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Versions: (draft-ietf-sip-199) 00 01 02 03 04 05 06 RFC 6228

SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                                  Ericsson
Expires: June 12, 2010                                  December 9, 2009


           Response Code for Indication of Terminated Dialog
                     draft-ietf-sipcore-199-02.txt

Abstract

   This specification defines a new SIP response code, 199 Early Dialog
   Terminated, which a SIP forking proxy and a UAS can use to indicate
   upstream towards the UAC that an early dialog has been terminated,
   before a final response is sent towards the UAC.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 12, 2010.

Copyright Notice

   Copyright (c) 2009 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



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   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 BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  User Agent Client behavior . . . . . . . . . . . . . . . . . .  4
     4.1.  Examples of resource types . . . . . . . . . . . . . . . .  5
     4.2.  Examples of policy procedures  . . . . . . . . . . . . . .  6
   5.  User Agent Server behavior . . . . . . . . . . . . . . . . . .  7
   6.  Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Backward compatibility . . . . . . . . . . . . . . . . . . . .  9
   8.  Usage with SDP offer/answer  . . . . . . . . . . . . . . . . .  9
   9.  Usage with 100rel  . . . . . . . . . . . . . . . . . . . . . . 10
   10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Example with a forking proxy which generates 199 . . . . . 10
     10.2. Example with a forking proxy which receives 200 OK . . . . 11
     10.3. Example with two forking proxies, of which one
           generates 199  . . . . . . . . . . . . . . . . . . . . . . 12
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     12.1. IANA Registration of the 199 response code . . . . . . . . 14
     12.2. IANA Registration of the 199 Option Tag  . . . . . . . . . 14
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     14.2. Informational References . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15


















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1.  Introduction

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

   When a proxy receives an initial request (outside an existing dialog,
   and without a pre-defined route set), it can forward it towards
   multiple remote destinations.  When the proxy does that, it performs
   forking.

   When a forking proxy receives non-100 provisional responses, it
   forwards the responses upstream towards the sender of the associated
   request.  When a forking proxy receives a 2xx final response, it
   forwards the response upstream towards the sender of the associated
   request.  At that point the proxy normally sends a CANCEL request
   downstream towards all remote destinations where it previously sent
   the request associated with the 2xx final response, and from which it
   has yet not received a final response, in order to terminate
   associated outstanding early dialogs.  It is possible to receive
   multiple 2xx final responses.  When SIP entities upstream receive the
   first 2xx final response, and they do not to intend to accept
   subsequent 2xx final responses, they will automatically terminate
   other associated outstanding early dialogs.  If additional 2xx final
   responses are received, those SIP entities will normally send a BYE
   request using the dialog identifier retrieved from the subsequent 2xx
   final response.

   NOTE: A UAC can use the Request-Disposition header [RFC3841] to
   request that proxies do not send CANCEL requests downstream once they
   have received the first final 2xx 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 forking proxy "stores" it
   and waits for further final responses from remote destinations where
   the forked request was forwarded.  At some point the proxy uses a
   specified mechanism to determine the "best" final response code, and
   forwards that final response upstream towards the sender of the
   associated request.  When SIP entities upstream receive the non-2xx
   final response they will release resources associated with the
   session, and the UAC will terminate, or retry, the session setup.

   Since the forking proxy does not always immediately forward non-2xx
   final responses, SIP entities upstream (including the UAC that
   initiated the request) do not know that a specific early dialog has



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   been terminated, and the SIP entities keep possible resources
   associated with the early dialog until they receive a final response
   from the forking proxy.

   This specification defines a new SIP response code, 199 Early Dialog
   Terminated, which a forking proxy and a UAS can use to indicate
   upstream that an early dialog has been terminated.  The 199 response
   can also be sent by a UAS, prior to sending a non-2xx final response.
   SIP entities that receive the 199 provisional response can release
   resources associated with the specific early dialog.  The SIP
   entities can also use the 199 provisional response to make policy
   related decisions related to early dialogs.

   The 199 response code is an optimization, which allows the UAC to be
   informed about terminated early dialogs.  However, since the support
   of the 199 response is optional, a UAC cannot assume that it will
   always receive a 199 provisional response for all terminated early
   dialogs.


2.  Conventions

   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 BCP 14, RFC 2119
   [RFC2119].


3.  Requirements

   REQ 1: It must be possible to indicate to the UAC that an early
   dialog has been terminated before a final response is sent.


4.  User Agent Client behavior

   When a UAC sends an initial request, and if it wants to receive 199
   responses, it MUST insert the 199 option-tag in the Supported header,
   which indicates that the client supports the 199 Early Dialog
   Terminated response code.  The UAC SHOULD NOT insert the 199 option-
   tag in the Require header, unless the particular session usage
   requires the UAS to support the response code.  Also, the UAC SHOULD
   NOT insert the 199 option-tag in the Proxy-Require header, unless the
   particular session usage requires every proxy on the path to support
   the response code.  Using Require or Proxy-Require with the 199
   option-tag will in many cases result in unnecessary session
   establishment failures.




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   When a UAC receives a 199 response it MAY release resources and
   procedures associated with the early dialog on which the 199 response
   is received.  Examples of resources and procedures are e.g.
   procedures for the establishment of media plane resources (bandwidth,
   radio, codecs etc), media security procedures or procedures related
   to NAT traversal.  In addition, the UAC may use the 199 response for
   policy decisions related to early dialogs, e.g. when choosing to
   process media associated with a particular early dialog.

   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 client receives an unreliable 199 response on a dialog which has
   not previously been created (this can happen if a 199 response
   reaches the client before a 18x response) the client SHALL discard
   the 199 responses.  If a client receives a reliable 199 response on a
   dialog which has not previously been created the UAC SHOULD
   acknowledge the 199 response, as described in [RFC3262].

4.1.  Examples of resource types

   Examples which benefit from resource-release are:

   1.  Codec release - when resources for a specific codec has been
   reserved only for the stream that is terminated.  In that case the
   resources associated with that codec can be released.

   2.  Pre-conditions - when the dialog is terminated, procedures and
   resources associated to the pre-conditions for that dialog can be
   released.

   3.  In-band security negotiation - when the dialog is terminated,
   procedures and resources associated with the in-band security
   negotiation for that dialog can be released.

   4.  ICE [I-D.ietf-mmusic-ice] mechanism - when the dialog is
   terminated, procedures and resources associated with the ICE related
   in-band procedures for that dialog can be released.

   5.  Limited access resources - in case of forking and multiple stream
   it may not be possible to allow early media on all dialogs, so media
   sessions associated with some dialogs may e.g. be set to "inactive".
   When a dialog is terminated, media sessions associated with other
   dialogs can be allowed.

   6.  Secure media selection - when SRTP is used to encrypt the media.



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   In some cases SIP entities are only able to render media associated
   with a single early dialog.  If a 199 response is received on a
   dialog, and media associated with that media has been rendered, the
   SIP entity can start rendering media associated with another early
   dialog.

   If the client is able to associate the 199 response with a specific
   media stream, it MAY choose to discard media on that specific media
   stream, it MAY release all resources associated with that media
   stream and it MAY start to process media streams received on other
   early dialogs.  When the P-Early-Media header [RFC5009] is used, a UA
   MAY trigger different actions depending on whether the header has
   been used for the terminated dialog.  How the association between the
   dialog and the associated media stream is done is outside the scope
   of this document.

   NOTE: When using SRTP [RFC3711], the secure media stream is bound to
   the crypto context setup for the dialog, and can be identified using
   the MKI (if used) of SRTP.

   If the client only has a single early dialog (other early dialogs MAY
   not have been established, or they MAY have been established and
   later terminated) and a 199 response is received for that early
   dialog, the client terminates the early dialog.  Afterwards, the
   client SHOULD act as before the first early dialog was established.

4.2.  Examples of policy procedures

   1.  UAC early media selection - when media associated with multiple
   early dialogs is received, SIP endpoint normally chooses to process
   media associated with a single early dialog (e.g. the recently
   established early dialog).  If a 199 response is received on such
   early dialog, the SIP endpoint can start processing media associated
   with another early dialog.  For example, an early dialog may be used
   for an announcement message, and when the message is finished a 199
   response will be sent on that dialog, in order for the SIP endpoint
   to stop processing media associated with that early dialog.  This
   kind of policy is normal especially in PSTN gateways, where the
   calling user cannot control which media is processed.

   2.  SBC early media selection - when an SBC is used to control which
   media is processed and forwarded.  In many cases, the SBC only
   processes media associated with a single early dialog.  Typical for
   NAT traversal, the SBC often "latches" onto a media stream.  If a 199
   response is received, the SBC can choose to start processing media
   associated with another dialog.  If the SBC performs latching, it can
   trigger a "re-latch" onto a new media stream when the 199 response is
   received.



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   3.  UAC ringing tone generation - when a UAC receives a 180 response,
   it may choose to generate a local ringing tone.  If early media is
   received, the UAC may stop the local ringing tone generation and play
   the incoming early media packets.  If a 199 response is received on
   the early dialog associated with the early media, and the UAC has
   previously received a 180 response for another early dialog, it can
   start to generate local ringing tone again.  Having knowledge that
   the early dialog associated with early media has been terminated, the
   UAC can also start generating local ringing tone if a 180 is received
   on another early dialog after the early dialog has been terminated.


5.  User Agent Server behavior

   If the received initial request contains an 199 option tag, the UAS
   SHOULD NOT send a 199 response on a dialog on which it intends to
   send a final response, unless it e.g. has been configured to do so
   due to lack of 199 support by forking proxies or other intermediate
   SIP entities.

   NOTE: If the UAS has created multiple early dialogs (the UAS is
   acting similar to a forking proxy), it does not always intend to send
   a final response for all of those dialogs.

   When a UAS generates a 199 response, the response MUST contain a To
   header tag parameter, which identifies the early dialog that has been
   terminated.  The UAS MUST also insert a Reason header [RFC3326] which
   contains a response code which describes the reason why the dialog
   was terminated.

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

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

   If the INVITE request did 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 unreliable, or include an SDP offer with no m- lines
   in the reliable 199 response.

   When a 199 response is sent by a UAS, since the provisional response
   is only used for information purpose, the UAS SHOULD send it
   unreliably even if the 100rel option tag [RFC3262] is present in the
   Require header of the associated request.



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6.  Proxy behavior

   When a proxy receives a 199 response, it MUST process the response as
   any other non-100 provisional responses.  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
   dialog on which the response is received.  If a proxy receives a 199
   response out of dialog, it processes it as other non-100 provisional
   responses received out of dialog.

   When a forking proxy receives a non-2xx final response that it
   recognizes as terminating one or more early dialogs, it SHOULD
   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 (using the 199 option tag) for the
   199 response code

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

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

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

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

   Based on implementation policy, the 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 which triggered the 199 response.

   When a forking proxy generates a 199 response, the response MUST



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   contain a To header tag parameter, which identifies the early dialog
   that has been terminated.  The proxy MUST also insert a Reason header
   [RFC3326] which contains the response code of the response that
   triggered the 199 response.

   A forking proxy which supports generating 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, the 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.  In case of a unreliably
   sent 199 response, the proxy MAY forward the 199 response, or it MAY
   discard it.

   When a forking proxy generates a 199 response, the response MUST NOT
   be sent 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 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, as specified in [RFC3261].
   A UAC that receives a 199 response for an early dialog MUST NOT send
   any further requests on that dialog, except for requests which
   acknowledge reliable responses.  A proxy MUST forward requests
   according to [RFC3261], even if the proxy has knowledge that the
   early dialog has been terminated.

   The 199 Early Dialog Terminated response code does not "replace" a
   final response.  RFC 3261 [RFC3261] specifies when a final response
   is sent.


8.  Usage with SDP offer/answer

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



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   If the INVITE request did 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 unreliable, or include an SDP offer with no m- lines
   in the reliable 199 response.


9.  Usage with 100rel

   When a 199 Early Dialog Terminated provisional response is sent by a
   UAS, since the provisional response is only used for information
   purpose, the UAS SHOULD send it unreliably even if the 100rel option
   tag [RFC3262] is present in the Require header of the associated
   request.

   When a forking proxy generates a 199 response, the response MUST NOT
   be sent reliably.


10.  Examples

10.1.  Example with a forking proxy which generates 199

   The figure shows an example, where a proxy (P1) forks an INVITE
   received from UAC.  The forked INVITE reaches UAS_2, UAS_3 and UAS_4,
   which send 18x provisional responses in order to create early dialogs
   between themselves and the UAC.  UAS_2 and UAS_3 reject the INVITE by
   sending a 4xx error response each.  When P1 receives the 4xx
   responses it immediately sends 199 responses, associated with the
   dialogs where the 4xx responses were received, towards the UAC.  The
   early dialog leg is shown in parenthesis.




















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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) --|                 |       |       |
     |             |<-- 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

10.2.  Example with a forking proxy which receives 200 OK

   The figure shows an example, where a proxy (P1) forks an INVITE
   received from UAC.  The forked INVITE reaches UAS_2, UAS_3 and UAS_4,
   which send 18x provisional responses in order to create early dialogs
   between themselves and the UAC.  UAS_4 accepts the session by sending
   a 200 OK final response.  When P1 receives the 200 OK responses 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
   yet not received any final responses on those early dialogs (even if
   P1 sends CANCEL request to UAS_2 and UAS_3 P1 may still receive 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 parenthesis.











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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

10.3.  Example with two forking proxies, of which one generates 199

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
















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    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


11.  Security Considerations

   General security issues related to SIP responses are described in
   [RFC3261].  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 sends a 199 response to the UAC,
   which terminates a specific dialog, it can take a while until the UAS
   finds out that the UAC, and possbile stateful intermediates, have
   terminated the dialog.  SIP security mechanisms (e.g. hop-to-hop TLS)
   can be used to minimize, or eliminate, the risk for such attacks.


12.  IANA Considerations

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



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12.1.  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 XXXX [[NOTE TO IANA: Please replace XXXX with the RFC number of this specification]]

    Response Code Number: 199

    Default Reason Phrase: Early Dialog Terminated

12.2.  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, it indicates that the UA supports the
         response code. When present in a Require header in a request,
         it indicates that the UAS MUST support the sending of the
         response code.


13.  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 and Hans Erik van Elburg for their feedback and suggestions.


14.  References

14.1.  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



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              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.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

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

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

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

14.2.  Informational References

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

   [RFC5009]  Ejza, R., "Private Header (P-Header) Extension to the
              Session Initiation Protocol (SIP) for Authorization of
              Early Media", RFC 5009, September 2007.














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Author's Address

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com










































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