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Re: [lwip-users] socket write hangs, in LwIP 1.4.? (ppp-new branch but n

From: address@hidden
Subject: Re: [lwip-users] socket write hangs, in LwIP 1.4.? (ppp-new branch but not ppp related)
Date: Fri, 21 Dec 2012 11:11:54 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:16.0) Gecko/20121010 Thunderbird/16.0.1

Mark Lakata wrote:
I'm not sure how to get the size of the "connections's TX/RX window sizes".

The easiest way is wiresharp. Posting a pcap here might help us to look at what's going on.

Here is a snippet from the lwipopts.h.
#define TCP_SND_BUF             (2*TCP_MSS)
#define TCP_WND                 (2*TCP_MSS)

Using half the MSS should work (if it doesn't, it should be a bug in lwIP), but it's not recommended to do so. TCP needs at leasst 4*MSS to work efficiently. I bet with TCP_SND_BUF and TCP_WND set to 2*TCP_MSS, your problems are gone.

The problem here is that TCP tries to prevent sending too many ACKs/window-updates. Most TCP implementations only send an ACK if the resulting change for the remote client is one MSS, this is why you easily run into problems with WND==2*MSS and lwIP splitting up send-data into MSS-sized chunks.


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