SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                                  Ericsson
Intended status: Informational                              May 31, Standards Track                          August 5, 2010
Expires: December 2, 2010 February 6, 2011

                  Indication of support for keep-alive
                     draft-ietf-sipcore-keep-04.txt
                     draft-ietf-sipcore-keep-05.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 is 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
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   This Internet-Draft will expire on December 2, 2010. February 6, 2011.

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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.  Lifetime of keep-alives  . . . . . . . . . . . . . . . . .  5
       4.2.1.  General  . . . . . . . . . . . . . . . . . . . . . . .  5
       4.2.2.  Keep-alives associated with registration . . . . . . .  5
       4.2.3.  Keep-alives associated with 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.
     6.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.2.
     6.2.  Keep-alive negotiation associated with registration:
           UA-proxy . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.3.
     6.3.  Keep-alive negotiation associated with dialog: UA-proxy  . 10
     7.4.
     6.4.  Keep-alive negotiation associated with dialog: UA-UA . . . 12
   8.
   7.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     9.1.
     8.1.  keep . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   10.
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   11.
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     12.1.
   11. Normative References . . . . . . . . . . . . . . . . . . . 14
     12.2. Informative References . . . . . . . . . . . . . . . . . . 15 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.  Introduction

   Section 3.5 of SIP Outbound [RFC5626] defines two keep-alive
   mechanisms.  Eventhough  Even though the keep-alive mechanisms are separated from
   the rest of the SIP Outbound mechanism, SIP Outbound does not define
   a mechanism to explicitly negotiate usage of the keep-alive
   mechanisms, since
   mechanisms.  In some cases usage of keep-alives in most cases are can be implicitly
   negotiated as part of the SIP Outbound negotiation.

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

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

   The following sections describe use-cases where a mechanism to
   explicitly negotiate 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 in order to maintain NAT
   bindings open during the session it still needs to be able to
   negotiate sending of keep-alives towards its adjacent upstream SIP
   entity.  A typical example is an emergency call, where a registration
   is not always required in order to make the call.

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 might not support SIP Outbound.  However, they
   might still support the keep-alive mechanisms defined in SIP
   Outbound, and need to be able to negotiate usage of them.

1.3.  Use-case: SIP dialog initiated Outbound flows

   SIP Outbound allows the establishment of flows using the initial
   request for a dialog.  As specified in [RFC5626], usage keep-alives of keep-
   alives is not implicitly negotiated for such flows, why usage needs to be
   explicitly negotiated. flows.

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 towards it its adjacent upstream
   downstream SIP entity.  If a  A SIP entity is willing to receive keep-alives from
   its adjacent downstream SIP entity, and can also insert the Via header field
   associated with the adjacent downstream SIP entity contains
   in a "keep"
   parameter, the SIP entity willing to receive keep-alives can add an
   integer parameter value to that "keep" parameter.  The integer
   parameter value can be used response to explicitly indicate a recommended keep-alive
   frequency.  A zero value indicates that the SIP entity is willing willingness to receive keep-alives, but that a recommended keep-alive frequency is
   not provided. keep-
   alives from its adjacent upstream SIP entity.

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

   Adjacent downstream SIP entity: The adjacent SIP entity in the
   direction towards which a SIP request is sent.

   Adjacent upstream SIP entity: The adjacent SIP entity in the
   direction from which a SIP request is received.

4.  User Agent and Proxy behavior

4.1.  General

   This section describes how SIP UAs and proxies negotiate usage of
   keep-alives associated with a registration, or a dialog, which types
   of SIP requests types can be used in order to negotiate the usage, and the
   lifetime of the negotiated keep-alives.

   SIP entities indicate willingness to send keep-alives towards the
   adjacent upstream downstream SIP entity using SIP requests.  The associated
   responses are used by SIP entities to indicate willingness to receive
   keep-alives.  SIP entities that indicate willingness to receive keep-
   alives can provide a recommended keep-alive frequency.

   The procedures to negotiate usage of keep-alives are identical for
   SIP UAs and proxies.

   NOTE: Usage of keep-alives is negotiated per direction.  If a SIP
   entity has indicated willingness to receive keep-alives from its an
   adjacent downstream SIP entity, sending of keep-alives towards the same SIP
   entity needs to be separately negotiated.

   NOTE: Since there are SIP entities that already use a combination of
   Carriage Return and Line Feed (CRLF) as keep-alive messages, and SIP
   entities are expected to be able to receive those, this specification
   does not forbid the sending of CRLF keep-alive messages towards an
   adjacent upstream SIP entity even if usage of keep-alives have with that SIP entity
   has not been negotiated.  However, the "keep" parameter is still
   important in order for a SIP entity to indicate that it supports
   sending of CRLF keep-alive messages, so that the adjacent upstream downstream
   SIP entity does not use other mechanisms (e.g. short registration
   refresh intervals) in order to keep NAT bindings open.

4.2.  Lifetime of keep-alives

4.2.1.  General

   The lifetime of negotiated keep-alives depends on whether the keep-
   alives are associated with a registration or a dialog.  The  This section
   describes the lifetime of negotiated keep-alives.

4.2.2.  Keep-alives associated with registration

   SIP entities use a registration request in order to negotiate usage
   of keep-alives associated with a registration.  Usage of keep-alives
   can be negotiated when the registration is established, or later
   during the lifetime of the registration.  Once negotiated, keep-
   alives are sent until the registration is terminated, or until a
   subsequent registration refresh request is sent or forwarded.  When a
   subsequent registration refresh request is sent or forwarded, if a
   SIP entity is willing to continue sending keep-alives associated with
   the registration, usage of keep-alives MUST be re-negotiated.  If
   usage is not successfully re-negotiated, the SIP entity MUST cease
   sending of keep-alives associated with the registration.

   In case a SIP entity establishes multiple registration flows
   [RFC5626], usage of keep-alives needs to be negotiated separately for
   each individual registration flow.  A SIP entity MUST NOT send keep-
   alives associated with a registration flow for which usage of keep-
   alives has not been negotiated.

4.2.3.  Keep-alives associated with dialog

   SIP entities use an initial request for a dialog, or a mid-dialog
   target refresh request [RFC3261], in order to negotiate sending and
   receiving of keep-alives associated with a dialog.  Usage of keep-
   alives can be negotiated when the dialog is established, or later
   during the lifetime of the dialog.  Once negotiated, keep-alives MUST
   be sent for the lifetime of the dialog, until the dialog is
   terminated.  Once usage of keep-alives associated with a dialog has
   been negotiated, it is not possible to re-negotiate the usage
   associated with the dialog.

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

   As defined in [RFC5626], a SIP entity that supports sending of keep-
   alives must act as a Session Traversal Utilities for NAT (STUN)
   client [RFC5389].  The SIP entity must support the amount those aspects of STUN
   which is
   that are required in order 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 for the lifetime of a
   registration (in case of a REGISTER request),
   registration, or a dialog (in case of
   an initial request for a dialog, or a mid-dialog target refresh
   request), it MUST insert a "keep" parameter in its
   Via header field of the request. request to indicate willingness to send keep-
   alives.

   When the SIP entity receives the associated response, if the "keep"
   parameter in its Via header field of the response contains a "keep"
   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 requests (if the keep-
   alive negotiation is for a dialog).  Subsequent mid-dialog requests
   are addressed based on the dialog route set.

   Once a SIP entity has negotiated sending of keep-alives associated
   with a dialog towards its an adjacent upstream SIP entity, it MUST NOT insert a
   "keep" parameter in any subsequent SIP requests requests, associated with the dialog.
   dialog, towards that adjacent SIP entity.  Such "keep" parameter MUST
   be ignored, if received.

   Since an ACK request does not have an associated response, it can not
   be used to negotiate usage of keep-alives.  Therefore, a SIP entity
   MUST NOT insert a "keep" parameter in its Via header field of an ACK
   request.  Such "keep" parameter MUST be ignored, if received.

   When a SIP entity is about to send a keep-alive, if the

   A SIP entity at
   the same time is also about MUST NOT indicates willingness to send or forward a SIP request keep-alives
   associated with the same registration or a 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 unless it has also inserted itself in the same keep-alive action.
   dialog route set [RFC3261].

   NOTE: When a SIP entity sends an initial request for a dialog, if the
   adjacent upstream downstream SIP entity does not insert itself in the dialog
   route set using a Record-Route header field [RFC3261], the adjacent
   upstream
   downstream 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 parameter
   value, the SIP entity might choose to insert a "keep" parameter in
   its Via header field of a subsequent SIP request associated with the
   dialog, in case the new adjacent SIP downstream entity (based on the
   dialog route set) is willing to receive keep-alives (in which case it
   will add a 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, request is used to indicate willingness to send keep-
   alives, as long as at least one of response (provisional or final) to
   the responses, per dialog, INVITE request contains a "keep" parameter with a parameter value
   value, it is seen as an indication that the adjacent
   upstream downstream SIP
   entity is willing to receive keep-alives associated with the dialog
   on which the response is received.

   A SIP entity that has negotiated sending of keep-alives associated
   with a registration MUST be prepared to receive, from the receiver of
   the keep-alives, SIP messages (requests and responses) associated
   with the registration on the same IP port:address from where the
   keep-alives are sent.  If the SIP entity sends the keep-alives over a
   connection-oriented transport protocol, it MUST be prepared to
   receive SIP messages associated with the registration over the same
   connection.  The same rules apply for SIP messages associated with a
   dialog, if the SIP entity has negotiated sending of keep-alives
   associated with a 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 those aspects of STUN which is that are required in order to apply the
   STUN keep-
   alive keep-alive mechanism defined in [RFC5626], and it must support
   the CRLF keep-alive mechanism defined in [RFC5626].

   When a SIP entity creates sends or receives a response to a request that
   can be used in order to indicate willingness to send keep-alives
   associated with forwards a registration or dialog, response, and the Via header field
   associated with the adjacent downstream SIP entity (that is, the top-
   most Via header field once a
   upstream SIP entity that received the response
   has removed its own Via header field from the response) contains a
   "keep" parameter, indicated willingness to send keep-alives, if the
   SIP entity is willing to receive keep-alives associated with the registration (in case of a REGISTER response),
   registration, or the dialog (in case of a response to an initial request for a dialog,
   or a response to a mid-dialog target refresh request), from the adjacent downstream upstream SIP entity it
   MUST add a parameter value to the "keep" parameter, before sending or
   forwarding the response.  The parameter can contain a recommended
   keep-alive frequency, or a zero value.

   When a SIP entity indicates willingness to receive keep-alives in a
   response to an INVITE request, it MUST insert a "keep" parameter in
   at least one reliable response to the request.  The SIP entity MAY
   insert an identical "keep" parameter value in other responses to the
   same request.  The SIP entity MUST NOT insert "keep" parameters with
   differing values in responses to a single INVITE request.  The SIP
   entity SHOULD indicate the willingness to receive keep-alives as soon
   as possible.

   A SIP entity MUST NOT indicates willingness to receive keep-alives
   associated with a dialog, unless it has also inserted itself in the
   dialog route set [RFC3261].

   A SIP entity that has negotiated receiving of keep-alives associated
   with a registration MUST send, towards the sender of the keep-alives,
   SIP messages (requests and responses) associated with the
   registration using the IP address:port from where the keep-alives are
   received.  If the SIP entity receives the keep-alives over a
   connection-oriented transport protocol, it MUST send SIP messages
   associated with the registration over the same connection, rather
   than creating a new connection.  The same rules apply for SIP
   messages associated with a dialog, if the SIP entity has negotiated
   receiving of keep-alives associated with a dialog.

5.  Keep-alive frequency

   If a SIP entity receives a SIP response, where its Via header field
   contains a "keep" parameter with a non-zero value that indicates a
   recommended keep-alive frequency, it MUST use the procedures defined
   for the Flow-Timer header field [RFC5626].  According to the
   procedures, the SIP entity must send keep-alives at least as often as
   the indicated recommended keep-alive frequency, and if the SIP entity
   uses the recommended keep-alive frequency then it should send its
   keep-alives so that the interval between each keep-alive is randomly
   distributed between 80% and 100% of the recommended keep-alive
   frequency.

   If the received "keep" parameter value is zero, the SIP entity can
   send keep-alives at its discretion.  [RFC5626] provides additional
   guidance on selecting the keep-alive frequency in case a recommended
   keep-alive frequency is not provided.

   A

   If a SIP entity that uses the "keep" parameter to indicate willingness
   to receive keep-alives to indicate
   willingness to receive keep-alives also inserts a Flow-Timer header
   field (that can happen if the SIP entity is using both the Outbound
   mechanism and the keep-alive mechanism) in the same SIP message, the
   header field value and the "keep" parameter value MUST NOT use the Flow-Timer header field in
   order to provide a recommended keep-alive frequency. be identical.

   SIP Outbound uses the Flow-Timer header field to indicate the server-
   recommended keep-alive frequency.  However, it will only be sent
   between a UA and an edge proxy.  Using the "keep" parameter, however,
   the sending and receiving of keep-alives might be negotiated between
   multiple entities on the signalling path.  Since  In addition, since the server-
   recommended
   server-recommended keep-alive frequency might vary between different
   SIP entities, those would have to re-write the Flow-Timer header field
   value.  In addition, if a SIP entity does not indicate willingness to
   receive keep-alives from its adjacent downstream SIP entity, and
   receives a Flow-Timer header field in a response, it would have to
   remove the single Flow-Timer header field from the response.  This issue
   does not exist for the "keep" parameter, as each SIP entity has its
   own individual Via header field.

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 be used 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 all the
   entity supports keep-alives in order different frequency values, without forcing entities
   to keep re-write the connection open, no
   inference can be drawn on whether that connection can be used for
   requests in value of the backwards direction.

7. Flow-Timer header field.

6.  Examples

7.1.

6.1.  General

   This section shows example flows where usage of keep-alives,
   associated with a registration and a dialog, is negotiated between
   different SIP entities.

7.2.

6.2.  Keep-alive negotiation associated with registration: UA-proxy

   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) forwards the request towards the registrar.

   P1 is willing to receive keep-alives from Alice for the duration of
   the registration, so When P1 receives the associated response it adds
   a keep parameter value, which indicates a recommended keep-alive
   frequency of 30 seconds, to Alice's Via header field, 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 associated with the
   registration.  For the lifetime of the registration, Alice then sends
   periodic keep-alives (in this example using the STUN keep-alive
   technique) towards P1, using the recommended keep-alive frequency
   indicated by the keep parameter value.

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

                        Figure 1: Example call flow

7.3.

6.3.  Keep-alive negotiation associated with dialog: UA-proxy

   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, before it forwards the
   request towards Bob.

   P1 is willing to receive keep-alives from Alice for the duration of
   the dialog, so When P1 receives the associated response it adds a
   keep parameter value, which indicates a recommended keep-alive
   frequency of 30 seconds, to Alice's Via header field, 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 associated with the
   dialog.  For the lifetime of the dialog, Alice then sends periodic
   keep-alives (in this example using the STUN keep-alive technique)
   towards P1, using the recommended keep-alive frequency indicated by
   the keep parameter value.

     Alice                        P1                         Bob
       |                          |                           |
       |--- INVITE -------------->|                           |
       |    Via: Alice;keep       |                           |
       |                          |--- INVITE --------------->|
       |                          |    Via: P1                |
       |                          |    Via: Alice;keep        |
       |                          |    Record-Route: P1       |
       |                          |                           |
       |                          |<-- 200 OK ----------------|
       |                          |    Via: P1                |
       |                          |    Via: Alice;keep        |
       |                          |    Record-Route: P1       |
       |<-- 200 OK ---------------|                           |
       |    Alice: UAC;keep=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

7.4.

6.4.  Keep-alive negotiation associated with dialog: UA-UA

   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, before it
   forwards the request towards Bob. .

   When Alice receives the response, she determines from her Via header
   field that P1 is not willing to receive keep-alives associated with
   the dialog 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 associated with the
   dialog from Alice, so he creates a response and adds a keep parameter
   value, which indicates a recommended keep-alive frequency of 30
   seconds, to Alice's Via header field, before he 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 associated with the
   dialog.  For the lifetime 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 by
   the keep parameter value.

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

                        Figure 3: Example call flow

8.

7.  Grammar

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

   The ABNF [RFC5234] is:

   via-params =/ keep

   keep       = "keep" [ EQUAL 1*(DIGIT) ]

9.

8.  IANA Considerations

9.1.

8.1.  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. 7.  The required information is:

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

10.

9.  Security Considerations

   This specification does not introduce security considerations in
   addition to those specified in [RFC5626].

11.

10.  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 Willis, John Elwell and Paul
   Kyzivat for their comments on the list.  Thanks to Vijay Gurbani for
   providing text (later removed) about the relationship with the
   connect-reuse specification.

12.  References

12.1.

11.  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.

   [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