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Versions: 00 01 02 03 04 05 06 RFC 4572

Multiparty Multimedia Session                                  J. Lennox
Control                                                      Columbia U.
Internet-Draft                                                 July 2005
Expires: January 2, 2006

 Connection-Oriented Media Transport over the Transport Layer Security
        (TLS) Protocol in the Session Description Protocol (SDP)

Status of this Memo

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Copyright Notice

   Copyright (C) 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 SDP 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

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   established securely, so long as the integrity of session
   descriptions is assured.

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  . . . . . . . . . . . . . . . . . . .  8
     6.1.  Certificate Choice . . . . . . . . . . . . . . . . . . . .  8
     6.2.  Certificate Presentation . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   Appendix A.   Changes From Earlier Versions  . . . . . . . . . . . 12
   Appendix A.1. Changes From Draft -04 . . . . . . . . . . . . . . . 12
   Appendix A.2. Changes From Draft -03 . . . . . . . . . . . . . . . 12
   Appendix A.3. Changes From Draft -02 . . . . . . . . . . . . . . . 12
   Appendix A.4. Changes From Draft -01 . . . . . . . . . . . . . . . 13
   Appendix A.5. Changes From Draft -00 . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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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 which uses a connection-
   oriented transport.  The document 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
   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.  Section 4 gives the
   basic mechanism for establishing TLS-based connected-oriented media
   in SDP.  Section 5 describes the SDP fingerprint attribute, which,

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   assuming 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",
   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 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.

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 describing 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)

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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 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

   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 [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 certificate authority.

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   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 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 [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 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
   a=fingerprint:SHA-1 \

   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 by this document.)  The 'TCP/TLS' protocol
   identifier describes only the transport protocol, not the upper-layer

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   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.

   Media sessions described with this identifier follow the procedures
   defined in the connection-oriented media specification [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 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

   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 [18], 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 [10] in Figure 2.  This syntax extends the BNF
   syntax of SDP [1].

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   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 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.  Additional hash functions can be defined only by
   standards-track RFCs which 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

6.  Endpoint Identification

6.1.  Certificate Choice

   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 [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.

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   The certificate presented by an endpoint MUST correspond to one of
   the following identities:

   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 which exactly matches the IP in the
      connection-address in the session description's 'c' line.
   o  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  If the SDP session description describing 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.  The details of what
      URIs are appropriate are dependent on the transmitting protocol.
      (For more details on this, see Section 7.)

   If the SDP session description describing the session was transmitted
   over an end-to-end secure protocol which uses X.509 certificates, and
   the certificates sent fulfill the requirements above (as they
   normally would be expected to), the endpoint MAY use the same
   certificate to certify the media connection.  For example, an SDP
   description sent over HTTP/TLS [20] or secured by S/MIME [17] MAY use
   the same certificate to secure the media connection.  (Note, however,
   that the sips protocol [16] (SIP over TLS) provides only hop-by-hop
   security, so its TLS certificates do not satisfy this criterion.)  To
   support the use of certificate caches, as described in Section 7,
   endpoints SHOULD 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, 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 [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, if the

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   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 [2]) 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 MUST NOT definitively accept 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 [15]), 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.

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.

   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

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   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.)

   Depending on how SDP messages are transmitted, it is not always
   possible to determine whether a uniformResourceIdentifier
   subjectAltName presented in a remote certificate is expected or not
   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 some cases this determination may need to be
   made by a human, as automated logic may not be able to determine
   correctness.  (For example, "You placed this call to
   sip:alice@example.com, but the remote certificate presented belongs
   to sip:bob@example.com.  Continue?")  This issue is not one specific
   to this specification; the same consideration applies for S/MIME-
   signed SDP carried over SIP.

   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 [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.

8.  IANA Considerations

   This document defines an SDP proto value: 'TCP/TLS'.  Its format is
   defined in Section 4.  This proto value should be registered by IANA
   on under "Session Description Protocol (SDP) Parameters" under

   This document defines an SDP session and media level attribute:
   'fingerprint'.  Its format is defined in Section 5.  This attribute
   should be registered by IANA under "Session Description Protocol
   (SDP) Parameters" under "att-field (both session and media level)".

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   The SDP specification, RFC2327, states that specifications defining
   new proto values, like the 'TCP/TLS' proto value defined in 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 [14] by production of, or reference to, a standards-track RFC
   that defines the transport protocol for the format.

Appendix A.  Changes From Earlier Versions

   Note to the RFC-Editor: please remove this section prior to
   publication as an RFC.

Appendix A.1.  Changes From Draft -04

   The section discussing the difficulty of knowing what URI identities
   are appropriate for SDP was expanded, adding a reference to the
   Gateway Control Protocol.

   An un-cited informative reference was removed.

Appendix A.2.  Changes From Draft -03

   The number of options in the protocol were significantly reduced: a
   number of SHOULD requirements were elevated to MUST.  Notably, the
   use of the 'fingerprint' attribute, strict certificate identity
   choices, and the use of the same digest algorithm for fingerprints as
   for certificates were all made mandatory.

   Support for the digest algorithms from FIPS 180-2 [11] / RFC 4055 [9]
   ('SHA-224', 'SHA-256', 'SHA-384', and 'SHA-512') was added.

   Discussion was added about the difficulty of automatically
   determining the URI a remote endpoint's certificate should assert,
   especially in SIP in the presence of call forwarding or third-party
   call control.

   The document was aligned with version -10 of
   draft-ietf-mmusic-comedia [2].  This consisted mostly of wording and
   formatting changes.

Appendix A.3.  Changes From Draft -02

   None, other than IPR boilerplate and reference updates.  Draft -03
   was a resubmission to refresh the draft's presence in the Internet-

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   Drafts repository.

Appendix A.4.  Changes From Draft -01

   o  Made the use of SHA-1 fingerprints mandatory in self-signed
   o  Aligned with version -09 of draft-ietf-mmusic-comedia [2], also
      drawing some wording changes from that document.
   o  Forbid the use of wildcards for the dNS subjectAltName.
   o  Eliminated requirements on identities provided with self-signed
   o  Recommended the use of a certificate cache when SDP integrity
      protection cannot be assured.
   o  Explained that there is no currently supported mechanism for
      securely sending RTP over connection-oriented media.
   o  Described the procedure for establishing media formats for TCP/

Appendix A.5.  Changes From Draft -00

   o  Significantly expanded introduction and motivation sections.
   o  Significant clarifications to other sections.
   o  Aligned with version -07 of draft-ietf-mmusic-comedia [2].
      Protocol identifier changed from TLS to TCP/TLS at that document's

9.  References

9.1.  Normative References

   [1]   Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-25 (work in progress), July 2005.

   [2]   Yon, D., "Connection-Oriented Media Transport in the Session
         Description Protocol  (SDP)", draft-ietf-mmusic-sdp-comedia-10
         (work in progress), November 2004.

   [3]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
         RFC 2246, January 1999.

   [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

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         - 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., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [11]  National Institute of Standards and Technology, "Secure Hash
         Standard", FIPS PUB 180-2, August 2002, <http://csrc.nist.gov/

   [12]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
         April 1992.

   [13]  Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319,
         April 1992.

   [14]  Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
         Mail Extensions (MIME) Part Four: Registration Procedures",
         BCP 13, RFC 2048, November 1996.

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., "S/MIME Version 3 Message Specification",
         RFC 2633, June 1999.

   [18]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,

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         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, "SSH Protocol Architecture",
         draft-ietf-secsh-architecture-22 (work in progress),
         March 2005.

   [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 RTP and RTCP Packets over Connection-
         Oriented Transport", draft-ietf-avt-rtp-framing-contrans-06
         (work in progress), September 2005.

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Author's Address

   Jonathan Lennox
   Columbia University Department of Computer Science
   450 Computer Science
   1214 Amsterdam Ave., M.C. 0401
   New York, NY  10027

   Phone: +1 212 939 7018
   Email: lennox@cs.columbia.edu

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