Re: [tcpm] TCP-AO Option Layout - Alignment and Padding

[Joe Touch <[e-mail]>]

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YOSHIFUJI Hideaki / ???? wrote:
...
<what if we...>
|> Use 2 bits for the KID; 1 is insufficient, and 2 is more than sufficient
|> IMO.
|>
|> The MAC' is a half-word aligned MAC with the top 2 bits overwritten
|> (i.e., it is NOT shifted right); the KID occupies those top 2 bits.
|>
|> If a user wants word alignment, pick a MAC length of N-2 bytes.
|>
|> Wouldn't that be simpler and more space efficient?
|
| We have 256 keys, and it is not enough.

Can you explain this further? We're talking about the number of keys
used within a single TCP connection; there isn't a clear need (AFAICT)
for more than a very small number of these.

| And is NOT simple and more complex from implemntation point of view
| - eats a lot of cpu cycles.

It would consume only two additional cycles in the following:
	header && 0011111111.... || keyID

(store keyIDs as values 0, 64, 128, 192 so you don't need to shift them
in/out)

I.e., 2 opcodes to encode the bits in the location desired.

Joe
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