What does it mean for a MAP to be "mandatory"?

I believe there has already been some discussion of this, and it relates 
mainly to the WSDL binding in any case, but it also has to do with the 
semantics of MAPs as (currently) defined in the core.  I've approached 
much the same issue from a different angle previously in [1].

Section 4 of the WSDL binding begins

"This section describes which of the core message properties are 
mandatory or optional for messages in the various MEPs defined by WSDL 
1.1 and WSDL 2.0."

What does "mandatory" mean in this context?  Does it mean that 
<wsa:ReplyTo> and <wsa:MessageID> headers MUST appear in a request 
message, or would it mean that it MUST be possible for the receiver of 
the request to determine values from them (whether they are present as 
SOAP headers or not), or does it mean something else?

As far as I can tell, existing request/reply services would /not/ be WSA 
compliant in either case, as there generally isn't a message ID in an 
HTTP request, nor any clear way to construct one, nor any particular 
need to construct one.

One reason for asking this is to understand better the migration path 
from current request/reply services to WSA aware "async request/reply" 
services.

[1] 
http://lists.w3.org/Archives/Public/public-ws-addressing/2005Mar/0171.html