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

[Matthias Weisser]

>>> Kieran Mansley <address@hidden> schrieb am 20.06.2007 um 12:14:
> 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

No. There where no ACKs from the PC side.
BTW:
Its a simple echo server.

> 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?

Attached to this mail you find a pcap file. 210 is the PC (echo client) and 34 
is the lwIP device running the echo server on port 6666.

>> 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.

I ask because when I send big data packets (10k+) at once to my echo server I 
have lost data from time to time and I think that has something to do with bad 
settings in my lwipopt.h file.

Matthias

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