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Re: [lwip-devel] HW checksum support and fragmented packets

From: James G. Smith
Subject: Re: [lwip-devel] HW checksum support and fragmented packets
Date: Wed, 18 Jul 2012 13:12:16 +0100
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On 07/18/2012 11:52 AM, Kieran Mansley wrote:
> I wonder if we could overload the checksum field itself to indicate whether a 
> checksum has already been calculated/verified?

I doubt it.

A 0-checksum means something specific in the case of UDP, but other than
that a checksum value could be anything in the bit-range available.

If using up all the pbuf flags is an issue; and since we do NOT want to
increase the size of pbuf objects (after all we want to be running in
deeply embedded, low RAM, worlds) then it all depends what the "unused"
pbuf flag bits are there for?

Spare bits for "future expansion" covers this proposal for using 2bits
as IP and PAYLOAD checksum flags... but it would mean that any further
pbuf enhancements will be up against the wall and will have to consider
either an incompatible change (e.g. swapping 8-bits of "ref" for 8-bits
of new "flags"), or forward compatible (but higher RAM cost) of
appending a new pbuf field covered by using one of the existing "unused"
flag bits.

-- Jamie

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