Network Working Group                                   C. Jennings, Ed.
Internet-Draft                                             Cisco Systems
Updates:  3261,3327 (if approved)                           R. Mahy, Ed.
Expires:  April 25,  May 26, 2007                                       Plantronics
                                                       November 22, 2006

Managing Client Initiated Connections in the Session Initiation Protocol

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Copyright Notice

   Copyright (C) The Internet Society (2006).


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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
     2.1   Definitions  . . . . . . . . . . . . . . . . . . . . . . .  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   Keepalive Technique  . . . . . . . . . . . . . . . . . . . 10
   4.  User Agent Mechanisms  . . . . . . . . . . . . . . . . . . . . 12
     4.1   Instance ID Creation . . . . . . . . . . . . . . . . . . . 12
     4.2   Initial Registrations  . . . . . . . . . . . . . . . . . . 13
       4.2.1   Registration by Other Instances  . . . . . . . . . . . 14 15
     4.3   Sending Requests . . . . . . . . . . . . . . . . . . . . . 15
     4.4   Detecting Flow Failure . . . . . . . . . . . . . . . . . . 15
       4.4.1   Keepalive with TCP KEEPALIVE . . . . . . . . . . . . . 16
       4.4.2   Keepalive with STUN  . . . . . . . . . . . . . . . . . 16
       4.4.3   Flow Recovery  . . . . . . . . . . . . . . . . . . . . 16
   5.  Edge Proxy Mechanisms  . . . . . . . . . . . . . . . . . . . . 17 18
     5.1   Processing Register Requests . . . . . . . . . . . . . . . 17 18
     5.2   Generating Flow Tokens . . . . . . . . . . . . . . . . . . 18
     5.3   Forwarding Requests  . . . . . . . . . . . . . . . . . . . 18 19
     5.4   Edge Proxy Keepalive Handling  . . . . . . . . . . . . . . 19 20
   6.  Registrar Mechanisms: Processing REGISTER Requests . . . . . . 20
   7.  Authoritative Proxy Mechansims: Forwarding Requests  . . . . . 21 22
   8.  STUN Keepalive Processing  . . . . . . . . . . . . . . . . . . 22 23
     8.1   Explicit Probes  . . . . . . . . . . . . . . . . . . . . . 24 25
     8.2   Use with Sigcomp . . . . . . . . . . . . . . . . . . . . . 24 25
   9.  Example Message Flow . . . . . . . . . . . . . . . . . . . . . 25 26
   10.   Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . 28 29
   11.   Definition of 430 Flow Failed response code  . . . . . . . . 29 30
   12.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 29 30
     12.1  Contact Header Field . . . . . . . . . . . . . . . . . . . 29 30
     12.2  SIP/SIPS URI Parameters  . . . . . . . . . . . . . . . . . 30 31
     12.3  SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 30 31
     12.4  Response Code  . . . . . . . . . . . . . . . . . . . . . . 30 31
     12.5  Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 30 31
   13.   Security Considerations  . . . . . . . . . . . . . . . . . . 31 32
   14.   Requirements . . . . . . . . . . . . . . . . . . . . . . . . 32 33
   15.   Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . 32 33
     15.1  Changes from 04 05 Version  . . . . . . . . . . . . . . . . . 32 33
     15.2  Changes from 03 04 Version  . . . . . . . . . . . . . . . . . 33 34
     15.3  Changes from 02 03 Version  . . . . . . . . . . . . . . . . . 34 35
     15.4  Changes from 01 02 Version  . . . . . . . . . . . . . . . . . 34 36
     15.5  Changes from 01 Version  . . . . . . . . . . . . . . . . . 36
     15.6  Changes from 00 Version  . . . . . . . . . . . . . . . . . 35 36
   16.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 35 36
   A.  Default Flow Registration Backoff Times  . . . . . . . . . . . 35 37
   17.   References . . . . . . . . . . . . . . . . . . . . . . . . . 36 37
     17.1  Normative References . . . . . . . . . . . . . . . . . . . 36 37
     17.2  Informative References . . . . . . . . . . . . . . . . . . 37 39
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 38 40
       Intellectual Property and Copyright Statements . . . . . . . . 39 41

1.  Introduction

   There are many environments for SIP [RFC3261] 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 there may be a NAT, NAT could be present,
   which are is only capable of allowing new connections from the private
   address side to the public side.  This specification allows SIP
   registration when the UA is behind such a firewall or NAT.

   Most IP phones and personal computers get their network
   configurations dynamically via a protocol such as DHCP (Dynamic Host
   Configuration Protocol).  These systems typically do not have a
   useful name in the Domain Name System (DNS), and they definitely do
   not almost never
   have a long-term, stable DNS name that is appropriate for use in the
   subjectAltName of a certificate, as required by [RFC3261].  However,
   these systems can still act as a TLS client and form connections to a
   proxy or registrar which authenticates with a server certificate.
   The server can authenticate the UA using a shared secret in a digest
   challenge over that TLS connection.

   The key idea of this specification is that when a UA sends a REGISTER
   request, the proxy can later use this same network "flow", whether "flow"--whether
   this is a bidirectional stream of UDP datagrams, a TCP connection, or
   an analogous concept of another transport protocol to 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 specification also uses a
   keep alive
   defines how SIP implements the STUN keepalive usage.  The keepalive
   mechanism over each flow so that is used to keep NAT bindings fresh, and to allow the UA can to
   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 [RFC2119].

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 simultaneous registration gets a unique reg-id value.
   instance-id: This specification uses the word instance-id to refer to
      the value of the "sip.instance" media feature tag in the Contact
      header field.  This is a Uniform Resource Name (URN) that uniquely
      identifies this specific UA instance.
   outbound-proxy-set A set of SIP URIs (Uniform Resource Identifiers)
      that represents each of the outbound proxies (often Edge Proxies)
      with which the UA will attempt to maintain a direct flow.  The
      first URI in the set is often referred to as the primary outbound
      proxy and the second as the secondary outbound proxy.  There is no
      difference between any of the URIs in this set, nor does the
      primary/secondary terminology imply that one is preferred over the

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 or a network

   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 keepalive mechanism to keep their flow to the proxy or registrar

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

   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
   ("urn:uuid:00000000-0000-0000-0000-000A95A0E128") 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 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.

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 outbound proxy/registrar function for the
   domain.  Reliability is achieved by having the UA form two TCP
   connections to the domain.

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

      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 [I-D.ietf-sipping-config-
      framework], from adding other ways for a User Agent to discover
      its outbound-proxy-set.

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

   The UA is configured with multiple outbound proxy registration URIs.
   These URIs are configured into the UA through whatever the normal
   mechanism is to configure the proxy or registrar address in the UA.
   If the AOR is, the outbound-proxy-set might look
   something like ";keepalive=stun" and "sip:;keepalive=stun".  The "keepalive=stun" tag
   indicates that a SIP server supports STUN and SIP multiplexed over
   the same flow, as described later in this specification.  Note that
   each URI in the outbound-proxy-set could resolve to several different
   physical hosts.  The administrative domain that created these URIs
   should ensure that the two URIs resolve to separate hosts.  These
   URIs are handled according to normal SIP processing rules, so
   mechanisms like SRV can be used to do load balancing across a proxy

   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 4.4.3.  The multiple
   flows to many servers help reduce the load caused by the avalanche
   restart.  If a UA has multiple flows, and one of the servers fails,
   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 [RFC3327] 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
   identifier has two purposes.  First, it allows the Edge Proxy to map
   future requests back to the correct flow.  Second, because the
   identifier will only be returned if the user authentication with the
   registrar succeeds, it allows the Edge Proxy to indirectly check the
   user's authentication information via the registrar.  The identifier
   SHOULD be 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 [RFC3327].

3.5  Keepalive Technique

   A keepalive 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 describes using STUN
   [I-D.ietf-behave-rfc3489bis] over the same flow as the SIP traffic to
   perform the keepalive.  For connection-oriented transports (e.g.  TCP
   and TLS over TCP), the UAC MAY use TCP keepalives to detect flow
   failure if the UAC can send these keepalives and detect a keepalive
   failure according to the time frames described in Section 4.4.

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

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

   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 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 use a UUID URN [RFC4122]. [RFC4122] as its instance-id.  The UUID
   URN allows for non-
   centralized 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 [RFC4122] 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 [RFC3188].  Since
   there is no clear relationship between a SIP UA instance and a URN in
   this namespace, there is no way a selection of a value can be
   performed that guarantees that another UA instance doesn't choose the
   same value.

   The UA SHOULD include a "sip.instance" media feature tag as a UA
   characteristic [RFC3840] in requests and responses.  As described in
   [RFC3840], 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 [RFC2141].  One case where
   a UA may not want to include the URN in the sip.instance media
   feature tag is when it is making an anonymous request or some other
   privacy concern requires that the UA not reveal its identity.

      RFC 3840 [RFC3840] 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 [RFC3840] and in the caller
      preferences specification [RFC3841].  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 [RFC2141].  Lexical equality may could result in two URNs being
      considered unequal when they are actually equal.  In this specific
      usage of URNs, the only element which provides the URN is the SIP
      UA instance identified by that URN.  As a result, the UA instance
      SHOULD provide lexically equivalent URNs in each registration it
      generates.  This is likely to be normal behavior in any case;
      clients are not likely to modify the value of the instance ID so
      that it remains functionally equivalent yet lexigraphically
      different from previous registrations.

4.2  Initial Registrations

   UAs obtain at

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

   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 MUST also add include 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
   sequence of reg-id values does not have to be sequential but MUST be
   exactly the same reg-id sequence of reg-id values each time the device UA instance
   power cycles or reboots so that the reg-id values will collide with
   the previously used reg-id values.  This is so the registrar can
   replace the older registration.

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

   The UAC MUST indicate that it supports the Path header [RFC3327]
   mechanism, by including the 'path' option-tag in a Supported header
   field value in its REGISTER requests.  Other than optionally
   examining the Path vector in the response, this is all that is
   required of the UAC to support Path.

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

   Note that the UA needs to honor 503 (Service Unavailable) responses
   to registrations as described in RFC 3261 and RFC 3263 [RFC3263].  In
   particular, implementors should note that when receiving a 503
   (Service Unavailable) response with a Retry-After header field, the
   UA should 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 should 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

   Use of outbound with UDP flows typically prevents messages larger
   than 1300 bytes from being delivered using TCP as described in
   Section 18.1.1 RFC 3261.  Use of outbound in conjunction with UDP
   flows in environments where requests are expected to be larger than
   this size is therefore NOT RECOMMENDED.

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

4.2.1  Registration by Other Instances

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

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

4.3  Sending Requests

   When a UA is about to send a request, it first performs normal
   processing to select the next hop URI.  The UA can use a variety of
   techniques to compute the route set and accordingly the next hop URI.
   Discussion of these techniques is outside the scope of this document
   but could include mechanisms specified in RFC 3608 [RFC3608] (Service
   Route) and [I-D.rosenberg-sip-route-construct].

   The UA performs normal DNS resolution on the next hop URI (as
   described in RFC 3263 [RFC3263]) 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, there must 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.

      Note that if the UA wants its flow to work through NATs or
      firewalls it still needs to put the 'rport' parameter [RFC3581] 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 [I-D.ietf-sipping-nat-scenarios].

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 this section. 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.

   The time between keepalive 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  Issues such as battery
   consumption might motivate longer keepalive intervals.

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

4.4.1  Keepalive with TCP KEEPALIVE

   User Agents that are capable of generating per-connection TCP
   keepalives with timer values consistent with those in this section
   MAY use TCP keepalives instead of using STUN keepalives for TCP-based

4.4.2  Keepalive with STUN

   User Agents that form flows, check if the configured URI they are
   connecting to has a 'keepalive' URI parameter (defined in Section 12)
   with the value of 'stun'.  If the parameter is present and the UA is
   not already performing keepalives using another supported mechanism,
   the UA needs to periodically perform keepalive checks by sending STUN
   [I-D.ietf-behave-rfc3489bis] Binding Requests over the flow as
   described in Section 8.

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

4.4.3  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 keepalives have not timed out for min-regtime seconds
   (default of 120 seconds) after a registration.  For STUN-based
   keepalives, this means three successful STUN transactions over UDP or
   one successful STUN transaction over TCP.  If a flow is established
   and is alive after this amount of time, the number of consecutive
   registration failures is set to zero.  Each time a flow fails before
   two minutes, the number of consecutive registration failures is
   incremented by one.  Note that a failure during the initial STUN
   validation does not count against the number of consecutive
   registration failures.

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

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

   These times MAY be configurable in the UA.  The four times are:
   o  max-time with a default of 1800 seconds
   o  base-time-all-fail with a default of 30 seconds
   o  base-time-not-failed with a default of 90 seconds
   o  min-regtime with a default of 120 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 (900 to 1800
   seconds) between each attempt.

5.  Edge Proxy Mechanisms

5.1  Processing Register Requests

   When an Edge Proxy receives a registration request with a reg-id
   header parameter in the Contact header field, it typically needs to
   store a "flow token", containing information about determine
   if it (the edge proxy) will have to be visited for any subsequent
   requests sent to the flow from user agent identified in the
   previous hop, 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 [RFC3327].  The token MAY be placed in the userpart of the URI.  If the edge proxy Edge Proxy is the first SIP node after the
   UAC, it either MUST store a "flow token"--containing information
   about the flow token in a from the previous hop--in its Path header, URI, or reject the
   request.  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.

   Each subsequent edge proxy examines the first Path header field
   value.  If this URI does the Edge Proxy is not contain an 'ob' parameter, the edge
   proxy MUST ignore first SIP node
   after the reg-id parameter and UAC it MUST NOT include place an 'ob' URI parameter if it adds in a Path
   header field value.  If the first Path
   header field value contains an 'ob' parameter, this indicates that
   the first edge proxy performed outbound processing.  In this case the
   edge proxy MUST store a flow token in a Path header, unless  The Edge Proxy can determine if it has
   positive knowledge that the URI in previous Path header is reachable
   from any node on the public Internet, and that the next first
   hop SIP node
   can reach any node on the public Internet.  This insures that there
   is a reachable path from the authoritative proxy back to the User
   Agent.  Regardless if by examining the proxy includes a flow token, if it adds a
   Path Via header field value, it MUST include the 'ob' parameter in its
   Path URI.

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.  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 identifier approximately 16 octets
      long.  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.  (No NTP time is needed for UDP.)
      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-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 [RFC2104].  The concatenation of
      the HMAC and S are base64 encoded, as defined in [RFC3548], and
      used as the flow identifier.  When using IPv4 addresses, this will
      result in a 32-octet identifier.

5.3  Forwarding Requests

   When an Edge Proxy receives a request, it applies normal routing
   procedures with the following addition.  If the Edge Proxy receives a
   request where the edge proxy is the host in the topmost Route header
   field value, and the Route header contains a flow token, the proxy
   compares the flow in the flow token with the source of the request.
   If these refer to the same flow, the Edge Proxy removes the Route
   header and continues processing the request.  Otherwise, 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 remove the the
   Route header and forward the request over the flow that received the
   REGISTER request that caused 'logical flow'
   identified by the flow identifier token token, that is known to be created. 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 (Flow Failed)
   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 a TCP-based transport, 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 a UDP-
      based transport, 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
      (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 (Flow Failed) response to the

   Note that this specification needs mid-dialog requests to be routed
   over the same flows as those stored in the Path vector from the
   initial registration, but techniques to ensure that mid-dialog
   requests are routed over an existing flow are not part of this
   specification.  However, an approach such as having the Edge Proxy
   Record-Route with a flow token is one way to ensure that mid-dialog
   requests are routed over the correct flow.

5.4  Edge Proxy Keepalive Handling

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

6.  Registrar Mechanisms: Processing REGISTER Requests

   This specification updates the definition of a binding in RFC 3261
   [RFC3261] Section 10 and RFC 3327 [RFC3327] Section 5.3.

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

   Registrars which implement this specification MUST support the Path
   header mechanism [RFC3327].

   In addition to the normal information stored in the binding record,
   some additional information needs to be stored for any registration
   that contains an instance-id and a reg-id header parameter in the
   Contact header field value.  First the registrar examines all the first
   Path header field values, value, if any.  If any of these the Path header field exists and
   the first URI does not have an 'ob' URI parameter, the registrar MUST
   ignore the reg-id parameter and continue processing the request as if
   it did not support this specification.  Likewise if the REGISTER
   request visited an edge proxy, but no Path header field values are
   present, the registrar MUST ignore the reg-id parameter.
   Specifically, the registrar MUST use RFC 3261 Contact binding rules,
   and MUST NOT include the 'outbound' option-tag in its Supported
   header field.  The registrar can determine if it is the first hop by
   examining the Via header field.

   If the UAC has a direct flow with the registrar, the registrar MUST
   store enough information to uniquely identify the network flow over
   which the request arrived.  For common operating systems with TCP,
   this would typically just be the file descriptor and the time the
   file descriptor was opened.  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.

   In addition, unless the registrar has positive knowledge that the
   topmost Path header URI is reachable from the authoritative proxy, it
   must store the flow information for the previous hop.  The registrar
   MAY store this information by adding itself to the Path header field
   with an appropriate flow token.

   The registrar MUST also store all the Contact header field
   information including the reg-id and instance-id parameters and
   SHOULD also store the time at which the binding was last updated.  If
   a Path header field is present, RFC 3327 [RFC3327] requires the
   registrar to store this information as well.  If the registrar
   receives a re-registration, it MUST update any information that
   uniquely identifies the network flow over which the request arrived
   if that information has changed, and SHOULD update the time the
   binding was last updated.

   The Registrar MUST include the 'outbound' option-tag (defined in
   Section (Section 12.1)) in a Supported header field value in its
   responses to REGISTER requests for which it has performed outbound
   processing.  The Registrar MAY be configured with local policy to
   reject any registrations that do not include the instance-id and
   reg-id.  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.

   To be compliant with this specification, registrars which can receive
   SIP requests directly from a UAC without intervening edge proxies
   MUST implement support the STUN NAT Keepalive usage on its SIP ports as
   described in Section 8.

7.  Authoritative Proxy Mechansims: 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 430 Flow Failed (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 targetted instance has already
      provided its response.

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

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

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

8.  STUN Keepalive Processing

   This section describes changes to the SIP transport layer that allow
   SIP and the STUN [I-D.ietf-behave-rfc3489bis] NAT Keepalive usage to
   be mixed over the same flow.  The STUN messages are used to verify
   connectivity is still available over a flow and to provide periodic
   keepalives.  Note that these STUN keepalives are always sent to the
   next SIP hop.  STUN messages are not delivered end-to-end.

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

   If a server compliant to this section receives SIP requests on a
   given interface and port, it MUST also provide a limited version of a
   STUN server on the same interface and port as described in Section
   12.3 of [I-D.ietf-behave-rfc3489bis].  When STUN messages are sent
   with a SIP over TLS over TCP flow, the STUN messages are sent inside
   the TLS-protected channel.

      It is easy to distinguish STUN and SIP packets sent over UDP,
      because the first octet of a STUN packet has a value of 0 or 1
      while the first octet of a SIP message is never a 0 or 1.  For TCP
      or TLS over TCP flows, determining if the first octet of the next
      message in a stream is SIP or STUN is still straightforward,
      however straightforward.  As
      with any stream-based protocol, implementations need to be preared
      to receive STUN messages which cross a stream buffer boundary, and
      SIP and STUN messages which share the same stream buffer.

   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 misconfiguration) despite these warnings, the node may
   could be blacklisted for UDP traffic, or cause its TCP server to
   loose framing over its connection.  For each target node (as
   determined by IP address, address family, and port number), the
   sender needs to determine if that destination is validated to support
   STUN, that it does not support STUN, or that it needs to be

   When a URI is created that refers to a SIP device node that supports STUN as
   described in this section, the 'keepalive' URI parameter, as defined
   in Section 12 SHOULD be added to the 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.  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.

   A SIP node MUST NOT send STUN requests over a flow unless it has an
   explicit indication that the target next hop SIP server claims to
   support STUN.  For example, automatic or manual configuration of an
   outbound-proxy-set which contains the keepalive=stun parameter 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.  A SIP node MAY also
   probe the next hop using a SIP OPTIONS request to check for support
   of the 'sip-stun' option tag in a Supported header field.

   Futhermore, even with explicit indication of next hop STUN support, a
   SIP node needs to validate support for STUN the first time it sends
   traffic to a specific unvalidated target destination.  If an explicit
   probe indicates support for the 'sip-stun' option-tag, that
   destination is validated for STUN support.  If an explicit probe does
   not indicate support for the 'sip-stun' option-tag, the target
   destination does not support STUN request, and the UAC MUST NOT send
   further STUN requests to this destination.  A SIP node MAY send one
   STUN request and its retransmissions to an unvalidated destination.
   If a STUN request ever succeeds to a destination, that destination is
   thereafter validated for STUN support.  If this initial STUN request
   does not result in a STUN response, the SIP node MUST NOT send
   additional STUN requests over this flow, unless and until a next-hop
   probe later validates the destination.  In addition, the SIP node
   SHOULD remember unvalidated destination nodes that have been used
   within one hour and SHOULD NOT send additional STUN messages to any
   of these destinations.  Note that until STUN support has been
   verified, an initial STUN failure over UDP is not considered a flow
   failure.  For UDP flows, an unvalidated flow can still be reused for
   SIP traffic, however for unvalidated TCP or TLS over TCP flows, the
   connection over which STUN requests were sent MUST be closed.

      Typically, a SIP node first sends a SIP request and waits to
      receive a final response (other than a 408 response) over a flow
      to a new target destination, before sending any STUN messages.
      When scheduled for the next NAT refresh, the SIP node sends a STUN
      request to the target.  If none of the STUN requests succeed
      (result in a STUN success response), and the UAC has not already
      done so, the UAC sends an OPTIONS request to the next hop to
      verify support for the 'sip-stun' option-tag.

   Once a destination is validated to support STUN messages, 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.  A 408 response to a next-hop OPTIONS probe is also
   considered a flow failure.

8.1  Explicit Probes

   This section defines a new SIP option-tag called 'sip-stun'.
   Advertising this option-tag indicates that the server can receive SIP
   messages and STUN messages as part of the NAT Keepalive usage on the
   same port.  Clients that want to probe a SIP server to determine
   support for STUN, can send an OPTIONS request to the next hop by
   setting the Max-Forwards header field to 0.  The OPTIONS response
   will contain a Supported header field with a list of the server's
   supported option-tags.

      A UAC SHOULD NOT include the 'sip-stun' option-tag in a Proxy-
      Require header.  This is because a request with this header will
      fail in some topologies where the first proxy support sip-stun,
      but a subsequent proxy does not.  Note that RFC 3261 does not
      allow proxies to remove option-tags from a Proxy-Require header

8.2  Use with Sigcomp

   When STUN is used together with SigComp [RFC3320] compressed SIP
   messages over the same flow, how the STUN messages are sent depends
   on the transport protocol.  For UDP flows, the STUN messages are
   simply sent uncompressed, "outside" of SigComp.  This is supported by
   multiplexing STUN messages with SigComp messages by checking the two
   topmost bits of the message.  These bits are always one for SigComp,
   or zero for STUN.

      All SigComp messages contain a prefix (the five most-significant
      bits of the first byte are set to one) that does not occur in
      UTF-8 encoded text messages [RFC-2279], so for applications which
      use this encoding (or ASCII encoding) it is possible to multiplex
      uncompressed application messages and SigComp messages on the same
      UDP port.
      The most significant two bits of every STUN message are both
      zeroes.  This, combined with the magic cookie, aids in
      differentiating STUN packets from other protocols when STUN is
      multiplexed with other protocols on the same port.

   For TCP-based flows, SigComp requires that all messages are processed
   by the SigComp compressor to facilitate framing.  For these
   transports, STUN messages are sent encapsulated in the SigComp "well-
   known shim header" as described in Appendix A Section 11 of RFC 3320 [RFC3320]. [I-D.ietf-rohc-

      Because the bytecodes expressed in the well-known shim header do
      not store any state, correlation of such SigComp requests to a
      compartment is not necessary.  To avoid ambiguity, we add the
      following requirement:  any SigComp state that might result from a
      message that, once decompressed, turns out to be a STUN message,
      MUST be discarded.

9.  Example Message Flow

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

                   [ 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 ";lr;keepalive=stun"
   and ";lr;keepalive=stun".  The Callee REGISTER in
   message (1) looks like:

   Via: SIP/2.0/UDP;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <>;tag=7F94778B653B
   To: Callee <>
   Call-ID: 16CB75F21C70
   Supported: path
   Route: <;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=7F94778B653B
   To: Callee <>;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:

   Via: SIP/2.0/UDP;branch=z9hG4bKnqr9bym
   Max-Forwards: 70
   From: Callee <>;tag=755285EABDE2
   To: Callee <>
   Call-ID: E05133BD26DD
   Supported: path
   Route: <;lr;keepalive=stun>
   Contact: <sip:callee@>
   Content-Length: 0

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP;branch=z9hG4bKnqr9bym
   From: Callee <>;tag=755285EABDE2
   To: Callee <>;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.

10.  Grammar

   This specification defines new Contact header field parameters,
   reg-id and +sip.instance.  The grammar includes the definitions from
   RFC 3261 [RFC3261] and includes the definition of uric from RFC 2396

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

   The ABNF[RFC4234] is:

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

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

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

    instance-val   = *uric ; defined in RFC 2396

   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 may 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 [RFC3968].  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.]

12.2  SIP/SIPS URI Parameters

   This specification arguments the "SIP/SIPS URI Parameters" sub-
   registry as per the registry created by [RFC3969].  The required
   information is:

       Parameter Name  Predefined Values  Reference
       keepalive        stun               [RFC AAAA]
       ob                                  [RFC AAAA]

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

12.3  SIP Option Tag

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

   Name: outbound
   Description: This option-tag is used to identify Registrars which
      support extensions for Client Initiated Connections.  A Registrar
      places this option-tag in a Supported header to communicate the
      Registrar's support for this extension to the registering User
   Name: sip-stun
   Description: This option-tag is used to identify SIP servers which
      can receive STUN requests described in the STUN NAT Keepalive
      usage on the same ports they use to receive SIP messages.

12.4  Response Code

   This section registers a new SIP Response Code, as per the guidelines
   in Section 27.1 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 [RFC2506].  The tag is placed into the sip tree,
   which is defined in RFC 3840 [RFC3840].

   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 [RFC3841] 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 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.

   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 registrar stores the
   value of the Path header field.  Later when the registrar forwards a
   request destined for the UA, it copies the stored value of the Path
   header field into the Route header field of the request and forwards
   the request to the Edge Proxy.

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

14.  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.

15.  Changes

   Note to RFC Editor:  Please remove this whole section.

15.1  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 keepalive=stun could be added to Path.

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

   Scrubbed document for avoidable lowercase mays, shoulds, and musts.

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

15.2  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 reregistration typically happening over same flow
   as original registration.

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

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

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

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


15.3  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 failover 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 misteaks.


15.4  Changes from 02 Version

   Removed Double CRLF Keepalive

   Changed ;sip-stun syntax to ;keepalive=stun

   Fixed incorrect text about TCP keepalives.


15.5  Changes from 01 Version

   Moved definition of instance-id from GRUU[I-D.ietf-sip-gruu] 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


15.6  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.

16.  Acknowledgments

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

Appendix A.  Default Flow Registration Backoff Times

   The base-time used for the flow re-registration backoff times
   described in Section 4.4.3 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         |

17.  References

17.1  Normative References

              Rosenberg, J., "Simple Traversal of UDP Through Underneath Network
              Address Translators (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-05 (work in progress),
              July 2005.
              October 2006.

              Surtees, A., "Implementer's Guide for SigComp",
              draft-ietf-rohc-sigcomp-impl-guide-08 (work in progress),
              October 2006.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2506]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
              Registration Procedure", BCP 31, RFC 2506, March 1999.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3320]  Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
              Liu, Z., and J. Rosenberg, "Signaling Compression
              (SigComp)", RFC 3320, January 2003.

   [RFC3327]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol
              (SIP) Extension Header Field for Registering Non-Adjacent
              Contacts", RFC 3327, December 2002.

   [RFC3581]  Rosenberg, J. and H. Schulzrinne, "An Extension to the
              Session Initiation Protocol (SIP) for Symmetric Response
              Routing", RFC 3581, August 2003.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

   [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968,
              December 2004.

   [RFC3969]  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.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

17.2  Informative References

              Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
              (SIP)", draft-ietf-sip-gruu-04 draft-ietf-sip-gruu-11 (work in progress),
              July 2005.
              October 2006.

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

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

              Rosenberg, J., "Clarifying Construction "Construction of the Route Header Field in
              the Session Initiation Protocol  (SIP)",
              draft-rosenberg-sip-route-construct-02 (work in progress),
              July 2005.
              October 2006.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC3188]  Hakala, J., "Using National Bibliography Numbers as
              Uniform Resource Names", RFC 3188, October 2001.

   [RFC3548]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

   [RFC3608]  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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