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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                      November 18, 2007
Expires:  May 21, 2008

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 IETF Trust (2007).


   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 from the User Agent
   to its Registrar.

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  . . . . . . . . .  8
     3.4.   Edge Proxies  . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.   Keepalive Technique . . . . . . . . . . . . . . . . . . . 11
       3.5.1.  CRLF Keepalive Technique . . . . . . . . . . . . . . . 11
       3.5.2.  STUN Keepalive Technique . . . . . . . . . . . . . . . 12
   4.  User Agent Mechanisms  . . . . . . . . . . . . . . . . . . . . 12
     4.1.   Instance ID Creation  . . . . . . . . . . . . . . . . . . 12
     4.2.   Registrations . . . . . . . . . . . . . . . . . . . . . . 13
       4.2.1.  Non Outbound Registrations . . . . . . . . . . . . . . 15
     4.3.   Sending Non-REGISTER Requests . . . . . . . . . . . . . . 15
     4.4.   Detecting Flow Failure  . . . . . . . . . . . . . . . . . 16
       4.4.1.  Keepalive with CRLF  . . . . . . . . . . . . . . . . . 17
       4.4.2.  Keepalive with STUN  . . . . . . . . . . . . . . . . . 18
     4.5.   Flow Recovery . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Edge Proxy Mechanisms  . . . . . . . . . . . . . . . . . . . . 19
     5.1.   Processing Register Requests  . . . . . . . . . . . . . . 19
     5.2.   Generating Flow Tokens  . . . . . . . . . . . . . . . . . 20
     5.3.   Forwarding Non-REGISTER Requests  . . . . . . . . . . . . 20
     5.4.   Edge Proxy Keepalive Handling . . . . . . . . . . . . . . 21
   6.  Registrar Mechanisms: Processing REGISTER Requests . . . . . . 21
   7.  Authoritative Proxy Mechanisms: Forwarding Requests  . . . . . 23
   8.  STUN Keepalive Processing  . . . . . . . . . . . . . . . . . . 24
     8.1.   Use with Sigcomp  . . . . . . . . . . . . . . . . . . . . 25
   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
     12.5.  Media Feature Tag . . . . . . . . . . . . . . . . . . . . 31

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   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   14. Operational Notes on Transports  . . . . . . . . . . . . . . . 33
   15. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 34
   16. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     16.1.  Changes from 09 Version . . . . . . . . . . . . . . . . . 34
     16.2.  Changes from 08 Version . . . . . . . . . . . . . . . . . 34
     16.3.  Changes from 07 Version . . . . . . . . . . . . . . . . . 35
     16.4.  Changes from 06 Version . . . . . . . . . . . . . . . . . 35
     16.5.  Changes from 05 Version . . . . . . . . . . . . . . . . . 35
     16.6.  Changes from 04 Version . . . . . . . . . . . . . . . . . 36
     16.7.  Changes from 03 Version . . . . . . . . . . . . . . . . . 37
     16.8.  Changes from 02 Version . . . . . . . . . . . . . . . . . 38
     16.9.  Changes from 01 Version . . . . . . . . . . . . . . . . . 38
     16.10. Changes from 00 Version . . . . . . . . . . . . . . . . . 38
   17. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 38
   Appendix A.  Default Flow Registration Backoff Times . . . . . . . 39
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     18.1.  Normative References  . . . . . . . . . . . . . . . . . . 39
     18.2.  Informative References  . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 43

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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 a SIP User Agent behind
   such a firewall or NAT to receive inbound traffic associated with
   registrations or dialogs that it initiates.

   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 Transport Layer Security (TLS) [18] 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 (as defined in Section 22 of RFC
   3261) over that TLS connection.

   The key idea of this specification is that when a UA sends a REGISTER
   or a dialog-forming 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 incoming requests that need to go
   to this UA in the context of the registration or dialog.

   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 register over multiple flows at the same time.  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",

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   document are to be interpreted as described in RFC 2119 [2].

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
   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 concurrent 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

3.  Overview

   The mechanisms defined in this document are useful in several
   scenarios 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

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   reliability.  Each registration includes the instance-id for the UA
   and a reg-id label that is different for each flow.  The registrar
   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 whether a UA is creatin a new flow or refreshing or
   replacing an old one, possibly 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 to deliver a request 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 a SIP-specific usage profile
   of STUN (Session Traversal Utilities for NAT) [3].

   The UA can also ask its first hop proxy to use an specific flow for
   subsequent messages when sending a dialog-forming request.  This
   allows the UA to setup a subscription dialog for the SIP
   configuration package [19] before the UA registers.

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

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   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:example.com;keep SIP/2.0
   Via: SIP/2.0/TCP;rport;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
   Supported: path, outbound
   Contact: <sip:line1@;transport=tcp>; reg-id=1;
   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

   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 existing flow, instead of resolving the Contact URI using the
   procedures in RFC 3263 [4] and trying to form a new flow to that

   As described in the next section, if the proxy has multiple flows
   that all go to this UA, the proxy can choose any one of the
   registration bindings for this AOR that has the same instance-id as
   the selected UA.

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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

   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 authoritative 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 [20] 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|

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   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 address and AOR in the UA.  If
   the AOR is alice@example.com, the outbound-proxy-set might look
   something like "sip:primary.example.com;keep" and "sip:
   secondary.example.com;keep".  The "keep" tag indicates that a SIP
   server will respond correctly to the mandatory to implement keepalive
   mechanisms 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

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   between the UA and the Edge Proxy.
                |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 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 authenticates with the
   registrar successfully, 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.

   UAs are also free to use native transport keepalives, however the UA
   application may not be able to set these timers on a per-connection
   basis, and the server certainly cannot make any assumption about what
   values are used.  Use of native transport keepalives is therefore
   outside the scope of this document.

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

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   (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.

3.5.2.  STUN Keepalive Technique

   This technique can only be used for connection-less transports, such
   as UDP.

   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

      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 many believed
      that good performance and full backwards compatibility using this
      method were mutually exclusive.

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

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      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 on this device.

   If a URN scheme other than UUID is used, the UA MUST only use URNs
   for which an IETF consensus RFC defines how the specific URN needs to
   be constructed and used in the sip.instance Contact parameter for
   outbound behavior.

   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 sip.instance media feature tag
   at all is when it is making an anonymous request or some other
   privacy concern requires that the UA not reveal its identity.

      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
      MUST 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

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   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

   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.  (The
   UA could limit the number of flows formed to conserve battery power,
   for example).  All of these REGISTER requests will use the same
   Call-ID.  [OPEN ISSUE:  This is for consistency with GRUU, Section
   5.1 paragraph 5.  Is this a bad idea?  Alternatively GRUU could check
   all reg-ids and preserve temporary GRUU if a registration used the
   same Call-ID as used by any of the current bindings for the same
   instance.]  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
   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 registrations.

      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 not register
      with EP2 at all.

   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.

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   The UAC MAY examine successful registration responses for the
   presence of an 'outbound' option-tag in a Require header field value.
   Presence of this option-tag indicates that the registrar is compliant
   with this specification, and that any edge proxies which needed to
   participate are also compliant.  If the registrar did not support
   outbound, the UA may have unintentionally registered an unroutable
   contact.  It is the responsiblity of the UA to remove any
   inappropriate Contacts.

   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

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

4.2.1.  Non Outbound Registrations

   A User Agent MUST NOT include a reg-id header parameter in the
   Contact header field of a registration with a non-zero expiration, 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.)

   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 Non-REGISTER 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.

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   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.

   If the UA is sending a dialog-forming request, and wants all
   subsequent requests in the dialog to arrive over the same flow, the
   UA adds an 'ob' parameter to its Contact header.  Typically this is
   desirable, but it is not necessary for example if the Contact is a
   GRUU [23].  The flow used for the request is typically the same flow
   the UA registered over, but it could be a new flow, for example the
   initial subcription dialog for the configuration framework [19] needs
   to exist before registration.

      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 [24].


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 or
   Section 4.4.2.  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

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   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, as 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, if the 'keep' parameter is present in the URI,
   the UA selects the keepalive approach that is recommended for that

4.4.1.  Keepalive with CRLF

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

   A User Agent that forms flows, checks if the configured URI to which
   the UA is connecting resolves to a stream-based transport (ex:  TCP
   and TLS over TCP) and has a 'keep' URI parameter (defined in
   Section 12).  If the parameter is present, 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:

   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 TLS, the CRLFs
   are sent inside the TLS protected channel.  If sending over a SigComp
   [25] compressed data stream, the CRLF keepalives are sent inside the
   compressed stream.  The double CRLF is considered a single SigComp

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   message.  The specific mechanism for representing these characters is
   an implementation specific matter to be handled by the SigComp
   compressor at the sending end.

   If a pong is not received within 10 seconds then the client MUST
   treat the flow as failed.  Clients MUST support this CRLF keepalive.

4.4.2.  Keepalive with STUN

   This approach MUST only be used with connection-less transports, such
   as UDP.

   A User Agent that forms flows, checks if the configured URI to which
   the UA is connecting resolve to use the UDP transport, and has a
   'keep' URI parameter (defined in Section 12).  If the parameter is
   present, the UA can 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 a STUN Binding Error Response is received, or if no Binding
   Response is received after 7 retransmissions (16 times the STUN "RTO"
   timer--RTO is an estimate of round-trip time), the UA considers the
   flow failed.  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
   subsequent 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

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   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 (if all failed) with a default of 30 seconds
   o  base-time (if all have 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
   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

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.

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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 [26].  The concatenation of the
      HMAC and S are base64 encoded, as defined in [27], and used as the
      flow identifier.  When using IPv4 addresses, this will result in a
      32-octet identifier.

5.3.  Forwarding Non-REGISTER 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 decodes the flow token and 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.

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   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 specific procedures at the edge proxy 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 add a Record-Route header 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 dialog-
   forming requests in the UAC's Contact URI to determine if it should
   add a flow token.

5.4.  Edge Proxy Keepalive Handling

   All edge proxies compliant with this specification MUST implement
   support for STUN NAT Keepalives 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.

   Registrars which implement this specification MUST support the Path
   header mechanism RFC 3327 [5].

   When receiving a REGISTER request, the registrar first checks from
   its Via header field if the registrar is the first hop or not.  If
   the registrar is not the first hop, it examines the Path header of
   the request.  If the Path header field is missing or it exists but
   the first URI does not have an 'ob' URI parameter, the registrar MUST
   ignore the reg-id parameter of the Contact header.

   A Contact header field value with an instance-id but no reg-id is

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   valid (this combination can be used in the GRUU [23] specification),
   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.  If the Contact
   header contains more than one header field value with a non-zero
   expiration and a 'reg-id' parameter, the entire registration SHOULD
   be rejected with a 400 Bad Request response.  If the Contact header
   did not contain a 'reg-id' parameter or if that parameter became
   ignored (as described above) the registrar MUST NOT include the
   'outbound' option-tag in the Require header field of its response.

   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.  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.  If the Contact header field does not
   contain a '+sip.instance' media feature parameter, the registrar
   processes the request using the Contact binding rules in RFC 3261

   When a '+sip.instance' media feature parameter is present in a
   Contact header field of a REGISTER request (after the Contact header
   validation as described above), 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.  The registrar MUST store in the binding the Contact URI,
   all the Contact head field parameters, and any Path header field
   values and SHOULD also store the time at which the binding was last
   updated.  (Even though the Contact URI is not used for binding
   comparisons, it is still needed by the authoritative proxy to form
   the target set.)  The Registrar MUST include the 'outbound' option-
   tag (defined in Section 12.1) in a Require header field value in its
   response to the REGISTER request.

   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.

   If the registrar receives a re-registration for a specific
   combination of AOR, instance-id and reg-id values, the registrar MUST

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   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.

   To be compliant with this specification, registrars which can receive
   SIP requests directly from a UAC without intervening edge proxies
   MUST implement the same keepalive mechanisms as Edge Proxies
   (Section 5.4).

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 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 and populate the Route header in the request.
   If the proxy doubles as a registrar and stored information about the
   flow that created the binding, then the proxy MUST send the request
   over the same 'logical flow' saved with the binding, since that flow
   is known to deliver data to the specific target UA instance's network
   flow that was saved with the binding.

      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

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   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] Binding Requests to be mixed over the same flow.
   This constitues a new STUN usage.  The STUN messages are used to
   verify that connectivity is 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

   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
   except the XOR-MAPPED-ADDRESS and never use any form of
   authentication.  The UAS, proxy, or registrar 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 UDP port, it MUST also provide a limited version
   of a STUN server on the same interface and UDP port.

      It is easy to distinguish STUN and SIP packets sent over UDP,
      because the first octet of a STUN Binding method 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' URI parameter, as defined in
   Section 12 SHOULD 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

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   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' 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 200-class 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 request to the

   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.

   The SIP keepalive STUN usage requires no backwards compatibility with
   RFC 3489 [11].

8.1.  Use with Sigcomp

   When STUN is used together with SigComp [25] compressed SIP messages
   over the same flow.  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 [12] 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 Binding method 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.

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9.  Example Message Flow

      [----example.com domain------]
     Bob         EP1   EP2     Proxy     Alice
      |           |     |        |         |
    1)|-REGISTER->|     |        |         |
    2)|           |---REGISTER-->|         |
    3)|           |<----200 OK---|         |
    4)|<-200 OK---|     |        |         |
    5)|----REGISTER---->|        |         |
    6)|           |     |--REG-->|         |
    7)|           |     |<-200---|         |
    8)|<----200 OK------|        |         |
      |           |     |        |         |
      |    CRASH  X     |        |         |
      |        Reboot   |        |         |
    9)|           |     |        |<-INVITE-|
   10)|           |<---INVITE----|         |
   11)|           |----430------>|         |
   12)|           |     |<-INVITE|         |
   13)|<---INVITE-------|        |         |
   14)|----200 OK------>|        |         |
   15)|           |     |200 OK->|         |
   16)|           |     |        |-200 OK->|
   17)|           |     |        |<-ACK----|
   18)|           |     |<-ACK---|         |
   19)|<---ACK----------|        |         |
      |           |     |        |         |
   20)|--2CRLF->X |     |        |         |
      |           |     |        |         |
   21)|-REGISTER->|     |        |         |
   22)|<-200 OK---|     |        |         |
      |           |     |        |         |

   [TODO FIX example] 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;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=7F94778B653B
   To: Callee <sip:callee@example.com>
   Call-ID: 16CB75F21C70
   Supported: path
   Route: <sip:pri.example.com;lr;keep-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 reg-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 <sip:callee@example.com>;tag=7F94778B653B
   To: Callee <sip:callee@example.com>;tag=6AF99445E44A
   Call-ID: 16CB75F21C70
   Supported: outbound
   Contact: <sip:callee@>
   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;branch=z9hG4bKnqr9bym
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=755285EABDE2
   To: Callee <sip:callee@example.com>
   Call-ID: E05133BD26DD
   Supported: path
   Route: <sip:sec.example.com;lr;keep-stun>
   Contact: <sip:callee@>
   Content-Length: 0

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP;branch=z9hG4bKnqr9bym
   From: Callee <sip:callee@example.com>;tag=755285EABDE2
   To: Callee <sip:callee@example.com>;tag=49A9AD0B3F6A
   Call-ID: E05133BD26DD
   Supported: outbound
   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 8 contains a Record-Route
   header for the Secondary proxy, with its flow token.


   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 [13].

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

   The ABNF[14] is:

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

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

    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 [15].  The required
   information is:

    Header Field                  Parameter Name   Predefined  Reference
    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 [16].  The required
   information is:

       Parameter Name  Predefined Values  Reference
       keep                No            [RFC AAAA]
       timed-keepalives    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 UAs and 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, and vice versa.

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 [17].  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

   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.

   The Security Considerations discussed in [1] and [5] are also
   relevant to this document.  For the security considerations of
   generating flow tokens, please also see Section 5.2.  A discussion of
   preventing the avalanche restart problem is in Section 4.5.

   This document does not change the mandatory to implement security
   mechanisms in SIP.  User Agents are already required to implement
   Digest authentication while support of TLS is recommended; proxy
   servers are already required to implement Digest and TLS.

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

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   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.

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 09 Version

   Make outbound consistent with the latest version of STUN 3489bis
   draft.  The STUN keepalive section of outbound is now a STUN usage
   (much less formal).

   Fixed references.

16.2.  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.

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   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

16.3.  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.

16.4.  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.

16.5.  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.

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   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

16.6.  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.

   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-

   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

   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-

   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

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   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.

16.7.  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
   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

   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

   Incorporated numerous clarifications and rewordings for better

   Fixed many typos and spelling steaks.

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16.8.  Changes from 02 Version

   Removed Double CRLF Keepalive

   Changed ;sip-stun syntax to ;keepalive=stun

   Fixed incorrect text about TCP keepalives.

16.9.  Changes from 01 Version

   Moved definition of instance-id from GRUU[23] 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

   Described the use of STUN as a proper STUN usage

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

16.10.  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, Erkki Koivusalo, and Byron Campben 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

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   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,
   Jeroen van Bemmel, and Derek MacDonald.

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.

      | # 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-12
         (work in progress), November 2007.

   [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.

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Internet-Draft     Client Initiated Connections in SIP     November 2007

   [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.

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

   [11]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
         - Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

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

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

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

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

   [16]  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.

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

18.2.  Informative References

   [18]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.1", RFC 4346, April 2006.

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   [19]  Petrie, D., "A Framework for Session Initiation Protocol User
         Agent Profile Delivery", draft-ietf-sipping-config-framework-13
         (work in progress), October 2007.

   [20]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
         specifying the location of services (DNS SRV)", RFC 2782,
         February 2000.

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

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

   [23]  Rosenberg, J., "Obtaining and Using Globally Routable User
         Agent (UA) URIs (GRUU) in the Session Initiation Protocol
         (SIP)", draft-ietf-sip-gruu-15 (work in progress),
         October 2007.

   [24]  Boulton, C., "Best Current Practices for NAT Traversal for
         SIP", draft-ietf-sipping-nat-scenarios-07 (work in progress),
         July 2007.

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

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

   [27]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 4648, October 2006.

Authors' Addresses

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

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

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Internet-Draft     Client Initiated Connections in SIP     November 2007

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

   Email:  rohan@ekabal.com

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