Subject: RE: [rohc] sigcomp issue cabo-15

From: "Price, Richard" <[email-masked]>
Date: Fri, 15 Mar 2002 17:35:10 -0000

> > Is 1000 bits sufficiently close to the per-message overhead of an arbitrary
> > signalling protocol? If so then we should just fix it in the standard. If not
> > then we should leave it as one of the (non-negotiable) application-defined
> > parameters.
> 
> The per-message overhead becomes very small with ROHC compression of the
> headers.
> So this may not be the best guidance here.

We can certainly offer more CPU cycles than the per-message overhead would
normally dictate (particularly if ROHC is also in use), but this might introduce
a slight security risk from malicious endpoints who send a large number of
very small SigComp messages. Is this anything we need to worry about?

> > For a request-response protocol such as SIP the endpoint only needs to store
> > 2 copies of the UDVM memory as state for each remote sender. A similar amount
> > of memory is required by each compressor, so the total working memory for an
> > endpoint is roughly as follows:
> 
> I don't get it.  Why do you need to store a whole UDVM core for each remote
> sender?
> You'd store a circular buffer and some code, but 64K?

The reason for this is simply that the circular buffer typically expands to fill
however much UDVM memory is offered by the receiving endpoint. If 64K is available
then most compression algorithms will make good use of it!

The receiving endpoint will always need to store 2 UDVM cores for each remote
sender, so if it offers a 64K UDVM then it will need to make sure that 128K of
state information can be stored for each sender. If this is not possible due to
memory constraints then the endpoint should offer a UDVM_memory_size of less than
64K.

It might be useful to plot a graph of e.g. DEFLATE performance vs memory required
at a SigComp endpoint, so that we can see whether it is possible to fix the amount
of working memory offered by all implementations of SigComp.

Regards,

Richard

---

From: "Dr. Carsten Bormann" <[email-masked]>
Date: Fri, 15 Mar 2002 19:56:03 +0100

Actually, I have no idea how SIP exchanges would grow to ever benefit from
64K.
This means you would have 40 1500 byte SIP packets to look at.
Hmm, does that help?

Gruesse, Carsten

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