draft-ietf-mmusic-comedia-tls-03.txt   draft-ietf-mmusic-comedia-tls-04.txt 
Multiparty Multimedia Session J. Lennox Multiparty Multimedia Session J. Lennox
Control Columbia U. Control Columbia U.
Expires: December 18, 2005 Expires: January 7, 2006
Connection-Oriented Media Transport over the Transport Layer Security Connection-Oriented Media Transport over the Transport Layer Security
(TLS) Protocol in the Session Description Protocol (SDP) (TLS) Protocol in the Session Description Protocol (SDP)
draft-ietf-mmusic-comedia-tls-03 draft-ietf-mmusic-comedia-tls-04
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
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have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 18, 2005. This Internet-Draft will expire on January 7, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document specifies how to establish secure connection-oriented This document specifies how to establish secure connection-oriented
media transport sessions over the Transport Layer Security (TLS) media transport sessions over the Transport Layer Security (TLS)
protocol using the Session Description Protocol (SDP). It defines a protocol using the Session Description Protocol (SDP). It defines a
new protocol identifier, TCP/TLS. It also defines the syntax and new SDP protocol identifier, 'TCP/TLS'. It also defines the syntax
semantics for an SDP "fingerprint" attribute that identifies the and semantics for an SDP 'fingerprint' attribute that identifies the
certificate which will be presented for the TLS session. This certificate which will be presented for the TLS session. This
mechanism allows media transport over TLS connections to be mechanism allows media transport over TLS connections to be
established securely, so long as the integrity of session established securely, so long as the integrity of session
descriptions is assured. descriptions is assured.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
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3.2 Threat Model . . . . . . . . . . . . . . . . . . . . . . . 4 3.2 Threat Model . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 The Need For Self-Signed Certificates . . . . . . . . . . 5 3.3 The Need For Self-Signed Certificates . . . . . . . . . . 5
3.4 Example SDP Description For TLS Connection . . . . . . . . 6 3.4 Example SDP Description For TLS Connection . . . . . . . . 6
4. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 6 4. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 6
5. Fingerprint Attribute . . . . . . . . . . . . . . . . . . . . 7 5. Fingerprint Attribute . . . . . . . . . . . . . . . . . . . . 7
6. Endpoint Identification . . . . . . . . . . . . . . . . . . . 8 6. Endpoint Identification . . . . . . . . . . . . . . . . . . . 8
6.1 Certificate Choice . . . . . . . . . . . . . . . . . . . . 8 6.1 Certificate Choice . . . . . . . . . . . . . . . . . . . . 8
6.2 Certificate Presentation . . . . . . . . . . . . . . . . . 9 6.2 Certificate Presentation . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
A. Changes From Earlier Versions . . . . . . . . . . . . . . . . 11 A. Changes From Earlier Versions . . . . . . . . . . . . . . . . 12
A.1 Changes From Draft -02 . . . . . . . . . . . . . . . . . . 11 A.1 Changes From Draft -03 . . . . . . . . . . . . . . . . . . 12
A.2 Changes From Draft -01 . . . . . . . . . . . . . . . . . . 11 A.2 Changes From Draft -02 . . . . . . . . . . . . . . . . . . 12
A.3 Changes From Draft -00 . . . . . . . . . . . . . . . . . . 12 A.3 Changes From Draft -01 . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 A.4 Changes From Draft -00 . . . . . . . . . . . . . . . . . . 13
9.1 Normative References . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.2 Informative References . . . . . . . . . . . . . . . . . . 13 9.1 Normative References . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14 9.2 Informative References . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 16
1. Introduction 1. Introduction
The Session Description Protocol (SDP) [1] provides a general purpose The Session Description Protocol (SDP) [1] provides a general purpose
format for describing multimedia sessions in announcements or format for describing multimedia sessions in announcements or
invitations. For many applications, it is desirable to establish, as invitations. For many applications, it is desirable to establish, as
part of a multimedia session, a media stream which uses a connection- part of a multimedia session, a media stream which uses a connection-
oriented transport. The document Connection-Oriented Media Transport oriented transport. The document Connection-Oriented Media Transport
in the Session Description Protocol (SDP) [2] specifies a general in the Session Description Protocol (SDP) [2] specifies a general
mechanism for describing and establishing such connection-oriented mechanism for describing and establishing such connection-oriented
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for connection-oriented media streams. It also discusses in more for connection-oriented media streams. It also discusses in more
detail the need for end systems to use self-signed certificates. detail the need for end systems to use self-signed certificates.
3.1 SDP Operational Modes 3.1 SDP Operational Modes
There are two principal operational modes for multimedia sessions: There are two principal operational modes for multimedia sessions:
advertised and offer-answer. Advertised sessions are the simpler advertised and offer-answer. Advertised sessions are the simpler
mode. In this mode, a server publishes, in some manner, an SDP mode. In this mode, a server publishes, in some manner, an SDP
session description describing a multimedia session it is making session description describing a multimedia session it is making
available. The classic example of this mode of operation is the available. The classic example of this mode of operation is the
Session Announcment Protocol (SAP) [14], in which SDP session Session Announcement Protocol (SAP) [15], in which SDP session
descriptions are periodically transmitted to a well-known multicast descriptions are periodically transmitted to a well-known multicast
group. Traditionally, these descriptions involve multicast group. Traditionally, these descriptions involve multicast
conferences, but unicast sessions are also possible. (Connection- conferences, but unicast sessions are also possible. (Connection-
oriented media, obviously, cannot use multicast.) Recipients of a oriented media, obviously, cannot use multicast.) Recipients of a
session description connect to the addresses published in the session session description connect to the addresses published in the session
description. These recipients may not previously have been known to description. These recipients may not previously have been known to
the advertiser of the session description. the advertiser of the session description.
Alternatively, SDP conferences can operate in offer-answer mode [5]. Alternatively, SDP conferences can operate in offer-answer mode [5].
This mode allows two participants in a multimedia session to This mode allows two participants in a multimedia session to
negotiate the multimedia session between them. In this model, one negotiate the multimedia session between them. In this model, one
participant offers the other a description of the desired session participant offers the other a description of the desired session
from its perspective, and the other participant answers with the from its perspective, and the other participant answers with the
desired session from its own perspective. In this mode, each of 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 participants in the session has knowledge of the other one. This is
the mode of operation used by the Session Initiation Protocol (SIP) the mode of operation used by the Session Initiation Protocol (SIP)
[15]. [16].
3.2 Threat Model 3.2 Threat Model
Participants in multimedia conferences often wish to guarantee Participants in multimedia conferences often wish to guarantee
confidentiality, data integrity, and authentication for their media confidentiality, data integrity, and authentication for their media
sessions. This section describes various types of attackers and the sessions. This section describes various types of attackers and the
ways they attempt to violate these guarantees. It then describes how ways they attempt to violate these guarantees. It then describes how
the TLS protocol can be used to thwart the attackers. the TLS protocol can be used to thwart the attackers.
The simplest type of attacker is one who listens passively to the The simplest type of attacker is one who listens passively to the
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(most commonly, a mutually-trusted certificate authority) to validate (most commonly, a mutually-trusted certificate authority) to validate
certificates, and the endpoints know what certificate identity to certificates, and the endpoints know what certificate identity to
expect, endpoints can be certain that such an attack has not taken expect, endpoints can be certain that such an attack has not taken
place. place.
Finally, the most serious type of attacker is one who can modify or Finally, the most serious type of attacker is one who can modify or
redirect session descriptions: for example, a compromised or redirect session descriptions: for example, a compromised or
malicious SIP proxy server. Neither TLS itself, nor any mechanisms malicious SIP proxy server. Neither TLS itself, nor any mechanisms
which use it, can protect an SDP session against such an attacker. which use it, can protect an SDP session against such an attacker.
Instead, the SDP description itself must be secured through some Instead, the SDP description itself must be secured through some
mechanism; SIP, for example, defines how S/MIME [16] can be used to mechanism; SIP, for example, defines how S/MIME [17] can be used to
secure session descriptions. secure session descriptions.
3.3 The Need For Self-Signed Certificates 3.3 The Need For Self-Signed Certificates
SDP session descriptions are created by any endpoint that needs to SDP session descriptions are created by any endpoint that needs to
participate in a multimedia session. In many cases, such as SIP participate in a multimedia session. In many cases, such as SIP
phones, such endpoints have dynamically-configured IP addresses and phones, such endpoints have dynamically-configured IP addresses and
host names, and must be deployed with nearly zero configuration. For host names, and must be deployed with nearly zero configuration. For
such an endpoint, it is for practical purposes impossible to obtain a such an endpoint, it is for practical purposes impossible to obtain a
certificate signed by a well-known certificate authority. certificate signed by a well-known certificate authority.
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prior relationship: certificates can be securely described in the prior relationship: certificates can be securely described in the
session description itself. This is done by providing a secure hash session description itself. This is done by providing a secure hash
of a certificate, or "certificate fingerprint", as an SDP attribute; of a certificate, or "certificate fingerprint", as an SDP attribute;
this mechanism is described in Section 5. this mechanism is described in Section 5.
3.4 Example SDP Description For TLS Connection 3.4 Example SDP Description For TLS Connection
Figure 1 illustrates an SDP offer which signals the availability of a 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 T.38 fax session over TLS. For the purpose of brevity, the main
portion of the session description is omitted in the example, showing portion of the session description is omitted in the example, showing
only the m= line and its attributes. (This example is the same as 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 the first one in [2], except for the proto parameter and the
fingerprint attribute.) See the subsequent sections for explanations fingerprint attribute.) See the subsequent sections for explanations
of the example's TLS-specific attributes. of the example's TLS-specific attributes.
(Note: due to RFC formatting conventions, this draft splits SDP (Note: due to RFC formatting conventions, this draft splits SDP
across lines whose content would exceed 72 characters. A backslash across lines whose content would exceed 72 characters. A backslash
character marks where this line folding has taken place. This character marks where this line folding has taken place. This
backslash and its trailing CRLF and whitespace would not appear in backslash and its trailing CRLF and whitespace would not appear in
actual SDP content.) actual SDP content.)
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c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
a=setup:passive a=setup:passive
a=connection:new a=connection:new
a=fingerprint:SHA-1 \ a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 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 Figure 1: Example SDP Description Offering a TLS Media Stream
4. Protocol Identifiers 4. Protocol Identifiers
The m= line in SDP specifies, among other items, the transport The 'm' line in SDP specifies, among other items, the transport
protocol to be used for the media in the session. See the "Media protocol to be used for the media in the session. See the "Media
Descriptions" section of SDP [1] for a discussion on transport Descriptions" section of SDP [1] for a discussion on transport
protocol identifiers. protocol identifiers.
This specification defines a new protocol identifier, TCP/TLS, which This specification defines a new protocol identifier, 'TCP/TLS',
indicates that the media described will use the Transport Layer which indicates that the media described will use the Transport Layer
Security protocol [3] over TCP. (Using TLS over other transport Security protocol [3] over TCP. (Using TLS over other transport
protocols is not discussed by this document.) The TCP/TLS protocol protocols is not discussed by this document.) The 'TCP/TLS' protocol
identifier describes only the transport protocol, not the upper-layer identifier describes only the transport protocol, not the upper-layer
protocol. An m= line that specifies TCP/TLS MUST further qualify the protocol. An 'm' line that specifies 'TCP/TLS' MUST further qualify
protocol using a fmt identifier, to indicate the application being the protocol using a fmt identifier, to indicate the application
run over TLS. being run over TLS.
As TLS sessions are connection-oriented, media sessions described in Media sessions described with this identifier follow the procedures
this manner follow the procedures defined in the connection-oriented defined in the connection-oriented media specification [2]. They
media specification [2]. They also use the attributes defined in also use the SDP attributes defined in that specification, 'setup'
that specification, "a=setup" and "a=connection". and 'connection'.
5. Fingerprint Attribute 5. Fingerprint Attribute
Parties to a TLS session indicate their identities by presenting Parties to a TLS session indicate their identities by presenting
authentication certificates as part of the TLS handshake procedure. authentication certificates as part of the TLS handshake procedure.
Authentication certificates are X.509 [6] certificates, as profiled Authentication certificates are X.509 [6] certificates, as profiled
by RFC 3279 [7] and RFC 3280 [8]. by RFC 3279 [7], RFC 3280 [8] and RFC 4055 [9].
In order to associate media streams with connections, and to prevent In order to associate media streams with connections, and to prevent
unauthorized barge-in attacks on the media streams, endpoints MAY unauthorized barge-in attacks on the media streams, endpoints MUST
provide a certificate fingerprint. If the X.509 certificate provide a certificate fingerprint. If the X.509 certificate
presented for the TLS connection matches the fingerprint presented in presented for the TLS connection matches the fingerprint presented in
the SDP, the endpoint can be confident that the author of the SDP is the SDP, the endpoint can be confident that the author of the SDP is
indeed the initiator of the connection. indeed the initiator of the connection.
A certificate fingerprint is a secure one-way hash of the DER A certificate fingerprint is a secure one-way hash of the DER
(distinguished encoding rules) form of the certificate. (Certificate (distinguished encoding rules) form of the certificate. (Certificate
fingerprints are widely supported by tools which manipulate X.509 fingerprints are widely supported by tools which manipulate X.509
certificates; for instance, the command "openssl x509 -fingerprint" certificates; for instance, the command "openssl x509 -fingerprint"
causes the command-line tool of the openssl package to print a causes the command-line tool of the openssl package to print a
certificate fingerprint, and the certificate managers for Mozilla and certificate fingerprint, and the certificate managers for Mozilla and
Internet Explorer display them when viewing the details of a Internet Explorer display them when viewing the details of a
certificate.) certificate.)
A fingerprint is represented in SDP as an attribute (an "a=" line). 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 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 hash value itself. The hash value is represented as a sequence of
upper-case hexadecimal bytes, separated by colons. The number of upper-case hexadecimal bytes, separated by colons. The number of
bytes is defined by the hash function. (This is the syntax used by bytes is defined by the hash function. (This is the syntax used by
openssl and by the browsers' certificate managers. It is different openssl and by the browsers' certificate managers. It is different
from the syntax used to represent hash values in, e.g., HTTP digest from the syntax used to represent hash values in, e.g., HTTP digest
authentication [17], which uses unseparated lower-case hexadecimal authentication [18], which uses unseparated lower-case hexadecimal
bytes. It was felt that consistency with other applications of bytes. It was felt that consistency with other applications of
fingerprints was more important.) fingerprints was more important.)
The formal syntax of the fingerprint attribute is given in Augmented The formal syntax of the fingerprint attribute is given in Augmented
Backus-Naur Form [9] in Figure 2. This syntax extends the BNF syntax Backus-Naur Form [10] in Figure 2. This syntax extends the BNF
of SDP [1]. syntax of SDP [1].
attribute =/ fingerprint-attribute attribute =/ fingerprint-attribute
fingerprint-attribute = "fingerprint" ":" hash-func SP fingerprint fingerprint-attribute = "fingerprint" ":" hash-func SP fingerprint
hash-func = "sha-1" / "md5" / "md2" / token hash-func = "sha-1" / "sha-224" / "sha-256" /
"sha-384" / "sha-512" /
"md5" / "md2" / token
; Additional hash functions can only come ; Additional hash functions can only come
; from updates to RFC 3279 ; from updates to RFC 3279
fingerprint = 2UHEX *(":" 2UHEX) fingerprint = 2UHEX *(":" 2UHEX)
; Each byte in upper-case hex, separated ; Each byte in upper-case hex, separated
; by colons. ; by colons.
UHEX = DIGIT / %x41-46 ; A-F uppercase UHEX = DIGIT / %x41-46 ; A-F uppercase
Figure 2: Augmented Backus-Naur Syntax for the Fingerprint Attribute Figure 2: Augmented Backus-Naur Syntax for the Fingerprint Attribute
A certificate fingerprint SHOULD be computed using the same one-way A certificate fingerprint MUST be computed using the same one-way
hash function as is used in the certificate's signature algorithm. hash function as is used in the certificate's signature algorithm.
(This guarantees that the fingerprint will be usable by the other (This guarantees that the fingerprint will be usable by the other
endpoint, so long as the certificate itself is.) Following RFC 3279 endpoint, so long as the certificate itself is.) Following RFC 3279
[7], therefore, the defined hash functions are SHA-1 [10] [18], MD5 [7] as updated by RFC 4055 [9], therefore, the defined hash functions
[11], and MD2 [12], with SHA-1 preferred. Additional hash functions are 'SHA-1' [11] [19], 'SHA-224' [11], 'SHA-256' [11], 'SHA-384'
can be defined only by standards-track RFCs which update or obsolete [11], 'SHA-512' [11], 'MD5' [12], and 'MD2' [13], with 'SHA-1'
RFC 3279 [7]. Self-signed certificates (for which legacy preferred. Additional hash functions can be defined only by
certificates are not a consideration) MUST use SHA-1 in their standards-track RFCs which update or obsolete RFC 3279 [7]. Self-
signature algorithm, and thus also MUST use it to calculate signed certificates (for which legacy certificates are not a
certificate fingerprints. 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- The fingerprint attribute may be either a session-level or a media-
level SDP attribute. If it is a session-level attribute, it applies level SDP attribute. If it is a session-level attribute, it applies
to all TLS sessions for which no media-level fingerprint attribute is to all TLS sessions for which no media-level fingerprint attribute is
defined. defined.
6. Endpoint Identification 6. Endpoint Identification
6.1 Certificate Choice 6.1 Certificate Choice
X.509 certificates certify identities. The certificate provided for X.509 certificates certify identities. The certificate provided for
a TLS connection needs to certify an appropriate identity for the a TLS connection needs to certify an appropriate identity for the
connection. Identity matching is performed using the matching rules connection. Identity matching is performed using the matching rules
specified by RFC 3280 [8]. If more than one identity of a given type 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 is present in the certificate (e.g., more than one dNSName name), a
match in any one of the set is considered acceptable. match in any one of the set is considered acceptable.
If an endpoint does not provide a certificate fingerprint in its SDP, The certificate presented by an endpoint MUST correspond to one of
its certificate MUST correspond to one of the following identities, the following identities:
and MUST be signed by a certificate authority known to the other
endpoint.
o If the connection address for the media description is specified o If the connection address for the media description is specified
as an IP address, the endpoint MAY use a certificate with an as an IP address, the endpoint MAY use a certificate with an
iPAddress subjectAltName which exactly matches the IP in the iPAddress subjectAltName which exactly matches the IP in the
connection-address in the session description's c= line. connection-address in the session description's 'c' line.
o If the connection address for the media description is specified o If the connection address for the media description is specified
as a fully-qualified domain name, the endpoint MAY use a as a fully-qualified domain name, the endpoint MAY use a
certificate with a dNSName subjectAltName matching the specified certificate with a dNSName subjectAltName matching the specified
c= line connection-address exactly. (Wildcard patterns MUST NOT 'c' line connection-address exactly. (Wildcard patterns MUST NOT
be used.) be used.)
o If the SDP session description describing the session was o If the SDP session description describing the session was
transmitted over an end-to-end secure protocol which uses X.509 transmitted over a protocol (such as SIP [16]) for which the
certificates, the endpoint MAY use the same certificate to certify identities of session participants are defined by uniform resource
the media connection. For example, an SDP description sent over identifiers (URIs), the endpoint MAY use a certificate with a
HTTP/TLS [19] or secured by S/MIME [16] MAY use the same uniformResourceIdentifier subjectAltName. The details of what
certificate to secure the media connection. (Note, however, that URIs are appropriate are dependent on the transmitting protocol.
the sips protocol [15] (SIP over TLS) provides only hop-by-hop (For more details on this, see Section 7.)
security, so its TLS certificates do not satisfy this criterion.)
In this case, the certificate must be one that is allowed in this
context by the transmitting protocol.
In those cases where an endpoint provides a certificate fingerprint, If the SDP session description describing the session was transmitted
the certificate MAY be self-signed. The certificate MUST be well- over an end-to-end secure protocol which uses X.509 certificates, and
formed (and thus MUST include a syntactically valid SubjectAltName), the certificates sent fulfill the requirements above (as they
but no further requirements are imposed upon this field's contents. normally would be expected to), the endpoint MAY use the same
To support the use of certificate caches, however, as described in certificate to certify the media connection. For example, an SDP
Section 7, endpoints SHOULD consistently provide the same certificate description sent over HTTP/TLS [20] or secured by S/MIME [17] MAY use
for each identity they support. 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 6.2 Certificate Presentation
In all cases, an endpoint acting as the TLS server, i.e., one taking 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 the 'setup:passive' role, in the terminology of connection-oriented
media, MUST present a certificate during TLS initiation, following media, MUST present a certificate during TLS initiation, following
the rules presented in Section 6.1. If the certificate does not the rules presented in Section 6.1. If the certificate does not
match the original fingerprint, or, if there is no fingerprint, the match the original fingerprint, or, if there is no fingerprint, the
certificate identity is incorrect, the client endpoint MUST either certificate identity is incorrect, the client endpoint MUST either
notify the user, if possible, or terminate the media connection with notify the user, if possible, or terminate the media connection with
a bad certificate error. a bad certificate error.
If the SDP offer/answer model [5] is being used, the client (the If the SDP offer/answer model [5] is being used, the client (the
endpoint with the setup:active role) MUST also present a certificate endpoint with the 'setup:active' role) MUST also present a
following the rules of Section 6.1. The server MUST request a certificate following the rules of Section 6.1. The server MUST
certificate, and if the client does not provide one, if the request a certificate, and if the client does not provide one, if the
certificate does not match the provided fingerprint, or, if there was certificate does not match the provided fingerprint, or, if there was
no fingerprint, the certificate identity is incorrect, the server no fingerprint, the certificate identity is incorrect, the server
endpoint MUST either notify the user or terminate the media endpoint MUST either notify the user or terminate the media
connection with a bad certificate error. connection with a bad certificate error.
Note that when the offer/answer model is being used, it is possible 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. for a media connection to outrace the answer back to the offerer.
Thus, if the offerer has offered a setup:passive or setup:actpass Thus, if the offerer has offered a 'setup:passive' or 'setup:actpass'
role, it MUST (as specified in the Connection-Oriented Media role, it MUST (as specified in the Connection-Oriented Media
specification [2]) begin listening for an incoming connection as soon specification [2]) begin listening for an incoming connection as soon
as it sends its offer. However, because its peer's media connection as it sends its offer. However, because its peer's media connection
may outrace its answer, it SHOULD NOT definitively accept or reject may outrace its answer, it MUST NOT definitively accept the peer's
the peer's certificate until it has received and processed the SDP certificate until it has received and processed the SDP answer.
answer.
If offer/answer is not being used (e.g., if the SDP was sent over the If offer/answer is not being used (e.g., if the SDP was sent over the
Session Announcement Protocol [14]), the TLS server typically has no Session Announcement Protocol [15]), the TLS server typically has no
external knowledge of what the TLS client's identity ought to be. In external knowledge of what the TLS client's identity ought to be. In
this case, no client certificate need be presented, and no this case, no client certificate need be presented, and no
certificate validation can be performed, unless the server has certificate validation can be performed, unless the server has
knowledge of valid clients through some external means. knowledge of valid clients through some external means.
7. Security Considerations 7. Security Considerations
This entire document concerns itself with security. The problem to This entire document concerns itself with security. The problem to
be solved is addressed in Section 1, and a high-level overview is be solved is addressed in Section 1, and a high-level overview is
presented in Section 3. See the SDP specification [1] for security presented in Section 3. See the SDP specification [1] for security
skipping to change at page 10, line 49 skipping to change at page 11, line 4
However, such integrity protection is not always possible. For these However, such integrity protection is not always possible. For these
cases, end systems SHOULD maintain a cache of certificates which cases, end systems SHOULD maintain a cache of certificates which
other parties have previously presented using this mechanism. If other parties have previously presented using this mechanism. If
possible, users SHOULD be notified when an unsecured certificate possible, users SHOULD be notified when an unsecured certificate
associated with a previously unknown end system is presented, and associated with a previously unknown end system is presented, and
SHOULD be strongly warned if a different and unauthenticated SHOULD be strongly warned if a different and unauthenticated
certificate is presented by a party with which they have communicated certificate is presented by a party with which they have communicated
in the past. In this way, even in the absence of integrity in the past. In this way, even in the absence of integrity
protection for SDP, the security of this document's mechanism is protection for SDP, the security of this document's mechanism is
equivalent to that of the Secure Shell (ssh) protocol [20], which is equivalent to that of the Secure Shell (ssh) protocol [21], which is
vulnerable to man-in-the-middle attacks when two parties first vulnerable to man-in-the-middle attacks when two parties first
communicate, but can detect ones that occur subsequently. (Note that communicate, but can detect ones that occur subsequently. (Note that
a precise definition of the "other party" depends on the application a precise definition of the "other party" depends on the application
protocol carrying the SDP message.) 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 and
third-party call control, 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- TLS is not always the most appropriate choice for secure connection-
oriented media; in some cases, a higher- or lower-level security oriented media; in some cases, a higher- or lower-level security
protocol may be appropriate. protocol may be appropriate.
This document does not define any mechanism for securely transporting This document does not define any mechanism for securely transporting
RTP and RTCP packets over a connection-oriented channel. There was 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 no consensus in the working group as to whether it would be better to
send Secure RTP packets [21] over a connection-oriented transport send Secure RTP packets [22] over a connection-oriented transport
[22], or whether it would be better to send standard unsecured RTP [23], or whether it would be better to send standard unsecured RTP
packets over TLS using the mechanisms described in this document. packets over TLS using the mechanisms described in this document.
The group consensus was to wait until a use-case requiring secure The group consensus was to wait until a use-case requiring secure
connection-oriented RTP was presented. connection-oriented RTP was presented.
8. IANA Considerations 8. IANA Considerations
This document defines an SDP proto value: TCP/TLS. Its format is This document defines an SDP proto value: 'TCP/TLS'. Its format is
defined in Section 4. This proto value should be registered by IANA defined in Section 4. This proto value should be registered by IANA
on http://www.iana.org/assignments/sdp-parameters under "proto". on under "Session Description Protocol (SDP) Parameters" under
"proto".
This document defines an SDP session and media level attribute: This document defines an SDP session and media level attribute:
fingerprint. Its format is defined in Section 5. This attribute 'fingerprint'. Its format is defined in Section 5. This attribute
should be registered by IANA on should be registered by IANA under "Session Description Protocol
http://www.iana.org/assignments/sdp-parameters under "att-field (both (SDP) Parameters" under "att-field (both session and media level)".
session and media level)".
Specifications defining new proto values, like this one, must define The SDP specification, RFC2327, states that specifications defining
the rules by which their media format (fmt) namespace is managed. new proto values, like the 'TCP/TLS' proto value defined in this one,
For the TCP/TLS protocol, new formats SHOULD have an associated MIME must define the rules by which their media format (fmt) namespace is
registration. Use of an existing MIME subtype for the format is managed. For the TCP/TLS protocol, new formats SHOULD have an
encouraged. If no MIME subtype exists, it is RECOMMENDED that a associated MIME registration. Use of an existing MIME subtype for
suitable one be registered through the IETF process [13] by the format is encouraged. If no MIME subtype exists, it is
production of, or reference to, a standards-track RFC that defines RECOMMENDED that a suitable one be registered through the IETF
the transport protocol for the format. 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 Appendix A. Changes From Earlier Versions
Appendix A.1 Changes From Draft -02 Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
Appendix A.1 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.2 Changes From Draft -02
None, other than IPR boilerplate and reference updates. Draft -03 None, other than IPR boilerplate and reference updates. Draft -03
was a resubmission to refresh the draft's presence in the Internet- was a resubmission to refresh the draft's presence in the Internet-
Drafts repository. Drafts repository.
Appendix A.2 Changes From Draft -01 Appendix A.3 Changes From Draft -01
o Made the use of SHA-1 fingerprints mandatory in self-signed o Made the use of SHA-1 fingerprints mandatory in self-signed
certificates. certificates.
o Aligned with version -09 of draft-ietf-mmusic-comedia [2], also o Aligned with version -09 of draft-ietf-mmusic-comedia [2], also
drawing some wording changes from that document. drawing some wording changes from that document.
o Forbid the use of wildcards for the dNS subjectAltName. o Forbid the use of wildcards for the dNS subjectAltName.
o Eliminated requirements on identities provided with self-signed o Eliminated requirements on identities provided with self-signed
certificates. certificates.
o Recommended the use of a certificate cache when SDP integrity o Recommended the use of a certificate cache when SDP integrity
protection cannot be assured. protection cannot be assured.
o Explained that there is no currently supported mechanism for o Explained that there is no currently supported mechanism for
securely sending RTP over connection-oriented media. securely sending RTP over connection-oriented media.
o Described the procedure for establishing media formats for TCP/ o Described the procedure for establishing media formats for TCP/
TLS. TLS.
Appendix A.3 Changes From Draft -00 Appendix A.4 Changes From Draft -00
o Significantly expanded introduction and motivation sections. o Significantly expanded introduction and motivation sections.
o Significant clarifications to other sections. o Significant clarifications to other sections.
o Aligned with version -07 of draft-ietf-mmusic-comedia [2]. o Aligned with version -07 of draft-ietf-mmusic-comedia [2].
Protocol identifier changed from TLS to TCP/TLS at that document's Protocol identifier changed from TLS to TCP/TLS at that document's
recommendation. recommendation.
9. References 9. References
9.1 Normative References 9.1 Normative References
skipping to change at page 13, line 12 skipping to change at page 14, line 5
[7] Bassham, L., Polk, W., and R. Housley, "Algorithms and [7] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure Identifiers for the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile", Certificate and Certificate Revocation List (CRL) Profile",
RFC 3279, April 2002. RFC 3279, April 2002.
[8] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 [8] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002. Revocation List (CRL) Profile", RFC 3280, April 2002.
[9] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [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. Specifications: ABNF", RFC 2234, November 1997.
[10] National Institute of Standards and Technology, "Secure Hash [11] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995, Standard", FIPS PUB 180-2, August 2002, <http://csrc.nist.gov/
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>. publications/fips/fips180-2/fips180-2.pdf>.
[11] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [12] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992. April 1992.
[12] Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319, [13] Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319,
April 1992. April 1992.
[13] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet [14] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", Mail Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 2048, November 1996. BCP 13, RFC 2048, November 1996.
9.2 Informative References 9.2 Informative References
[14] Handley, M., Perkins, C., and E. Whelan, "Session Announcement [15] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000. Protocol", RFC 2974, October 2000.
[15] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [16] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[16] Ramsdell, B., "S/MIME Version 3 Message Specification", [17] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999. RFC 2633, June 1999.
[17] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., [18] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999. Basic and Digest Access Authentication", RFC 2617, June 1999.
[18] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", [19] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
RFC 3174, September 2001. RFC 3174, September 2001.
[19] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [20] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[20] Ylonen, T. and C. Lonvick, "SSH Protocol Architecture", [21] Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
draft-ietf-secsh-architecture-22 (work in progress), draft-ietf-secsh-architecture-22 (work in progress),
March 2005. March 2005.
[21] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [22] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
[22] Lazzaro, J., "Framing RTP and RTCP Packets over Connection- [23] Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
Oriented Transport", draft-ietf-avt-rtp-framing-contrans-05 Oriented Transport", draft-ietf-avt-rtp-framing-contrans-05
(work in progress), January 2005. (work in progress), January 2005.
[23] Andreasen, F., "Session Description Protocol Security [24] Andreasen, F., "Session Description Protocol Security
Descriptions for Media Streams", Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-11 (work in progress), draft-ietf-mmusic-sdescriptions-11 (work in progress),
June 2005. June 2005.
Author's Address Author's Address
Jonathan Lennox Jonathan Lennox
Columbia University Department of Computer Science Columbia University Department of Computer Science
450 Computer Science 450 Computer Science
1214 Amsterdam Ave., M.C. 0401 1214 Amsterdam Ave., M.C. 0401
 End of changes. 

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