Network Working Group R. Sparks

Network Working Group R. Sparks
Request for Comments: 5057 Estacado Systems
Category: Informational November 2007

Multiple Dialog Usages in the Session Initiation Protocol

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Abstract

Several methods in the Session Initiation Protocol (SIP) can create
an association between endpoints known as a dialog. Some of these
methods can also create a different, but related, association within
an existing dialog. These multiple associations, or dialog usages,
require carefully coordinated processing as they have independent
life-cycles, but share common dialog state. Processing multiple
dialog usages correctly is not completely understood. What is
understood is difficult to implement.

This memo argues that multiple dialog usages should be avoided. It
discusses alternatives to their use and clarifies essential behavior
for elements that cannot currently avoid them.

This is an informative document and makes no normative statements of
any kind.

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Table of Contents

1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Examples of Multiple Usages . . . . . . . . . . . . . . . . . 4
3.1. Transfer . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Reciprocal Subscription . . . . . . . . . . . . . . . . . 6
4. Usage Creation and Destruction . . . . . . . . . . . . . . . . 9
4.1. Invite Usages . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Subscribe usages . . . . . . . . . . . . . . . . . . . . . 9
5. Proper Handling of Multiple Usages . . . . . . . . . . . . . . 9
5.1. A Survey of the Effect of Failure Responses on Usages
and Dialogs . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Transaction Timeouts . . . . . . . . . . . . . . . . . . . 15
5.3. Matching Requests to Usages . . . . . . . . . . . . . . . 16
5.4. Target Refresh Requests . . . . . . . . . . . . . . . . . 17
5.5. Refreshing and Terminating Usages . . . . . . . . . . . . 17
5.6. Refusing New Usages . . . . . . . . . . . . . . . . . . . 18
5.7. Replacing Usages . . . . . . . . . . . . . . . . . . . . . 18
6. Avoiding Multiple Usages . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 24
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
10. Informative References . . . . . . . . . . . . . . . . . . . . 24

1. Overview

This is an informative document. It makes no normative statements of
any kind. This document refines the concept of a dialog usage in the
Session Initiation Protocol (SIP [1]), and discusses what led to its
existence. It explores ambiguity associated with processing multiple
dialog usages that share a dialog. In particular, it surveys the
effect of SIP failure responses on transaction, dialog usage, and
dialog state. This document will help the implementer understand
what is required to process multiple dialog usages correctly, and
will provide information for future standards-track work that will
clarify RFC 3261 and other related documents. Finally, the document
explores single-usage dialog alternatives (using SIP extensions) to
multiple dialog usages.

2. Introduction

Several methods in SIP can establish a dialog. When they do so, they
also establish an association between the endpoints within that
dialog. This association has been known for some time as a "dialog
usage" in the developer community. A dialog initiated with an INVITE
request has an invite usage. A dialog initiated with a SUBSCRIBE

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request has a subscribe usage. A dialog initiated with a REFER
request has a subscribe usage.

Dialogs with multiple usages arise when a usage-creating action
occurs inside an existing dialog. Such actions include accepting a
REFER or SUBSCRIBE issued inside a dialog established with an INVITE
request. Multiple REFERs within a dialog create multiple
subscriptions, each of which is a new dialog usage sharing common
dialog state. (Note that any REFER issued utilizing the
subscription-suppression mechanism specified in [2] creates no new
usage.) Similarly, an endpoint in a dialog established with an
INVITE might subscribe to its peer's Key Press Markup Language (KPML)
[3] and later issue a REFER, resulting in three dialog usages sharing
common dialog state.

The common state in the dialog shared by any usages is exactly:

o the Call-ID

o the local Tag

o the remote Tag

o the local CSeq

o the remote CSeq

o the Route-set

o the local contact

o the remote target

o the secure flag

Usages have state that is not shared in the dialog. For example, a
subscription has a duration, along with other usage-specific state.
Multiple subscriptions in the same dialog each have their own
duration.

A dialog comes into existence with the creation of the first usage,
and continues to exist until the last usage is terminated (reference
counting). Unfortunately, many of the usage management aspects of
SIP, such as authentication, were originally designed with the
implicit assumption that there was one usage per dialog. The
resulting mechanisms have mixed effects, some influencing the usage,
and some influencing the entire dialog.

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The current specifications define two usages, invite and subscribe.
A dialog can share up to one invite usage and arbitrarily many
subscribe usages.

Because RFC 3261 [1] states that user-agents should reuse Call-ID and
increment CSeq across a series of registration requests (and that to-
tags appear in register responses in some of the examples), some
implementations have treated REGISTER as if it were in a dialog.
However, RFC 3261 explicitly calls out that REGISTER does not create
a dialog. A series of REGISTER requests does not create any usage or
dialog. Similarly, PUBLISH [4] does not create any usage or dialog.

3. Examples of Multiple Usages

3.1. Transfer

In Figure 1, Alice transfers a call she received from Bob to Carol.
A dialog (and an invite dialog usage) between Alice and Bob comes
into being with the 200 OK labeled F1. A second usage (a
subscription to event refer) comes into being with the NOTIFY labeled
F2. This second usage ends when the subscription is terminated by
the NOTIFY transaction labeled F3. The dialog still has one usage
(the invite usage), which lasts until the BYE transaction labeled F4.
At this point, the dialog has no remaining usages, so it ceases to
exist. Details of each of these messages are shown in Figure 2.

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Alice Bob Carol
| INVITE | |
|<----------------| |
Dialog 1 Usage 1 | 200 OK (F1) | |
-start- -start- ----------->|---------------->| |
| | | ACK | |
| | |<----------------| |
| | | reINVITE/200/ACK| |
| | | (hold) | |
| | |---------------->| |
| | | REFER | |
| | Dialog 1 |---------------->| |
| | Usage 2 | NOTIFY (F2) | |
| | -start- -->|<----------------| INVITE |
| | | | 200 NOTIFY |----------->|
| | | |---------------->| 200 OK |
| | | | 200 REFER |<-----------|
| | | |<----------------| ACK |
| | | | NOTIFY (F3) |----------->|
| | | |<----------------| |
| | | | 200 | . |
| | -end- -->|---------------->| . |
| | | BYE (F4) | Dialog 2 |
| | |<----------------| proceeds |
| | | 200 | . |
-end- -end- ------------>|---------------->| . |

Figure 1

Message Details (abridged to show only dialog or usage details)
F1
SIP/2.0 200 OK
Call-ID: [email protected]
CSeq: 100 INVITE
To: <sip:[email protected]>;tag=alicetag1
From: <sip:[email protected]>;tag=bobtag1
Contact: <sip:[email protected]>

F2
NOTIFY sip:[email protected] SIP/2.0
Event: refer
Call-ID: [email protected]
CSeq: 101 NOTIFY
To: <sip:[email protected]>;tag=alicetag1
From: <sip:[email protected]>;tag=bobtag1
Contact: <sip:[email protected]>

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F3
NOTIFY sip:[email protected] SIP/2.0
Event: refer
Subscription-State: terminated;reason=noresource
Call-ID: [email protected]
CSeq: 102 NOTIFY
To: <sip:[email protected]>;tag=alicetag1
From: <sip:[email protected]>;tag=bobtag1
Contact: <sip:[email protected]>
Content-Type: message/sipfrag

SIP/2.0 200 OK

F4
BYE sip:[email protected] SIP/2.0
Call-ID: [email protected]
CSeq: 103 BYE
To: <sip:[email protected]>;tag=alicetag1
From: <sip:[email protected]>;tag=bobtag1
Contact: <sip:[email protected]>

Figure 2

3.2. Reciprocal Subscription

In Figure 3, Alice subscribes to Bob's presence. For simplicity,
assume Bob and Alice are both serving their presence from their
endpoints instead of a presence server. To focus on the essential
points, the figure leaves out any rendezvous signaling through which
Alice discovers Bob's endpoint.

Bob is interested in Alice's presence too, so he subscribes to Alice
(in most deployed presence/IM systems, people watch each other). He
decides to skip the rendezvous step since he's already in a dialog
with Alice, and sends his SUBSCRIBE inside that dialog (a few early
SIMPLE clients behaved exactly this way).

The dialog and its first usage comes into being at F1, which
establishes Alice's subscription to Bob. Its second usage begins at
F2, which establishes Bob's subscription to Alice. These two
subscriptions are independent - they have distinct and different
expirations, but they share all the dialog state.

The first usage ends when Alice decides to unsubscribe at F3. Bob's
subscription to Alice, and thus the dialog, continues to exist.
Alice's UA must maintain this dialog state even though the
subscription that caused it to exist in the first place is now over.
The second usage ends when Alice decides to terminate Bob's

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subscription at F4 (she's probably going to reject any attempt on
Bob's part to resubscribe until she's ready to subscribe to Bob
again). Since this was the last usage, the dialog also terminates.
Details of these messages are shown in Figure 4.

Alice Bob
| |
| SUBSCRIBE |
|------------------->|
Dialog Usage 1 | NOTIFY (F1) |
-start- -start- --------->|<-------------------|
| | | 200 SUBSCRIBE |
| | |<-------------------|
| | | 200 NOTIFY |
| | |------------------->|
| | | SUBSCRIBE |
| | |<-------------------|
| | Usage 2 | NOTIFY (F2) |
| | -start- -->|------------------->|
| | | | 200 SUBSCRIBE
| | | |------------------->|
| | | | 200 NOTIFY |
| | | |<-------------------|
| | | | : |
| | | | : |
| | | | (un)SUBSCRIBE (F3) |
| | | |------------------->|
| | | | 200 |
| | | |<-------------------|
| | | | NOTIFY |
| | | |<-------------------|
| | | | 200 |
| -end- ----------->|------------------->|
| | | : |
| | | : |
| | | NOTIFY (F4) |
| | | (Terminated) |
| | |------------------->|
| | | 200 |
-end- -end- -->|<-------------------|
| |

Figure 3

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Message Details (abridged to show only dialog or usage details)
F1
NOTIFY sip:[email protected] SIP/2.0
Event: presence
Subscription-State: active;expires=600
Call-ID: [email protected]
From: <sip:[email protected]>;tag=bobtag2
To: <sip:[email protected]>;tag=alicetag2
CSeq: 100 NOTIFY
Contact: <sip:[email protected]>

F2
NOTIFY sip:[email protected] SIP/2.0
Event: presence
Subscription-State: active;expires=1200
Call-ID: [email protected]
To: <sip:[email protected]>;tag=bobtag2
From: <sip:[email protected]>;tag=alicetag2
CSeq: 500 NOTIFY
Contact: <sip:[email protected]>

F3
SUBSCRIBE sip:[email protected] SIP/2.0
Event: presence
Expires: 0
Call-ID: [email protected]
To: <sip:[email protected]>;tag=bobtag2
From: <sip:[email protected]>;tag=alicetag2
CSeq: 501 SUBSCRIBE
Contact: <sip:[email protected]>

F4
NOTIFY sip:[email protected] SIP/2.0
Event: presence
Subscription-State: terminated;reason=deactivated
Call-ID: [email protected]
To: <sip:[email protected]>;tag=bobtag2
From: <sip:[email protected]>;tag=alicetag2
CSeq: 502 NOTIFY
Contact: <sip:[email protected]>

Figure 4

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4. Usage Creation and Destruction

Dialogs come into existence along with their first usage. Dialogs
terminate when their last usage is destroyed. The messages that
create and destroy usages vary per usage. This section provides a
high-level categorization of those messages. The section does not
attempt to explore the REGISTER pseudo-dialog.

4.1. Invite Usages

Created by: non-100 provisional responses to INVITE; 200 response to
INVITE

Destroyed by: 200 responses to BYE; certain failure responses to
INVITE, UPDATE, PRACK, INFO, or BYE; anything that destroys a
dialog and all its usages

4.2. Subscribe usages

Created by: 200 class responses to SUBSCRIBE; 200 class responses to
REFER; NOTIFY requests

Destroyed by: 200 class responses to NOTIFY-terminated; NOTIFY or
refresh-SUBSCRIBE request timeout; certain failure responses to
NOTIFY or SUBSCRIBE; expiration without refresh if network issues
prevent the terminal NOTIFY from arriving; anything that destroys
a dialog and all its usages

5. Proper Handling of Multiple Usages

The examples in Section 3 show straightforward cases where it is
fairly obvious when the dialog begins and ends. Unfortunately, there
are many scenarios where such clarity is not present. For instance,
in Figure 1, what would it mean if the response to the NOTIFY (F2)
were a 481? Does that simply terminate the refer subscription, or
does it destroy the entire dialog? This section explores the problem
areas with multiple usages that have been identified to date.

5.1. A Survey of the Effect of Failure Responses on Usages and Dialogs

For this survey, consider a subscribe usage inside a dialog
established with an invite usage. Unless stated otherwise, we'll
discuss the effect on each usage and the dialog when a client issuing
a NOTIFY inside the subscribe usage receives a failure response (such
as a transferee issuing a NOTIFY to event refer). Further, unless
otherwise stated, the conclusions apply to arbitrary multiple usages.
This survey is written from the perspective of a client receiving the

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error response. The effect on dialogs and usages at the server
issuing the response is the same.

3xx responses: Redirection mid-dialog is not well understood in SIP,
but whatever effect it has impacts the entire dialog and all of
its usages equally. In our example scenario, both the
subscription and the invite usage would be redirected by this
single response.

For the failure responses with code 400 and greater, there are three
common ways the failure can affect the transaction, usage, and dialog
state.

Transaction Only The error affects only the transaction, not the
usage or dialog the transaction occurs in (beyond affecting the
local CSeq). Any other usage of the dialog is unaffected. The
error is a complaint about this transaction, not the usage or
dialog that the transaction occurs in.

Destroys Usage The error destroys the usage, but not the dialog.
Any other usages sharing this dialog are not affected.

Destroys Dialog The error destroys the dialog and all usages sharing
it.

Table 1 and Table 2 display how the various codes affect transaction,
usage, or dialog state. Response code specific comments or
exceptions follow the table.

+----------------------+----------------+-----------------+
| Transaction Only | Destroys Usage | Destroys Dialog |
+----------------------+----------------+-----------------+
| 400 (or unknown 4xx) | 405, 480 | 404, 410, 416 |
| 401, 402, 403, 406 | 481, 489 | 482, 483 |
| 407, 408, 412-415 | 501 | 484, 485 |
| 417, 420, 421, 422 | | 502, 604 |
| 423, 428, 429 | | |
| 436-438, 486, 487 | | |
| 488, 491, 493, 494 | | |
| 500 (or unknown 5xx) | | |
| 503, 504, 505 | | |
| 513, 580 | | |
| 600 (or unknown 6xx) | | |
| 603, 606 | | |
+----------------------+----------------+-----------------+

Table 1

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+---------+---------------------------------+-------------+-------+
| Code | Reason | Impact | Notes |
+---------+---------------------------------+-------------+-------+
| 400/4xx | Bad Request | Transaction | |
| 401 | Unauthorized | Transaction | |
| 402 | Payment Required | Transaction | (1) |
| 403 | Forbidden | Transaction | |
| 404 | Not Found | Dialog | (2) |
| 405 | Method Not Allowed | Usage | (3) |
| 406 | Not Acceptable | Transaction | |
| 407 | Proxy Authentication Required | Transaction | |
| 408 | Request Timeout | Transaction | (4) |
| 410 | Gone | Dialog | (2) |
| 412 | Conditional Request Failed | Transaction | |
| 413 | Request Entity Too Large | Transaction | |
| 414 | Request-URI Too Long | Transaction | |
| 415 | Unsupported Media Type | Transaction | |
| 416 | Unsupported URI Scheme | Dialog | (2) |
| 417 | Unknown Resource-Priority | Transaction | |
| 420 | Bad Extension | Transaction | |
| 421 | Extension Required | Transaction | |
| 422 | Session Interval Too Small | Transaction | (5) |
| 423 | Interval Too Brief | Transaction | |
| 428 | Use Identity Header | Transaction | |
| 429 | Provide Referrer Identity | Transaction | (6) |
| 436 | Bad Identity-Info | Transaction | |
| 437 | Unsupported Certificate | Transaction | |
| 438 | Invalid Identity Header | Transaction | |
| 480 | Temporarily Unavailable | Usage | (7) |
| 481 | Call/Transaction Does Not Exist | Usage | (8) |
| 482 | Loop Detected | Dialog | (9) |
| 483 | Too Many Hops | Dialog | (10) |
| 484 | Address Incomplete | Dialog | (2) |
| 485 | Ambiguous | Dialog | (2) |
| 486 | Busy Here | Transaction | (11) |
| 487 | Request Terminated | Transaction | |
| 488 | Not Acceptable Here | Transaction | |
| 489 | Bad Event | Usage | (12) |
| 491 | Request Pending | Transaction | |
| 493 | Undecipherable | Transaction | |
| 494 | Security Agreement Required | Transaction | |
| 500/5xx | Server Internal Error | Transaction | (13) |
| 501 | Not Implemented | Usage | (3) |
| 502 | Bad Gateway | Dialog | (14) |
| 503 | Service Unavailable | Transaction | (15) |
| 504 | Server Time-Out | Transaction | (16) |
| 505 | Version Not Supported | Transaction | |
| 513 | Message Too Large | Transaction | |

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| 580 | Precondition Failure | Transaction | |
| 600/6xx | Busy Everywhere | Transaction | (17) |
| 603 | Decline | Transaction | |
| 604 | Does Not Exist Anywhere | Dialog | (2) |
| 606 | Not Acceptable | Transaction | |
+---------+---------------------------------+-------------+-------+

Table 2

(1) 402 Payment Required: This is a reserved response code. If
encountered, it should be treated as an unrecognized 4xx.

(2) 404 Not Found:

410 Gone:

416 Unsupported URI Scheme:

484 Address Incomplete:

485 Ambiguous:

604 Does Not Exist Anywhere:

The Request-URI that is being rejected is the remote target set by
the Contact provided by the peer. Getting this response means
that something has gone fundamentally wrong with the dialog state.

(3) 405 Method Not Allowed:

501 Not Implemented:

Either of these responses would be aberrant in our example
scenario since support for the NOTIFY method is required by the
usage. In this case, the UA knows the condition is unrecoverable
and should stop sending NOTIFYs on the usage. Any refresh
subscriptions should be rejected. In general, these errors will
affect at most the usage. If the request was not integral to the
usage (it used an unknown method, or was an INFO inside an INVITE
usage, for example), only the transaction will be affected.

(4) 408 Request Timeout: Receiving a 408 will have the same effect
on usages and dialogs as a real transaction timeout as described
in Section 5.2.

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(5) 422 Session Interval Too Small: This response does not make
sense for any mid-usage request. If it is received, an element in
the path of the request is violating protocol, and the recipient
should treat this as it would an unknown 4xx response.

(6) 429 Provide Referrer Identity: This response won't be returned
to a NOTIFY as in our example scenario, but when it is returned to
a REFER, it is objecting only to the REFER request itself.

(7) 480 Temporarily Unavailable: RFC 3261 is unclear on what this
response means for mid-usage requests. Future updates to that
specification are expected to clarify that this response affects
only the usage in which the request occurs. No other usages are
affected. If the response included a Retry-After header field,
further requests in that usage should not be sent until the
indicated time has past. Requests in other usages may still be
sent at any time.

(8) 481 Call/Transaction Does Not Exist: This response indicates
that the peer has lost its copy of the dialog usage state. The
dialog itself should not be destroyed unless this was the last
usage.

The effects of a 481 on a dialog and its usages are the most
ambiguous of any final response. There are implementations that
have chosen the meaning recommended here, and others that destroy
the entire dialog without regard to the number of outstanding
usages. Going forward with this clarification will allow those
deployed implementations that assumed only the usage was destroyed
to work with a wider number of implementations. Existing
implementations that destroy all other usages in the dialog will
continue to function as they do now, except that peers following
the recommendation will attempt to do things with the other usages
and this element will return 481s for each of them until they are
all gone. However, the necessary clarification to RFC 3261 needs
to make it very clear that the ability to terminate usages
independently from the overall dialog using a 481 is not
justification for designing new applications that count on
multiple usages in a dialog.

The 481 response to a CANCEL request has to be treated
differently. For CANCEL, a 481 means the UAS can't find a
matching transaction. A 481 response to a CANCEL affects only the
CANCEL transaction. The usage associated with the INVITE is not
affected.

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(9) 482 Loop Detected: This response is aberrant mid-dialog. It
will only occur if the Record-Route header field were improperly
constructed by the proxies involved in setting up the dialog's
initial usage, or if a mid-dialog request forks and merges (which
should never happen). Future requests using this dialog state
will also fail.

An edge condition exists during RFC 3263 failover at the
element sending a request, where the request effectively forks
to multiple destinations from the client. Some implementations
increase risk entering this edge condition by trying the next
potential location as determined by RFC 3263 very rapidly if
the first does not immediately respond. In any situation where
a client sends the same request to more than one endpoint, it
must be prepared to receive a response from each branch (and
should choose a "best" response to act on following the same
guidelines as a forking proxy). In this particular race
condition, if multiple branches respond, all but one will most
likely return a 482 Merged Request. The client should select
the remaining non-482 response as the "best" response.

(10) 483 Too Many Hops: Similar to 482, receiving this mid-dialog is
aberrant. Unlike 482, recovery may be possible by increasing Max-
Forwards (assuming that the requester did something strange like
using a smaller value for Max-Forwards in mid-dialog requests than
it used for an initial request). If the request isn't tried with
an increased Max-Forwards, then the agent should follow the
Destroy Dialog actions.

(11) 486 Busy Here: This response is nonsensical in our example
scenario, or in any scenario where this response comes inside an
established usage. If it occurs in that context, it should be
treated as an unknown 4xx response.

(12) 489 Bad Event: In our example scenario, [5] declares that the
subscription usage in which the NOTIFY is sent is terminated.
This response is only valid in the context of SUBSCRIBE and
NOTIFY. UAC behavior for receiving this response to other methods
is not specified, but treating it as an unknown 4xx is a
reasonable practice.

(13) 500 and 5xx unrecognized responses: If the response contains a
Retry-After header field value, the server thinks the condition is
temporary, and the request can be retried after the indicated
interval. If the response does not contain a Retry-After header
field value, the UA may decide to retry after an interval of its
choosing or attempt to gracefully terminate the usage. Whether or
not to terminate other usages depends on the application. If the

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UA receives a 500 (or unrecognized 5xx) in response to an attempt
to gracefully terminate this usage, it can treat this usage as
terminated. If this is the last usage sharing the dialog, the
dialog is also terminated.

(14) 502 Bad Gateway: This response is aberrant mid-dialog. It will
only occur if the Record-Route header field were improperly
constructed by the proxies involved in setting up the dialog's
initial usage. Future requests using this dialog state will also
fail.

(15) 503 Service Unavailable: As per [6], the logic handling
locating SIP servers for transactions may handle 503 requests
(effectively, sequentially forking at the endpoint based on DNS
results). If this process does not yield a better response, a 503
may be returned to the transaction user. Like a 500 response, the
error is a complaint about this transaction, not the usage.
Because this response occurred in the context of an established
usage (hence an existing dialog), the route-set has already been
formed and any opportunity to try alternate servers (as
recommended in [1]) has been exhausted by the RFC3263 logic.

(16) 504 Server Time-out: It is not obvious under what circumstances
this response would be returned to a request in an existing
dialog.

(17) 600 and 6xx unrecognized responses: Unlike 400 Bad Request, a
600 response code says something about the recipient user, not the
request that was made. This end user is stating an unwillingness
to communicate. If the response contains a Retry-After header
field value, the user is indicating willingness to communicate
later and the request can be retried after the indicated interval.
This usage, and any other usages sharing the dialog are
unaffected. If the response does not contain a Retry-After header
field value, the UA may decide to retry after an interval of its
choosing or attempt to gracefully terminate the usage. Whether or
not to terminate other usages depends on the application. If the
UA receives a 600 (or unrecognized 6xx) in response to an attempt
to gracefully terminate this usage, it can treat this usage as
terminated. If this is the last usage sharing the dialog, the
dialog is also terminated.

5.2. Transaction Timeouts

[1] states that a UAC should terminate a dialog (by sending a BYE) if
no response is received for a request sent within a dialog. This
recommendation should have been limited to the invite usage instead
of the whole dialog. [5] states that a timeout for a NOTIFY removes a

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subscription, but a SUBSCRIBE that fails with anything other than a
481 does not. Given these statements, it is unclear whether a
refresh SUBSCRIBE issued in a dialog shared with an invite usage
destroys either usage or the dialog if it times out.

Generally, a transaction timeout should affect only the usage in
which the transaction occurred. Other uses sharing the dialog should
not be affected. In the worst case of timeout due to total transport
failure, it may require multiple failed messages to remove all usages
from a dialog (at least one per usage).

There are some mid-dialog messages that never belong to any usage.
If they timeout, they will have no effect on the dialog or its
usages.

5.3. Matching Requests to Usages

For many mid-dialog requests, identifying the usage they belong to is
obvious. A dialog can have at most one invite usage, so any INVITE,
UPDATE, PRACK, ACK, CANCEL, BYE, or INFO requests belong to it. The
usage (i.e. the particular subscription) SUBSCRIBE, NOTIFY, and REFER
requests belong to can be determined from the Event header field of
the request. REGISTER requests within a (pseudo)-dialog belong to
the registration usage. (As mentioned before, implementations aren't
mixing registration usages with other usages, so this document isn't
exploring the consequences of that bad behavior).

According to [1], "an OPTIONS request received within a dialog
generates a 200 OK response that is identical to one constructed
outside a dialog and does not have any impact on that dialog". Thus,
OPTIONS does not belong to any usage. Only those failures discussed
in Section 5.1 and Section 5.2 that destroy entire dialogs will have
any effect on the usages sharing the dialog with a failed OPTIONS
request.

MESSAGE requests are discouraged inside a dialog. Implementations
are restricted from creating a usage for the purpose of carrying a
sequence of MESSAGE requests (though some implementations use it that
way, against the standard recommendation). A failed MESSAGE
occurring inside an existing dialog will have similar effects on the
dialog and its usages as a failed OPTIONS request.

Mid-dialog requests with unknown methods cannot be matched with a
usage. Servers will return a failure response (likely a 501). The
effect on the dialog and its usages at either the client or the
server should be similar to that of a failed OPTIONS request.

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These guidelines for matching messages to usages (or determining
there is no usage) apply equally when acting as a UAS, a UAC, or any
third party tracking usage and dialog state by inspecting all
messages between two endpoints.

5.4. Target Refresh Requests

Target refresh requests update the remote target of a dialog when
they are successfully processed. The currently defined target
refresh requests are INVITE, UPDATE, SUBSCRIBE, NOTIFY, and REFER
[7]).

The remote target is part of the dialog state. When a target refresh
request affects it, it affects it for ALL usages sharing that dialog.
If a subscription and invite usage are sharing a dialog, sending a
refresh SUBSCRIBE with a different contact will cause reINVITEs from
the peer to go to that different contact.

A UAS will only update the remote target if it sends a 200 class
response to a target refresh request. A UAC will only update the
remote target if it receives a 200 class response to a target refresh
request. Again, any update to a dialog's remote target affects all
usages of that dialog.

There is known ambiguity around the effects of provisional responses
on remote targets that a future specification will attempt to
clarify. Furthermore, because the remote target is part of the
dialog state, not any usage state, there is ambiguity in having
target refresh requests in progress simultaneously on multiple usages
in the same dialog. Implementation designers should consider these
conditions with care.

5.5. Refreshing and Terminating Usages

Subscription and registration usages expire over time and must be
refreshed (with a refresh SUBSCRIBE, for example). This expiration
is usage state, not dialog state. If several subscriptions share a
dialog, refreshing one of them has no effect on the expiration of the
others.

Normal termination of a usage has no effect on other usages sharing
the same dialog. For instance, terminating a subscription with a
NOTIFY/Subscription-State: terminated will not terminate an invite
usage sharing its dialog. Likewise, ending an invite usage with a
BYE does not terminate any active Event: refer subscriptions
established on that dialog.

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5.6. Refusing New Usages

As the survey of the effect of failure responses shows, care must be
taken when refusing a new usage inside an existing dialog. Choosing
the wrong response code will terminate the dialog and all of its
usages. Generally, returning a 603 Decline is the safest way to
refuse a new usage.

5.7. Replacing Usages

[8] defines a mechanism through which one usage can replace another.
It can be used, for example, to associate the two dialogs in which a
transfer target is involved during an attended transfer. It is
written using the term "dialog", but its intent was only to affect
the invite usage of the dialog it targets. Any other usages inside
that dialog are unaffected. For some applications, the other usages
may no longer make sense, and the application may terminate them as
well.

However, the interactions between Replaces and multiple dialog usages
have not been well explored. More discussion of this topic is
needed. Implementers should avoid this scenario completely.

6. Avoiding Multiple Usages

Processing multiple usages correctly is not completely understood.
What is understood is difficult to implement and is very likely to
lead to interoperability problems. The best way to avoid the trouble
that comes with such complexity is to avoid it altogether.

When designing new applications or features that use SIP dialogs, do
not require endpoints to construct multiple usages to participate in
the application or use the feature. When designing endpoints,
address the existing multiple usage scenarios as best as possible.
Outside those scenarios, if a peer attempts to create a second usage
inside a dialog, refuse it.

Unfortunately, there are existing applications, like transfer, that
currently entail multiple usages, so the simple solution of "don't do
it" will require some transitional work. This section looks at the
pressures that led to these existing multiple usages and suggests
alternatives.

When executing a transfer, the transferor and transferee currently
share an invite usage and a subscription usage within the dialog
between them. This is a result of sending the REFER request within
the dialog established by the invite usage. Implementations were led
to this behavior by these primary problems:

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1. There was no way to ensure that a REFER on a new dialog would
reach the particular endpoint involved in a transfer. Many
factors, including details of implementations and changes in
proxy routing between an INVITE and a REFER could cause the REFER
to be sent to the wrong place. Sending the REFER down the
existing dialog ensured it got to the same endpoint with which
the dialog was established.

2. It was unclear how to associate an existing invite usage with a
REFER arriving on a new dialog, where it was completely obvious
what the association was when the REFER came on the invite
usage's dialog.

3. There were concerns with authorizing out-of-dialog REFERs. The
authorization policy for REFER in most implementations piggybacks
on the authorization policy for INVITE (which is, in most cases,
based simply on "I placed or answered this call").

Globally Routable User Agent (UA) URIs (GRUUs) [9] have been defined
specifically to address problem 1 by providing a URI that will reach
one specific user-agent. The Target-Dialog header field [10] was
created to address problems 2 and 3. This header field allows a
request to indicate the dialog identifiers of some other dialog,
providing association with the other dialog that can be used in an
authorization decision.

The Join [11] and Replaces [8] mechanisms can also be used to address
problem 1. When using this technique, a new request is sent outside
any dialog with the expectation that it will fork to possibly many
endpoints, including the one we're interested in. This request
contains a header field listing the dialog identifiers of a dialog in
progress. Only the endpoint holding a dialog matching those
identifiers will accept the request. The other endpoints the request
may have forked to will respond with an error. This mechanism is
reasonably robust, failing only when the routing logic for out-of-
dialog requests changes such that the new request does not arrive at
the endpoint holding the dialog of interest.

The reachability aspects of using a GRUU to address problem 1 can be
combined with the association-with-other-dialogs aspects of the Join/
Replaces and Target-Dialog mechanisms. A REFER request sent out-of-
dialog can be sent towards a GRUU, and identify an existing dialog as
part of the context the receiver should use. The Target-Dialog
header field can be included in the REFER listing the dialog this
REFER is associated with. Figure 5 sketches how this could be used
to achieve transfer without reusing a dialog. For simplicity, the
diagram and message details do not show the server at example.com

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that will be involved in routing the GRUU. Refer to [9] for those
details.

Alice Bob Carol
| | |
| F1 INVITE (Bob's AOR) | |
| Call-ID: (call-id one) | |
| Contact: (Alice's-GRUU) | |
|------------------------------->| |
| F2 200 OK | |
| To: <>;tag=totag1 | |
| From: <>;tag=fromtag1 | |
| Call-ID: (call-id one) | |
| Contact: (Bob's-GRUU) | |
|<-------------------------------| |
| ACK | |
|------------------------------->| |
| : | |
| (Bob places Alice on hold) | |
| : | F3 INVITE (Carol's AOR) |
| | Call-ID: (call-id two) |
| | Contact: (Bob's-GRUU) |
| |----------------------------->|
| | F4 200 OK |
| | To: <>;tag=totag2 |
| | From: <>;tag=fromtag2 |
| | Call-ID: (call-id two) |
| | Contact: (Carol's-GRUU) |
| |<-----------------------------|
| | ACK |
| |----------------------------->|
| | : |
| | (Bob places Carol on hold) |
| F5 REFER (Alice's-GRUU) | : |
| Call-ID: (call-id three) | |
| Refer-To: (Carol's-GRUU) | |
| Target-Dialog: (call-id one,totag1,fromtag1) |
| Contact: (Bob's-GRUU) | |
|<-------------------------------| |
| 202 Accepted | |
|------------------------------->| |

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| NOTIFY (Bob's-GRUU) | |
| Call-ID: (call-id three) | |
|------------------------------->| |
| 200 OK | |
|<-------------------------------| |
| | |
| F6 INVITE (Carol's-GRUU) |
| Call-ID: (call-id four) |
| Contact: (Alice's-GRUU) |
|-------------------------------------------------------------->|
| 200 OK |
| Contact: (Carol's-GRUU) |
|<--------------------------------------------------------------|
| ACK |
|-------------------------------------------------------------->|
| | |
| F7 NOTIFY (Bob's-GRUU) | |
| Call-ID: (call-id three) | |
|------------------------------->| |
| 200 OK | |
|<-------------------------------| |
| BYE (Alice's-GRUU) | |
| Call-ID: (call-id one) | |
|<-------------------------------| BYE (Carol's-GRUU) |
| | Call-ID: (call-id two) |
| 200 OK |----------------------------->|
|------------------------------->| 200 OK |
| |<-----------------------------|
| | |

Figure 5: Transfer without dialog reuse

In message F1, Alice invites Bob indicating support for GRUUs (and
offering a GRUU for herself):

Message F1 (abridged, detailing pertinent fields)

INVITE sip:[email protected] SIP/2.0
Call-ID: [email protected]
Supported: gruu
Contact: <sip:[email protected];gr=urn:uuid:(Alice's UA's bits)>

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Message F2 carries Bob's GRUU to Alice.

Message F2 (abridged, detailing pertinent fields)

SIP/2.0 200 OK
Supported: gruu
To: <sip:[email protected]>;tag=totag1
From: <sip:[email protected]>;tag=fromtag1
Contact: <sip:[email protected];gr=urn:uuid:(Bob's UA's bits)>

Bob decides to try to transfer Alice to Carol, so he puts Alice on
hold and sends an INVITE to Carol. Carol and Bob negotiate GRUU
support similar to what happened in F1 and F2.

Message F3 (abridged, detailing pertinent fields)

INVITE sip:[email protected] SIP/2.0
Supported: gruu
Call-ID: [email protected]
Contact: <sip:[email protected];gr=urn:uuid:(Bob's UA's bits)>

Message F4 (abridged, detailing pertinent fields)

SIP/2.0 200 OK
Supported: gruu
To: <sip:[email protected]>;tag=totag2
From: <sip:[email protected]>;tag=fromtag2
Call-ID: [email protected]
Contact: <sip:[email protected];gr=urn:uuid:(Carol's UA's bits)>

After consulting Carol, Bob places her on hold and refers Alice to
her using message F5. Notice that the Refer-To URI is Carol's GRUU,
and that this is on a different Call-ID than message F1. (The URI in
the Refer-To header is line-broken for readability in this document;
it would not be valid to break the URI this way in a real message.)

Message F5 (abridged, detailing pertinent fields)

REFER sip:[email protected] SIP/2.0
Call-ID: [email protected]
Refer-To: <sip:[email protected];g=urn:uid:(Carol's UA's bits)
[email protected];
to-tag=totag2;from-tag=fromtag2>
Target-Dialog: [email protected];
local-tag=fromtag1;remote-tag=totag1
Supported: gruu
Contact: <sip:[email protected];gr=urn:uuid:(Bob's UA's bits)>

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Alice uses the information in the Target-Dialog header field to
determine that this REFER is associated with the dialog she already
has in place with Bob. Alice is now in a position to use the same
admission policy she used for in-dialog REFERs: "Do I have a call
with this person?". She accepts the REFER, sends Bob the obligatory
immediate NOTIFY, and proceeds to INVITE Carol with message F6.

Message F6 (abridged, detailing pertinent fields)

sip:[email protected];gr=urn:uuid:(Carol's UA's bits)
\ /
\ /
| |
v v
INVITE SIP/2.0
Call-ID: [email protected]
Replaces: [email protected];
to-tag=totag2;from-tag=fromtag2
Supported: gruu
Contact: <sip:[email protected];gr=urn:uuid:(Alice's UA's bits)>

Carol accepts Alice's invitation to replace her dialog (invite usage)
with Bob, and notifies him that the REFERenced INVITE succeeded with
F7:

Message F7 (abridged, detailing pertinent fields)

NOTIFY sip:[email protected] SIP/2.0
Subscription-State: terminated;reason=noresource
Call-ID: [email protected]
Contact: <sip:[email protected];gr=urn:uuid:(Alice's UA's bits)>
Content-Type: message/sipfrag

SIP/2.0 200 OK

Bob then ends his invite usages with both Alice and Carol using BYEs.

7. Security Considerations

Handling multiple usages within a single dialog is complex and
introduces scenarios where the right thing to do is not clear. The
ambiguities described here can result in unexpected disruption of
communication if response codes are chosen carelessly. Furthermore,
these ambiguities could be exploited, particularly by third-parties
injecting unauthenticated requests or inappropriate responses.
Implementations choosing to create or accept multiple usages within a
dialog should give extra attention to the security considerations in

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[1], especially those concerning the authenticity of requests and
processing of responses.

Service implementations should carefully consider the effects on
their service of peers making different choices in these areas of
ambiguity. A service that requires multiple usages needs to pay
particular attention to the effect on service and network utilization
when a client fails to destroy a dialog the service believes should
be destroyed. A service that disallows multiple usages should
consider the effect on clients that, for instance, destroy the entire
dialog when only a usage should be torn down. In the worst case of a
service deployed into a network with a large number of misbehaving
clients trying to create multiple usages in an automated fashion, a
retry storm similar to an avalanche restart could be induced.

8. Conclusion

Handling multiple usages within a single dialog is complex and
introduces scenarios where the right thing to do is not clear.
Implementations should avoid entering into multiple usages whenever
possible. New applications should be designed to never introduce
multiple usages.

There are some accepted SIP practices, including transfer, that
currently require multiple usages. Recent work, most notably GRUU,
makes those practices unnecessary. The standardization of those
practices and the implementations should be revised as soon as
possible to use only single-usage dialogs.

9. Acknowledgments

The ideas in this document have been refined over several IETF
meetings with many participants. Significant contribution was
provided by Adam Roach, Alan Johnston, Ben Campbell, Cullen Jennings,
Jonathan Rosenberg, Paul Kyzivat, and Rohan Mahy. Members of the
reSIProcate project also shared their difficulties and discoveries
while implementing multiple-usage dialog handlers.

10. Informative References

[1] 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.

[2] Levin, O., "Suppression of Session Initiation Protocol (SIP)
REFER Method Implicit Subscription", RFC 4488, May 2006.

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RFC 5057 Multiple Dialog Usages November 2007

[3] Burger, E. and M. Dolly, "A Session Initiation Protocol (SIP)
Event Package for Key Press Stimulus (KPML)", RFC 4730,
November 2006.

[4] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.

[5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.

[6] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.

[7] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.

[8] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.

[9] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", Work in Progress, June 2006.

[10] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol (SIP)",
RFC 4538, June 2006.

[11] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.

Author's Address

Robert J. Sparks
Estacado Systems

EMail: [email protected]

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Full Copyright Statement

Copyright (C) The IETF Trust (2007).

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