[lwip-devel] [bug #49631] Zero window and refused data problem

[Joel Cunningham]

Follow-up Comment #11, bug #49631 (project lwip):

I'm fine with that.  I re-read RFC 793 and it doesn't specify that a zero
window probe MUST only be 1 byte.  I must have remembered incorrectly:


The sending TCP must be prepared to accept from the user and send at
least one octet of new data even if the send window is zero.  The
sending TCP must regularly retransmit to the receiving TCP even when
the window is zero.  Two minutes is recommended for the retransmission
interval when the window is zero.  This retransmission is essential to
guarantee that when either TCP has a zero window the re-opening of the
window will be reliably reported to the other.

When the receiving TCP has a zero window and a segment arrives it must
still send an acknowledgment showing its next expected sequence number
and current window (zero).


My only remaining concern was that if LwIP is the sender and it's engage in
treating the window as zero rather than splitting the next unsent segment to
fit within the remaining window, this logic will not correctly identify the
packet as a "zero window probe".

This fix is still an improvement though as it does fix the issue for other
stacks which will fully close the receive window


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