I think the optional in the table is due to the fact that record-route on
18x/2xx is possible only if it was present in the INVITE.

Unfortunately, this is not readily obvious in the RFC2361 since 12.1.1 does
not refer to early dialog -

   When a UAS responds to a request with a response that establishes a
   dialog (such as a 2xx to INVITE), the UAS MUST copy all Record-Route
   header field values from the request into the response (including the
   URIs, URI parameters, and any Record-Route header field parameters,
   whether they are known or unknown to the UAS) and MUST maintain the
   order of those values.

However, in 13.3.1.1, it says -
13.3.1.1 Progress

   If the UAS is not able to answer the invitation immediately, it can
   choose to indicate some kind of progress to the UAC (for example, an
   indication that a phone is ringing).  This is accomplished with a
   provisional response between 101 and 199.  These provisional
   responses establish early dialogs and therefore follow the procedures
   of Section 12.1.1 in addition to those of Section 8.2.6.

So, it can be divined by putting these together. (an academic exercise, of
sorts, for the reader!)

In fact, this posting was made last week for the question of CONTACT on a
provisional response.
See (thread on RFC3261 says UAC can send BYE for an early dialog?)

-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Brett Tate
Sent: Wednesday, July 24, 2002 7:09 PM
To: 'Baniel Uri-CUB001'; [EMAIL PROTECTED]
Subject: RE: [Sip-implementors] record-route in 183?


> Is record-route returned in 183?
> (or only in the final response that follows
> such as 200)?

Table 3 of rfc3261 reflects that record-route
is optional within 18x/2xx.

If you plan to record-route the 2xx, you
should also record-route the 18x.  Otherwise
requests after 18x and prior to 2xx will
bypass your proxy.  Such requests might be
BYE, UPDATE, INFO, PRACK, etcetera.

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