XCON Working Group G. Camarillo Internet-Draft Ericsson Expires: June 3, 2006 November 30, 2005 Connection Establishment in the Binary Floor Control Protocol (BFCP) draft-ietf-xcon-bfcp-connection-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 3, 2006. Copyright Notice Copyright (C) The Internet Society (2005). 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. Camarillo Expires June 3, 2006 [Page 1]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 2]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 3]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 4]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 5]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 6]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 7]
Internet-Draft BFCP November 2005 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. Camarillo Expires June 3, 2006 [Page 8]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 9]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 10]
Internet-Draft BFCP November 2005 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 Camarillo Expires June 3, 2006 [Page 11]
Internet-Draft BFCP November 2005 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-05 (work in progress), July 2005. [7] Camarillo, G., "Session Description Protocol (SDP) Format for Binary Floor Control Protocol (BFCP) Streams", draft-ietf-mmusic-sdp-bfcp-02 (work in progress), July 2005. Camarillo Expires June 3, 2006 [Page 12]
Internet-Draft BFCP November 2005 Author's Address Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: Gonzalo.Camarillo@ericsson.com Camarillo Expires June 3, 2006 [Page 13]
Internet-Draft BFCP November 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Camarillo Expires June 3, 2006 [Page 14]
