Computing window sizes and adjustments

[JCA]

   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.

   What is it that OpenSSH does in this respect?