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rfc:rfc4572

Network Working Group J. Lennox Request for Comments: 4572 Columbia U. Updates: 4145 July 2006 Category: Standards Track

Connection-Oriented Media Transport over the Transport Layer Security

      (TLS) Protocol in the Session Description Protocol (SDP)

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.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 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 SDP protocol identifier, 'TCP/TLS'.  It also defines the syntax
 and semantics for an SDP 'fingerprint' attribute that identifies the
 certificate that 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.
 This document extends and updates RFC 4145.

Lennox Standards Track [Page 1] RFC 4572 Comedia over TLS in SDP July 2006

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................4
 3. Overview ........................................................4
    3.1. SDP Operational Modes ......................................4
    3.2. Threat Model ...............................................5
    3.3. The Need for Self-Signed Certificates ......................5
    3.4. Example SDP Description for TLS Connection .................6
 4. Protocol Identifiers ............................................6
 5. Fingerprint Attribute ...........................................7
 6. Endpoint Identification .........................................9
    6.1. Certificate Choice .........................................9
    6.2. Certificate Presentation ..................................10
 7. Security Considerations ........................................10
 8. IANA Considerations ............................................12
 9. References .....................................................14
    9.1. Normative References ......................................14
    9.2. Informative References ....................................15

Lennox Standards Track [Page 2] RFC 4572 Comedia over TLS in SDP July 2006

1. Introduction

 The Session Description Protocol (SDP) [1] 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 that uses a connection-
 oriented transport.  RFC 4145, Connection-Oriented Media Transport in
 the Session Description Protocol (SDP) [2], 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 [3].
 The TLS protocol allows applications to communicate over a channel
 that provides confidentiality 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 that 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 certification 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 certification 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 that 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 that 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.  Section 4 gives the
 basic mechanism for establishing TLS-based connected-oriented media

Lennox Standards Track [Page 3] RFC 4572 Comedia over TLS in SDP July 2006

 in SDP.  Section 5 describes the SDP fingerprint attribute, which,
 assuming that the integrity of SDP content is assured, allows the
 secure use of self-signed certificates.  Section 6 describes which
 X.509 certificates are presented, and how they are used in TLS.
 Section 7 discusses additional security considerations.

2. Terminology

 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 [4] and
 indicate requirement levels for compliant implementations.

3. Overview

 This section discusses the threat model that motivates TLS transport
 for connection-oriented media streams.  It also discusses in more
 detail the need for end systems to use self-signed certificates.

3.1. SDP Operational Modes

 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 of a multimedia session it is making available.
 The classic example of this mode of operation is the Session
 Announcement Protocol (SAP) [15], 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 [5].
 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)
 [16].

Lennox Standards Track [Page 4] RFC 4572 Comedia over TLS in SDP July 2006

3.2. Threat Model

 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 certification 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
 that 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 [17] can be used to
 secure session descriptions.

3.3. The Need for Self-Signed Certificates

 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 certification authority.

Lennox Standards Track [Page 5] RFC 4572 Comedia over TLS in SDP July 2006

 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.

3.4. Example SDP Description for TLS Connection

 Figure 1 illustrates an SDP offer that 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 RFC 4145 [2], 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 document 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

4. Protocol Identifiers

 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 [1] 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 [3] over TCP.  (Using TLS over other transport
 protocols is not discussed in 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

Lennox Standards Track [Page 6] RFC 4572 Comedia over TLS in SDP July 2006

 the protocol using a fmt identifier to indicate the application being
 run over TLS.
 Media sessions described with this identifier follow the procedures
 defined in RFC 4145 [2].  They also use the SDP attributes defined in
 that specification, 'setup' and 'connection'.

5. Fingerprint Attribute

 Parties to a TLS session indicate their identities by presenting
 authentication certificates as part of the TLS handshake procedure.
 Authentication certificates are X.509 [6] certificates, as profiled
 by RFC 3279 [7], RFC 3280 [8], and RFC 4055 [9].
 In order to associate media streams with connections and to prevent
 unauthorized barge-in attacks on the media streams, endpoints MUST
 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 that 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
 uppercase 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 [18], which uses unseparated lowercase 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 [10] in Figure 2.  This syntax extends the BNF
 syntax of SDP [1].

Lennox Standards Track [Page 7] RFC 4572 Comedia over TLS in SDP July 2006

 attribute              =/ fingerprint-attribute
 fingerprint-attribute  =  "fingerprint" ":" hash-func SP fingerprint
 hash-func              =  "sha-1" / "sha-224" / "sha-256" /
                           "sha-384" / "sha-512" /
                           "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 MUST be computed using the same one-way
 hash function as is used in the certificate's signature algorithm.
 (This ensures that the security properties required for the
 certificate also apply for the fingerprint.  It also guarantees that
 the fingerprint will be usable by the other endpoint, so long as the
 certificate itself is.)  Following RFC 3279 [7] as updated by RFC
 4055 [9], therefore, the defined hash functions are 'SHA-1' [11]
 [19], 'SHA-224' [11], 'SHA-256' [11], 'SHA-384' [11], 'SHA-512' [11],
 'MD5' [12], and 'MD2' [13], with 'SHA-1' preferred.  A new IANA
 registry of Hash Function Textual Names, specified in Section 8,
 allows for addition of future tokens, but they may only be added if
 they are included in RFCs that update or obsolete RFC 3279 [7].
 Self-signed certificates (for which legacy certificates are not a
 consideration) MUST use one of the FIPS 180 algorithms (SHA-1,
 SHA-224, SHA-256, SHA-384, or SHA-512) as 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.

Lennox Standards Track [Page 8] RFC 4572 Comedia over TLS in SDP July 2006

6. Endpoint Identification

6.1. Certificate Choice

 An X.509 certificate binds an identity and a public key.  If SDP
 describing a TLS session is transmitted over a mechanism that
 provides integrity protection, a certificate asserting any
 syntactically valid identity MAY be used.  For example, an SDP
 description sent over HTTP/TLS [20] or secured by S/MIME [17] MAY
 assert any identity in the certificate securing the media connection.
 Security protocols that provide only hop-by-hop integrity protection
 (e.g., the sips protocol [16], SIP over TLS) are considered
 sufficiently secure to allow the mode in which any valid identity is
 accepted.  However, see Section 7 for a discussion of some security
 implications of this fact.
 In situations where the SDP is not integrity-protected, however, the
 certificate provided for a TLS connection MUST certify an appropriate
 identity for the connection.  In these scenarios, the certificate
 presented by an endpoint MUST certify either the SDP connection
 address, or the identity of the creator of the SDP message, as
 follows:
 o  If the connection address for the media description is specified
    as an IP address, the endpoint MAY use a certificate with an
    iPAddress subjectAltName that exactly matches the IP in the
    connection-address in the session description's 'c' line.
    Similarly, if the connection address for the media description is
    specified as a fully-qualified domain name, the endpoint MAY use a
    certificate with a dNSName subjectAltName matching the specified
    'c' line connection-address exactly.  (Wildcard patterns MUST NOT
    be used.)
 o  Alternately, if the SDP session description of the session was
    transmitted over a protocol (such as SIP [16]) for which the
    identities of session participants are defined by uniform resource
    identifiers (URIs), the endpoint MAY use a certificate with a
    uniformResourceIdentifier subjectAltName corresponding to the
    identity of the endpoint that generated the SDP.  The details of
    what URIs are valid are dependent on the transmitting protocol.
    (For more details on the validity of URIs, see Section 7.)
 Identity matching is performed using the matching rules specified by
 RFC 3280 [8].  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.  To support the use of
 certificate caches, as described in Section 7, endpoints SHOULD

Lennox Standards Track [Page 9] RFC 4572 Comedia over TLS in SDP July 2006

 consistently provide the same certificate for each identity they
 support.

6.2. Certificate Presentation

 In all cases, an endpoint acting as the TLS server (i.e., one taking
 the '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.  If the certificate does not
 match the original fingerprint, the client endpoint MUST terminate
 the media connection with a bad_certificate error.
 If the SDP offer/answer model [5] is being used, the client (the
 endpoint with the 'setup:active' role) MUST also present a
 certificate following the rules of Section 6.1.  The server MUST
 request a certificate, and if the client does not provide one, or if
 the certificate does not match the provided fingerprint, the server
 endpoint MUST 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 RFC 4145 [2]) begin listening for an
 incoming connection as soon as it sends its offer.  However, it MUST
 NOT assume that the data transmitted over the TLS connection is valid
 until it has received a matching fingerprint in an SDP answer.  If
 the fingerprint, once it arrives, does not match the client's
 certificate, the server endpoint MUST terminate the media connection
 with a bad_certificate error, as stated in the previous paragraph.
 If offer/answer is not being used (e.g., if the SDP was sent over the
 Session Announcement Protocol [15]), there is no secure channel
 available for clients to communicate certificate fingerprints to
 servers.  In this case, servers MAY request client certificates,
 which SHOULD be signed by a well-known certification authority, or
 MAY allow clients to connect without a certificate.

7. Security Considerations

 This entire document concerns itself with security.  The problem to
 be solved is addressed in Section 1, and a high-level overview is
 presented in Section 3.  See the SDP specification [1] for security
 considerations applicable to SDP in general.
 Offering a TCP/TLS connection in SDP (or agreeing to one in SDP
 offer/answer mode) does not create an obligation for an endpoint to
 accept any TLS connection with the given fingerprint.  Instead, the

Lennox Standards Track [Page 10] RFC 4572 Comedia over TLS in SDP July 2006

 endpoint must engage in the standard TLS negotiation procedure to
 ensure that the TLS stream cipher and MAC algorithm chosen meet the
 security needs of the higher-level application.  (For example, an
 offered stream cipher of TLS_NULL_WITH_NULL_NULL SHOULD be rejected
 in almost every application scenario.)
 Like all SDP messages, SDP messages describing TLS streams are
 conveyed in an encapsulating application protocol (e.g., SIP, Media
 Gateway Control Protocol (MGCP), etc.).  It is the responsibility of
 the encapsulating protocol to ensure the integrity 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 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 that 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 unsecured 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 [21], 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.)  Users
 SHOULD NOT, however, in any circumstances be notified about
 certificates described in SDP descriptions sent over an integrity-
 protected channel.
 To aid interoperability and deployment, security protocols that
 provide only hop-by-hop integrity protection (e.g., the sips protocol
 [16], SIP over TLS) are considered sufficiently secure to allow the
 mode in which any syntactically valid identity is accepted in a
 certificate.  This decision was made because sips is currently the
 integrity mechanism most likely to be used in deployed networks in
 the short to medium term.  However, in this mode, SDP integrity is
 vulnerable to attacks by compromised or malicious middleboxes, e.g.,
 SIP proxy servers.  End systems MAY warn users about SDP sessions
 that are secured in only a hop-by-hop manner, and definitions of
 media formats running over TCP/TLS MAY specify that only end-to-end
 integrity mechanisms be used.

Lennox Standards Track [Page 11] RFC 4572 Comedia over TLS in SDP July 2006

 Depending on how SDP messages are transmitted, it is not always
 possible to determine whether or not a subjectAltName presented in a
 remote certificate is expected for the remote party.  In particular,
 given call forwarding, third-party call control, or session
 descriptions generated by endpoints controlled by the Gateway Control
 Protocol [22], it is not always possible in SIP to determine what
 entity ought to have generated a remote SDP response.  In general,
 when not using authenticity and integrity protection of SDP
 descriptions, a certificate transmitted over SIP SHOULD assert the
 endpoint's SIP Address of Record as a uniformResourceIndicator
 subjectAltName.  When an endpoint receives a certificate over SIP
 asserting an identity (including an iPAddress or dNSName identity)
 other than the one to which it placed or received the call, it SHOULD
 alert the user and ask for confirmation.  This applies whether
 certificates are self-signed, or signed by certification authorities;
 a certificate for sip:bob@example.com may be legitimately signed by a
 certification authority, but may still not be acceptable for a call
 to sip:alice@example.com.  (This issue is not one specific to this
 specification; the same consideration applies for S/MIME-signed SDP
 carried over SIP.)
 This document does not define any mechanism for securely transporting
 RTP and RTP Control Protocol (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
 [23] over a connection-oriented transport [24], 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.
 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.

8. IANA Considerations

 This document defines an SDP proto value: 'TCP/TLS'.  Its format is
 defined in Section 4.  This proto value has been registered by IANA
 under "Session Description Protocol (SDP) Parameters" under "proto".
 This document defines an SDP session and media-level attribute:
 'fingerprint'.  Its format is defined in Section 5.  This attribute
 has been registered by IANA under "Session Description Protocol (SDP)
 Parameters" under "att-field (both session and media level)".
 The SDP specification [1] states that specifications defining new
 proto values, like the 'TCP/TLS' proto value defined in this one,

Lennox Standards Track [Page 12] RFC 4572 Comedia over TLS in SDP July 2006

 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 [14] by production of, or reference to, a standards-track RFC
 that defines the transport protocol for the format.
 This specification creates a new IANA registry named "Hash Function
 Textual Names".  It will not be part of the SDP Parameters.
 The names of hash functions used for certificate fingerprints are
 registered by the IANA.  Hash functions MUST be defined by standards-
 track RFCs that update or obsolete RFC 3279 [7].
 When registering a new hash function textual name, the following
 information MUST be provided:
 o  The textual name of the hash function.
 o  The Object Identifier (OID) of the hash function as used in X.509
    certificates.
 o  A reference to the standards-track RFC, updating or obsoleting RFC
    3279 [7], defining the use of the hash function in X.509
    certificates.
 Figure 3 contains the initial values of this registry.
 Hash Function Name     OID                         Reference
 ------------------     ---                         ---------
 "md2"                  1.2.840.113549.2.2          RFC 3279
 "md5"                  1.2.840.113549.2.5          RFC 3279
 "sha-1"                1.3.14.3.2.26               RFC 3279
 "sha-224"              2.16.840.1.101.3.4.2.4      RFC 4055
 "sha-256"              2.16.840.1.101.3.4.2.1      RFC 4055
 "sha-384"              2.16.840.1.101.3.4.2.2      RFC 4055
 "sha-512"              2.16.840.1.101.3.4.2.3      RFC 4055
          Figure 3: IANA Hash Function Textual Name Registry

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9. References

9.1. Normative References

 [1]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
       Description Protocol", RFC 4566, July 2006.
 [2]   Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
       Session Description Protocol (SDP)", RFC 4145, September 2005.
 [3]   Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
       Protocol Version 1.1", RFC 4346, April 2006.
 [4]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [5]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
       Session Description Protocol (SDP)", RFC 3264, June 2002.
 [6]   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.
 [7]   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.
 [8]   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.
 [9]   Schaad, J., Kaliski, B., and R. Housley, "Additional Algorithms
       and Identifiers for RSA Cryptography for use in the Internet
       X.509 Public Key Infrastructure Certificate and Certificate
       Revocation List (CRL) Profile", RFC 4055, June 2005.
 [10]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 4234, October 2005.
 [11]  National Institute of Standards and Technology, "Secure Hash
       Standard", FIPS PUB 180-2, August 2002, <http://csrc.nist.gov/
       publications/fips/fips180-2/fips180-2.pdf>.
 [12]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
       April 1992.

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 [13]  Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319,
       April 1992.
 [14]  Freed, N. and J. Klensin, "Media Type Specifications and
       Registration Procedures", BCP 13, RFC 4288, December 2005.

9.2. Informative References

 [15]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
       Protocol", RFC 2974, October 2000.
 [16]  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.
 [17]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
       (S/MIME) Version 3.1 Message Specification", RFC 3851, July
       2004.
 [18]  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.
 [19]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
       RFC 3174, September 2001.
 [20]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [21]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol
       Architecture", RFC 4251, January 2006.
 [22]  Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, "Gateway
       Control Protocol Version 1", RFC 3525, June 2003.
 [23]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
       Norrman, "The Secure Real-time Transport Protocol (SRTP)",
       RFC 3711, March 2004.
 [24]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and
       RTP Control Protocol (RTCP) Packets over Connection-Oriented
       Transport", RFC 4571, July 2006.

Lennox Standards Track [Page 15] RFC 4572 Comedia over TLS in SDP July 2006

Author's Address

 Jonathan Lennox
 Columbia University Department of Computer Science
 450 Computer Science
 1214 Amsterdam Ave., M.C. 0401
 New York, NY  10027
 US
 EMail: lennox@cs.columbia.edu

Lennox Standards Track [Page 16] RFC 4572 Comedia over TLS in SDP July 2006

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Lennox Standards Track [Page 17]

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