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List:       ipng
Subject:    (IPng 4977) Re: IPv6 over ATM encapsulation
From:       "Andrew G. Malis" <amalis () alpo ! casc ! com>
Date:       1997-11-25 16:08:52
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Dan,

> Obviously, an IPV6 header barely fits into an ATM cell, even if no extension 
> headers are used.  However, has anybody taken a look at whether there are 
> frequent three-cell scenarios which could become two-cell scenarios by 
> removing the SNAP header (common combinations of IPV6 extension headers, TCP 
> options and/or very common user data that add up to 82-88 bytes?)  There was a 
> thread earlier (which unfortunately got dropped) on the possibility of header 
> compression for IPV4;  if we did something like that for IPV6, could we get 
> TCP ACKs back down to one cell?

IPv6 header compression, as specified in draft-degermark-ipv6-hc-04.txt,
has the goal of compressing the IPv6 and UDP/TCP headers down to 4-7
octets.  However, it is really aimed at "low- and medium-speed links",
to quote the draft, and you certainly wouldn't want to have to be doing
the compression and decompression in software at OC3+ rates, or have to
build hardware to do it.  Uncompressed TCP acks will fit in two cells
whether or not LLC/SNAP headers are used.  I'm not close enough into an
IPv6 implementation to say if there are any common packets that are
exactly 82-88 octets in length, but I would tend to doubt it.

I would prefer to keep implementations and configuration simpler by
not including null encapsulation on PVCs.

Cheers,
Andy
________________________________________________________________________
Andrew G. Malis  malis@ascend.com  phone:978 952-7414   fax:978 392-1484
Ascend Communications, Inc.        1 Robbins Road     Westford, MA 01886


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