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RFC 3261


SIP: Session Initiation Protocol

Part 13 of 13, p. 252 to 269
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27 IANA Considerations

   All method names, header field names, status codes, and option tags
   used in SIP applications are registered with IANA through
   instructions in an IANA Considerations section in an RFC.

   The specification instructs the IANA to create four new sub-
   registries under
   Option Tags, Warning Codes (warn-codes), Methods and Response Codes,
   added to the sub-registry of Header Fields that is already present

27.1 Option Tags

   This specification establishes the Option Tags sub-registry under

   Option tags are used in header fields such as Require, Supported,
   Proxy-Require, and Unsupported in support of SIP compatibility
   mechanisms for extensions (Section 19.2).  The option tag itself is a
   string that is associated with a particular SIP option (that is, an
   extension).  It identifies the option to SIP endpoints.

   Option tags are registered by the IANA when they are published in
   standards track RFCs.  The IANA Considerations section of the RFC
   must include the following information, which appears in the IANA
   registry along with the RFC number of the publication.

      o  Name of the option tag.  The name MAY be of any length, but
         SHOULD be no more than twenty characters long.  The name MUST
         consist of alphanum (Section 25) characters only.

      o  Descriptive text that describes the extension.

27.2 Warn-Codes

   This specification establishes the Warn-codes sub-registry under and initiates its
   population with the warn-codes listed in Section 20.43.  Additional
   warn-codes are registered by RFC publication.

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Updated by:  RFC 5630/A 
   The descriptive text for the table of warn-codes is:

   Warning codes provide information supplemental to the status code in
   SIP response messages when the failure of the transaction results
   from a Session Description Protocol (SDP) (RFC 2327 [1]) problem.

   The "warn-code" consists of three digits.  A first digit of "3"
   indicates warnings specific to SIP.  Until a future specification
   describes uses of warn-codes other than 3xx, only 3xx warn-codes may
   be registered.

   Warnings 300 through 329 are reserved for indicating problems with
   keywords in the session description, 330 through 339 are warnings
   related to basic network services requested in the session
   description, 370 through 379 are warnings related to quantitative QoS
   parameters requested in the session description, and 390 380 through 399
   are miscellaneous SIP-related warnings that do not fall into one of the above

27.3 Header Field Names

   This obsoletes the IANA instructions about the header sub-registry

   The following information needs to be provided in an RFC publication
   in order to register a new header field name:

      o  The RFC number in which the header is registered;

      o  the name of the header field being registered;

      o  a compact form version for that header field, if one is

   Some common and widely used header fields MAY be assigned one-letter
   compact forms (Section 7.3.3).  Compact forms can only be assigned
   after SIP working group review, followed by RFC publication.

27.4 Method and Response Codes

   This specification establishes the Method and Response-Code sub-
   registries under and
   initiates their population as follows.  The initial Methods table is:

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         INVITE                   [RFC3261]
         ACK                      [RFC3261]
         BYE                      [RFC3261]
         CANCEL                   [RFC3261]
         REGISTER                 [RFC3261]
         OPTIONS                  [RFC3261]
         INFO                     [RFC2976]

   The response code table is initially populated from Section 21, the
   portions labeled Informational, Success, Redirection, Client-Error,
   Server-Error, and Global-Failure.  The table has the following

      Type (e.g., Informational)
            Number    Default Reason Phrase         [RFC3261]

   The following information needs to be provided in an RFC publication
   in order to register a new response code or method:

      o  The RFC number in which the method or response code is

      o  the number of the response code or name of the method being

      o  the default reason phrase for that response code, if

27.5 The "message/sip" MIME type.

   This document registers the "message/sip" MIME media type in order to
   allow SIP messages to be tunneled as bodies within SIP, primarily for
   end-to-end security purposes.  This media type is defined by the
   following information:

      Media type name: message
      Media subtype name: sip
      Required parameters: none

      Optional parameters: version
         version: The SIP-Version number of the enclosed message (e.g.,
         "2.0").  If not present, the version defaults to "2.0".
      Encoding scheme: SIP messages consist of an 8-bit header
         optionally followed by a binary MIME data object.  As such, SIP
         messages must be treated as binary.  Under normal circumstances
         SIP messages are transported over binary-capable transports, no
         special encodings are needed.

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      Security considerations: see below
         Motivation and examples of this usage as a security mechanism
         in concert with S/MIME are given in 23.4.

27.6 New Content-Disposition Parameter Registrations

   This document also registers four new Content-Disposition header
   "disposition-types": alert, icon, session and render.  The authors
   request that these values be recorded in the IANA registry for

   Descriptions of these "disposition-types", including motivation and
   examples, are given in Section 20.11.

   Short descriptions suitable for the IANA registry are:

      alert     the body is a custom ring tone to alert the user
      icon      the body is displayed as an icon to the user
      render    the body should be displayed to the user
      session   the body describes a communications session, for
                example, as RFC 2327 SDP body

28 Changes From RFC 2543

   This RFC revises RFC 2543.  It is mostly backwards compatible with
   RFC 2543.  The changes described here fix many errors discovered in
   RFC 2543 and provide information on scenarios not detailed in RFC
   2543.  The protocol has been presented in a more cleanly layered
   model here.

   We break the differences into functional behavior that is a
   substantial change from RFC 2543, which has impact on
   interoperability or correct operation in some cases, and functional
   behavior that is different from RFC 2543 but not a potential source
   of interoperability problems.  There have been countless
   clarifications as well, which are not documented here.

28.1 Major Functional Changes

   o  When a UAC wishes to terminate a call before it has been answered,
      it sends CANCEL.  If the original INVITE still returns a 2xx, the
      UAC then sends BYE.  BYE can only be sent on an existing call leg
      (now called a dialog in this RFC), whereas it could be sent at any
      time in RFC 2543.

   o  The SIP BNF was converted to be RFC 2234 compliant.

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   o  SIP URL BNF was made more general, allowing a greater set of
      characters in the user part.  Furthermore, comparison rules were
      simplified to be primarily case-insensitive, and detailed handling
      of comparison in the presence of parameters was described.  The
      most substantial change is that a URI with a parameter with the
      default value does not match a URI without that parameter.

   o  Removed Via hiding.  It had serious trust issues, since it relied
      on the next hop to perform the obfuscation process.  Instead, Via
      hiding can be done as a local implementation choice in stateful
      proxies, and thus is no longer documented.

   o  In RFC 2543, CANCEL and INVITE transactions were intermingled.
      They are separated now.  When a user sends an INVITE and then a
      CANCEL, the INVITE transaction still terminates normally.  A UAS
      needs to respond to the original INVITE request with a 487

   o  Similarly, CANCEL and BYE transactions were intermingled; RFC 2543
      allowed the UAS not to send a response to INVITE when a BYE was
      received.  That is disallowed here.  The original INVITE needs a

   o  In RFC 2543, UAs needed to support only UDP.  In this RFC, UAs
      need to support both UDP and TCP.

   o  In RFC 2543, a forking proxy only passed up one challenge from
      downstream elements in the event of multiple challenges.  In this
      RFC, proxies are supposed to collect all challenges and place them
      into the forwarded response.

   o  In Digest credentials, the URI needs to be quoted; this is unclear
      from RFC 2617 and RFC 2069 which are both inconsistent on it.

   o  SDP processing has been split off into a separate specification
      [13], and more fully specified as a formal offer/answer exchange
      process that is effectively tunneled through SIP.  SDP is allowed
      in INVITE/200 or 200/ACK for baseline SIP implementations; RFC
      2543 alluded to the ability to use it in INVITE, 200, and ACK in a
      single transaction, but this was not well specified.  More complex
      SDP usages are allowed in extensions.

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   o  Added full support for IPv6 in URIs and in the Via header field.
      Support for IPv6 in Via has required that its header field
      parameters allow the square bracket and colon characters.  These
      characters were previously not permitted.  In theory, this could
      cause interop problems with older implementations.  However, we
      have observed that most implementations accept any non-control
      ASCII character in these parameters.

   o  DNS SRV procedure is now documented in a separate specification
      [4].  This procedure uses both SRV and NAPTR resource records and
      no longer combines data from across SRV records as described in
      RFC 2543.

   o  Loop detection has been made optional, supplanted by a mandatory
      usage of Max-Forwards.  The loop detection procedure in RFC 2543
      had a serious bug which would report "spirals" as an error
      condition when it was not.  The optional loop detection procedure
      is more fully and correctly specified here.

   o  Usage of tags is now mandatory (they were optional in RFC 2543),
      as they are now the fundamental building blocks of dialog

   o  Added the Supported header field, allowing for clients to indicate
      what extensions are supported to a server, which can apply those
      extensions to the response, and indicate their usage with a
      Require in the response.

   o  Extension parameters were missing from the BNF for several header
      fields, and they have been added.

   o  Handling of Route and Record-Route construction was very
      underspecified in RFC 2543, and also not the right approach.  It
      has been substantially reworked in this specification (and made
      vastly simpler), and this is arguably the largest change.
      Backwards compatibility is still provided for deployments that do
      not use "pre-loaded routes", where the initial request has a set
      of Route header field values obtained in some way outside of
      Record-Route.  In those situations, the new mechanism is not

   o  In RFC 2543, lines in a message could be terminated with CR, LF,
      or CRLF.  This specification only allows CRLF.

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   o  Usage of Route in CANCEL and ACK was not well defined in RFC 2543.
      It is now well specified; if a request had a Route header field,
      its CANCEL or ACK for a non-2xx response to the request need to
      carry the same Route header field values.  ACKs for 2xx responses
      use the Route values learned from the Record-Route of the 2xx

   o  RFC 2543 allowed multiple requests in a single UDP packet.  This
      usage has been removed.

   o  Usage of absolute time in the Expires header field and parameter
      has been removed.  It caused interoperability problems in elements
      that were not time synchronized, a common occurrence.  Relative
      times are used instead.

   o  The branch parameter of the Via header field value is now
      mandatory for all elements to use.  It now plays the role of a
      unique transaction identifier.  This avoids the complex and bug-
      laden transaction identification rules from RFC 2543.  A magic
      cookie is used in the parameter value to determine if the previous
      hop has made the parameter globally unique, and comparison falls
      back to the old rules when it is not present.  Thus,
      interoperability is assured.

   o  In RFC 2543, closure of a TCP connection was made equivalent to a
      CANCEL.  This was nearly impossible to implement (and wrong) for
      TCP connections between proxies.  This has been eliminated, so
      that there is no coupling between TCP connection state and SIP

   o  RFC 2543 was silent on whether a UA could initiate a new
      transaction to a peer while another was in progress.  That is now
      specified here.  It is allowed for non-INVITE requests, disallowed
      for INVITE.

   o  PGP was removed.  It was not sufficiently specified, and not
      compatible with the more complete PGP MIME.  It was replaced with

   o  Added the "sips" URI scheme for end-to-end TLS.  This scheme is
      not backwards compatible with RFC 2543.  Existing elements that
      receive a request with a SIPS URI scheme in the Request-URI will
      likely reject the request.  This is actually a feature; it ensures
      that a call to a SIPS URI is only delivered if all path hops can
      be secured.

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   o  Additional security features were added with TLS, and these are
      described in a much larger and complete security considerations

   o  In RFC 2543, a proxy was not required to forward provisional
      responses from 101 to 199 upstream.  This was changed to MUST.
      This is important, since many subsequent features depend on
      delivery of all provisional responses from 101 to 199.

   o  Little was said about the 503 response code in RFC 2543.  It has
      since found substantial use in indicating failure or overload
      conditions in proxies.  This requires somewhat special treatment.
      Specifically, receipt of a 503 should trigger an attempt to
      contact the next element in the result of a DNS SRV lookup.  Also,
      503 response is only forwarded upstream by a proxy under certain

   o  RFC 2543 defined, but did no sufficiently specify, a mechanism for
      UA authentication of a server.  That has been removed.  Instead,
      the mutual authentication procedures of RFC 2617 are allowed.

   o  A UA cannot send a BYE for a call until it has received an ACK for
      the initial INVITE.  This was allowed in RFC 2543 but leads to a
      potential race condition.

   o  A UA or proxy cannot send CANCEL for a transaction until it gets a
      provisional response for the request.  This was allowed in RFC
      2543 but leads to potential race conditions.

   o  The action parameter in registrations has been deprecated.  It was
      insufficient for any useful services, and caused conflicts when
      application processing was applied in proxies.

   o  RFC 2543 had a number of special cases for multicast.  For
      example, certain responses were suppressed, timers were adjusted,
      and so on.  Multicast now plays a more limited role, and the
      protocol operation is unaffected by usage of multicast as opposed
      to unicast.  The limitations as a result of that are documented.

   o  Basic authentication has been removed entirely and its usage

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   o  Proxies no longer forward a 6xx immediately on receiving it.
      Instead, they CANCEL pending branches immediately.  This avoids a
      potential race condition that would result in a UAC getting a 6xx
      followed by a 2xx.  In all cases except this race condition, the
      result will be the same - the 6xx is forwarded upstream.

   o  RFC 2543 did not address the problem of request merging.  This
      occurs when a request forks at a proxy and later rejoins at an
      element.  Handling of merging is done only at a UA, and procedures
      are defined for rejecting all but the first request.

28.2 Minor Functional Changes

   o  Added the Alert-Info, Error-Info, and Call-Info header fields for
      optional content presentation to users.

   o  Added the Content-Language, Content-Disposition and MIME-Version
      header fields.

   o  Added a "glare handling" mechanism to deal with the case where
      both parties send each other a re-INVITE simultaneously.  It uses
      the new 491 (Request Pending) error code.

   o  Added the In-Reply-To and Reply-To header fields for supporting
      the return of missed calls or messages at a later time.

   o  Added TLS and SCTP as valid SIP transports.

   o  There were a variety of mechanisms described for handling failures
      at any time during a call; those are now generally unified.  BYE
      is sent to terminate.

   o  RFC 2543 mandated retransmission of INVITE responses over TCP, but
      noted it was really only needed for 2xx.  That was an artifact of
      insufficient protocol layering.  With a more coherent transaction
      layer defined here, that is no longer needed.  Only 2xx responses
      to INVITEs are retransmitted over TCP.

   o  Client and server transaction machines are now driven based on
      timeouts rather than retransmit counts.  This allows the state
      machines to be properly specified for TCP and UDP.

   o  The Date header field is used in REGISTER responses to provide a
      simple means for auto-configuration of dates in user agents.

   o  Allowed a registrar to reject registrations with expirations that
      are too short in duration.  Defined the 423 response code and the
      Min-Expires for this purpose.

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29 Normative References

   [1]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

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

   [3]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [4]  Rosenberg, J. and H. Schulzrinne, "SIP: Locating SIP Servers",
        RFC 3263, June 2002.

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

   [6]  Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
        Transport Layer Security (TLS)", RFC 3268, June 2002.

   [7]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [8]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
        Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [9]  Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April

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

   [11] Freed, F. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November

   [12] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
        Recommendations for Security", RFC 1750, December 1994.

   [13] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        SDP", RFC 3264, June 2002.

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

   [15] Postel, J., "DoD Standard Transmission Control Protocol", RFC
        761, January 1980.

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   [16] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
        H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
        "Stream Control Transmission Protocol", RFC 2960, October 2000.

   [17] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A. and L. Stewart, "HTTP authentication:
        Basic and Digest Access Authentication", RFC 2617, June 1999.

   [18] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
        Information in Internet Messages: The Content-Disposition Header
        Field", RFC 2183, August 1997.

   [19] Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet, F.,
        Watson, M. and M. Zonoun, "MIME media types for ISUP and QSIG
        Objects", RFC 3204, December 2001.

   [20] Braden, R., "Requirements for Internet Hosts - Application and
        Support", STD 3, RFC 1123, October 1989.

   [21] Alvestrand, H., "IETF Policy on Character Sets and Languages",
        BCP 18, RFC 2277, January 1998.

   [22] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
        Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
        RFC 1847, October 1995.

   [23] Housley, R., "Cryptographic Message Syntax", RFC 2630, June

   [24] Ramsdell B., "S/MIME Version 3 Message Specification", RFC 2633,
        June 1999.

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

   [26] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

30 Informative References

   [27] R. Pandya, "Emerging mobile and personal communication systems,"
        IEEE Communications Magazine, Vol. 33, pp. 44--52, June 1995.

   [28] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP:  A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

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   [29] Schulzrinne, H., Rao, R. and R. Lanphier, "Real Time Streaming
        Protocol (RTSP)", RFC 2326, April 1998.

   [30] Cuervo, F., Greene, N., Rayhan, A., Huitema, C., Rosen, B. and
        J. Segers, "Megaco Protocol Version 1.0", RFC 3015, November

   [31] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [32] Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL
        scheme", RFC 2368, July 1998.

   [33] E. M. Schooler, "A multicast user directory service for
        synchronous rendezvous," Master's Thesis CS-TR-96-18, Department
        of Computer Science, California Institute of Technology,
        Pasadena, California, Aug. 1996.

   [34] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.

   [35] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April

   [36] Dawson, F. and T. Howes, "vCard MIME Directory Profile", RFC
        2426, September 1998.

   [37] Good, G., "The LDAP Data Interchange Format (LDIF) - Technical
        Specification", RFC 2849, June 2000.

   [38] Palme, J., "Common Internet Message Headers",  RFC 2076,
        February 1997.

   [39] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
        Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
        Digest Access Authentication", RFC 2069, January 1997.

   [40] Johnston, A., Donovan, S., Sparks, R., Cunningham, C., Willis,
        D., Rosenberg, J., Summers, K. and H. Schulzrinne, "SIP Call
        Flow Examples", Work in Progress.

   [41] E. M. Schooler, "Case study: multimedia conference control in a
        packet-switched teleconferencing system," Journal of
        Internetworking:  Research and Experience, Vol. 4, pp. 99--120,
        June 1993.  ISI reprint series ISI/RS-93-359.

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   [42] H. Schulzrinne, "Personal mobility for multimedia services in
        the Internet," in European Workshop on Interactive Distributed
        Multimedia Systems and Services (IDMS), (Berlin, Germany), Mar.

   [43] Floyd, S., "Congestion Control Principles", RFC 2914, September

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Updated by:  RFC 6026/8.11 
A. Table of Timer Values

   Table 4 summarizes the meaning and defaults of the various timers
   used by this specification.

Timer    Value            Section               Meaning
T1       500ms default    Section     RTT Estimate
T2       4s               Section     The maximum retransmit
                                               interval for non-INVITE
                                               requests and INVITE
T4       5s               Section     Maximum duration a
                                               message will
                                               remain in the network
Timer A  initially T1     Section     INVITE request retransmit
                                               interval, for UDP only
Timer B  64*T1            Section     INVITE transaction
                                               timeout timer
Timer C  > 3min           Section 16.6         proxy INVITE transaction
                           bullet 11            timeout
Timer D  > 32s for UDP    Section     Wait time for response
         0s for TCP/SCTP                       retransmits
Timer E  initially T1     Section     non-INVITE request
                                               retransmit interval,
                                               UDP only
Timer F  64*T1            Section     non-INVITE transaction
                                               timeout timer
Timer G  initially T1     Section 17.2.1       INVITE response
                                               retransmit interval
Timer H  64*T1            Section 17.2.1       Wait time for
                                               ACK receipt
Timer I  T4 for UDP       Section 17.2.1       Wait time for
         0s for TCP/SCTP                       ACK retransmits
Timer J  64*T1 for UDP    Section 17.2.2       Wait time for
         0s for TCP/SCTP                       non-INVITE request
Timer K  T4 for UDP       Section     Wait time for
         0s for TCP/SCTP                       response retransmits
      Timer L  64*T1            Section 17.2.1       Wait time for
                                                     accepted INVITE
                                                     request retransmits

      Timer M  64*T1            Section 17.1.1       Wait time for
                                                     retransmission of
                                                     2xx to INVITE or
                                                     additional 2xx from
                                                     other branches of
                                                     a forked INVITE
                   Table 4: Summary of timers

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   We wish to thank the members of the IETF MMUSIC and SIP WGs for their
   comments and suggestions.  Detailed comments were provided by Ofir
   Arkin, Brian Bidulock, Jim Buller, Neil Deason, Dave Devanathan,
   Keith Drage, Bill Fenner, Cedric Fluckiger, Yaron Goland, John
   Hearty, Bernie Hoeneisen, Jo Hornsby, Phil Hoffer, Christian Huitema,
   Hisham Khartabil, Jean Jervis, Gadi Karmi, Peter Kjellerstedt, Anders
   Kristensen, Jonathan Lennox, Gethin Liddell, Allison Mankin, William
   Marshall, Rohan Mahy, Keith Moore, Vern Paxson, Bob Penfield, Moshe
   J. Sambol, Chip Sharp, Igor Slepchin, Eric Tremblay, and Rick

   Brian Rosen provided the compiled BNF.

   Jean Mahoney provided technical writing assistance.

   This work is based, inter alia, on [41,42].

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Authors' Addresses

   Authors addresses are listed alphabetically for the editors, the
   writers, and then the original authors of RFC 2543.  All listed
   authors actively contributed large amounts of text to this document.

   Jonathan Rosenberg
   72 Eagle Rock Ave
   East Hanover, NJ 07936


   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027


   Gonzalo Camarillo
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas


   Alan Johnston
   100 South 4th Street
   St. Louis, MO 63102


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   Jon Peterson
   NeuStar, Inc
   1800 Sutter Street, Suite 570
   Concord, CA 94520


   Robert Sparks
   dynamicsoft, Inc.
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024


   Mark Handley
   International Computer Science Institute
   1947 Center St, Suite 600
   Berkeley, CA 94704


   Eve Schooler
   AT&T Labs-Research
   75 Willow Road
   Menlo Park, CA 94025


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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
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