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Versions: 00 01 02 03 04 05 RFC 5018

XCON Working Group                                          G. Camarillo
Internet-Draft                                                  Ericsson
Expires: September 4, 2007                                 March 3, 2007


  Connection Establishment in the Binary Floor Control Protocol (BFCP)
                 draft-ietf-xcon-bfcp-connection-04.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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.  Client and server
   authentication are based on Transport Layer Security (TLS).








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  TCP Connection Establishment . . . . . . . . . . . . . . . . .  3
   4.  TLS Usage  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Authentication . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.1.  Certificate-based Server Authentication  . . . . . . . . .  5
     5.2.  Client Authentication based on a Pre-shared Secret . . . .  6
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  7
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  8
   Intellectual Property and Copyright Statements . . . . . . . . . . 10


































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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 (Session Description Protocol) [8] offer/answer [3] exchange
   between a client and a floor control server is specified in RFC 4583
   [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 specifies how to authenticate clients using PSK (Pre-Shared
   Key)-TLS (Transport Layer Security) [6] and how to authenticate
   servers using server certificates.


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 [1] 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.



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   Once the client has the transport address (i.e., IP address and port)
   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 target to the corresponding set of IP
   addresses, IPv6-only or dual-stack clients MUST use name resolution
   functions that implement the Source and Destination Address Selection
   algorithms specified in RFC3484 [5] (on many hosts that support IPv6,
   APIs like getaddrinfo() provide this functionality and subsume
   existing APIs like gethostbyname().)

   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 result in the selection of a preferred destination address.
   However, the Source and Destination Selection algorithms of [5] 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. [12] 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 RFC
   4582 [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.





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4.  TLS Usage

   All BFCP entities implement TLS [7] 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 client authentication based on pre-shared secrets and
   server authentication based on server certificates.

5.1.  Certificate-based Server Authentication

   At TLS connection establishment, the floor control server MUST
   present its certificate to the client.  The certificate 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 [4].

   A client establishing a connection to a server knows the server's
   hostname or IP address.  If the client knows the server's hostname,
   the client MUST check it against the server's identity as presented
   in the server's Certificate message, in order to prevent man-in-the-
   middle attacks.

   If a subjectAltName extension of type dNSName is present, that MUST
   be used as the identity.  Otherwise, the (most specific) Common Name
   field in the Subject field of the certificate MUST be used.  Although
   the use of the Common Name is existing practice, it is deprecated and
   Certification Authorities are encouraged to use the subjectAltName
   instead.

   Matching is performed using the matching rules specified by RFC 3280
   [4].  If more than one identity of a given type is present in the
   certificate (e.g., more than one dNSName name), a match in any one of
   the set is considered acceptable.  Names in Common Name fields may
   contain the wildcard character *, which is considered to match any
   single domain name component or component fragment (e.g., *.a.com
   matches foo.a.com but not bar.foo.a.com. f*.com matches foo.com but
   not bar.com).

   If the client knows the server's IP address, the iPAddress



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   subjectAltName must be present in the certificate and must exactly
   match the IP address known to the client.

   If the hostname or IP address known to the client does not match the
   identity in the certificate, user oriented clients MUST either notify
   the user (clients MAY give the user the opportunity to continue with
   the connection in any case) or terminate the connection with a bad
   certificate error.  Automated clients MUST log the error to an
   appropriate audit log (if available) and SHOULD terminate the
   connection (with a bad certificate error).  Automated clients MAY
   provide a configuration setting that disables this check, but MUST
   provide a setting which enables it.

5.2.  Client Authentication based on a Pre-shared Secret

   Client authentication is based on a pre-shared secret between client
   and server.  Authentication is performed using PSK-TLS [6].

   The BFCP specification mandates support for the
   TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite.  Additionally, clients and
   servers supporting this specification MUST support the
   TLS_RSA_PSK_WITH_AES_128_CBC_SHA ciphersuite as well.


6.  Security Considerations

   Client and server authentication as specified in this document are
   based on the use of TLS.  Therefore, it is strongly RECOMMENDED that
   TLS with non-null encryption is always used.  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 client and server authentication).

   TLS PSK mode is subject to offline dictionary attacks.  In DHE and
   RSA modes, an attacker who can mount a single man-in-the-middle
   attack on a client/server pair can then mount a dictionary attack on
   the password.  In modes without DHE or RSA, an attacker who can
   record communications between a client/server pair can mount a
   dictionary attack on the password.  Accordingly, it is RECOMMENDED
   that where possible clients use certificate-based server
   authentication ciphersuites with PSK in order to defend against
   dictionary attacks.

   In addition, passwords SHOULD be chosen with enough entropy to
   provide some protection against dictionary attacks.  Because the
   entropy of text varies dramatically and is generally far less than
   that of an equivalent random bitstring, no hard and fast rules about
   password length are possible.  However, in general passwords SHOULD



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   be chosen to be at least 8 characters and selected from a pool
   containing both upper and lower case, numbers, and special keyboard
   characters (note that an 8-character ASCII password has a maximum
   entropy of 56 bits and in general far lower).  FIPS PUB 112 [11]
   provides some guidance on the relevant issues.  If possible,
   passphrases are preferable to passwords.  In addition, a cooperating
   client and server pair MAY choose to derive the TLS PSK shared key
   from the passphrase via a password-based key derivation function such
   as PBKDF2 [2].

   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 using TLS.  The floor control server assumes that
   attackers cannot hickjack TLS connections from authenticated clients.

   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.

   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.  Therefore, it is RECOMMENDED that TLS is used
   with a non-null encryption algorithm.


7.  IANA Considerations

   This specification does not contain any actions for the IANA.


8.  Acknowledgments

   Sam Hartman, Karim El Malki, and Vijay Gurbani provided useful
   comments on this document.  Eric Rescorla performed a detailed
   security analysis of this document.



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9.  References

9.1.  Normative References

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

   [2]   Kaliski, B., "PKCS #5: Password-Based Cryptography
         Specification Version 2.0", RFC 2898, September 2000.

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

   [4]   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.

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

   [6]   Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for
         Transport Layer Security (TLS)", RFC 4279, December 2005.

   [7]   Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.1", RFC 4346, April 2006.

   [8]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

   [9]   Camarillo, G., Ott, J., and K. Drage, "The Binary Floor Control
         Protocol (BFCP)", RFC 4582, November 2006.

   [10]  Camarillo, G., "Session Description Protocol (SDP) Format for
         Binary Floor Control Protocol (BFCP) Streams", RFC 4583,
         November 2006.

   [11]  National Institute of Standards and Technology (NIST),
         "Password Usage", FIPS PUB 112, May 1985.

9.2.  Informative References

   [12]  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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