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PROPOSED STANDARD
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
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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.
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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.
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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).
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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.
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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.
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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.
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[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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RFC 5018 BFCP September 2007
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