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Versions: (draft-holmberg-sipcore-keep) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 6223

SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                                  Ericsson
Intended status: Informational                              May 24, 2010
Expires: November 25, 2010


                  Indication of support for keep-alive
                     draft-ietf-sipcore-keep-03.txt

Abstract

   This specification defines a new Session Initiation Protocol (SIP)
   Via header field parameter, "keep", which allows adjacent SIP
   entities to explicitly negotiate usage of the Network Address
   Translation (NAT) keep-alive mechanisms defined in SIP Outbound, in
   cases where SIP Outbound is not supported, cannot be applied, or
   where usage of keep-alives are not implicitly negotiated as part of
   the SIP Outbound negotiation.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 25, 2010.

Copyright Notice

   Copyright (c) 2010 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Use-case: Session from non-registered UAs  . . . . . . . .  3
     1.2.  Use-case: SIP Outbound not supported . . . . . . . . . . .  3
     1.3.  Use-case: SIP dialog initiated Outbound flows  . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  User Agent and Proxy behavior  . . . . . . . . . . . . . . . .  4
     4.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  Scope and duration of keep-alives  . . . . . . . . . . . .  5
       4.2.1.  General  . . . . . . . . . . . . . . . . . . . . . . .  5
       4.2.2.  Keep-alives within registration  . . . . . . . . . . .  5
       4.2.3.  Keep-alives within dialog  . . . . . . . . . . . . . .  6
     4.3.  Behavior of a SIP entity willing to send keep-alives . . .  6
     4.4.  Behavior of a SIP entity willing to receive keep-alives  .  7
   5.  Keep-alive frequency . . . . . . . . . . . . . . . . . . . . .  8
   6.  Overlap with connection reuse  . . . . . . . . . . . . . . . .  8
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.2.  Keep-alive negotiation: UA-proxy within registration . . .  9
     7.3.  Keep-alive negotiation: UA-proxy within dialog . . . . . . 10
     7.4.  Keep-alive negotiation: UA-UA within dialog  . . . . . . . 12
   8.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  keep . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     12.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15















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1.  Introduction

   Section 3.5 of SIP Outbound [RFC5626] defines two keep-alive
   mechanisms.  Eventhough the keep-alive mechanisms are separated from
   the rest of the SIP Outbound mechanism, it is currently not possible
   to explicitly negotiate usage of the keep-alive mechanisms, since
   usage of keep-alives in most cases are implicitly negotiated as part
   of the SIP Outbound negotiation.

   However, there are SIP Outbound use-cases where the usage of keep-
   alives are not implicitly negotiated as part of the SIP Outbound
   negotiation.  In addition, there are cases where SIP Outbound is not
   supported, where it cannot be applied, but where there is still a
   need to be able to negotiate usage of keep-alives.  For those cases,
   a mechanism to explicitly negotiate the usage of keep-alives is
   needed.

   This specification defines a new Session Initiation Protocol (SIP)
   [RFC3261] Via header field parameter, "keep", which allows adjacent
   SIP entities can use to explicitly negotiate the usage of the NAT
   keep-alive mechanisms defined in SIP Outbound.  The "keep" parameter
   allows SIP entities to indicate willingness to send keep-alives, and
   it allows SIP entities to indciate willingness to receive keep-
   alives.

   The following sections describe use-cases where a mechanism to
   explicitly negotiate the usage of keep-alives is needed.

1.1.  Use-case: Session from non-registered UAs

   In some cases a User Agent Client (UAC) does not register itself
   before it establishes a session, but where it still needs to be able
   to establish a session and send keep-alives in order to maintain NAT
   bindings open during the duration of the call.  A typical example is
   an emergency calls, where a registration is not always required.

1.2.  Use-case: SIP Outbound not supported

   In some cases all SIP entities that need to be able to negotiate the
   usage of keep-alives do not support SIP Outbound.  However, they
   still support the keep-alive mechanisms defined in SIP Outbound, and
   need to be able to negotiate the usage of them.

1.3.  Use-case: SIP dialog initiated Outbound flows

   SIP Outbound allows the establishment of flows using initial SIP
   dialog requests.  As specified in [RFC5626], the usage keep-alives
   are not implicitly negotiated for such flows.  Therefor there is a



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   need to be able to explicitly negotiate the usage of the keep-alives.


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.  Definitions

   Edge proxy: As defined in [RFC5626], a SIP proxy that is located
   topologically between the registering User Agent (UA) and the
   Authoritative Proxy.

   NOTE: In some deployments the edge proxy might physically be located
   in the same entity as the Authoritative Proxy.

   Keep-alives: Refers to keep-alive messages as defined in SIP Outbound
   [RFC5626].

   "keep" parameter: A SIP Via header field parameter that a SIP entity
   can insert in its Via header field of a request to explicitly
   indicate willingness to send keep-alives.  A SIP entity can add a
   "yes" parameter value to a "keep" parameter in the top-most Via
   header field of a recieved SIP request, to indicate willingness to
   receive keep-alives from the adjacent downstream SIP entity
   (associated with the top-most Via header field of the received
   request) from which it received the request.

   SIP entity: SIP User Agent (UA), or proxy, as defined in [RFC3261].


4.  User Agent and Proxy behavior

4.1.  General

   This section describes of SIP UAs and proxies negotiate the sending
   or receiving of keep-alives within a registration and within a
   dialog.  It also describes which types of SIP requests and responses
   can be used in order to negotiate the sending and receiving of keep-
   alives, and the lifetime of the negotiated keep-alives.

   SIP requests are used by SIP entities to indicate willingness to send
   keep-alives towards the adjacent upstream SIP entity.  The associated
   responses are used by SIP entities to indicate willingness to receive



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   keep-alives.  The procedures to indicate willingness to send and
   received keep-alives are identical for UAs and proxies.

   NOTE: Since there are SIP entities that already use CRLF keep-alives,
   and SIP entities are expected to be able to receive those, this
   specification does not forbid the sending of CRLF keep-alives towards
   an SIP entity even if it has not indicated willingess to receive
   keep-alives using the "keep" parameter.  However, the "keep"
   parameter is still important in order for a SIP entity to indicate
   that it supports CRLF keep-alives, so that the adjacent SIP entity
   does not use other mechanisms (e.g. short registration refresh
   intervals) in order to make sure the NAT bindings are kept open.

4.2.  Scope and duration of keep-alives

4.2.1.  General

   The sending and receving of keep-alives can be negotiated within a
   registration, or within a dialog.  The scope of the negotiated keep-
   alives depends on what SIP request methods are used for the keep-
   alive negotiation.

   The sending and receiving of keep-alives can be negotiated when a
   registration or dialog is initiated, or later during the registration
   or dialog.  However, once a SIP entity has negotiated the sending of
   keep-alives within a registration or dialog, it can not re-negotiate
   the sending of keep-alives within the same registration or dialog.
   Likewise, once a SIP entity has indicated willingness to receive
   keep-alives within a registration or dialog, it MUST NOT indicate
   willingness to receive keep-alives in a response to a subsequent
   request within that registration or dialog.

   A SIP entity that has indicated willingess to receive keep-alives
   within a dialog can still, in a subsequent request within the dialog,
   indicate willingness to send keep-alives within the same dialog.
   Likewise, a SIP entity that has negotiated the sending of keep-alives
   within a dialog can in a response to a subsequent request indicate
   willingness to receive keep-alives within the same dialog.

4.2.2.  Keep-alives within registration

   SIP entities use the REGISTER method in order to negotiate the
   sending and reciving of keep-alives within a registration.  The keep-
   alives can be negotiated when the registration is established, or
   later within the registration.  Once negotiated, the keep-alives are
   sent during the lifetime of the registration, until it is terminated.

   In case a SIP entity establishes multiple registration flows



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   [RFC5626], the sending and receiving of keep-alives is done
   separately for each individual registration flow.  The SIP entity
   MUST NOT send keep-alives on registration flows where it has not
   received an indicator that the adjacent upstream SIP entity is
   willing to receive keep-alives withing that registration flow.

4.2.3.  Keep-alives within dialog

   SIP entities use a initial request for a dialog, or a mid-dialog
   target refresh request [RFC3261] in order to negotiate the sending
   and reciving of keep-alives within a dialog.  The keep-alives can be
   negotiated when the dialog is established, or later within the
   dialog.  Once negotiated, the keep-alives are sent during the
   lifetime of the dialog, until it is terminated.

   Since an ACK request does not have an associated response, it can not
   be used to negotiate the sending and reciving of keep-alives.
   Therefor a SIP entity MUST NOT insert a "keep" parameter in its Via
   header field of an ACK request.  If a SIP entity receives a "keep"
   parameter in an ACK request, it MUST ignore the parameter.

4.3.  Behavior of a SIP entity willing to send keep-alives

   As defined in [RFC5626], a SIP entity that supports the sending of
   keep-alives must act as a Session Traversal Utilities for NAT (STUN)
   client [RFC5389].  The SIP entity must support the amount of STUN
   which is required to apply the STUN keep-alive mechanism defined in
   [RFC5626], and it must support the CRLF keep-alive mechanism defined
   in [RFC5626].

   When a SIP entity sends or forwards a request, if it wants to
   negotiate the sending of keep-alives within the registration (in case
   of a REGISTER request) or dialog (in case of an initial request for a
   dialog, or a mid-dialog target refresh request), and if it has not
   previously negotiated the sending of keep-alives within the same
   registration or dialog, it MUST insert a "keep" parameter in its Via
   header field of the request.

   When the SIP entity receives the associated response, if the "keep"
   parameter in its Via header field in the response contains a "yes"
   parameter value, it MUST start to send keep-alives towards the same
   destination where it would send a subsequent request (e.g.  REGISTER
   requests and initial requests for dialog) associated with the
   registration (if the keep-alive negotiation is for a registration),
   or where it would send subsequent mid-dialog reuqests (if the keep-
   alive negotiation is for a dialog).  Subsequent mid-dialog requests
   are addressed based on the dialog route set.




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   If the response contains a Flow-Timer header field, the SIP entity
   MUST remove the header field before it forwards the response towards
   another SIP entity.

   When a SIP entity is about to send a keep-alive, if the SIP entity at
   the same time is also about to send or forward a SIP request within
   the same registration or dialog, for which the keep-alive is to be
   sent, the SIP entity MAY choose not to send the keep-alive, as the
   SIP request will perform the same keep-alive action.

   NOTE: When a SIP entity sends an initial request for a dialog, if the
   adjacent upstream SIP entity does not insert itself in the dialog
   route set using a Record-Route header field, the adjacent upstream
   SIP entity will change once the dialog route set has been
   established.  If a SIP entity inserts a "keep" parameter in its Via
   header field of an initial request for a dialog, and the "keep"
   parameter in the associated response does not contain a "yes"
   parameter value, the SIP entity can insert a "keep" parameter in its
   Via header field of a subsequent request within the dialog, in case
   the new adjacent SIP entity is willing to receive keep-alives (in
   which case it will add a "yes" parameter value to the "keep"
   parameter).

   NOTE: If a SIP entity inserts a "keep" parameter in its Via header
   field of an INVITE request, and it receives multiple responses
   (provisional or final) associated with the request, as long as at
   least one of the responses, for a specific dialog, contains a "keep"
   parameter with a "yes" value it is seen as an indication that the
   adjacent upstream SIP entity is willing to receive keep-alives within
   the dialog.

4.4.  Behavior of a SIP entity willing to receive keep-alives

   As defined in [RFC5626], a SIP entity that supports receiving of
   keep-alives must act as a STUN server [RFC5389].  The SIP entity must
   support the amount of STUN which is required to apply the STUN keep-
   alive mechanism defined in [RFC5626], and it must support the CRLF
   keep-alive mechanism defined in [RFC5626].

   When a SIP entity receives request that can be used in order to
   negotiate the sending and receiveing of keep-alives, the top-most Via
   header field of the request contains a "keep" parameter, and the SIP
   entity has not previously indicated willingess to receive keep-alives
   from the adjacent downstream SIP entity within the registration (in
   case of a REGISTER request) or dialog (in case of a initial request
   for a dialog, or a mid-dialog target refresh request), if it is
   willing to receive keep-alives from the adjacent downstream SIP
   entity it MUST add a "yes" parameter value to the "keep" parameter of



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   the top-most Via header field of the request, before forwarding the
   request or creating a response.  In addition, the SIP entity MAY
   insert a Flow-Timer header field [RFC5626] in the associated
   response, which indicates the recommended keep-alive frequency for
   the registration or dialog.


5.  Keep-alive frequency

   If a SIP entity receives a SIP response, where its Via header field
   contains a "keep" parameter with a "yes" value, also contains a Flow-
   Timer header field [RFC5626], according to [RFC5626] the SIP entity
   MUST send keep-alives at least as often as this number of seconds,
   and if the SIP entity uses the server-recommended keep-alive
   frequency it should send its keep-alives so that the interval between
   each keep-alive israndomly distributed between 80% and 100% of the
   server-provided time.

   If the SIP entity does not receive a Flow-Timer header field from the
   edge proxy, it can send keep-alives at its discretion.  [RFC5626]
   provides additional guidance on selecting the keep-alive frequency in
   case a Flow-Timer header field is not received.

   OPEN ISSUE: It has been suggested that, instead of using the Flow-
   Timer header field in order to provide the recommented keep-alive
   frequency value, the value would be added as a parameter to the
   "keep" parameter, instead of the "yes" value.


6.  Overlap with connection reuse

   The connect-reuse specification [I-D.ietf-sip-connect-reuse]
   specifies how to use connection-oriented transports to send requests
   in the reverse direction.  SIP entity A opens a connection to entity
   B in order to send a request.  Under certain conditions entity B can
   reuse that connection for sending requests in the backwards direction
   to A as well.  However, the connect-reuse specification does not
   define a keep-alive mechanism for this connection.

   The mechanism specified in this draft is thus orthogonal to the
   purpose of connection reuse.  An entity that wants to use connection-
   reuse as well as indicate keep-alive mechanism on that connection
   will insert both the "alias" parameter defined in [connect-reuse] as
   well as the "keep" parameter defined in this memo.  Inserting only
   one of these parameters is not a substitute for the other.  Thus,
   while the presence of a "keep" parameter will indicate that the enity
   supports keep-alives in order to keep the connection open, no
   inference can be drawn on whether that connection can be used for



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   requests in the backwards direction.


7.  Examples

7.1.  General

   This section shows example flows where the usage of keep-alives is
   negotiated between different SIP entities, within a registration or
   within a dialog.

7.2.  Keep-alive negotiation: UA-proxy within registration

   The figure shows an example where Alice sends an REGISTER request.
   She indicates willingness of sending keep-alive by inserting a "keep"
   parameter in her Via header field of the request.  The edge proxy
   (P1) supports the keep-alive mechanism, and is willing to receive
   keep-alives from Alice during the registration, so it adds a "yes"
   value to the "keep" parameter in the Via header field of the UAC,
   before it forwards the request towards the registrar.

   When P1 receives the associated response, it inserts a Flow-Timer
   header field, with a recommended keep-alive frequency interval of 30
   seconds, in the response, before it forwards the response towards
   Alice.

   When Alice receives the response, she determines from her Via header
   field that P1 is willing to receive keep-alives within the
   registration.  For the duration of the registration, the UAC then
   sends periodic keep-alives (in this example using the STUN keep-alive
   technique) towards P1, using the recommended keep-alive frequency
   indicated in the Flow-Timer header field of the response.



















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     Alice                        P1                      REGISTRAR
       |                          |                           |
       |--- REGISTER------------->|                           |
       |    Via: UAC;keep         |                           |
       |                          |--- REGISTER-------------->|
       |                          |    Via: P1                |
       |                          |    Via: UAC;keep=yes      |
       |                          |                           |
       |                          |<-- 200 OK ----------------|
       |                          |    Via: P1                |
       |                          |    Via: UAC;keep=yes      |
       |<-- 200 OK ---------------|                           |
       |    Via: UAC;keep=yes     |                           |
       |    Flow-Timer: 30        |                           |
       |                          |                           |
       |                          |                           |
       |                   *** Timeout ***                    |
       |                          |                           |
       |=== STUN request ========>|                           |
       |<== STUN response ========|                           |
       |                          |                           |
       |                   *** Timeout ***                    |
       |                          |                           |
       |=== STUN request ========>|                           |
       |<== STUN response ========|                           |
       |                          |                           |



                        Figure 1: Example call flow

7.3.  Keep-alive negotiation: UA-proxy within dialog

   The figure shows an example where Alice sends an initial INVITE
   request for a dialog.  She indicates willingness to send keep-alive
   by inserting a "keep" parameter in her Via header field of the
   request.  The edge proxy (P1) adds itself to the dialog route set by
   adding itself to a Record-Route header field.  P1 also supports the
   keep-alive mechanism, and is willing to receive keep-alives from
   Alice during the dialog, so it adds a "yes" value to the "keep"
   parameter in the Via header field of Alice, before it forwards the
   request towards Bob.

   When P1 receives the associated response, it inserts a Flow-Timer
   header field, with a recommended keep-alive frequency interval of 30
   seconds, in the response, before it forwards the response towards
   Alice.




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   When Alice receives the response, she determines from its Via header
   field that P1 is willing to receive keep-alives within the dialog.
   For the duration of the dialog, she then sends periodic keep-alives
   (in this example using the STUN keep-alive technique) towards P1,
   using the recommended keep-alive frequency indicated in the Flow-
   Timer header field of the response.

     Alice                        P1                         Bob
       |                          |                           |
       |--- INVITE -------------->|                           |
       |    Via: UAC;keep         |                           |
       |                          |--- INVITE --------------->|
       |                          |    Via: P1                |
       |                          |    Via: UAC;keep=yes      |
       |                          |    Record-Route: P1       |
       |                          |                           |
       |                          |<-- 200 OK ----------------|
       |                          |    Via: P1                |
       |                          |    Via: UAC;keep=yes      |
       |                          |    Record-Route: P1       |
       |<-- 200 OK ---------------|                           |
       |    Via: UAC;keep=yes     |                           |
       |    Flow-Timer: 30        |                           |
       |    Record-Route: P1      |                           |
       |                          |                           |
       |--- ACK ----------------->|                           |
       |                          |                           |
       |                          |--- ACK ------------------>|
       |                          |                           |
       |                   *** Timeout ***                    |
       |                          |                           |
       |=== STUN request ========>|                           |
       |<== STUN response ========|                           |
       |                          |                           |
       |                   *** Timeout ***                    |
       |                          |                           |
       |=== STUN request ========>|                           |
       |<== STUN response ========|                           |
       |                          |                           |
       |                          |                           |
       |--- BYE ----------------->|                           |
       |                          |                           |
       |                          |--- BYE ------------------>|
       |                          |                           |
       |                          |<-- 200 OK ----------------|
       |                          |                           |

                        Figure 2: Example call flow



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7.4.  Keep-alive negotiation: UA-UA within dialog

   The figure shows an example where Alice sends an initial INVITE
   request for a dialog.  She indicates willingness to send keep-alive
   by inserting a "keep" parameter in her Via header field of the
   request.  The edge proxy (P1) does not add itself to the dialog route
   set by adding itself to a Record-Route header field, and it does not
   indicate willingness to receive keep-alives from Alice.

   When Alice receives the response, she determines from her Via header
   field that P1 is not willing to receive keep-alives from her.  When
   the dialog route set has been established, Alice sends a mid-dialog
   UPDATE request towards Bob (since P1 did not insert itself in the
   dialog route set), and she once again indicates willingness to send
   keep-alives by inserting a "keep" parameter in her Via header field
   of the request.  Bob supports the keep-alive mechanism, and is
   willing to receive keep-alives from Alice during the dialog, so he
   adds a "yes" value to the "keep" parameter in the Via header field of
   Alice, before he creates and sends a response towards her.  Bob also
   inserts a Flow-Timer header field in the response, with a recommended
   keep-alive frequency interval of 30 seconds.

   When Alice receives the response, she determines from her Via header
   field that Bob is willing to receive keep-alives from her within the
   dialog.  For the duration of the dialog, Alice then sends periodic
   keep-alives (in this example using the STUN keep-alive technique)
   towards Bob, using the recommended keep-alive frequency indicated in
   the Flow-Timer header field of the response.























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     Alice                        P1                         Bob
       |                          |                           |
       |--- INVITE -------------->|                           |
       |    Via: UAC;keep         |                           |
       |                          |--- INVITE --------------->|
       |                          |    Via: P1                |
       |                          |    Via: UAC:keep          |
       |                          |                           |
       |                          |<-- 200 OK ----------------|
       |                          |    Via: P1                |
       |                          |    Via: UAC;keep          |
       |<-- 200 OK ---------------|                           |
       |    Via: UAC;keep         |                           |
       |                          |                           |
       |                                                      |
       |--- ACK --------------------------------------------->|
       |                                                      |
       |--- UPDATE ------------------------------------------>|
       |    Via: UAC;keep                                     |
       |                                                      |
       |<-- 200 OK ------------------------------------------>|
       |    Via: UAC;keep=yes                                 |
       |    Flow-Timer: 30                                    |
       |                                                      |
       |                                                      |
       |                   *** Timeout ***                    |
       |                                                      |
       |=== STUN request ====================================>|
       |<== STUN response ====================================|
       |                                                      |
       |                   *** Timeout ***                    |
       |                                                      |
       |=== STUN request ====================================>|
       |<== STUN response ====================================|
       |                                                      |
       |                                                      |
       |--- BYE --------------------------------------------->|
       |                                                      |
       |<-- 200 OK -------------------------------------------|
       |                                                      |

                        Figure 3: Example call flow


8.  Grammar

   This specification defines a new Via header field parameter, "keep".
   The grammar includes the definitions from [RFC5626].



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   The ABNF [RFC5234] is:


   via-params =/ keep

   keep       = "keep" [ EQUAL "yes" ]



9.  IANA Considerations

9.1.  keep

   This specification defines a new Via header field parameter called
   keep in the "Header Field Parameters and Parameter Values" sub-
   registry as per the registry created by [RFC5626].  The syntax is
   defined in Section 8.  The required information is:


                                                  Predefined
   Header Field            Parameter Name         Values      Reference
   ----------------------  ---------------------  ----------  ---------
   Via                     keep                   No          [RFCXXXX]



10.  Security Considerations

   This specification does not introduce security consideritions in
   additions to those specified in [RFC5626].


11.  Acknowledgements

   Thanks to Staffan Blau, Francois Audet, Hadriel Kaplan, Sean Schneyer
   and Milo Orsic for their comments on the initial draft.  Thanks to
   Juha Heinaenen, Jiri Kuthan, Dean Willis and John Elwell for their
   comments on the list.  Thanks to Vijay Gurbani for providing text
   about the relationship with the connect-reuse specification.


12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [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.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

12.2.  Informative References

   [I-D.ietf-sip-connect-reuse]
              Gurbani, V., Mahy, R., and B. Tate, "Connection Reuse in
              the Session Initiation Protocol (SIP)",
              draft-ietf-sip-connect-reuse-14 (work in progress),
              August 2009.


Author's Address

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com

















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