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AW: [lwip-users] TCPH_OFFSET


From: Zschocke, Florian
Subject: AW: [lwip-users] TCPH_OFFSET
Date: Wed, 5 Mar 2003 14:57:48 +0100

Kieran Mansley wrote on Mittwoch, 5. März 2003 14:47:

> I think that would work fine.  I guess it was designed to
> provide access to both the offset and the other bits (I think
> they're reserved, not sure what if anything they're used for)

The problem is that that wouldn't work since the partitioning of these 16
bits in the header is different from what is used in lwIP. The TCP header
has these fields (at offset 96):

4-bit header length | 6-bit reserved | 6-bit flags

The lwIP TCP header and macros partition these 16 bits in two octets which
results in the boundary lying in the middle of the reserved bits. Both the
"offset" (actually header length) and the flags fields aren't extracted
correctly by the macros. I wasn't sure if this is due to some limitations in
some processor architectures though I would doubt that.

Florian.




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