3.3 SACK with Window Scaling
Connected: An Internet Encyclopedia
3.3 SACK with Window Scaling
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3. TCP SELECTIVE ACKNOWLEDGMENT OPTIONS
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3.3 SACK with Window Scaling
3.3 SACK with Window Scaling
If window scaling is in effect, then 16 bits may not be sufficient
for the SACK option fields that define the origin and length of a
block. There are two possible ways to handle this:
- Expand the SACK origin and length fields to 24 or 32 bits.
- Scale the SACK fields by the same factor as the window.
The first alternative would significantly reduce the number of
blocks possible in a SACK option; therefore, we have chosen the
second alternative, scaling the SACK information as well as the
window.
Scaling the SACK information introduces some loss of precision,
since a SACK option must report queued data blocks whose origins
and lengths are multiples of the window scale factor rcv.scale.
These reported blocks must be equal to or smaller than the actual
blocks of queued data.
Specifically, suppose that the receiver has a contiguous block of
queued data that occupies sequence numbers L, L+1, ... L+N-1, and
that the window scale factor is S = rcv.scale. Then the
corresponding block that will be reported in a SACK option will
be:
Relative Origin = int((L+S-1)/S)
Block Size = int((L+N)/S) - (Relative Origin)
where the function int(x) returns the greatest integer contained
in x.
The resulting loss of precision is not a serious problem for the
sender. If the data-sending TCP keeps track of the boundaries of
all segments in its retransmission queue, it will generally be
able to infer from the imprecise SACK data which full segments
don't need to be retransmitted. This will fail only if S is
larger than the maximum segment size, in which case some segments
may be retransmitted unnecessarily. If the sending TCP does not
keep track of transmitted segment boundaries, the imprecision of
the scaled SACK quantities will only result in retransmitting a
small amount of unneeded sequence space. On the average, the data
sender will unnecessarily retransmit J*S bytes of the sequence
space for each SACK received; here J is the number of blocks
reported in the SACK, and S = snd.scale.
Next: 3.4 SACK Option Examples
Connected: An Internet Encyclopedia
3.3 SACK with Window Scaling
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