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Versions: (draft-mahy-sip-connect-reuse) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 5923

SIP WG                                                           R. Mahy
Internet-Draft                                              SIP Edge LLC
Expires: August 5, 2006                                  V. Gurbani, Ed.
                                          Lucent Technologies, Inc./Bell
                                                            Laboratories
                                                                 B. Tate
                                                               BroadSoft
                                                           February 2006


       Connection Reuse in the Session Initiation Protocol (SIP)
                  draft-ietf-sip-connect-reuse-05.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on August 5, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   When SIP entities use a connection oriented protocol to send a
   request, they typically originate their connections from an ephemeral
   port.  The SIP protocol includes mechanisms which insure that
   responses to a request, and new requests sent in the original



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   direction reuse an existing connection.  However, new requests sent
   in the opposite direction are unlikely to reuse the existing
   connection.  This frequently causes a pair of SIP entities to use one
   connection for requests sent in each direction, and can result in
   potential scaling and performance problems.  This document proposes
   requirements and a mechanism which address this deficiency in
   environments where the connection could be opened in either
   direction.

Table of Contents

   1.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Applicability Statement  . . . . . . . . . . . . . . . . . .   3
   3.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   Benefits of Connection Reuse . . . . . . . . . . . . . . . .   5
   5.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   6
   6.   Requirements . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.   Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . .   8
   8.   Normative Behavior . . . . . . . . . . . . . . . . . . . . .   8
     8.1  Client Behavior  . . . . . . . . . . . . . . . . . . . . .   9
     8.2  Server Behavior  . . . . . . . . . . . . . . . . . . . . .  10
   9.   Security Considerations  . . . . . . . . . . . . . . . . . .  11
     9.1  Authenticating TLS Client Connections  . . . . . . . . . .  11
     9.2  Authenticating TLS Server Connections  . . . . . . . . . .  11
     9.3  Security Considerations for the TCP Transport  . . . . . .  11
   10.  Connection Reuse and SRV Interaction . . . . . . . . . . . .  13
   11.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  13
   12.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  14
   13.  References . . . . . . . . . . . . . . . . . . . . . . . . .  14
     13.1   Normative References . . . . . . . . . . . . . . . . . .  14
     13.2   Informational References . . . . . . . . . . . . . . . .  14
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  15
        Intellectual Property and Copyright Statements . . . . . . .  16


















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1.  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 [3].

   Additional terminology used in this document:

      Advertised address: The address that occurs in the Via sent-by
      production rule, including the port number and transport.

      Alias: A transport layer connection associated with a resolved
      address.

      Resolved address: The address of a user agent (including port
      number and transport) retrieved from the DNS resolution contained
      in RFC3263 [5].

2.  Applicability Statement

   The applicability of the mechanism described in this document is for
   two adjacent SIP entities to reuse connections when they are agnostic
   about the direction of the connection, i.e., either end can initiate
   the connection.  SIP entities that can only open a connection in a
   specific direction -- perhaps because of Network Address Translation
   (NAT) and firewall reasons -- reuse their connections using the
   mechanism described in [1].

   The connect reuse mechanism described in this document is defined
   only for Transport Layer Security (TLS) transports.  Specifically,
   implementations MUST NOT use this mechanism for the TCP transport due
   to the possible attacks that can be launched with connection reuse
   over TCP.  Such attacks and alternative methods for connection reuse
   over TCP are described in Section 9.3.

3.  Introduction

   SIP [2] entities can communicate using either unreliable/
   connectionless (e.g., UDP) or reliable/connection-oriented (e.g.,
   TCP, SCTP [11]) transport protocols.  When SIP entities use a
   connection-oriented protocol (such as TCP or SCTP) to send a request,
   they typically originate their connections from an ephemeral port.

   In the following example, Entity A listens for SIP requests over TLS
   [4] on TCP port 5061 (the default port for SIP over TLS over TCP),
   but uses an ephemeral port (port 8293) for a new connection to Entity
   B. These entities could be SIP User Agents or SIP Proxy Servers.




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          +-----------+ 8293 (UAC)      5061 (UAS) +-----------+
          |           |--------------------------->|           |
          |  Entity   |                            |  Entity   |
          |     A     |                            |     B     |
          |           | 5061 (UAS)                 |           |
          +-----------+                            +-----------+

   Figure 1: Uni-directional connection for requests from A to B.


   The SIP protocol includes mechanisms which insure that responses to a
   request reuse the existing connection which is typically still
   available, and also includes provisions for reusing existing
   connections for other requests sent by the originator of the
   connection.  However, new requests sent in the opposite direction --
   in the example above, requests from B destined to A --  are unlikely
   to reuse the existing connection.  This frequently causes a pair of
   SIP entities to use one connection for requests sent in each
   direction, as shown below.

          +-----------+ 8293              5061 +-----------+
          |           |.......................>|           |
          |  Entity   |                        |  Entity   |
          |     A     | 5061              9741 |     B     |
          |           |<-----------------------|           |
          +-----------+                        +-----------+

   Figure 2: Two connections for requests between A and B.


   Opening an extra connection where an existing one is sufficient can
   result in potential scaling and performance problems.  Consider the
   call flow shown below where Proxy A and Proxy B use the Record-Route
   mechanism to stay involved in a dialog.  Proxy B will establish a new
   TLS connection just to send a BYE request.
















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                      Proxy A    Proxy B
                         |          |
     Create connection 1 +---INV--->|
                         |          |
                         |<---200---+ Response over connection 1
                         |          |
     Re-use connection 1 +---ACK--->|
                         |          |
                         =          =
                         |          |
                         |<---BYE---+ Create connection 2
                         |          |
          Response over  +---200--->|
          connection 2

   Figure 3: Multiple connections for requests.

   Thus, it is advantageous to reuse connections whenever possible.

4.  Benefits of Connection Reuse

   Opening an extra connection where an existing one is sufficient can
   result in potential scaling and performance problems.  For example,
   each new connection using TLS requires a TCP 3-way handshake, a
   handful of round-trips to establish TLS, typically expensive
   asymmetric authentication and key generation algorithms, and
   certificate verification.  This effectively doubles the load on each
   entity.  Setting up a second connection (from B to A above) for
   subsequent requests, even requests in the context of an existing
   dialog (e.g., re-INVITE or BYE after an initial INVITE, or a NOTIFY
   after a SUBSCRIBE [10] or a REFER [9]), can also cause excessive
   delay (especially in networks with long round-trip times).

   ReINVITEs or UPDATE [7] requests are expected to be handled
   automatically and rapidly in order to avoid media and session state
   from being out of step.  If a reINVITE requires a new TLS connection,
   the reINVITE could be delayed by several extra round-trip times.
   Depending on the round-trip time, this combined delay could be
   perceptible or even annoying to a human user.  This is especially
   problematic for some common SIP call flows (for example, the
   recommended example flow in figure number 4 in RFC3725 [8]  use many
   reINVITEs).

   The mechanism described in this document can mitigate the delays
   associated with subsequent requests.






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5.  Overview of Operation

   This section is tutorial in nature, and does not specify any
   normative behavior.

   The act of reusing a connection is initiated by an user agent client
   (UAC, or the proxy half of the UAC) when it adds an "alias" parameter
   to the added Via header (the parameter itself is defined later).
   When a user agent server (UAS, or the proxy half of the UAS) receives
   the request, it examines the topmost Via header.  If the header
   contained an "alias" parameter, the UAS establishes a binding such
   that subsequent requests going to the UAC will reuse the connection.
   We now explain this working in more detail in the context of
   communication between two adjacent proxies.  Without any loss of
   generality, it should be clear that the same technique can be used
   for connection reuse between a UAC and an edge proxy, or between an
   edge proxy and a UAS, or between an UAC and an UAS.

   P1 and P2 are proxies responsible for routing SIP requests through
   user agents that use them as edge proxies (see Figure 4).

                   P1                       P2
              p1.example.com          p2.example.com
               (192.0.2.1)              (192.0.2.128)

   Figure 4: Proxy setup.

   This document is concerned with specifying an extension to SIP for
   connection reuse at the receiving end; i.e., reusing the connection
   when P2 wants to send a request to P1.  However, it should be clear
   that P1 can reuse a connection previously established with P2.  In
   fact, the SIP community recommends that clients reuse a connection
   previously established with a server for subsequent transactions
   going to the same resolved address.  Thus, the reuse property of a
   connection, once it is established, is bi-directional and alias
   tables may be maintained at both P1 and P2.

   P1 gets a request from one of its upstream user agents, and after
   performing RFC3263 server selection, arrives at a destination address
   of P2.  P1 maintains an alias table, and it populates the alias table
   with the IP address, port number, and transport of P2 as determined
   through RFC3263 server selection.  P1 adds an "alias" parameter to
   the topmost Via header (inserted by it) before sending the request to
   P2.  The value in the sent-by production rule of the Via header
   (including the port number), and the transport over which the request
   was sent becomes the advertised address of P1:

   Via: SIP/2.0/TLS p1.example.com;branch=z9hG4bKa7c8dze;alias



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   Assuming that P1 does not have an existing aliased connection with
   P2, P1 now opens a connection with P2.  Upon connection
   authentication and acceptance, it adds P2s to its alias table.  P1's
   alias table now looks like:

   Destination      Destination      Destination     Alias Connection
   IP Address       Port             Transport       Descriptor
   ...
   192.0.2.128      5061              TLS            25

   Subsequent requests that traverse from P1 to P2 will reuse this
   connection; i.e., the requests will be sent over the descriptor 25.

   When P2 receives the request, it may add a "received" parameter to
   the topmost Via and examines the topmost Via to determine whether P1
   supports aliased connections.  The Via at P2 now looks like:

   Via: SIP/2.0/TLS p1.example.com;branch=z9hG4bKa7c8dze;alias;
     received=192.0.2.1

   The presence of the "alias" parameter indicates that P1 does support
   aliasing.  P2 now authenticates the connection and if the
   authentication was successful, P2 creates an alias to P1 using the
   advertised address in the topmost Via. P2's alias table looks like:

   Destination      Destination      Destination     Alias Connection
   IP Address       Port             Transport       Descriptor
   ...
   192.0.2.1        5061             TLS             18

   There are two items of interest here:
   1.  Note that the entry in the last column for P2's alias table is
       the descriptor over which the connection was passively accepted.
       When P2 gets a request from one of its user agents, and
       determines through RFC3263 server resolution that the request
       should be sent to P1 over TLS using the default port (5061), it
       will reuse the aliased connection accessible to it through
       descriptor 18 instead of opening a new connection.
   2.  The network address inserted in the "Destination IP Address"
       column should be the source address as seen by P2 (i.e., the
       "received" parameter).  It could be the case that the host name
       of P1 resolves to different IP addresses due to round-robin DNS.
       However, the aliased connection is to be established with the
       original sender of the request.

   To implement connection aliases for resolved addresses, a SIP node
   could (for example) search an additional data structure (the alias
   table) prior to opening a new connection, or could modify the data



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   structure in which it keeps active connection state so that aliases,
   active connections, and blacklisted nodes are all discovered when
   looking for an active connection.

6.  Requirements

   1.  A connection sharing mechanism SHOULD allow SIP entities to reuse
       existing connections for requests and responses originated from
       either peer in the connection.
   2.  A connection sharing mechanism MUST NOT require UACs (clients) to
       send all traffic from well-know SIP ports.
   3.  A connection sharing mechanism MUST NOT require configuring
       ephemeral port numbers in DNS.
   4.  A connection sharing mechanism MUST prevent unauthorized
       hijacking of other connections.
   5.  Connection sharing SHOULD persist across SIP transactions and
       dialogs.
   6.  There is no requirement to share a complete path for ordinary
       connection reuse.  Hop-by-hop connection sharing is more
       appropriate.

7.  Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC 4234 [6].  This document extends the
   via-params to include a new via-alias defined below.

      via-params = via-ttl / via-maddr / via-received / via-branch /
                   via-alias / via-extension

      via-alias  = "alias"


8.  Normative Behavior

   This document specifies how to reuse connections.  The SIP community
   recommends that servers keep connections up unless they need to
   reclaim resources, and that clients keep connections up as long as
   they are needed.  Connection reuse works best when the client and the
   server maintain their connections for long periods of time.  SIP
   entities therefore SHOULD NOT automatically drop connections on
   completion of a transaction or termination of a dialog.

   An alias is formed at the receiver of a request when it gets a
   request with the "alias" parameter in the topmost Via header.  If the
   receiver decides to accept the alias, then the alias corresponds to
   the source IP address, transport, and port (if one exists in the Via
   sent-by, or the default port if it does not) of the sender of the



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   request.  Whenever the RFC3263 server selection mechanism executed at
   the receiver results in the choice of this IP address, port, and
   transport tuple, the alias MUST be used instead.

      Note that at the receiver, the responses are sent over the same
      connection as specified by RFC3261.  The aliasing mechanism at the
      receiver allows subsequent requests going from the receiver to the
      original sender of the request to reuse the same connection.

   An alias is formed at the sender of the request when it executes the
   RFC3263 server selection mechanism to arrive at an IP address, port,
   and transport tuple to send a request to.  Subsequent requests going
   to the same destination address MUST use the alias instead.

   Only one alias SHOULD exist for the resolved address.  If more than
   one alias is requested because of race conditions (or any other
   reasons), the receiver SHOULD consider the latest alias to be the
   desired alias.  The receiver MUST NOT interpret the situation as a
   desire for load balancing between the aliases.

   Because an alias connection might be reclaimed during a transaction,
   clients SHOULD NOT enforce the RFC 3261 requirement of sending CANCEL
   and ACK (for non 2xx responses) to the same port.  If the alias
   connection no longer exists, the client SHOULD open a new connection
   to the resolved address and send the CANCEL or ACK there instead.
   The newly opened connection MAY be inserted into the alias table.

8.1  Client Behavior

   The proposed mechanism uses a new Via header field parameter.  The
   "alias" parameter is included in a Via header field value to indicate
   that the client wants to create a transport layer alias.  The client
   places its advertised address in the Via header field value (in the
   "sent-by" production).

   The implications of placing an "alias" parameter in the topmost Via
   header of a request must be understood by the client.  Specifically,
   this means that the client MUST keep the connection open for as long
   as the resources on the host operating system allow it to, and that
   it MUST accept requests over this connection -- as opposed to a
   default listening port -- from its downstream peer.  And furthermore,
   it MUST reuse the connection when subsequent requests in the same or
   different transactions are destined to the same resolved address.

      Note that RFC3261 states that a response should arrive over the
      same connection that was opened for a request.

   Whether or not to allow an aliased connection ultimately depends on



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   the recepient of the request.  Thus, clients MUST NOT assume that the
   acceptance of a request by a server automatically enables connection
   aliasing.  They MUST continue receiving requests on their default
   port.

   Clients must be prepared for the case that the connection no longer
   exists when they are ready to send a subsequent request over it.
   This may happen if the peer ran out of operating system resources and
   had to close the connection.  In such a case, a new connection MUST
   be opened to the resolved address and the alias table updated
   accordingly.

   Clients must authenticate the connection before forming an alias.
   Section 9.1 discusses the authentication steps in more detail.

8.2  Server Behavior

   When a server receives a request whose topmost Via header contains an
   "alias" parameter, it signifies that the upstream client will leave
   the connection open beyond the transaction and dialog lifetime, and
   that subsequent transactions and dialogs that are destined to a
   resolved address that matches the identifiers in the advertised
   address in the topmost Via header can reuse this connection.

   Whether or not to honor an aliased connection ultimately depends on
   the policies of the server.  It MAY choose to honor it, and thereby
   send subsequent requests over the aliased connection.  If the server
   chooses not to honor an aliased connection, it MUST allow the request
   to proceed as though the "alias" parameter was not present in the
   topmost Via header.

      This assures interoperability with RFC3261 server behavior.
      Clients should feel comfortable including the "alias" parameter
      without fear that the server will reject the SIP request because
      of its presence.

   Servers MUST be prepared to deal with the case that the aliased
   connection no longer exist when they are ready to send a subsequent
   request over it.  This may happen if the peer ran out of operating
   system resources and had to close the connection.  In such a case, a
   new connection MUST be opened to the resolved address and the alias
   table updated accordingly.

   If the Via sent-by contains a port, it MUST be used as a destination
   port.  Otherwise the default port is the destination port.

   Servers must authenticate the connection before forming an alias.
   Section 9.2 discusses the authentication steps in more detail.



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9.  Security Considerations

   This document presents requirements and a mechanism for reusing
   existing connections easily.  Unauthenticated connection reuse would
   present many opportunities for rampant abuse and hijacking.
   Authenticating connection aliases is essential to prevent connection
   hijacking.  For example, a program run by a malicious user of a
   multiuser system could attempt to hijack SIP requests destined for
   the well-known SIP port from a large relay proxy.

9.1  Authenticating TLS Client Connections

   When a TLS client establishes a connection with a server, it is
   presented with the server's X.509 certificate.  The client MUST
   ensure that the canonical host name of the server is present either
   as the distinguished name (DN) of the Subject field or as a DNS URI
   in the subjectAltName X.509v3 extension before updating its alias
   table with the resolved address.

9.2  Authenticating TLS Server Connections

   A TLS server conformant to this specification MUST ask for a client
   certificate; if the client possesses a certificate, it will be
   presented to the server for mutual authentication.  The server MUST
   ensure that the canonical host name of the client is present either
   as the distinguished name (DN) of the Subject field or as a DNS URI
   in the subjectAltName X.509v3 extension before updating its alias
   table.  If the client does not have a certificate, it is RECOMMENDED
   that servers issue a 403 response with the reason phrase set to
   "Certificate Required for Alias" to provide a more descriptive reason
   for rejection to a human user.  The TLS connection should be closed
   immediately since accepting such a connection and establishing an
   alias would be tantamount to using an encrypted channel for TCP but
   still exposing the server to the same types of attacks described in
   Section 9.3.

9.3  Security Considerations for the TCP Transport

   Connection reuse over TCP is inherently insecure.  Because the nature
   of the aliasing mechanism is such that it redirects requests destined
   for one port at a host to another port, service hi-jacking can result
   if adequate care is not taken to ensure that the redirected port is
   indeed authorized to receive the requests that would normally have
   gone to another, authorized port.  Consider the following scenario to
   understand the service hi-jacking attack that can be mounted when
   using connection reuse over TCP.

   A TCP server receives a request with the "alias" parameter as follows



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   (the "received" parameter is added by the server after getting the
   request):

   Via: SIP/2.0/TCP uac.example.com;branch=z9hG4bKa7c8dze;alias;
      received=192.0.4.33

   From the server's perspective, its alias table is updated such that
   whenever a request is destined to 192.0.4.33, port 5060, it will
   instead be sent to the peer at the end of the aliased connection.
   The security attack can now be mounted as follows: assume a malware
   program is running on a multi-user computer.  The malware program
   knows that a user on the computer runs a SIP user agent, but the SIP
   user agent is currently not active (possibly by scanning ports on the
   local machine to seek a busy port 5060).  Note that the malware
   program does not need to wait until the legitimate user agent was not
   running, however, doing so increases the chances that the server will
   not reject the malware program's request.  Once the malware program
   decides that a legitimate user agent is not running, it sends sends a
   request to the server with an "alias" parameter.  The server believes
   it is accepting a request from a legitimate user agent and sends
   subsequent requests to the aliased connection.  The SIP service on
   the computer has now effectively been hi-jacked for the default port.
   The malware program does not need administrative privileges to
   execute, and in fact, can masquerade as any user (legitimate or not)
   of the computer.

   Later on, when the legitimate user agent is started, it may also send
   a request with an "alias" parameter to the server, which may detect
   that it now has two aliased connections.  Making matters much worse,
   it cannot determine which of the two is the legitimate one and may
   well reject the request from the legitimate user.

   In another form of this attack, the legitimate user agent may not
   support connection aliasing, but the malware program may use the
   mechanism to usurp the SIP service on the computer.

   In yet another form of an attack, the malware program uses the
   aliasing mechanism to shortcut registering with a proxy to receive
   requests.  In this case, it sends a request to the edge proxy (who
   may also substitute as the inbound proxy with access to a location
   service for that domain).  In the request is a bogus request URI that
   will cause the edge proxy to fail the request, however, the edge
   proxy keeps the connection open and any subsequent requests destined
   to that host on the default port are instead sent to the malware
   program.  Registration is thus not needed in order to receive
   incoming requests.

   HTTP Digest is useful to mitigate only a subset of these attacks over



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   TCP.  For instance, HTTP Digest helps in authenticating a user agent
   to a proxy server before the alias table is updated.  However, HTTP
   Digest is of no help when one proxy desires to enter an aliasing
   agreement with another downstream proxy.

   Keeping in view the possible attacks for TCP connection reuse
   documented here and the limited help provided by HTTP Digest to
   mitigate these attacks, it is recommended that TCP peers that want to
   avail of connection reuse do so such that each peer actively opens up
   a TCP connection in the direction of the other (as depicted in Figure
   2).  This manner of opening connections, while still not secure, is
   at least much more apparent and direct than using the connection
   reuse mechanism over TCP in an unauthenticated fashion.

10.  Connection Reuse and SRV Interaction

   Connection reuse has an interaction with the DNS SRV load balancing
   mechanism.  To understand the interaction, consider the following
   figure:


             /+---- S1
   +-------+/
   | Proxy |------- S2
   +-------+\
             \+---- S3

   Figure 5: Load balancing.

   Here, the proxy uses DNS SRV to load balance across the three
   servers, S1, S2, and S3.  Using the connect reuse mechanism specified
   in this document, over time the proxy will maintain a distinct
   aliased connection to each of the servers.  However, once this is
   done, subsequent traffic is load balanced across the three downstream
   servers in the normal manner.

11.  IANA Considerations

   This document adds a parameter to the SIP header field parameters
   registry:

   Header field in which parameter can appear: Via
   Name of the parameter: alias
   Reference:  This document







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12.  Acknowledgments

   Thanks to Jon Peterson for helpful answers about certificate behavior
   with SIP, Jonathan Rosenberg for his initial support of this concept,
   and Cullen Jennings for providing a sounding board for this idea.

13.  References

13.1  Normative References

   [1]  Jennings, C. and R. Mahy, "Managing Client Initiated Connections
        in the Session Initiation  Protocol (SIP)",
        draft-ietf-sip-outbound-01.txt (work in progress), October 2005.

   [2]  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.

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

   [4]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
        RFC 2246, January 1999.

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

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

13.2  Informational References

   [7]   Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, September 2002.

   [8]   Rosenberg, J., Peterson, J., Schulzrinne, H., and H. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", RFC 3725, April 2004.

   [9]   Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

   [10]  Roach, A., "The Session Initiation Protocol (SIP)-Specific
         Event Notification", RFC 3265, June 2002.

   [11]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.



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         Paxson, "The Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 2960, October 2000.


Authors' Addresses

   Rohan Mahy
   SIP Edge LLC

   Email: rohan@ekabal.com


   Vijay K. Gurbani (editor)
   Lucent Technologies, Inc./Bell Laboratories

   Email: vkg at acm dot org


   Brett Tate
   BroadSoft

   Email: brett@broadsoft.com





























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