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Copyright © The Internet Society (2005).
This document specifies how to establish secure connection-oriented media transport sessions over the Transport Layer Security (TLS) protocol using the Session Description Protocol (SDP). It defines a new protocol identifier, TCP/TLS. It also defines the syntax and semantics for an SDP "fingerprint" attribute that identifies the certificate which will be presented for the TLS session. This mechanism allows media transport over TLS connections to be established securely, so long as the integrity of session descriptions is assured.
3.1 SDP Operational Modes
3.2 Threat Model
3.3 The Need For Self-Signed Certificates
3.4 Example SDP Description For TLS Connection
4. Protocol Identifiers
5. Fingerprint Attribute
6. Endpoint Identification
6.1 Certificate Choice
6.2 Certificate Presentation
7. Security Considerations
8. IANA Considerations
A. Changes From Earlier Versions
A.1 Changes From Draft -02
A.2 Changes From Draft -01
A.3 Changes From Draft -00
9.1 Normative References
9.2 Informative References
§ Author's Address
§ Intellectual Property and Copyright Statements
The Session Description Protocol (SDP) (Handley, M., “SDP: Session Description Protocol,” February 2005.) provides a general purpose format for describing multimedia sessions in announcements or invitations. For many applications, it is desirable to establish, as part of a multimedia session, a media stream which uses a connection-oriented transport. The document Connection-Oriented Media Transport in the Session Description Protocol (SDP) (Yon, D., “Connection-Oriented Media Transport in the Session Description Protocol (SDP),” November 2004.) specifies a general mechanism for describing and establishing such connection-oriented streams; however, the only transport protocol it directly supports is TCP. In many cases, session participants wish to provide confidentiality, data integrity, and authentication for their media sessions. This document therefore extends the Connection-Oriented Media specification to allow session descriptions to describe media sessions that use the Transport Layer Security (TLS) protocol (Dierks, T. and C. Allen, “The TLS Protocol Version 1.0,” January 1999.).
The TLS protocol allows applications to communicate over a channel which provides privacy and data integrity. The TLS specification, however, does not specify how specific protocols establish and use this secure channel; particularly, TLS leaves the question of how to interpret and validate authentication certificates as an issue for the protocols which run over TLS. This document specifies such usage for the case of connection-oriented media transport.
Complicating this issue, endpoints exchanging media will often be unable to obtain authentication certificates signed by a well-known root certificate authority (CA). Most certificate authorities charge for signed certificates, particularly host-based certificates; additionally, there is a substantial administrative overhead to obtaining signed certificates, as certificate authorities must be able to confirm that they are issuing the signed certificates to the correct party. Furthermore, in many cases endpoints' IP addresses and host names are dynamic: they may be obtained from DHCP, for example. It is impractical to obtain a CA-signed certificate valid for the duration of a DHCP lease. For such hosts, self-signed certificates are usually the only option. This specification defines a mechanism which allows self-signed certificates can be used securely, provided that the integrity of the SDP description is assured. It provides for endpoints to include a secure hash of their certificate, known as the "certificate fingerprint", within the session description. Provided the fingerprint of the offered certificate matches the one in the session description, end hosts can trust even self-signed certificates.
The rest of this document is laid out as follows. An overview of the problem and threat model is given in Section 3 (Overview). Section 4 (Protocol Identifiers) gives the basic mechanism for establishing TLS-based connected-oriented media in SDP. Section 5 (Fingerprint Attribute) describes the SDP fingerprint attribute, which, assuming the integrity of SDP content is assured, allows the secure use of self-signed certificates. Section 6 (Endpoint Identification) describes which X.509 certificates are presented, and how they are used in TLS. Section 7 (Security Considerations) discusses additional security considerations.
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) and indicate requirement levels for compliant implementations.
This section discusses the threat model which motivates TLS transport for connection-oriented media streams. It also discusses in more detail the need for end systems to use self-signed certificates.
There are two principal operational modes for multimedia sessions: advertised and offer-answer. Advertised sessions are the simpler mode. In this mode, a server publishes, in some manner, an SDP session description describing a multimedia session it is making available. The classic example of this mode of operation is the Session Announcment Protocol (SAP) (Handley, M., Perkins, C., and E. Whelan, “Session Announcement Protocol,” October 2000.), in which SDP session descriptions are periodically transmitted to a well-known multicast group. Traditionally, these descriptions involve multicast conferences, but unicast sessions are also possible. (Connection-oriented media, obviously, cannot use multicast.) Recipients of a session description connect to the addresses published in the session description. These recipients may not previously have been known to the advertiser of the session description.
Alternatively, SDP conferences can operate in offer-answer mode (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.). This mode allows two participants in a multimedia session to negotiate the multimedia session between them. In this model, one participant offers the other a description of the desired session from its perspective, and the other participant answers with the desired session from its own perspective. In this mode, each of the participants in the session has knowledge of the other one. This is the mode of operation used by the Session Initiation Protocol (SIP) (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.).
Participants in multimedia conferences often wish to guarantee confidentiality, data integrity, and authentication for their media sessions. This section describes various types of attackers and the ways they attempt to violate these guarantees. It then describes how the TLS protocol can be used to thwart the attackers.
The simplest type of attacker is one who listens passively to the traffic associated with a multimedia session. This attacker might, for example, be on the same local-area or wireless network as one of the participants in a conference. This sort of attacker does not threaten a connection's data integrity or authentication, and almost any operational mode of TLS can provide media stream confidentiality.
More sophisticated is an attacker who can send his own data traffic over the network, but who cannot modify or redirect valid traffic. In SDP's 'advertised' operational mode, this can barely be considered an attack; media sessions are expected to be initiated from anywhere on the network. In SDP's offer-answer mode, however, this type of attack is more serious. An attacker could initiate a connection to one or both of the endpoints of a session, thus impersonating an endpoint, or acting as a man in the middle to listen in on their communications. To thwart these attacks, TLS uses endpoint certificates. So long as the certificates' private keys have not been compromised, the endpoints have an external trusted mechanism (most commonly, a mutually-trusted certificate authority) to validate certificates, and the endpoints know what certificate identity to expect, endpoints can be certain that such an attack has not taken place.
Finally, the most serious type of attacker is one who can modify or redirect session descriptions: for example, a compromised or malicious SIP proxy server. Neither TLS itself, nor any mechanisms which use it, can protect an SDP session against such an attacker. Instead, the SDP description itself must be secured through some mechanism; SIP, for example, defines how S/MIME (Ramsdell, B., “S/MIME Version 3 Message Specification,” June 1999.) can be used to secure session descriptions.
SDP session descriptions are created by any endpoint that needs to participate in a multimedia session. In many cases, such as SIP phones, such endpoints have dynamically-configured IP addresses and host names, and must be deployed with nearly zero configuration. For such an endpoint, it is for practical purposes impossible to obtain a certificate signed by a well-known certificate authority.
If two endpoints have no prior relationship, self-signed certificates cannot generally be trusted, as there is no guarantee that an attacker is not launching a man-in-the-middle attack. Fortunately, however, if the integrity of SDP session descriptions can be assured, it is possible to consider those SDP descriptions themselves as a prior relationship: certificates can be securely described in the session description itself. This is done by providing a secure hash of a certificate, or "certificate fingerprint", as an SDP attribute; this mechanism is described in Section 5 (Fingerprint Attribute).
Figure 1 (Example SDP Description Offering a TLS Media Stream) illustrates an SDP offer which signals the availability of a T.38 fax session over TLS. For the purpose of brevity, the main portion of the session description is omitted in the example, showing only the m= line and its attributes. (This example is the same as the first one in  (Yon, D., “Connection-Oriented Media Transport in the Session Description Protocol (SDP),” November 2004.), except for the proto parameter and the fingerprint attribute.) See the subsequent sections for explanations of the example's TLS-specific attributes.
(Note: due to RFC formatting conventions, this draft splits SDP across lines whose content would exceed 72 characters. A backslash character marks where this line folding has taken place. This backslash and its trailing CRLF and whitespace would not appear in actual SDP content.)
m=image 54111 TCP/TLS t38 c=IN IP4 192.0.2.2 a=setup:passive a=connection:new a=fingerprint:SHA-1 \ 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
| Figure 1: Example SDP Description Offering a TLS
Media Stream |
The m= line in SDP specifies, among other items, the transport protocol to be used for the media in the session. See the "Media Descriptions" section of SDP (Handley, M., “SDP: Session Description Protocol,” February 2005.) for a discussion on transport protocol identifiers.
This specification defines a new protocol identifier, TCP/TLS, which indicates that the media described will use the Transport Layer Security protocol (Dierks, T. and C. Allen, “The TLS Protocol Version 1.0,” January 1999.) over TCP. (Using TLS over other transport protocols is not discussed by this document.) The TCP/TLS protocol identifier describes only the transport protocol, not the upper-layer protocol. An m= line that specifies TCP/TLS MUST further qualify the protocol using a fmt identifier, to indicate the application being run over TLS.
As TLS sessions are connection-oriented, media sessions described in this manner follow the procedures defined in the connection-oriented media specification (Yon, D., “Connection-Oriented Media Transport in the Session Description Protocol (SDP),” November 2004.). They also use the attributes defined in that specification, "a=setup" and "a=connection".
Parties to a TLS session indicate their identities by presenting authentication certificates as part of the TLS handshake procedure. Authentication certificates are X.509 (International Telecommunications Union, “Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks,” March 2000.) certificates, as profiled by RFC 3279 (Bassham, L., Polk, W., and R. Housley, “Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) and RFC 3280 (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.).
In order to associate media streams with connections, and to prevent unauthorized barge-in attacks on the media streams, endpoints MAY provide a certificate fingerprint. If the X.509 certificate presented for the TLS connection matches the fingerprint presented in the SDP, the endpoint can be confident that the author of the SDP is indeed the initiator of the connection.
A certificate fingerprint is a secure one-way hash of the DER (distinguished encoding rules) form of the certificate. (Certificate fingerprints are widely supported by tools which manipulate X.509 certificates; for instance, the command "openssl x509 -fingerprint" causes the command-line tool of the openssl package to print a certificate fingerprint, and the certificate managers for Mozilla and Internet Explorer display them when viewing the details of a certificate.)
A fingerprint is represented in SDP as an attribute (an "a=" line). It consists of the name of the hash function used, followed by the hash value itself. The hash value is represented as a sequence of upper-case hexadecimal bytes, separated by colons. The number of bytes is defined by the hash function. (This is the syntax used by openssl and by the browsers' certificate managers. It is different from the syntax used to represent hash values in, e.g., HTTP digest authentication (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), which uses unseparated lower-case hexadecimal bytes. It was felt that consistency with other applications of fingerprints was more important.)
The formal syntax of the fingerprint attribute is given in Augmented Backus-Naur Form (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” November 1997.) in Figure 2 (Augmented Backus-Naur Syntax for the Fingerprint Attribute). This syntax extends the BNF syntax of SDP (Handley, M., “SDP: Session Description Protocol,” February 2005.).
attribute =/ fingerprint-attribute fingerprint-attribute = "fingerprint" ":" hash-func SP fingerprint hash-func = "sha-1" / "md5" / "md2" / token ; Additional hash functions can only come ; from updates to RFC 3279 fingerprint = 2UHEX *(":" 2UHEX) ; Each byte in upper-case hex, separated ; by colons. UHEX = DIGIT / %x41-46 ; A-F uppercase
| Figure 2: Augmented Backus-Naur Syntax
for the Fingerprint Attribute |
A certificate fingerprint SHOULD be computed using the same one-way hash function as is used in the certificate's signature algorithm. (This guarantees that the fingerprint will be usable by the other endpoint, so long as the certificate itself is.) Following RFC 3279 (Bassham, L., Polk, W., and R. Housley, “Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.), therefore, the defined hash functions are SHA-1 (National Institute of Standards and Technology, “Secure Hash Standard,” April 1995.)  (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.), MD5 (Rivest, R., “The MD5 Message-Digest Algorithm,” April 1992.), and MD2 (Kaliski, B., “The MD2 Message-Digest Algorithm,” April 1992.), with SHA-1 preferred. Additional hash functions can be defined only by standards-track RFCs which update or obsolete RFC 3279 (Bassham, L., Polk, W., and R. Housley, “Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.). Self-signed certificates (for which legacy certificates are not a consideration) MUST use SHA-1 in their signature algorithm, and thus also MUST use it to calculate certificate fingerprints.
The fingerprint attribute may be either a session-level or a media-level SDP attribute. If it is a session-level attribute, it applies to all TLS sessions for which no media-level fingerprint attribute is defined.
X.509 certificates certify identities. The certificate provided for a TLS connection needs to certify an appropriate identity for the connection. Identity matching is performed using the matching rules specified by RFC 3280 (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.). 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.
If an endpoint does not provide a certificate fingerprint in its SDP, its certificate MUST correspond to one of the following identities, and MUST be signed by a certificate authority known to the other endpoint.
In those cases where an endpoint provides a certificate fingerprint, the certificate MAY be self-signed. The certificate MUST be well-formed (and thus MUST include a syntactically valid SubjectAltName), but no further requirements are imposed upon this field's contents. To support the use of certificate caches, however, as described in Section 7 (Security Considerations), endpoints SHOULD consistently provide the same certificate for each identity they support.
In all cases, an endpoint acting as the TLS server, i.e., one taking the a=setup:passive role, in the terminology of connection-oriented media, MUST present a certificate during TLS initiation, following the rules presented in Section 6.1 (Certificate Choice). If the certificate does not match the original fingerprint, or, if there is no fingerprint, the certificate identity is incorrect, the client endpoint MUST either notify the user, if possible, or terminate the media connection with a bad certificate error.
If the SDP offer/answer model (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) is being used, the client (the endpoint with the setup:active role) MUST also present a certificate following the rules of Section 6.1 (Certificate Choice). The server MUST request a certificate, and if the client does not provide one, if the certificate does not match the provided fingerprint, or, if there was no fingerprint, the certificate identity is incorrect, the server endpoint MUST either notify the user or terminate the media connection with a bad certificate error.
Note that when the offer/answer model is being used, it is possible for a media connection to outrace the answer back to the offerer. Thus, if the offerer has offered a setup:passive or setup:actpass role, it MUST (as specified in the Connection-Oriented Media specification (Yon, D., “Connection-Oriented Media Transport in the Session Description Protocol (SDP),” November 2004.)) begin listening for an incoming connection as soon as it sends its offer. However, because its peer's media connection may outrace its answer, it SHOULD NOT definitively accept or reject the peer's certificate until it has received and processed the SDP answer.
If offer/answer is not being used (e.g., if the SDP was sent over the Session Announcement Protocol (Handley, M., Perkins, C., and E. Whelan, “Session Announcement Protocol,” October 2000.)), the TLS server typically has no external knowledge of what the TLS client's identity ought to be. In this case, no client certificate need be presented, and no certificate validation can be performed, unless the server has knowledge of valid clients through some external means.
This entire document concerns itself with security. The problem to be solved is addressed in Section 1 (Introduction), and a high-level overview is presented in Section 3 (Overview). See the SDP specification (Handley, M., “SDP: Session Description Protocol,” February 2005.) for security considerations applicable to SDP in general.
Like all SDP messages, SDP messages describing TLS streams are conveyed in an encapsulating application protocol (e.g., SIP, MGCP, etc.). It is the responsibility of the encapsulating protocol to ensure the integrity and confidentiality of the SDP security descriptions. Therefore, the application protocol SHOULD either invoke its own security mechanisms (e.g., secure multiparts) or alternatively utilize a lower-layer security service (e.g., TLS or IPSec). This security service SHOULD provide strong message authentication and packet-payload encryption as well as effective replay protection.
However, such integrity protection is not always possible. For these cases, end systems SHOULD maintain a cache of certificates which other parties have previously presented using this mechanism. If possible, users SHOULD be notified when an unsecured certificate associated with a previously unknown end system is presented, and SHOULD be strongly warned if a different and unauthenticated certificate is presented by a party with which they have communicated in the past. In this way, even in the absence of integrity protection for SDP, the security of this document's mechanism is equivalent to that of the Secure Shell (ssh) protocol (Ylonen, T. and C. Lonvick, “SSH Protocol Architecture,” March 2005.), which is vulnerable to man-in-the-middle attacks when two parties first communicate, but can detect ones that occur subsequently. (Note that a precise definition of the "other party" depends on the application protocol carrying the SDP message.)
TLS is not always the most appropriate choice for secure connection-oriented media; in some cases, a higher- or lower-level security protocol may be appropriate.
This document does not define any mechanism for securely transporting RTP and RTCP packets over a connection-oriented channel. There was no consensus in the working group as to whether it would be better to send Secure RTP packets (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) over a connection-oriented transport (Lazzaro, J., “Framing RTP and RTCP Packets over Connection-Oriented Transport,” January 2005.), or whether it would be better to send standard unsecured RTP packets over TLS using the mechanisms described in this document. The group consensus was to wait until a use-case requiring secure connection-oriented RTP was presented.
This document defines an SDP proto value: TCP/TLS. Its format is defined in Section 4 (Protocol Identifiers). This proto value should be registered by IANA on http://www.iana.org/assignments/sdp-parameters under "proto".
This document defines an SDP session and media level attribute: fingerprint. Its format is defined in Section 5 (Fingerprint Attribute). This attribute should be registered by IANA on http://www.iana.org/assignments/sdp-parameters under "att-field (both session and media level)".
Specifications defining new proto values, like this one, must define the rules by which their media format (fmt) namespace is managed. For the TCP/TLS protocol, new formats SHOULD have an associated MIME registration. Use of an existing MIME subtype for the format is encouraged. If no MIME subtype exists, it is RECOMMENDED that a suitable one be registered through the IETF process (Freed, N., Klensin, J., and J. Postel, “Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures,” November 1996.) by production of, or reference to, a standards-track RFC that defines the transport protocol for the format.
None, other than IPR boilerplate and reference updates. Draft -03 was a resubmission to refresh the draft's presence in the Internet-Drafts repository.
|||Handley, M., “SDP: Session Description Protocol,” draft-ietf-mmusic-sdp-new-24 (work in progress), February 2005.|
|||Yon, D., “Connection-Oriented Media Transport in the Session Description Protocol (SDP),” draft-ietf-mmusic-sdp-comedia-10 (work in progress), November 2004.|
|||Dierks, T. and C. Allen, “The TLS Protocol Version 1.0,” RFC 2246, January 1999.|
|||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|||Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” RFC 3264, June 2002.|
|||International Telecommunications Union, “Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks,” ITU-T Recommendation X.509, ISO Standard 9594-8, March 2000.|
|||Bassham, L., Polk, W., and R. Housley, “Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” RFC 3279, April 2002.|
|||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.|
|||Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” RFC 2234, November 1997 (TXT, HTML, XML).|
|||National Institute of Standards and Technology, “Secure Hash Standard,” FIPS PUB 180-1, April 1995.|
|||Rivest, R., “The MD5 Message-Digest Algorithm,” RFC 1321, April 1992.|
|||Kaliski, B., “The MD2 Message-Digest Algorithm,” RFC 1319, April 1992.|
|||Freed, N., Klensin, J., and J. Postel, “Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures,” BCP 13, RFC 2048, November 1996 (TXT, HTML, XML).|
|||Handley, M., Perkins, C., and E. Whelan, “Session Announcement Protocol,” RFC 2974, October 2000.|
|||Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” RFC 3261, June 2002.|
|||Ramsdell, B., “S/MIME Version 3 Message Specification,” RFC 2633, June 1999.|
|||Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” RFC 2617, June 1999 (TXT, HTML, XML).|
|||Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” RFC 3174, September 2001.|
|||Rescorla, E., “HTTP Over TLS,” RFC 2818, May 2000.|
|||Ylonen, T. and C. Lonvick, “SSH Protocol Architecture,” draft-ietf-secsh-architecture-22 (work in progress), March 2005.|
|||Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” RFC 3711, March 2004.|
|||Lazzaro, J., “Framing RTP and RTCP Packets over Connection-Oriented Transport,” draft-ietf-avt-rtp-framing-contrans-05 (work in progress), January 2005.|
|||Andreasen, F., “Session Description Protocol Security Descriptions for Media Streams,” draft-ietf-mmusic-sdescriptions-11 (work in progress), June 2005.|
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