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Versions: (draft-jennings-sipping-outbound) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 RFC 5626

Network Working Group                                   C. Jennings, Ed.
Internet-Draft                                             Cisco Systems
Updates:  3261,3327                                         R. Mahy, Ed.
(if approved)                                                Plantronics
Intended status:  Standards Track                          June 25, 2007
Expires:  December 27, 2007


Managing Client Initiated Connections in the Session Initiation Protocol
                                 (SIP)
                       draft-ietf-sip-outbound-09

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on December 27, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Session Initiation Protocol (SIP) allows proxy servers to
   initiate TCP connections and send asynchronous UDP datagrams to User
   Agents in order to deliver requests.  However, many practical
   considerations, such as the existence of firewalls and Network
   Address Translators (NATs), prevent servers from connecting to User



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   Agents in this way.  This specification defines behaviors for User
   Agents, registrars and proxy servers that allow requests to be
   delivered on existing connections established by the User Agent.  It
   also defines keep alive behaviors needed to keep NAT bindings open
   and specifies the usage of multiple connections.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Summary of Mechanism . . . . . . . . . . . . . . . . . . .  5
     3.2.  Single Registrar and UA  . . . . . . . . . . . . . . . . .  6
     3.3.  Multiple Connections from a User Agent . . . . . . . . . .  7
     3.4.  Edge Proxies . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Keepalive Technique  . . . . . . . . . . . . . . . . . . . 11
       3.5.1.  CRLF Keepalive Technique . . . . . . . . . . . . . . . 11
       3.5.2.  TCP Keepalive Technique  . . . . . . . . . . . . . . . 12
       3.5.3.  STUN Keepalive Technique . . . . . . . . . . . . . . . 12
   4.  User Agent Mechanisms  . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Instance ID Creation . . . . . . . . . . . . . . . . . . . 13
     4.2.  Registrations  . . . . . . . . . . . . . . . . . . . . . . 14
       4.2.1.  Registration by Other Instances  . . . . . . . . . . . 16
     4.3.  Sending Requests . . . . . . . . . . . . . . . . . . . . . 16
     4.4.  Detecting Flow Failure . . . . . . . . . . . . . . . . . . 17
       4.4.1.  Keepalive with TCP KEEPALIVE . . . . . . . . . . . . . 18
       4.4.2.  Keepalive with CRLF  . . . . . . . . . . . . . . . . . 18
       4.4.3.  Keepalive with STUN  . . . . . . . . . . . . . . . . . 18
     4.5.  Flow Recovery  . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Edge Proxy Mechanisms  . . . . . . . . . . . . . . . . . . . . 20
     5.1.  Processing Register Requests . . . . . . . . . . . . . . . 20
     5.2.  Generating Flow Tokens . . . . . . . . . . . . . . . . . . 20
     5.3.  Forwarding Requests  . . . . . . . . . . . . . . . . . . . 21
     5.4.  Edge Proxy Keepalive Handling  . . . . . . . . . . . . . . 22
   6.  Registrar Mechanisms: Processing REGISTER Requests . . . . . . 22
   7.  Authoritative Proxy Mechanisms: Forwarding Requests  . . . . . 24
   8.  STUN Keepalive Processing  . . . . . . . . . . . . . . . . . . 24
     8.1.  Use with Sigcomp . . . . . . . . . . . . . . . . . . . . . 26
   9.  Example Message Flow . . . . . . . . . . . . . . . . . . . . . 26
   10. Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   11. Definition of 430 Flow Failed response code  . . . . . . . . . 30
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
     12.1. Contact Header Field . . . . . . . . . . . . . . . . . . . 30
     12.2. SIP/SIPS URI Parameters  . . . . . . . . . . . . . . . . . 31
     12.3. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 31
     12.4. Response Code  . . . . . . . . . . . . . . . . . . . . . . 31



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     12.5. Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 31
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   14. Operational Notes on Transports  . . . . . . . . . . . . . . . 33
   15. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 34
   16. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     16.1. Changes from 08 Version  . . . . . . . . . . . . . . . . . 34
     16.2. Changes from 07 Version  . . . . . . . . . . . . . . . . . 34
     16.3. Changes from 06 Version  . . . . . . . . . . . . . . . . . 35
     16.4. Changes from 05 Version  . . . . . . . . . . . . . . . . . 35
     16.5. Changes from 04 Version  . . . . . . . . . . . . . . . . . 35
     16.6. Changes from 03 Version  . . . . . . . . . . . . . . . . . 36
     16.7. Changes from 02 Version  . . . . . . . . . . . . . . . . . 37
     16.8. Changes from 01 Version  . . . . . . . . . . . . . . . . . 37
     16.9. Changes from 00 Version  . . . . . . . . . . . . . . . . . 38
   17. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 38
   Appendix A.  Default Flow Registration Backoff Times . . . . . . . 38
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 39
     18.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 42






























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1.  Introduction

   There are many environments for SIP [1] deployments in which the User
   Agent (UA) can form a connection to a Registrar or Proxy but in which
   connections in the reverse direction to the UA are not possible.
   This can happen for several reasons.  Connections to the UA can be
   blocked by a firewall device between the UA and the proxy or
   registrar, which will only allow new connections in the direction of
   the UA to the Proxy.  Similarly a NAT could be present, which is only
   capable of allowing new connections from the private address side to
   the public side.  This specification allows SIP registration when the
   UA is behind such a firewall or NAT.

   Most IP phones and personal computers get their network
   configurations dynamically via a protocol such as DHCP (Dynamic Host
   Configuration Protocol).  These systems typically do not have a
   useful name in the Domain Name System (DNS), and they almost never
   have a long-term, stable DNS name that is appropriate for use in the
   subjectAltName of a certificate, as required by [1].  However, these
   systems can still act as a TLS client and form connections to a proxy
   or registrar which authenticates with a server certificate.  The
   server can authenticate the UA using a shared secret in a digest
   challenge over that TLS connection.

   The key idea of this specification is that when a UA sends a REGISTER
   request, the proxy can later use this same network "flow"--whether
   this is a bidirectional stream of UDP datagrams, a TCP connection, or
   an analogous concept of another transport protocol--to forward any
   requests that need to go to this UA.  For a UA to receive incoming
   requests, the UA has to connect to a server.  Since the server can't
   connect to the UA, the UA has to make sure that a flow is always
   active.  This requires the UA to detect when a flow fails.  Since
   such detection takes time and leaves a window of opportunity for
   missed incoming requests, this mechanism allows the UA to use
   multiple flows to the proxy or registrar.  This specification also
   defines multiple keepalive schemes.  The keepalive mechanism is used
   to keep NAT bindings fresh, and to allow the UA to detect when a flow
   has failed.


2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].






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2.1.  Definitions

   Authoritative Proxy:  A proxy that handles non-REGISTER requests for
      a specific Address-of-Record (AOR), performs the logical Location
      Server lookup described in RFC 3261, and forwards those requests
      to specific Contact URIs.
   Edge Proxy:  An Edge Proxy is any proxy that is located topologically
      between the registering User Agent and the Authoritative Proxy.
   Flow:  A Flow is a network protocol layer (layer 4) association
      between two hosts that is represented by the network address and
      port number of both ends and by the protocol.  For TCP, a flow is
      equivalent to a TCP connection.  For UDP a flow is a bidirectional
      stream of datagrams between a single pair of IP addresses and
      ports of both peers.  With TCP, a flow often has a one to one
      correspondence with a single file descriptor in the operating
      system.
   reg-id:  This refers to the value of a new header field parameter
      value for the Contact header field.  When a UA registers multiple
      times, each simultaneous registration gets a unique reg-id value.
   instance-id:  This specification uses the word instance-id to refer
      to the value of the "sip.instance" media feature tag in the
      Contact header field.  This is a Uniform Resource Name (URN) that
      uniquely identifies this specific UA instance.
   outbound-proxy-set  A set of SIP URIs (Uniform Resource Identifiers)
      that represents each of the outbound proxies (often Edge Proxies)
      with which the UA will attempt to maintain a direct flow.  The
      first URI in the set is often referred to as the primary outbound
      proxy and the second as the secondary outbound proxy.  There is no
      difference between any of the URIs in this set, nor does the
      primary/secondary terminology imply that one is preferred over the
      other.


3.  Overview

   Several scenarios in which this technique is useful are discussed
   below, including the simple co-located registrar and proxy, a User
   Agent desiring multiple connections to a resource (for redundancy,
   for example), and a system that uses Edge Proxies.

3.1.  Summary of Mechanism

   The overall approach is fairly simple.  Each UA has a unique
   instance-id that stays the same for this UA even if the UA reboots or
   is power cycled.  Each UA can register multiple times over different
   connections for the same SIP Address of Record (AOR) to achieve high
   reliability.  Each registration includes the instance-id for the UA
   and a reg-id label that is different for each flow.  The registrar



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   can use the instance-id to recognize that two different registrations
   both reach the same UA.  The registrar can use the reg-id label to
   recognize that a UA is registering after a reboot or a network
   failure.

   When a proxy goes to route a message to a UA for which it has a
   binding, it can use any one of the flows on which a successful
   registration has been completed.  A failure on a particular flow can
   be tried again on an alternate flow.  Proxies can determine which
   flows go to the same UA by comparing the instance-id.  Proxies can
   tell that a flow replaces a previously abandoned flow by looking at
   the reg-id.

   UAs can use a simple periodic message as a keepalive mechanism to
   keep their flow to the proxy or registrar alive.  For connection
   oriented transports such as TCP this is based on CRLF or a transport
   specific keepalive while for transports that are not connection
   oriented this is accomplished by using the keepalive usage profile of
   STUN (Session Traversal Utilities for NAT) [3].

3.2.  Single Registrar and UA

   In the topology shown below, a single server is acting as both a
   registrar and proxy.

      +-----------+
      | Registrar |
      | Proxy     |
      +-----+-----+
            |
            |
       +----+--+
       | User  |
       | Agent |
       +-------+

   User Agents which form only a single flow continue to register
   normally but include the instance-id as described in Section 4.1.
   The UA can also include a reg-id parameter which is used to allow the
   registrar to detect and avoid keeping invalid contacts when a UA
   reboots or reconnects after its old connection has failed for some
   reason.

   For clarity, here is an example.  Bob's UA creates a new TCP flow to
   the registrar and sends the following REGISTER request.






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   REGISTER sip:example.com;keep-crlf SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.1;branch=z9hG4bK-bad0ce-11-1036
   Max-Forwards: 70
   From: Bob <sip:bob@example.com>;tag=d879h76
   To: Bob <sip:bob@example.com>
   Call-ID: 8921348ju72je840.204
   CSeq: 1 REGISTER
   Supported: path
   Contact: <sip:line1@192.168.0.2>; reg-id=1;
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-000A95A0E128>"
   Content-Length: 0

   The registrar challenges this registration to authenticate Bob. When
   the registrar adds an entry for this contact under the AOR for Bob,
   the registrar also keeps track of the connection over which it
   received this registration.

   The registrar saves the instance-id
   ("urn:uuid:00000000-0000-1000-8000-000A95A0E128") and reg-id ("1")
   along with the rest of the Contact header field.  If the instance-id
   and reg-id are the same as a previous registration for the same AOR,
   the registrar replaces the old Contact URI and flow information.
   This allows a UA that has rebooted to replace its previous
   registration for each flow with minimal impact on overall system
   load.

   When Alice sends a request to Bob, his authoritative proxy selects
   the target set.  The proxy forwards the request to elements in the
   target set based on the proxy's policy.  The proxy looks at the
   target set and uses the instance-id to understand if two targets both
   end up routing to the same UA.  When the proxy goes to forward a
   request to a given target, it looks and finds the flows over which it
   received the registration.  The proxy then forwards the request on
   that flow, instead of resolving the Contact URI using the procedures
   in RFC 3263 [4] and trying to form a new flow to that contact.  This
   allows the proxy to forward a request to a particular contact over
   the same flow that the UA used to register this AOR.  If the proxy
   has multiple flows that all go to this UA, it can choose any one of
   the registration bindings for this AOR that has the same instance-id
   as the selected UA.

3.3.  Multiple Connections from a User Agent

   There are various ways to deploy SIP to build a reliable and scalable
   system.  This section discusses one such design that is possible with
   the mechanisms in this specification.  Other designs are also
   possible.




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   In the example system below, the logical outbound proxy/registrar for
   the domain is running on two hosts that share the appropriate state
   and can both provide registrar and outbound proxy functionality for
   the domain.  The UA will form connections to two of the physical
   hosts that can perform the outbound proxy/registrar function for the
   domain.  Reliability is achieved by having the UA form two TCP
   connections to the domain.

   Scalability is achieved by using DNS SRV to load balance the primary
   connection across a set of machines that can service the primary
   connection, and also using DNS SRV to load balance across a separate
   set of machines that can service the secondary connection.  The
   deployment here requires that DNS is configured with one entry that
   resolves to all the primary hosts and another entry that resolves to
   all the secondary hosts.  While this introduces additional DNS
   configuration, the approach works and requires no additional SIP
   extensions.

      Note:  Approaches which select multiple connections from a single
      DNS SRV set were also considered, but cannot prevent two
      connections from accidentally resolving to the same host.  The
      approach in this document does not prevent future extensions, such
      as the SIP UA configuration framework [19], from adding other ways
      for a User Agent to discover its outbound-proxy-set.

       +-------------------+
       | Domain            |
       | Logical Proxy/Reg |
       |                   |
       |+-----+     +-----+|
       ||Host1|     |Host2||
       |+-----+     +-----+|
       +---\------------/--+
            \          /
             \        /
              \      /
               \    /
              +------+
              | User |
              | Agent|
              +------+

   The UA is configured with multiple outbound proxy registration URIs.
   These URIs are configured into the UA through whatever the normal
   mechanism is to configure the proxy or registrar address in the UA.
   If the AOR is Alice@example.com, the outbound-proxy-set might look
   something like "sip:primary.example.com;keep-stun" and "sip:
   secondary.example.com;keep-stun".  The "keep-stun" tag indicates that



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   a SIP server supports STUN and SIP multiplexed over the same flow, as
   described later in this specification.  Note that each URI in the
   outbound-proxy-set could resolve to several different physical hosts.
   The administrative domain that created these URIs should ensure that
   the two URIs resolve to separate hosts.  These URIs are handled
   according to normal SIP processing rules, so mechanisms like SRV can
   be used to do load balancing across a proxy farm.

   The domain also needs to ensure that a request for the UA sent to
   host1 or host2 is then sent across the appropriate flow to the UA.
   The domain might choose to use the Path header approach (as described
   in the next section) to store this internal routing information on
   host1 or host2.

   When a single server fails, all the UAs that have a flow through it
   will detect a flow failure and try to reconnect.  This can cause
   large loads on the server.  When large numbers of hosts reconnect
   nearly simultaneously, this is referred to as the avalanche restart
   problem, and is further discussed in Section 4.5.  The multiple flows
   to many servers help reduce the load caused by the avalanche restart.
   If a UA has multiple flows, and one of the servers fails, the UA
   delays the specified time before trying to form a new connection to
   replace the flow to the server that failed.  By spreading out the
   time used for all the UAs to reconnect to a server, the load on the
   server farm is reduced.

   When used in this fashion to achieve high reliability, the operator
   will need to configure DNS such that the various URIs in the outbound
   proxy set do not resolve to the same host.

   Another motivation for maintaining multiple flows between the UA and
   its registrar is related to multihomed UAs.  Such UAs can benefit
   from multiple connections from different interfaces to protect
   against the failure of an individual access link.

3.4.  Edge Proxies

   Some SIP deployments use edge proxies such that the UA sends the
   REGISTER to an Edge Proxy that then forwards the REGISTER to the
   Registrar.  The Edge Proxy includes a Path header [5] so that when
   the registrar later forwards a request to this UA, the request is
   routed through the Edge Proxy.  There could be a NAT or firewall
   between the UA and the Edge Proxy.








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                +---------+
                |Registrar|
                |Proxy    |
                +---------+
                 /      \
                /        \
               /          \
            +-----+     +-----+
            |Edge1|     |Edge2|
            +-----+     +-----+
               \           /
                \         /
        ----------------------------NAT/FW
                  \     /
                   \   /
                  +------+
                  |User  |
                  |Agent |
                  +------+

   These systems can use effectively the same mechanism as described in
   the previous sections but need to use the Path header.  When the Edge
   Proxy receives a registration, it needs to create an identifier value
   that is unique to this flow (and not a subsequent flow with the same
   addresses) and put this identifier in the Path header URI.  This
   identifier has two purposes.  First, it allows the Edge Proxy to map
   future requests back to the correct flow.  Second, because the
   identifier will only be returned if the user authentication with the
   registrar succeeds, it allows the Edge Proxy to indirectly check the
   user's authentication information via the registrar.  The identifier
   is placed in the user portion of a loose route in the Path header.
   If the registration succeeds, the Edge Proxy needs to map future
   requests that are routed to the identifier value from the Path
   header, to the associated flow.

   The term Edge Proxy is often used to refer to deployments where the
   Edge Proxy is in the same administrative domain as the Registrar.
   However, in this specification we use the term to refer to any proxy
   between the UA and the Registrar.  For example the Edge Proxy may be
   inside an enterprise that requires its use and the registrar could be
   from a service provider with no relationship to the enterprise.
   Regardless if they are in the same administrative domain, this
   specification requires that Registrars and Edge proxies support the
   Path header mechanism in RFC 3327 [5].







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3.5.  Keepalive Technique

   This document describes three keepalive mechanisms.  Each of these
   mechanisms uses a client-to-server "ping" keepalive and a
   corresponding server-to-client "pong" message.  This ping-pong
   sequence allows the client, and optionally the server, to tell if its
   flow is still active and useful for SIP traffic.  The server responds
   to pings by sending pongs.  If the client does not receive a pong in
   response to its ping, it declares the flow dead and opens a new flow
   in its place.

   This document also suggests timer values for two of these client
   keepalive mechanisms.  These timer values were chosen to keep most
   NAT and firewall bindings open, to detect unresponsive servers within
   2 minutes, and to prevent the avalanche restart problem.  However,
   the client may choose different timer values to suit its needs, for
   example to optimize battery life.  In some environments, the server
   can also keep track of the time since a ping was received over a flow
   to guess the likelihood that the flow is still useful for delivering
   SIP messages.  In this case, the server provides an indicator (the
   'timed-keepalives' parameter) that the server requires the client to
   use the suggested timer values.

   When the UA detects that a flow has failed or that the flow
   definition has changed, the UA needs to re-register and will use the
   back-off mechanism described in Section 4 to provide congestion
   relief when a large number of agents simultaneously reboot.

   A keepalive mechanism needs to keep NAT bindings refreshed; for
   connections, it also needs to detect failure of a connection; and for
   connectionless transports, it needs to detect flow failures including
   changes to the NAT public mapping.  For connection oriented
   transports such as TCP and SCTP, this specification describes a
   keepalive approach based on sending CRLFs, and for TCP, a usage of
   TCP transport-layer keepalives.  For connectionless transport, such
   as UDP, this specification describes using STUN [3] over the same
   flow as the SIP traffic to perform the keepalive.

3.5.1.  CRLF Keepalive Technique

   This approach can only be used with connection-oriented transports
   such as TCP or SCTP.  The client periodically sends a double-CRLF
   (the "ping") then waits to receive a single CRLF (the "pong").  If
   the client does not receive a "pong" within an appropriate amount of
   time, it considers the flow failed.






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3.5.2.  TCP Keepalive Technique

   This approach can only be used when the transport protocol is TCP.

   User Agents that are capable of generating per-connection TCP
   keepalives can use TCP keepalives.  When using this approach the
   values of the keepalive timer are left to the client.  Servers cannot
   make any assumption about what values are used.

      Note:  when TCP is being used, it's natural to think of using TCP
      KEEPALIVE.  Unfortunately, many operating systems and programming
      environments do not allow the keepalive time to be set on a per-
      connection basis.  Thus, applications may not be able to set an
      appropriate time.

3.5.3.  STUN Keepalive Technique

   This technique can only be used for transports, such as UDP, that are
   not connection oriented.

   For connection-less transports, a flow definition could change
   because a NAT device in the network path reboots and the resulting
   public IP address or port mapping for the UA changes.  To detect
   this, STUN requests are sent over the same flow that is being used
   for the SIP traffic.  The proxy or registrar acts as a Session
   Traversal Utilities for NAT (STUN) [3] server on the SIP signaling
   port.

      Note:  The STUN mechanism is very robust and allows the detection
      of a changed IP address.  Many other options were considered, but
      the SIP Working Group selected the STUN-based approach.
      Approaches using SIP requests were abandoned because to achieve
      the required performance, the server needs to deviate from the SIP
      specification in significant ways.  This would result in many
      undesirable and non-deterministic behaviors in some environments.
      Another approach considered to detect a changed flow was using
      OPTIONS messages and the rport parameter.  Although the OPTIONS
      approach has the advantage of being backwards compatible, it also
      significantly increases the load on the proxy or registrar server.
      Related to this idea was an idea of creating a new SIP PING method
      that was like OPTIONS but faster.  It would be critical that this
      PING method did not violate the processing requirements of a
      proxies and UAS so it was never clear how it would be
      significantly faster than OPTIONS given it would still have to
      obey things like checking the Proxy-Require header.  After
      considerable consideration the working group came to some
      consensus that the STUN approach was a better solution than these
      alternative designs.



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4.  User Agent Mechanisms

4.1.  Instance ID Creation

   Each UA MUST have an Instance Identifier URN that uniquely identifies
   the device.  Usage of a URN provides a persistent and unique name for
   the UA instance.  It also provides an easy way to guarantee
   uniqueness within the AOR.  This URN MUST be persistent across power
   cycles of the device.  The Instance ID MUST NOT change as the device
   moves from one network to another.

   A UA SHOULD create a UUID URN [6] as its instance-id.  The UUID URN
   allows for non-centralized computation of a URN based on time, unique
   names (such as a MAC address), or a random number generator.

      A device like a soft-phone, when first installed, can generate a
      UUID [6] and then save this in persistent storage for all future
      use.  For a device such as a hard phone, which will only ever have
      a single SIP UA present, the UUID can include the MAC address and
      be generated at any time because it is guaranteed that no other
      UUID is being generated at the same time on that physical device.
      This means the value of the time component of the UUID can be
      arbitrarily selected to be any time less than the time when the
      device was manufactured.  A time of 0 (as shown in the example in
      Section 3.2) is perfectly legal as long as the device knows no
      other UUIDs were generated at this time.

   If a URN scheme other than UUID is used, the URN MUST be selected
   such that the instance can be certain that no other instance
   registering against the same AOR would choose the same URN value.  An
   example of a URN that would not meet the requirements of this
   specification is the national bibliographic number [20].  Since there
   is no clear relationship between a SIP UA instance and a URN in this
   namespace, there is no way a selection of a value can be performed
   that guarantees that another UA instance doesn't choose the same
   value.

   To convey its instance-id in both requests and responses, the UA
   includes a "sip.instance" media feature tag as a UA characteristic
   [7] .  As described in [7], this media feature tag will be encoded in
   the Contact header field as the "+sip.instance" Contact header field
   parameter.  The value of this parameter MUST be a URN [8].  One case
   where a UA may not want to include the URN in the sip.instance media
   feature tag is when it is making an anonymous request or some other
   privacy concern requires that the UA not reveal its identity.






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      RFC 3840 [7] defines equality rules for callee capabilities
      parameters, and according to that specification, the
      "sip.instance" media feature tag will be compared by case-
      sensitive string comparison.  This means that the URN will be
      encapsulated by angle brackets ("<" and ">") when it is placed
      within the quoted string value of the +sip.instance Contact header
      field parameter.  The case-sensitive matching rules apply only to
      the generic usages defined in RFC 3840 [7] and in the caller
      preferences specification [9].  When the instance ID is used in
      this specification, it is effectively "extracted" from the value
      in the "sip.instance" media feature tag.  Thus, equality
      comparisons are performed using the rules for URN equality that
      are specific to the scheme in the URN.  If the element performing
      the comparisons does not understand the URN scheme, it performs
      the comparisons using the lexical equality rules defined in RFC
      2141 [8].  Lexical equality could result in two URNs being
      considered unequal when they are actually equal.  In this specific
      usage of URNs, the only element which provides the URN is the SIP
      UA instance identified by that URN.  As a result, the UA instance
      SHOULD provide lexically equivalent URNs in each registration it
      generates.  This is likely to be normal behavior in any case;
      clients are not likely to modify the value of the instance ID so
      that it remains functionally equivalent yet lexicographically
      different from previous registrations.

4.2.  Registrations

   At configuration time UAs obtain one or more SIP URIs representing
   the default outbound-proxy-set.  This specification assumes the set
   is determined via any of a number of configuration mechanisms, and
   future specifications can define additional mechanisms such as using
   DNS to discover this set.  How the UA is configured is outside the
   scope of this specification.  However, a UA MUST support sets with at
   least two outbound proxy URIs and SHOULD support sets with up to four
   URIs.  For each outbound proxy URI in the set, the UA SHOULD send a
   REGISTER in the normal way using this URI as the default outbound
   proxy.  Forming the route set for the request is outside the scope of
   this document, but typically results in sending the REGISTER such
   that the topmost Route header field contains a loose route to the
   outbound proxy URI.  Other issues related to outbound route
   construction are discussed in [21].

   Registration requests, other than those described in Section 4.2.1,
   MUST include an instance-id media feature tag as specified in
   Section 4.1.

   These ordinary registration requests include a distinct reg-id
   parameter in the Contact header field.  Each one of these



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   registrations will form a new flow from the UA to the proxy.  The
   sequence of reg-id values does not have to be sequential but MUST be
   exactly the same sequence of reg-id values each time the UA instance
   power cycles or reboots so that the reg-id values will collide with
   the previously used reg-id values.  This is so the registrar can
   replace the older registration.

      The UAC can situationally decide whether to request outbound
      behavior by including or omitting the 'reg-id' parameter.  For
      example, imagine the outbound-proxy-set contains two proxies in
      different domains, EP1 and EP2.  If an outbound-style registration
      succeeded for a flow through EP1, the UA might decide to include
      'outbound' in its Require header field when registering with EP2,
      in order to insure consistency.  Similarly, if the registration
      through EP1 did not support outbound, the UA might decide to omit
      the 'reg-id' parameter when registering with EP2.

   The UAC MUST indicate that it supports the Path header [5] mechanism,
   by including the 'path' option-tag in a Supported header field value
   in its REGISTER requests.  Other than optionally examining the Path
   vector in the response, this is all that is required of the UAC to
   support Path.

   The UAC MAY examine successful registrations for the presence of an
   'outbound' option-tag in a Supported header field value.  Presence of
   this option-tag indicates that the registrar is compliant with this
   specification, and that any edge proxies which need to participate
   are also compliant.

   Note that the UA needs to honor 503 (Service Unavailable) responses
   to registrations as described in RFC 3261 and RFC 3263 [4].  In
   particular, implementors should note that when receiving a 503
   (Service Unavailable) response with a Retry-After header field, the
   UA is expected to wait the indicated amount of time and retry the
   registration.  A Retry-After header field value of 0 is valid and
   indicates the UA is expected to retry the REGISTER immediately.
   Implementations need to ensure that when retrying the REGISTER, they
   revisit the DNS resolution results such that the UA can select an
   alternate host from the one chosen the previous time the URI was
   resolved.

   Finally, re-registrations which merely refresh an existing valid
   registration SHOULD be sent over the same flow as the original
   registration.







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4.2.1.  Registration by Other Instances

   A User Agent MUST NOT include a reg-id header parameter in the
   Contact header field of a registration if the registering UA is not
   the same instance as the UA referred to by the target Contact header
   field.  (This practice is occasionally used to install forwarding
   policy into registrars.)

   Note that a UAC also MUST NOT include an instance-id or reg-id
   parameter in a request to unregister all Contacts (a single Contact
   header field value with the value of "*").

4.3.  Sending Requests

   When a UA is about to send a request, it first performs normal
   processing to select the next hop URI.  The UA can use a variety of
   techniques to compute the route set and accordingly the next hop URI.
   Discussion of these techniques is outside the scope of this document
   but could include mechanisms specified in RFC 3608 [22] (Service
   Route) and [21].

   The UA performs normal DNS resolution on the next hop URI (as
   described in RFC 3263 [4]) to find a protocol, IP address, and port.
   For protocols that don't use TLS, if the UA has an existing flow to
   this IP address, and port with the correct protocol, then the UA MUST
   use the existing connection.  For TLS protocols, there MUST also be a
   match between the host production in the next hop and one of the URIs
   contained in the subjectAltName in the peer certificate.  If the UA
   cannot use one of the existing flows, then it SHOULD form a new flow
   by sending a datagram or opening a new connection to the next hop, as
   appropriate for the transport protocol.

   The contact is formed normally in that the UAC uses the IP address of
   the device (even if the device is behind a NAT).  Unless there are
   privacy reason not to include an instance-id, the contact SHOULD
   include the instance-id media feature tag as specified in
   Section 4.1.  The UAC MUST also include an "ob" parameter in the
   Contact URI if, and only if, the UAC is sending the request over a
   flow for which the Registrar applied outbound processing.

      Note that if the UA wants its flow to work through NATs or
      firewalls it still needs to put the 'rport' parameter [10] in its
      Via header field value, and send from the port it is prepared to
      receive on.  More general information about NAT traversal in SIP
      is described in [23].






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4.4.  Detecting Flow Failure

   The UA needs to detect when a specific flow fails.  The UA actively
   tries to detect failure by periodically sending keepalive messages
   using one of the techniques described in Section 4.4.1,
   Section 4.4.2, or Section 4.4.3.  If a flow has failed, the UA
   follows the procedures in Section 4.2 to form a new flow to replace
   the failed one.

   When the outbound-proxy-set contains the "timed-keepalives"
   parameter, the UA MUST send its keepalives according to the time
   periods described in this section.  The server can specify this so
   the server can detect liveness of the client within a predictable
   time scale.  If the parameter is not present, the UA can send
   keepalives at its discretion.

   The time between each keepalive request when using non connection
   based transports such as UDP SHOULD be a random number between 24 and
   29 seconds while for connection based transports such as TCP it
   SHOULD be a random number between 95 and 120 seconds.  These times
   MAY be configurable.  To clarify, the random number will be different
   for each request.  Issues such as battery consumption might motivate
   longer keepalive intervals.  If the 'timed-keepalives' parameter is
   set on the outbound-proxy-set, the UA MUST use these recommended
   timer values.

      Note on selection of time values:  For UDP, the upper bound of 29
      seconds was selected so that multiple STUN packets could be sent
      before 30 seconds based on information that many NATs have UDP
      timeouts as low as 30 seconds.  The 24 second lower bound was
      selected so that after 10 minutes the jitter introduced by
      different timers will make the keepalive requests unsynchronized
      to evenly spread the load on the servers.  For TCP, the 120
      seconds upper bound was chosen based on the idea that for a good
      user experience, failures normally will be detected in this amount
      of time and a new connection set up.  Operators that wish to
      change the relationship between load on servers and the expected
      time that a user might not receive inbound communications will
      probably adjust this time.  The 95 seconds lower bound was chosen
      so that the jitter introduced will result in a relatively even
      load on the servers after 30 minutes.

   The client needs to perform normal RFC 3263 [4] SIP DNS resolution on
   the URI from the outbound-proxy-set to pick a transport.  Once a
   transport is selected, the UA selects a keepalive approach that is
   allowed for that transport and that is allowed by the server based on
   the tags in the URI from the outbound-proxy-set.




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4.4.1.  Keepalive with TCP KEEPALIVE

   This approach MUST only be used with TCP.

   User Agents that form flows, check if the configured URI they are
   connecting to has a 'timed-keepalives' URI parameter (defined in
   Section 12).  If the parameter is not present and the UA is not
   already performing keepalives using another supported mechanism, the
   UA needs to periodically perform keepalive checks by using the TCP
   Keepalive mechanism.  Not all environments can use this approach and
   it is not mandatory to implement.  Deployments that use it should
   also include keep-crlf so that clients that do not implement this
   option but are using TCP have an alternative approach to use.

4.4.2.  Keepalive with CRLF

   This approach MUST only be used with connection oriented transports
   such as TCP or SCTP.

   User Agents that form flows check if the configured URI they are
   connecting to has a 'keep-crlf' URI parameter (defined in
   Section 12).  If the parameter is present and the UA is not already
   performing keepalives using another supported mechanism, the UA can
   send keep alives as described in this section.

   For this mechanism, the client "ping" is a double-CRLF sequence, and
   the server "pong" is a single CRLF, as defined in the ABNF below:

   CRLF = CR LF
   double-CRLF = CR LF CR LF
   CR = 0x0d
   LF = 0x0a

   The ping and pong need to be sent between SIP messages and cannot be
   sent in the middle of a SIP message.  If sending over a TLS protected
   channel, the CRLFs are sent inside the TLS record layer.  If a pong
   is not received within 10 seconds then the client MUST treat the flow
   as failed.  Clients MUST support this CRLF keepalive.

4.4.3.  Keepalive with STUN

   This approach MUST only be used with transports, such as UDP, that
   are not connection oriented.

   User Agents that form flows, check if the configured URI they are
   connecting to has a 'keep-stun' URI parameter (defined in
   Section 12).  If the parameter is present and the UA is not already
   performing keepalives using another supported mechanism, the UA can



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   periodically perform keepalive checks by sending STUN [3] Binding
   Requests over the flow as described in Section 8.  Clients MUST
   support STUN based keepalives.

   If the XOR-MAPPED-ADDRESS in the STUN Binding Response changes, the
   UA MUST treat this event as a failure on the flow.

4.5.  Flow Recovery

   When a flow to a particular URI in the outbound-proxy-set fails, the
   UA needs to form a new flow to replace the old flow and replace any
   registrations that were previously sent over this flow.  Each new
   registration MUST have the same reg-id as the registration it
   replaces.  This is done in much the same way as forming a brand new
   flow as described in Section 4.2; however, if there is a failure in
   forming this flow, the UA needs to wait a certain amount of time
   before retrying to form a flow to this particular next hop.

   The amount of time to wait depends if the previous attempt at
   establishing a flow was successful.  For the purposes of this
   section, a flow is considered successful if outbound registration
   succeeded, and if keepalives are in use on this flow, at least one
   consecutive keepalive response was received.

   The number of seconds to wait is computed in the following way.  If
   all of the flows to every URI in the outbound proxy set have failed,
   the base time is set to 30 seconds; otherwise, in the case where at
   least one of the flows has not failed, the base time is set to 90
   seconds.  The wait time is computed by taking two raised to the power
   of the number of consecutive registration failures for that URI, and
   multiplying this by the base time, up to a maximum of 1800 seconds.

   wait-time = min( max-time, (base-time * (2 ^ consecutive-failures)))

   These times MAY be configurable in the UA.  The three times are:
   o  max-time with a default of 1800 seconds
   o  base-time-all-fail with a default of 30 seconds
   o  base-time-not-failed with a default of 90 seconds
   For example, if the base time is 30 seconds, and there were three
   failures, then the wait time is min(1800,30*(2^3)) or 240 seconds.
   The delay time is computed by selecting a uniform random time between
   50 and 100 percent of the wait time.  The UA MUST wait for the value
   of the delay time before trying another registration to form a new
   flow for that URI.

   To be explicitly clear on the boundary conditions:  when the UA boots
   it immediately tries to register.  If this fails and no registration
   on other flows succeed, the first retry happens somewhere between 30



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   and 60 seconds after the failure of the first registration request.
   If the number of consecutive-failures is large enough that the
   maximum of 1800 seconds is reached, the UA will keep trying
   indefinitely with a random time of 15 to 30 minutes between each
   attempt.


5.  Edge Proxy Mechanisms

5.1.  Processing Register Requests

   When an Edge Proxy receives a registration request with a reg-id
   header parameter in the Contact header field, it needs to determine
   if it (the edge proxy) will have to be visited for any subsequent
   requests sent to the user agent identified in the Contact header
   field, or not.  If the Edge Proxy determines that this is the case,
   it inserts its URI in a Path header field value as described in RFC
   3327 [5].  If the Edge Proxy is the first SIP node after the UAC, it
   either MUST store a "flow token"--containing information about the
   flow from the previous hop--in its Path URI, or reject the request.
   The flow token MUST be an identifier that is unique to this network
   flow.  The flow token MAY be placed in the userpart of the URI.  In
   addition, the first node MUST include an 'ob' URI parameter in its
   Path header field value.  If the Edge Proxy is not the first SIP node
   after the UAC it MUST NOT place an 'ob' URI parameter in a Path
   header field value.  The Edge Proxy can determine if it is the first
   hop by examining the Via header field.

5.2.  Generating Flow Tokens

   A trivial but impractical way to satisfy the flow token requirement
   in Section 5.1 involves storing a mapping between an incrementing
   counter and the connection information; however this would require
   the Edge Proxy to keep an impractical amount of state.  It is unclear
   when this state could be removed and the approach would have problems
   if the proxy crashed and lost the value of the counter.  A stateless
   example is provided below.  A proxy can use any algorithm it wants as
   long as the flow token is unique to a flow, the flow can be recovered
   from the token, and the token cannot be modified by attackers.

   Example Algorithm:  When the proxy boots it selects a 20-octet crypto
      random key called K that only the Edge Proxy knows.  A byte array,
      called S, is formed that contains the following information about
      the flow the request was received on:  an enumeration indicating
      the protocol, the local IP address and port, the remote IP address
      and port.  The HMAC of S is computed using the key K and the HMAC-
      SHA1-80 algorithm, as defined in [11].  The concatenation of the
      HMAC and S are base64 encoded, as defined in [12], and used as the



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      flow identifier.  When using IPv4 addresses, this will result in a
      32-octet identifier.

5.3.  Forwarding Requests

   When an Edge Proxy receives a request, it applies normal routing
   procedures with the following addition.  If the Edge Proxy receives a
   request where the edge proxy is the host in the topmost Route header
   field value, and the Route header field value contains a flow token,
   the proxy compares the flow in the flow token with the source of the
   request to determine if this is an "incoming" or "outgoing" request.
   If the flow in the flow token in the topmost Route header field value
   matches the source of the request, the request in an "outgoing"
   request.  For an "outgoing" request, the edge proxy just removes the
   Route header and continues processing the request.  Otherwise, this
   is an "incoming" request.  For an incoming request, the proxy removes
   the Route header field value and forwards the request over the
   'logical flow' identified by the flow token, that is known to deliver
   data to the specific target UA instance.  For connection-oriented
   transports, if the flow no longer exists the proxy SHOULD send a 430
   (Flow Failed) response to the request.

   Proxies which used the example algorithm described in this document
   to form a flow token follow the procedures below to determine the
   correct flow.

   Example Algorithm:  To decode the flow token, take the flow
      identifier in the user portion of the URI and base64 decode it,
      then verify the HMAC is correct by recomputing the HMAC and
      checking that it matches.  If the HMAC is not correct, the proxy
      SHOULD send a 403 (Forbidden) response.  If the HMAC is correct
      then the proxy SHOULD forward the request on the flow that was
      specified by the information in the flow identifier.  If this flow
      no longer exists, the proxy SHOULD send a 430 (Flow Failed)
      response to the request.

   Note that this specification needs mid-dialog requests to be routed
   over the same flows as those stored in the Path vector from the
   initial registration, but techniques to ensure that mid-dialog
   requests are routed over an existing flow are not part of this
   specification.  However, an approach such as having the Edge Proxy
   Record-Route with a flow token is one way to ensure that mid-dialog
   requests are routed over the correct flow.  The Edge Proxy can use
   the presence of the "ob" parameter in the UAC's Contact URI to
   determine if it should add a flow token.






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5.4.  Edge Proxy Keepalive Handling

   All edge proxies compliant with this specification MUST implement
   support for the STUN NAT Keepalive usage on its SIP UDP ports as
   described in Section 8.

   When a server receives a double CRLF sequence on a connection
   oriented transport such as TCP or SCTP, it MUST immediately respond
   with a single CRLF over the same connection.


6.  Registrar Mechanisms: Processing REGISTER Requests

   This specification updates the definition of a binding in RFC 3261
   [1] Section 10 and RFC 3327 [5] Section 5.3.

   When no +sip.instance media feature parameter is present in a Contact
   header field value in a REGISTER request, the corresponding binding
   is still between an AOR and the URI from that Contact header field
   value.  When a +sip.instance media feature parameter is present in a
   Contact header field value in a REGISTER request, the corresponding
   binding is between an AOR and the combination of the instance-id
   (from the +sip.instance media feature parameter) and the value of
   reg-id parameter if it is present.  For a binding with an
   instance-id, the registrar still stores the Contact header field
   value URI with the binding, but does not consider the Contact URI for
   comparison purposes.  A Contact header field value with an
   instance-id but no reg-id is valid, but one with a reg-id but no
   instance-id is not.  If the registrar processes a Contact header
   field value with a reg-id but no instance-id, it simply ignores the
   reg-id parameter.  The registrar MUST be prepared to receive,
   simultaneously for the same AOR, some registrations that use
   instance-id and reg-id and some registrations that do not.

   Registrars which implement this specification MUST support the Path
   header mechanism [5].

   In addition to the normal information stored in the binding record,
   some additional information needs to be stored for any registration
   that contains an instance-id and a reg-id header parameter in the
   Contact header field value.  First the registrar examines the first
   Path header field value, if any.  If the Path header field exists and
   the first URI does not have an 'ob' URI parameter, the registrar MUST
   ignore the reg-id parameter and continue processing the request as if
   it did not support this specification.  Likewise if the REGISTER
   request visited an edge proxy, but no Path header field values are
   present, the registrar MUST ignore the reg-id parameter.
   Specifically, if it ignores the 'reg-id' parameter the registrar MUST



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   use RFC 3261 Contact binding rules, and MUST NOT include the
   'outbound' option-tag in its Supported header field.  The registrar
   can determine if it is the first hop by examining the Via header
   field.

   If the UAC has a direct flow with the registrar, the registrar MUST
   store enough information to uniquely identify the network flow over
   which the request arrived.  For common operating systems with TCP,
   this would typically just be the handle to the file descriptor where
   the handle would become invalid if the TCP session was closed.  For
   common operating systems with UDP this would typically be the file
   descriptor for the local socket that received the request, the local
   interface, and the IP address and port number of the remote side that
   sent the request.  The registrar MAY store this information by adding
   itself to the Path header field with an appropriate flow token.

   The registrar MUST also store all the Contact header field
   information including the reg-id and instance-id parameters and
   SHOULD also store the time at which the binding was last updated.  If
   a Path header field is present, RFC 3327 [5] requires the registrar
   to store this information as well.  If the registrar receives a re-
   registration, it MUST update any information that uniquely identifies
   the network flow over which the request arrived if that information
   has changed, and SHOULD update the time the binding was last updated.

   The Registrar MUST include the 'outbound' option-tag (defined in
   Section (Section 12.1)) in a Supported header field value in its
   responses to REGISTER requests for which it has performed outbound
   processing, and MUST NOT include this option-tag if it did not
   perform outbound processing.  Furthermore, the Registrar MUST NOR
   include this option-tag in its response if the Registrar skipped
   outbound processing by ignoring the reg-id parameter as described in
   this specification.  Note that the requirements in this section
   applies to both REGISTER requests received from an Edge Proxy as well
   as requests received directly from the UAC.  The Registrar MAY be
   configured with local policy to reject any registrations that do not
   include the instance-id and reg-id, or with Path header field values
   that do not contain the 'ob' parameter.

   To be compliant with this specification, registrars which can receive
   SIP requests directly from a UAC without intervening edge proxies
   MUST implement the STUN NAT Keepalive usage on its SIP UDP ports as
   described in Section 8 and when it receives a double-CRLF sequence on
   a connection oriented transport such as TCP or SCTP, it MUST
   immediately respond with a single CRLF over the same connection.






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7.  Authoritative Proxy Mechanisms: Forwarding Requests

   When a proxy uses the location service to look up a registration
   binding and then proxies a request to a particular contact, it
   selects a contact to use normally, with a few additional rules:

   o  The proxy MUST NOT populate the target set with more than one
      contact with the same AOR and instance-id at a time.
   o  If a request for a particular AOR and instance-id fails with a 430
      (Flow Failed) response, the proxy SHOULD replace the failed branch
      with another target (if one is available) with the same AOR and
      instance-id, but a different reg-id.
   o  If the proxy receives a final response from a branch other than a
      408 (Request Timeout) or a 430 (Flow Failed) response, the proxy
      MUST NOT forward the same request to another target representing
      the same AOR and instance-id.  The targeted instance has already
      provided its response.

   The proxy uses normal forwarding rules looking at the next-hop target
   of the message and the value of any stored Path header field vector
   in the registration binding to decide how to forward the request and
   populate the Route header in the request.  If the proxy stored
   information about the flow over which it received the REGISTER for
   the binding, then the proxy MUST send the request over the same
   'logical flow' saved with the binding that is known to deliver data
   to the specific target UA instance.

      Typically this means that for TCP, the request is sent on the same
      TCP socket that received the REGISTER request.  For UDP, the
      request is sent from the same local IP address and port over which
      the registration was received, to the same IP address and port
      from which the REGISTER was received.

   If a proxy or registrar receives information from the network that
   indicates that no future messages will be delivered on a specific
   flow, then the proxy MUST invalidate all the bindings in the target
   set that use that flow (regardless of AOR).  Examples of this are a
   TCP socket closing or receiving a destination unreachable ICMP error
   on a UDP flow.  Similarly, if a proxy closes a file descriptor, it
   MUST invalidate all the bindings in the target set with flows that
   use that file descriptor.


8.  STUN Keepalive Processing

   This section describes changes to the SIP transport layer that allow
   SIP and the STUN [3] NAT Keepalive usage to be mixed over the same
   flow.  The STUN messages are used to verify that connectivity is



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   still available over a UDP flow, and to provide periodic keepalives.
   Note that these STUN keepalives are always sent to the next SIP hop.
   STUN messages are not delivered end-to-end.

   The only STUN messages required by this usage are Binding Requests,
   Binding Responses, and Binding Error Responses.  The UAC sends
   Binding Requests over the same UDP flow that is used for sending SIP
   messages.  These Binding Requests do not require any STUN attributes.
   The UAS responds to a valid Binding Request with a Binding Response
   which MUST include the XOR-MAPPED-ADDRESS attribute.

   If a server compliant to this section receives SIP requests on a
   given interface and port, it MUST also provide a limited version of a
   STUN server on the same interface and port as described in Section
   12.3 of [3].

      It is easy to distinguish STUN and SIP packets sent over UDP,
      because the first octet of a STUN packet has a value of 0 or 1
      while the first octet of a SIP message is never a 0 or 1.

   When a URI is created that refers to a SIP node that supports STUN as
   described in this section, the 'keep-stun' URI parameter, as defined
   in Section 12 MUST be added to the URI.  This allows a UA to inspect
   the URI to decide if it should attempt to send STUN requests to this
   location.  For example, an edge proxy could insert this parameter
   into its Path URI so that the registering UA can discover the edge
   proxy supports STUN keepalives.

   Because sending and receiving binary STUN data on the same ports used
   for SIP is a significant and non-backwards compatible change to RFC
   3261, this section requires a number of checks before sending STUN
   messages to a SIP node.  If a SIP node sends STUN requests (for
   example due to incorrect configuration) despite these warnings, the
   node could be blacklisted for UDP traffic.

   A SIP node MUST NOT send STUN requests over a flow unless it has an
   explicit indication that the target next hop SIP server claims to
   support STUN.  For example, automatic or manual configuration of an
   outbound-proxy-set which contains the 'keep-stun' parameter, or
   receiving the parameter in the Path header of the edge proxy, is
   considered sufficient explicit indication.  Note that UACs MUST NOT
   use an ambiguous configuration option such as "Work through NATs?" or
   "Do Keepalives?" to imply next hop STUN support.

      Typically, a SIP node first sends a SIP request and waits to
      receive a final response (other than a 408 response) over a flow
      to a new target destination, before sending any STUN messages.
      When scheduled for the next NAT refresh, the SIP node sends a STUN



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      request to the target.

   Once a flow is established, failure of a STUN request (including its
   retransmissions) is considered a failure of the underlying flow.  For
   SIP over UDP flows, if the XOR-MAPPED-ADDRESS returned over the flow
   changes, this indicates that the underlying connectivity has changed,
   and is considered a flow failure.

8.1.  Use with Sigcomp

   When STUN is used together with SigComp [24] compressed SIP messages
   over the same flow, how the STUN messages are sent depends on the
   transport protocol.  For UDP flows, the STUN messages are simply sent
   uncompressed, "outside" of SigComp.  This is supported by
   multiplexing STUN messages with SigComp messages by checking the two
   topmost bits of the message.  These bits are always one for SigComp,
   or zero for STUN.

      All SigComp messages contain a prefix (the five most-significant
      bits of the first byte are set to one) that does not occur in
      UTF-8 [13] encoded text messages, so for applications which use
      this encoding (or ASCII encoding) it is possible to multiplex
      uncompressed application messages and SigComp messages on the same
      UDP port.
      The most significant two bits of every STUN message are both
      zeroes.  This, combined with the magic cookie, aids in
      differentiating STUN packets from other protocols when STUN is
      multiplexed with other protocols on the same port.


9.  Example Message Flow

   The following call flow shows a basic registration and an incoming
   call.  At some point, the flow to the Primary proxy is lost.  An
   incoming INVITE tries to reach the Callee through the Primary flow,
   but receives an ICMP Unreachable message.  The Caller retries using
   the Secondary Edge Proxy, which uses a separate flow.  Later, after
   the Primary reboots, The Callee discovers the flow failure and
   reestablishes a new flow to the Primary.












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                   [-----example.com domain -------------------]
   Caller           Secondary             Primary            Callee
     |                 |                  |     (1) REGISTER |
     |                 |                  |<-----------------|
     |                 |                  |(2) 200 OK        |
     |                 |                  |----------------->|
     |                 |                  |     (3) REGISTER |
     |                 |<------------------------------------|
     |                 |(4) 200 OK        |                  |
     |                 |------------------------------------>|
     |                 |                  |                  |
     |                 |           CRASH  X                  |
     |(5) INVITE       |                  |                  |
     |----------------------------------->|                  |
     |(6) ICMP Unreachable                |                  |
     |<-----------------------------------|                  |
     |(7) INVITE       |                  |                  |
     |---------------->|                  |                  |
     |                 |(8) INVITE        |                  |
     |                 |------------------------------------>|
     |                 |(9) 200 OK        |                  |
     |                 |<------------------------------------|
     |(10) 200 OK      |                  |                  |
     |<----------------|                  |                  |
     |(11) ACK         |                  |                  |
     |---------------->|                  |                  |
     |                 |(12) ACK          |                  |
     |                 |------------------------------------>|
     |                 |                  |                  |
     |                 |          REBOOT  |                  |
     |                 |                  |(13) REGISTER     |
     |                 |                  |<-----------------|
     |                 |                  |(14) 200 OK       |
     |                 |                  |----------------->|
     |                 |                  |                  |
     |(15) BYE         |                  |                  |
     |---------------->|                  |                  |
     |                 | (16) BYE         |                  |
     |                 |------------------------------------>|
     |                 |                  |      (17) 200 OK |
     |                 |<------------------------------------|
     |     (18) 200 OK |                  |                  |
     |<----------------|                  |                  |
     |                 |                  |                  |

   This call flow assumes that the Callee has been configured with a
   proxy set that consists of "sip:pri.example.com;lr;keep-stun" and
   "sip:sec.example.com;lr;keep-stun".  The Callee REGISTER in message



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   (1) looks like:


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=7F94778B653B
   To: Callee <sip:callee@example.com>
   Call-ID: 16CB75F21C70
   CSeq: 1 REGISTER
   Supported: path
   Route: <sip:pri.example.com;lr;keep-stun>
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=1
   Content-Length: 0

   In the message, note that the Route is set and the Contact header
   field value contains the instance-id and reg-id.  The response to the
   REGISTER in message (2) would look like:


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: Callee <sip:callee@example.com>;tag=7F94778B653B
   To: Callee <sip:callee@example.com>;tag=6AF99445E44A
   Call-ID: 16CB75F21C70
   CSeq: 1 REGISTER
   Supported: outbound
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=1
     ;expires=3600
   Content-Length: 0

   The second registration in message 3 and 4 are similar other than the
   Call-ID has changed, the reg-id is 2, and the route is set to the
   secondary instead of the primary.  They look like:













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   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnqr9bym
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=755285EABDE2
   To: Callee <sip:callee@example.com>
   Call-ID: E05133BD26DD
   CSeq: 1 REGISTER
   Supported: path
   Route: <sip:sec.example.com;lr;keep-stun>
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=2
   Content-Length: 0


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnqr9bym
   From: Callee <sip:callee@example.com>;tag=755285EABDE2
   To: Callee <sip:callee@example.com>;tag=49A9AD0B3F6A
   Call-ID: E05133BD26DD
   Supported: outbound
   CSeq: 1 REGISTER
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=1
     ;expires=3600
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=2
     ;expires=3600
   Content-Length: 0

   The messages in the call flow are very normal.  The only interesting
   thing to note is that the INVITE in message 8 contains a Record-Route
   header for the Secondary proxy, with its flow token.

   Record-Route:
    <sip:PQPbqQE+Ynf+tzRPD27lU6uxkjQ8LLUG@sec.example.com;lr>

   The registrations in message 13 and 14 are the same as message 1 and
   2 other than the Call-ID and tags have changed.  Because these
   messages will contain the same instance-id and reg-id as those in 1
   and 2, this flow will partially supersede that for messages 1 and 2
   and will be tried first by Primary.







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10.  Grammar

   This specification defines new Contact header field parameters,
   reg-id and +sip.instance.  The grammar includes the definitions from
   RFC 3261 [1] and includes the definition of uric from RFC 3986 [14].

      Note:  The "=/" syntax used in this ABNF indicates an extension of
      the production on the left hand side.

   The ABNF[15] is:

    contact-params =/ c-p-reg / c-p-instance

    c-p-reg        = "reg-id" EQUAL 1*DIGIT ; 1 to 2**31

    c-p-instance   =  "+sip.instance" EQUAL
                      LDQUOT "<" instance-val ">" RDQUOT

    instance-val   = *uric ; defined in RFC 3986

   The value of the reg-id MUST NOT be 0 and MUST be less than 2**31.


11.  Definition of 430 Flow Failed response code

   This specification defines a new SIP response code '430 Flow Failed'.
   This response code is used by an Edge Proxy to indicate to the
   Authoritative Proxy that a specific flow to a UA instance has failed.
   Other flows to the same instance could still succeed.  The
   Authoritative Proxy SHOULD attempt to forward to another target
   (flow) with the same instance-id and AOR.


12.  IANA Considerations

12.1.  Contact Header Field

   This specification defines a new Contact header field parameter
   called reg-id in the "Header Field Parameters and Parameter Values"
   sub-registry as per the registry created by [16].  The required
   information is:

    Header Field                  Parameter Name   Predefined  Reference
                                                     Values
    ____________________________________________________________________
    Contact                       reg-id               No     [RFC AAAA]

    [NOTE TO RFC Editor: Please replace AAAA with



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                         the RFC number of this specification.]

12.2.  SIP/SIPS URI Parameters

   This specification augments the "SIP/SIPS URI Parameters" sub-
   registry as per the registry created by [17].  The required
   information is:

       Parameter Name  Predefined Values  Reference
       ____________________________________________
       keep-crlf           No            [RFC AAAA]
       keep-stun           No            [RFC AAAA]
       timed-keepalive     No            [RFC AAAA]
       ob                  No            [RFC AAAA]

       [NOTE TO RFC Editor: Please replace AAAA with
                            the RFC number of this specification.]

12.3.  SIP Option Tag

   This specification registers a new SIP option tag, as per the
   guidelines in Section 27.1 of RFC 3261.

   Name:  outbound
   Description:  This option-tag is used to identify Registrars which
      support extensions for Client Initiated Connections.  A Registrar
      places this option-tag in a Supported header to communicate the
      Registrar's support for this extension to the registering User
      Agent.

12.4.  Response Code

   This section registers a new SIP Response Code, as per the guidelines
   in Section 27.4 of RFC 3261.

   Code:  430
   Default Reason Phrase:  Flow Failed
   Reference:  This document

12.5.  Media Feature Tag

   This section registers a new media feature tag, per the procedures
   defined in RFC 2506 [18].  The tag is placed into the sip tree, which
   is defined in RFC 3840 [7].

   Media feature tag name:  sip.instance

   ASN.1 Identifier:  New assignment by IANA.



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   Summary of the media feature indicated by this tag:  This feature tag
   contains a string containing a URN that indicates a unique identifier
   associated with the UA instance registering the Contact.

   Values appropriate for use with this feature tag:  String.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms:  This
   feature tag is most useful in a communications application, for
   describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use:  Routing a call to a specific device.

   Related standards or documents:  RFC XXXX

   [[Note to IANA:  Please replace XXXX with the RFC number of this
   specification.]]

   Security Considerations:  This media feature tag can be used in ways
   which affect application behaviors.  For example, the SIP caller
   preferences extension [9] allows for call routing decisions to be
   based on the values of these parameters.  Therefore, if an attacker
   can modify the values of this tag, they might be able to affect the
   behavior of applications.  As a result, applications which utilize
   this media feature tag SHOULD provide a means for ensuring its
   integrity.  Similarly, this feature tag should only be trusted as
   valid when it comes from the user or user agent described by the tag.
   As a result, protocols for conveying this feature tag SHOULD provide
   a mechanism for guaranteeing authenticity.


13.  Security Considerations

   One of the key security concerns in this work is making sure that an
   attacker cannot hijack the sessions of a valid user and cause all
   calls destined to that user to be sent to the attacker.  Note that
   the intent is not to prevent existing active attacks on SIP UDP and
   TCP traffic, but to insure that no new attacks are added by
   introducing the outbound mechanism.

   The simple case is when there are no edge proxies.  In this case, the
   only time an entry can be added to the routing for a given AOR is
   when the registration succeeds.  SIP already protects against
   attackers being able to successfully register, and this scheme relies
   on that security.  Some implementers have considered the idea of just
   saving the instance-id without relating it to the AOR with which it
   registered.  This idea will not work because an attacker's UA can
   impersonate a valid user's instance-id and hijack that user's calls.



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   The more complex case involves one or more edge proxies.  When a UA
   sends a REGISTER request through an Edge Proxy on to the registrar,
   the Edge Proxy inserts a Path header field value.  If the
   registration is successfully authenticated, the registrar stores the
   value of the Path header field.  Later when the registrar forwards a
   request destined for the UA, it copies the stored value of the Path
   header field into the Route header field of the request and forwards
   the request to the Edge Proxy.

   The only time an Edge Proxy will route over a particular flow is when
   it has received a Route header that has the flow identifier
   information that it has created.  An incoming request would have
   gotten this information from the registrar.  The registrar will only
   save this information for a given AOR if the registration for the AOR
   has been successful; and the registration will only be successful if
   the UA can correctly authenticate.  Even if an attacker has spoofed
   some bad information in the Path header sent to the registrar, the
   attacker will not be able to get the registrar to accept this
   information for an AOR that does not belong to the attacker.  The
   registrar will not hand out this bad information to others, and
   others will not be misled into contacting the attacker.


14.  Operational Notes on Transports

   This entire section is non-normative.

   RFC 3261 requires proxies, registrars, and User Agents to implement
   both TCP and UDP but deployments can chose which transport protocols
   they want to use.  Deployments need to be careful in choosing what
   transports to use.  Many SIP features and extensions, such as large
   presence notification bodies, result in SIP requests that can be too
   large to be reasonably transported over UDP.  RFC 3261 states that
   when a request is too large for UDP, the device sending the request
   attempts to switch over to TCP.  No known deployments currently use
   this feature but it is important to note that when using outbound,
   this will only work if the UA has formed both UDP and TCP outbound
   flows.  This specification allows the UA to do so but in most cases
   it will probably make more sense for the UA to form a TCP outbound
   connection only, rather than forming both UDP and TCP flows.  One of
   the key reasons that many deployments choose not to use TCP has to do
   with the difficulty of building proxies that can maintain a very
   large number of active TCP connections.  Many deployments today use
   SIP in such a way that the messages are small enough that they work
   over UDP but they can not take advantage of all the functionality SIP
   offers.  Deployments that use only UDP outbound connections are going
   to fail with sufficiently large SIP messages.




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15.  Requirements

   This specification was developed to meet the following requirements:

   1.  Must be able to detect that a UA supports these mechanisms.
   2.  Support UAs behind NATs.
   3.  Support TLS to a UA without a stable DNS name or IP address.
   4.  Detect failure of a connection and be able to correct for this.
   5.  Support many UAs simultaneously rebooting.
   6.  Support a NAT rebooting or resetting.
   7.  Minimize initial startup load on a proxy.
   8.  Support architectures with edge proxies.


16.  Changes

   Note to RFC Editor:  Please remove this whole section.

16.1.  Changes from 08 Version

   UAs now include the 'ob' parameter in their Contact header for non-
   REGISTER requests, as a hint to the Edge Proxy (so the EP can Record-
   Route with a flow-token for example).

   Switched to CRLF for keepalives of connection-oriented transports
   after brutal consensus at IETF 68.

   Added timed-keepalive parameter and removed the unnecessary keep-tcp
   param, per consensus at IETF68.

   Removed example "Algorithm 1" which only worked over SIPS, per
   consensus at IETF68.

   Deleted text about probing and validating with options, per consensus
   at IETF68.

   Deleted provision for waiting 120 secs before declaring flow stable,
   per consensus at IETF68.

   fixed example UUIDs

16.2.  Changes from 07 Version

   Add language to show the working group what adding CRLF keepalives
   would look like.

   Changed syntax of keep-alive=stun to keep-stun so that it was easier
   to support multiple tags in the same URI.



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16.3.  Changes from 06 Version

   Added the section on operational selection of transports.

   Fixed various editorial typos.

   Put back in requirement flow token needs to be unique to flow as it
   had accidentally been dropped in earlier version.  This did not
   change any of the flow token algorithms.

   Reordered some of the text on STUN keepalive validation to make it
   clearer to implementors.  Did not change the actual algorithm or
   requirements.  Added note to explain how if the proxy changes, the
   revalidation will happen.

16.4.  Changes from 05 Version

   Mention the relevance of the 'rport' parameter.

   Change registrar verification so that only first-hop proxy and the
   registrar need to support outbound.  Other intermediaries in between
   do not any more.

   Relaxed flow-token language slightly.  Instead of flow-token saving
   specific UDP address/port tuples over which the request arrived, make
   language fuzzy to save token which points to a 'logical flow' that is
   known to deliver data to that specific UA instance.

   Added comment that keep-stun could be added to Path.

   Added comment that battery concerns could motivate longer TCP
   keepalive intervals than the defaults.

   Scrubbed document for avoidable lowercase may, should, and must.

   Added text about how Edge Proxies could determine they are the first
   hop.

16.5.  Changes from 04 Version

   Moved STUN to a separate section.  Reference this section from within
   the relevant sections in the rest of the document.

   Add language clarifying that UA MUST NOT send STUN without an
   explicit indication the server supports STUN.

   Add language describing that UA MUST stop sending STUN if it appears
   the server does not support it.



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   Defined a 'sip-stun' option tag.  UAs can optionally probe servers
   for it with OPTIONS.  Clarified that UAs SHOULD NOT put this in a
   Proxy-Require.  Explain that the first-hop MUST support this option-
   tag.

   Clarify that SIP/STUN in TLS is on the "inside".  STUN used with
   Sigcomp-compressed SIP is "outside" the compression layer for UDP,
   but wrapped inside the well-known shim header for TCP-based
   transports.

   Clarify how to decide what a consecutive registration timer is.  Flow
   must be up for some time (default 120 seconds) otherwise previous
   registration is not considered successful.

   Change UAC MUST-->SHOULD register a flow for each member of outbound-
   proxy-set.

   Reworded registrar and proxy in some places (introduce the term
   "Authoritative Proxy").

   Loosened restrictions on always storing a complete Path vector back
   to the registrar/authoritative proxy if a previous hop in the path
   vector is reachable.

   Added comment about re-registration typically happening over same
   flow as original registration.

   Changed 410 Gone to new response code 430 Flow Failed.  Was going to
   change this to 480 Temporarily Unavailable.  Unfortunately this would
   mean that the authoritative proxy deletes all flows of phones who use
   480 for Do Not Disturb.  Oops!

   Restored sanity by restoring text which explains that registrations
   with the same reg-id replace the old registration.

   Added text about the 'ob' parameter which is used in Path header
   field URIs to make sure that the previous proxy that added a Path
   understood outbound processing.  The registrar doesn't include
   Supported:  outbound unless it could actually do outbound processing
   (ex:  any Path headers have to have the 'ob' parameter).

   Added some text describing what a registration means when there is an
   instance-id, but no reg-id.

16.6.  Changes from 03 Version

   Added non-normative text motivating STUN vs. SIP PING, OPTIONS, and
   Double CRLF.  Added discussion about why TCP Keepalives are not



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   always available.

   Explained more clearly that outbound-proxy-set can be "configured"
   using any current or future, manual or automatic configuration/
   discovery mechanism.

   Added a sentence which prevents an Edge Proxy from forwarding back
   over the flow over which the request is received if the request
   happens to contain a flow token for that flow.  This was an
   oversight.

   Updated example message flow to show a fail-over example using a new
   dialog-creating request instead of a mid-dialog request.  The old
   scenario was leftover from before the outbound / gruu reorganization.

   Fixed tags, Call-IDs, and branch parameters in the example messages.

   Made the ABNF use the "=/" production extension mechanism recommended
   by Bill Fenner.

   Added a table in an appendix expanding the default flow recovery
   timers.

   Incorporated numerous clarifications and rewordings for better
   comprehension.

   Fixed many typos and spelling steaks.

16.7.  Changes from 02 Version

   Removed Double CRLF Keepalive

   Changed ;sip-stun syntax to ;keepalive=stun

   Fixed incorrect text about TCP keepalives.

16.8.  Changes from 01 Version

   Moved definition of instance-id from GRUU[25] draft to this draft.

   Added tentative text about Double CRLF Keepalive

   Removed pin-route stuff

   Changed the name of "flow-id" to "reg-id"

   Reorganized document flow




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   Described the use of STUN as a proper STUN usage

   Added 'outbound' option-tag to detect if registrar supports outbound

16.9.  Changes from 00 Version

   Moved TCP keepalive to be STUN.

   Allowed SUBSCRIBE to create flow mappings.  Added pin-route option
   tags to support this.

   Added text about updating dialog state on each usage after a
   connection failure.


17.  Acknowledgments

   Jonathan Rosenberg provided many comments and useful text.  Dave Oran
   came up with the idea of using the most recent registration first in
   the proxy.  Alan Hawrylyshen co-authored the draft that formed the
   initial text of this specification.  Additionally, many of the
   concepts here originated at a connection reuse meeting at IETF 60
   that included the authors, Jon Peterson, Jonathan Rosenberg, Alan
   Hawrylyshen, and Paul Kyzivat.  The TCP design team consisting of
   Chris Boulton, Scott Lawrence, Rajnish Jain, Vijay K. Gurbani, and
   Ganesh Jayadevan provided input and text.  Nils Ohlmeier provided
   many fixes and initial implementation experience.  In addition,
   thanks to the following folks for useful comments:  Francois Audet,
   Flemming Andreasen, Mike Hammer, Dan Wing, Srivatsa Srinivasan, Dale
   Worely, Juha Heinanen, Eric Rescorla, Lyndsay Campbell, Christer
   Holmberg, Kevin Johns, and Erkki Koivusalo.


Appendix A.  Default Flow Registration Backoff Times

   The base-time used for the flow re-registration backoff times
   described in Section 4.5 are configurable.  If the base-time-all-fail
   value is set to the default of 30 seconds and the base-time-not-
   failed value is set to the default of 90 seconds, the following table
   shows the resulting delay values.











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      +-------------------+--------------------+--------------------+
      | # of reg failures | all flows unusable | >1 non-failed flow |
      +-------------------+--------------------+--------------------+
      | 0                 | 0 secs             | 0 secs             |
      | 1                 | 30-60 secs         | 90-180 secs        |
      | 2                 | 1-2 mins           | 3-6 mins           |
      | 3                 | 2-4 mins           | 6-12 mins          |
      | 4                 | 4-8 mins           | 12-24 mins         |
      | 5                 | 8-16 mins          | 15-30 mins         |
      | 6 or more         | 15-30 mins         | 15-30 mins         |
      +-------------------+--------------------+--------------------+


18.  References

18.1.  Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

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

   [3]   Rosenberg, J., "Simple Traversal Underneath Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05
         (work in progress), October 2006.

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

   [5]   Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
         Extension Header Field for Registering Non-Adjacent Contacts",
         RFC 3327, December 2002.

   [6]   Leach, P., Mealling, M., and R. Salz, "A Universally Unique
         IDentifier (UUID) URN Namespace", RFC 4122, July 2005.

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

   [8]   Moats, R., "URN Syntax", RFC 2141, May 1997.

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




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   [10]  Rosenberg, J. and H. Schulzrinne, "An Extension to the Session
         Initiation Protocol (SIP) for Symmetric Response Routing",
         RFC 3581, August 2003.

   [11]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [12]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 3548, July 2003.

   [13]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.

   [14]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Sy ntax", STD 66, RFC 3986,
         January 2005.

   [15]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.

   [16]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Header Field Parameter Registry for the Session Initiation
         Protocol (SIP)", BCP 98, RFC 3968, December 2004.

   [17]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Uniform Resource Identifier (URI) Parameter Registry for the
         Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
         December 2004.

   [18]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
         Registration Procedure", BCP 31, RFC 2506, March 1999.

18.2.  Informative References

   [19]  Petrie, D., "A Framework for Session Initiation Protocol User
         Agent Profile Delivery", draft-ietf-sipping-config-framework-09
         (work in progress), October 2006.

   [20]  Hakala, J., "Using National Bibliography Numbers as Uniform
         Resource Names", RFC 3188, October 2001.

   [21]  Rosenberg, J., "Construction of the Route Header Field in the
         Session Initiation Protocol (SIP)",
         draft-rosenberg-sip-route-construct-02 (work in progress),
         October 2006.

   [22]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
         Extension Header Field for Service Route Discovery During



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         Registration", RFC 3608, October 2003.

   [23]  Boulton, C., "Best Current Practices for NAT Traversal for
         SIP", draft-ietf-sipping-nat-scenarios-05 (work in progress),
         June 2006.

   [24]  Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu,
         Z., and J. Rosenberg, "Signaling Compression (SigComp)",
         RFC 3320, January 2003.

   [25]  Rosenberg, J., "Obtaining and Using Globally Routable User
         Agent (UA) URIs (GRUU) in the Session Initiation Protocol
         (SIP)", draft-ietf-sip-gruu-11 (work in progress),
         October 2006.


Authors' Addresses

   Cullen Jennings (editor)
   Cisco Systems
   170 West Tasman Drive
   Mailstop SJC-21/2
   San Jose, CA  95134
   USA

   Phone:  +1 408 902-3341
   Email:  fluffy@cisco.com


   Rohan Mahy (editor)
   Plantronics
   345 Encincal St
   Santa Cruz, CA  95060
   USA

   Email:  rohan@ekabal.com















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