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Versions: 00 01 02 03 04 05 RFC 5018

XCON Working Group                                          G. Camarillo
Internet-Draft                                                  Ericsson
Expires: March 19, 2007                               September 15, 2006


  Connection Establishment in the Binary Floor Control Protocol (BFCP)
                 draft-ietf-xcon-bfcp-connection-02.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies how a Binary Floor Control Protocol (BFCP)
   client establishes a connection to a BFCP floor control server
   outside the context of an offer/answer exchange.  This document also
   specifies a digest authentication mechanism for BFCP based on shared
   secrets.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  TCP Connection Establishment . . . . . . . . . . . . . . . . .  3
   4.  TLS Usage  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Authentication . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.1.  Certificate-based Mutual Authentication  . . . . . . . . .  5
     5.2.  Digest-based Client Authentication . . . . . . . . . . . .  5
       5.2.1.  Client Behavior  . . . . . . . . . . . . . . . . . . .  6
       5.2.2.  Floor Control Server Behavior  . . . . . . . . . . . .  7
     5.3.  Attribute Definitions  . . . . . . . . . . . . . . . . . .  8
       5.3.1.  NONCE  . . . . . . . . . . . . . . . . . . . . . . . .  8
       5.3.2.  DIGEST . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.4.  Error Code Definitions . . . . . . . . . . . . . . . . . . 10
     5.5.  Security Considerations  . . . . . . . . . . . . . . . . . 10
     5.6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 12
       5.6.1.  Attribute Registration . . . . . . . . . . . . . . . . 12
       5.6.2.  Error Code Registration  . . . . . . . . . . . . . . . 12
       5.6.3.  Digest Algorithm Subregistry . . . . . . . . . . . . . 12
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

























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1.  Introduction

   As discussed in the BFCP (Binary Floor Control Protocol)
   specification [9], a given BFCP client needs a set of data in order
   to establish a BFCP connection to a floor control server.  These data
   include the transport address of the server, the conference
   identifier, and the user identifier.

   Once a client obtains this information, it needs to establish a BFCP
   connection to the floor control server.  The way this connection is
   established depends on the context of the client and the floor
   control server.  How to establish such a connection in the context of
   an SDP [8] offer/answer [4] exchange between a client and a floor
   control server is specified in [10].  This document specifies how a
   client establishes a connection to a floor control server outside the
   context of an SDP offer/answer exchange.

   BFCP entities establishing a connection outside an SDP offer/answer
   exchange need different authentication mechanisms than entities using
   offer/answer exchanges.  This is because offer/answer exchanges
   provide parties with an initial integrity-protected channel that
   clients and floor control servers can use to exchange the
   fingerprints of their self-signed certificates.  Outside the offer/
   answer model, such a channel is not typically available.  This
   document defines a digest mechanism for BFCP that is based on shared
   secrets.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [2] and indicate requirement levels for
   compliant implementations.


3.  TCP Connection Establishment

   As stated in Section 1, a given BFCP client needs a set of data in
   order to establish a BFCP connection to a floor control server.
   These data include the transport address of the server, the
   conference identifier, and the user identifier.  It is outside the
   scope of this document to specify how a client obtains this
   information.  This document assumes that the client obtains this
   information using an out-of-band method.

   Once the client has the transport address (i.e., IP address and port)



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   of the floor control server, it initiates a TCP connection towards
   it.  That is, the client performs an active TCP open.

   If the client is provided with the floor control server's host name
   instead of with its IP address, the client MUST perform a DNS lookup
   in order to resolve the host name into an IP address.  Clients
   eventually perform an A or AAAA DNS lookup (or both) on the host
   name.

   In order to translate the host name to the corresponding set of IP
   addresses, IPv6-only or dual-stack clients MUST use the newer
   getaddrinfo() name lookup function, instead of gethostbyname() [7].
   The new function implements the Source and Destination Address
   Selection algorithms specified in [12], and is expected to be
   supported by all IPv6 hosts.

   The advantage of the additional complexity is that this technique
   will output an ordered list of IPv6/IPv4 destination addresses based
   on the relative merits of the corresponding source/destination pairs.
   This will guarantee optimal routing.  However, the Source and
   Destination Selection algorithms of [6] are dependent on broad
   operating system support and uniform implementation of the
   application programming interfaces that implement this behavior.

      Developers should carefully consider the issues described by Roy
      et al. [11] with respect to address resolution delays and address
      selection rules.  For example, implementations of getaddrinfo()
      may return address lists containing IPv6 global addresses at the
      top of the list and IPv4 addresses at the bottom, even when the
      host is only configured with an IPv6 local scope (e.g., link-
      local) and an IPv4 address.  This will, of course, introduce a
      delay in completing the connection.

   The BFCP specification [9] describes a number of situations when the
   TCP connection between a client and the floor control server needs to
   be reestablished.  However, that specification does not describe the
   reestablishment process because this process depends on how the
   connection was established in the first place.

   When the existing TCP connection is closed following the rules in
   [9], the client SHOULD reestablish the connection towards the floor
   control server.  If a TCP connection cannot deliver a BFCP message
   from the client to the floor control server and times out, the client
   SHOULD reestablish the TCP connection.


4.  TLS Usage




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   All BFCP entities implement TLS and SHOULD use it in all their
   connections.  TLS provides integrity and replay protection, and
   optional confidentiality.  The floor control server MUST always act
   as the TLS server.

   A floor control server that receives a BFCP message over TCP (no TLS)
   can request the use of TLS by generating an Error message with an
   Error code with a value of 9 (Use TLS).


5.  Authentication

   BFCP supports certificate-based mutual authentication between clients
   and floor control servers, as specified in Section 5.1.
   Additionally, BFCP also provides a digest mechanism based on a shared
   secret to provide client authentication for clients without
   certificates.  This digest mechanism is described in Section 5.2.

5.1.  Certificate-based Mutual Authentication

   At TLS connection establishment, the floor control server MUST
   present its certificate to the client.  Clients with certificates
   SHOULD also present their certificates to the floor control server.

   The certificates provided at the TLS-level MUST either be directly
   signed by one of the other party's trust anchors or be validated
   using a certification path that terminates at one of the other
   party's trust anchors [5].

5.2.  Digest-based Client Authentication

   Clients without certificates can authenticate themselves to the floor
   control server using a digest-based mechanism instead.  BFCP supports
   digest-based client authentication based on a shared secret between a
   client and the floor control server.  The floor control server of a
   conference shares a secret with each of the participants in the
   conference and can request them to sign their messages using that
   shared secret.  Consequently, there is a need for a mechanism to
   generate such a shared secret.  However, such mechanism is outside
   the scope of this document.  This document assumes that shared
   secrets are generated and exchanged using out-of-band means.
   However, shared secrets MUST be at least as long as the length of the
   output of the digest algorithm used, as recommended in [1].

   Digest-based client authentication in BFCP is based on the DIGEST
   attribute, which is defined in Section 5.3.2.  This attribute, which
   always appears as the last attribute in a message, contains an
   algorithm identifier and a keyed digest of the BFCP message using



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   that algorithm.  The text used as input to the digest algorithm is
   the BFCP message, including the common header, up to and including
   the attribute preceding the DIGEST attribute.  Depending on the
   algorithm, this text may need to be padded with zeroes.
   Section 5.3.2 lists the algorithms specified in BFCP.

   The key used as input to the keyed digest is the secret shared
   between the server and the user identified by the User ID in the
   common header of the message.

   Section 5.2.1 and Section 5.2.2 discuss how to achieve client
   authentication using the DIGEST attribute.

5.2.1.  Client Behavior

   To achieve client authentication, a client needs to prove to the
   floor control server that the client can produce a DIGEST attribute
   for a message using their shared secret and that the message is fresh
   (to avoid replay attacks).  Clients prove the freshness of a message
   by including a NONCE attribute in the message.

   Clients can obtain the digest algorithms supported by the floor
   control server in an Error response from the floor control server
   with Error Code 10 (DIGEST Attribute Required).  A client SHOULD use
   the first digest algorithm in the list that it supports.

   The nonce to be placed in the NONCE attribute by the client is
   typically provided by the floor control server in an Error response
   with Error Code 10 (DIGEST Attribute Required) or 6 (Invalid Nonce).
   If a client generates a message without a DIGEST attribute and
   receives an Error response with Error Code 10 (DIGEST Attribute
   Required), the client SHOULD resend the message with a DIGEST
   attribute and a NONCE attribute with the nonce received in the Error
   response.

   If after sending a message with a DIGEST attribute, a client receives
   an Error response with Error Code 11 (Invalid Nonce), the client
   SHOULD resend the message using the new nonce received in the Error
   response.  If the Error Code is 12 (Authentication Failed) instead,
   the client MUST NOT send further messages to the floor control server
   until it has obtained a different (hopefully valid) shared secret
   than the one used in the original message.

   If a client receives a nonce in a message from the floor control
   server, the client SHOULD add a NONCE attribute with this nonce and a
   DIGEST attribute to its next message to the floor control server.





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5.2.2.  Floor Control Server Behavior

   If the floor control server receives a message without DIGEST
   attribute from an unauthenticated client, the floor control server
   responds with an Error message with Error Code 10 (DIGEST Attribute
   Required).  The floor control message MUST include a list with the
   digest algorithms supported by the floor control server in order of
   preference (i.e., the first algorithm is the most preferred) and a
   NONCE attribute with a nonce value.  Floor control servers MUST NOT
   use the same nonce for the same shared secret more than once.

   When a floor control server receives a BFCP message with a DIGEST
   attribute, it checks whether the Algorithm identifier in the DIGEST
   attribute corresponds to an algorithm that is supported by the floor
   control server.  If it does not, the floor control server SHOULD
   return an Error message with Error Code 10 (DIGEST Attribute
   Required) with a list with the digest algorithms supported by the
   floor control server.

   If the algorithm identifier is valid, the floor control server checks
   whether the NONCE attribute carries a nonce which was generated by
   the floor control server for this client and which still has not
   expired.  If the nonce is not valid, authentication is considered to
   have failed, in which case the floor control server SHOULD return an
   Error message with Error Code 11 (Invalid Nonce) with a new nonce in
   a NONCE attribute.

   If the nonce is valid, the floor control server calculates the keyed
   digest of the message using the algorithm identified by the DIGEST
   attribute.  The key used as input to the keyed digest is the secret
   shared between the server and the user identified by the User ID in
   the common header of the message.  If the resulting value is the same
   as the one in the DIGEST attribute, authentication is considered
   successful.

   If the resulting value is different than the one in the DIGEST
   attribute, authentication is considered to have failed, in which case
   the server SHOULD return an Error message with Error Code 12
   (Authentication Failed).  Messages from a client that cannot be
   authenticated MUST NOT be processed further.

   Floor control servers MAY include a NONCE attribute in their
   responses to provide the nonce to be used in the next message by the
   client.  However, when TLS is used, floor control servers MAY choose
   to only authenticate the first message sent over the TLS connection.
   This way, the client does not need to sign every message it sends
   (message signatures can be long when compared with BFCP messages).
   Reducing the size of BFCP messages can considerably reduce



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   transmission times over low-bandwidth links.

5.3.  Attribute Definitions

   The following new attribute types are defined:

                    +------+-----------+-------------+
                    | Type | Attribute | Format      |
                    +------+-----------+-------------+
                    |  19  | NONCE     | Unsigned16  |
                    |  20  | DIGEST    | OctetString |
                    +------+-----------+-------------+

                         Table 1: BFCP attributes

   Both are EXTENSION-ATTRIBUTES as specified in [9].

5.3.1.  NONCE

   The NONCE attribute can appear in any message.  The NONCE attribute
   MUST be the last attribute of messages that do not contain a DIGEST
   attribute and the second to last attribute of messages that contain a
   DIGEST attribute (the DIGEST attribute is always the last).  The
   following is the format of the NONCE attribute.


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 1 0 0 1 1|M|0 0 0 0 0 1 0 0|          Nonce Value          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: NONCE format

   Nonce Value: this 16-bit field contains a nonce.

5.3.2.  DIGEST

   The DIGEST attribute can only appear in messages sent by clients.
   The DIGEST attribute MUST be the last attribute of the message in
   which it appears.  The following is the format of the DIGEST
   attribute.









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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 1 0 1 0 0|M|    Length     |   Algorithm   |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
     |                                                               |
     |                                                               |
     |                                                               |
     /                           Digest                              /
     /                                                               /
     |                                                               |
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Figure 2: DIGEST format

   Algorithm: this 8-bit field contains the identifier of the algorithm
      used to calculate the keyed digest.  The following are the
      algorithm identifiers defined:

         +------------+-----------+---------------+--------------+
         | Identifier | Algorithm | Digest Length | Reference    |
         +------------+-----------+---------------+--------------+
         |      0     | HMAC-SHA1 |    20 bytes   | RFC 2104 [1] |
         +------------+-----------+---------------+--------------+

                        Table 2: Digest algorithms

      The text used as input to the digest algorithm is the BFCP
      message, including the common header, up to and including the
      attribute preceding the DIGEST attribute.  Depending on the
      algorithm, this text may need to be padded with zeroes.

      The key used as input to the keyed digest is the secret shared
      between the server and the user identified by the User ID in the
      common header of the message.

   Digest: this field contains a keyed digest of the BFCP message.  Its
      calculation is described in Section 5.2.

   Padding: padding added so that the contents of the DIGEST attribute
      is 32-bit aligned.  The Padding bits SHOULD be set to zero by the
      sender and MUST be ignored by the receiver.





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5.4.  Error Code Definitions

   This specification defines the following new BFCP Error Codes:

                   +-------+---------------------------+
                   | Value | Meaning                   |
                   +-------+---------------------------+
                   |   10  | DIGEST Attribute Required |
                   |   11  | Invalid Nonce             |
                   |   12  | Authentication Failed     |
                   +-------+---------------------------+

                        Table 3: Error Code meaning

   The following is the definition of Error Specific Details for Error
   Code 10 (DIGEST Attribute Needed)


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Algorithm ID  | Algorithm ID  | Algorithm ID  | Algorithm ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     /                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               | Algorithm ID  | Algorithm ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Algorithm ID  | Algorithm ID  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: Digest algorithms format

   Algorithm ID: these 8-bit fields contain the identifiers of the
      digest algorithms supported by the floor control server in order
      of preference (i.e., the first algorithm is the most preferred).

5.5.  Security Considerations

   BFCP can use TLS or message signatures to provide client
   authentication.  Floor control server authentication is based on TLS,
   which also provides replay and integrity protection, and
   confidentiality.  It is RECOMMENDED that TLS with non-null encryption
   is always used and that the first message from an unauthenticated
   client over a given TLS connection is challenged by the floor control
   server.  Clients and floor control servers MAY use other security
   mechanisms as long as they provide similar security properties (i.e.,
   replay and integrity protection, confidentiality, and server
   authentication).



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   The remainder of this Section analyzes some of the threats against
   BFCP and how they are addressed.

   An attacker may attempt to impersonate a client (a floor participant
   or a floor chair) in order to generate forged floor requests or to
   grant or deny existing floor requests.  Client impersonation is
   avoided by having clients sign their messages.  A nonce is included
   in the signature to ensure the freshness of the message.  If the
   client is using a TLS connection to communicate with the floor
   control server, it is enough that the client signs its first message
   over the TLS connection.  The floor control server assumes that
   attackers cannot hickjack the TLS connection and, therefore, that
   subsequent messages over the TLS connection come from the client that
   was initially authenticated.  If TLS-based client authentication is
   used, there is not need for the client to sign BFCP messages over the
   connection.

   An attacker may attempt to impersonate a floor control server.  A
   successful attacker would be able to make clients think that they
   hold a particular floor so that they would try to access a resource
   (e.g., sending media) without having legitimate rights to access it.
   Floor control server impersonation is avoided by having floor control
   servers present their server certificates at TLS connection
   establishment time.  Clients MUST NOT send any signed BFCP message to
   an unauthenticated floor control server in order to prevent man-in-
   the-middle attacks.

   Attackers may attempt to modify messages exchanged by a client and a
   floor control server.  The integrity protection provided by TLS
   connections prevents this attack.

   An attacker may attempt to fetch a valid message sent by a client to
   a floor control server and replay it at a later point.  If the
   message was signed, the attacker may attempt to establish a new TLS
   connection with the floor control server and replay the message over
   the new connection.  The use of nonces avoids this type of attack.
   As stated in Section 5.2.2, floor control servers do not use the same
   nonce for the same shared secret more than once.

   Using TLS confidentiality also prevents that attack because the
   attacker cannot access the contents of the message in the first
   place.  Additionally, TLS provides replay protection within a given
   connection.  Therefore, it is RECOMMENDED that TLS is used with a
   non-null encryption algorithm.

   Attackers may attempt to pick messages from the network to get access
   to confidential information between the floor control server and a
   client (e.g., why a floor request was denied).  TLS confidentiality



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   prevents this attack.

5.6.  IANA Considerations

   The following sections instruct the IANA to perform a set of actions.

5.6.1.  Attribute Registration

   The IANA is instructed to register the following new values under the
   Attribute subregistry under the BFCP Parameters registry.

                     +------+-----------+------------+
                     | Type | Attribute | Reference  |
                     +------+-----------+------------+
                     |  19  | NONCE     | [RFC XXXX] |
                     |  20  | DIGEST    | [RFC XXXX] |
                     +------+-----------+------------+

           Table 4: New values of the BFCP Attribute subregistry

   [Note to the RFC editor: please, replace RFCxxxx with the RFC number
   that will be assigned to this document.]

5.6.2.  Error Code Registration

   The IANA is instructed to register the following new values under the
   Error Code subregistry under the BFCP Parameters registry.

            +-------+---------------------------+------------+
            | Value | Meaning                   | Reference  |
            +-------+---------------------------+------------+
            |   10  | DIGEST Attribute Required | [RFC XXXX] |
            |   11  | Invalid Nonce             | [RFC XXXX] |
            |   12  | Authentication Failed     | [RFC XXXX] |
            +-------+---------------------------+------------+

             Table 5: New Values of the Error Code subregistry

   [Note to the RFC editor: please, replace RFCxxxx with the RFC number
   that will be assigned to this document.]

5.6.3.  Digest Algorithm Subregistry

   This Section establishes the Digest Algorithm subregistry under the
   BFCP Parameters registry.  As per the terminology in RFC 2434 [3],
   the registration policy for BFCP digest algorithms shall be
   "Specification Required".




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   For each BFCP digest algorithm, the IANA registers its numeric
   identifier, its name, and the reference to the specification where
   the algorithm is defined.  The following table contains the initial
   values of this subregistry.

                  +------------+-----------+-----------+
                  | Identifier | Algorithm | Reference |
                  +------------+-----------+-----------+
                  |      0     | HMAC-SHA1 | RFC 2104  |
                  +------------+-----------+-----------+

       Table 6: Initial values of the Digest Algorithms subregistry


6.  Acknowledgments

   Sam Hartman and Karim El Malki provided useful comments on this
   document.


7.  References

7.1.  Normative References

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

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

   [3]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

   [4]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [5]   Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [6]   Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

   [7]   Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E. Castro,
         "Application Aspects of IPv6 Transition", RFC 4038, March 2005.

   [8]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session



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         Description Protocol", RFC 4566, July 2006.

   [9]   Camarillo, G., "The Binary Floor Control Protocol (BFCP)",
         draft-ietf-xcon-bfcp-06 (work in progress), December 2005.

   [10]  Camarillo, G., "Session Description Protocol (SDP) Format for
         Binary Floor Control Protocol  (BFCP) Streams",
         draft-ietf-mmusic-sdp-bfcp-03 (work in progress),
         December 2005.

7.2.  Informative References

   [11]  Roy, S., "IPv6 Neighbor Discovery On-Link Assumption Considered
         Harmful", draft-ietf-v6ops-onlinkassumption-04 (work in
         progress), January 2006.




































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Author's Address

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com










































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Intellectual Property Statement

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Camarillo                Expires March 19, 2007                [Page 16]


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