Network Working Group                                   C. Jennings, Ed.
Internet-Draft                                             Cisco Systems
Updates:  3261,3327 (if approved)                           R. Mahy, Ed.
Expires:  September 6, 21, 2006                                 Plantronics
                                                          March 5, 20, 2006

Managing Client Initiated Connections in the Session Initiation Protocol

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   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 Agents in this way.
   Even when a proxy server can open a TCP connection to a User Agent,
   most User Agents lack a certificate suitable to act as a TLS
   (Transport Layer Security) server.  This specification defines
   behaviors for User Agents, registrars and proxy servers that allow
   requests to be delivered on existing connections established by the
   User Agent.  It also defines keep alive behaviors needed to keep NAT
   bindings open and specifies the usage of multiple connections.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
     2.1   Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1   Summary of Mechanism . . . . . . . . . . . . . . . . . . .  5
     3.2   Single Registrar and UA  . . . . . . . . . . . . . . . . .  6
     3.3   Multiple Connections from a User Agent . . . . . . . . . .  7
     3.4   Edge Proxies . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5   Keep Alive Technique . . . . . . . . . . . . . . . . . . . 10
   4.  User Agent Mechanisms  . . . . . . . . . . . . . . . . . . . . 10
     4.1   Instance ID Creation . . . . . . . . . . . . . . . . . . . 10
     4.2   Initial Registrations  . . . . . . . . . . . . . . . . . . 12
       4.2.1   Registration by Other Instances  . . . . . . . . . . . 13
     4.3   Sending Requests . . . . . . . . . . . . . . . . . . . . . 13
       4.3.1   Selecting the First Hop  . . . . . . . . . . . . . . . 13
       4.3.2   Forming Flows  . . . . . . . . . . . . . . . . . . . . 13
     4.4   Detecting Flow Failure . . . . . . . . . . . . . . . . . . 14
       4.4.1   Keep Alive with STUN . . . . . . . . . . . . . . . . . 14
       4.4.2.  Keep Alive with Double CRLF  . . . . . . . . . . . . . 15
       4.4.2   Flow Recovery  . . . . . . . . . . . . . . . . . . . . . . 15
   5.  Edge Proxy Mechanisms  . . . . . . . . . . . . . . . . . . . . 16
     5.1. 15
     5.1   Processing Register Requests . . . . . . . . . . . . . . . 16
     5.2. 15
     5.2   Generating Flow Tokens . . . . . . . . . . . . . . . . . . 16
     5.3   Forwarding Requests  . . . . . . . . . . . . . . . . . . . 17 16
   6.  Registrar and Location Server Mechanisms . . . . . . . . . . . 17
     6.1   Processing Register Requests . . . . . . . . . . . . . . . 18
     6.2. 17
     6.2   Forwarding Requests  . . . . . . . . . . . . . . . . . . . 19 18
   7.  Mechanisms for All Servers (Proxys, Registars, UAS)  . . . . . 19
     7.1   STUN Processing  . . . . . . . . . . . . . . . . . . . . . 19
     7.2.  Double CRLF Processing . . . . . . . . . . . . . . . . . . 20
   8.  Example Message Flow . . . . . . . . . . . . . . . . . . . . . 20
   9.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     10.1  Contact Header Field . . . . . . . . . . . . . . . . . . . 24
     10.2  SIP/SIPS URI Paramters . . . . . . . . . . . . . . . . . . 24
     10.3  SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 24
     10.4  Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 25
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   12. Open Issues  .   Requirements . . . . . . . . . . . . . . . . . . . . . . . . 26
   13. Requirements   Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     13.1  Changes  . . . . . . . . . . from 02 Version  . . . . . . . . . . . . . . . . . 27
     13.2  Changes from 01 Version  . . . . . . . . . . . . . . . . . 27
     13.3  Changes from 00 Version  . . . . . . . . . . . . . . . . . 27
   14.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27
   15.   References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     15.1  Normative References . . . . . . . . . . . . . . . . . . . 28
     15.2  Informative References . . . . . . . . . . . . . . . . . . 29
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
       Intellectual Property and Copyright Statements . . . . . . . . . . 31

1.  Introduction

   There are many environments for SIP [5] deployments in which the User
   Agent (UA) can form a connection to a Registrar or Proxy but in which
   the connections in the reverse direction to the UA are not possible.
   This can happen for several reasons.  Connection to the UA can be
   blocked by a firewall device between the UA and the proxy or
   registrar, which will only allow new connections in the direction of
   the UA to the Proxy.  Similarly there may be a NAT, which are only
   capable of allowing new connections from the private address side to
   the public side.  This specification allows SIP registration when the
   UA is behind such a firewall or NAT.

   Most IP phones and personal computers get their network
   configurations dynamically via a protocol such as DHCP (Dynamic Host
   Configuration Protocol).  These systems typically do not have a
   useful name in the Domain Name System (DNS), and they definitely do
   not have a long-term, stable DNS name that is appropriate for binding
   to a certificate.  It is impractical for them to have a certificate
   that can be used as a client-side TLS certificate for SIP.  However,
   these systems can still form TLS connections to a proxy or registrar
   which authenticates with a server certificate.  The server can
   authenticate the UA using a shared secret in a digest challenge over
   that TLS connection.

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

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [4].


2.1  Definitions
   Edge Proxy: An Edge Proxy is any proxy that is located topologically
      between the registering User Agent and the registrar.
   flow: A Flow is a network protocol layer (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 simultaneous registration gets a unique reg-id value.
   instance-id: This specification uses the word instance-id to refer to
      the value of the "sip.instance" media feature tag in the Contact
      header field.  This is a Uniform Resource Name (URN) that uniquely
      identifies this specific UA instance.
   outbound-proxy-set A configured 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

3.  Overview

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


3.1  Summary of Mechanism

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

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

   UAs use the STUN (Simple Traversal of UDP through NATs) protocol as
   the keep alive mechanism to keep their flow to the proxy or registrar


3.2  Single Registrar and UA

   In this example, a single server is acting as both a registrar and

      | 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 is used to allow the
   registrar to detect and avoid using invalid contacts when a UA
   reboots or reconnects after its old connection has failed for some

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

   Via: SIP/2.0/TCP;branch=z9hG4bK-bad0ce-11-1036
   Max-Forwards: 70
   From: Bob <>;tag=d879h76
   To: Bob <>
   Call-ID: 8921348ju72je840.204
   Supported: path
   Contact: <sip:line1@>; 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 and reg-id 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 proxy uses the most
   recently created registration first.  This allows a UA that has
   rebooted to replace its previous registration for each flow with
   minimal impact on overall system load.

   When Alice sends a request to Bob, his proxy selects the target set.
   The proxy forwards the request to elements in the target set based on
   the proxy's policy.  The proxy looks at the target set and uses the
   instance-id to understand that two targets both end up routing to the
   same UA.  When the proxy goes to forward a request to a given target,
   it looks and finds the flows that received the registration.  The
   proxy then forwards the request on that flow instead of trying to
   form a new flow to that contact.  This allows the proxy to forward a
   request to a particular contact over the same flow that the UA used
   to register this AOR.  If the proxy has multiple flows that all go to
   this UA, it can choose any one of registration bindings for this AOR
   that has the same instance-id as the selected UA.  In general, if two
   registrations have the same reg-id and instance-id, the proxy will
   favor the most recently registered flow.  This is so that if a UA
   reboots, the proxy will prefer to use the most recent flow that goes
   to this UA instead of trying one of the old flows which would
   presumably fail.


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 this example system, the logical proxy/registrar for the domain is
   running on two hosts that share the appropriate state and can both
   provide registrar and proxy functionality for the domain.  The UA
   will form connections to two of the physical hosts that can perform
   the proxy/registrar function for the domain.  Reliability is achieved
   by having the UA form two TCP connections to the domain.  Scalability
   is achieved by using DNS SRV to load balance the primary connection
   across a set of machines that can service the primary connection and
   also using DNS SRV to load balance across a separate set of machines
   that can service the backup 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 backup
   hosts.  Designs having only one set were also considered, but in this
   case there would have to be some way to ensure that the two
   connection did not accidentally resolve to the same host.  Various
   approaches for this are possible but all probably require extensions
   to the SIP protocol so they were not included in this specification.

   This approach can work with the disadvantage that slightly more
   configuration of DNS is required.

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

   The UA is configured with a primary and backup registration URI.
   These URIs are configured into the UA through whatever the normal
   mechanism is to configure the proxy or registrar address in the UA.
   If the AOR is, the outbound-proxy-set might look
   something like ";sip-stun" ";keepalive=stun" and "sip:;sip-stun".;keepalive=stun".  The "sip-stun" "keepalive=stun" tag
   indicates that a SIP server supports STUN and SIP muxed over the same
   flow, as described later in this specification.  Note that each URI
   in the outbound-proxy-set could resolve to several different physical
   hosts.  The administrative domain that created these URIs should
   ensure that the two URIs resolve to separate hosts.  These URIs are
   handled according to normal SIP processing rules, so things 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 (as described in the
   next section) approach 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. 4.4.2.  The multiple
   flows to many servers help reduce the load caused by the avalanche
   restart.  If a UA has multiple flows, and one of the servers fails,
   it can delay some significant time before trying to form a new
   connection to replace the flow to the server that failed.  By
   spreading out the time used for all the UAs to reconnect to a server,
   the load on the server farm is reduced.


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 [12] so that when
   the registrar later forwards a request to this UA, the request is
   routed through the Edge Proxy.  There could be a NAT or firewall
   between the UA and the Edge Proxy.
                |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 can
   be done by putting the value in the user portion of a loose route in
   the path header.  If the registration succeeds, the Edge Proxy needs
   to map future requests that are routed to the identifier value 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
   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 [12].


3.5  Keep Alive Technique

   A keep alive mechanism needs to detect failure of a connection and
   changes to the NAT public mapping, as well as keeping any NAT
   bindings refreshed.  This specification uses describes using STUN [7] over
   the same flow as the SIP traffic to perform the keep alive.  A  For
   connection-oriented transports (e.g.  TCP and TLS over TCP), the UAC
   MAY use TCP keep-alives to detect flow failure if the UAC can send
   these keep alives and detect a keep alive failure according to the
   timeframes described in Section 4.4.

   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, requests are sent over the same flow that is being used for the
   SIP traffic.  The proxy or registrar acts as a STUN server on the SIP
   signaling port.

      Note:  The STUN mechanism is very robust and allows the detection
      of a changed IP address.  Many other options were considered.  It
      may also be possible to detect a changes flow with OPTIONS
      messages and the rport parameter.  Although considered, but
      the OPTIONS approach
      has SIP Working Group selected the advantage of being backwards compatible, STUN-based approach, since it also
      significantly increases
      works over any transport.  Approaches using SIP requests were
      abandoned because to achieve the load on required performance, the proxy or registrar server. server
      needs to deviate from the SIP specification in significant ways.
      This would result in many undesirable and non-deterministic
      behaviors in some environments.  The TCP KEEP_ALIVE mechanism was will
      not used because most always work, since some operating systems and programming
      environments do not allow the keep alive time to be set on a per
      connection basis.
      Linux, Solaris, OS X, and Windows all allow KEEP_ALIVEs to be
      turned on or off on a single socket using the SO_KEEPALIVE socket
      options but can not change the duration of the timer for an
      individual socket.  The length of the timer typically defaults to
      7200 seconds.  The length of the timer can be changed to a smaller
      value by setting a kernel parameter but that affects all TCP
      connections on the host and thus is not appropriate to use.

   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.

4.  User Agent Mechanisms


4.1  Instance ID Creation

   Each UA MUST have an Instance Identifer 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 persitant across power
   cylces of the device.

   A UA SHOULD use a UUID URN [9].  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 [9] and then save this in persistent storage for all future
      use.  For a device such as a hard phone, which will only ever have
      a single SIP UA present, the UUID can include the MAC address and
      be generated at any time because it is guaranteed that no other
      UUID is being generated at the same time on that physical device.
      This means the value of the time component of the UUID can be
      arbitrarily selected to be any time less than the time when the
      device was manufactured.  A time of 0 (as shown in the example in
      Section 3.2) is perfectly legal as long as the device knows no
      other UUIDs were generated at this time.

   If a URN scheme other than UUID is used, the URN MUST be selected
   such that the instance can be certain that no other instance
   registering against the same AOR would choose the same URN value.  An
   example of a URN that would not meet the requirements of this
   specification is the national bibliographic number [15].  Since there
   is no clear relationship between a SIP UA instance and a URN in this
   namespace, there is no way a selection of a value can be performed
   that guarantees that another UA instance doesn't choose the same

   The UA SHOULD include a "sip.instance" media feature tag as a UA
   characteristic [10] in requests and responses.  As described in [10],
   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 [3].  One case where a UA may not want
   to include the URN in the sip.instance media feature tag is when it
   is making an anoymous request or some other privacy concern requires
   that the UA not reveal its identity.

      RFC 3840 [10] 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 [10] and in the caller
      preferences specification [2].  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 [3].  Lexical equality may result in two URNs being
      considered unequal when they are actually equal.  In this specific
      usage of URNs, the only element which provides the URN is the SIP
      UA instance identified by that URN.  As a result, the UA instance
      SHOULD provide lexically equivalent URNs in each registration it
      generates.  This is likely to be normal behavior in any case;
      clients are not likely to modify the value of the instance ID so
      that it remains functionally equivalent yet lexigraphically
      different to previous registrations.


4.2  Initial Registrations

   UAs are configured with one or more SIP URIs representing the default
   outbound-proxy-set.  The specification assumes the set is determined
   via configuration but future specifications may define other
   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 (primary and
   backup) and SHOULD support sets with up to four URIs.  For each
   outbound proxy URI in the set, the UA MUST send a REGISTER in the
   normal way using this URI as the default outbound proxy.  Forming the
   route set for the request is 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 [20].

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

   These ordinary registration requests MUST also add a distinct reg-id
   parameter to the Contact header field.  Each one of these
   registrations will form a new flow from the UA to the proxy.  The
   reg-id sequence does not have to be sequential but MUST be exactly
   the same reg-id sequence each time the device power cycles or reboots
   so that the reg-id values will collide with the previously used
   reg-id values.  This is so the proxy can realize that the older
   registrations are probably not useful.

   The UAC MUST indicate that it supports the Path header [12]
   mechanism, by including the 'path' option-tag in a Supported header
   field value in its REGISTER requests.  Other than optionally
   examining the Path vector in the response, this is all that is
   required of the UAC to support Path.

   The UAC MAY examine successful registrations for the presence of an
   'outbound' option-tag in a Supported header field value.  Presence of
   this option-tag indicates that the registrar is compliant with this

   Note that the UA needs to honor 503 responses to registrations as
   described in RFC 3261 and RFC 3263 [6].  In particular, implementors
   should note that when receiving a 503 response with a Retry-After
   header field, the UA should wait the indicated amount of time and
   retry the registration.  A Retry-After header field value of 0 is
   valid and indicates the UA should retry the REGISTER immediately.
   Implementations need to ensure that when retrying the REGISTER they
   revisit the DNS resolution results such that the UA can select an
   alternate host from the one chosen the previous time the URI was


4.2.1  Registration by Other Instances

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

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


4.3  Sending Requests

   As described in Section 4.1, all requests need to include the
   instance-id media feature tag unless privacy concerns require


4.3.1  Selecting the First Hop

   When an 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 [21] (Service
   Route) and [20].


4.3.2  Forming Flows

   The UA performs normal DNS resolution on the next hop URI (as
   described in RFC 3263 [6]) to find a protocol, IP address, and port.
   For non TLS protocols, 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, the existing flow is only
   used if, in addition to matching the IP address, port, and protocol,
   the host production in the next hop URI MUST match 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.


4.4  Detecting Flow Failure

   The UA needs to detect when a specific flow fails.  If a flow has
   failed, the UA follows the procedures in Section 4.2 to form a new
   flow to replace the failed one.  The UA proactively tries to detect
   failure by periodically sending keep alive messages using one of the
   techniques described in this section.

   The time between keep alive requests when using UDP based transports
   SHOULD be a random number between 24 and 29 seconds while for TCP
   based transports it SHOULD be a random number between 95 and 120
   seconds.  These times MAY be configurable.

   o  Note on selection of time values:  For UDP, the upper bound of 29
      seconds was selected so that multiple STUN packets could be sent
      before 30 seconds based on information that many NATs have UDP
      timeouts as low as 30 seconds.  The 24 second lower bound was
      selected so that after 10 minutes the jitter introduced by
      different timers will the keep alive requests unsynchronized to
      evenly spread the load on the servers.  For TCP, the 120 seconds
      was chosen based on the idea that for a good user experience,
      failures should 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 may 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.

4.4.1.  Keep Alive with STUN

   User Agents that form flows MUST check if the configured URI they are
   connecting to has the "sip-stun" URI parameter (defined in
   Section 10).  If the parameter is present, the UA needs to
   periodically perform keep alive checks by sending a STUN [7] Binding
   Requests over the flow.

   If the XOR-MAPPED-ADDRESS in the STUN Binding Response changes, the
   UA MUST treat this event as was chosen so that the jitter
      introduced will result in a failure relatively even load on the flow.

4.4.2. servers
      after 30 minutes.

4.4.1  Keep Alive with Double CRLF STUN

   User Agents that form flows MUST check if the configured URI they are
   connecting to has the "crlf-ping" a 'keepalive' URI parameter (defined in Section 10). 10)
   with the value of 'stun'.  If the parameter is present, the UA needs
   to send periodically perform keep alive requests checks by sending a CRLF STUN [7]
   Binding Requests over the flow.

   If the UA does not receive any data back over XOR-MAPPED-ADDRESS in the flow within 7
   seconds of sending STUN Binding Response changes, the CRLF, then it
   UA MUST consider the lack of
   response to be treat this event as a flow failure.

4.5. failure on the flow.

4.4.2  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 4.3.2; 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 time to wait is computed in the following way.  If all of the
   flows to every URI in the proxy set have failed, the base time is set
   to 30 seconds; otherwise, in the case where at least one of the flows
   has not failed, the base time is set to 90 seconds.  The wait time is
   computed by taking two raised to power of the number of consecutive
   registration failures for that URI, and multiplying this by the base
   time, up to a maximum of 1800 seconds.
     wait-time = min( 1800, (base-time * (2 ^ consecutive-failures)))

   These three times MAY be configurable in the UA.  The three times are
   the max-time with a default of 1800 seconds, the base-time-all-fail
   with a default of 30 seconds, and the base-time-not-failed with a
   default of 60 seconds.  For example if the base time was 30 seconds,
   and there had been three failures, then the wait time would be
   min(1800,30*(2^3)) or 240 seconds.  The delay time is computed by
   selecting a uniform random time between 50 and 100 percent of the
   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 forever
   with a random time between 900 and 1800 seconds between the attempts.

5.  Edge Proxy Mechanisms


5.1  Processing Register Requests

   When an Edge Proxy receives a registration request with a
   sip.instance media feature tag in the Contact header field, it MUST
   form a flow identifier token that is unique to this network flow.
   The Edge Proxy MUST insert this token into a URI referring to this
   proxy and place this URI into a Path header field as described in RFC
   3327 [12].  The token MAY be placed in the userpart of the URI.


5.2  Generating Flow Tokens

   A trivial but impractical way to satisfy the flow token requirement
   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.  Two
   stateless examples are provided below.  A proxy can use any algorithm
   it wants as long as the flow token is unique to a flow, the flow can
   be recovered from the token, and the token can not be modified by

   Algorithm 1: The proxy generates a flow token for connection-oriented
      transports by concatenating the file descriptor (or equivalent)
      with the NTP time the connection was created, and base64 encoding
      the result.  This results in an approximately 16 octet identifier.
      The proxy generates a flow token for UDP by concatenating the file
      descriptor and the remote IP address and port, then base64
      encoding the result.  This algorithm MUST NOT be used unless all
      messages between the Edge proxy and Registrar use a SIPS protected
      transport.  If the SIPS level of integrity protection is not
      available, an attacker can hijack another user's calls.
   Algorithm 2: When the proxy boots it selects a 20 byte crypto random
      key called K that only the Edge Proxy knows.  A byte array, called
      S, is formed that contains the following information about the
      flow the request was received on:  an enumeration indicating the
      protocol, the local IP address and port, the remote IP address and
      port.  The HMAC of S is computed using the key K and the HMAC-
      SHA1-80 algorithm, as defined in [16].  The concatenation of the
      HMAC and S are base64 encoded, as defined in [18], and used as the
      flow identifier.  When using IPv4 addresses, this will result in a
      32 octet identifier.


5.3  Forwarding Requests

   When the Edge Proxy receives a request, it applies normal routing
   procedures with the following addition.  If the top-most Route header
   refers to the Edge Proxy and contains a valid flow identifier token
   created by this proxy, the proxy MUST forward the request over the
   flow that received the REGISTER request that caused the flow
   identifier token to be created.  For connection-oriented transports,
   if the flow no longer exists the proxy SHOULD send a 410 response to
   the request.

      The advantage to a stateless approach to managing the flow
      information is that there is no state on the edge proxy that
      requires clean up or that has to be synchronized with the

   Proxies which used one of the two algorithms described in this
   document to form a flow token follow the procedures below to
   determine the correct flow.

   Algorithm 1: The proxy base64 decodes the user part of the Route
      header.  For TCP, if a connection specified by the file descriptor
      is present and the creation time of the file descriptor matches
      the creation time encoded in the Route header, the proxy forwards
      the request over that connection.  For UDP, the proxy forwards the
      request from the encoded file descriptor to the source IP address
      and port.
   Algorithm 2: To decode the flow token take the flow identifier in the
      user portion of the URI, and base64 decode it, then verify the
      HMAC is correct by recomputing the HMAC and checking it matches.
      If the HMAC is not correct, the proxy SHOULD send a 403 response.
      If the HMAC was correct then the proxy should forward the request
      on the flow that was specified by the information in the flow
      identifier.  If this flow no longer exists, the proxy SHOULD send
      a 410 response to the request.

   Note that techniques to ensure that mid-dialog requests are routed
   over an existing flow are out of scope and therefore not part of this
   specification.  However, an approach such as having the Edge Proxy
   Record-Route with a flow token is one way to ensure that mid-dialog
   requests are routed over the correct flow.

6.  Registrar and Location Server Mechanisms


6.1  Processing Register Requests

   This specification updates the definition of a binding in RFC 3261
   [5] Section 10 and RFC 3327 [12] Section 5.3.

   When no instance-id is present in a Contact header field value in a
   REGISTER request, the corresponding binding is still between an AOR
   and the URI from that Contact header field value.  When an
   instance-id is present in a Contact header field value in a REGISTER
   request, the corresponding binding is between an AOR and the
   combination of instance-id and reg-id.  For a binding with an
   instance-id, the registrar still stores the Contact header field
   value URI with the binding, but does not consider the Contact URI for
   comparison purposes (the Contact URI is not part of the "key" for the
   binding).  The registrar MUST be prepared to receive, simultaneously
   for the same AOR, some registrations that use instance-id and reg-id
   and some that do not.

   Registrars which implement this specification, MUST support the Path
   header mechanism [12].

   In addition to the normal information stored in the binding record,
   some additional information MUST be stored for any registration that
   contains a reg-id header parameter in the Contact header field value.
   The registrar MUST store enough information to uniquely identify the
   network flow over which the request arrived.  For common operating
   systems with TCP, this would typically just be the file descriptor.
   For common operating systems with UDP this would typically be the
   file descriptor for the local socket that received the request, the
   local interface, and the IP address and port number of the remote
   side that sent the request.

   The registrar MUST also store all the Contact header field
   information including the reg-id and instance-id parameters and
   SHOULD also store the time at which the binding was last updated.  If
   a Path header field is present, RFC 3327 [12] requires the registrar
   to store this information as well.  If the registrar receives a re-
   registration, it MUST update the information that uniquely identifies
   the network flow over which the request arrived and SHOULD update the
   time the binding was last updated.

   The Registrar MUST include the 'outbound' option-tag in a Supported
   header field value in its responses to REGISTER requests.  The
   Registrar MAY be configured with local policy to reject any
   registrations that do not include the instance-id and reg-id to
   eliminate the amplification attack described in [19].  Note that the
   requirements in this section applies to both REGISTER requests
   received from an Edge Proxy as well as requests received directly
   from the UAC.


6.2  Forwarding Requests

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

   o  The proxy MUST NOT populate the target set with more than one
      contact with the same AOR and instance-id at a time.  If a request
      for a particular AOR and instance-id fails with a 410 response,
      the proxy SHOULD replace the failed branch with another target (if
      one is available) with the same AOR and instance-id, but a
      different reg-id.
   o  If two bindings have the same instance-id and reg-id, the proxy
      SHOULD prefer the contact that was most recently updated.

   The proxy uses normal forwarding rules looking at the Route of the
   message and the value of any stored Path header field vector in the
   registration binding to decide how to forward the request and
   populate the Route header in the request.  Additionally, when the
   proxy forwards a request to a binding that contains a reg-id, the
   proxy MUST send the request over the same network flow that was saved
   with the binding.  This means that for TCP, the request MUST be sent
   on the same TCP socket that received the REGISTER request.  For UDP,
   the request MUST be sent from the same local IP address and port over
   which the registration was received, to the same IP address and port
   from which the REGISTER was received.

   If a proxy or registrar receives 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 that use that
   flow (regardless of AOR).  Examples of this are a TCP socket closing
   or receiving a destination unreachable ICMP error on a UDP flow.
   Similarly, if a proxy closes a file descriptor, it MUST invalidate
   all the bindings with flows that use that file descriptor.

7.  Mechanisms for All Servers (Proxys, Registars, UAS)

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


7.1  STUN Processing

   This document defines a new STUN usage for inband connectivity
   checks.  The only STUN messages required by this usage are Binding
   Requests, Binding Responses, and Error Responses.  The UAC sends
   Binding Requests over the same UDP flow, TCP connection, or TLS
   channel used for sending SIP messages, once a SIP registration has
   been successfully processed on that flow.  These Binding Requests do
   not require any STUN attributes.  The UAS responds to a valid Binding
   Request with a Binding Response which MUST include the XOR-MAPPED-
   ADDRESS attribute.  After a successful STUN response is received over
   TCP or TLS over TCP, the underlying TCP connection is left in the
   active state.

   If the server receives SIP requests on a given interface and port, it
   MUST also provide a limited version of a STUN server on the same
   interface and port.  Specifically it MUST be capable of receiving and
   responding to STUN Binding Requests.

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

   When a URI is created that refers to a SIP device that supports STUN
   as described in this section, the 'keepalive' URI parameter "sip-stun", parameter, as
   defined in Section 10 MUST be added to the URI. URI, with a value of
   'stun'.  This allows a UA to inspect the URI to decide if it should
   attempt to send STUN requests to this location.  The sip-stun 'keepalive' tag
   typically would be present in the URI in the Route header field value
   of a REGISTER request and not be in the Request URI.

7.2.  Double CRLF Processing

   If the SIP server is acting as the TCP client and initiated the TCP
   connection (meaning that this host did the active open), then the SIP
   server MUST NOT perform any of the processing in this section.  The
   following only applies when the SIP server is acting as the TCP
   server (meaning that this host did the passive open).

   When the server receives a CRLF before the start line of a message on
   a flow, it MUST send some data back on that same flow within 3
   seconds.  If no message is actively being sent, it SHOULD send back a
   CRLF after waiting at least 1 second.  The reason for waiting at
   least 1 second is that if the other end has an incorrect
   implementation and incorrectly echoes the CRLF, this will stop the
   flow from going into a live-lock state.

8.  Example Message Flow

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

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

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

   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <>;tag=a73kszlfl
   To: Callee <>
   Call-ID: 1j9FpLxk3uxtm8tn@
   Supported: path
   Route: <;lr;sip-stun> <;lr;keepalive=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 <>;tag=a73kszlfl
   To: Callee <> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn@
   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
   backup instead of the primary.  They look like:

   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <>;tag=a73kszlfl
   To: Callee <>
   Call-ID: 1j9FpLxk3uxtm8tn-2@
   Supported: path
   Route: <;lr;sip-stun> <;lr;keepalive=stun>
   Contact: <sip:callee@>
   Content-Length: 0

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   From: Callee <>;tag=a73kszlfl
   To: Callee <> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn-2@
   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 6 contains the following
   Record-Route header field:

   Record-Route: <;lr>

   Message 11 seems seams strange in that it goes to the backup instead
   of the primary.  The Caller actually sends the message to the domain
   of the callee to a host (primary or backup) that is currently
   available.  How the domain does this is an implementation detail up
   to the domain and not part of this specification.

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

9.  Grammar

   This specification defines new Contact header field parameters,
   reg-id and +sip.instance.  The grammar includes the definitions from
   RFC 3261 [5] and includes the definition of uric from RFC 2396 [11].
   The ABNF[8] is:

    contact-params = c-p-q / c-p-expires / c-p-flow / c-p-instance
                     / contact-extension

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

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

    instance-val   = *uric ; defined in RFC 2396

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

10.  IANA Considerations


10.1  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 [13] .  The required
   information is:

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

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


10.2  SIP/SIPS URI Paramters

   This specification arguments the "SIP/SIPS URI Parameters" sub-
   registry as per the registry created by [14] .  The required
   information is:

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

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


10.3  SIP Option Tag

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

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


10.4  Media Feature Tag

   This section registers a new media feature tag, per the procedures
   defined in RFC 2506 [1].  The tag is placed into the sip tree, which
   is defined in RFC 3840 [10].

   Media feature tag name:  sip.instance

   ASN.1 Identifier:  New assignment by IANA.

   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 [23] 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 may 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.

11.  Security Considerations

   One of the key security concerns in this work is making sure that an
   attacker cannot hijack the sessions of a valid user and cause all
   calls destined to that user to be sent to the attacker.

   The simple case is when there are no edge proxies.  In this case, the
   only time an entry can be added to the routing for a given AOR is
   when the registration succeeds.  SIP 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.

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

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

12.  Open Issues

   Do we want to include the Double CRLF keep alive option?

   Are thre any deployments that could use Algorithm 1 and if not can we
   remove it?

   We should change syntax from "sip-stun" to "keep-alive=sip-stun".

13.  Requirements

   This specification was developed to meet the following requirements:

   1.  Must be able to detect that a UA supports these mechanisms.
   2.  Support UAs behind NATs.
   3.  Support TLS to a UA without a stable DNS name or IP address.
   4.  Detect failure of connection and be able to correct for this.
   5.  Support many UAs simultaneously rebooting.

   6.  Support a NAT rebooting or resetting.
   7.  Minimize initial startup load on a proxy.
   8.  Support architectures with edge proxies.


13.  Changes

   Note to RFC Editor:  Please remove this whole section.


13.1  Changes from 02 Version

   Removed Double CRLF Keepalive

   Changed ;sip-stun syntax to ;keepalive=stun

   Fixed incorrect text about TCP keepalives.

13.2  Changes from 01 Version

   Moved definition of instance-id from GRUU[17] draft to this draft.

   Added tentative text about Double CRLF Keep Alive

   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


13.3  Changes from 00 Version

   Moved TCP keep alive to be STUN.

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

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


14.  Acknowledgments

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


15.  References


15.1  Normative References

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

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

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

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

   [5]   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.

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

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

   [8]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

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

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

   [11]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396,
         August 1998.

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

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

   [14]  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.


15.2  Informative References

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

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

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

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

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

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

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

Authors' Addresses

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

   Phone:  +1 408 902-3341

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


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