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[lwip-devel] [task #6849] Test how checksum on copy could be integrated

From: Bill Auerbach
Subject: [lwip-devel] [task #6849] Test how checksum on copy could be integrated into the stack
Date: Mon, 11 May 2009 14:52:59 +0000
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: Gecko/2009042316 Firefox/3.0.10

Follow-up Comment #11, task #6849 (project lwip):

I wanted to add a point.  There are 2 checksum options to consider:

1) Offloading that the driver or stack does at some level.  I've seen an lwIP
port with driver offloading.

2) Offloading where the application generates data which also can have the
checksum calculated per block of data (not exceeding mss).

Although case 2 is less general it's no less important for applications which
get data from some other device and the data can be streamed with a checksums
included as part of that protocol.  I will be implementing case 2 and envision
a tcp_enqueue where I can include the checksum.  If a segment has a
'check_sum' valid field (defaulting to off/0), then a tcp_enqueue function
which receives the checksum can set this field to on/1.  Perhaps this field
can be the flag too that means do not chain on this segment.  I don't know
lwIP well enough to propose anything more than an idea like this.

Case 2 offloading can also be more efficient because if the checksum is done
at the time the data is being queued, it's likely to be in cache and quickly
accessed.  Driver offloading usually means the data has been replaced in the
cache and there is less benefit offloading this late.  Of course this is
system dependent.


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