SIP WG                                                  C. Jennings, Ed.
Internet-Draft                                             Cisco Systems
Expires:  April 26, 2006                                    R. Mahy, Ed.
                                                            SIP Edge LLC
                                                        October 23, 2005

Managing Client Initiated Connections in the Session Initiation Protocol

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The Internet Society (2005).


   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 NATs, prevent servers from
   connecting to User Agents in this way.  Even when a proxy server can
   open a TCP connection to a User Agent, most User Agents lack a

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   certificate suitable to act as a TLS server.  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 for
   high availability systems.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  3
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Summary of Mechanism . . . . . . . . . . . . . . . . . . .  4
     3.2.  Single Registrar and UA  . . . . . . . . . . . . . . . . .  5
     3.3.  Multiple Connections from a User Agent . . . . . . . . . .  6
     3.4.  Edge Proxies . . . . . . . . . . . . . . . . . . . . . . .  8
     3.5.  Keep Alive Technique . . . . . . . . . . . . . . . . . . .  9
   4.  User Agent Mechanisms  . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Forming Flows  . . . . . . . . . . . . . . . . . . . . . . 10
       4.1.1.  Request without GRUU . . . . . . . . . . . . . . . . . 11
     4.2.  Detecting Flow Failure . . . . . . . . . . . . . . . . . . 11
     4.3.  Flow Failure Recovery  . . . . . . . . . . . . . . . . . . 12
     4.4.  Registration by Other Instances  . . . . . . . . . . . . . 13
   5.  Registrar Mechanisms . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Processing Register Requests . . . . . . . . . . . . . . . 13
     5.2.  Forwarding Requests  . . . . . . . . . . . . . . . . . . . 14
   6.  Edge Proxy Mechanisms  . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Processing Register Requests . . . . . . . . . . . . . . . 15
     6.2.  Forwarding Requests  . . . . . . . . . . . . . . . . . . . 16
   7.  Mechanisms for All Servers . . . . . . . . . . . . . . . . . . 17
     7.1.  STUN Processing  . . . . . . . . . . . . . . . . . . . . . 17
     7.2.  Pin-Route Processing . . . . . . . . . . . . . . . . . . . 17
   8.  Example Message Flow . . . . . . . . . . . . . . . . . . . . . 18
   9.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 23
   13. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 24
   14. Changes from 00 Version  . . . . . . . . . . . . . . . . . . . 24
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     16.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28

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1.  Introduction

   There are many environments for SIP deployments in which the User
   Agent (UA) can form a connection to a Registrar or Proxy but in which
   the connections in the reverse direction to the UA are not possible.
   This can happen for several reasons.  Connection 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 there may be a NAT, which are 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 a firewall or NAT.

   Most IP phones and personal computers get their network
   configurations dynamically via a protocol such as DHCP.  These
   systems typically do not have a useful name in DNS, and they
   definitely do not have a long-term, stable DNS name that is
   appropriate for binding to a certificate.  It is impractical for them
   to have a certificate that can be used as a client-side TLS
   certificate for SIP.  However, these systems can still form TLS
   connections to a proxy or registrar such that the UA authenticates
   the server certificate, and the server authenticates 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 connection, be it UDP,
   TCP, or 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 the server.  Since the server can't connect to the UA,
   the UA has to make sure that a connection is always active.  This
   requires the UA to detect when a connection fails.  Since, such
   detection takes time and leaves a window of opportunity for missed
   incoming requests, this mechanism allows the UA to use multiple
   connections, referred to as "flows", to the proxy or registrar and
   using a keep alive mechanism on each flow so that the UA can detect
   when a flow has failed.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [2].

2.1.  Definitions

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   Edge Proxy: An Edge Proxy is any proxy that is located topologically
      between the registering User Agent and the registrar.
   flow: A Flow is a network protocol layer connection between two hosts
      that is represented by the network address of both ends and the
      protocol.  For TCP and UDP this would include the IP addresses and
      ports of both ends and the protocol (TCP or UDP).  With TCP, a
      flow would often have a one to one correspondence with a single
      file descriptor in the operating system.
   flow-id: This refers to the value of a new header field parameter
      value for the contact header.  When a UA register multiple times,
      each registration gets a unique flow-id value.  This does not
      refer to flow.
   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 as defined in [1].  This is a URN that uniquely
      identifies the UA.

3.  Overview

   Several scenarios in which this technique is useful are discussed
   below, including the simple collocated 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 (found in the GRUU[1]) that stays the same for this UA
   even if the UA reboots or is power cycled.  Each UA can register
   multiple times for the same AOR to achieve high reliability.  Each
   registration includes the instance-id for the UA and a flow-id label
   that is different for each connection.

   UAs use STUN as the keep alive mechanism to keep their flow to the
   proxy or registrar alive.  A UA can create more than one flow using
   multiple registrations for the same AOR.  The instance-id parameter
   is used by the proxy to identify which UA a flow is associated with.
   The flow-id is used by the proxy and registrar to tell the difference
   between a UA re-registering and one that is registering over an
   additional flow.  The proxies keep track of the flows used for
   successful registrations.

   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 looking at the instance-id.  Proxies can

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   tell that a flow replaces a previously abandoned flow by looking at
   the flow-id.

3.2.  Single Registrar and UA

   In this example there a single server is acting as both a registrar
   and proxy.

      | Registrar |
      | Proxy     |
       | User  |
       | Agent |

   User Agents forming only a single connection continue to register
   normally but include the instance-id as described in the GRUU [1]
   specification and can also add a flow-id parameter to the Contact
   header field value.  The flow-id parameter is used to allow the
   registrar to detect and avoid using invalid contacts when a UA
   reboots or reconnects after its old connection has failed for some

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

      REGISTER SIP/2.0
      Via: SIP/2.0/UDP;branch=z9hG4bK-bad0ce-11-1036
      Max-Forwards: 70
      From: Bob <>;tag=d879h76
      To: Bob <>
      Call-ID: 8921348ju72je840.204
      CSeq: 1 REGISTER
      Contact: <sip:line1@>; flow-id=1;
      Content-Length: 0

      Note:  Implementors often ask why the value of the sip.instance is
      inside angle brackets.  This is a requirement of RFC 3840 [7]
      which defines media feature tags in SIP.  Feature tags which are
      strings are compared by case sensitive string comparison.  To
      differentiate these tags from tokens (which are not case
      sensitive), case sensitive parameters such as the sip.instance

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      media feature tag are placed inside angle brackets.

   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 (as defined in [1]) and flow-id
   (as defined in Section 9) along with the rest of the Contact header
   field.  If the instance-id and flow-id are the same as a previous
   registration for the same AOR, the proxy uses the most recently
   created registration first.  This allows a UA that has rebooted to
   replace its previous registration for each flow with minimal impact
   on overall system load.

   Later when Alice sends a request to Bob, his 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 the target set
   and uses the instance-id to understand that 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 that received the
   registration.  The proxy then forwards the request on that flow
   instead of trying to form a new flow to that contact.  This allows
   the proxy to forward a request to a particular contact down the same
   flow that did the registration for this AOR.  If the proxy had
   multiple flows that all went to this UA, it would choose any one of
   registration bindings that it had for this AOR and that had the same
   instance-id as the selected UA.  In general, if two registrations
   have the same flow-id and instance-id, the proxy would favor the most
   recently registered flow.  This is so that if a UA reboots, the proxy
   will prefer to use the most recent flow that goes to this UA instead
   of trying one of the old flows which will presumably fail.

3.3.  Multiple Connections from a User Agent

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

   In this example system, the logical proxy/registrar for the domain is
   running on two hosts that share the appropriate state and can both
   provide registrar and proxy functionality for the domain.  The UA
   will form connections to two of the physical hosts that can perform
   the proxy/registrar function for the domain.  Reliability is achieved
   by having the UA form two connections to the domain.  Scaleability is
   achieved by using DNS SRV to load balance the primary connection
   across a set of machines that can service the primary connection and

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   also using DNS SRV to load balance across a separate set of machines
   that can service the backup connection.  The deployment here requires
   that DNS be configured with an entry that resolves to all the primary
   hosts and another that resolves to all the backup hosts.  Designs
   having only one set were also considered but in this case, there
   would have to be some way to ensure that the two connection did not
   accidentally resolve to the same host.  Various approaches for this
   are possible but all probably require extensions to the SIP protocol
   so they were not included in this specification.  This approach can
   work with the disadvantage that slightly more configuration of DNS is

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

   The UA is configured with a primary and backup registration URI.
   These URIs are configured into the UA through whatever the normal
   mechanism is to configure the proxy or registrar for the UA.  They
   might look something like ";sip-stun" and
   ";sip-stun" if the domain was  The
   "sip-stun" tag indicates that they support STUN as described later in
   this specification.  Note that each of them could resolve to several
   different hosts.  The administrative domain that created these URIs
   MUST ensure that the two URIs resolve to separate hosts.  These URIs
   have normal SIP processing so things like SRV can be used to do load
   balancing across a proxy farm.

   The User Agent would get a GRUU from the domain to use at its
   contact.  The GRUU would refer to the domain, not host1 or host2.
   Regardless of which host received a request to GRUU, the domain would
   need to ensure that the request got sent to host1 or host2 and then
   sent across the appropriate flow to the UA.  The domain might choose
   to use the Path header (as described in the next section) approach to
   form this internal routing to host1 or host2.

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   When a single server fails, all the UAs that have a registration with
   it will detect this and try to reconnect.  This can cause large loads
   on the server and is referred to as the avalanche restart problem
   further discussed in Section 4.3.  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, it can delay some
   significant 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 is

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 [10] 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 for FW between
   the UA and the Edge Proxy and there could also be one between the
   Edge Proxy and the Registrar.  This second case typically happens
   when the Edge Proxy is in an enterprise the Registrar is located at a
   service provider.

                |Proxy    |
                 /      \
               /          \
            +-----+     +-----+
            |Edge1|     |Edge2|
            +-----+     +-----+
               \           /
                \         /
                  \     /
                   \   /
                  |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.  This is done

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   by putting the value 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 that was put
   in the Path header to the associated flow.

   The term Edge Proxy is often used to refer to deployments where the
   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
   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 [10].

3.5.  Keep Alive Technique

   A keep alive mechanism needs to detect both failure of a connection
   and changes to the NAT public mapping as well as keeping any NAT
   bindings refreshed.  This specification uses STUN [5] over the same
   flow as the SIP traffic to perform the keep alive.  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, requests are sent over the connection that is being used
   for the SIP traffic.  The proxy or registrar acts as a STUN 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.  It
      may also be possible to do this with OPTIONS messages and rport;
      although this approach has the advantage of being backwards
      compatible, it also increases the load on the proxy or registrar
      server.  The TCP KEEP_ALIVE mechanism is not used because most
      operating systems do not allow the time to be set on a per
      connection basis.  Linux, Solaris, OS X, and Windows all allow
      KEEP_ALIVEs to be turned on or off on a single socket using the
      SO_KEEPALIVE socket options but can not change the duration of the
      timer for an individual socket.  The length of the timer typically
      defaults to 7200 seconds.  The length of the timer can be changed
      to a smaller value by setting a kernel parameter but that affects
      all TCP connections on the host and thus is not appropriate to

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

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4.  User Agent Mechanisms

   The UA behavior is divided up into sections.  The first describes
   what a client must do when forming a new connection, the second when
   detecting failure of a connection, and the third on failure recovery.

4.1.  Forming Flows

   When a User Agent initiates a dialog, it MUST provide a Contact URI
   which has GRUU properties if it is in possession of an appropriate
   GRUU.  If it can not provide a GRUU, it needs to follow the procedure
   specified in Section 4.1.1.

   UAs are configured with one or more SIP URIs representing the default
   outbound proxies with which to register.  A UA MUST support sets with
   at least two outbound proxy URIs (primary and backup) and SHOULD
   support sets with up to four URIs.  For each outbound proxy URI in
   the set, the UA MUST send a REGISTER in the normal way using this URI
   as the default outbound proxy.  Forming the route set for the request
   is discussed in [15] but typically results in sending the REGISTER
   with the Route header field containing a loose route to the outbound
   proxy URI.  The UA MUST include the instance-id as described in [1].
   The UA MUST also add a distinct flow-id parameter to the Contact
   header field.  The UA SHOULD use a flow-id value of 1 for the first
   URI in the set, and a flow-id value of 2 for the second, and so on.
   Each one of these registrations will form a new flow from the UA to
   the proxy.  The flow-id sequence does not have to be exactly 1,2,3
   but it does have to be exactly the same flow-id sequence each time
   the device power cycles or reboots so that the flow-id values will
   collide with the previously used flow-id values and the proxy can
   realize that the older registrations are probably not useful.

   If the 200 response to a REGISTER contains a Service Route header
   field value as defined in RFC 3608 [16], then whichever proxy sends
   the 200 response last will affect where all future requests from this
   UA are directed.

   Note that the UA needs to honor 503 responses to registrations as
   described in RFC 3261 and RFC 3263 [4].  In particular, implementors
   should note that when receiving a 503 with a Retry-After, the UA
   should wait the indicated amount of time and retry the registration.
   A Retry-After header field value of 0 is valid and indicates the UA
   should retry the REGISTER immediately.  Implementations need to
   ensure that when retrying the REGISTER they redo the DNS resolution
   process such that if multiple hosts are reachable from the URI, there
   is a chance that the UA will select an alternate host from the one it
   chose the previous time the URI was resolved.

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   Note on Instance-ID Selection:  The instance-id needs to be a URN but
   there are many ways one can be generated.  A particularly simple way
   for both "hard" phones and "soft" phones is to use a UUID as defined
   in [6].  A device like a soft-phone, when first installed, should
   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 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.

4.1.1.  Request without GRUU

   If the UA does not have a GRUU, it MUST send the request with a
   Contact header field containing a +sip.instance media feature
   parameter, and it MUST include the "pin-route" option-tag in both a
   Proxy-Require and a Require header field value.  A User Agent
   compliant with this specification MUST NOT initiate a dialog with an
   INVITE without a GRUU in the Contact header field.  (At the time of
   this writing this is allowed only for dialogs initiated with the
   SUBSCRIBE method.)

   This mechanism without a GRUU is not reliable if any of the proxies
   on the path fail so it SHOULD not be used for long lived
   subscriptions.  Once a UA acquires an appropriate GRUU, it should
   terminate these subscriptions and re-subscribe using the normal GRUU
   based approach.

4.2.  Detecting Flow Failure

   The UA needs to detect if a given flow has failed, and if it has
   failed, follow the procedures in Section 4.1 to form a new flow to
   replace the failed one.

   User Agents that form flows MUST check if the configured URI they are
   connecting to has the "sip-stun" tag (defined in Section 10) and, if
   the tag is present, then the UA needs to periodically perform STUN
   [5] requests over the flow.  The time between STUN requests when
   using UDP SHOULD be a random number between 24 and 29 seconds while
   for other transport protocols it SHOULD be a random number between 95
   and 120 seconds.  The times MAY be configurable.

   Note on selection of time values:  For UDP, the upper bound of 29
   seconds was selected so that multiple STUN packets would be sent
   before 30 seconds based on information that some NATs had UDP

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   timeouts as low as 30 seconds.  The 24 second lower bound was
   selected so that after 10 minutes the jitter this introduce would
   have unsyncronized the STUN requests from different devices to evenly
   spread the load on the servers.  For TCP, the 120 seconds was chosen
   based on the idea that for a good user experience, failures would be
   detected in this 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 may not receive inbound communications will
   probably adjust this time widely.  The 95 seconds lower bound was
   chosen so that the jitter introduced would result in a relatively
   even load on the servers after 30 minutes.

   If the mapped address in the STUN response changes, the UA must treat
   this as a failure on the flow.  Any time a SIP message is sent and
   the proxy does not respond, this is also considered a failure, the
   flow is discarded and the procedures in Section 4.3 are followed to
   form a new flow.

4.3.  Flow Failure Recovery

   When a flow to a particular URI in the proxy set fails, the UA needs
   to form a new flow to replace it.  The new flow MUST have the same
   flow-id as the flow it is replacing.  This is done in much the same
   way as the flows are described as being formed in Section 4.1;
   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 URI in the proxy set.  The time to wait is computed in the
   following way.  If all of the flows to every URI in the 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 the base time
   multiplied by two to power of the number of consecutive registration
   failures to that URI up to a maximum of 1800 seconds.

       wait-time = min( 1800, (base-time * (2 ^ consecutive-failures)))

   These three times SHOULD be configurable in the UA.  The three times
   are the max-time with a default of 1800 seconds, the base-time-all-
   fail with a default of 30 seconds, and the base-time-not-failed with
   a default of 60 seconds.  For example if the base time was 30
   seconds, and there had been three failures, then the wait time would
   be 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 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

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   on other flows had succeeded, the first retry would happen somewhere
   between 30 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 being reached, then the UA keep trying
   forever with a random time between 900 and 1800 seconds between the

   SIP dialogs can be used for one or more "usages".  For example, a
   session created with INVITE (a session "usage") and a subscription (a
   subscription "usage") can share a dialog.  On failure of a flow, a
   User Agent might wish to resynchronizing the state of any active
   usages on any dialogs using the flow.  For example, the User Agent
   could send a new subscription for each subscription usage and an
   INVITE with replaces for each session usage.  Note that when a flow
   was obtained via a REGISTER request, the flow might be used by many
   dialogs and dialog usages.  A flow obtained via another request (e.g.
   a SUBSCRIBE request) only has usages from a single dialog.  The only
   reason to do this is that a message may have been lost while the flow
   was being reestablished.  The GRUU will ensure that any future
   messages are still delivered to the UA even if it does not re-
   subscribe, re-INVITE, or otherwise refresh the usage.  Deployments
   need to carefully consider the implications of these sorts of
   operations.  This approach only helps in a very narrow corner case
   and it will cause a huge load on the system if a single proxy
   crashes.  In some deployments, this will cause more harm than good.

4.4.  Registration by Other Instances

   A User Agent MUST NOT include an instance-id or flow-id 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.)

5.  Registrar Mechanisms

5.1.  Processing Register Requests

   Registrars which implement this specification, MUST support the Path
   header mechanism[10] and processes REGISTER requests as described in
   Section 10 of RFC 3261 with the following change.  Any time the
   registrar checks if a new contact matches an existing contact in the
   location database, it MUST also check and see if both the instance-id
   and flow-id match.  If they do not both match, then they are not the
   same contact.  Additionally, if the both the instance-id and flow-id
   are present and do match, then it is considered a match regardless of
   if the value of the contact header field value matches.  The

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   registrar MUST be prepared to receive some registrations that use
   instance-id and flow-id and some that do not, simultaneously for the
   same AOR.

   In addition to the normal information stored in the binding record,
   some additional information MUST be stored for any registration that
   contains a flow-id header parameter in the Contact header field
   value.  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 file
   descriptor.  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 MUST also store all the Contact header field
   information including the flow-id and instance-id and SHOULD also
   store the time at which the binding was last updated.  If a Path
   header field is present RFC 3327 [10] requires this to be stored and
   the registrar MUST store the Path header field value with the binding
   record.  Any time a messages is forwarded over the flow that created
   this binding, this stored Path header field value will be used to
   route the message.  If the registrar receives a re-registration, it
   MUST update the information that uniquely identifies the network flow
   over which the request arrived and SHOULD update the time the binding
   was last updated.

   The REGISTRAR MAY be configured with local policy to reject any
   registrations that do not include the instance-id and flow-id to
   eliminate the amplification attack described in [14].

5.2.  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.  If a request
      for a particular AOR and instance-id fails with a 410 response,
      the proxy SHOULD replace the failed branch with another target
      with the same AOR and instance-id, but a different flow-id.
   o  If two bindings have the same instance-id and flow-id, it SHOULD
      prefer the contact that was most recently updated.

   Note that if the request URI is a GRUU, the proxy will only select
   contacts with the AOR and instance-id associated with the GRUU.  The
   rules above still apply to a GRUU.  This allows a request routed to a

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   GRUU to first try one of the flows to a UA, then if that fails, try
   another flow to the same UA instance.

   The proxy uses normal forwarding rules looking at the Route of the
   message and any values of the of the stored Path header field value
   in the registration binding to decide how to forward the request and
   populate the Route header in the request.  Additionally, when the
   proxy forwards a request to a binding that contains a flow-id, the
   proxy MUST send the request over the same network flow that was saved
   with the binding.  This means that for TCP, the request MUST be sent
   on the same TCP socket that received the REGISTER request.  For UDP,
   the request MUST be 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 an indication from the network that
   indicates that no future messages on this flow will work, then it
   MUST remove all the bindings 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 remove all the bindings that use that flow.

6.  Edge Proxy Mechanisms

6.1.  Processing Register Requests

   When an Edge Proxy receives a registration request it MUST form a
   flow identifier token that is unique to this network flow and use
   this token as the user part of the URI that this proxy inserts into
   the Path header.  Edge proxies MUST use a Path header.  A trivial way
   to satisfy this requirement 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.
   Two stateless examples are 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 can not be
   modified by attackers.

   Algorithm 1: The proxy generates a flow token for connection-oriented
      transports by concatenating the file descriptor (or equivalent)
      with the NTP time the connection was created, and base64 encoding
      the result.  This results in an approximately 16 octet identifier.
      The proxy generates a flow token for UDP by concatenating the file
      descriptor and the remote IP address and port, then base64
      encoding the result.

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   Algorithm 2: When the proxy boots it selects a 20 byte 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 [8].  The concatenation of the
      HMAC and S are base64 encoded, as defined in [9], and used as the
      flow identifier.  With IPv4 address, this will result in a 32
      octet identifier.

   Algorithm 1 MUST NOT be used unless the REGISTER request is over a
   SIPS protected transport.  If the SIPS level of integrity protection
   is not available, an attacker can hijack another user's calls.

6.2.  Forwarding Requests

   When the Edge Proxy receives a request it applies normal routing
   procedures with the addition that it is routed to a URI with a flow
   identifier token that this proxy created, then the proxy MUST forward
   the request over the flow that received the REGISTER request that
   caused the flow identifier token to be created.  For connection-
   oriented transports, if the flow no longer exists the proxy SHOULD
   send a 410 response to the request.  The advantage to a stateless
   approach to managing the flow information is that there is no state
   on the edge proxy that requires clean up that has to be synchronized
   with the registrar.

   Algorithm 1: The proxy base64 decodes the user part of the Route
      header.  For TCP, if a connection specified by the file descriptor
      is present and the creation time of the file descriptor matches
      the creation time encoded in the Route header, the proxy forwards
      the request over that connection.  For UDP, the proxy forwards the
      request from the encoded file descriptor to the source IP address
      and port.
   Algorithm 2: To decode the flow token take the flow identifier in the
      user portion of the URI, and base64 decode it, then verity the
      HMAC is correct by recomputing the HMAC and checking it matches.
      If the HMAC is not correct, the proxy SHOULD send a 403 response.
      If the HMAC was 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 410 response to the request.

   Edge Proxies MUST Record-Route so that mid-dialog requests still are
   routed over the correct flow.

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7.  Mechanisms for All Servers

7.1.  STUN Processing

   TODO:  This section needs to be brought into sync with the STUN draft
   and check there are not issues for SIP and STUN on TCP or UDP

   A SIP device that receives SIP messages directly from a UA needs to
   behave as specified in this section.  Such devices would generally
   include a Registrar and an Edge Proxy, as they both receive register
   requests directly from a UA.

   If the server 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.  Specifically it MUST be capable of receiving and
   responding to STUN requests with the exception that it does not need
   to support STUN requests with the changed port or changed address
   flag set.  This allows the STUN server to run with only one port and
   IP address.

   It is easy to distinguish STUN and SIP packets 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 device that supports STUN
   as described in this section, the URI parameter "sip-stun", as
   defined in Section 10 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.  The sip-stun tag would typically show up in the
   URI in the Route header field value of a REGISTER request and would
   not be in the request URI.

7.2.  Pin-Route Processing

   A sip device receives a request with the "pin-route" options tag set
   in the Proxy-Require header field or the Require header field needs
   to follow the procedures in this section.

   A UAS that receives a request with the "pin-route" option tag in the
   Require header MUST either reject the request if pin-route is not
   supported, or if pin-route is supported by this UAS, the UAS MUST
   ensure that any message send in the dialog formed by this request is
   sent on the same flow as the initial request.  This specification
   does not mandate that all UAs support this option but certain UAs,
   such as the NOTIFIER in the configuration framework, will want to
   support this so they can form subscriptions with devices that do not
   have a GRUU.

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   A proxy that receives a request with the "pin-route" option tag in
   the Proxy-Require header MUST add a record-route header field value
   that resolves to this proxy and it MUST ensure that any future
   requests or responses in this dialog are forwarded on the same flow
   as the original request.  The suggested way to do this is to form a
   flow identifier token in the same way that an Edge Proxy would form
   this for the Path header and insert this flow identifier token in the
   user portion of the URI used in the record route header field value.

8.  Example Message Flow

   The following call flow shows a basic registration and an incoming
   call.  Part way through the call, the flow to the Primary proxy is
   lost.  The BYE message for the call is rerouted to the callee via the
   Backup proxy.  When connectivity to the primary proxy is established,
   the Callee registers again to replace the lost flow as shown in
   message 15.

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

   This call flow assumes that the Callee has been configured with a
   proxy set that consists of ";lr;sip-stun" and
   ";lr;sip-stun".  The Callee REGISTER in message
   (1) looks like:

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   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <>;tag=a73kszlfl
   To: Callee <>
   Call-ID: 1j9FpLxk3uxtm8tn@
   Route: <;lr;sip-stun>
   Contact: <sip:callee@>
   Content-Length: 0

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

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   From: Callee <>;tag=a73kszlfl
   To: Callee <> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn@
   Contact: <sip:callee@>
   Content-Length: 0

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

   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <>;tag=a73kszlfl
   To: Callee <>
   Call-ID: 1j9FpLxk3uxtm8tn-2@
   Route: <;lr;sip-stun>
   Contact: <sip:callee@>

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   Content-Length: 0

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   From: Callee <>;tag=a73kszlfl
   To: Callee <> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn-2@
   Contact: <sip:callee@>
   Contact: <sip:callee@>
   Content-Length: 0

   The messages in the call flow are very normal.  The only interesting
   thing to note is that the INVITE in message 6 will have a:

   Record-Route: <;lr>

   Message 11 seems seams strange in that it goes to the backup instead
   of the primary.  The Caller actually sends the message to the domain
   of the callee based on the GRUU that the callee provided in their
   Contact header field value when the dialog was formed and the domain
   selected a host (primary or backup) that was currently available.
   How the domain does this is an implementation detail up to the

   The registrations in message 15 and 16 are the same as message 1 and
   2 other than the Call-ID has changed.

9.  Grammar

   This specification defines a new Contact header field parameter,
   flow-id.  The grammar for DIGIT and EQUAL is obtained from RFC 3261

    contact-params = c-p-q / c-p-expires / c-p-flow / contact-extension
    c-p-flow       = "flow-id" EQUAL 1*DIGIT

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

10.  IANA Considerations

   This specification defines a new Contact header field parameter
   called flow-id in the "Header Field Parameters and Parameter Values"
   sub-registry as per the registry created by [11] at  The required
   information is:

    Header Field                  Parameter Name   Predefined  Reference
    Contact                       flow-id              Yes    [RFC AAAA]

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

   This specification defines a new value in the "SIP/SIPS URI
   Parameters" sub-registry as per the registry created by [12] at  The required
   information is:

       Parameter Name  Predefined Values  Reference
       sip-stun        No                 [RFC AAAA]

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

   TODO:  Add IANA section for "pin-route" option tag.

11.  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.

   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 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

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   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.

   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 proxy 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.

12.  Open Issues

   Service Route:  The current interaction of this draft and
   draft-rosenberg-sip-route-construct [15] does not work.  Currently
   the Service Route specification, RCFC 3608, suggests that the service
   route is appended to the outbound proxy set.  That will work with
   this specification.  However the [15] draft is suggesting to change
   the behavior so that the Service Route replaces the outbound proxy.
   This is basically so that SIP can be used to make configuration
   changes to the UA.  The problem is that this specification requires
   two or more URIs for the outbound configuration (so that reliability
   is possible) and the Service Route would only be able to provide a
   single URI.  If it is desirable to use Service Route this way, it
   probably needs to be modified in many ways including allowing it to
   return different Service Routes to different devices registering for
   the same AOR.

   Record Routing Edge Proxies:  If an Edge Proxy record routes with a
   name that resolves explicitly to it and then crashes, all future
   requests in that dialog will fail.  If an Edge Proxy record routes
   with a name that resolves to many edge proxies or does not record
   route at all, then requests that do not have GRUU as a contact will

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   not work.  A suggested resolution to this is to require GRUU for long
   lived dialogs and have the Edge proxies use path headers and not
   record route.

   SUBSCRIBEs without a GRUU.  Earlier version of draft assumed that a
   REGISTER was always the first message.  However the configuration
   framework[13] needs to perform a SUBSCRIBE to get the configuration
   that will allow the UA to register.  This specification needs to deal
   with situations where there is a SUBSCRIBE but no REGISTER.  The
   current resolution is to record route for these special cases and
   mitigate the reliability implications of this by not allowing these
   dialogs to be long lived.

   The terminology of flow, flow-id, connection is confusing.  Do we
   want to change it?

13.  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.
   4.   Detect failure of connection and be able to correct for this.
   5.   Support many UAs simultaneously rebooting.
   6.   Support a NAT rebooting or resetting.
   7.   Support proxy farms with multiple hosts for scaling and
        reliability purposes.
   8.   Minimize initial startup load on a proxy.
   9.   Support proxies that provide geographic redundancy.
   10.  Support architectures with edge proxies.
   11.  Must be able to receive notifications over the same flow used to
        send a subscription, even before any registrations have been
        established.  This ensures compatibility with the SIP
        configuration framework [13].

14.  Changes from 00 Version

   Moved TCP keep alive 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.

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15.  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, and
   Lyndsay Campbell.

16.  References

16.1.  Normative References

   [1]  Rosenberg, J., "Obtaining and Using Globally Routable User Agent
        (UA) URIs (GRUU) in the Session Initiation Protocol (SIP)",
        draft-ietf-sip-gruu-04 (work in progress), July 2005.

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

   [3]  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.

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

   [5]  Rosenberg, J., "Simple Traversal of UDP Through Network Address
        Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02 (work
        in progress), July 2005.

   [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.

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16.2.  Informative References

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

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

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

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

   [12]  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.

   [13]  Petrie, D., "A Framework for Session Initiation Protocol User
         Agent Profile Delivery", draft-ietf-sipping-config-framework-07
         (work in progress), July 2005.

   [14]  Lawrence, S., Hawrylyshen, A., and R. Sparks, "Problems with
         Max-Forwards Processing (and Potential Solutions)",
         October 2005.

   [15]  Rosenberg, J., "Clarifying Construction of the Route Header
         Field in the Session Initiation Protocol (SIP)",
         draft-rosenberg-sip-route-construct-00 (work in progress),
         July 2005.

   [16]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
         Extension Header Field for Service Route Discovery During
         Registration", RFC 3608, October 2003.

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Authors' Addresses

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

   Phone:  +1 408 902-3341

   Rohan Mahy (editor)
   SIP Edge LLC
   5617 Scotts Valley Drive, Suite 200
   Scotts Valley, CA  95066


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Internet-Draft     Client Initiated Connections in SIP      October 2005

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