K 10
svn:author
V 5
andre
K 8
svn:date
V 27
2012-04-04T10:33:54.915469Z
K 7
svn:log
V 1910
Simplify and enhance the window change/update acceptance logic,
especially in the presence of bi-directional data transfers.

snd_wl1 tracks the right edge, including data in the reassembly
queue, of valid incoming data.  This makes it like rcv_nxt plus
reassembly.  It never goes backwards to prevent older, possibly
reordered segments from updating the window.

snd_wl2 tracks the left edge of sent data.  This makes it a duplicate
of snd_una.  However joining them right now is difficult due to
separate update dependencies in different places in the code flow.

snd_wnd tracks the current advertized send window by the peer.  In
tcp_output() the effective window is calculated by subtracting the
already in-flight data, snd_nxt less snd_una, from it.

ACK's become the main clock of window updates and will always update
the window when the left edge of what we sent is advanced.  The ACK
clock is the primary signaling mechanism in ongoing data transfers.
This works reliably even in the presence of reordering, reassembly
and retransmitted segments.  The ACK clock is most important because
it determines how much data we are allowed to inject into the network.

Zero window updates get us out of persistence mode are crucial.  Here
a segment that neither moves ACK nor SEQ but enlarges WND is accepted.

When the ACK clock is not active (that is we're not or no longer
sending any data) any segment that moves the extended right SEQ edge,
including out-of-order segments, updates the window.  This gives us
updates especially during ping-pong transfers where the peer isn't
done consuming the already acknowledged data from the receive buffer
while responding with data.

The SSH protocol is a prime candidate to benefit from the improved
bi-directional window update logic as it has its own windowing
mechanism on top of TCP and is frequently sending back protocol ACK's.

Tcpdump provided by:	darrenr

END