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

From: "Tom Taylor" <[email-masked]>
Date: Mon, 1 Dec 1997 09:23:53 -0500

We at Nortel wish to add our support to the position expressed in 
draft-manchester-pppext-transper-00.txt.  We have put two contributions 
into ANSI T1X1.5, quantifying the exposures resulting from failure to 
scramble, and comparing the effectiveness of various possible solutions.

In brief summary:
*	outages can be caused by adverse patterns even when interleaved with
other traffic on higher speed systems; 
*	the problem can't be fixed by tightening up timing in the SONET/SDH
systems, because that would increase vulnerability to jitter;
*	prophylactic byte stuffing can be countered by the attacker;
*	set/reset scrambling as proposed in
draft-ietf-pppext-pppsonet-scrambler-00.txt is overly complex and forces
close coupling between the SONET/SDH path and scrambling functions.

The conclusions of our submissions to ANSI T1X1.5 are as follows:

Because scrambling offers the best security in the considered solution
set, it 
is selected.

Because of the numerous advantages with respect to set-reset scramblers
of 
self-synchronous scramblers, a self-synchronous scrambler represents the
best solution.

Because of its selection as the scrambler for the ATM cell payload
mapping 
into SONET/SDH, the 1 + x**43 polynomial is familiar. It is also simple,
which 
makes it an obvious choice as a solution with which interoperability can
quickly 
and easily be achieved.

The most effective implementation would scramble the entire HDLC frame. 
This removes any concerns with deterministic patterns due to HDLC
overhead, 
or due to post-scrambling escape byte insertion.

Because of its robustness relative to CRC-16, CRC-32 should be used as
the 
HDLC Frame Check Sequence.