Computing window sizes and adjustments

Damien Miller djm at
Tue Jul 17 08:17:26 EST 2007

On Mon, 16 Jul 2007, JCA wrote:

>    In SSHv2, the data that consumes window space is that sent in the
> channel data and channel data extended messages. My question is, how
> is the data that consumes window space reckoned? One would have
> thought that it is the total length of the message itself, but the
> standard seems to imply that only the data contained in the data
> string field in the messages above is to be taken into account. That
> is, things like eg the padding and HMAC fields do not consume window
> space.

Windows in the SSH protocol are per-channel, so it only makes sense to use
the data that is sent over a channel. This does not include MAC and padding
as these are protocol-level, not channel level.

>    What is it that OpenSSH does in this respect?

OpenSSH counts the data sent over a channel against the window, not
including the protocol-level framing used to send it.


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