Subject: Re: draft-ietf-pppext-sonet-ds-00.txt

From: Stuart Venters <[email-masked]>
Date: Thu, 20 Nov 97 9:50:26 CST

Bill,

I have read you draft "draft-ietf-pppext-sonet-ds-00.txt" and thought about
it and i have two fundamental suggestions concerning it.

First the "Applicability" section. IMHO, (and given the benefit of 20-20
hind site,) I think you are solving the wrong problem.  While there may
be a need for an intra-campus Sonet spec, there is definitely a need for
a full wide area spec which is fully Sonet compatible.  Wouldn't overall
interoperability be better served if we gave up interoperability with the
original 1619 and cut over to some other scheme which is fully compatible
with the actual wide area Sonet installed base?

Second, the "Prophylactic Octet Stuffing" section.  I still think
some sort of scrambler is the best final solution, but if you insist that 
interoperability with the original 1619 is an overiding concern, then i
see two major holes in this section.

One is that the scheme is too predictable.  If an 
attacker can guess how the countermeasures will react then he has a high
probability of defeating them.  Perhaps the stuffing threshold should be
random?

The other is the limited choice of expected attack patterns.  Consider an
attack where the byte pattern on the fiber after the sonet scrambler is 
mostly zeros with a one sprinkled in every 10 to 20 bits.  The pattern
detector would see nothing but the physical fiber interfaces would have a
problem due to the duty cycle of the signal.

Cheers,
Stuart Venters
(The views expressed are not necessarily those of mu employer.)