PuTTY sometimes sends a special request to SSH servers in the middle of channel data, with the name
[email protected] (see section F.1). The purpose of this request is to measure the round-trip time to the server, which PuTTY uses to tune its flow control. The server does not actually have to understand the message; it is expected to send back a
SSH_MSG_CHANNEL_FAILURE message indicating that it didn't understand it. (All PuTTY needs for its timing calculations is some kind of response.)
It has been known for some SSH servers to get confused by this message in one way or another – because it has a long name, or because they can't cope with unrecognised request names even to the extent of sending back the correct failure response, or because they handle it sensibly but fill up the server's log file with pointless spam, or whatever. PuTTY therefore supports this bug-compatibility flag: if it believes the server has this bug, it will never send its ‘
[email protected]’ request, and will make do without its timing data.