Subject: RE: [rohc] RE: The 'b'-bit

From: "Price, Richard" <[email-masked]>
Date: Fri, 15 Mar 2002 11:47:00 -0000

> > The only requirement is that the identity of the
> > sending endpoint MUST be verified by the application just as for a
> > state creation request.
> 
> A quick question: is this about case a), b), or c)? 
> 
> a) determine if a *received* announcement sent by remote
> endpoint is legitimate;
> b) determine if an announcement *created* locally by a 
> received sigcomp message is allowed to be sent back to 
> the remote endpoint that sent the sigcomp message
> c) both a) and b)

The above statement covers case a) only. SigComp-05 leaves case b) as
an implementation decision at the compressor - once a received
message has been authenticated as per case a) then the co-located
compressor can choose to send some announcement data in return without
requiring a further security check at the same endpoint.

> I'm asking this because this thread started as case b) 
> (see below), but it seems what you said above is about a)?
> 
> endpoint 1                     endpoint 2
>                  m(S0)
> map M to S1  ------------>    create M, and S1=S0+M
> 
>         announcement: b=0, shared_id=M
>            <--------------------

I think that the above diagram is missing the 's' bit, which is sent in
the first message to indicate that Endpoint 1 has saved message M as
state. In fact the thread is not concerned with whether the 'b' bit is
discarded, but whether the 's' bit is discarded. This is case a) as
described above.

Looking at this in more detail, in order to set the 'b' bit to 0 the
following two conditions must be satisfied:

i)  The announcement request for the 's' bit must succeed
    (otherwise shared_id cannot be set to M)

ii) The state creation request for S1=S0+M must fail

In SigComp-05 however, if the state creation request fails then the
announcement request fails also. This prevents the 'b' bit from ever
being set to 0, because if condition ii) is met then the 's' bit is
discarded and so condition i) is not met.

Regards,

Richard