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

Network Working Group                                   C. Jennings, Ed.
Internet-Draft                                             Cisco Systems
Updates:  3261,3327                                         R. Mahy, Ed.
(if approved)                                               Unaffiliated
Intended status:  Standards Track                           May 11, 2009
Expires:  November 12, 2009


Managing Client Initiated Connections in the Session Initiation Protocol
                                 (SIP)
                       draft-ietf-sip-outbound-17

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on November 12, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The Session Initiation Protocol (SIP) allows proxy servers to
   initiate TCP connections or to send asynchronous UDP datagrams to
   User Agents in order to deliver requests.  However, in a large number
   of real deployments, many practical considerations, such as the
   existence of firewalls and Network Address Translators (NATs) or the
   use of TLS with server-provided certificates, 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 from
   the User Agent to its Registrar.





























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  5
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Summary of Mechanism . . . . . . . . . . . . . . . . . . .  7
     3.2.  Single Registrar and UA  . . . . . . . . . . . . . . . . .  7
     3.3.  Multiple Connections from a User Agent . . . . . . . . . .  9
     3.4.  Edge Proxies . . . . . . . . . . . . . . . . . . . . . . . 11
     3.5.  Keep alive Technique . . . . . . . . . . . . . . . . . . . 12
       3.5.1.  CRLF Keep alive Technique  . . . . . . . . . . . . . . 13
       3.5.2.  STUN Keep alive Technique  . . . . . . . . . . . . . . 13
   4.  User Agent Procedures  . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Instance ID Creation . . . . . . . . . . . . . . . . . . . 13
     4.2.  Registrations  . . . . . . . . . . . . . . . . . . . . . . 15
       4.2.1.  Initial Registrations  . . . . . . . . . . . . . . . . 15
       4.2.2.  Subsequent REGISTER requests . . . . . . . . . . . . . 17
       4.2.3.  Third Party Registrations  . . . . . . . . . . . . . . 17
     4.3.  Sending Non-REGISTER Requests  . . . . . . . . . . . . . . 17
     4.4.  Keep-alives and Detecting Flow Failure . . . . . . . . . . 18
       4.4.1.  Keep alive with CRLF . . . . . . . . . . . . . . . . . 19
       4.4.2.  Keep alive with STUN . . . . . . . . . . . . . . . . . 21
     4.5.  Flow Recovery  . . . . . . . . . . . . . . . . . . . . . . 21
   5.  Edge Proxy Procedures  . . . . . . . . . . . . . . . . . . . . 22
     5.1.  Processing Register Requests . . . . . . . . . . . . . . . 22
     5.2.  Generating Flow Tokens . . . . . . . . . . . . . . . . . . 23
     5.3.  Forwarding Non-REGISTER Requests . . . . . . . . . . . . . 23
       5.3.1.  Processing Incoming Requests . . . . . . . . . . . . . 24
       5.3.2.  Processing Outgoing Requests . . . . . . . . . . . . . 24
     5.4.  Edge Proxy Keep alive Handling . . . . . . . . . . . . . . 25
   6.  Registrar Procedures . . . . . . . . . . . . . . . . . . . . . 25
   7.  Authoritative Proxy Procedures: Forwarding Requests  . . . . . 27
   8.  STUN Keep alive Processing . . . . . . . . . . . . . . . . . . 28
     8.1.  Use with Sigcomp . . . . . . . . . . . . . . . . . . . . . 29
   9.  Example Message Flow . . . . . . . . . . . . . . . . . . . . . 30
     9.1.  Subscription to configuration package  . . . . . . . . . . 30
     9.2.  Registration . . . . . . . . . . . . . . . . . . . . . . . 32
     9.3.  Incoming call and proxy crash  . . . . . . . . . . . . . . 34
     9.4.  Re-registration  . . . . . . . . . . . . . . . . . . . . . 37
     9.5.  Outgoing call  . . . . . . . . . . . . . . . . . . . . . . 38
   10. Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
     11.1. Flow-Timer Header Field  . . . . . . . . . . . . . . . . . 40
     11.2. 'reg-id' Contact Header Field Parameter  . . . . . . . . . 40
     11.3. SIP/SIPS URI Parameters  . . . . . . . . . . . . . . . . . 41
     11.4. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 41
     11.5. 430 (Flow Failed) Response Code  . . . . . . . . . . . . . 41



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     11.6. 439 (First Hop Lacks Outbound Support) Response Code . . . 42
     11.7. Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 42
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 43
   13. Operational Notes on Transports  . . . . . . . . . . . . . . . 44
   14. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 45
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 45
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 46
     16.2. Informational References . . . . . . . . . . . . . . . . . 47
   Appendix A.  Default Flow Registration Backoff Times . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48








































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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, but the most likely
   is a NAT or a firewall in between the SIP UA and the proxy.  Many
   such devices will only allow outgoing connections.  This
   specification allows a SIP User Agent behind such a firewall or NAT
   to receive inbound traffic associated with registrations or dialogs
   that it initiates.

   Most IP phones and personal computers get their network
   configurations dynamically via a protocol such as DHCP (Dynamic Host
   Configuration Protocol).  These systems typically do not have a
   useful name in the Domain Name System (DNS), and they almost never
   have a long-term, stable DNS name that is appropriate for use in the
   subjectAltName of a certificate, as required by [RFC3261].  However,
   these systems can still act as a Transport Layer Security (TLS)
   [RFC4346] client and form outbound connections to a proxy or
   registrar which authenticates with a server certificate.  The server
   can authenticate the UA using a shared secret in a digest challenge
   (as defined in Section 22 of RFC 3261) over that TLS connection.
   This specification allows a SIP User Agent who has to initiate the
   TLS connection to receive inbound traffic associated with
   registrations or dialogs that it initiates.

   The key idea of this specification is that when a UA sends a REGISTER
   request or a dialog-forming request, the proxy can later use this
   same network "flow"--whether this is a bidirectional stream of UDP
   datagrams, a TCP connection, or an analogous concept in another
   transport protocol--to forward any incoming requests that need to go
   to this UA in the context of the registration or dialog.

   For a UA to receive incoming requests, the UA has to connect to a
   server.  Since the server can't connect to the UA, the UA has to make
   sure that a flow is always active.  This requires the UA to detect
   when a flow fails.  Since such detection takes time and leaves a
   window of opportunity for missed incoming requests, this mechanism
   allows the UA to register over multiple flows at the same time.  This
   specification also defines two keep alive schemes.  The keep alive
   mechanism is used to keep NAT bindings fresh, and to allow the UA to
   detect when a flow has failed.


2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [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 [RFC3261], and forwards those requests
      to specific Contact URIs.  (In [RFC3261], the role which is
      authoritative for REGISTER requests for a specific AOR is a
      Registration Server.)
   Edge Proxy:  An Edge Proxy is any proxy that is located topologically
      between the registering User Agent and the Authoritative Proxy.
      The "first" edge proxy refers to the first edge proxy encountered
      when a UA sends a request.
   Flow:  A Flow is a network transport layer association between two
      hosts that is represented by the network address and port number
      of both ends and by the transport protocol.  For TCP, a flow is
      equivalent to a TCP connection.  For UDP a flow is a bidirectional
      stream of datagrams between a single pair of IP addresses and
      ports of both peers.  With TCP, a flow often has a one to one
      correspondence with a single file descriptor in the operating
      system.
   Flow Token:  An identifier which uniquely identifies a flow which can
      be included in a SIP URI (Uniform Resource Identifier).
   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 for a different flow, each concurrent registration
      gets a unique reg-id value.
   instance-id:  This specification uses the word instance-id to refer
      to the value of the "sip.instance" media feature tag in the
      Contact header field.  This is a Uniform Resource Name (URN) that
      uniquely identifies this specific UA instance.
   ob Parameter:  The 'ob' parameter is a SIP URI parameter which has
      different meaning depending on context.  In a Path header field
      value it is used by the first edge proxy to indicate that a flow
      token was added to the URI.  In a Contact or Route header field
      value it indicates that the UA would like other requests in the
      same dialog routed over the same flow.
   outbound-proxy-set:  A set of SIP URIs (Uniform Resource Identifiers)
      that represents each of the outbound proxies (often Edge Proxies)
      with which the UA will attempt to maintain a direct flow.  The
      first URI in the set is often referred to as the primary outbound
      proxy and the second as the secondary outbound proxy.  There is no
      difference between any of the URIs in this set, nor does the
      primary/secondary terminology imply that one is preferred over the
      other.




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3.  Overview

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

   This entire section is non-normative.

3.1.  Summary of Mechanism

   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 flows 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 correspond to the same UA.  The
   registrar can use the reg-id label to recognize whether a UA is
   creating a new flow or refreshing or replacing an old one, possibly
   after a reboot or a network failure.

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

   When sending a dialog-forming request, a UA can also ask its first
   edge proxy to route subsequent requests in that dialog over the same
   flow.  This is necessary whether the UA has registered or not.

   UAs use a simple periodic message as a keep alive mechanism to keep
   their flow to the proxy or registrar alive.  For connection oriented
   transports such as TCP this is based on carriage-return and line-feed
   sequences (CRLF), while for transports that are not connection
   oriented this is accomplished by using a SIP-specific usage profile
   of STUN (Session Traversal Utilities for NAT) [RFC5389].

3.2.  Single Registrar and UA

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






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      +-----------+
      | 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 also includes a reg-id Contact header field which is used to
   allow the registrar to detect and avoid keeping invalid contacts when
   a UA reboots or reconnects after its old connection has failed for
   some reason.

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

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bK-bad0ce-11-1036
   Max-Forwards: 70
   From: Bob <sip:bob@example.com>;tag=d879h76
   To: Bob <sip:bob@example.com>
   Call-ID: 8921348ju72je840.204
   CSeq: 1 REGISTER
   Supported: path, outbound
   Contact: <sip:line1@192.0.2.2;transport=tcp>; reg-id=1;
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-000A95A0E128>"
   Content-Length: 0

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

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

   When Alice sends a request to Bob, his authoritative proxy selects



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   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 over
   an existing flow, instead of resolving the Contact URI using the
   procedures in [RFC3263] and trying to form a new flow to that
   contact.

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

3.3.  Multiple Connections from a User Agent

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

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

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

   The UA is configured with multiple outbound proxy registration URIs.



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   These URIs are configured into the UA through whatever the normal
   mechanism is to configure the proxy address and AOR in the UA.  If
   the AOR is alice@example.com, the outbound-proxy-set might look
   something like "sip:primary.example.com" and "sip:
   secondary.example.com".  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 DNS SRV [RFC2782]
   can be used to do load balancing across a proxy farm.  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.

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

   When a single server fails, all the UAs that have a flow through it
   will detect a flow failure and try to reconnect.  This can cause
   large loads on the server.  When large numbers of hosts reconnect
   nearly simultaneously, this is referred to as the avalanche restart
   problem, and is further discussed in Section 4.5.  The multiple flows
   to many servers help reduce the load caused by the avalanche restart.
   If a UA has multiple flows, and one of the servers fails, the UA
   delays a recommended amount of 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.

   Scalability is achieved by using DNS SRV [RFC2782] to load balance
   the primary connection across a set of machines that can service the
   primary connection, and also using DNS SRV to load balance across a
   separate set of machines that can service the secondary connection.
   The deployment here requires that DNS is configured with one entry
   that resolves to all the primary hosts and another entry that
   resolves to all the secondary hosts.  While this introduces
   additional DNS configuration, the approach works and requires no
   additional SIP extensions to [RFC3263].

   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.




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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.  There could be a NAT or firewall between the UA and the
   Edge Proxy.

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

   The Edge Proxy includes a Path header [RFC3327] so that when the
   proxy/registrar later forwards a request to this UA, the request is
   routed through the Edge Proxy.

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

   The term Edge Proxy is often used to refer to deployments where the
   Edge Proxy is in the same administrative domain as the Registrar.



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   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 [RFC3327].

3.5.  Keep alive Technique

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

   This document also suggests timer values for these client keep alive
   mechanisms.  These timer values were chosen to keep most NAT and
   firewall bindings open, to detect unresponsive servers within 2
   minutes, and to mitigate against the avalanche restart problem.
   However, the client may choose different timer values to suit its
   needs, for example to optimize battery life.  In some environments,
   the server can also keep track of the time since a ping was received
   over a flow to guess the likelihood that the flow is still useful for
   delivering SIP messages.

   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 4.5 to provide congestion
   relief when a large number of agents simultaneously reboot.

   A keep alive mechanism needs to keep NAT bindings refreshed; for
   connections, it also needs to detect failure of a connection; and for
   connectionless transports, it needs to detect flow failures including
   changes to the NAT public mapping.  For connection oriented
   transports such as TCP [RFC0793] and SCTP [RFC4960], this
   specification describes a keep alive approach based on sending CRLFs.
   For connectionless transport, such as UDP [RFC0768], this
   specification describes using STUN [RFC5389] over the same flow as
   the SIP traffic to perform the keep alive.

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



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   outside the scope of this document.

3.5.1.  CRLF Keep alive Technique

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

      Sending a CRLF over a connection-oriented transport is backwards
      compatible (because of requirements in Section 7.5 of [RFC3261]),
      but only implementations which support this specification will
      respond to a "ping" with a "pong".

3.5.2.  STUN Keep alive Technique

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

   For connection-less transports, a flow definition could change
   because a NAT device in the network path reboots and the resulting
   public IP address or port mapping for the UA changes.  To detect
   this, STUN requests are sent over the same flow that is being used
   for the SIP traffic.  The proxy or registrar acts as a limited
   Session Traversal Utilities for NAT (STUN) [RFC5389] server on the
   SIP signaling port.

      Note:  The STUN mechanism is very robust and allows the detection
      of a changed IP address and port.  Many other options were
      considered, but the SIP Working Group selected the STUN-based
      approach.  Approaches using SIP requests were abandoned because
      many believed that good performance and full backwards
      compatibility using this method were mutually exclusive.


4.  User Agent Procedures

4.1.  Instance ID Creation

   Each UA MUST have an Instance Identifier Uniform Resource Name (URN)
   [RFC2141] 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.




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   A UA SHOULD create a UUID URN [RFC4122] as its instance-id.  The UUID
   URN allows for non-centralized computation of a URN based on time,
   unique names (such as a MAC address), or a random number generator.

      A device like a soft-phone, when first installed, can generate a
      UUID [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 on this device.

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

   To convey its instance-id in both requests and responses, the UA
   includes a "sip.instance" media feature tag as a UA characteristic
   [RFC3840].  This media feature tag is encoded in the Contact header
   field as the "+sip.instance" Contact header field parameter.  One
   case where a UA could prefer to omit 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.

      [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 the callee capabilities [RFC3841]
      and the caller preferences [RFC3841] specifications.  When the
      instance ID is used in this specification, it is "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
      [RFC2141].  Lexical equality could result in two URNs being
      considered unequal when they are actually equal.  In this specific
      usage of URNs, the only element which provides the URN is the SIP
      UA instance identified by that URN.  As a result, the UA instance



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      has to provide lexically equivalent URNs in each registration it
      generates.  This is likely to be normal behavior in any case;
      clients are not likely to modify the value of the instance ID so
      that it remains functionally equivalent yet lexicographically
      different from previous registrations.

4.2.  Registrations

4.2.1.  Initial Registrations

   At configuration time, UAs obtain one or more SIP URIs representing
   the default outbound-proxy-set.  This specification assumes the set
   is determined via any of a number of configuration mechanisms, and
   future specifications can define additional mechanisms such as using
   DNS to discover this set.  How the UA is configured is outside the
   scope of this specification.  However, a UA MUST support sets with at
   least two outbound proxy URIs and SHOULD support sets with up to four
   URIs.

   For each outbound proxy URI in the set, the UAC SHOULD send a
   REGISTER request using this URI as the default outbound proxy.
   (Alternatively, the UA could limit the number of flows formed to
   conserve battery power, for example).  If the set has more than one
   URI, the UAC MUST send a REGISTER request to at least two of the
   default outbound proxies from the set.  UAs that support this
   specification MUST include the outbound option tag in a Supported
   header field in a REGISTER request.  Each of these REGISTER requests
   will use a unique Call-ID.  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.

   REGISTER requests, other than those described in Section 4.2.3, MUST
   include an instance-id media feature tag as specified in Section 4.1.

   For registration requests in accordance to this specification, the UA
   MUST include reg-id parameter in the Contact header field that is
   distinct from other reg-id parameters used from the same
   +sip.instance and AOR.  Each one of these registrations will form a
   new flow from the UA to the proxy.  The sequence of reg-id values
   does not have to be sequential but MUST be exactly the same sequence
   of reg-id values each time the UA instance power cycles or reboots so
   that the reg-id values will collide with the previously used reg-id
   values.  This is so the registrar can replace the older
   registrations.






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

   The UAC MUST support the Path header [RFC3327] mechanism, and
   indicate its support 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 examines successful registration responses for the presence
   of an outbound option-tag in a Require header field value.  Presence
   of this option-tag indicates that the registrar is compliant with
   this specification, and that any edge proxies which needed to
   participate are also compliant.  If the registrar did not support
   outbound, the UA has potentially registered an un-routable contact.
   It is the responsibility of the UA to remove any inappropriate
   Contacts.

   If outbound registration succeeded, as indicated by the presence of
   the outbound option-tag in the Require header field of a successful
   registration response, the UA begins sending keep alives as described
   in Section 4.4.

   Note that the UA needs to honor 503 (Service Unavailable) responses
   to registrations as described in [RFC3261] and [RFC3263].  In
   particular, implementors should note that when receiving a 503
   (Service Unavailable) response with a Retry-After header field, the
   UA is expected to wait the indicated amount of time and retry the
   registration.  A Retry-After header field value of 0 is valid and
   indicates the UA is expected to retry the REGISTER request
   immediately.  Implementations need to ensure that when retrying the
   REGISTER request, they revisit the DNS resolution results such that
   the UA can select an alternate host from the one chosen the previous
   time the URI was resolved.

   If the registering UA receives a 439 (First Hop Lacks Outbound
   Support) response to a REGISTER request, it MAY re-attempt
   registration without using the outbound mechanism (subject to local
   policy at the client).  If the client has one or more alternate
   outbound proxies available, it MAY re-attempt registration through
   such outbound proxies.  See Section 11.6 for more information on the



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   439 response code.

4.2.2.  Subsequent REGISTER requests

   Registrations for refreshing a binding and for removing a binding use
   the same instance-id and reg-id values as the corresponding initial
   registration where the binding was added.  Registrations which merely
   refresh an existing binding are sent over the same flow as the
   original registration where the binding was added.

   If a re-registration is rejected with a recoverable error response,
   for example by a 503 (Service Unavailable) containing a Retry-After
   header, the UAC SHOULD NOT tear down the corresponding flow if the
   flow uses a connection-oriented transport such as TCP.  As long as
   "pongs" are received in response to "pings", the flow SHOULD be kept
   active until a non-recoverable error response is received.  This
   prevents unnecessary closing and opening of connections.

4.2.3.  Third Party Registrations

   In an initial registration or re-registration, a UA MUST NOT include
   a reg-id header parameter in the Contact header field if the
   registering UA is not the same instance as the UA referred to by the
   target Contact header field.  (This practice is occasionally used to
   install forwarding policy into registrars.)

   A UAC also MUST NOT include an instance-id feature tag or reg-id
   Contact header field parameter in a request to un-register all
   Contacts (a single Contact header field value with the value of "*").

4.3.  Sending Non-REGISTER Requests

   When a UAC 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.
   UAs that support this specification SHOULD include the outbound
   option tag in a Supported header field in a request that is not a
   REGISTER request.

   The UAC performs normal DNS resolution on the next hop URI (as
   described in [RFC3263]) to find a protocol, IP address, and port.
   For protocols that don't use TLS, if the UAC has an existing flow to
   this IP address, and port with the correct protocol, then the UAC
   MUST use the existing connection.  For TLS protocols, there MUST also
   be a match between the host production in the next hop and one of the
   URIs contained in the subjectAltName in the peer certificate.  If the
   UAC cannot use one of the existing flows, then it SHOULD form a new



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   flow by sending a datagram or opening a new connection to the next
   hop, as appropriate for the transport protocol.

   Typically, a UAC using the procedures of this document and sending a
   dialog-forming request will want all subsequent requests in the
   dialog to arrive over the same flow.  If the UAC is using a GRUU
   [I-D.ietf-sip-gruu] that was instantiated using a Contact header
   field value that included an "ob" parameter, the UAC sends the
   request over the flow used for registration and susequent requests
   will arrive over that same flow.  If the UAC is not using such a
   GRUU, then the UAC adds an "ob" parameter to its Contact header field
   value.  This will cause all subsequent requests in the dialog to
   arrive over the flow instantiated by the dialog-forming request.
   This case is typical when the request is sent prior to registration,
   such as in the the initial subcription dialog for the configuration
   framework [I-D.ietf-sipping-config-framework].

      Note that if the UAC wants a UDP 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.  Keep-alives and Detecting Flow Failure

   Keep alives are used for refreshing NAT/firewall bindings and
   detecting flow failure.  Flows can fail for many reasons including
   NATs rebooting and Edge Proxies crashing.

   As described in Section 4.2, a UA that registers will begin sending
   keep alives after an appropriate registration response.  A UA that
   does not register (for example, a PSTN gateway behind a firewall) can
   also send keep alives under certain circumstances.

   Under specific circumstances, a UAC might be allowed to send STUN
   keep alives even if the procedures in Section 4.2 were not completed,
   provided that there is an explicit indication that the target first
   hop SIP node supports STUN keep alives.  This applies for example to
   a non-registering UA or to a case where the UA registration
   succeeded, but the response did not include the outbound option-tag
   in the Require header field.

      Note that a UA can "always" send a double CRLF (a "ping") over
      connection-oriented transports as this is already allowed by
      Section 7.5/[RFC3261], However a UA that did not register using
      outbound registration cannot expect a CRLF in response (a "pong")
      unless the UA has an explicit indication that CRLF keep alives are
      supported as described in this section.  Likewise, a UA that did



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      not successfully register with outbound procedures needs explicit
      indication that the target first hop SIP node supports STUN keep
      alives before it can send any STUN messages.

   A configuration option indicating keep alive support for a specific
   target is considered an explicit indication.  If these conditions are
   satisfied, the UA sends its keep alives according to the same
   guidelines described in the rest of this section as UAs which
   register.

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

   When a successful registration response contains the Flow-Timer
   header field, the value of this header field is the number of seconds
   the server is prepared to wait without seeing keep alives before it
   could consider the corresponding flow dead.  Note that the server
   would wait for an amount of time larger than the Flow-Timer in order
   to have a grace period to account for transport delay.  The UA MUST
   send keep alives at least as often as this number of seconds.  If the
   UA uses the server recommended keep alive frequency it SHOULD send
   its keep alives so that the interval between each keep alive is
   randomly distributed between 80% and 100% of the server provided
   time.  For example, if the server suggests 120 seconds, the UA would
   send each keep alive with a different frequency between 95 and 120
   seconds.

   If no Flow-Timer header field was present in a register response for
   this flow, the UA can send keep alives at its discretion.  The
   sections below provide RECOMMENDED default values for these keep
   alives.

   The client needs to perform normal [RFC3263] SIP DNS resolution on
   the URI from the outbound-proxy-set to pick a transport.  Once a
   transport is selected, the UA selects the keep alive approach that is
   recommended for that transport.

4.4.1.  Keep alive with CRLF

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

   A User Agent that forms flows, checks if the configured URI to which
   the UA is connecting resolves to a connection-oriented transport (ex:



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   TCP and TLS over TCP).

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

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

   The ping and pong need to be sent between SIP messages and cannot be
   sent in the middle of a SIP message.  If sending over TLS, the CRLFs
   are sent inside the TLS protected channel.  If sending over a SigComp
   [RFC3320] compressed data stream, the CRLF keep alives are sent
   inside the compressed stream.  The double CRLF is considered a single
   SigComp message.  The specific mechanism for representing these
   characters is an implementation specific matter to be handled by the
   SigComp compressor at the sending end.

   If a pong is not received within 10 seconds after sending a ping (or
   immediately after processing any incoming message being received when
   the pong was expected), then the client MUST treat the flow as
   failed.  Clients MUST support this CRLF keep alive.

   When a Flow-Timer header field is not provided in the most recent
   success registration response, the proper selection of keep alive
   frequency is primarily a trade-off between battery usage and
   availability.  The UA MUST select a random number between a fixed or
   configurable upper bound and a lower bound, where the lower bound is
   20% less then the upper bound.  The fixed upper bound or the default
   configurable upper bound SHOULD be 120 seconds (95 seconds lower
   bound) where battery power is not a concern and 840 seconds (672
   seconds lower bound) where battery power is a concern.  The random
   number will be different for each keep alive ping.

      Note on selection of time values:  the 120 seconds upper bound was
      chosen based on the idea that for a good user experience, failures
      normally will be detected in this amount of time and a new
      connection set up.  The 14 minute upper-bound for battery-powered
      devices was selected based on NATs with TCP timeouts as low as 15
      minutes.  Operators that wish to change the relationship between
      load on servers and the expected time that a user might not
      receive inbound communications will probably adjust this time.
      The 95 seconds lower bound was chosen so that the jitter
      introduced will result in a relatively even load on the servers
      after 30 minutes.





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4.4.2.  Keep alive with STUN

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

   A User Agent that forms flows, checks if the configured URI to which
   the UA is connecting resolves to use the UDP transport.  The UA can
   periodically perform keep alive checks by sending STUN [RFC5389]
   Binding Requests over the flow as described in Section 8.  Clients
   MUST support STUN based keep alives.

   When a Flow-Timer header field is not included in a successful
   registration response,the time between each keep alive request SHOULD
   be a random number between 24 and 29 seconds.

      Note on selection of time values:  the upper bound of 29 seconds
      was selected, as many NATs have UDP timeouts as low as 30 seconds.
      The 24 second lower bound was selected so that after 10 minutes
      the jitter introduced by different timers will make the keep alive
      requests unsynchronized to evenly spread the load on the servers.

   If a STUN Binding Error Response is received, or if no Binding
   Response is received after 7 retransmissions (16 times the STUN "RTO"
   timer--RTO is an estimate of round-trip time), the UA considers the
   flow failed.  If the XOR-MAPPED-ADDRESS in the STUN Binding Response
   changes, the UA MUST treat this event as a failure on the flow.

4.5.  Flow Recovery

   When a flow used for registration (through 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
   value as the registration it replaces.  This is done in much the same
   way as forming a brand new flow as described in Section 4.2; however,
   if there is a failure in forming this flow, the UA needs to wait a
   certain amount of time before retrying to form a flow to this
   particular next hop.

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

   The number of seconds to wait is computed in the following way.  If
   all of the flows to every URI in the outbound proxy set have failed,
   the base-time is set to 30 seconds; otherwise, in the case where at



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   least one of the flows has not failed, the base-time is set to 90
   seconds.  The upper-bound wait time (W) 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.

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

   These times MAY be configurable in the UA.  The three times are:
   o  max-time with a default of 1800 seconds
   o  base-time (if all failed) with a default of 30 seconds
   o  base-time (if all have not failed) with a default of 90 seconds
   For example, if the base time is 30 seconds, and there were three
   failures, then the upper-bound wait time is min(1800,30*(2^3)) or 240
   seconds.  The actual amount of time the UA waits before retrying
   registration (the retry delay time) is computed by selecting a
   uniform random time between 50 and 100 percent of the upper-bound
   wait time.  The UA MUST wait for at least the value of the retry
   delay time before trying another registration to form a new flow for
   that URI (a 503 response to an earlier failed registration attempt
   with a Retry-After header field value may cause the UA to wait
   longer)..

   To be explicitly clear on the boundary conditions:  when the UA boots
   it immediately tries to register.  If this fails and no registration
   on other flows succeed, the first retry happens somewhere between 30
   and 60 seconds after the failure of the first registration request.
   If the number of consecutive-failures is large enough that the
   maximum of 1800 seconds is reached, the UA will keep trying
   indefinitely with a random time of 15 to 30 minutes between each
   attempt.


5.  Edge Proxy Procedures

5.1.  Processing Register Requests

   When an Edge Proxy receives a registration request with a reg-id
   header field parameter in the Contact header field, it needs to
   determine if it (the edge proxy) will have to be visited for any
   subsequent requests sent to the user agent identified in the Contact
   header field, or not.  If the edge proxy is the first hop, as
   indicated by the Via header field, it MUST insert its URI in a Path
   header field value as described in [RFC3327].  If it is not the first
   hop, it might still decide to add itself to the Path header based on
   local policy.  In addition, if the Edge Proxy is the first SIP node
   after the UAC, the edge proxy either MUST store a "flow token"
   (containing information about the flow from the previous hop) in its



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   Path URI or reject the request.  The flow token MUST be an identifier
   that is unique to this network flow.  The flow token MAY be placed in
   the userpart of the URI.  In addition, the first node MUST include an
   'ob' URI parameter in its Path header field value.  If the Edge Proxy
   is not the first SIP node after the UAC it MUST NOT place an ob URI
   parameter in a Path header field value.  The Edge Proxy can determine
   if it is the first hop by examining the Via header field.

5.2.  Generating Flow Tokens

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

   Example Algorithm:  When the proxy boots it selects a 20-octet crypto
      random key called K that only the Edge Proxy knows.  A byte array,
      called S, is formed that contains the following information about
      the flow the request was received on:  an enumeration indicating
      the protocol, the local IP address and port, the remote IP address
      and port.  The HMAC of S is computed using the key K and the HMAC-
      SHA1-80 algorithm, as defined in [RFC2104].  The concatenation of
      the HMAC and S are base64 encoded, as defined in [RFC4648], and
      used as the flow identifier.  When using IPv4 addresses, this will
      result in a 32-octet identifier.

5.3.  Forwarding Non-REGISTER Requests

   When an Edge Proxy receives a request, it applies normal routing
   procedures with the following additions.  If the Edge Proxy receives
   a request where the edge proxy is the host in the topmost Route
   header field value, and the Route header field value contains a flow
   token, the proxy follows the procedures of this section.  Otherwise
   the edge proxy skips the procedures in this section, removes itself
   from the Route header field, and continues processing the request.

   The proxy decodes the flow token and compares the flow in the flow
   token with the source of the request to determine if this is an
   "incoming" or "outgoing" request.

   If the flow in the flow token identified by the topmost Route header
   field value matches the source IP address and port of the request,
   the request is an "outgoing" request, otherwise, it is an "incoming"



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   request.

5.3.1.  Processing Incoming Requests

   If the Route header value contains an ob URI parameter, the Route
   header was probably copied from the Path header in a registration.
   If the Route header value contains an ob URI parameter, and the
   request is a new dialog-forming request, the proxy needs to adjust
   the route set to insure that subsequent requests in the dialog can be
   delivered over a valid flow to the UA instance identified by the flow
   token.

      A simple approach to satisfy this requirement is for the proxy to
      add a Record-Route header field value that contains the flow-
      token, by copying the URI in the Route header minus the 'ob'
      parameter.

   Next, whether the Route header field contained an ob URI parameter or
   not, the proxy removes the Route header field value and forwards the
   request over the 'logical flow' identified by the flow token, that is
   known to deliver data to the specific target UA instance.  If the
   flow token has been tampered with, the proxy SHOULD send a 403
   (Forbidden) response.  If the flow no longer exists the proxy SHOULD
   send a 430 (Flow Failed) response to the request.

   Proxies which used the example algorithm described in Section 5.2 to
   form a flow token follow the procedures below to determine the
   correct flow.  To decode the flow token, take the flow identifier in
   the user portion of the URI and base64 decode it, then verify the
   HMAC is correct by recomputing the HMAC and checking that it matches.
   If the HMAC is not correct, the request has been tampered with.

5.3.2.  Processing Outgoing Requests

   For mid-dialog requests to work with outbound UAs, the requests need
   to be forwarded over some valid flow to the appropriate UA instance.
   If the Edge Proxy receives an outgoing dialog-forming request, the
   Edge Proxy can use the presence of the ob URI parameter in the UAC's
   Contact URI (or topmost Route header field) to determine if the Edge
   Proxy needs to assist in mid-dialog request routing.

   Implementation note:  Specific procedures at the edge proxy to ensure
      that mid-dialog requests are routed over an existing flow are not
      part of this specification.  However, an approach such as having
      the Edge Proxy add a Record-Route header with a flow token is one
      way to ensure that mid-dialog requests are routed over the correct
      flow.




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5.4.  Edge Proxy Keep alive Handling

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

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

   The last proxy to forward a successful registration response to a UA
   MAY include a Flow-Timer header field if the response contains the
   outbound option-tag in a Require header field value in the response.
   The reason a proxy would send a Flow-Timer is if it wishes to detect
   flow failures proactively and take appropriate action (e.g., log
   alarms, provide alternative treatment if incoming requests for the UA
   are received, etc.).  The server MUST wait for an amount of time
   larger than the Flow-Timer in order to have a grace period to account
   for transport delay.


6.  Registrar Procedures

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

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

   When receiving a REGISTER request, the registrar MUST check from its
   Via header field if the registrar is the first hop or not.  If the
   registrar is not the first hop, it MUST examine the Path header of
   the request.  If the Path header field is missing or it exists but
   the first URI does not have an ob URI parameter, then outbound
   processing MUST NOT be applied to the registration.  In this case,
   the following processing applies:  if the REGISTER request contains
   the reg-id and the outbound option tag in a Supported header field,
   then the registrar MUST respond to the REGISTER request with a 439
   (First Hop Lacks Outbound Support) response; otherwise, the registrar
   MUST ignore the reg-id parameter of the Contact header.  See
   Section 11.6 for more information on the 439 response code.

   A Contact header field value with an instance-id media feature tag
   but no reg-id header field parameter is valid (this combination will
   result in the creation of a GRUU, as described in GRUU
   [I-D.ietf-sip-gruu] specification), but one with a reg-id but no
   instance-id is not.  If the registrar processes a Contact header
   field value with a reg-id but no instance-id, it simply ignores the



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   reg-id parameter.

   A registration containing a reg-id header field parameter and a non-
   zero expiration is used to register a single UA instance over a
   single flow, and can also de-register any Contact header fields with
   zero expiration.  Therefore if the Contact header field contains more
   than one header field value with a non-zero expiration and any of
   these header field values contain a reg-id Contact header field
   parameter, the entire registration SHOULD be rejected with a 400 (Bad
   Request) response.  The justification for recommending rejection
   versus making it mandatory is that the receiver is allowed by
   [RFC3261] to squelch (not respond to) excessively malformed or
   malicious messages.

   If the Contact header did not contain a reg-id Contact header field
   parameter or if that parameter was ignored (as described above) the
   registrar MUST NOT include the outbound option-tag in the Require
   header field of its response.

   The registrar MUST be prepared to receive, simultaneously for the
   same AOR, some registrations that use instance-id and reg-id and some
   registrations that do not.  The Registrar MAY be configured with
   local policy to reject any registrations that do not include the
   instance-id and reg-id, or with Path header field values that do not
   contain the ob URI parameter.  If the Contact header field does not
   contain a '+sip.instance' media feature parameter, the registrar
   processes the request using the Contact binding rules in [RFC3261].

   When a '+sip.instance' media feature parameter and a reg-id Contact
   header field parameter are present in a Contact header field of a
   REGISTER request (after the Contact header validation as described
   above), the corresponding binding is between an AOR and the
   combination of the instance-id (from the +sip.instance media feature
   parameter) and the value of reg-id Contact header field parameter
   parameter.  The registrar MUST store in the binding the Contact URI,
   all the Contact header field parameters, and any Path header field
   values.  (Even though the Contact URI is not used for binding
   comparisons, it is still needed by the authoritative proxy to form
   the target set.)  Provided that the UAC had included an oubound
   option-tag (defined in Section 11.4) in a Supported header field
   value in the REGISTER request, the Registrar MUST include the
   outbound option-tag in a Require header field value in its response
   to that REGISTER request.

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



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   the handle would become invalid if the TCP session was closed.  For
   common operating systems with UDP this would typically be the file
   descriptor for the local socket that received the request, the local
   interface, and the IP address and port number of the remote side that
   sent the request.  The registrar MAY store this information by adding
   itself to the Path header field with an appropriate flow token.

   If the registrar receives a re-registration for a specific
   combination of AOR, instance-id and reg-id values, the registrar MUST
   update any information that uniquely identifies the network flow over
   which the request arrived if that information has changed, and SHOULD
   update the time the binding was last updated.

   To be compliant with this specification, registrars which can receive
   SIP requests directly from a UAC without intervening edge proxies
   MUST implement the same keep alive mechanisms as Edge Proxies
   (Section 5.4).  Registrars with a direct flow with a UA MAY include a
   Flow-Timer header in a 2XX class registration response which includes
   the outbound option-tag in the Require header.


7.  Authoritative Proxy Procedures: Forwarding Requests

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

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

   The proxy uses the next-hop target of the message and the value of
   any stored Path header field vector in the registration binding to
   decide how to forward and populate the Route header in the request.
   If the proxy is colocated with the registrar and stored information
   about the flow to the UA that created the binding, then the proxy
   MUST send the request over the same 'logical flow' saved with the
   binding, since that flow is known to deliver data to the specific
   target UA instance's network flow that was saved with the binding.




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

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


8.  STUN Keep alive Processing

   This section describes changes to the SIP transport layer that allow
   SIP and STUN [RFC5389] Binding Requests to be mixed over the same
   flow.  This constitutes a new STUN usage.  The STUN messages are used
   to verify that connectivity is still available over a UDP flow, and
   to provide periodic keep alives.  These STUN keep alives are always
   sent to the next SIP hop.  STUN messages are not delivered end-to-
   end.

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

   If a server compliant to this section receives SIP requests on a
   given interface and UDP port, it MUST also provide a limited version
   of a STUN server on the same interface and UDP port.

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

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



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   node could be blacklisted for UDP traffic.

   A SIP node MUST NOT send STUN requests over a flow unless it has an
   explicit indication that the target next hop SIP server claims to
   support this specification.  UACs MUST NOT use an ambiguous
   configuration option such as "Work through NATs?" or "Do Keep
   alives?" to imply next hop STUN support.  A UAC MAY use the presence
   of an ob URI parameter in the Path header in a registration response
   as an indication that its first edge proxy supports the keep alives
   defined in this document.

      Typically, a SIP node first sends a SIP request and waits to
      receive a 2XX class response over a flow to a new target
      destination, before sending any STUN messages.  When scheduled for
      the next NAT refresh, the SIP node sends a STUN request to the
      target.

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

   The SIP keep alive STUN usage requires no backwards compatibility
   with [RFC3489].

8.1.  Use with Sigcomp

   When STUN is used together with SigComp [RFC3320] compressed SIP
   messages over the same flow, 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 [RFC3629] encoded text messages, so for applications which
      use this encoding (or ASCII encoding) it is possible to multiplex
      uncompressed application messages and SigComp messages on the same
      UDP port.
      The most significant two bits of every STUN Binding method are
      both zeroes.  This, combined with the magic cookie, aids in
      differentiating STUN packets from other protocols when STUN is
      multiplexed with other protocols on the same port.






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9.  Example Message Flow

   Below is an example message flow illustrating most of the concepts
   discussed in this specification.  In many cases, Via, Content-Length
   and Max-Forwards headers are omitted for brevity and readability.

   In these examples, "EP1" and "EP2" are outbound proxies, and "Proxy"
   is the authoritativeProxy.

   The section is subdivided into independent calls flows:  however,
   they are structured in sequential order of an hypothetical sequence
   of call flows.

9.1.  Subscription to configuration package

   If the outbound proxy set is already configured on Bob's UA, then
   this subsection can be skipped.  Otherwise, if the outbound proxy set
   is learned through the configuration package, Bob's UA sends a
   SUBSCRIBE request for the UA profile configuration package
   [I-D.ietf-sipping-config-framework].  This request is a poll (Expires
   is zero).  After receiving the NOTIFY request, Bob's UA fetches the
   external configuration using HTTPS (not shown) and obtains a
   configuration file which contains the outbound-proxy-set "sip:
   ep1.example.com;lr" and "sip:ep2.example.com;lr.

     [----example.com domain-------------------------]
     Bob         EP1   EP2     Proxy             Config
      |           |     |        |                  |
    1)|SUBSCRIBE->|     |        |                  |
    2)|           |---SUBSCRIBE Event: ua-profile ->|
    3)|           |<--200 OK -----------------------|
    4)|<--200 OK--|     |        |                  |
    5)|           |<--NOTIFY------------------------|
    6)|<--NOTIFY--|     |        |                  |
    7)|---200 OK->|     |        |                  |
    8)|           |---200 OK ---------------------->|
      |           |     |        |                  |

   In this example, the DNS server happens to be configured so that sip:
   example.com resolves to EP1 and EP2.











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   Example Message #1:

   SUBSCRIBE sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com
     SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnlsdkdj2
   Max-Forwards: 70
   From: <anonymous@example.com>;tag=23324
   To: <sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com>
   Call-ID: nSz1TWN54x7My0GvpEBj
   CSeq: 1 SUBSCRIBE
   Event: ua-profile ;profile-type=device
    ;vendor="example.com";model="uPhone";version="1.1"
   Expires: 0
   Supported: path, outbound
   Accept: message/external-body, application/x-uPhone-config
   Contact: <sip:192.0.2.2;transport=tcp;ob>
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
   Content-Length: 0

   In message #2, EP1 adds the following Record-Route header:

   Record-Route:
    <sip:GopIKSsn0oGLPXRdV9BAXpT3coNuiGKV@ep1.example.com;lr>

   In message #5, the configuration server sends a NOTIFY with an
   external URL for Bob to fetch his configuration.  The NOTIFY has a
   Subscription-State header that ends the subscription.

   Message #5

   NOTIFY sip:192.0.2.2;transport=tcp;ob SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.5;branch=z9hG4bKn81dd2
   Max-Forwards: 70
   To: <anonymous@example.com>;tag=23324
   From: <sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com>;tag=0983
   Call-ID: nSz1TWN54x7My0GvpEBj
   CSeq: 1 NOTIFY
   Route: <sip:GopIKSsn0oGLPXRdV9BAXpT3coNuiGKV@ep1.example.com;lr>
   Subscription-State: terminated;reason=timeout
   Event: ua-profile
   Content-Type: message/external-body; access-type="URL"
    ;expiration="Thu, 01 Jan 2009 09:00:00 UTC"
    ;URL="http://example.com/uPhone.cfg"
    ;size=9999;hash=10AB568E91245681AC1B
   Content-Length: 0

   EP1 receives this NOTIFY request, strips off the Route header,
   extracts the flow-token, calculates the correct flow and forwards the



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   request (Message #6) over that flow to Bob.

   Bob's UA fetches the configuration file and learns the outbound proxy
   set.

9.2.  Registration

   Now that Bob's UA is configured with the outbound-proxy-set whether
   through configuration or using the configuration framework procedures
   of the previous section, Bob's UA sends REGISTER requests through
   each edge proxy in the set.  Once the registrations succeed, Bob's UA
   begins sending CRLF keep alives about every 2 minutes.

     Bob         EP1   EP2     Proxy     Alice
      |           |     |        |         |
    9)|-REGISTER->|     |        |         |
   10)|           |---REGISTER-->|         |
   11)|           |<----200 OK---|         |
   12)|<-200 OK---|     |        |         |
   13)|----REGISTER---->|        |         |
   14)|           |     |--REG-->|         |
   15)|           |     |<-200---|         |
   16)|<----200 OK------|        |         |
      |           |     |        |         |
      |  about 120 seconds later...        |
      |           |     |        |         |
   17)|--2CRLF--->|     |        |         |
   18)|<--CRLF----|     |        |         |
   19)|------2CRLF----->|        |         |
   20)|<------CRLF------|        |         |
      |           |     |        |         |

   In message #9, Bob's UA sends its first registration through the
   first edge proxy in the outbound-proxy-set by including a loose
   route.  The UA includes an instance-id and reg-id in its Contact
   header field value.  Note the option-tags in the Supported header.















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   Message #9

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Bob <sip:bob@example.com>;tag=7F94778B653B
   To: Bob <sip:bob@example.com>
   Call-ID: 16CB75F21C70
   CSeq: 1 REGISTER
   Supported: path, outbound
   Route: <sip:ep1.example.com;lr>
   Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
   Content-Length: 0

   Message #10 is similar.  EP1 removes the Route header field value,
   decrements Max-Forwards, and adds its Via header field value.  Since
   EP1 is the first edge proxy, it adds a Path header with a flow token
   and includes the 'ob' parameter.

   Path: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob>

   Since the response to the REGISTER (message #11) contains the
   outbound option-tag in the Require header field, Bob's UA will know
   that the registrar used outbound binding rules.  The response also
   contains the currently active Contacts, the Path for the current
   registration.

   Message #11

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP 192.0.2.15;branch=z9hG4bKnuiqisi
   Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnashds7
   From: Bob <sip:bob@example.com>;tag=7F94778B653B
   To: Bob <sip:bob@example.com>;tag=6AF99445E44A
   Call-ID: 16CB75F21C70
   CSeq: 1 REGISTER
   Supported: path, outbound
   Require: outbound
   Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1;expires=3600
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
   Path: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob>
   Content-Length: 0

   The second registration through EP2 (message #13) is similar other
   than the Call-ID has changed, the reg-id is 2, and the Route header
   goes through EP2.




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   Message #13

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnqr9bym
   Max-Forwards: 70
   From: Bob <sip:bob@example.com>;tag=755285EABDE2
   To: Bob <sip:bob@example.com>
   Call-ID: E05133BD26DD
   CSeq: 1 REGISTER
   Supported: path, outbound
   Route: <sip:ep2.example.com;lr>
   Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=2
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
   Content-Length: 0

   Likewise in message #14, EP2 adds a Path header with flow token and
   'ob' parameter.

   Path: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob>

   Message #16 tells Bob's UA that outbound registration was successful,
   and shows both Contacts.  Note that only the Path corresponding to
   the current registration is returned.

   Message #16

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bKnqr9bym
   From: Bob <sip:bob@example.com>;tag=755285EABDE2
   To: Bob <sip:bob@example.com>;tag=49A9AD0B3F6A
   Call-ID: E05133BD26DD
   Supported: path, outbound
   Require: outbound
   CSeq: 1 REGISTER
   Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1;expires=3600
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
   Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=2;expires=3600
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
   Path: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob>
   Content-Length: 0

9.3.  Incoming call and proxy crash

   In this example, after registration, EP1 crashes and reboots.  Before
   Bob's UA notices that its flow to EP1 is no longer responding, Alice
   calls Bob. Bob's authoritative proxy first tries the flow to EP1, but
   EP1 no longer has a flow to Bob so it responds with a 430 Flow Failed
   response.  The proxy removes the stale registration and tries the



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   next binding for the same instance.

     Bob         EP1   EP2     Proxy     Alice
      |           |     |        |         |
      |    CRASH  X     |        |         |
      |        Reboot   |        |         |
      |           |     |        |         |
   21)|           |     |        |<-INVITE-|
   22)|           |<---INVITE----|         |
   23)|           |----430------>|         |
   24)|           |     |<-INVITE|         |
   25)|<---INVITE-------|        |         |
   26)|----200 OK------>|        |         |
   27)|           |     |200 OK->|         |
   28)|           |     |        |-200 OK->|
   29)|           |     |<----------ACK----|
   30)|<---ACK----------|        |         |
      |           |     |        |         |
   31)|           |     |<----------BYE----|
   32)|<---BYE----------|        |         |
   33)|----200 OK------>|        |         |
   34)|           |     |--------200 OK--->|
      |           |     |        |         |


   Message #21

   INVITE sip:bob@example.com SIP/2.0
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 INVITE

   Bob's proxy rewrites the Request-URI to the Contact URI used in Bob's
   registration, and places the path for one of the registrations
   towards Bob's UA instance into a Route header field.  This Route goes
   through EP1.

   Message #22

   INVITE sip:bob@192.0.2.2;transport=tcp SIP/2.0
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 INVITE
   Route: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob>

   Since EP1 just rebooted, it does not have the flow described in the



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   flow token.  It returns a 430 Flow Failed response.

   Message #23

   SIP/2.0 430 Flow Failed
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 INVITE

   The proxy deletes the binding for this path and tries to forward the
   INVITE again, this time with the path through EP2.

   Message #24

   INVITE sip:bob@192.0.2.2;transport=tcp SIP/2.0
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 INVITE
   Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob>

   In message #25, EP2 needs to add a Record-Route header field value,
   so that any subsequent in-dialog messages from Alice's UA arrive at
   Bob's UA.  EP2 can determine it needs to Record-Route since the
   request is a dialog-forming request and the Route header contained a
   flow token and an 'ob' parameter.  This Record-Route information is
   passed back to Alice's UA in the responses (messages #26, 27, and 28)

   Message #25

   INVITE sip:bob@192.0.2.2;transport=tcp SIP/2.0
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 INVITE
   Record-Route:
     <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>

   Message #26

   SIP/2.0 200 OK
   To: Bob <sip:bob@example.com>;tag=skduk2
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 INVITE
   Record-Route:
     <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>



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   At this point, both UAs have the correct route-set for the dialog.
   Any subsequent requests in this dialog will route correctly.  For
   example, the ACK request in message #29 is sent form Alice's UA
   directly to EP2.  The BYE request in message #31 uses the same route-
   set.

   Message #29

   ACK sip:bob@192.0.2.2;transport=tcp SIP/2.0
   To: Bob <sip:bob@example.com>;tag=skduk2
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 1 ACK
   Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>

   Message #31

   BYE sip:bob@192.0.2.2;transport=tcp SIP/2.0
   To: Bob <sip:bob@example.com>;tag=skduk2
   From: Alice <sip:alice@a.example>;tag=02935
   Call-ID: klmvCxVWGp6MxJp2T2mb
   CSeq: 2 BYE
   Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>

9.4.  Re-registration

   Somewhat later, Bob's UA sends keep alives to both its edge proxies,
   but it discovers that the flow with EP1 failed.  Bob's UA re-
   registers through EP1 using the same reg-id and Call-ID it previously
   used.





















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     Bob         EP1   EP2     Proxy     Alice
      |           |     |        |         |
   35)|------2CRLF----->|        |         |
   36)|<------CRLF------|        |         |
   37)|--2CRLF->X |     |        |         |
      |           |     |        |         |
   38)|-REGISTER->|     |        |         |
   39)|           |---REGISTER-->|         |
   40)|           |<----200 OK---|         |
   41)|<-200 OK---|     |        |         |
      |           |     |        |         |

   Message #38

   REGISTER sip:example.com SIP/2.0
   From: Bob <sip:bob@example.com>;tag=7F94778B653B
   To: Bob <sip:bob@example.com>
   Call-ID: 16CB75F21C70
   CSeq: 2 REGISTER
   Supported: path, outbound
   Route: <sip:ep1.example.com;lr>
   Contact: <sip:bob@192.0.2.2;transport=tcp>;reg-id=1
    ;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"

   In message #39, EP1 inserts a Path header with a new flow token:

   Path: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr;ob>

9.5.  Outgoing call

   Finally, Bob makes an outgoing call to Alice.  Bob's UA includes an
   'ob' parameter in its Contact URI in message #42.  EP1 adds a Record-
   Route with a flow-token in message #43.  The route-set is returned to
   Bob in the response (messages #45, 46, and 47) and either Bob or
   Alice can send in-dialog requests.
















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     Bob         EP1   EP2     Proxy     Alice
      |           |     |        |         |
   42)|--INVITE-->|     |        |         |
   43)|           |---INVITE---->|         |
   44)|           |     |        |-INVITE->|
   45)|           |     |        |<--200---|
   46)|           |<----200 OK---|         |
   47)|<-200 OK---|     |        |         |
   48)|--ACK----->|     |        |         |
   49)|           |-----ACK--------------->|
      |           |     |        |         |
   50)|-- BYE---->|     |        |         |
   51)|           |-----------BYE--------->|
   52)|           |<----------200 OK-------|
   53)|<--200 OK--|     |        |         |
      |           |     |        |         |

   Message #42

   INVITE sip:alice@a.example SIP/2.0
   From: Bob <sip:bob@example.com>;tag=ldw22z
   To: Alice <sip:alice@a.example>
   Call-ID: 95KGsk2V/Eis9LcpBYy3
   CSeq: 1 INVITE
   Route: <sip:ep1.example.com;lr>
   Contact: <sip:bob@192.0.2.2;transport=tcp;ob>

   In message #43, EP1 adds the following Record-Route header.

   Record-Route:
     <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr>

   When EP1 receives the BYE (message #50) from Bob's UA, it can tell
   that the request is an "outgoing" request (since the source of the
   request matches the flow in the flow token) and simply deletes its
   Route header field value and forwards the request on to Alice's UA.

   Message #50

   BYE sip:alice@a.example SIP/2.0
   From: Bob <sip:bob@example.com>;tag=ldw22z
   To: Alice <sip:alice@a.example>;tag=plqus8
   Call-ID: 95KGsk2V/Eis9LcpBYy3
   CSeq: 2 BYE
   Route: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr>
   Contact: <sip:bob@192.0.2.2;transport=tcp;ob>





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10.  Grammar

   This specification defines a new header field "Flow-Timer", new
   Contact header field parameters, reg-id and +sip.instance.  The
   grammar includes the definitions from [RFC3261] and includes the
   definition of uric from [RFC3986].  Flow-Timer is an extension-header
   from the message-header in the [RFC3261] ABNF.

   The ABNF[RFC5234] is:

    Flow-Timer     = "Flow-Timer" HCOLON 1*DIGIT

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

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

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

    instance-val   = *uric ; defined in RFC 3986

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


11.  IANA Considerations

11.1.  Flow-Timer Header Field

   This specification defines a new SIP header field "Flow-Timer" whose
   syntax is defined in Section 10.

     Header Name        compact    Reference
     -----------------  -------    ---------
     Flow-Timer                    [RFCXXXX]

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

11.2.  'reg-id' Contact Header Field Parameter

   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 syntax is
   defined in Section 10.  The required information is:







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                                                  Predefined
   Header Field            Parameter Name         Values      Reference
   ----------------------  ---------------------  ----------  ---------
   Contact                 reg-id                 No          [RFCXXXX]

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

11.3.  SIP/SIPS URI Parameters

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

   Parameter Name     Predefined Values     Reference
   --------------     -----------------     ---------
   ob                 No                    [RFCXXXX]

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

11.4.  SIP Option Tag

   This specification registers a new SIP option tag, as per the
   guidelines in Section 27.1 of [RFC3261].

   Name:  outbound
   Description:  This option-tag is used to identify UAs and Registrars
      which support extensions for Client Initiated Connections.  A UA
      places this option in a Supported header to communicate its
      support for this extension.  A Registrar places this option-tag in
      a Require header to indicate to the registering User Agent that
      the Registrar used registrations using the binding rules defined
      in this extension.

11.5.  430 (Flow Failed) Response Code

   This document registers a new SIP response code (430 Flow Failed), as
   per the guidelines in Section 27.4 of [RFC3261].  This response code
   is used by an Edge Proxy to indicate to the Authoritative Proxy that
   a specific flow to a UA instance has failed.  Other flows to the same
   instance could still succeed.  The Authoritative Proxy SHOULD attempt
   to forward to another target (flow) with the same instance-id and
   AOR.  Endpoints should never receive a 430 response.  If an endpoint
   receives a 430 response it should treat it as a 400 (Bad Request) per
   normal 8.1.3.2/[RFC3261] procedures.  This response code is defined
   by the following information, which has been added to the method and
   response-code sub-registry under



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   http://www.iana.org/assignments/sip-parameters.

     Response Code                               Reference
     ------------------------------------------  ---------
     Request Failure 4xx
       430 Flow Failed                           [RFCXXXX]

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

11.6.  439 (First Hop Lacks Outbound Support) Response Code

   This document registers a new SIP response code (439 First Hop Lacks
   Outbound Support), as per the guidelines in Section 27.4 of
   [RFC3261].  This response code is used by a registrar to indicate
   that it supports the 'outbound' feature described in this
   specification, but that the first outbound proxy that the user is
   attempting to register through does not.  Note that this response
   code is only appropriate in the case that the registering user agent
   advertises support for outbound processing by including the outbound
   option tag in a Supported header field.  Proxies MUST NOT send a 439
   response to any requests that do not contain a reg-id parameter and
   an outbound option tag in a Supported header field.  This response
   code is defined by the following information, which has been added to
   the method and response-code sub-registry under
   http://www.iana.org/assignments/sip-parameters.

     Response Code                               Reference
     ------------------------------------------  ---------
     Request Failure 4xx
       439 First Hop Lacks Outbound Support      [RFCXXXX]

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

11.7.  Media Feature Tag

   This section registers a new media feature tag, per the procedures
   defined in [RFC2506].  The tag is placed into the sip tree, which is
   defined in [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.



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   Values appropriate for use with this feature tag:  String.

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

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

   Related standards or documents:  RFC XXXX

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

   Security Considerations:  This media feature tag can be used in ways
   which affect application behaviors.  For example, the SIP caller
   preferences extension [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 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.


12.  Security Considerations

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

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

   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



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

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

   The Security Considerations discussed in [RFC3261] and [RFC3327] are
   also relevant to this document.  For the security considerations of
   generating flow tokens, please also see Section 5.2.  A discussion of
   preventing the avalanche restart problem is in Section 4.5.

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


13.  Operational Notes on Transports

   This entire section is non-normative.

   [RFC3261] requires proxies, registrars, and User Agents to implement
   both TCP and UDP but deployments can chose which transport protocols
   they want to use.  Deployments need to be careful in choosing what
   transports to use.  Many SIP features and extensions, such as large
   presence notification bodies, result in SIP requests that can be too
   large to be reasonably transported over UDP.  [RFC3261] states that
   when a request is too large for UDP, the device sending the request
   attempts to switch over to TCP.  It is important to note that when
   using outbound, this will only work if the UA has formed both UDP and
   TCP outbound flows.  This specification allows the UA to do so but in
   most cases it will probably make more sense for the UA to form a TCP
   outbound connection only, rather than forming both UDP and TCP flows.
   One of the key reasons that many deployments choose not to use TCP
   has to do with the difficulty of building proxies that can maintain a



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   very large number of active TCP connections.  Many deployments today
   use SIP in such a way that the messages are small enough that they
   work over UDP but they can not take advantage of all the
   functionality SIP offers.  Deployments that use only UDP outbound
   connections are going to fail with sufficiently large SIP messages.


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.  Acknowledgments

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


16.  References







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16.1.  Normative References

   [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.

   [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.

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

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 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.




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   [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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

16.2.  Informational References

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

   [I-D.ietf-sipping-config-framework]
              Channabasappa, S., "A Framework for Session Initiation
              Protocol User Agent Profile Delivery",
              draft-ietf-sipping-config-framework-15 (work in progress),
              February 2008.

   [I-D.ietf-sipping-nat-scenarios]
              Boulton, C., Rosenberg, J., Camarillo, G., and F. Audet,
              "Best Current Practices for NAT Traversal for Client-
              Server SIP", draft-ietf-sipping-nat-scenarios-09 (work in
              progress), September 2008.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

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



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              February 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.


Appendix A.  Default Flow Registration Backoff Times

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

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











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Authors' Addresses

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

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


   Rohan Mahy (editor)
   Unaffiliated

   Email:  rohan@ekabal.com


































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