[clue-tech] Downside of increasing TCP window size?

[Chris Schock]

If you never have issues with your connection then increasing the window
size is fine; if you do have the occasional error then increasing window
size will actually make your connection slower because it will take longer
for TCP to recognize there is a problem and recover from it.

TCP can negotiate the window size already, you're probably better off just
letting TCP handle it.

> Hi everyone,
>
> We have a very low bandwidth, very high latency TCP/IP connection
> that we're trying to optimize.  Is there a downside to using large
> TCP windows on an Important web server, when only a few of the clients
> are on high latency connections and most clients are on low latency
> connections?
>
> In other words, if we change the window sizes on our Important web
> server, what will (or could) break?
>
> The Layer 2 is a 4800 baud MSAT satellite dialup to the PSTN and our
> own terminal server (with an analog modem) using Linux pppd on both
> ends.  The TCP connection is an SSL request to an apache web server,
> using the Perl Net::SSL libraries via LWP::UserAgent and possibly
> HTTP::Request.
>
> Here's a diagram:
>
> Client <-RS232-> MSAT satmodem <-> PSTN <-> terminal server <-> SSL web
> server
> PPP CLIENT <------------------------------> PPP SERVER
> TCP CLIENT <------------------------------------------------->  TCP SERVER
>
> The client, terminal server, and SSL web server are all Linux of
> varying flavors.
>
> Anyway it appears that if we can make the TCP socket from the remote
> to the server using large TCP windows, the connection is more reliable
> (less likely to time out) and faster (data moves more quickly overall).
> I think this is due to the well known TCP ACK issue on high latency
> networks.
>
> I'm tuning the kernel TCP as follows:
>
> echo "4096 131072 131072" > /proc/sys/net/ipv4/tcp_wmem
>
> We've tried changing the tcp_rmem to some small value but it doesn't
> seem to make too much of a difference.
>
> I know the window size should be a multiple of the MTU (576) but I'm
> not sure exactly how to do the calculation.  Round trip ping times are
> 1700 to 1900ms.  Small MTUs mean that less data has to be resent if
> a TCP retransmit is necessary; a 1500 byte MTU is a really huge
> packet on a 4800 baud connection.
>
> Anyway the Important web server shall remain unchanged unless I can
> provide some proof that Nothing Bad Will Happen if we change its TCP
> window sizes.
>
> Thanks for any advice or clues,
> Jim
>
> --
> Jim Ockers, P.Eng. (ockers at ockers.net)
> Contact info: please see http://www.ockers.net/
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