Subject: A comment on the siganling part of draft-ietf-pwe3-fcs-retention- 00.txt

From: Sasha Vainshtein <[email-masked]>
Date: Thu, 27 Nov 2003 09:20:10 +0200

Andy and all,

IMHO, there is a couple of (inor) problems with the proposed 
signaling of FCS retention in draft-ietf-pwe3-fcs-retention-00.txt.

1. PEs only signal that they agree to receive retained FCS as the proposed 
   interface attribute does not have any value, just an ID. This is OK with 
   Ethernet, but the draft also discusses FR (port mode), HDLC and PPP - 
   and, AFAIK, these may use 16-bit or 32-bit FCS (even if 32-bit FCS is
rare). 
   Surely retaining a 16-bit FCS value coming from the AC at one end of the
PW 
   will not be of much use at the other end if the corresponding AC uses 
   32-bit FCS!

   This problem is easy to fix by adding VALUE to the proposed attribute.
   This value would specify the FCS type used by the AC in question (e.g., 
   Ethernet, 16-bit HDLC, 32-bit HDLC), and limiting applicability of
retention 
   to the case when the two ACs use the same kind of FCS. (The size of the
attribute 
   should be increased accordingly to account for this value).

2. It would also be nice to specify that a PE includes the FCS retention
attribute 
   if it is capable of both receiving (as in the text) and SENDING retained
FCS. 

   This would prevent bizarre situations where both PEs have agreed to
RECEIVE 
   retained FCS (and hence presumably expect FCS in the PW packets) without 
   one (and possibly both) really SENDING it.

   Of course, the signaling could be also enhanced to allow independent FCS
retention
   in each direction of the PW, but this hardly seems justified.

My 2c.
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With best regards,
                          Sasha Vainshtein
email:   [email-masked]
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