [RFC6141]. In these
RFCs, only the six patterns shown in Table 1 are defined for
exchanging an offer and an answer with SIP messages.
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
INVITE causes multiple dialogs due to forking, an offer/answer
exchange is carried out independently in each distinct dialog. When
an INVITE request contains no offer, only Pattern 2 or Pattern 4
apply. According to Section 13.2.1 of [RFC3261], 'The first reliable
non-failure message' must have an offer if there is no offer in the
INVITE request. This means that the User Agent (UA) that receives
the INVITE request without an offer must include an offer in the
first reliable response with 100rel extension. If no reliable
provisional response has been sent, the User Agent Server (UAS) must
include an offer when sending 2xx response.
In Pattern 3, the first reliable provisional response may or may not
have an answer. When a reliable provisional response contains a
session description, and is the first to do so, then that session
description is the answer to the offer in the INVITE request. The
answer cannot be updated, and a new offer cannot be sent in a
subsequent reliable response for the same INVITE transaction.
In Pattern 5, a Provisional Response ACKnowledgement (PRACK) request
can contain an offer only if the reliable response that it
acknowledges contains an answer to the previous offer/answer
exchange.
NOTE: It is legal to have UPDATE/2xx exchanges without offer/
answer exchanges (Pattern 6). However, when re-INVITEs are sent
for non-offer/answer purposes, an offer/answer exchange is
required. In that case, the prior SDP will typically be repeated.
Okumura, et al. Informational [Page 4]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
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
this. When an INVITE results in multiple dialogs, each has a
separate offer/answer negotiation.
NOTE: This is when using a Content-Disposition of "session".
There may be a second offer/answer negotiation in progress using a
Content-Disposition of "early-session" [RFC3959]. That is not
addressed by this document.
Offer Answer RFC Ini Est Early
-------------------------------------------------------------------
1. INVITE Req. 2xx INVITE Resp. 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
4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N
5. PRACK Req. 200 PRACK Resp. RFC 3262 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
In Table 1, '1xx-rel' corresponds to the reliable provisional
response that contains the 100rel option defined in [RFC3262].
The 'Ini' column shows the ability to exchange the offer/answer to
initiate the session. 'Y' indicates that the pattern can be used in
the initial offer/answer exchange, while 'N' indicates that it
cannot. Only the initial INVITE transaction can be used to exchange
the offer/answer to establish a multimedia session.
The 'Est' column shows the ability to update the established session.
The 'Early' column indicates which patterns may be used to modify the
established session in an early dialog. There are two ways to
exchange a subsequent offer/answer in an early dialog.
2.3. Rejection of an Offer
It is not always clear how to reject an offer when it is
unacceptable, and some methods do not allow explicit rejection of an
offer. For each of the patterns in Table 1, Table 2 shows how to
reject an offer.
When a UA receives an INVITE request with an unacceptable offer, it
should respond with a 488 response, preferably with Warning header
field indicating the reason of the rejection, unless another response
code is more appropriate to reject it (Pattern 1 and Pattern 3).
Okumura, et al. Informational [Page 5]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
If this is a re-INVITE, extra care must be taken, as detailed in
[RFC6141]. Specifically, if the offer contains any changes or
additions to media stream properties, and those have already been
used to transmit/receive media before the final response is sent,
then a 2xx response should be sent, with a syntactically correct
session description. This may optionally be followed by an UPDATE
request to rearrange the session parameters if both ends support the
UPDATE method. Alternatively, the UA may send an error response to
the (re-)INVITE request to terminate the dialog or to roll back the
offer/answer status before sending re-INVITE request. In this case,
the UAS should not continue to retransmit the unacknowledged reliable
provisional response; the User Agent Client (UAC) should not continue
to retransmit a PRACK request.
When a UA receives an UPDATE request with an offer that it cannot
accept, it should respond with a 488 response, preferably with
Warning header field indicating the reason of the rejection, unless
another response code is more appropriate to reject it (Pattern 6).
When a UA receives a PRACK request with an offer that it cannot
accept, it may respond with a 200 response with a syntactically
correct session description. Optionally, this may be followed by an
UPDATE request to rearrange the session parameters if both ends
support the UPDATE method. Alternatively, the UA may terminate the
dialog and send an error response to the INVITE request (Pattern 5).
In addition, there is a possibility for UAC to receive a 488 response
for an PRACK request. In that case, UAC may send again a PRACK
request without an offer or send a CANCEL request to terminate the
INVITE transaction.
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 restriction is not clear. There are cases where it is
unacceptable to send a 2xx response. For example, the UAS may
need to send an authentication challenge in a 401 response. This
is an open issue and out of scope for this document.
When a UA receives a response with an offer that it cannot accept,
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
rearrange the session parameters by initiating a new offer/answer
Okumura, et al. Informational [Page 6]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
exchange, or alternatively terminate the session (Pattern 2 and
Pattern 4). When initiating a new offer/answer, a UA should take
care not to cause an infinite offer/answer loop.
Section 14.2 of [RFC3261], "UAS Behavior", states:
The UAS MUST ensure that the session description overlaps with its
previous session description in media formats, transports, or
other parameters that require support from the peer. This is to
avoid the need for the peer to reject the session description.
This is a rule for an offer within 2xx response to a re-INVITE. This
rule should be applied to an offer within a reliable provisional
response and a PRACK request.
Offer Rejection
------------------------------------------------------------------
1. INVITE Req. (*) 488 INVITE Response
2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer
OR termination of dialog
3. INVITE Req. 488 INVITE Response (same as Pattern 1)
4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer
5. PRACK Req. (**) 200 PRACK Resp. followed by new offer
OR termination of dialog
6. UPDATE Req. 488 UPDATE Response
(*) If this was a re-INVITE, a failure response should not be sent if
media has already been exchanged using the new offer.
(**) A UA should only use PRACK to send an offer when it has strong
reasons to expect the receiver will accept the offer.
Table 2: Rejection of an Offer
2.4. Session Description That Is Not an Offer or an Answer
As previously stated, a session description in a SIP message is not
necessarily an offer or an answer. For example, SIP can use a
session description to describe capabilities apart from offer/answer
exchange. Examples of this are a 200 OK response for OPTIONS and a
488 response for INVITE.
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3. Detailed Discussion of the Offer/Answer Model for SIP
3.1. Offer/Answer for the INVITE method with 100rel Extension
The INVITE method provides the basic procedure for offer/answer
exchange in SIP. Without the 100rel option, the rules are simple as
described in [RFC3261]. If an INVITE request includes a session
description, Pattern 1 is applied and if an INVITE request does not
include a session description, Pattern 2 is applied.
With 100rel, Patterns 3, 4, and 5 are added and this complicates the
rules. An INVITE request may cause multiple responses. Note that
even if both UAs support the 100rel extension, not all the
provisional responses may be sent reliably.
3.1.1. INVITE Request with SDP
When a UAC includes an SDP body in the INVITE request as an offer,
only the first SDP in a reliable non-failure response to the INVITE
request is the real answer. No other offer/answer exchanges can
occur within the messages (other responses and ACK) of the INVITE
transaction.
In [RFC3261] there are some descriptions about an offer/answer
exchange, but those cause a little confusion. We interpret those
descriptions as follows,
UAC behavior:
1. If the first SDP that the UAC received is included in an
unreliable provisional response to the INVITE request,
[RFC3261] (Section 13.2.1, second bullet) requires that this
be treated as an answer. However, because that same section
states that the answer has to be in a reliable non-failure
message, this SDP is not the true answer and therefore the
offer/answer exchange is not yet completed.
2. After the UAC has received the answer in a reliable
provisional response to the INVITE, [RFC3261] requires that
any SDP in subsequent responses be ignored.
3. If the second and subsequent SDP (including a real answer) is
different from the first SDP, the UAC should consider that the
SDP is equal to the first SDP. Therefore, the UAC should not
switch to the new SDP.
Okumura, et al. Informational [Page 8]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
UAS behavior:
1. [RFC3261] requires all SDP in the responses to the INVITE
request to be identical.
2. After the UAS has sent the answer in a reliable provisional
response to the INVITE, the UAS should not include any SDPs in
subsequent responses to the INVITE.
3. [RFC3261] permits the UAS to send any provisional response
without 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
offer/answer exchange. In parallel offer/answer exchanges (caused
by forking), a UA may obviously receive a different "preview" of
an answer in each dialog. UAs are expected to deal with this.
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
preview of it) is required in order for media to be accepted. Two
examples of why this might be required are as follows:
o To avoid receiving media from undesired sources, some User Agents
assume symmetric RTP will be used, ignore all incoming media
packets until an address/port has been received from the other
end, and then use that address/port to filter incoming media
packets.
o In some networks, an intermediate node must authorize a media
stream before it can flow and requires a confirming answer to the
offer before doing so.
Therefore, a UAS should send an SDP answer reliably (if possible)
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
starts sending media.
Okumura, et al. Informational [Page 9]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
UAC UAS
| F1 INVITE (SDP) | <- The offer in the offer/answer model.
|-------------------->|
| F2 1xx (SDP) | <- The offer/answer exchange is not
|<--------------------| closed yet, but UAC acts as if it
| | ^ receives the answer.
| F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer
|<--------------------| | SDP.
| F4 PRACK (no SDP) | |
|-------------------->| | The UAC must not send a new offer.
| F5 2xx PRA (no SDP) | |
|<--------------------| v
| |
| F6 1xx-rel (SDP) | <- The answer in the offer/ answer model.
|<--------------------| -
| F7 PRACK | | The UAC can send a new offer in a PRACK
|-------------------->| | request to acknowledge F6.
| F8 2xx PRA | | After F7, the UAC and UAS can send a new
|<--------------------| v offer in an UPDATE request.
| |
| F9 1xx-rel | <- SDP should not be included in the
|<--------------------| subsequent 1xx-rel once offer/answer
| F10 PRACK | has been completed.
|-------------------->|
| F11 2xx PRA |
|<--------------------|
| |
| F12 2xx INV | <- SDP should not be included in the
|<--------------------| final response once offer/answer has
| F13 ACK | been completed.
|-------------------->|
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
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
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
receives the same SDP in a reliable non-failure response, which is
the real answer. After sending the SDP in F6, the UAS must prepare
to receive a new offer from the UAC in a PRACK request or in an
UPDATE request if the UAS supports UPDATE.
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
ignore them, to handle a peer that does not conform to the
recommended implementation.
Okumura, et al. Informational [Page 10]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
3.1.2. INVITE Request without SDP
When a UAC does not include an SDP body in the INVITE request,
[RFC3261] (Section 13.2.1, first bullet) requires that the UAS
include an offer in the first reliable non-failure response.
However, a UAC might not expect an SDP in the other responses to the
INVITE request because RFC 3261 simply does not anticipate the
possibility. Therefore, the UAS ought not include any SDP in the
other responses to the INVITE request.
NOTE: In Figure 2, 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 F9, and just ignore them to handle a peer that does
not conform to the recommended implementation.
UAC UAS
| F1 INVITE (no SDP) |
|-------------------->|
| F2 1xx |
|<--------------------|
| |
| F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP
|<--------------------| as the offer.
| F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel
|-------------------->| must contain SDP as the answer.
| F5 2xx PRA (no SDP) | -
|<--------------------| |
| | |
| F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not
|<--------------------| | contain SDP.
| F7 PRACK | |
|-------------------->| | The UAC can send a new offer in an UPDATE
| F8 2xx PRA | | request after F4.
|<--------------------| v
| |
| F9 2xx INV (no SDP) | <- The final response should not
|<--------------------| contain SDP.
| F10 ACK |
|-------------------->|
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
or reject the offer, the reliable provisional response with SDP as an
offer (Pattern 4) can result in the retransmission until the PRACK
request can be sent. The UAC should take care to avoid this
situation when it sends the INVITE request without SDP.
Okumura, et al. Informational [Page 11]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
3.2. Offer/Answer Exchange in Early Dialog
When both UAs support the 100rel extension, they can update the
session in the early dialog once the first offer/answer exchange has
been completed.
From a UA sending an INVITE request:
A UA can send an UPDATE request with a new offer if both ends support
the UPDATE method. Note that if the UAS needs to prompt the user to
accept or reject the offer, the delay can result in retransmission of
the UPDATE request.
A UA can send a PRACK request with a new offer only when
acknowledging the reliable provisional response carrying the answer
to an offer in the INVITE request. Compared to using the UPDATE
method, using PRACK can reduce the number of messages exchanged
between the UAs. However, to avoid problems or delays caused by
PRACK offer rejection, the UA is recommended to send a PRACK request
only when it has strong reasons to expect the receiver will accept
it. For example, the procedure used in precondition extension
[RFC3312] is a case where a PRACK request should be used for updating
the session status in an early dialog. Note also that if a UAS needs
to prompt the user to accept or reject the offer, the delay can
result in retransmission of the PRACK request.
From a UA receiving an INVITE request:
A UA can send an UPDATE request with a new offer if both ends support
the UPDATE method. A UAS cannot send a new offer in the reliable
provisional response, so the UPDATE method is the only method for a
UAS to update an early session.
3.3. Offer/Answer Exchange in an Established Dialog
Both the re-INVITE and UPDATE methods can be used in an established
dialog to update the session.
The UPDATE method is simpler and can save at least one message
compared with the INVITE method. But both ends must support the
UPDATE method for it to be used.
The INVITE method needs at least three messages to complete but no
extensions are needed. Additionally, the INVITE method allows the
peer to take time to decide whether or not it will accept a session
update by sending provisional responses. That is, re-INVITE allows
the UAS to interact with the user at the peer, while UPDATE needs to
be answered automatically by the UAS. It is noted that re-INVITE
Okumura, et al. Informational [Page 12]
RFC 6337 SIP Usage of the Offer/Answer Model August 2011
should be answered immediately unless such a user interaction is
needed. Otherwise, some Third Party Call Control (3PCC) [RFC3725]
flows will break.
3.4. Recovering from a Failed Re-INVITE
Section 14.1 of [RFC3261] requires that the session parameters in
effect prior to a re-INVITE remain unchanged if the re-INVITE fails,
as if no re-INVITE had been issued. This remains the case even if
multiple offer/answer exchanges have occurred between the sending of
the re-INVITE and its failure, and even if media has been exchanged
using the proposed changes in the session. Because this can be
difficult to achieve in practice, a newer specification [RFC6141]
recommends the UAS to send a 2xx response to a re-INVITE in cases
where rolling back changes would be problematic.
Nevertheless, a UAC may receive a failure response to a re-INVITE
after proposed changes that must be rolled back have already been
used. In such a case, the UAC should send an UPDATE offering the SDP
that has been reinstated. (See [RFC6141] for details.)
4. Exceptional Case Handling
In [RFC3264], the following restrictions are defined with regard to
sending a new offer.
At any time, either agent MAY generate a new offer that updates
the session. However, it MUST NOT generate a new offer if it has
received an offer which it has not yet answered or rejected.
Furthermore, it MUST NOT generate a new offer if it has generated
a prior offer for which it has not yet received an answer or a
rejection.
Assuming that the above rules are guaranteed, there seem to be two
possible 'exceptional' cases to be considered in SIP offer/answer
usage: the 'message crossing' case and the 'glare' case. One of the
reasons why the usage of SIP methods to exchange offer/answer needs
to be carefully restricted in the RFCs is to ensure that the UA can
detect and handle appropriately the 'exceptional' cases to avoid
incompatible behavior.
4.1. Message Crossing Case Handling
When message packets cross in the transport network, an offer may be
received before the answer for the previous offer/answer exchange, as
shown in Figure 3. In such a case, UA A must detect that the session
description SDP-2 is not the answer to offer1.
Okumura, et al. Informational [Page 13]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
A B
|SDP-1 (offer1)|
M1 |----------------->|
|SDP-2 (answer1)|
M2 |<------\ /-------|
| \/ |
|SDP-3 /\(offer2)|
M3 |<------/ \-------|
Figure 3: Message Crossing Case
Because of the restrictions on placement of offers and answers
(summarized in Table 1), there are a limited number of valid
exchanges of messages that may lead to this message crossing case.
These are enumerated in Table 3. (This table only shows messages
containing offers or answers. There could be other messages, without
session descriptions, which are not shown.)
When a response to an UPDATE request crosses a reliable response to
an INVITE request, there are variants shown in Figures 4 and 5, which
are dependent on an INVITE (Mx) that contains no offer. These are
also included in Table 3.
A B
| |
|UPDATE(offer1) |
M1 |==============================>|
|re-INVITE(no offer) |
Mx |------------------------------>| --+
| 2xx-UPD(answer1)| |
M2 |<===========\ /===============| | first reliable
| \/ 1xx-rel/2xx-INV| | response
| /\ (offer2)| |
M3 |<===========/ \===============| <-+
|PRACK/ACK(answer2) |
My |------------------------------>|
| |
Figure 4: Avoidable Message Crossing Cases
To avoid the message crossing condition shown in Figure 4, UA A
should not send this re-INVITE request until an UPDATE transaction
has been completed. If UA B encounters this message crossing
condition, it should reject this re-INVITE request with a 500
response.
Okumura, et al. Informational [Page 14]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
A B
| |
|re-INVITE(no offer) |
Mx |------------------------------>| --+
|UPDATE(offer1) | |
M1 |==============================>| |
| 2xx-UPD(answer1)| |
M2 |<===========\ /===============| | first reliable
| \/ 1xx-rel/2xx-INV| | response
| /\ (offer2)| |
M3 |<===========/ \===============| <-+
|PRACK/ACK(answer2) |
My |------------------------------>|
| |
Figure 5: Avoidable Message Crossing Cases
To avoid the message crossing condition shown in Figure 5, UA A
should not send this UPDATE request until an ACK or a PRACK
transaction associated with an offer/answer has been completed. If
UA B encounters this message crossing condition, it should reject
this UPDATE request with a 500 response.
The situation when a PRACK request crosses UPDATE request is shown in
Figure 6.
A B
| |
| re-INVITE (no offer)|
1st reliable+-- |<------------------------------|
response | M1|1xx-rel(offer1) |
+-> |==============================>| --+
| PRACK(answer1)| M3| Acknowledge
|<===========\ /===============| <-+
| \/ |
| /\ UPDATE(offer2)|
|<===========/ \===============| M2
|500-UPD |
|------------------------------>|
|2xx-PRA |
|------------------------------>|
| |
Figure 6: Avoidable Message Crossing Cases
Okumura, et al. Informational [Page 15]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
To avoid the message crossing condition shown in Figure 6, UA B
should not send this UPDATE request until a PRACK transaction
associated with an offer/answer has been completed. If UA A
encounters this message crossing condition, it should reject this
UPDATE request with a 500 response.
The situation when a reliable provisional response to an INVITE
request crosses UPDATE request is shown in Figure 7.
A B
| |
|re-INVITE(offer1) |
M1 |==============================>|
| 1xx-rel(answer1)|
|<===========\ /===============| M3
| \/ |
| /\ UPDATE(offer2)|
+-- |<===========/ \===============| 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 until a PRACK transaction
associated with an offer/answer has been completed. 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 an INVITE request crosses UPDATE
request is shown in Figure 8.
Okumura, et al. Informational [Page 16]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
A B
| |
|re-INVITE(offer1) |
|==============================>|
| 2xx-INV(answer1)|
|<===========\ /===============|
| \/ |
| /\ UPDATE(offer2)|
+-- |<===========/ \===============|
| |491-UPD |
Acknowledge | |------------------------------>|
| |ACK |
+-> |------------------------------>|
| |
Figure 8: Avoidable Message Crossing Cases
This is a true glare. To avoid the message crossing condition shown
in Figure 8, UA B should not send the UPDATE request until it has
received an ACK request. But there is no problem even if UA B sends
it. If UA A encounters this message crossing condition, it should
reject this UPDATE request with a 491 response.
The situation when a response to an UPDATE request crosses a PRACK
request is shown in Figure 9.
A B
| |
| re-INVITE(offer0)|
|<------------------------------|
|1xx-rel(answer0) |
|------------------------------>| --+
|UPDATE(offer1) | |
M1 |==============================>| |
| 2xx-UPD(answer1)| | Acknowledge
|<===========\ /===============| M3|
| \/ | |
| /\ PRACK(offer2)| 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 until a PRACK transaction
associated with an offer/answer has been completed. If UA B
encounters this message crossing condition, it should reject this
UPDATE request with a 491 response.
Okumura, et al. Informational [Page 17]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
Table 3 summarizes 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| | 6 |
+--------+----------+-----------+-------+UAC-IU +------+
| INVITE | 1xx-rel | UPDATE (*)| | | 7 |
| +----------+-----------+UAS-IcU+-------+------+
| | 2xx-INV | UPDATE (*)| | - | 8 |
+--------+----------+-----------+-------+-------+------+
(*) invalid sequences if INVITE request is an initial one
Table 3: Offer/Answer Crossing Message Sequences
4.2. Glare Case Handling
When both ends in a dialog send a new offer at nearly the same time,
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
called a 'glare' case.
A B
|offer1 offer2|
M1 |-------\ /-------| M2
| \/ |
| /\ |
|<------/ \------>|
Figure 10: Glare Case
Okumura, et al. Informational [Page 18]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
When offer2 is in an UPDATE request or (re-)INVITE request, it must
be rejected with a 491 or 500 response.
There is a variant of Figure 7. When offer2 is in a PRACK request
(within the current rules, only possible if offer1 is in an UPDATE
request), as shown in Figure 11, UA A has a dilemma.
A B
| |
| re-INVITE(offer0)|
|<------------------------------|
|1xx-rel(answer0) |
|------------------------------>| --+
|UPDATE(offer1) PRACK(offer2)| M2| Acknowledge
M1 |============\ /===============| <-+
| \/ |
| /\ |
|<===========/ \==============>|
| 491-UPD|
|<------------------------------|
| |
Figure 11: Avoidable Glare Case
All PRACKs are supposed to be accepted with a 200 response, yet there
is no way to indicate the problem with a 200 response. At best, it
could proceed on the assumption that the UPDATE will be rejected with
a 491. To avoid the glare condition shown in Figure 11, UA A should
not send this UPDATE request until a PRACK transaction associated
with an offer/answer has been completed. 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.
Okumura, et al. Informational [Page 19]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
A B
| |
|re-INVITE(no offer) |
|------------------------------>| --+
| 1xx-rel/2xx-INV| | 1st reliable
|UPDATE(offer1) (offer2)| M2| response
M1 |============\ /===============| <-+
| \/ |
| /\ |
|<===========/ \==============>|
| 500-UPD|
|<------------------------------|
| |
Figure 12: Avoidable Glare Case
To avoid the glare condition shown in Figure 12, UA A should not send
this UPDATE request until an ACK or a PRACK transaction associated
with an offer/answer has been completed. 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
| |
|UPDATE(offer1) |
|==========\ |
|re-INVITE \ (no offer) |
|------------\----------------->| --+
| \ 1xx-rel/2xx-INV| | 1st reliable
| \ (offer2)| | response
|<==============\===============| <-+
| \ |
| \============>|
| 500-UPD|
|<------------------------------|
| |
Figure 13: Avoidable Glare Case
To avoid the glare condition shown in Figure 13, UA A should not send
this re-INVITE request until an UPDATE transaction has been
completed. If UA B encounters this glare condition, it should reject
this UPDATE request with a 500 response.
Okumura, et al. Informational [Page 20]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
Table 4 summarizes this section. Each action is described in
Section 4.3.
| offer1 | offer2 |Action |Action |Figure|
| M1 | M2 | of A | of B | |
+-----------+-----------+-------+-------+------+
| | re-INVITE |UAS-IcI|UAS-IcI| |
| re-INVITE +-----------+-------+-------+ |
| | 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 Re-INVITE
Almost all exceptional cases are caused by an interworking of UPDATE
and re-INVITE. 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 re-INVITE. But there is no particular reason why the rules
are separated. The lack of restrictions for sending a re-INVITE
request cause a lot of 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 [RFC3261], we interpret
it as follows:
UAC-II: While an INVITE transaction is incomplete or ACK
transaction associated with an offer/answer is incomplete,
a UA must not send another INVITE request.
Okumura, et al. Informational [Page 21]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
UAC-UU: While an UPDATE transaction is incomplete, a UA must not
send another UPDATE request.
UAC-UI: While an UPDATE transaction is incomplete, a UA should not
send a re-INVITE request.
UAC-IU: While an INVITE transaction is incomplete, and an ACK or a
PRACK transaction associated with an offer/answer is
incomplete, a UA should not send an UPDATE request.
When a 2xx response to an INVITE includes an offer, the ACK
transaction is considered to be associated with an offer/answer.
When a reliable provisional response to an INVITE includes an offer
or an answer, the PRACK transaction is considered to be associated
with an offer/answer.
UAS behavior receiving an UPDATE is described in Section 5.2 of
[RFC3311]. There are two concerns in this section:
1. There is no description about the interworking of an UPDATE
request and an INVITE request without an offer.
2. There is no description about the interworking of an UPDATE
request and reliable response to an INVITE with an offer.
We interpret this section as follows:
UAS-IcI: While an INVITE client transaction is incomplete or ACK
transaction associated with an offer/answer is incomplete,
a UA must reject another INVITE request with a 491
response.
UAS-IsI: While an INVITE server transaction is incomplete or ACK
transaction associated with an offer/answer is incomplete,
a UA must reject another INVITE request with a 500
response.
UAS-UcU: While an UPDATE client transaction is incomplete, a UA must
reject another UPDATE request with a 491 response.
UAS-UsU: While an UPDATE server transaction is incomplete, a UA must
reject another UPDATE request with a 500 response.
UAS-UcI: While an UPDATE client transaction is incomplete, a UA
should reject a re-INVITE request with a 491 response.
Okumura, et al. Informational [Page 22]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
UAS-UsI: While an UPDATE server transaction is incomplete, a UA
should reject a re-INVITE request with a 500 response.
UAS-IcU: While an INVITE client transaction is incomplete, and an
ACK or a PRACK transaction associated with an 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 an
ACK or a PRACK transaction associated with an offer/answer
is incomplete, a UA should reject an UPDATE request with a
500 response.
These rules are shown in following figures.
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
Okumura, et al. Informational [Page 23]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
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
Okumura, et al. Informational [Page 24]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
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
includes an offer. The interworking of a re-INVITE and an UPDATE
without an offer is out of scope for this document.
5. Content of Offers and Answers
While [RFC3264] and [RFC3312] give some guidance, questions remain
about exactly what should be included in an offer or answer. This is
especially a problem when the common "hold" feature has been
activated, and when there is the potential for a multimedia call.
Details of behavior depend on the capabilities and state of the User
Agent. The kinds of recommendations that can be made are limited by
the model of device capabilities and state that is 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:
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:
o NAT traversal and Interactive Connectivity Establishment (ICE)
o specific codecs and their parameters
o the negotiation of secure media streams
o grouping of media streams
o preconditions
Okumura, et al. Informational [Page 25]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
5.1. General Principle for Constructing Offers and Answers
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
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-
INVITE that contains no offer. (However, in the case of re-INVITE,
the constraints of [RFC3261] and [RFC3264] must be observed.)
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
remaining consistent with the offer/answer rules of [RFC3264] and
other RFCs.
NOTE: "at that time" is important. The device may permit the user
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
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
apply a default, such as assuming it wants to use all of its
capabilities.
5.2. Choice of Media Types and Formats to Include and Exclude
5.2.1. Sending an Initial INVITE with Offer
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
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
capable of supporting, with the particular subset being determined by
the design and configuration (e.g., via [RFC6080]) of the UAC
combined with input from the user interface of the UAC.
The media formats may be all or a subset of the media formats the UAC
is capable of supporting for the corresponding media type, with the
particular subset being determined by the design and configuration of
the UAC combined with input from the user interface of the UAC.
Including all supported media formats will maximize the possibility
that the other party will have a supported format in common. But
including many can result in an unacceptably large SDP body.
Okumura, et al. Informational [Page 26]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
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
include an offer in the first reliable response to the INVITE. It
has largely the same options as when sending an initial INVITE with
an offer, but there are some differences. The choice may be governed
by both static (default) selections of media types as well as dynamic
selections made by a user via interaction with the device while it is
alerting.
NOTE: The offer may be sent in a reliable provisional response,
before the user of the device has been alerted and had an
opportunity to select media options for the call. In this case,
the UAS cannot include any call-specific options from the user of
the device. If there is a possibility that the user of the device
will wish to change what is offered before answering the call,
then special care should be taken. If PRACK and UPDATE are
supported by caller and callee then an initial offer can be sent
reliably, and changed with an UPDATE if the user desires a change.
If PRACK and UPDATE are not supported, then the initial offer
cannot be changed until the call is fully established. In that
case, the offer in a 200 response for the initial INVITE should
include only the media types and formats believed to be acceptable
to the user.
5.2.3. Answering an Initial INVITE with Offer
When a UAS receives an initial INVITE with an offer, what media lines
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 media types. Each media line may be accepted, by
including a non-zero port number, or rejected by including a zero
port number in the answer. The media lines that are accepted should
typically be those with types and formats the UAS would have included
if it were the offerer.
The media formats the answer may contain are constrained by
[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 offer. The UAS may also include other media formats it is
able to support at this time. Doing so establishes an asymmetric
media 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.
Okumura, et al. Informational [Page 27]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
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
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
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
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
The guidelines above (Sections 5.1 and 5.2.1 through Section 5.2.4)
apply, but constraints in [RFC3264] must also be followed. The
following are of particular note because they have proven
troublesome:
o The number of "m=" lines may not be reduced in a subsequent offer.
Previously rejected media streams must remain, or be reused to
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
changes, it must increment by one from the one previously sent as
an offer or answer. (Section 8 of [RFC3264].) If it doesn't
change, then the entire SDP body must be identical to what was
previously sent as an offer or answer. Changing the "o=" line,
except version number value, during the session is an error case.
The behavior when receiving such a non-compliant offer/answer SDP
body is implementation dependent. If a UA needs to negotiate a
'new' SDP session, it should use the INVITE/Replaces method.
o In the case of RTP, the mapping from a particular dynamic payload
type number to a particular codec within that media stream ("m="
line) must not change for the duration of the session. (Section
8.3.2 of [RFC3264].)
Okumura, et al. Informational [Page 28]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
NOTE: This may be impossible for a back-to-back user agent
(B2BUA) to follow in some cases (e.g., 3PCC transfer) if it
does not terminate media.
When the new offer is sent in response to an offerless (re-)INVITE,
it should be constructed according to the General Principle for
Constructing Offers and Answers (Section 5.1 ): all codecs the UA is
currently willing and able to use should be included, not just the
ones that were negotiated by previous offer/answer exchanges. The
same is true for media types -- so if UA A initially offered audio
and video to UA B, and they end up with only audio, and UA B sends an
offerless (re-)INVITE to UA A, A's resulting offer should most likely
re-attempt video, by reusing the zeroed "m=" line used previously.
NOTE: The behavior above is recommended, but it is not always
achievable, for example, in some interworking scenarios. Or, the
offerer may simply not have enough resources to offer "everything"
at that point. Even if the UAS is not able to offer any other SDP
that the one currently being used, it should not reject the re-
INVITE. Instead, it should generate an offer with the currently
used SDP with "o=" line unchanged.
5.3. Hold and Resume of Media
[RFC3264] specifies (using non-normative language) that "hold" should
be indicated in an established session by sending a new offer
containing "a=sendonly" attribute for each media stream to be held.
An answerer is then to respond with "a=recvonly" attribute to
acknowledge that the hold request has been understood.
Note that the use of sendonly/recvonly is not limited to hold. These
may be used for other reasons, such as devices that are only capable
of sending or receiving. So receiving an offer with "a=sendonly"
attribute must not be treated as a certain indication that the
offerer has placed the media stream on hold.
This model is based on an assumption that the UA initiating the hold
will want to play Music on Hold, which is not always the case. A UA
may, if desired, initiate hold by offering "a=inactive" attribute if
it does not intend to transmit any media while in hold status.
The rules of [RFC3264] constrain what may be in an answer when the
offer contains "sendonly", "recvonly", or "inactive" in an "a=" line.
But they do not constrain what must be in a subsequent offer. The
"General Principle for Constructing Offers and Answers" (Section 5.1)
is important here. The initiation of "hold" is a local action. It
should reflect the desired state of the UA. It then affects what the
UA includes in offers and answers until the local state is reset.
Okumura, et al. Informational [Page 29]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
The receipt of an offer containing "a=sendonly" attribute or
"a=inactive" attribute and the sending of a compatible answer should
not change the desired 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 input.
If UA2 has previously been "placed on hold" by UA1, via receipt of
"a=sendonly" attribute, then it may initiate its own hold by sending
a new offer containing "a=sendonly" attribute to UA1. Upon receipt
of that, UA1 will answer with "a=inactive" attribute because that is
the only valid answer that reflects its desire not to receive media.
NOTE: Section 8.4 of [RFC3264] contains a conflicting
recommendation that the offer contain "a=inactive" attribute in
this case. We interpret that recommendation to be non-normative.
The use of "a=sendonly" attribute in this case will never produce
a worse outcome, and can produce a better outcome in useful cases.
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
will then send an offer with "a=sendrecv" attribute. The UA2 will
respond with its desired state of "a=sendonly" attribute because that
is a permitted response. When UA2 desires to also resume, it will
send an offer with "a=sendrecv" attribute. In this case, because UA1
has the same desire it will respond with "a=sendrecv" attribute. In
the same case, when UA2 receives the offer with "a=sendrecv"
attribute, if it has decided it wants to reset its local hold but has
not yet signaled the intent, it may send "a=sendrecv" attribute in
the answer.
If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive"
attribute, and subsequently wants to initiate its own hold, also
using "a=inactive" attribute, it need not send a new offer, since the
only valid response is "a=inactive" attribute and that is already in
effect. However, its local desired state will now be either
"inactive" or "a=sendonly" attribute. This affects what it will send
in future offers and answers.
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-
INVITE without an offer, or when sending a re-INVITE to refresh the
session timer), it should follow the "General Principle for
Constructing Offers and Answers" (Section 5.1). If it previously
initiated a "hold" by sending "a=sendonly" attribute or "a=inactive"
attribute, then it should offer that again. If it had not previously
initiated "hold", then it should offer "a=sendrecv" attribute, even
Okumura, et al. Informational [Page 30]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
if it had previously been forced to answer something else. Without
this behavior it is possible to get "stuck on hold" in some cases,
especially when a 3pcc is involved.
5.4. Behavior on Receiving SDP with c=0.0.0.0
[RFC3264] requires that an agent be capable of receiving SDP with a
connection address of 0.0.0.0, in which case it means that neither
RTP nor RTCP should be sent to the peer.
If a UA generates an answer to the offer received with "c=IN IP4
0.0.0.0", the direction attribute of the accepted media stream in the
answer must still be based on direction attribute of the offered
stream and rules specified in [RFC3264] to form the direction "a="
line in the answer. There is no clear rule about the use of "c=IN
IP4 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. Security Considerations
This document clarifies ambiguities in the intended behavior of the
two SIP User Agents engaged in a dialog. The primary specification
of offer/answer behavior that is being clarified resides in [RFC3261]
and [RFC3264], with extensions in [RFC3311], [RFC3312], and
[RFC6141]. The focus of this document is on cases where ambiguities
can result failed or degraded calls when there is no attacker. The
clarifications exclude call flows that lead to difficulties, without
legitimizing any formerly invalid call flows. Thus, the security
considerations of the above mentioned documents continue to apply and
need not be extended to handle any additional cases.
The offer/answer process can be disrupted in numerous ways by an
attacker. SIP provides mechanisms to protect the offer/answer
exchange from tampering by third parties. Of note is "Enhancements
for Authenticated Identity Management in the Session Initiation
Protocol (SIP)" [RFC4474], as well as Section 26.3.2, "Security
Solutions", of [RFC3261].
7. Acknowledgements
The authors would like to thank Christer Holmberg, Rajeev Seth,
Nataraju A B, Byron Campen, Jonathan Rosenberg, Gonzalo Camarillo,
and Gao Yang for their thorough reviews and comments. Many of their
suggestions and ideas have been incorporated in this document.
Okumura, et al. Informational [Page 31]
RFC 6337 SIP Usage of the Offer/Answer Model August 2011
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC6141] Camarillo, G., Holmberg, C., and Y. Gao, "Re-INVITE and
Target-Refresh Request Handling in the Session Initiation
Protocol (SIP)", RFC 6141, March 2011.
Okumura, et al. Informational [Page 32]RFC 6337 SIP Usage of the Offer/Answer Model August 2011
8.2. Informative References
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, April 2004.
[RFC3959] Camarillo, G., "The Early Session Disposition Type for the
Session Initiation Protocol (SIP)", RFC 3959,
December 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC6080] Petrie, D. and S. Channabasappa, "A Framework for Session
Initiation Protocol User Agent Profile Delivery",
RFC 6080, March 2011.
Authors' Addresses
OKUMURA Shinji
Softfront
28-196, Noth9, West15, Chuo-ku
Sapporo, Hokkaido 060-0009
Japan
EMail: shinji.okumura@softfront.jp
Takuya Sawada
KDDI Corporation
3-10-10, Iidabashi, Chiyoda-ku
Tokyo
Japan
EMail: tu-sawada@kddi.com
Paul H. Kyzivat
Hudson, MA 01749
USA
EMail: pkyzivat@alum.mit.edu
Okumura, et al. Informational [Page 33]
