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#### RFC 7323, "TCP Extensions for High Performance", September 2014

Source of RFC: tcpm (tsv)

Errata ID: 5586
Status: Rejected
Type: Technical
Publication Format(s) : TEXT

Reported By: Marco Caspers
Date Reported: 2018-12-27
Rejected by: Mirja Kühlewind
Date Rejected: 2020-03-06

Section 2.3 says:

```Since the max window is 2^S (where S is the scaling shift count)
times at most 2^16 - 1 (the maximum unscaled window), the maximum
window is guaranteed to be < 2^30 if S <= 14.  Thus, the shift count
MUST be limited to 14 (which allows windows of 2^30 = 1 GiB).  If a
Window Scale option is received with a shift.cnt value larger than
14, the TCP SHOULD log the error but MUST use 14 instead of the
specified value.  This is safe as a sender can always choose to only
scaling by a factor larger than 14 and the sender is only scaling by
14, then the receive window used by the sender will appear smaller
than it is in reality.

```

It should say:

```Since the max window is 2^S (where S is the scaling shift count)
times at most 2^16 - 1 (the maximum unscaled window), the maximum
window is guaranteed to be < 2^30-2^14 if S <= 14.  Thus, the shift
count
MUST be limited to 14 (which allows windows of 2^30-2^14 ~ 1 GiB).
If a
Window Scale option is received with a shift.cnt value larger than
14, the TCP SHOULD log the error but MUST use 14 instead of the
specified value.  This is safe as a sender can always choose to only
scaling by a factor larger than 14 and the sender is only scaling by
14, then the receive window used by the sender will appear smaller
than it is in reality.

```

Notes:

Shifting is inserting zeroes on the right hand side. Thus for S = 14 the 14 right most bits are zero and thus the calculation 2^30 is invalid for the guaranteed maximum window size.

Correct calculation formulae is (2^30 - 1) - (2^14 -1).
Which can be simplified to 2^30 - 2^14.
--VERIFIER NOTES--
That section is for illustration purposes only, and not intended as an exact value for the maximum supported window size.

It is correct that the maximum supported window size is 2^30-2^14, but the requirement is, that the window size has to remain smaller than 2^30.

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