[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

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 (if approved)                           R. Mahy, Ed.
Expires:  December 27, 2006                                  Plantronics
                                                           June 25, 2006


Managing Client Initiated Connections in the Session Initiation Protocol
                                 (SIP)
                       draft-ietf-sip-outbound-04

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 27, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Session Initiation Protocol (SIP) allows proxy servers to
   initiate TCP connections and send asynchronous UDP datagrams to User
   Agents in order to deliver requests.  However, many practical
   considerations, such as the existence of firewalls and Network
   Address Translators (NATs), prevent servers from connecting to User
   Agents in this way.  This specification defines behaviors for User



Jennings & Mahy         Expires December 27, 2006               [Page 1]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Summary of Mechanism . . . . . . . . . . . . . . . . . . .  5
     3.2.  Single Registrar and UA  . . . . . . . . . . . . . . . . .  6
     3.3.  Multiple Connections from a User Agent . . . . . . . . . .  7
     3.4.  Edge Proxies . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Keepalive Technique  . . . . . . . . . . . . . . . . . . . 10
   4.  User Agent Mechanisms  . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Instance ID Creation . . . . . . . . . . . . . . . . . . . 11
     4.2.  Initial Registrations  . . . . . . . . . . . . . . . . . . 13
       4.2.1.  Registration by Other Instances  . . . . . . . . . . . 14
     4.3.  Sending Requests . . . . . . . . . . . . . . . . . . . . . 14
     4.4.  Detecting Flow Failure . . . . . . . . . . . . . . . . . . 14
       4.4.1.  Keepalive with STUN  . . . . . . . . . . . . . . . . . 15
       4.4.2.  Flow Recovery  . . . . . . . . . . . . . . . . . . . . 15
   5.  Edge Proxy Mechanisms  . . . . . . . . . . . . . . . . . . . . 16
     5.1.  Processing Register Requests . . . . . . . . . . . . . . . 16
     5.2.  Generating Flow Tokens . . . . . . . . . . . . . . . . . . 16
     5.3.  Forwarding Requests  . . . . . . . . . . . . . . . . . . . 17
   6.  Registrar and Location Server Mechanisms . . . . . . . . . . . 18
     6.1.  Processing REGISTER Requests . . . . . . . . . . . . . . . 18
     6.2.  Forwarding Requests  . . . . . . . . . . . . . . . . . . . 19
   7.  Mechanisms for All Servers (Proxys, Registars, UASs) . . . . . 20
     7.1.  STUN Processing  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Example Message Flow . . . . . . . . . . . . . . . . . . . . . 21
   9.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
     10.1. Contact Header Field . . . . . . . . . . . . . . . . . . . 25
     10.2. SIP/SIPS URI Parameters  . . . . . . . . . . . . . . . . . 25
     10.3. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 26
     10.4. Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 26
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 28
   13. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     13.1. Changes from 03 Version  . . . . . . . . . . . . . . . . . 28
     13.2. Changes from 02 Version  . . . . . . . . . . . . . . . . . 29
     13.3. Changes from 01 Version  . . . . . . . . . . . . . . . . . 29
     13.4. Changes from 00 Version  . . . . . . . . . . . . . . . . . 29



Jennings & Mahy         Expires December 27, 2006               [Page 2]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Default Flow Registration Backoff Times . . . . . . . 30
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     15.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
   Intellectual Property and Copyright Statements . . . . . . . . . . 34












































Jennings & Mahy         Expires December 27, 2006               [Page 3]

Internet-Draft     Client Initiated Connections in SIP         June 2006


1.  Introduction

   There are many environments for SIP [RFC3261] deployments in which
   the User Agent (UA) can form a connection to a Registrar or Proxy but
   in which connections in the reverse direction to the UA are not
   possible.  This can happen for several reasons.  Connections to the
   UA can be blocked by a firewall device between the UA and the proxy
   or registrar, which will only allow new connections in the direction
   of the UA to the Proxy.  Similarly there may be a NAT, which are only
   capable of allowing new connections from the private address side to
   the public side.  This specification allows SIP registration when the
   UA is behind such a firewall or NAT.

   Most IP phones and personal computers get their network
   configurations dynamically via a protocol such as DHCP (Dynamic Host
   Configuration Protocol).  These systems typically do not have a
   useful name in the Domain Name System (DNS), and they definitely do
   not have a long-term, stable DNS name that is appropriate for use in
   the subjectAltName of a certificate, as required by [RFC3261].
   However, these systems can still act as a TLS client and form
   connections to a proxy or registrar which authenticates with a server
   certificate.  The server can authenticate the UA using a shared
   secret in a digest challenge over that TLS connection.

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


2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.1.  Definitions





Jennings & Mahy         Expires December 27, 2006               [Page 4]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   Edge Proxy: An Edge Proxy is any proxy that is located topologically
      between the registering User Agent and the registrar.
   Flow: A Flow is a network protocol layer (layer 4) association
      between two hosts that is represented by the network address and
      port number of both ends and by the protocol.  For TCP, a flow is
      equivalent to a TCP connection.  For UDP a flow is a bidirectional
      stream of datagrams between a single pair of IP addresses and
      ports of both peers.  With TCP, a flow often has a one to one
      correspondence with a single file descriptor in the operating
      system.
   reg-id: This refers to the value of a new header field parameter
      value for the Contact header field.  When a UA registers multiple
      times, each simultaneous registration gets a unique reg-id value.
   instance-id: This specification uses the word instance-id to refer to
      the value of the "sip.instance" media feature tag in the Contact
      header field.  This is a Uniform Resource Name (URN) that uniquely
      identifies this specific UA instance.
   outbound-proxy-set A set of SIP URIs (Uniform Resource Identifiers)
      that represents each of the outbound proxies (often Edge Proxies)
      with which the UA will attempt to maintain a direct flow.  The
      first URI in the set is often refereed 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.


3.  Overview

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

3.1.  Summary of Mechanism

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

   When a proxy goes to route a message to a UA for which it has a



Jennings & Mahy         Expires December 27, 2006               [Page 5]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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

3.2.  Single Registrar and UA

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

      +-----------+
      | Registrar |
      | Proxy     |
      +-----+-----+
            |
            |
       +----+--+
       | User  |
       | Agent |
       +-------+

   User Agents which form only a single flow continue to register
   normally but include the instance-id as described in Section 4.1.
   The UA can also include a reg-id parameter which is used to allow the
   registrar to detect and avoid using 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 192.0.2.1;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
   Contact: <sip:line1@192.168.0.2>; reg-id=1;
    ;+sip.instance="<urn:uuid:00000000-0000-0000-0000-000A95A0E128>"
   Content-Length: 0



Jennings & Mahy         Expires December 27, 2006               [Page 6]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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

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

3.3.  Multiple Connections from a User Agent

   There are various ways to deploy SIP to build a reliable and scalable
   system.  This section discusses one such design that is possible with
   the mechanisms in this specification.  Other designs are also
   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 outbound proxy/registrar function for the
   domain.  Reliability is achieved by having the UA form two TCP
   connections to the domain.

   Scalability is achieved by using DNS SRV to load balance the primary
   connection across a set of machines that can service the primary
   connection and also using DNS SRV to load balance across a separate



Jennings & Mahy         Expires December 27, 2006               [Page 7]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   set of machines that can service the secondary connection.  The
   deployment here requires that DNS is configured with one entry that
   resolves to all the primary hosts and another entry that resolves to
   all the secondary hosts.  While this introduces additional DNS
   configuration, the approach works and requires no addition SIP
   extensions.

      Note:  Approaches which select multiple connections from a single
      DNS SRV set were also considered, but cannot prevent two
      connections from accidentally resolving to the same host.  The
      approach in this document does not prevent future extensions, such
      as the SIP UA configuration framework [I-D.ietf-sipping-config-
      framework], from adding other ways for a User Agent to discover
      its outbound-proxy-set.

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

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

   The domain also needs to ensure that a request for the UA sent to



Jennings & Mahy         Expires December 27, 2006               [Page 8]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

   When a single server fails, all the UAs that have a flow through it
   will detect a flow failure and try to reconnect.  This can cause
   large loads on the server.  When large numbers of hosts reconnect
   nearly simultaneously, this is referred to as the avalanche restart
   problem, and is further discussed in Section 4.4.2.  The multiple
   flows to many servers help reduce the load caused by the avalanche
   restart.  If a UA has multiple flows, and one of the servers fails,
   the UA delays the specified time before trying to form a new
   connection to replace the flow to the server that failed.  By
   spreading out the time used for all the UAs to reconnect to a server,
   the load on the server farm is reduced.

   When used in this fashion to achieve high reliability, the operator
   will need to configure DNS such that the various URIs in the outbound
   proxy set do not resolve to the same host.

3.4.  Edge Proxies

   Some SIP deployments use edge proxies such that the UA sends the
   REGISTER to an Edge Proxy that then forwards the REGISTER to the
   Registrar.  The Edge Proxy includes a Path header [RFC3327] so that
   when the registrar later forwards a request to this UA, the request
   is routed through the Edge Proxy.  There could be a NAT or firewall
   between the UA and the Edge Proxy.
                +---------+
                |Registrar|
                |Proxy    |
                +---------+
                 /      \
                /        \
               /          \
            +-----+     +-----+
            |Edge1|     |Edge2|
            +-----+     +-----+
               \           /
                \         /
        ----------------------------NAT/FW
                  \     /
                   \   /
                  +------+
                  |User  |
                  |Agent |
                  +------+



Jennings & Mahy         Expires December 27, 2006               [Page 9]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

   The term Edge Proxy is often used to refer to deployments where the
   Edge Proxy is in the same administrative domain as the Registrar.
   However, in this specification we use the term to refer to any proxy
   between the UA and the Registrar.  For example the Edge Proxy may be
   inside an enterprise that requires its use and the registrar could be
   a service provider with no relationship to the enterprise.
   Regardless if they are in the same administrative domain, this
   specification requires that Registrars and Edge proxies support the
   Path header mechanism in RFC 3327 [RFC3327].

3.5.  Keepalive Technique

   A keepalive mechanism needs to detect failure of a connection and
   changes to the NAT public mapping, as well as keeping any NAT
   bindings refreshed.  This specification describes using STUN
   [I-D.ietf-behave-rfc3489bis] over the same flow as the SIP traffic to
   perform the keepalive.  For connection-oriented transports (e.g.  TCP
   and TLS over TCP), the UAC MAY use TCP keepalives to detect flow
   failure if the UAC can send these keepalives and detect a keepalive
   failure according to the time frames described in Section 4.4.

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

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



Jennings & Mahy         Expires December 27, 2006              [Page 10]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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


4.  User Agent Mechanisms

4.1.  Instance ID Creation

   Each UA MUST have an Instance Identifier URN that uniquely identifies
   the device.  Usage of a URN provides a persistent and unique name for
   the UA instance.  It also provides an easy way to guarantee
   uniqueness within the AOR.  This URN MUST be persistent across power
   cycles of the device.  The Instance ID MUST NOT change as the device
   moves from one network to another.

   A UA SHOULD use a UUID URN [RFC4122].  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



Jennings & Mahy         Expires December 27, 2006              [Page 11]

Internet-Draft     Client Initiated Connections in SIP         June 2006


      no other UUID is being generated at the same time on that physical
      device.  This means the value of the time component of the UUID
      can be arbitrarily selected to be any time less than the time when
      the device was manufactured.  A time of 0 (as shown in the example
      in Section 3.2) is perfectly legal as long as the device knows no
      other UUIDs were generated at this time.

   If a URN scheme other than UUID is used, the URN MUST be selected
   such that the instance can be certain that no other instance
   registering against the same AOR would choose the same URN value.  An
   example of a URN that would not meet the requirements of this
   specification is the national bibliographic number [RFC3188].  Since
   there is no clear relationship between a SIP UA instance and a URN in
   this namespace, there is no way a selection of a value can be
   performed that guarantees that another UA instance doesn't choose the
   same value.

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

      RFC 3840 [RFC3840] defines equality rules for callee capabilities
      parameters, and according to that specification, the
      "sip.instance" media feature tag will be compared by case-
      sensitive string comparison.  This means that the URN will be
      encapsulated by angle brackets ("<" and ">") when it is placed
      within the quoted string value of the +sip.instance Contact header
      field parameter.  The case-sensitive matching rules apply only to
      the generic usages defined in RFC 3840 [RFC3840] and in the caller
      preferences specification [RFC3841].  When the instance ID is used
      in this specification, it is effectively "extracted" from the
      value in the "sip.instance" media feature tag.  Thus, equality
      comparisons are performed using the rules for URN equality that
      are specific to the scheme in the URN.  If the element performing
      the comparisons does not understand the URN scheme, it performs
      the comparisons using the lexical equality rules defined in RFC
      2141 [RFC2141].  Lexical equality may result in two URNs being
      considered unequal when they are actually equal.  In this specific
      usage of URNs, the only element which provides the URN is the SIP
      UA instance identified by that URN.  As a result, the UA instance
      SHOULD provide lexically equivalent URNs in each registration it
      generates.  This is likely to be normal behavior in any case;
      clients are not likely to modify the value of the instance ID so



Jennings & Mahy         Expires December 27, 2006              [Page 12]

Internet-Draft     Client Initiated Connections in SIP         June 2006


      that it remains functionally equivalent yet lexigraphically
      different from previous registrations.

4.2.  Initial Registrations

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

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

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

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

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

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



Jennings & Mahy         Expires December 27, 2006              [Page 13]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   time and retry the registration.  A Retry-After header field value of
   0 is valid and indicates the UA should retry the REGISTER
   immediately.  Implementations need to ensure that when retrying the
   REGISTER they revisit the DNS resolution results such that the UA can
   select an alternate host from the one chosen the previous time the
   URI was resolved.

4.2.1.  Registration by Other Instances

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

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

4.3.  Sending Requests

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

   The UA performs normal DNS resolution on the next hop URI (as
   described in RFC 3263 [RFC3263]) to find a protocol, IP address, and
   port.  For non-TLS protocols, if the UA has an existing flow to this
   IP address, and port with the correct protocol, then the UA MUST use
   the existing connection.  For TLS protocols, there must also be a
   match between the host production in the next hop and one of the URIs
   contained in the subjectAltName in the peer certificate.  If the UA
   cannot use one of the existing flows, then it SHOULD form a new flow
   by sending a datagram or opening a new connection to the next hop, as
   appropriate for the transport protocol.

4.4.  Detecting Flow Failure

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

   The time between keepalive requests when using UDP-based transports



Jennings & Mahy         Expires December 27, 2006              [Page 14]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   SHOULD be a random number between 24 and 29 seconds while for TCP-
   based transports it SHOULD be a random number between 95 and 120
   seconds.  These times MAY be configurable.

   o  Note on selection of time values:  For UDP, the upper bound of 29
      seconds was selected so that multiple STUN packets could be sent
      before 30 seconds based on information that many NATs have UDP
      timeouts as low as 30 seconds.  The 24 second lower bound was
      selected so that after 10 minutes the jitter introduced by
      different timers will make the keepalive requests unsynchronized
      to evenly spread the load on the servers.  For TCP, the 120
      seconds upper bound was chosen based on the idea that for a good
      user experience, failures should be detected in this amount of
      time and a new connection set up.  Operators that wish to change
      the relationship between load on servers and the expected time
      that a user may not receive inbound communications will probably
      adjust this time.  The 95 seconds lower bound was chosen so that
      the jitter introduced will result in a relatively even load on the
      servers after 30 minutes.

4.4.1.  Keepalive with STUN

   User Agents that form flows MUST check if the configured URI they are
   connecting to has a 'keepalive' URI parameter (defined in Section 10)
   with the value of 'stun'.  If the parameter is present, the UA needs
   to periodically perform keepalive checks by sending a STUN [I-D.ietf-
   behave-rfc3489bis] Binding Requests over the flow.

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

4.4.2.  Flow Recovery

   When a flow to a particular URI in the outbound-proxy-set fails, the
   UA needs to form a new flow to replace the old flow and replace any
   registrations that were previously sent over this flow.  Each new
   registration MUST have the same reg-id as the registration it
   replaces.  This is done in much the same way as forming a brand new
   flow as described in Section 4.2; however, if there is a failure in
   forming this flow, the UA needs to wait a certain amount of time
   before retrying to form a flow to this particular next hop.

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



Jennings & Mahy         Expires December 27, 2006              [Page 15]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   this by the base time, up to a maximum of 1800 seconds.

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

   These three times MAY be configurable in the UA.  The three times
   are:
   o  max-time with a default of 1800 seconds
   o  base-time-all-fail with a default of 30 seconds
   o  base-time-not-failed with a default of 90 seconds
   For example, if the base time was 30 seconds, and there had been
   three failures, then the wait time would be min(1800,30*(2^3)) or 240
   seconds.  The delay time is computed by selecting a uniform random
   time between 50 and 100 percent of the wait time.  The UA MUST wait
   for the value of the delay time before trying another registration to
   form a new flow for that URI.

   To be explicitly clear on the boundary conditions:  when the UA boots
   it immediately tries to register.  If this fails and no registration
   on other flows succeed, the first retry happens somewhere between 30
   and 60 seconds after the failure of the first registration request.
   If the number of consecutive-failures is large enough that the
   maximum of 1800 seconds is reached, the UA will keep trying forever
   with a random time between 900 and 1800 seconds between the attempts.


5.  Edge Proxy Mechanisms

5.1.  Processing Register Requests

   When an Edge Proxy receives a registration request with a reg-id
   header parameter in the Contact header field, it MUST form a flow
   identifier token that is unique to this network flow.  The Edge Proxy
   MUST insert this token into a URI referring to this proxy and place
   this URI into a Path header field as described in RFC 3327 [RFC3327].
   The token MAY be placed in the userpart of the URI.

5.2.  Generating Flow Tokens

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



Jennings & Mahy         Expires December 27, 2006              [Page 16]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   Algorithm 1: The proxy generates a flow token for connection-oriented
      transports by concatenating the file descriptor (or equivalent)
      with the NTP time the connection was created, and base64 encoding
      the result.  This results in an identifier approximately 16 octets
      long.  The proxy generates a flow token for UDP by concatenating
      the file descriptor and the remote IP address and port, then
      base64 encoding the result.  (No NTP time is needed for UDP.)
      This algorithm MUST NOT be used unless all messages between the
      Edge proxy and Registrar use a SIPS protected transport.  If the
      SIPS level of integrity protection is not available, an attacker
      can hijack another user's calls.
   Algorithm 2: When the proxy boots it selects a 20-octet crypto random
      key called K that only the Edge Proxy knows.  A byte array, called
      S, is formed that contains the following information about the
      flow the request was received on:  an enumeration indicating the
      protocol, the local IP address and port, the remote IP address and
      port.  The HMAC of S is computed using the key K and the HMAC-
      SHA1-80 algorithm, as defined in [RFC2104].  The concatenation of
      the HMAC and S are base64 encoded, as defined in [RFC3548], and
      used as the flow identifier.  When using IPv4 addresses, this will
      result in a 32-octet identifier.

5.3.  Forwarding Requests

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

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

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







Jennings & Mahy         Expires December 27, 2006              [Page 17]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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


6.  Registrar and Location Server Mechanisms

6.1.  Processing REGISTER Requests

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

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

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

   In addition to the normal information stored in the binding record,
   some additional information MUST be stored for any registration that



Jennings & Mahy         Expires December 27, 2006              [Page 18]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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

   The Registrar MUST include the 'outbound' option-tag (defined in
   Section (Section 10.1)) in a Supported header field value in its
   responses to REGISTER requests.  The Registrar MAY be configured with
   local policy to reject any registrations that do not include the
   instance-id and reg-id.  Note that the requirements in this section
   applies to both REGISTER requests received from an Edge Proxy as well
   as requests received directly from the UAC.

6.2.  Forwarding Requests

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

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

   The proxy uses normal forwarding rules looking at the next-hop target
   of the message and the value of any stored Path header field vector
   in the registration binding to decide how to forward the request and
   populate the Route header in the request.  Additionally, when the
   proxy forwards a request to a binding that contains a reg-id, the
   proxy MUST send the request over the same network flow that was saved
   with the binding.  This means that for TCP, the request MUST be sent



Jennings & Mahy         Expires December 27, 2006              [Page 19]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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


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

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

7.1.  STUN Processing

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

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

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

   When a URI is created that refers to a SIP device that supports STUN
   as described in this section, the 'keepalive' URI parameter, as
   defined in Section 10 MUST be added to the URI, with a value of
   'stun'.  This allows a UA to inspect the URI to decide if it should



Jennings & Mahy         Expires December 27, 2006              [Page 20]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   attempt to send STUN requests to this location.  The 'keepalive' tag
   typically would be present in the URI in the Route header field value
   of a REGISTER request and not be in the Request URI.


8.  Example Message Flow

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





































Jennings & Mahy         Expires December 27, 2006              [Page 21]

Internet-Draft     Client Initiated Connections in SIP         June 2006


                   [-----example.com domain -------------------]
   Caller           Secondary             Primary            Callee
     |                 |                  |     (1) REGISTER |
     |                 |                  |<-----------------|
     |                 |                  |(2) 200 OK        |
     |                 |                  |----------------->|
     |                 |                  |     (3) REGISTER |
     |                 |<------------------------------------|
     |                 |(4) 200 OK        |                  |
     |                 |------------------------------------>|
     |                 |                  |                  |
     |                 |           CRASH  X                  |
     |(5) INVITE       |                  |                  |
     |----------------------------------->|                  |
     |(6) ICMP Unreachable                |                  |
     |<-----------------------------------|                  |
     |(7) INVITE       |                  |                  |
     |---------------->|                  |                  |
     |                 |(8) INVITE        |                  |
     |                 |------------------------------------>|
     |                 |(9) 200 OK        |                  |
     |                 |<------------------------------------|
     |(10) 200 OK      |                  |                  |
     |<----------------|                  |                  |
     |(11) ACK         |                  |                  |
     |---------------->|                  |                  |
     |                 |(12) ACK          |                  |
     |                 |------------------------------------>|
     |                 |                  |                  |
     |                 |          REBOOT  |                  |
     |                 |                  |(13) REGISTER     |
     |                 |                  |<-----------------|
     |                 |                  |(14) 200 OK       |
     |                 |                  |----------------->|
     |                 |                  |                  |
     |(15) BYE         |                  |                  |
     |---------------->|                  |                  |
     |                 | (16) BYE         |                  |
     |                 |------------------------------------>|
     |                 |                  |      (17) 200 OK |
     |                 |<------------------------------------|
     |     (18) 200 OK |                  |                  |
     |<----------------|                  |                  |
     |                 |                  |                  |

   This call flow assumes that the Callee has been configured with a
   proxy set that consists of "sip:
   primary.example.com;lr;keepalive=stun" and "sip:



Jennings & Mahy         Expires December 27, 2006              [Page 22]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   secondary.example.com;lr;keepalive=stun".  The Callee REGISTER in
   message (1) looks like:


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=7F94778B653B
   To: Callee <sip:callee@example.com>
   Call-ID: 16CB75F21C70
   CSeq: 1 REGISTER
   Supported: path
   Route: <sip:primary.example.com;lr;keepalive=stun>
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=1
   Content-Length: 0

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


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: Callee <sip:callee@example.com>;tag=7F94778B653B
   To: Callee <sip:callee@example.com>;tag=6AF99445E44A
   Call-ID: 16CB75F21C70
   CSeq: 1 REGISTER
   Supported: outbound
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=1
     ;expires=3600
   Content-Length: 0

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












Jennings & Mahy         Expires December 27, 2006              [Page 23]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnqr9bym
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=755285EABDE2
   To: Callee <sip:callee@example.com>
   Call-ID: E05133BD26DD
   CSeq: 1 REGISTER
   Supported: path
   Route: <sip:secondary.example.com;lr;keepalive=stun>
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=2
   Content-Length: 0


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnqr9bym
   From: Callee <sip:callee@example.com>;tag=755285EABDE2
   To: Callee <sip:callee@example.com>;tag=49A9AD0B3F6A
   Call-ID: E05133BD26DD
   Supported: outbound
   CSeq: 1 REGISTER
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=1
     ;expires=3600
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
     ;reg-id=2
     ;expires=3600
   Content-Length: 0

   The messages in the call flow are very normal.  The only interesting
   thing to note is that the INVITE in message 8 contains a Record-Route
   header for the Secondary proxy, with its flow token.

   Record-Route:
    <sip:PQPbqQE+Ynf+tzRPD27lU6uxkjQ8LLUG@secondary.example.com;lr>

   The registrations in message 13 and 14 are the same as message 1 and
   2 other than the Call-ID and tags have changed.  Because these
   messages will contain the same instance-id and reg-id as those in 1
   and 2, this flow will partially supersede that for messages 1 and 2
   and will be tried first by Primary.


9.  Grammar




Jennings & Mahy         Expires December 27, 2006              [Page 24]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   This specification defines new Contact header field parameters,
   reg-id and +sip.instance.  The grammar includes the definitions from
   RFC 3261 [RFC3261] and includes the definition of uric from RFC 2396
   [RFC2396].

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

   The ABNF[RFC4234] is:

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

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

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

    instance-val   = *uric ; defined in RFC 2396

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


10.  IANA Considerations

10.1.  Contact Header Field

   This specification defines a new Contact header field parameter
   called reg-id in the "Header Field Parameters and Parameter Values"
   sub-registry as per the registry created by [RFC3968].  The required
   information is:

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

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

10.2.  SIP/SIPS URI Parameters

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







Jennings & Mahy         Expires December 27, 2006              [Page 25]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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

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

10.3.  SIP Option Tag

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

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

10.4.  Media Feature Tag

   This section registers a new media feature tag, per the procedures
   defined in RFC 2506 [RFC2506].  The tag is placed into the sip tree,
   which is defined in RFC 3840 [RFC3840].

   Media feature tag name:  sip.instance

   ASN.1 Identifier:  New assignment by IANA.

   Summary of the media feature indicated by this tag:  This feature tag
   contains a string containing a URN that indicates a unique identifier
   associated with the UA instance registering the Contact.

   Values appropriate for use with this feature tag:  String.

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

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

   Related standards or documents:  RFC XXXX

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

   Security Considerations:  This media feature tag can be used in ways



Jennings & Mahy         Expires December 27, 2006              [Page 26]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   which affect application behaviors.  For example, the SIP caller
   preferences extension [RFC3841] allows for call routing decisions to
   be based on the values of these parameters.  Therefore, if an
   attacker can modify the values of this tag, they may be able to
   affect the behavior of applications.  As a result, applications which
   utilize this media feature tag SHOULD provide a means for ensuring
   its integrity.  Similarly, this feature tag should only be trusted as
   valid when it comes from the user or user agent described by the tag.
   As a result, protocols for conveying this feature tag SHOULD provide
   a mechanism for guaranteeing authenticity.


11.  Security Considerations

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

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

   The more complex case involves one or more edge proxies.  When a UA
   sends a REGISTER request through an Edge Proxy on to the registrar,
   the Edge Proxy inserts a Path header field value.  If the
   registration is successfully authenticated, the 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.



Jennings & Mahy         Expires December 27, 2006              [Page 27]

Internet-Draft     Client Initiated Connections in SIP         June 2006


12.  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.


13.  Changes

   Note to RFC Editor:  Please remove this whole section.

13.1.  Changes from 03 Version

   Added non-normative text motivating STUN vs. SIP PING, OPTIONS, and
   Double CRLF.  Added discussion about why TCP Keepalives are not
   always available.

   Explained more clearly that outbound-proxy-set can be "configured"
   using any current or future, manual or automatic configuration/
   discovery mechanism.

   Added a sentence which prevents an Edge Proxy from forwarding back
   over the flow over which the request is received if the request
   happens to contain a flow token for that flow.  This was an
   oversight.

   Updated example message flow to show a failover example using a new
   dialog-creating request instead of a mid-dialog request.  The old
   scenario was leftover from before the outbound/gruu reorganization.

   Fixed tags, Call-IDs, and branch parameters in the example messages.

   Made the ABNF use the "=/" production extension mechanism recommended
   by Bill Fenner.

   Added a table in an appendix expanding the default flow recovery
   timers.

   Incorporated numerous clarifications and rewordings for better
   comprehension.




Jennings & Mahy         Expires December 27, 2006              [Page 28]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   Fixed many typos and spelling misteaks.

13.2.  Changes from 02 Version

   Removed Double CRLF Keepalive

   Changed ;sip-stun syntax to ;keepalive=stun

   Fixed incorrect text about TCP keepalives.

13.3.  Changes from 01 Version

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

   Added tentative text about Double CRLF Keepalive

   Removed pin-route stuff

   Changed the name of "flow-id" to "reg-id"

   Reorganized document flow

   Described the use of STUN as a proper STUN usage

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

13.4.  Changes from 00 Version

   Moved TCP keepalive to be STUN.

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

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


14.  Acknowledgments

   Jonathan Rosenberg provided many comments and useful text.  Dave Oran
   came up with the idea of using the most recent registration first in
   the proxy.  Alan Hawrylyshen co-authored the draft that formed the
   initial text of this specification.  Additionally, many of the
   concepts here originated at a connection reuse meeting at IETF 60
   that included the authors, Jon Peterson, Jonathan Rosenberg, Alan
   Hawrylyshen, and Paul Kyzivat.  The TCP design team consisting of
   Chris Boulton, Scott Lawrence, Rajnish Jain, Vijay K. Gurbani, and



Jennings & Mahy         Expires December 27, 2006              [Page 29]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   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, and Lyndsay Campbell.


Appendix A.  Default Flow Registration Backoff Times

   The base-time used for the flow re-registration backoff times
   described in Section 4.4.2 are configurable.  If the base-time-all-
   fail value is set to the default of 30 seconds and the base-time-not-
   failed value is set to the default of 90 seconds, the following table
   shows the resulting delay values.

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


15.  References

15.1.  Normative References

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

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

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

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

   [RFC2506]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag



Jennings & Mahy         Expires December 27, 2006              [Page 30]

Internet-Draft     Client Initiated Connections in SIP         June 2006


              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.

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

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

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

   [RFC3969]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Uniform Resource Identifier (URI) Parameter
              Registry for the Session Initiation Protocol (SIP)",
              BCP 99, RFC 3969, December 2004.

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

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

15.2.  Informative 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-04 (work in progress),
              July 2005.




Jennings & Mahy         Expires December 27, 2006              [Page 31]

Internet-Draft     Client Initiated Connections in SIP         June 2006


   [I-D.ietf-sipping-config-framework]
              Petrie, D., "A Framework for Session Initiation Protocol
              User Agent Profile Delivery",
              draft-ietf-sipping-config-framework-08 (work in progress),
              Mar 2006.

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

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

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

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

   [RFC3608]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol
              (SIP) Extension Header Field for Service Route Discovery
              During Registration", RFC 3608, October 2003.


























Jennings & Mahy         Expires December 27, 2006              [Page 32]

Internet-Draft     Client Initiated Connections in SIP         June 2006


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)
   Plantronics
   345 Encincal St
   Santa Cruz, CA  95060
   USA

   Email:  rohan@ekabal.com































Jennings & Mahy         Expires December 27, 2006              [Page 33]

Internet-Draft     Client Initiated Connections in SIP         June 2006


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




Jennings & Mahy         Expires December 27, 2006              [Page 34]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/