On Jun 17, 2009, at 07:54, Pascal Thubert (pthubert) wrote:
80 bytes
Well.
MAC frame mac: 127 bytes
MAC header/trailer: 5 bytes
Adresses (usually, if EUI-64 is used): 18 bytes
Security Header: 0..14 bytes (depending e.g. on key identifier)
Security Trailer (MIC): 0/4/8/16 bytes
Depending on how security is being used, the MAC payload may be as low
as 74 bytes.
With 6 bytes of fragment header overhead, we are at 68; with rounding
to a multiple of 8, we are at 64.
1280/64 = 20.
The first fragment may have some additional header overhead (somewhat
unlikely, but possible), so I'd say:
21 fragments
This is without a mesh header.
If a mesh header in the current worst case configuration is needed,
make that 28 (!).
I think rounding up to 32 is about right.
5 bits for the sequence number also happens to match up with the 11
bits needed for the datagram size.
Spending a bit or two of those for some form of huffman/rice coding
might be the more appropriate way to save bytes in the FRACK packet,
if that is a concern, because many packets will fit into much fewer
(eg., 7 or less, 14 or less) fragments. Or, simpler, trailing zeroes
might be suppressed in the FRACK.
All this pesky bit fiddling... But someone has to do it.
Gruesse, Carsten
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