Internet DRAFT - draft-ietf-simple-msrp-cema
draft-ietf-simple-msrp-cema
SIMPLE Working Group C. Holmberg
Internet-Draft S. Blau
Intended status: Standards Track Ericsson
Expires: January 4, 2013 E. Burger
Georgetown University
July 3, 2012
Connection Establishment for Media Anchoring (CEMA) for the Message
Session Relay Protocol (MSRP)
draft-ietf-simple-msrp-cema-07.txt
Abstract
This document defines a Message Session Relay Protocol (MSRP)
extension, Connection Establishment for Media Anchoring (CEMA).
Support of the extension is OPTIONAL. The extension allows
middleboxes to anchor the MSRP connection, without the need for
middleboxes to modify the MSRP messages, and thus also enables a
secure end-to-end MSRP communication in networks where such
middleboxes are deployed. The document also defines a Session
Description Protocol (SDP) attribute, 'msrp-cema', that MSRP
endpoints use to indicate support of the CEMA extension.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 4, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. Connection Establishment for Media Anchoring Mechanism . . . . 6
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. MSRP SDP Offerer Procedures . . . . . . . . . . . . . . . 7
4.3. MSRP SDP Answerer Procedures . . . . . . . . . . . . . . . 9
4.4. Address Information Matching . . . . . . . . . . . . . . . 10
4.5. Usage With the Alternative Connection Model . . . . . . . 11
5. The SDP 'msrp-cema' attribute . . . . . . . . . . . . . . . . 11
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Middlebox Assumptions . . . . . . . . . . . . . . . . . . . . 11
6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. MSRP Awareness . . . . . . . . . . . . . . . . . . . . . . 12
6.3. TCP Connection Reuse . . . . . . . . . . . . . . . . . . . 12
6.4. SDP Integrity . . . . . . . . . . . . . . . . . . . . . . 12
6.5. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 14
7.3. TLS Usage without Middleboxes . . . . . . . . . . . . . . 14
7.4. TLS Usage with Middleboxes . . . . . . . . . . . . . . . . 14
7.5. Authentication, Credentials and Key Management . . . . . . 15
7.6. Endpoint procedures for TLS negotiation . . . . . . . . . 16
7.7. Fingerprint Based Authentication . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8.1. IANA Registration of the SDP 'msrp-cema' attribute . . . . 18
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The Message Session Relay Protocol (MSRP) [RFC4975] expects to use
MSRP relays [RFC4976] as a means for Network Address Translation
(NAT) traversal and policy enforcement. However, many Session
Initiation Protocol (SIP) [RFC3261] networks, which deploy MSRP,
contain middleboxes. These middleboxes anchor and control media,
perform tasks such as NAT traversal, performance monitoring, address
domain bridging, interconnect Service Layer Agreement (SLA) policy
enforcement, and so on. One example is the Interconnection Border
Control Function (IBCF) [GPP23228], defined by the 3rd Generation
Partnership Project (3GPP). The IBCF controls a media relay that
handles all types of SIP session media such as voice, video, MSRP,
etc.
MSRP, as defined in RFC 4975 [RFC4975] and RFC 4976 [RFC4976], cannot
anchor through middleboxes. The reason is that MSRP messages have
routing information embedded in the message. Without an extension
such as CEMA, middleboxes must read the message to change the routing
information. This occurs because middleboxes modify the address:port
information in the Session Description Protocol (SDP) [RFC4566] c/m-
line in order to anchor media. An "active" [RFC6135] MSRP UA
establishes the MSRP TCP or TLS connection based on the MSRP URI of
the SDP 'path' attribute. This means that the MSRP connection will
not be routed through the middlebox, unless the middlebox also
modifies the MSRP URI of the topmost SDP 'path' attribute. In many
scenarios this will prevent the MSRP connection from being
established. In addition, if the middlebox modifies the MSRP URI of
the SDP 'path' attribute, then the MSRP URI comparison procedure
[RFC4975], which requires consistency between the address information
in the MSRP messages and the address information carried in the MSRP
URI of the SDP 'path' attribute, will fail.
The only way to achieve interoperability in this situation is for the
middlebox to act as an MSRP back-to-back User Agent (B2BUA). Here
the MSRP B2BUA acts as the endpoint for the MSRP signaling and media,
performs the corresponding modification in the associated MSRP
messages, and originates a new MSRP session towards the actual remote
endpoint. However, the enabling of MSRP B2BUA functionality requires
substantially more resource usage in the middlebox, that normally
result in negative performance impact. In addition, the MSRP message
needs to be exposed in clear text to the MSRP B2BUA, which violates
the end-to-end principle [RFC3724] .
This specification defines an MSRP extension, Connection
Establishment for Media Anchoring (CEMA). CEMA in most cases allows
MSRP endpoints to communicate through middleboxes, as defined in
Section 2, without a need for the middleboxes to be an MSRP B2BUA.
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In such cases, middleboxes, that want to anchor the MSRP connection
simply modify the SDP c/m-line address information, similar to what
the middleboxes do for non-MSRP media types. MSRP endpoints that
support the CEMA extension will use the SDP c/m-line address
information for establishing the TCP or TLS connection for sending
and receiving MSRP messages.
The CEMA extension is backward compatible, meaning that CEMA-enabled
MSRP endpoints can communicate with non-CEMA-enabled endpoints. In
scenarios where MSRP endpoints do not support the CEMA extension, an
MSRP endpoint that supports the CEMA extension behaves in the same
way as an MSRP endpoint that does not support it. The CEMA extension
only provides an alternative mechanism for negotiating and providing
address information for the MSRP TCP connection. After the creation
of the MSRP connection, an MSRP endpoint that supports the CEMA
extension acts according to the procedures for creating MSRP
messages, performing checks when receiving MSRP messages defined in
RFC 4975 and, when it is using a relay for MSRP communications, RFC
4976.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
Definitions:
Fingerprint Based TLS Authentication: An MSRP endpoint that uses a
self-signed certificate and sends a fingerprint (i.e., a hash of the
self-signed certificate)in SDP to the other MSRP endpoint. This
fingerprint binds the TLS key exchange to the signaling plane and
authenticates the other endpoint based on trust in the signaling
plane.
Name Based TLS Authentication: An MSRP endpoint that uses a
certificate which is bound to the endpoint's hostname or SIP address-
of-record. In the TLS session setup, the other MSRP endpoint
verifies that the identity associated with the certificate
corresponds to that of the peer (as indicated in SIP/SDP) and that
the binding of the identity to the public key was done by a party
which the endpoint trusts. This definition includes both traditional
certificates issued by a well-known certification authority as well
as self-signed certificates published via the SIP Certificate
Management Service [RFC6072] and other similar mechanisms.
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B2BUA: This is an abbreviation for back-to-back user agent.
MSRP B2BUA: A network element that terminates an MSRP connection from
one MSRP endpoint and reoriginates that connection towards another
MSRP endpoint. Note the MSRP B2BUA is distinct from a SIP B2BUA. A
SIP B2BUA terminates a SIP session and reoriginates that session
towards another SIP endpoint. In the context of MSRP, a SIP endpoint
initiates a SIP session towards another SIP endpoint. However, that
INVITE may go through, for example, an outbound Proxy or inbound
Proxy to route to the remote SIP endpoint. As part of that SIP
session an MSRP session, that may follow the SIP session path, is
negotiated. However, there is no requirement to co-locate the SIP
network elements with the MSRP network elements.
TLS B2BUA: A network element that terminates security associations
(SAs) from endpoints, and establishes separate SAs between itself and
each endpoint.
Middlebox: A SIP network device that modifies SDP media address:port
information in order to steer or anchor media flows described in the
SDP, including TCP and TLS connections used for MSRP communication,
through a media proxy function controlled by the SIP endpoint. In
most cases the media proxy function relays the MSRP messages without
modification, while in some circumstances it acts as a MSRP B2BUA.
Other SIP related functions, such as related to routing, modification
of SIP information etc, performed by the Middlebox, and whether it
acts a SIP B2BUA or not, is outside the scope of this document.
Section 5 describes additional assumptions regarding how the
Middlebox handles MSRP in order to support the extension defined in
this document.
Media anchor: An entity that performs media anchoring inserts itself
in the media path of a media communication session between two
entities. The entity will receive, and forward, the media sent
between the entities.
This document reuses the terms answer, answerer, offer and offerer as
defined in RFC 3264.
3. Applicability Statement
This document defines a Message Session Relay Protocol (MSRP)
extension, Connection Establishment for Media Anchoring (CEMA).
Support of the extension is optional. The extension allows
Middleboxes to anchor the MSRP connection, without the need for
Middleboxes to modify the MSRP messages, and thus also enables a
secure end-to-end MSRP communication in networks where such
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Middleboxes are deployed. The document also defines a Session
Description Protocol (SDP) attribute, 'msrp-cema', that MSRP
endpoints use to indicate support of the CEMA extension.
The CEMA extension is primarily intended for MSRP endpoints that
operate in networks in which Middleboxes that want to anchor media
connections are deployed, without the need for the Middleboxes to
enable MSRP B2BUA functionality. An example of such network is the
IP Multimedia Subsystem (IMS) defined by the 3rd Generation
Partnership Project (3GPP), which also has the capability for all
endpoints to use Name-based TLS Authentication. The extension is
also useful for other MSRP endpoints operating in other networks, but
that communicate with MSRP endpoints in networks with such
Middleboxes, unless there is a gateway between the networks that by
default always enable MSRP B2BUA functionality.
This document assumes certain behaviors on the part of Middleboxes,
as described in Section 6. These behaviors are not standardized. If
Middleboxes do not behave as assumed, then the CEMA extension does
not add any value over base MSRP behavior. MSRP endpoints that
support CEMA are required to use RFC 4975 behavior in cases where
they detect that the CEMA extension cannot be enabled.
4. Connection Establishment for Media Anchoring Mechanism
4.1. General
This section defines how an MSRP endpoint that supports the CEMA
extension generates SDP offers and answers for MSRP, and which SDP
information elements the MSRP endpoint uses when creating the TCP or
TLS connection for sending and receiving MSRP messages.
Based on the procedures described in sections 4.2 and 4.3, in the
following cases the CEMA extension will not be enabled, and there
will be a fallback to the MSRP connection establishment procedures
defined in RFC 4975 and RFC 4976:
- A non-CEMA-enabled MSRP endpoint becomes "active" [RFC6135] (no
matter whether it uses a relay for its MSRP communication or not), as
it will always establish the MSRP connection using the SDP 'path'
attribute, which contains the address information of the remote MSRP
endpoint, instead of using the SDP c/m-line which contains the
address information of the Middlebox.
- A non-CEMA-enabled MSRP endpoint that uses a relay for its MSRP
communication becomes "passive" [RFC6135], as it cannot be assumed
that the MSRP endpoint inserts the address information of the relay
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in the SDP c/m-line.
- A CEMA-enabled MSRP endpoint that uses a relay for its MSRP
communication becomes "active", since if it adds the received SDP
c/m-line address information to the ToPath header field of the MSRP
message (in order for the relay to establish the MSRP connection
towards the Middlebox), the session matching [RFC4975] performed by
the remote MSRP endpoint will fail.
4.2. MSRP SDP Offerer Procedures
When a CEMA-enabled offerer sends an SDP offer for MSRP, it generates
the SDP offer according to the procedures in RFC 4975. In addition,
the offerer follows RFC 4976 if it is using a relay for MSRP
communication. The offerer also performs the following additions and
modifications:
1. The offerer MUST include an SDP 'msrp-cema' attribute in the MSRP
media description of the SDP offer.
2. If the offerer is not using a relay for MSRP communication, it
MUST include an SDP 'setup' attribute in the MSRP media description
of the SDP offer, according to the procedures in RFC 6135 [RFC6135].
3. If the offerer is using a relay for MSRP communication, it MUST,
in addition to including the address information of the relay in the
topmost SDP 'path' attribute, also include the address information of
the relay, rather than the address information of itself, in the SDP
c/m-line associated with the MSRP media description. In addition, it
MUST include an SDP 'setup:actpass' attribute in the MSRP media
description of the SDP offer.
When the offerer receives an SDP answer, if the MSRP media
description of the SDP answer does not contain an SDP 'msrp-cema'
attribute, and if any of the following criteria below is met, the
offerer MUST fallback to RFC 4975 behavior, by sending a new SDP
offer according to the procedures in RFC 4975 and RFC 4976. The new
offer MUST NOT contain an SDP 'msrp-cema' attribute.
1. The SDP c/m-line address information associated with the MSRP
media description does not match Section 4.4 the information in the
MSRP URI of the 'path' attribute(s) (in which case is assumed that
the SDP c/m-line contains the address to a Middlebox), and the MSRP
endpoint will become "passive" (if the MSRP media description of the
SDP answer contains an SDP 'setup:active' attribute).
NOTE: If an MSRP URI contains a domain name, it needs to be resolved
into an IP address and port before it is checked against the SDP c/m-
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line address information, in order to determine whether the address
information matches.
2. The offerer uses a relay for its MSRP communication, the SDP c/m-
line address information associated with the MSRP media description
does not match the information in the MSRP URI of the SDP 'path'
attribute(s) (in which case is assumed that the SDP c/m-line contains
the address to a Middlebox), and the offerer will become "active"
(either by default or if the MSRP media description of the SDP answer
contains an SDP 'setup:passive' attribute).
3. The remote MSRP endpoint, acting as an answerer, uses a relay for
its MSRP communication, the SDP c/m-line address information
associated with the MSRP media description does not match the
information in the MSRP URI of the SDP 'path' attributes (in which
case is assumed that the SDP c/m-line contains the address to a
Middlebox), and the MSRP offerer will become "active" (either by
default or if the MSRP media description of the SDP answer contains
an SDP 'setup:passive' attribute).
NOTE: As described in section 5, in the absence of the SDP 'msrp-
cema' attribute in the new offer, it is assumed that a Middlebox will
act as an MSRP B2BUA in order to anchor MSRP media.
The offerer can send the new offer within the existing early dialog
[RFC3261], or it can terminate the early dialog and establish a new
dialog by sending the new offer in a new initial INVITE request.
The offerer MAY choose to terminate the session establishment if it
can detect that a Middlebox acting as an MSRP B2BUA is not the
desired remote MSRP endpoint.
If the answerer uses a relay for its MSRP communication, and the SDP
c/m-line address information associated with the MSRP media
description matches one of the SDP 'path' attributes, it is assumed
that there is no Middlebox in the network. In that case the offerer
MUST fallback to RFC 4975 behavior, but it does not need to send a
new SDP offer.
In other cases, where none of the criteria above is met, and where
the MSRP offerer becomes "active", it MUST use the SDP c/m-line for
establishing the MSRP TCP connection. If the offerer becomes
"passive", it will wait for the answerer to establish the TCP
connection, according to the procedures in RFC 4975.
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4.3. MSRP SDP Answerer Procedures
If the MSRP media description of the SDP offer does not contain an
SDP 'msrp-cema' attribute, and the SDP c/m-line address information
associated with the MSRP media description does not match the
information in the MSRP URI of the SDP 'path' attribute(s), the
answerer MUST either reject the offered MSRP connection (by using a
zero port value number in the generated SDP answer), or reject the
whole SDP offer carrying SIP request with a 488 Not Acceptable Here
[RFC3261] response.
NOTE: The reasons for the rejection is that the answerer assumes that
a middlebox, that do not support the CEMA extension, has modified the
c/m-line address information of the SDP offer, without enabling MSRP
B2BUA functionality.
NOTE: If an MSRP URI contains a domain name, it needs to be resolved
into an IP address and port before it is checked against the SDP c/m-
line address information, in order to determine whether the address
information matches.
If any of the criteria below is met, the answerer MUST fallback to
RFC 4975 behavior and generate the associated SDP answer according to
the procedures in RFC 4975 and RFC 4976. The answerer MUST NOT
insert an SDP 'msrp-cema' attribute in the MSRP media description of
the SDP answer.
1. Both MSRP endpoints are using relays for their MSRP
communication. The answerer can detect if the remote MSRP endpoint,
acting as an offerer, is using a relay for its MSRP communication if
the MSRP media description of the SDP offer contains multiple SDP
'path' attributes.
2. The offerer uses a relay for its MSRP communication, and will
become "active" (either by default or if the MSRP media description
of the SDP offer contains an SDP 'setup:active' attribute). Note
that a CEMA-enabled offerer would include an SDP 'setup:actpass'
attribute in the SDP offer, as described in Section 4.2.
3. The answerer uses a relay for MSRP communication and is not able
to become "passive" (if the MSRP media description of the offer
contains an SDP 'setup:passive' attribute. Note that an offerer is
not allowed to include an SDP 'setup:passive' attribute in an SDP
offer, as described in RFC 6135.
In all other cases, the answerer generates the associated SDP answer
according to the procedures in RFC 4975 and RFC 4976, with the
following additions and modifications:
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1. The answerer MUST include an SDP 'msrp-cema' attribute in the
MSRP media description of the SDP answer.
2. If the answerer is not using a relay for MSRP communication, it
MUST include an SDP 'setup' attribute in the MSRP media description
of the answer, according to the procedures in RFC 6135.
3. If the answerer is using a relay for MSRP communication, it MUST,
in addition to including the address information of the relay in the
topmost SDP 'path' attribute, also include the address information of
the relay, rather than the address information of itself, in the SDP
c/m-line associated with the MSRP media description. In addition,
the answerer MUST include an SDP 'setup:passive' attribute in the
MSRP media description of the SDP answer.
If the answerer included an SDP 'msrp-cema' attribute in the MSRP
media description of the SDP answer, and if the answerer becomes
"active", it MUST use the received SDP c/m-line for establishing the
MSRP TCP or TLS connection. If the answerer becomes "passive", it
will wait for the offerer to establish the MSRP TCP or TLS
connection, according to the procedures in RFC 4975.
4.4. Address Information Matching
When comparing address information in the SDP c/m-line and an MSRP
URI, for address and port equivalence, the address and port values
are retrieved in the following ways:
- SDP c/m-line address information: The IP address is retrieved from
the SDP c- line, and the port from the associated SDP m- line for
MSRP.
- In case the SDP c- line contains a Fully Qualified Domain Name
(FQDN), the IP address is retrieved using DNS.
- MSRP URI address information: The IP address and port are retrieved
from the authority part of the MSRP URI.
- In case the authority part of the MSRP URI contains a Fully
Qualified Domain Name (FQDN), the IP address is retrieved using DNS,
according to the procedures in section 6.2 of RFC 4975.
NOTE: According to RFC 4975, the authority part of the MSRP URI must
always contain a port.
Before IPv6 addresses are compared for equivalence, they need to be
converted into the same representation, using the mechanism defined
in RFC 5952 [RFC5952].
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NOTE: In case the DNS returns multiple records, each needs to be
compared against the SDP c/m- line address information, in order to
find at least one match.
NOTE: If the authority part of the MSRP URI contains special
characters, they are handled according to the procedures in section
6.1 of RFC 4975.
4.5. Usage With the Alternative Connection Model
An MSRP endpoint that supports the CEMA extension MUST support the
mechanism defined in RFC 6135, as it extends the number of scenarios
where one can use the CEMA extension. An example is where an MSRP
endpoint is using a relay for MSRP communication, and it needs to be
"passive" in order to use the CEMA extension, instead of doing a
fallback to RFC 4975 behavior.
5. The SDP 'msrp-cema' attribute
5.1. General
The SDP 'msrp-cema' attribute is used by MSRP entities to indicate
support of the CEMA extension, according to the procedures in
Sections 4.2 and 4.3.
5.2. Syntax
This section describes the syntax extensions to the ABNF syntax
defined in RFC 4566 required for the SDP 'msrp-cema' attribute. The
ABNF defined in this specification is conformant to RFC 5234
[RFC5234].
attribute /= msrp-cema-attr
;attribute defined in RFC 4566
msrp-cema-attr = "msrp-cema"
6. Middlebox Assumptions
6.1. General
This document does not specify explicit Middlebox behavior, even
though Middleboxes enable some of the procedures described here.
However, as MSRP endpoints are expected to operate in networks where
Middleboxes that want to anchor media are present, this document
makes certain assumptions regarding to how such Middleboxes behave.
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6.2. MSRP Awareness
In order to support interoperability between UAs that support the
CEMA extension and UAs that do not support the extension, the
Middlebox is MSRP aware. This means that it implements MSRP B2BUA
functionality. The Middlebox enables that functionality in cases
where the offerer does not support the CEMA extension. In cases
where the SDP offer indicates support of the CEMA extension, the
Middlebox can simply modify the SDP c/m-line address information for
the MSRP connection.
In cases where the Middlebox enables MSRP B2BUA functionality, it
acts as an MSRP endpoint. If it does not use the CEMA procedures it
will never forward the SDP 'msrp-cema' attribute in SDP offers and
answers.
If the Middlebox does not implement MSRP B2BUA functionality, or does
not enable it when the SDP 'msrp-cema' attribute is not present in
the SDP offer, CEMA-enabled MSRP endpoints will in some cases be
unable to interoperate with non-CEMA-enabled endpoints across the
Middlebox.
6.3. TCP Connection Reuse
Middleboxes do not need to parse and modify the MSRP payload when
endpoints use the CEMA extension. A Middlebox that does not parse
the MSRP payload probably will not be able to reuse TCP connections
for multiple MSRP sessions. Instead, in order to associate an MSRP
message with a specific session, the Middlebox often assigns a unique
local address:port combination for each MSRP session. Due to this,
between two Middleboxes there might be a separate connection for each
MSRP session.
If the Middlebox does not assign a unique address:port combination
for each MSRP session, and does not parse MSRP messages, it might end
up forwarding MSRP messages towards the wrong destination.
6.4. SDP Integrity
This document assumes that Middleboxes are able to modify the SDP
address information associated with the MSRP media.
NOTE: Eventhough the CEMA extension as such works with end-to-end SDP
protection, the main advantage of the extension is in networks where
Middleboxes are deployed.
If the Middlebox is unable to modify SDP payloads due to end-to-end
integrity protection, it will be unable to anchor MSRP media as the
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SIP signaling would fail due to integrity violations.
6.5. TLS
When UAs use the CEMA extension, this document assumes that
Middleboxes relay MSRP media packets at the transport layer. The TLS
handshake and resulting security association (SA) can be established
peer-to-peer between the MSRP endpoints. The Middlebox will see
encrypted MSRP media packets, but is unable to inspect the clear text
content.
When UAs fall back to RFC 4975 behavior Middleboxes act as TLS
B2BUAs. The Middlebox decrypts MSRP media packets received from one
MSRP endpoint, and then re-encrypts them before sending them toward
the other MSRP endpoint. Middleboxes can inspect and modify the MSRP
message content.
7. Security Considerations
7.1. General
Unless otherwise stated, the security considerations in RFC 4975 and
RFC 4976 still apply. This section only describes additions and
changes introduced by the CEMA extension.
The purpose of CEMA is to enable MSRP communication over Middleboxes.
These Middleboxes are commonly deployed by SIP network operators, who
also commonly deploy firewall and routing policies that prevent media
sessions from working unless they traverse the Middleboxes.
CEMA makes it possible for Middleboxes to tunnel TLS to allow end-to-
end security associations between endpoints. This is an improvement
over the status quo, since without CEMA, the Middleboxes would be
forced to both read and modify the cleartext MSRP messages, which
would make end-to-end confidentiality and integrity protection of the
MSRP transport channel impossible.
RFC 4975 suggests two ways for MSRP endpoints to verify that the TLS
connection is established end-to-end. The first option is to use
certificates from a well known certification authority and verify
that the SubjectAltName matches the MSRP URI of the other side. The
second option is to use self-signed certificates and include a
fingerprint of the certificate in the SDP offer/answer. Provided the
signalling is integrity protected, both endpoints can verify that the
TLS security association is established with the correct host by
matching the received certificate against the received fingerprint.
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Fingerprint based authentication is expected to be common for end
clients. In order to ensure the integrity of the fingerprint, RFC
4975 recommends using the SIP Identity mechanism [RFC4474]. However,
this mechanism may not be compatible with CEMA which operates under
the assumption that Middleboxes will modify the contents of SDP
offers and answers. Until a mechanism is available that enables a
subset of the SDP to be signed, end clients that support CEMA and use
fingerprint based authentication are forced to trust the entire
signalling path. In other words, end clients must accept the fact
that every signalling proxy could potentially replace the
fingerprints and insert a Middlebox that acts as a TLS B2BUA.
An alternative solution that only requires a limited trust in the
signaling plane is to use self-signed certificates together with the
SIP Certificate Management Service [RFC6072]. The security provided
by this solution is roughly equivalent to SIP Identity and
fingerprint based authentication (in fact, RFC 6072 is based on RFC
4474). Section 7.5 discusses this approach further.
In the remainder of this section we will assume that fingerprint-
based authentication is used without SIP Identity or similar
mechanisms which protects the SDP across several hops.
7.2. Man-in-the-Middle Attacks
If TLS is not used to protect MSRP, the CEMA extension might make it
easier for a man-in-the-middle to transparently insert itself in the
communication between MSRP endpoints in order to monitor or record
unprotected MSRP communication. This can be mitigated by the use of
TLS. It is therefore RECOMMENDED to use TLS [RFC5246]. It is also
recommended to use TLS e2e, which CEMA enables even in the case of
Middleboxes. According to RFC 4975, MSRP endpoints are required to
support TLS. This also apply to CEMA-enabled endpoints.
7.3. TLS Usage without Middleboxes
If TLS is used without Middleboxes, the security considerations in
RFC 4975 and RFC 4976 still apply unchanged. Note that this is not
the main use case for the CEMA extension.
7.4. TLS Usage with Middleboxes
This is the main use case for the CEMA extension; the endpoints
expect one or more Middleboxes.
The CEMA extension supports the usage of both name-based
authentication and fingerprint based authentication for TLS in the
presence of Middleboxes. The use of fingerprint based authentication
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requires signaling integrity protection. This can e.g. be hop-by-hop
cryptographic protection or cryptographic access protection combined
with a suitably protected core network. As stated in section 6.4,
this document assumes that Middleboxes are able to modify the SDP
address information associated with the MSRP media.
If a Middlebox acts as a TLS B2BUA, the security considerations are
the same as without the CEMA extension. In such case the Middlebox
acts as TLS endpoints.
If a Middlebox does not act as a TLS B2BUA, TLS is e2e and the
Middlebox just forwards the TLS packets. This requires that both
peers support the CEMA extension.
If fingerprint based authentication is used, the MSRP endpoints might
not be able to decide whether the Middlebox acts as a TLS B2BUA or
not. But this is not an issue as the signaling network is considered
trusted by the endpoint (a requirement to use fingerprint based
authentication).
7.5. Authentication, Credentials and Key Management
One issue with usage of TLS (not specific to CEMA) is the
availability of a PKI. Endpoints can always provide self-signed
certificates and include fingerprints in the SDP offer and answer.
However, this relies on SDP signaling being integrity protected,
which may not always be the case.
Therefore, in addition to the authentication mechanisms defined in
RFC 4975, it is RECOMMENDED that a CEMA-enabled MSRP endpoint also
supports self-signed certificates together with the Certificate
Management Service [RFC6072], to which it publishes its self-signed
certificate and from which it fetches on demand the self-signed
certificates of other endpoints.
Alternate key distribution mechanisms, such as DANE [DANE], PGP
[RFC6091], MIKEY-TICKET [RFC6043] or some other technology, might
become ubiquitous enough to solve the key distribution problem in the
future.
One of the target deployments for CEMA is the 3GPP IMS SIP network.
In this environment authentication and credential management is less
of a problem as the SDP signaling is mostly considered trusted,
service providers provision signed certificates or manage signed
certificates on behalf of their subscribers, and MIKEY-TICKET is
available. Some of these options require trusting the service
provider, but those issues are beyond the scope of this document.
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7.6. Endpoint procedures for TLS negotiation
The CEMA extension does not change the endpoint procedures for TLS
negotiation. As in RFC 4975, the MSRP endpoint uses the negotiation
mechanisms in SDP and then the TLS handshake to agree on a mechanisms
and algorithms that both support. The mechanisms can be divided in
three different security levels:
- MSRPS: Security Mechanisms that do not rely on trusted signaling
such as name based authentication
- MSRPS: Mechanisms that do rely on trusted signaling such as
fingerprint based authentication
- MSRP: Unprotected
If the endpoint uses security mechanisms that does not rely on
trusted signaling the endpoint can detect if a Middlebox that acts as
a B2BUA is inserted. It is therefore RECOMMENDED to use such a
mechanism.
If the endpoint uses security mechanisms that rely on trusted
signaling the endpoint may not be able to detect if a Middlebox that
acts as a B2BUA is inserted (by the trusted network operator). To be
able to eavesdrop a Middlebox must do an active "attack" on the setup
signaling. A Middlebox cannot insert itself at a later point.
If unprotected MSRP is used, the endpoint cannot detect if a
Middlebox that acts as a B2BUA is inserted and Middleboxes may be
inserted at any time during the session.
The mechanism in RFC 6072 [RFC6072] provides end-to-end security
without relying on trust in the signaling, and eases the use and
deployment of name based authentication.
The procedures for choosing and offering name based authentication,
fingerprint based authentication, and unprotected MSRP as described
in RFC 4975 still apply.
7.7. Fingerprint Based Authentication
If the endpoint cannot use a key management protocol that does not
rely on trust in the signaling such as name based authentication, the
only alternative is fingerprint based authentication.
The use of fingerprint based authentication requires integrity
protection of the signaling plane. This can e.g. be hop-by-hop
cryptographic protection or cryptographic access protection combined
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with a suitably protected core network. Unless cryptographic end-to-
end SDP integrity protection or encryption is used this may be hard
for the endpoint to decide. In the end it is up to the endpoint to
decide whether the signaling path is trusted or not.
How this decision is done is implementation specific, but normally
signaling over the internet SHOULD NOT be trusted. Signaling over a
local or closed network might be trusted. Such networks can e.g. be
a closed enterprise network or a network operated by an operator that
the end user trusts. In e.g. IMS the signaling traffic in the
access network is integrity protected and the traffic is routed over
a closed network separated from the Internet. If the network is not
trusted the endpoints SHOULD NOT use fingerprint authentication.
When an endpoint receives a fingerprint, that represents a binding
between the identity as established by TLS and that established via
SDP. As previously noted, the fingerprint is vulnerable to an active
MITM attack from any on-path proxy. Endpoints SHOULD therefore
locally store fingerprints associated with the relevant identities
when first seen, and SHOULD warn when a new fingerprint is seen for
what otherwise appears to be the same peer identity. While there are
valid reasons for keys to change from time to time, that ought be the
exception, hence the suggested warning.
It should however be noted that using fingerprint based
authentication over an insecure network increases the security
compared to unencrypted MSRP. In order to intercept the plaintext
media when fingerprint based authentication is used, the attacker is
required to be present on both the signaling and media paths and
actively modify the traffic. It is very hard for the endpoints to
detect when such an attack is taking place though. A client using
DTLS-SRTP [RFC5764] for VoIP media security might wish to use
fingerprint based authentication also for MSRP media security.
MSRPS with fingerprint based authentication is vulnerable to attacks
due to vulnerabilities in the SIP signaling. If there are weaknesses
in the integrity protections on the SIP signaling, an attacker may
insert malicious Middleboxes to alter, record, or otherwise harm the
media. With insecure signaling, it can be difficult for an endpoint
to even be aware the remote endpoint has any relationship to the
expected endpoint. Securing the SIP signaling does not solve all
problems. For example, in a SIPS environment, the endpoints have no
cryptographic way of validating that one or more SIP Proxies in the
proxy chain are not, in fact, malicious.
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8. IANA Considerations
8.1. IANA Registration of the SDP 'msrp-cema' attribute
This document instructs IANA to add a attribute to the 'att-field
(media level only)' registry of the SDP parameters registry,
according to the information provided in this section.
This section registers a new SDP attribute, 'msrp-cema'. The
required information for this registration, as specified in RFC 4566,
is:
Contact name: Christer Holmberg
Contact e-mail: christer.holmberg@ericsson.com
Attribute name: msrp-cema
Type of attribute: media level
Purpose: This attribute is used to indicate support of
the MSRP Connection Establishment for Media
Anchoring (CEMA) extension defined in
RFC XXXX. When present in an MSRP media
description of an SDP body, it indicates
that the creator of the SDP supports the CEMA
mechanism.
Values: The attribute does not carry a value
Charset dependency: none
9. Acknowledgements
Thanks to Ben Campbell, Remi Denis-Courmont, Nancy Greene, Hadriel
Kaplan, Adam Roach, Robert Sparks, Salvatore Loreto, Shida Schubert,
Ted Hardie, Richard L Barnes, Inaki Baz Castillo, Saul Ibarra
Corretge, Cullen Jennings, Adrian Georgescu, Miguel Garcia and Paul
Kyzivat for their guidance and input in order to produce this
document.
Thanks to John Mattsson, Oscar Ohlsson, Ben Campbell and Stephen
Farrell for their help to restructure the Security Considerations
section, based on the feedback from IESG.
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10. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-simple-msrp-cema-06
o Additional security considerations text added, based on input from
Stephen Farrell.
Changes from draft-ietf-simple-msrp-cema-05
o Changes based on additional IESG comments from Stephen Farrell.
o Changes based on IESG comments from Sean Turner.
o Changes based on WGLC comments from Paul Kyzivat.
o Changes based on WGLC comments from Nancy Greene.
o Changes based on WGLC comments from Ben Campbell.
o - Name based TLS authentication definition enchanced.
o - Section 7.1 modified and enhanced.
o - Required support of TLS explicitly added to section 7.2.
o - "Physical trust" wording removed from section 7.4 and 7.7.
Changes from draft-ietf-simple-msrp-cema-04
o Changes based on additional IESG comments from Stephen Farrell.
o - 'Media anchor' definition added.
o - TLS reference made normative.
o - MIKEY-TICKET recommendation removed.
o - Editorial clarifications.
Changes from draft-ietf-simple-msrp-cema-03
o Security Considerations sections re-written based on IESG
comments.
o Changes based on IESG comments from Peter Saint-Andre.
o Changes based on IESG comments from Robert Sparks.
o Changes based on IESG comments from Stephen Farrell.
o Changes based on IESG comments from Pete Resnick.
Changes from draft-ietf-simple-msrp-cema-02
o Changes based on WGLC comments.
o - Editorial changes based on comments from Nancy Greene.
o - Editorial changes based on comments from Saul Ibarra Corretge.
o - Editorial changes based on comments from Christian Schmidt.
o - Editorial changes based on comments from Miguel Garcia.
o Changes based on MMUSIC SDP impact review.
o - Editorial changes based on comments from Miguel Garcia.
Changes from draft-ietf-simple-msrp-cema-01
o Changes based on comment from Ben Campbell.
o - TLS B2BUA added to definitions section.
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o - Middlebox added.
o - Editorial changes.
Changes from draft-ietf-simple-msrp-sessmatch-13
o Changed the draft name, as was suggested by our AD and work group.
o Clean up language use, clarify language, and clean up editorial
and style issues.
o Formally defined an MSRP B2BUA.
Changes from draft-ietf-simple-msrp-sessmatch-12
o Extension name changed to Connection Establishment for Media
Anchoring (CEMA).
o Middlebox definition added.
o ALG terminology replaced with Middlebox.
o SDP attribute name changed to a=msrp-cema.
o Applicability Statement section expanded.
o Re-structuring of MSRP Answerer section.
o Changes based on comments from Saul Ibarra Corretge (1406111).
Changes from draft-ietf-simple-msrp-sessmatch-11
o Modification of the sessmatch mechanism.
o - Extension name changed to Alternative Connection Establishment
(ACE)
o - Session matching procedure no longer updated.
o - SDP c/m-line used for MSRP TCP connection.
o - sessmatch option-tag removed.
o - a=msrp-ace attribute defined.
o - Support of RFC 6135 mandatory.
Changes from draft-ietf-simple-msrp-sessmatch-10
o Sessmatch option-tag added, based on WG discussions and concensus.
Changes from draft-ietf-simple-msrp-sessmatch-08
o OPEN ISSUE regarding the need for a sessmatch option-tag removed.
Changes from draft-ietf-simple-msrp-sessmatch-07
o Sessmatch defined as an MSRP extension, rather than MSRP update
o Additional security considerations text added
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
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A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message
Session Relay Protocol (MSRP)", RFC 4975, September 2007.
[RFC4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
September 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6072] Jennings, C. and J. Fischl, "Certificate Management
Service for the Session Initiation Protocol (SIP)",
RFC 6072, February 2011.
[RFC6135] Holmberg, C. and S. Blau, "An Alternative Connection Model
for the Message Session Relay Protocol (MSRP)", RFC 6135,
February 2011.
11.2. Informative References
[RFC3724] Kempf, J., Austein, R., and IAB, "The Rise of the Middle
and the Future of End-to-End: Reflections on the Evolution
of the Internet Architecture", RFC 3724, March 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
[RFC6043] Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based
Modes of Key Distribution in Multimedia Internet KEYing
(MIKEY)", RFC 6043, March 2011.
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[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.
[GPP23228]
3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP
TS 23.228 10.5.0, June 2011.
[DANE] "DNS-based Authentication of Named Entities Work Group".
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Staffan Blau
Ericsson
Stockholm 12637
Sweden
Email: staffan.blau@ericsson.com
Eric Burger
Georgetown University
Department of Computer Science
37th and O Streets, NW
Washington, DC 20057-1232
United States of America
Phone:
Fax: +1 530 267 7447
Email: eburger@standardstrack.com
URI: http://www.standardstrack.com
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