Internet Engineering Task Force (IETF) C. Holmberg

Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 6228 Ericsson
Category: Standards Track May 2011
ISSN: 2070-1721

Session Initiation Protocol (SIP) Response Code for
Indication of Terminated Dialog

Abstract

This specification defines a new Session Initiation Protocol (SIP)
response code, 199 Early Dialog Terminated, that a SIP forking proxy
and a User Agent Server (UAS) can use to indicate to upstream SIP
entities (including the User Agent Client (UAC)) that an early dialog
has been terminated, before a final response is sent towards the SIP
entities.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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/rfc6228.

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
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.

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RFC 6228 199 May 2011

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................4
3. Applicability and Limitation ....................................4
4. User Agent Client Behavior ......................................4
5. User Agent Server Behavior ......................................6
6. Proxy Behavior ..................................................7
7. Backward Compatibility ..........................................9
8. Usage with SDP Offer/Answer .....................................9
9. Message Flow Examples ...........................................9
9.1. Example with a Forking Proxy that Generates 199 ............9
9.2. Example with a Forking Proxy that Receives 200 OK .........10
9.3. Example with Two Forking Proxies, of which One
Generates 199 .............................................11
10. Security Considerations .......................................12
11. IANA Considerations ...........................................13
11.1. IANA Registration of the 199 Response Code ...............13
11.2. IANA Registration of the 199 Option-Tag ..................13
12. Acknowledgements ..............................................13
13. References ....................................................14
13.1. Normative References .....................................14
13.2. Informative References ...................................14

1. Introduction

As defined in RFC 3261 [RFC3261], a Session Initiation Protocol (SIP)
early dialog is created when a non-100 provisional response is sent
to the initial dialog initiation request (e.g., INVITE, outside an
existing dialog). The dialog is considered to be in early state
until a final response is sent.

When a proxy receives an initial dialog initiation request, it can
forward the request towards multiple remote destinations. When the
proxy does that, it performs forking [RFC3261].

When a forking proxy receives a non-100 provisional response, or a
2xx final response, it forwards the response upstream towards the
sender of the associated request. After a forking proxy has
forwarded a 2xx final response, it normally generates and sends
CANCEL requests downstream towards all remote destinations where it
previously forked the request associated with the 2xx final response
and from which it has still not received a final response. The
CANCEL requests are sent in order to terminate any outstanding early
dialogs associated with the request.

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Upstream SIP entities might receive multiple 2xx final responses.
When a SIP entity receives the first 2xx final response, and it does
not intend to accept any subsequent 2xx final responses, it will
automatically terminate any other outstanding early dialog associated
with the request. If the SIP entity receives a subsequent 2xx final
response, it will normally generate and send an ACK request, followed
with a BYE request, using the dialog identifier retrieved from the
2xx final response.

NOTE: A User Agent Client (UAC) can use the Request-Disposition
header field [RFC3841] to request that proxies do not generate and
send CANCEL requests downstream once they have received the first
2xx final response.

When a forking proxy receives a non-2xx final response, it does not
always immediately forward the response upstream towards the sender
of the associated request. Instead, the proxy "stores" the response
and waits for subsequent final responses from other remote
destinations where the associated request was forked. At some point,
the proxy uses a specified mechanism to determine the "best" final
response code, and forwards a final response using that response code
upstream towards the sender of the associated request. When an
upstream SIP entity receives the non-2xx final response, it will
release resources associated with the session. The UAC will
terminate, or retry, the session setup.

Since the forking proxy does not always immediately forward non-2xx
final responses, upstream SIP entities (including the UAC that
initiated the request) are not immediately informed that an early
dialog has been terminated, and will therefore maintain resources
associated with the early dialog reserved until a final response is
sent by the proxy, even if the early dialog has already been
terminated. A SIP entity could use the resources for other things,
e.g., to accept subsequent early dialogs that it otherwise would
reject.

This specification defines a new SIP response code, 199 Early Dialog
Terminated. A forking proxy can send a 199 provisional response to
inform upstream SIP entities that an early dialog has been
terminated. A UAS can send a 199 response code, prior to sending a
non-2xx final response, for the same purpose. SIP entities that
receive the 199 response can use it to trigger the release of
resources associated with the terminated early dialog. In addition,
SIP entities might also use the 199 response to make policy decisions
related to early dialogs. For example, a media gate controlling a
SIP entity might use the 199 response when deciding for which early
dialogs media will be passed.

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Section 9 contains signalling examples that show when and how a
forking proxy generates 199 responses in different situations.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

3. Applicability and Limitation

The 199 response code is an optimization, and it only optimizes how
quickly recipients might be informed about terminated early dialogs.
The achieved optimization is limited. Since the response is normally
not sent reliably by a UAS, and cannot be sent reliably when
generated and sent by a proxy, it is possible that some or all of the
199 responses will get lost before they reach the recipients. In
such cases, recipients will behave the same as if the 199 response
code were not used at all.

One example for which a UAC could use the 199 response is that when
it receives a 199 response, it releases resources associated with the
terminated early dialog. The UAC could also use the 199 response to
make policy decisions related to early dialogs. For example, if a
UAC is playing media associated with an early dialog, and it then
receives a 199 response indicating the early dialog has been
terminated, it could start playing media associated with a different
early dialog.

Application designers utilizing the 199 response code MUST ensure
that the application's user experience is acceptable if all 199
responses are lost and not delivered to the recipients.

4. User Agent Client Behavior

When a UAC sends an initial dialog initiation request, and if it is
willing to receive 199 responses, it MUST insert a "199" option-tag
in the Supported header field [RFC3261] of the request. The option-
tag indicates that the UAC supports, and is willing to receive, 199
responses. A UAC SHOULD NOT insert a "199" option-tag in the Require
or the Proxy-Require header field [RFC3261] of the request, since in
many cases it would result in unnecessary session establishment
failures.

NOTE: The UAC always needs to insert a "199" option-tag in the
Supported header field, in order to indicate that it supports, and
is willing to receive, 199 responses, even if it also inserts the
option-tag in the Require or Proxy-Require header field.

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It is RECOMMENDED that a UAC not insert a "100rel" option-tag
[RFC3262] in the Require header field when it also indicates support
for 199 responses, unless the UAC also uses some other SIP extension
or procedure that mandates it to do so. The reason is that proxies
are not allowed to generate and send 199 responses when the UAC has
required provisional responses to be sent reliably.

When a UAC receives a 199 response, it might release resources
associated with the terminated early dialog. A UAC might also use
the 199 response to make policy decisions related to early dialogs.

NOTE: The 199 response indicates that the early dialog has been
terminated, so there is no need for the UAC to send a BYE request
in order to terminate the early dialog when it receives the 199
response.

NOTE: The 199 response does not affect other early dialogs
associated with the session establishment. For those dialogs, the
normal SIP rules regarding transaction timeout, etc., still apply.

Once a UAC has received and accepted a 199 response, it MUST NOT send
any media associated with the early dialog. In addition, if the UAC
is able to associate received media with early dialogs, it MUST NOT
process any received media associated with the early dialog that was
terminated.

If multiple usages [RFC5057] are used within an early dialog, and it
is not clear which dialog usage the 199 response terminates, SIP
entities that keep dialog state SHALL NOT release resources
associated with the early dialog when they receive the 199 response.

If a UAC receives an unreliably sent 199 response on a dialog that
has not previously been established (this can happen if a 199
response reaches the client before the 18x response that would
establish the early dialog) it SHALL discard the 199 response. If a
UAC receives a reliably sent 199 response on a dialog that has not
previously been created, it MUST acknowledge the 199 response, as
described in RFC 3262 [RFC3262].

If a UAC has received a 199 response for all early dialogs, and no
early dialogs associated with the session establishment remain, it
maintains the "Proceeding" state [RFC3261] and waits for possible
subsequent early dialogs to be established, and eventually for a
final response to be received.

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5. User Agent Server Behavior

If a UAS receives an initial dialog initiation request with a
Supported header field that contains a "199" option-tag, it SHOULD
NOT send a 199 response on an early dialog associated with the
request before it sends a non-2xx final response. Cases where a UAS
might send a 199 response are if it has been configured to do so due
to lack of support for the 199 response code by forking proxies or
other intermediate SIP entities, or if it is used in an environment
that specifies that it shall send a 199 response before sending a
non-2xx response.

NOTE: If a UAS has created multiple early dialogs associated with
an initial dialog initiation request (the UAS is acting similarly
to a forking proxy), it does not always intend to send a final
response on all of those early dialogs.

NOTE: If the Require header field of an initial dialog initiation
request contains a "100rel" option-tag, proxies will not be able
to generate and send 199 responses. In such cases, the UAS might
choose to send a 199 response on an early dialog before it sends a
non-2xx final response, even if it would not do so in other cases.

If the Supported header field of an initial dialog initiation request
does not contain a "199" option-tag, the UAC MUST NOT send a 199
response on any early dialog associated with the request.

When a UAS generates a 199 response, the response MUST contain a To
header field tag parameter [RFC3261], in order for other entities to
identify the early dialog that has been terminated. The UAS MUST
also insert a Reason header field [RFC3326] that contains a response
code describing the reason why the early dialog was terminated. The
UAS MUST NOT insert a "199" option-tag in the Supported, Require, or
Proxy-Require header field of the 199 response.

If a UAS intends to send 199 responses, and if it supports the
procedures defined in RFC 3840 [RFC3840], it MAY during the
registration procedure use the sip.extensions feature tag [RFC3840]
to indicate support for the 199 response code.

A 199 response SHOULD NOT contain a Session Description Protocol
(SDP) offer/answer message body, unless required by the rules in
RFC 3264 [RFC3264].

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RFC 6228 199 May 2011

According to RFC 3264, if an INVITE request does not contain an SDP
offer, and the 199 response is the a first reliably sent response
associated with the request, the 199 response is required to contain
an SDP offer. In this case, the UAS SHOULD send the 199 response
unreliably, or send the 199 response reliably and include an SDP
offer with no "m=" lines in the response.

Since a 199 response is only used for information purposes, the UAS
SHOULD send it unreliably, unless the "100rel" option-tag is present
in the Require header field of the associated request.

6. Proxy Behavior

When a proxy receives a 199 response to an initial dialog initiation
request, it MUST process the response as any other non-100
provisional response. The proxy will forward the response upstream
towards the sender of the associated request. The proxy MAY release
resources it has reserved associated with the early dialog that is
terminated. If a proxy receives a 199 response out of dialog, it
MUST process it as other non-100 provisional responses received out
of dialog.

When a forking proxy receives a non-2xx final response to an initial
dialog initiation request that it recognizes as terminating one or
more early dialogs associated with the request, it MUST generate and
send a 199 response upstream for each of the terminated early dialogs
that satisfy each of the following conditions:

- The forking proxy does not intend to forward the final response
immediately (in accordance with rules for a forking proxy).

- The UAC has indicated support (by inserting the "199" option-tag
in a Supported header field) for the 199 response code in the
associated request.

- The UAC has not required provisional responses to be sent reliably
(i.e., has not inserted the "100rel" option-tag in a Require or
Proxy-Require header field) in the associated request.

- The forking proxy has not already received and forwarded a 199
response for the early dialog.

- The forking proxy has not already sent a final response for any of
the early dialogs.

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RFC 6228 199 May 2011

As a consequence, once a final response to an initial dialog
initiation request has been issued by the proxy, no further 199
responses associated with the request will be generated or forwarded
by the proxy.

When a forking proxy forks an initial dialog initiation request, it
generates a unique Via header branch parameter value for each forked
leg. A proxy can determine whether additional forking has occurred
downstream of the proxy by storing the top Via branch value from each
response that creates an early dialog. If the same top Via branch
value is received for multiple early dialogs, the proxy knows that
additional forking has occurred downstream of the proxy. A non-2xx
final response received for a specific early dialog also terminates
all other early dialogs for which the same top Via branch value was
received in the responses that created those early dialogs.

Based on implementation policy, a forking proxy MAY wait before
sending the 199 response, e.g., if it expects to receive a 2xx final
response on another dialog shortly after it received the non-2xx
final response that triggered the 199 response.

When a forking proxy generates a 199 response, the response MUST
contain a To header field tag parameter that identifies the
terminated early dialog. A proxy MUST also insert a Reason header
field that contains the SIP response code of the response that
triggered the 199 response. The SIP response code in the Reason
header field informs the receiver of the 199 response about the SIP
response code that was used by the UAS to terminate the early dialog,
and the receiver might use that information for triggering different
types of actions and procedures. The proxy MUST NOT insert a "199"
option-tag in the Supported, Require, or Proxy-Require header field
of the 199 response.

A forking proxy that supports the generation of 199 responses MUST
keep track of early dialogs, in order to determine whether to
generate a 199 response when the proxy receives a non-2xx final
response. In addition, a proxy MUST keep track on which early
dialogs it has received and forwarded 199 responses, in order to not
generate additional 199 responses for those early dialogs.

If a forking proxy receives a reliably sent 199 response for a dialog
for which it has previously generated and sent a 199 response, it
MUST forward the 199 response. If a proxy receives an unreliably
sent 199 response for which it has previously generated and sent a
199 response, it MAY forward the response, or it MAY discard it.

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RFC 6228 199 May 2011

When a forking proxy generates and sends a 199 response, the response
SHOULD NOT contain a Contact header field or a Record-Route header
field [RFC3261].

If the Require header field of an initial dialog initiation request
contains a "100rel" option-tag, a proxy MUST NOT generate and send
199 responses associated with that request. The reason is that a
proxy is not allowed to generate and send 199 responses reliably.

7. Backward Compatibility

Since all SIP entities involved in a session setup do not necessarily
support the specific meaning of the 199 Early Dialog Terminated
provisional response, the sender of the response MUST be prepared to
receive SIP requests and responses associated with the dialog for
which the 199 response was sent (a proxy can receive SIP messages
from either direction). If such a request is received by a UA, it
MUST act in the same way as if it had received the request after
sending the final non-2xx response to the INVITE request, as
specified in RFC 3261. A UAC that receives a 199 response for an
early dialog MUST NOT send any further requests on that dialog,
except for requests that acknowledge reliable responses. A proxy
MUST forward requests according to RFC 3261, even if the proxy has
knowledge that the early dialog has been terminated.

A 199 response does not "replace" a final response. RFC 3261
specifies when a final response is sent.

8. Usage with SDP Offer/Answer

A 199 response SHOULD NOT contain an SDP offer/answer [RFC3264]
message body, unless required by the rules in RFC 3264.

If an INVITE request does not contain an SDP offer, and the 199
response is the first reliably sent response, the 199 response is
required to contain an SDP offer. In this case, the UAS SHOULD send
the 199 response unreliably, or include an SDP offer with no "m="
lines in a reliable 199 response.

9. Message Flow Examples

9.1. Example with a Forking Proxy that Generates 199

Figure 1 shows an example where a proxy (P1) forks an INVITE received
from a UAC. The forked INVITE reaches UAS_2, UAS_3, and UAS_4, which
send 18x provisional responses in order to establish early dialogs
between themselves and the UAC. UAS_2 and UAS_3 each reject the
INVITE by sending a 4xx error response. When P1 receives the 4xx

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responses, it immediately sends 199 responses towards the UAC, to
indicate that the early dialogs for which it received the 4xx
responses have been terminated. The early dialog leg is shown in
parentheses.

UAC P1 UAS_2 UAS_3 UAS_4
| | | | |
|-- INVITE -->| | | |
| |--- INVITE (2) ->| | |
| |--- INVITE (3) --------->| |
| |--- INVITE (4) ----------------->|
| |<-- 18x (2) -----| | |
|<- 18x (2) --| | | |
| |<-- 18x (3) -------------| |
|<- 18x (3) --| | | |
| |<-- 18x (4) ---------------------|
|<- 18x (4) --| | | |
| |<-- 4xx (2) -----| | |
| |--- ACK (2) ---->| | |
|<- 199 (2) --| | | |
| |<-- 4xx (3) -------------| |
| |--- ACK (3) ------------>| |
|<- 199 (3) --| | | |
| |<-- 200 (4) ---------------------|
|<- 200 (4) --| | | |
|-- ACK (4) ->| | | |
| |--- ACK (4) -------------------->|
| | | | |

Figure 1: Example Call Flow

9.2. Example with a Forking Proxy that Receives 200 OK

Figure 2 shows an example where a proxy (P1) forks an INVITE request
received from a UAC. The forked request reaches UAS_2, UAS_3, and
UAS_4, all of which send 18x provisional responses in order to
establish early dialogs between themselves and the UAC. Later, UAS_4
accepts the session and sends a 200 OK final response. When P1
receives the 200 OK response, it immediately forwards it towards the
UAC. P1 does not send 199 responses for the early dialogs from UAS_2
and UAS_3, since P1 has still not received any final responses on
those early dialogs (even if P1 sends CANCEL requests to UAS_2 and
UAS_3, P1 may still receive a 200 OK final response from UAS_2 or
UAS_3, which P1 would have to forward towards the UAC. The early
dialog leg is shown in parentheses.

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UAC P1 UAS_2 UAS_3 UAS_4
| | | | |
|-- INVITE -->| | | |
| |--- INVITE (2) ->| | |
| |--- INVITE (3) --------->| |
| |--- INVITE (4) ----------------->|
| |<-- 18x (2) -----| | |
|<- 18x (2) --| | | |
| |<-- 18x (3) -------------| |
|<- 18x (3) --| | | |
| |<-- 18x (4) ---------------------|
|<- 18x (4) --| | | |
| |<-- 200 (4) ---------------------|
|<- 200 (4) --| | | |
|-- ACK (4) ->| | | |
| |--- ACK (4) -------------------->|
| | | | |

Figure 2: Example Call Flow

9.3. Example with Two Forking Proxies, of which One Generates 199

Figure 3 shows an example where a proxy (P1) forks an INVITE request
received from a UAC. One of the forked requests reaches UAS_2. The
other requests reach another proxy (P2), which forks the request to
UAS_3 and UAS_4. UAS_3 and UAS_4 send 18x provisional responses in
order to establish early dialogs between themselves and the UAC.
Later, UAS_3 and UAS_4 each reject the INVITE request by sending a
4xx error response. P2 does not support the 199 response code and
forwards a single 4xx response. P1 supports the 199 response code,
and when it receives the 4xx response from P2, it also manages to
associate the early dialogs from both UAS_3 and UAS_4 with the
response. Therefore, P1 generates and sends two 199 responses to
indicate that the early dialogs from UAS_3 and UAS_4 have been
terminated. The early dialog leg is shown in parentheses.

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RFC 6228 199 May 2011

UAC P1 P2 UAS_2 UAS_3 UAS_4
| | | | | |
|-- INVITE -->| | | | |
| |-- INVITE (2) ------------------>| | |
| |-- INVITE ---->| | | |
| | |--- INVITE (3) --------->| |
| | |--- INVITE (4) ----------------->|
| | |<-- 18x (3) -------------| |
| |<- 18x (3) ----| | | |
|<- 18x (3) --| | | | |
| | |<-- 18x (4) ---------------------|
| |<- 18x (4) ----| | | |
|<- 18x (4) --| | | | |
| | |<-- 4xx (3) -------------| |
| | |--- ACK (3) ------------>| |
| | |<-- 4xx (4) ---------------------|
| | |--- ACK (4) -------------------->|
| |<- 4xx (3) ----| | | |
| |-- ACK (3) --->| | | |
|<- 199 (3) --| | | | |
|<- 199 (4) --| | | | |
| |<- 200 (2) ----------------------| | |
|<- 200 (2) --| | | | |
|-- ACK (2) ->| | | | |
| |-- ACK (2) --------------------->| | |
| | | | | |

Figure 3: Example Call Flow

10. Security Considerations

General security issues related to SIP responses are described in
RFC 3261. Due to the nature of the 199 response, it may be
attractive to use it for launching attacks in order to terminate
specific early dialogs (other early dialogs will not be affected).
In addition, if a man-in-the-middle generates and sends towards the
UAC a 199 response that terminates a specific dialog, it can take a
while until the UAS finds out that the UAC, and possible stateful
intermediates, have terminated the dialog. SIP security mechanisms
(e.g., hop-to-hop Transport Layer Security (TLS)) can be used to
minimize, or eliminate, the risk of such attacks.

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RFC 6228 199 May 2011

11. IANA Considerations

This section registers a new SIP response code and a new option-tag,
according to the procedures of RFC 3261.

11.1. IANA Registration of the 199 Response Code

This section registers a new SIP response code, 199. The required
information for this registration, as specified in RFC 3261, is:

RFC Number: RFC 6228

Response Code Number: 199

Default Reason Phrase: Early Dialog Terminated

11.2. IANA Registration of the 199 Option-Tag

This section registers a new SIP option-tag, 199. The required
information for this registration, as specified in RFC 3261, is:

Name: 199

Description: This option-tag is for indicating support of the 199
Early Dialog Terminated provisional response code. When
present in a Supported header of a request, it indicates that
the UAC supports the 199 response code. When present in a
Require or Proxy-Require header field of a request, it
indicates that the UAS, or proxies, MUST support the 199
response code. It does not require the UAS, or proxies, to
actually send 199 responses.

12. Acknowledgements

Thanks to Paul Kyzivat, Dale Worley, Gilad Shaham, Francois Audet,
Attila Sipos, Robert Sparks, Brett Tate, Ian Elz, Hadriel Kaplan,
Timothy Dwight, Dean Willis, Serhad Doken, John Elwell, Gonzalo
Camarillo, Adam Roach, Bob Penfield, Tom Taylor, Ya Ching Tan, Keith
Drage, Hans Erik van Elburg, and Cullen Jennings for their feedback
and suggestions.

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13. References

13.1. 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.

[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, December 2002.

[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.

13.2. Informative References

[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.

[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.

Author's Address

Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland

EMail: [email protected]

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