Clarifications for the use of REFER with RFC 6665
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The SIP REFER method relies on the SIP-Specific Event Notification framework.
That framework was revised by RFC 6665. This document highlights
the implications of the requirement changes in RFC 6665, and updates the definition of
the REFER method described in RFC 3515 to clarify and disambiguate the impact of those changes.
The SIP REFER method relies on the SIP-Specific Event Notification framework.
That framework was revised by . This document highlights
the implications of the requirement changes in RFC 6665, and updates to
clarify and disambiguate the impact of those changes.
Accepting a REFER request (without invoking extensions) results in an implicit
SIP-Events subscription. If that REFER was part of an existing dialog, the
implicit subscription creates a new, problematic dialog usage within that
dialog . The "norefersub" extension defined in asks to suppress this implicit subscription, but cannot
prevent its creation.
There are implementations in some known specialized environments (such as 3GPP) that use out-of-signaling agreements to
ensure that in-dialog REFER requests using the RFC 4488 extension do not create a new subscription inside that dialog. In the 3GPP
environment, the behavior is based on capabilities advertised using media feature tags. That mechanism does not, however, prevent
additional dialog usages when interoperating with implementations that do not support the mechanism. The extensions in
provide a standardized mechanism that allows avoiding any additional dialog usage.
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 .
Section 4.5.1 of makes GRUU mandatory
for notifiers to implement and use as the local target in the subscription created by the REFER request.
A user agent (UA) accepting a REFER that creates a subscription MUST populate its Contact header field with a GRUU.
A UA that might possibly become a notifier (e.g., by accepting a REFER
request that creates a subscription) needs to include a GRUU in the
Contact header field of dialog-forming and target-refresh methods (such as INVITE)
. This
ensures that out-of-dialog REFER requests corresponding to any
resulting INVITE dialogs arrive at this UA. Extensions can relax this
requirement by defining a REFER request that cannot create an implicit
subscription, thus not causing the accepting UA to become an RFC 6665 notifier
in the context of this dialog. is an example of such
an extension.
If a peer in an existing dialog has provided a GRUU as its Contact, sending
a REFER that might result in an additional dialog usage within that dialog
is prohibited. This is a direct consequence of requiring the use
of GRUU and the requirements in Section 4.5.2 of that document.
A user agent constructing a REFER request that could result in an implicit
subscription in a dialog MUST build it as an out-of-dialog message as defined
in , unless the remote endpoint is an older
implementation of RFC 3515 that has not been updated to conform to RFC 6665
(as determined by the absence of a GRUU in the remote target). Thus, the REFER
request will have no tag parameter in its To: header field.
Using the "norefersub"
option tag does not change this requirement, even
if used in a "Require" header field. Even if the recipient supports the
"norefersub" mechanism, and accepts the request with the option tag in the
"Require" header field, it is allowed to return a "Refer-Sub" header field
with a value of "true" in the response, and create an implicit subscription.
A user agent wishing to identify an existing dialog (such as for call transfer
as defined in ) MUST use the "Target-Dialog" extension
defined in to do so, and user agents accepting REFER
MUST be able to process that extension in requests they receive.
If a user agent can be certain that no implicit subscription will be created as a result of sending a REFER request (such as by requiring an extension that disallows any such subscription ), the REFER request MAY be sent within an existing dialog (whether or not the remote target is a GRUU). Such a REFER will be constructed with its Contact header field populated with the dialog's local URI as specified in Section 12 of .
As described in Section 4.5.2 of , there are cases
where a user agent may fall back to sharing existing dialogs for
backwards-compatibility purposes. This applies to a REFER only when the peer
has not provided a GRUU as its Contact in the existing dialog (i.e., when the
peer is an implementation of RFC 3515 that has not been updated to conform
with RFC 6665).
Section 8.3.1 of requires that elements not send a 202 response code to a subscribe request, but use the 200 response code instead. Any 202 response codes received to a subscribe request are treated as 200s. These changes also apply to REFER. Specifically, an element accepting a REFER request MUST NOT reply
with a 202 response code and MUST treat any 202 responses received as identical to a 200 response. Wherever requires sending a 202 response code, a 200 response code MUST be sent instead.
This document introduces no new security considerations directly. The updated considerations in apply to the implicit subscription created by an accepted REFER request.
Christer Holmberg provided the formulation for the final paragraph of the introduction. Christer Holmberg and Ivo Sedlacek provided detailed comments during working group discussion of the document.