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Subject: Re: socket receive buffer size & window updates
From: George Neville-Neil <gnn@neville-neil.com>
In-Reply-To: <CALCNsJTkQk41eXT0TLwVhHTuqbvk4iMsRttxwCsoWDTqbDVesA@mail.gmail.com>
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To: Vijay Singh <vijju.singh@gmail.com>
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On Mar 11, 2014, at 13:58 , Vijay Singh <vijju.singh@gmail.com> wrote:

> The socket option handler currently doesn't prevent connecting or connected
> sockets from changing their receive buffer sizes. In particular, I ran into
> a an application that sets the receive buffer size lower than what it
> originally was.
> 
> In tcp_output(), if no data is being sent, there is code that is trying to
> decide if a window update is needed.
> 
> If the socket receive buffer size was reduced after a larger window was
> already advertized, or perhaps even when there is data in the receive
> buffer, it seems to me that the computation in 592 could go -ve, and be
> interpreted as a large window update.
> 
> I was led to this issue after observing in packet traces that duplicate
> FINs were being sent on close. I tracked it down to this check. Should this
> be changed to a check like such?
> 

Interesting.  Do you have a bit of test code?

Best,
George