Re: [lwip-users] TCP retransmissions and some general questions

[Kieran Mansley]

On Wed, 2007-06-20 at 11:52 +0200, Matthias Weisser wrote:
> Hi
> 
> Today I found some time to do some deeper testing of my lwIP port. I
> used wireshark and tested around a bit.
> 
> The first thing I saw that there are TCP retransmissions from time to
> time. These retransmissions happens about 5-10ms after the original
> packet was transmitted but not for every packet. That seems a little
> to early to me.

Were there any ACKs from the other side between the sending of the
packet and its retransmission?  If so, it could be fast retransmission
and not a problem.  If not, it does seem a bit early.  Perhaps you could
send a wireshark/ethereal capture file to illustrate the problem?

> Another question is about the TCP window size. Which value makes sense
> together with all the other options in the lwipopts.h? Maybe its worth
> to write a little bit more about these options and how they depend on
> each other.

You're right that this the links between many options are not obvious,
and it's been a frequently asked question/problem on the mailing list.
I agree that more documentation would be good.  Any volunteers?  There
was a useful post on the mailing list a few months ago documenting some
"good" settings for everything, but I don't have a link to hand at the
moment.  

As for the window size, apart from making sure it's at least twice the
MSS, and that you have sufficient buffer space to accept a window worth
of data in one go, there's not a lot more to it.  Of course, bigger is
better in terms of performance, but will require more memory for
buffers.

> Another point is about mailboxes. I implemented them as FIFOs but how
> deep should this FIFOs be? I use 32 entries now but is this enough?

Not sure.

Kieran