[salud] Normative change to proxy behavior

[[e-mail] (Dale R. Worley)]

Since we're being particular about listing normative changes, I
thought we'd want to be more explicit about the change we're
permitting for proxies:

   14.  Proxy Behaviour

   A A SIP proxy MAY add an Alert-Info header field if none is present,
   and MAY add or remove URNs to an Alert-Info header field in a SIP
   request or a provisional 1xx response (excepting a 100 response) when
   it needs to provide additional information about the call or about
   the provided service.

By 3261, proxies are allowed to append a new header field value to a
header field that allows multiple values.  (This is done by an implied
redirection to an internal UA which provides a 4xx response with
"Contact:  <whatever?Alert-Info=urn:alert:additional:value>".)  But
removing header field values is not generally allowed.  Section 14 of
our draft enables removing values and adding values that are not at
the end of the value list.

We should document this change in the final paragraph of the abstract
and section 4 by adding something like this:

   Abstract

   This document normatively updates the RFC 3261, which defines the
   Session Initiation Protocol (SIP): It changes the usage of the Alert-
   Info header field defined in the RFC 3261 by additionally allowing
   its use in all provisional responses to INVITE (except the 100
   response).
+  This document also permits proxies to add and remove header field
+  values from the Alert-Info header field.

   4.  Update to RFC 3261

+  4.3.  Proxies may alter Alert-Info header fields
+
+  A SIP proxy MAY add or remove URNs to an Alert-Info header field in
+  a SIP request or a provisional 1xx response (excepting a 100
+  response).

Dale