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From: CLYNN@G.BBN.COM
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Re: A New TCP Retransmission Algorithm
Message-ID: <[G.BBN.COM]21-Apr-87.18:12:38.CLYNN>
Date: Tue, 21-Apr-87 17:12:00 EST
Article-I.D.: <[G.BBN.COM]21-Apr-87.18:12:38.CLYNN>
Posted: Tue Apr 21 17:12:00 1987
Date-Received: Wed, 22-Apr-87 06:16:51 EST
References: <870420213143.005848@MIT-MULTICS.ARPA>
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Sure, I'll admit to backing up the window, but not to doing it "illegally".
The spec says in the "Managing the Window" section that there is an
assumption that the window size is related to the currently available
data buffer space available for the connection;  that is in fact the
case - the user/application supplies one or more buffers to be filled
with data and the amount of space provided specifies the window.  The
spec also says in the "Data Communication" section that a Push requires
that any data be placed into the user's buffer and the buffer returned to
the user even if it is not full.  Consequently, the window is reduced by
the amount of unused space in the buffer returned to the user in response
to a Push.

Since there is justification in the spec for reducing the window, and the
spec requires senders to deal with it, it is also used in one other case.
If a packet is lost, as indicated by reception of several segments in the
window but not at the left edge, the window is reduced to the right edge
of the missing data to "encourage" the sender to retransmit the missing
data and to "discourage" it from sending additional new data or from
retransmitting data which was in fact received but couldn't be acked.
(If the sender ignores the hint, which we might call a NAK, nothing
goes wrong; the receiver will eventually send ICMP Source Quenches which
shouldn't be ignored and if that doesn't work packets will be flushed.)
This case can be viewed as a situation where the user has provided buffer
space but it cannot be used due to the missing segment(s).

If a connection is receiving something from the TCP at the other end,
even "obsolete acknowledgements", why shouldn't it be kept "alive"?

Charlie