Description
What version of Go are you using (go version)?
$ go version go version go1.18.4 linux/amd64
golang.org/x/crypto v0.0.0-20220829220503-c86fa9a7ed90
Does this issue reproduce with the latest release?
Yes, channel.go is unchanged since 2017.
What did you do?
Used tcpdump/Wireshark to observe packets on a long-lived SSH connection where the only application-level activity is sporadic small writes or a long stream of writes in the same direction, or other easy-to-understand patterns.
What did you expect to see?
TCP segments with data generally correspond to application-level writes, with only occasional window adjustments sent in the other direction (and rarely a rekeying, etc.).
What did you see instead?
A window adjustment is sent after every Channel.Read on the receiving end, which wastes bandwidth.
Solution
OpenSSH has logic for deferring window adjustments, which I think usually waits until about 5% of the window has been consumed. It seemingly hasn't changed since 2007, so I assume it works well enough for them.
I added similar logic to channel.adjustWindow and it seems to work as expected in local testing. If this sounds like a reasonable change, I'll see about signing the CLA and submitting a patch.