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Versions: 00 01 02 03 04 05 RFC 5018

XCON Working Group                                          G. Camarillo
Internet-Draft                                                  Ericsson
Expires: December 25, 2006                                 June 23, 2006


  Connection Establishment in the Binary Floor Control Protocol (BFCP)
                 draft-ietf-xcon-bfcp-connection-01.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 . . . . . . . . . . . . . . . . . . . . . . . .  4
     5.1.  Certificate-based Mutual Authentication  . . . . . . . . .  4
     5.2.  Digest-based Client Authentication . . . . . . . . . . . .  5
       5.2.1.  Client Behavior  . . . . . . . . . . . . . . . . . . .  5
       5.2.2.  Floor Control Server Behavior  . . . . . . . . . . . .  6
     5.3.  Attribute Definitions  . . . . . . . . . . . . . . . . . .  7
       5.3.1.  NONCE  . . . . . . . . . . . . . . . . . . . . . . . .  7
       5.3.2.  DIGEST . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.4.  Error Code Definitions . . . . . . . . . . . . . . . . . .  9
     5.5.  Security Considerations  . . . . . . . . . . . . . . . . .  9
     5.6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 11
       5.6.1.  Attribute Registration . . . . . . . . . . . . . . . . 11
       5.6.2.  Error Code Registration  . . . . . . . . . . . . . . . 11
       5.6.3.  Digest Algorithm Subregistry . . . . . . . . . . . . . 11
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14



























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1.  Introduction

   As discussed in the BFCP (Binary Floor Control Protocol)
   specification [6], 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 offer/answer [4] exchange between a client and a floor control
   server is specified in [7].  This document specifies how a client
   establishes a connection to a floor control server outside the
   context of an offer/answer exchange.

   BFCP entities establishing a connection outside an 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

   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 of the floor control
   server, it initiates a TCP connection towards it.  That is, the



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   client performs an active TCP open.

   OPEN ISSUE: do we want to define DNS procedures here?  If so, do we
   want to define SRV procedures or are A and AAAA RRs enough?

   The BFCP specification [6] 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 reseted following the rules in
   [6], 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.

   OPEN ISSUE: do we want to have floor control servers reestablish
   connections as well?


4.  TLS Usage

   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



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   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 servers 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.

   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
   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



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   typically provided by the floor control server in an Error response
   -- typically 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 re-send 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 re-send 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.

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 that is unguessable by attackers.

   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



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   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 typically
   authenticate only the first message sent over the TLS connection.

   OPEN ISSUE: do we want to state that servers typically authenticate
   only the first message sent over the TLS connection?

5.3.  Attribute Definitions

   The following new attribute types are defined:

                    +------+-----------+-------------+
                    | Type | Attribute | Format      |
                    +------+-----------+-------------+
                    |  17  | NONCE     | Unsigned16  |
                    |  18  | DIGEST    | OctetString |
                    +------+-----------+-------------+

                         Table 1: BFCP attributes

   Both are EXTENSION-ATTRIBUTES are specified in [6].

5.3.1.  NONCE

   The NONCE attribute can appear in any message.  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 0 1|M|0 0 0 0 0 1 0 0|          Nonce Value          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: NONCE format



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   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.


      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 0|M|0 0 0 1 1 0 0 0|   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.  When
      HMAC-SHA1 is used, the input text needs to be padded so as to be a
      multiple of 64 bytes.




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      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.

5.4.  Error Code Definitions

                   +-------+---------------------------+
                   | 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



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   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 signed using the DIGEST
   attribute.  Clients and floor control servers MAY use other security
   mechanisms as long as they provide similar security properties.

   OPEN ISSUE: same as before.  Do we want to say that we recommend to
   sign only the first message over a TLS connection?

   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.

   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.  Using TLS confidentiality prevents this attack
   because the attacker cannot access the contents of the message in the
   first place.  Additionally, TLS provides replay protection for a
   given connection.  Therefore, it is strongly RECOMMENDED that TLS is



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   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
   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  |
                     +------+-----------+------------+
                     |  17  | NONCE     | [RFC XXXX] |
                     |  18  | DIGEST    | [RFC XXXX] |
                     +------+-----------+------------+

           Table 4: New values of the BFCP Attribute subregistry

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

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".  For the purposes of this subregistry, the
   BFCP error codes for which IANA registration is requested MUST be
   defined by a standards-track RFC.  Such RFC MUST specify the value



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   and the semantics of the error code, and any Error Specific Details
   that apply to it.

   For each BFCP digest algorithm, the IANA registers its numeric
   identifier, its name, and the reference to the RFC 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 provided useful comments on this document.

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

   [7]  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.






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Author's Address

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com










































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