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

[Frédéric BERNON]

http://lists.gnu.org/archive/html/lwip-users/2006-11/msg00007.html // tune some 
TCP options (read all the thread)
http://lists.gnu.org/archive/html/lwip-users/2007-04/msg00056.html // Reduce 
footprint (read all the thread)

http://savannah.nongnu.org/task/?1549 // See the Simon Goldschmidt's big job to 
document lwIP functions (I have attach a RTF doxygen doc.)

http://savannah.nongnu.org/task/?6683 // Some tips about lwIP thread safety
http://savannah.nongnu.org/patch/index.php?5960 // Some tips about lwIP thread 
safety (more recent)

Perhaps it will be good to put a "news" on Savannah with these links waiting to 
document the next release?
  
====================================
Frédéric BERNON 
HYMATOM SA 
Chef de projet informatique 
Microsoft Certified Professional 
Tél. : +33 (0)4-67-87-61-10 
Fax. : +33 (0)4-67-70-85-44 
Email : address@hidden 
Web Site : http://www.hymatom.fr 
====================================
P Avant d'imprimer, penser à l'environnement
 


-----Message d'origine-----
De : address@hidden [mailto:address@hidden De la part de Kieran Mansley
Envoyé : mercredi 20 juin 2007 12:15
À : Mailing list for lwIP users
Objet : Re: [lwip-users] TCP retransmissions and some general questions


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



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