| SIMPLE Working Group | C.H. Holmberg |
| Internet-Draft | S.B. Blau |
| Intended status: Standards Track | Ericsson |
| Expires: March 01, 2012 | E.W. Burger |
| Georgetown University | |
| August 29, 2011 |
Connection Establishment for Media Anchoring (CEMA) for the Message Session Relay Protocol (MSRP)
draft-ietf-simple-msrp-cema-01.txt
This document defines an 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, a=msrp-cema, that MSRP endpoints use to indicate support of the CEMA extension.
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Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
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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, lawful intercept, 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. Since the active MSRP UA establishes the MSRP TCP or TLS connection based on the MSRP URI of the SDP a=path attribute, this means that the MSRP connection will not, unless the Middlebox also modifies the MSRP URI of the topmost SDP a=path attribute, be routed through the Middlebox. In many scenarios this will prevent the MSRP connection from being established. In addition, if the Middlebox modifies the MSRP URI of the SDP a=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 a=path attribute, will fail. Also the matching will fail if Middleboxes modify the address information in the MSRP URI of the SDP a=path attribute.
The only way to achieve interoperability in this situation is for the Middlebox to act as a 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 without a need for the Middleboxes to be a MSRP B2BUA. In such cases, Middleboxes that want to anchor the MSRP connection simply modify the SDP c/m-line address information, similar to what it does 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 fully backward compatible. 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.
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 TLS certificate and sends a certificate fingerprint in SDP.
Name Based TLS Authentication: An MSRP endpoint that uses a certificate from a well known certificate authority and the other endpoint matches the hostname in the received TLS communication SubjectAltName parameter towards the hostname received in the MSRP URI in SDP.
B2BUA: This is an abbreviation for back-to-back user agent.
MSRP B2BUA: A network element that terminates a 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 a 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.
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.
This document defines an 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, a=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 section defines how an MSRP endpoint that supports the CEMA extension generates SDP offers and answers for MSRP, and what SDP information elements the MSRP endpoint uses when creating the TCP or TLS connection for the MSRP messages.
In the following cases, where there is a Middlebox in the network, the CEMA extension can not be used, 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".
- A non-CEMA-enabled MSRP endpoint uses a relay for its MSRP communication.
- A CEMA-enabled MSRP endpoint that uses a relay for its MSRP communication becomes "active".
When a CEMA-enabled MSRP endpoint sends an SDP offer for MSRP, it generates the SDP offer according to the procedures in RFC 4975. In addition, the endpoint follows RFC 4976 if it is using a relay for MSRP communication. The endpoint also performs the following additions and modifications:
1. The MSRP endpoint MUST include an SDP a=msrp-cema attribute in the MSRP media description of the SDP offer.
2. If the MSRP endpoint is not using a relay for MSRP communication, it MUST include an SDP a=setup attribute in the MSRP media description of the SDP offer, according to the procedures in RFC 6135 [RFC6135].
3. If the MSRP endpoint is using a relay for MSRP communication, it MUST include the address information of the relay (the MSRP URI of the topmost SDP a=path attribute), 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 a=setup:actpass attribute in the MSRP media description of the SDP offer.
When the MSRP endpoint receives the associated SDP answer, the SDP answer indicates that the remote MSRP endpoint accepted the offered MSRP media if the port number of the MSRP media description is not zero.
If the MSRP media description of the SDP answer does not contain an SDP a=msrp-cema attribute, the MSRP endpoint MUST check the criteria below. If either or both of the criteria is met, the MSRP endpoint 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 a=msrp-cema attribute.
1. The SDP c/m-line address information associated with the MSRP media description does not match the information in the MSRP URI of the topmost SDP a=path attribute, and the MSRP media description contains an SDP a=setup:active attribute (indicating that the remote MSRP endpoint is "active").
2. The MSRP media description contains multiple SDP a=path attributes, indicating the use of MSRP relays.
NOTE: In the absence of the SDP a=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 MSRP endpoint MAY choose to terminate the session establishment if it can detect that a Middlebox acting as a MSRP B2BUA is not the desired remote endpoint.
The MSRP endpoint 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.
In all other cases, where the MSRP endpoint becomes "active", it MUST use the SDP c/m-line for establishing the MSRP TCP or TLS connection. If the MSRP endpoint becomes "passive", it will wait for the remote MSRP endpoint to establish the connection, according to the procedures in RFC 4975.
If any of the criteria below is met, the MSRP endpoint MUST fallback to RFC 4975 behavior and generate the associated SDP answer according to the procedures in RFC 4975 and RFC 4976. The MSRP endpoint MUST NOT insert an SDP a=msrp-cema attribute in the MSRP media description of the SDP answer.
1. Both MSRP endpoints are using relays for MSRP communication. An endpoint can detect the remote MSRP endpoint is using a relay for MSRP communication if the MSRP media description of the SDP offer contains multiple SDP a=path attributes.
2. The remote MSRP endpoint uses a relay for MSRP communication, and will become "active" either by default or if the MSRP media description of the SDP offer contains an SDP a=setup:active attribute. Note that a CEMA-enabled endpoint would include an SDP a=setup:actpass attribute in the SDP offer, as described in Section 4.2.
3. The MSRP endpoint uses a relay for MSRP communication and is not able to become "passive". The indication for this is the MSRP media description of the offer contains an SDP a=setup:passive attribute. Note that an MSRP endpoint is now allowed to include an SDP a=setup:passive attribute in an SDP offer, as described in RFC 6135.
4. The MSRP media description of the SDP offer does not contain an SDP a=msrp-cema attribute, the SDP c/m-line address information associated with the MSRP media description does not match the information in the MSRP URI of the topmost SDP a=path attribute, and the remote MSRP endpoint will become "active", either by default, or if the MSRP media description of the SDP offer contains an SDP a=setup:active attribute.
In all other cases, the MSRP endpoint generates the associated SDP answer according to the procedures in RFC 4975 and RFC 4976, with the following additions and modifications:
1. The MSRP endpoint MUST include an SDP a=msrp-cema attribute in the MSRP media description of the SDP answer.
2. If the MSRP endpoint is not using a relay for MSRP communication, it MUST include an SDP a=setup attribute in the MSRP media description of the answer, according to the procedures in RFC 6135.
3. If the MSRP endpoint is using a relay for MSRP communication, it MUST include the address information on the relay (the MSRP URI of the topmost SDP a=path attribute), 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 a=setup:passive attribute in the MSRP media description of the SDP answer.
If the MSRP endpoint included an SDP a=msrp-cema attribute in the MSRP media description of the SDP answer, and if the MSRP endpoint becomes "active", it MUST use the received SDP c/m-line for establishing the MSRP TLS connection. If the MSRP endpoint becomes "passive", it will wait for the remote MSRP endpoint to establish the TCP or TLS connection, according to the procedures in RFC 4975.
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 a 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.
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.
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 remote endpoint 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.
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.
This document assumes that Middleboxes are able to modify the SDP address information associated with the MSRP media. Middleboxes cannot be deployed in environments that require end-to-end SDP protection using SIP identity [RFC4916].
When UAs use the CEMA extension, Middleboxes relay MSRP media packets at the transport layer. The TLS handshake and resulting security association (SA) are 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 fallback to RFC 4975 behavior Middleboxes act as TLS B2BUAs, meaning that separate SAs are established between the Middlebox and each MSRP endpoint. 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.
In some cases, where MSRP B2BUA functionality does not need to be enabled, the CEMA extension makes it easier for a man in the middle (MiTM) to transparently insert itself in the communication between MSRP endpoints in order to monitor or record unprotected MSRP communication. It does not however make it easier for a MiTM to monitor TLS protected MSRP, or in any significant way modify TLS protected MSRP content or even find out that the packets contain MSRP messages, since that would require the MiTM to implement MSRP B2BUA functionality, no matter if UAs support the CEMA extension or not. It would thus require the MiTM to terminate the TCP/TLS/MSRP connection in both directions. MSRP endpoints SHOULD use encrypted channels, if possible. For backward compability, a CEMA-enabled MSRP endpoint MUST implement TLS.
The CEMA extension supports the usage of name-based authentication for TLS in the presence of Middleboxes.
If a Middlebox acts as a TLS B2BUA, MSRP endpoints will be able to use fingerprint based authentication for TLS, no matter if they support the CEMA extension or not. In such cases, as the Middlebox acts as TLS endpoints, MSRP endpoints might be given an incorrect impression that there is an end-to-end security association (SA) between the MSRP endpoints.
If a Middlebox does not act as a TLS B2BUA, fingerprint based authentication will not work, as the "SIP Identity" based integrity protection of SDP will break. Therefore, in addition to the authentication mechanisms defined in RFC 4975, it is RECOMMENDED that a CEMA-enabled MSRP endpoint also supports an authentication mechanism that does not rely on peer-to-peer SDP integrity.
It is RECOMMENDED that an MSRP endpoint support one of the following authentication mechanisms:
1. TLS certificates together with support of interacting with a Certificate Management Service [RFC6072], to which it publishes the public version of its own self-signed certificate and from which it fetches on demand the public certificates of other endpoints.
2. TLS-PSK managed by MIKEY-TICKET Based Key Management and Key Management Service [RFC6043]. Note that 3GPP has specified the MIKEY-TICKET based Key Management and Key Management Service authentication mechanism for the IP Multimedia Subsystem (IMS). Thus it will be available in that environment.
When an MSRP endpoint generates an SDP offer for MSRPS, in addition to the SDP attributes associated with the TLS authentication mechanisms described in RFC 4975, it MUST include any information elements associated with the other authentication mechanisms that it supports.
Unless the MSRP endpoints are able to use name-based authentication, and they support a common authentication mechanism, they MUST use that mechanism. If the MSRP endpoints do not support such common authentication mechanism, they MUST try fingerprint-based authentication, which will succeed if there are no Middleboxes present. If that also fails, the MSRP endpoints MUST either:
1. Consider the TLS authentication as failed, in accordance with RFC 4975; or
2. If something like SIPS protects the SIP signaling between the MSRP endpoints, use fingerprint based authentication without requiring peer-to-peer SDP integrity, and thus trust the network endpoints in the signaling path for SDP integrity.
As defined in RFC 4975, if TLS authentication fails, the user needs to be able to decide whether to try to establish an MSRP connection in the likely scenario of intercepted, altered,
One of the side effects of relieving Middleboxes from manipulating message content is CEMA provides an environment necessary for end-to-end integrity of MSRP media. However, while CEMA provides a prerequisite for end-to-end integrity, it is not sufficient.
CEMA recommends using an integrity-protected media channel, such as TLS. As defined in RFC 4975, all MSRP endpoints MUST support TLS. That applies also to CEMA-enabled endpoints.
One issue with usage of TLS is the availability of a certificate infrastructure. Endpoints can always provide self-signed certificates. However, this is problematic in that any endpoint can masquerade as another, by providing a self-signed certificate with the victim's information.
One of the target deployments for CEMA is the 3GPP IMS SIP network. In this environment service providers provision signed certificates or manage signed certificates on behalf of their subscribers. This does require trusting the service provider, but those issues are beyond the scope of this document.
Alternate key distribution mechanisms, such as DANE [DANE], PGP [RFC6091], or some other technology, might become ubiquitous enough to solve the key distribution problem in the future.
Even with seemingly end-to-end media integrity, at the time of the publication of this document there are other vulnerabilities in MSRP, due to vulnerabilities in the SIP signaling. If there are no integrity protections on the SIP signaling, it is easy to 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.
This section registers a new SDP attribute, a=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: a=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 sending UA supports the CEMA
mechanism.
Values: The attribute does not carry a value
Charset dependency: none
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, and Adrian Georgescu for their guidance and input in order to produce this document.
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-simple-msrp-sessmatch-13
Changes from draft-ietf-simple-msrp-sessmatch-12
Changes from draft-ietf-simple-msrp-sessmatch-11
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| [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, 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.B. Roach, "Relay Extensions for the Message Sessions Relay Protocol (MSRP)", RFC 4976, September 2007. |
| [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. |
| [RFC3724] | Kempf, J., Austein, R., 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. |
| [RFC4916] | Elwell, J., "Connected Identity in the Session Initiation Protocol (SIP)", RFC 4916, June 2007. |
| [RFC6043] | Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based Modes of Key Distribution in Multimedia Internet KEYing (MIKEY)", RFC 6043, March 2011. |
| [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", . | , "