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.
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
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
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].
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.
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
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].
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.
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
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
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.
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,