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IETF RFC 6714
Last modified on Saturday, August 25th, 2012
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Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 6714 S. Blau
Category: Standards Track Ericsson
ISSN: 2070-1721 E. Burger
Georgetown University
August 2012
Connection Establishment for Media Anchoring (CEMA)
for the Message Session Relay Protocol (MSRP)
 Abstract
This document defines a Message Session Relay Protocol (MSRP)
extension, Connection Establishment for Media Anchoring (CEMA).
Support of this extension is OPTIONAL. The extension allows
middleboxes to anchor the MSRP connection, without the need for
middleboxes to modify the MSRP messages; thus, it also enables secure
end-to-end MSRP communication in networks where such middleboxes are
deployed. This 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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6714.
Holmberg, et al. Standards Track PAGE 1 
RFC 6714 MSRP-CEMA August 2012
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Holmberg, et al. Standards Track PAGE 2
RFC 6714 MSRP-CEMA August 2012
Table of Contents
1. Introduction ....................................................3
2. Conventions .....................................................5
3. Applicability Statement .........................................6
4. Connection Establishment for Media Anchoring Mechanism ..........7
4.1. General ....................................................7
4.2. MSRP SDP Offerer Procedures ................................8
4.3. MSRP SDP Answerer Procedures ...............................9
4.4. Address Information Matching ..............................11
4.5. Usage with the Alternative Connection Model ...............12
5. The SDP 'msrp-cema' Attribute ..................................12
5.1. General ...................................................12
5.2. Syntax ....................................................12
6. Middlebox Assumptions ..........................................13
6.1. General ...................................................13
6.2. MSRP Awareness ............................................13
6.3. TCP Connection Reuse ......................................13
6.4. SDP Integrity .............................................14
6.5. TLS .......................................................14
7. Security Considerations ........................................14
7.1. General ...................................................14
7.2. Man-in-the-Middle (MITM) Attacks ..........................15
7.3. TLS Usage without Middleboxes .............................16
7.4. TLS Usage with Middleboxes ................................16
7.5. Authentication, Credentials, and Key Management ...........16
7.6. Endpoint Procedures for TLS Negotiation ...................17
7.7. Fingerprint-Based Authentication ..........................18
8. IANA Considerations ............................................19
8.1. IANA Registration of the SDP 'msrp-cema' Attribute ........19
9. Acknowledgements ...............................................20
10. References ....................................................20
10.1. Normative References .....................................20
10.2. Informative References ...................................21
1. Introduction
The Message Session Relay Protocol (MSRP) [RFC 4975] expects to use
MSRP relays [RFC 4976] as a means for Network Address Translation
(NAT) traversal and policy enforcement. However, many Session
Initiation Protocol (SIP) [RFC 3261] networks, which deploy MSRP,
contain middleboxes. These middleboxes anchor and control media;
perform tasks such as NAT traversal, performance monitoring, and
address domain bridging; interconnect Service Level Agreement (SLA)
policy enforcement; and so on. One example is the Interconnection
Holmberg, et al. Standards Track PAGE 3
RFC 6714 MSRP-CEMA August 2012
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 [RFC 4975] and RFC 4976 [RFC 4976], 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) [RFC 4566]
c/m-line in order to anchor media. An "active" [RFC 6135] MSRP User
Agent (UA) establishes the MSRP TCP or Transport Layer Security (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 [RFC 4975], 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 toward the actual remote
endpoint. However, the enabling of MSRP B2BUA functionality requires
substantially more resource usage in the middlebox, which normally
results in a negative impact on performance. In addition, the MSRP
message needs to be exposed in cleartext to the MSRP B2BUA, which
violates the end-to-end principle [RFC 3724].
This specification defines an MSRP extension, Connection
Establishment for Media Anchoring (CEMA). In most cases, CEMA allows
MSRP endpoints to communicate through middleboxes as defined in
Section 2, without a need for the middleboxes to be MSRP B2BUAs. 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.
Holmberg, et al. Standards Track PAGE 4
RFC 6714 MSRP-CEMA August 2012
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
[RFC 2119].
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 that 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 that the endpoint trusts. This definition
includes traditional certificates issued by a well-known
certification authority as well as self-signed certificates
published via the SIP Certificate Management Service [RFC 6072] and
other similar mechanisms.
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 toward
another MSRP endpoint. Note that the MSRP B2BUA is distinct from
a SIP B2BUA. A SIP B2BUA terminates a SIP session and
reoriginates that session toward another SIP endpoint. In the
Holmberg, et al. Standards Track PAGE 5
RFC 6714 MSRP-CEMA August 2012
context of MSRP, a SIP endpoint initiates a SIP session toward
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 an MSRP B2BUA. Other SIP-related functions -- such as
those related to routing, modification of SIP information, etc. --
performed by the Middlebox, and whether it acts as a SIP B2BUA or
not, are outside the scope of this document. Section 6 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 media anchor 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 this extension is OPTIONAL. The extension allows
Middleboxes to anchor the MSRP connection, without the need for
Middleboxes to modify the MSRP messages; thus, it also enables 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.
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
Holmberg, et al. Standards Track PAGE 6
RFC 6714 MSRP-CEMA August 2012
enable MSRP B2BUA functionality. An example of such a 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 that operate in other networks
but that communicate with MSRP endpoints in networks with such
Middleboxes, unless there is a gateway between these networks that by
default always enables 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" [RFC 6135] (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" [RFC 6135], as it cannot be assumed
that the MSRP endpoint inserts the address information of the
relay 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 toward the Middlebox), the session matching [RFC 4975]
performed by the remote MSRP endpoint will fail.
Holmberg, et al. Standards Track PAGE 7
RFC 6714 MSRP-CEMA August 2012
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 [RFC 6135].
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 its own address
information, 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 one or more of the criteria below are met, the
offerer MUST fall back 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 (see Section 4.4) the
information in the MSRP URI of the 'path' attribute(s) (in which
case it is assumed that the SDP c/m-line contains the address of
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-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 it is assumed that the SDP
Holmberg, et al. Standards Track PAGE 8
RFC 6714 MSRP-CEMA August 2012
c/m-line contains the address of 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 it is assumed that the SDP c/m-line contains the
address of 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 6, 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
[RFC 3261], 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 fall back to RFC 4975 behavior, but it does not need to send a
new SDP offer.
In other cases, where none of the criteria above are 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.
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
Holmberg, et al. Standards Track PAGE 9
RFC 6714 MSRP-CEMA August 2012
zero port value number in the generated SDP answer) or reject the
whole SIP request that carries the SDP offer with a 488 Not
Acceptable Here [RFC 3261] response.
NOTE: The reason for the rejection is that the answerer assumes that
a middlebox that does 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 one or more of the criteria below are met, the answerer MUST
fall back 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.
Holmberg, et al. Standards Track PAGE 10
RFC 6714 MSRP-CEMA August 2012
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:
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 its own address
information, 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 an FQDN, the
IP address is retrieved using DNS, according to the procedures in
Section 6.2 of RFC 4975.
Holmberg, et al. Standards Track PAGE 11
RFC 6714 MSRP-CEMA August 2012
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 [RFC 5952].
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
[RFC 5234].
attribute =/ msrp-cema-attr
;attribute defined in RFC 4566
msrp-cema-attr = "msrp-cema"
Holmberg, et al. Standards Track PAGE 12
RFC 6714 MSRP-CEMA August 2012
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 how such Middleboxes behave.
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 toward the wrong destination.
Holmberg, et al. Standards Track PAGE 13
RFC 6714 MSRP-CEMA August 2012
6.4. SDP Integrity
This document assumes that Middleboxes are able to modify the SDP
address information associated with the MSRP media.
NOTE: Even though 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
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 cleartext
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 SAs 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.
Holmberg, et al. Standards Track PAGE 14
RFC 6714 MSRP-CEMA August 2012
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
signaling is integrity protected, both endpoints can verify that the
TLS SA is established with the correct host by matching the received
certificate against the received fingerprint.
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 [RFC 4474].
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 signaling path. In other words, end clients must
accept the fact that every signaling 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 [RFC 6072]. 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 that protect the SDP across several hops.
7.2. Man-in-the-Middle (MITM) Attacks
If TLS is not used to protect MSRP, the CEMA extension might make it
easier for a MITM 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 that TLS [RFC 5246] be used. It is also
recommended that TLS be used end to end, which CEMA enables even in
the case of Middleboxes. According to RFC 4975, MSRP endpoints are
required to support TLS. This also applies to CEMA-enabled
endpoints.
Holmberg, et al. Standards Track PAGE 15
RFC 6714 MSRP-CEMA August 2012
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
requires signaling integrity protection. This can, for example, 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 those without the CEMA extension. In such a case, the
Middlebox acts as a TLS endpoint.
If a Middlebox does not act as a TLS B2BUA, TLS is end to end 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 or not the Middlebox acts as a TLS
B2BUA. 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 the 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.
Holmberg, et al. Standards Track PAGE 16
RFC 6714 MSRP-CEMA August 2012
Therefore, in addition to the authentication mechanisms defined in
RFC 4975, it is RECOMMENDED that a CEMA-enabled MSRP endpoint also
support self-signed certificates together with the Certificate
Management Service [RFC 6072], 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 DNS-Based
Authentication of Named Entities (DANE) [DANE], Pretty Good Privacy
(PGP) [RFC 6091], Ticket-Based Modes of Key Distribution in Multimedia
Internet KEYing (MIKEY-TICKET) [RFC 6043], 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 are
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.
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 mechanisms
and algorithms that both support. The mechanisms can be divided into
three different security levels:
1. MSRPS: Security mechanisms that do not rely on trusted signaling,
such as name-based authentication
2. MSRPS: Mechanisms that do rely on trusted signaling, such as
fingerprint-based authentication
3. MSRP: Unprotected
If the endpoint uses security mechanisms that do not rely on trusted
signaling, the endpoint can detect if a Middlebox that acts as a
B2BUA is inserted. It is therefore RECOMMENDED that such a mechanism
be used.
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.
Holmberg, et al. Standards Track PAGE 17
RFC 6714 MSRP-CEMA August 2012
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 [RFC 6072] provides end-to-end security
without relying on trust in the signaling plane 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 plane, 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, for example, be hop-by-
hop cryptographic protection or cryptographic access protection
combined 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, for
example, be a closed enterprise network or a network operated by an
operator that the end user trusts. In IMS, for example, 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-based authentication.
When an endpoint receives a fingerprint, that fingerprint 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 provide a warning
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 to be the exception -- hence the
suggested warning.
Holmberg, et al. Standards Track PAGE 18
RFC 6714 MSRP-CEMA August 2012
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 (a Secure Real-time Transport Protocol (SRTP) extension for
Datagram Transport Layer Security (DTLS)) [RFC 5764] for Voice over IP
(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 that the remote endpoint has any relationship to the
expected endpoint. Securing the SIP signaling does not solve all
problems. For example, in a SIP Secure (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.
8. IANA Considerations
8.1. IANA Registration of the SDP 'msrp-cema' Attribute
IANA has added an attribute to the 'att-field (media level only)'
registry of the Session Description Protocol (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 as follows:
Contact name: Christer Holmberg
Contact email: christer.holmberg@ericsson.com
Attribute name: msrp-cema
Type of attribute: media level
Holmberg, et al. Standards Track PAGE 19
RFC 6714 MSRP-CEMA August 2012
Purpose: This attribute is used to indicate support of
the MSRP Connection Establishment for Media
Anchoring (CEMA) extension defined in
RFC 6714. 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 in restructuring the Security Considerations
section, based on feedback from the IESG.
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC 3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC 4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
September 2007.
Holmberg, et al. Standards Track PAGE 20
RFC 6714 MSRP-CEMA August 2012
[RFC 4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
for the Message Sessions Relay Protocol (MSRP)",
RFC 4976, September 2007.
[RFC 5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC 6072] Jennings, C. and J. Fischl, Ed., "Certificate Management
Service for the Session Initiation Protocol (SIP)",
RFC 6072, February 2011.
[RFC 6135] Holmberg, C. and S. Blau, "An Alternative Connection
Model for the Message Session Relay Protocol (MSRP)",
RFC 6135, February 2011.
10.2. Informative References
[RFC 3724] Kempf, J., Ed., Austein, R., Ed., 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.
[RFC 4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC 5764] 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.
[RFC 5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
[RFC 6043] Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based
Modes of Key Distribution in Multimedia Internet KEYing
(MIKEY)", RFC 6043, March 2011.
[RFC 6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.
Holmberg, et al. Standards Track PAGE 21
RFC 6714 MSRP-CEMA August 2012
[GPP23228] 3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP
TS 23.228 11.5.0, June 2012,
<http://www.3gpp.org/ftp/Specs/html-info/23228.htm>.
[DANE] "DNS-Based Authentication of Named Entities (DANE)
Working Group",
<https://datatracker.ietf.org/wg/dane/charter/>.
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
Fax: +1 530 267 7447
EMail: eburger@standardstrack.com
URI: http://www.standardstrack.com
Holmberg, et al. Standards Track PAGE 22
RFC TOTAL SIZE: 50543 bytes
PUBLICATION DATE: Saturday, August 25th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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