Hi Jonathan:
It troubles me though that we cannot isolate the T from the VF because I
can see the need to report congestion and to apply diffserv in route
over.
To give you examples, ISA cares (sometimes) about the flow label but not
the ToS bits because being mesh under this is all carried in MAC
extensions. And the ISA flow label needs only 2 bytes. And in many route
over case, I expect the reverse, caring about the ToS bits but not the
Flow Label.
Proposal: The VTF bit becomes "another HC byte for VTF", if the bit is
reset VTF fields are all compressed, if set then how to compress VTF is
in the next byte. Then we'd have control bits to say how many bytes to
TF follow and what's in there
Combinations we need to support:
No compression: Need 4 bytes. Control is 1111 and then 24 bits F and
then T
Compressing T only: Need 3 bytes. Control is 1110 and then 24 bits F
Precedence + 2 bytes Flow: Need 3 bytes. Control is 110 and then 2 bit
ECN,
3 bits precedence and 16 bits
rightmost F
Compressing DSCP : Need 3 bytes. Control is 10 and and then 24 bits F.
Compress F: Need one byte. Control is 0 then 2 bit ECN and the next 5
bits are T striped from the rightmost bit. This is obtained by rotating
the byte by 3 so that the first bit is now the least significant DSCP
bit So that compression does not allow odd DSCPs but you'll notice that
most DSCPs are even)... This is also compatible with the precedence
model. See page 14 of attached doc for more.
What do you think?
Pascal
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