Cisco

Edison NJ US jdrosen@cisco.com http://www.jdrosen.net

RAI SIP SIP NAT This specification defines a media feature tag and an option tag for use with the Session Initiation Protocol (SIP). The media feature tag allows a UA to communicate to its registrar that it supports ICE. The option tag allows a User Agent (UA) to require support for ICE in order for a call to proceed.

RFC 3264 defines a two-phase exchange of Session Description Protocol (SDP) messages for the purposes of establishment of multimedia sessions. This offer/answer mechanism is used by protocols such as the Session Initiation Protocol (SIP). Protocols using offer/answer are difficult to operate through Network Address Translators (NAT). Because their purpose is to establish a flow of media packets, they tend to carry IP addresses within their messages, which is known to be problematic through NAT . To remedy this, an extension to SDP, called Interactive Connectivity Establishment (ICE) has been defined . ICE defines procedures by which agents gather a multiplicity of addresses, include all of them in an SDP offer or answer, and then use peer-to-peer Simple Traversal Underneath NAT (STUN) connectivity checks to determine a valid address. This specification defines a media feature tag, "sip.ice", and a SIP option tag, "ice", that can be used by SIP user agents that make use of ICE. motivates the need for the media feature tag and option tag, and and formally define them.

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 RFC 2119.

There are two primary motivations for defining an option tag and a media feature tag. They are support for gateways, and requiring ICE for a call.

Unfortunately, ICE requires both endpoints to support it in order for it to be used. Within a domain, there will typically be user agents that do and do not support ICE. In order to facilitate deployment of ICE, it is anticipated that domains will make use of gateways which act as ICE agents on one side, an non-ICE agents on the other side. This would allow a call from domain A into domain B to make use of ICE, even if the device in domain B does not itself yet support ICE. However, when domain B receives a call, it will need to know whether the call needs to pass through such a gateway, or whether it can go to the terminating UA directly. In order to make such a determination, this specification defines a media feature tag, "sip.ice", which can be included in the Contact header field of a REGISTER request . This allows the registrar to track whether a UA supports ICE or not. This information can be accessed by a proxy in order to determine whether a call needs to route through a gateway or not.

Although ICE provides a built in fall back to non-ICE operation when the answerer doesn't support it, there are cases where the offerer would rather abort the call rather than proceed without ICE. Typically, this is because they would like to choose a different m/c-line address for a non-ICE peer than they would for an ICE capable peer. To do this, the "ice" SIP option tag can be included in the Require header field of an INVITE request.

The "sip.ice" media feature tag indicates support for ICE. An agent supports ICE if it is either a lite or full implementation, and consequently, is capable of including candidate attributes in an SDP offer or answer for at least one transport protocol. An agent that supports ICE SHOULD include this media feature tag in the Contact header field of its REGISTER requests and OPTION responses. An agent MAY include the media feature tag in the Contact header field of an INVITE or INVITE response; however, doing so is redundant with ICE attributes in the SDP which indicate the same thing. In cases where an INVITE omits an offer, the lack or presence of the media feature tag in the Contact header field cannot be used by the callee (which will be the offerer) to determine whether the caller supports ICE. In cases of third party call control , the caller may be a controller that supports (or doesn't) ICE, while the answerer may be an agent which does (or doesn't) support ICE.

This "ice" OPTION tag SHOULD NOT be used in conjunction with the Supported header field (this SHOULD NOT includes responses to OPTIONS requests) The media feature tag is used as the one and only mechanism for indicating support for ICE. The option tag is meant to be used only with the Require header field. When placed in the Require header field of an INVITE request, it indicates that the UAS must support ICE in order to process the call. An agent supports ICE if it is either a full or lite implementation, and consequently, is capable of including candidate attributes in an SDP offer or answer for at least one transport protocol.

A malicious intermediary might attempt to modify a SIP message by inserting a Require header field containing the "ice" option tag. If ICE were not supported on the UAS, this would cause the call to fail when it would otherwise succeed. Of course, this attack is not specific to ICE, and can be done using any option tag. This attack is prevented by usage of the SIPS mechanism as defined in RFC 3261. Similarly, an intermediary might attempt to remove the media feature tag from a REGISTER request or OPTIONS request, which might cause a call to skip ICE processing when it otherwise might make use of it. This attack is also prevented using the SIPS mechanism.

This specification defines a new media feature tag and SIP option tag.

This section defines a new SIP option tag per the guidelines in Section 27.1 of RFC 3261. ice This option tag is used to identify the Interactive Connectivity Establishment (ICE) extension. When present in a Require header field, it indicates that ICE is required by an agent.

This section registers a new media feature tag in the SIP tree, defined in Section 12.1 of RFC 3840 . sip.ice 1.3.6.1.8.4.22 This feature tag indicates that the device supports Interactive Connectivity Establishment (ICE). Boolean. This feature tag is most useful in a communications application, for describing the capabilities of a device, such as a phone or PDA. Routing a call to a phone that can support ICE. RFC XXXX [[Note to IANA: Please replace XXXX with the RFC number of this specification.]] Security considerations for this media feature tag are discussed in of RFC XXXX . [[Note to IANA: Please replace XXXX with the RFC number of this specification.]]