Re: Regd window size

From: Matti Aarnio (
Date: Mon Jan 31 2000 - 04:18:48 EST

On Mon, Jan 31, 2000 at 02:21:50PM +0545, S. Malla wrote:
> Hi all,
> Our organization uses VSAT connectivity for Internet Access and we are
> advised to increase window sizing on our Linux (RedHat 6.1) servers to get
> better result. What is the maximum possible size of window and how can I
> increase it? Thanks for your time.

        You increase it at the code by using:
                int var = 200000;
                setsockopt(sock, SOL_SOCKET, SO_SNDBUF, &var)
                setsockopt(sock, SOL_SOCKET, SO_RCVBUF, &var)
        HOWEVER that is only benchmarkish thing, which doesn't
        help unless you have only *one* connection going at the link,
        and not at all at the usual fully saturated VSAT link case.

        In theory the window size you want is:
        e.g. 0.8s * 64kbps = 51200

        With that you can get full bandwidth to one TCP stream,
        but as soon as you have more of parallel streams, default
        window of 32000 is quite sufficient.

        This is even assuming that the source is able to send to you
        at that whopping 64kbps speed - very popular sites in the net
        are so saturated that they send some 8-16 kbps at most..

        There is cotcha at Linux SO_SNDBUF code, though. It includes
        also SKB overheads in the book-keeping, and that share depends
        on the MTU of the connection. Say 80 bytes per each IP packet
        carrying 1460 bytes of TCP data. (5.5% overhead)

> Rgds,
> SMalla

/Matti Aarnio
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