BFCP Connections: Connection Establishment in the Binary Floor Control Protocol (BFCP)

Network Working Group                                       G. Camarillo
Request for Comments: 5018                                      Ericsson
Category: Standards Track                                 September 2007


  Connection Establishment in the Binary Floor Control Protocol (BFCP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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).

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   3.  TCP Connection Establishment  . . . . . . . . . . . . . . . . . 2
   4.  TLS Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Authentication  . . . . . . . . . . . . . . . . . . . . . . . . 4
     5.1.  Certificate-Based Server Authentication . . . . . . . . . . 4
     5.2.  Client Authentication Based on a Pre-Shared Secret  . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8

1.  Introduction

   As discussed in the BFCP (Binary Floor Control Protocol)
   specification [RFC4582], 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) [RFC4566] offer/answer
   [RFC3264] exchange between a client and a floor control server is
   specified in RFC 4583 [RFC4583].  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) [RFC4279] and how to authenticate
   servers using server certificates.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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)
   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].  (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 [RFC3484]
   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.  [RFC4943] 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 [RFC4582] 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
   [RFC4582], 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

   [RFC4582] requires that all BFCP entities implement TLS [RFC4346] and
   recommends that they 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)
   SHOULD 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 [RFC3280].

   A client establishing a connection to a server knows the server's
   host name or IP address.  If the client knows the server's host name,
   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
   [RFC3280].  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 does not know the server's host name and contacts the
   server directly using the server's IP address, the iPAddress
   subjectAltName must be present in the certificate and must exactly
   match the IP address known to the client.

   If the host name 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 that 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 [RFC4279].

   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 simply relies on a pre-shared key without specifying the
   nature of the key.  In practice, such keys have two sources: text
   passwords and randomly generated binary keys.  When keys are derived
   from passwords, TLS PSK mode is subject to offline dictionary
   attacks.  In DHE (Diffie-Hellman Exchange) 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 password-
   derived PSKs 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
   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 [PUB112]
   provides some guidance on the relevant issues.  If possible,
   passphrases are preferable to passwords.  It is RECOMMENDED that
   implementations support, at minimum, 16-character passwords or
   passphrases.  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 [RFC2898].
   Because such key derivation functions may incorporate iteration
   functions for key strengthening, they provide some additional
   protection against dictionary attacks by increasing the amount of
   work that the attacker must perform.

   When the keys are randomly generated and of sufficient length,
   dictionary attacks are not effective because such keys are highly
   unlikely to be in the attacker's dictionary.  Where possible, keys
   SHOULD be generated using a strong random number generator as
   specified in [RFC4086].  A minimum key length of 80 bits SHOULD be
   used.

   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 hijack 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.  Acknowledgments

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

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3280]  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.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4582]  Camarillo, G., Ott, J., and K. Drage, "The Binary Floor
              Control Protocol (BFCP)", RFC 4582, November 2006.

   [RFC4583]  Camarillo, G., "Session Description Protocol (SDP) Format
              for Binary Floor Control Protocol (BFCP) Streams",
              RFC 4583, November 2006.

   [PUB112]   National Institute of Standards and Technology (NIST),
              "Password Usage", FIPS PUB 112, May 1985.

8.2.  Informative References

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898, September 2000.

   [RFC4943]  Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor
              Discovery On-Link Assumption Considered Harmful",
              RFC 4943, September 2007.

Author's Address

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com

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