Re: [lwip-users] enqueing problem

[address@hidden]

Noam weissman:
> I have a problem that I have seen lots or users straggling with, but 
> without any real solution.
>
> I am trying to send data in a loop. I have triad closing NAGLE as follows:
>
> // this should shut down the NAGLE algorithm
>
> pcb->flags |= TF_NODELAY | TF_ACK_NOW;
Please don't use stack-internal variables and defines like this. 
Instead, use the TCP API function tcp_nagle_disable() to disable the 
nagle algorithm.
> I am calling tcp_output() on every tcp_write() but this does not help 
> as well. I got ERR_MEM
>
> after the 20^th something call to tcp_write()
That suggests your memory settings are too low.
> I managed to find in one of the answers here that I should use a 
> smaller window, meaning
>
> change the settings in lwipopts.h… So I did, and it was a bit better.
>
> /* TCP Maximum segment size. */
>
> //#define TCP_MSS 1460
>
> #define TCP_MSS 512
>
> /* TCP sender buffer space (bytes). */
>
> //#define TCP_SND_BUF (3*TCP_MSS)
>
> #define TCP_SND_BUF (8*TCP_MSS)
Well, these two defines didn't change the window directly... But there's 
a good chance TCP_WND gets changed when changing TCP_MSS if you are 
using the default define from opt.h...
Besides TCP_SND_BUF, you might want to increase TCP_SND_QUEUELEN and 
maybe also MEMP_NUM_TCP_SEG. Also, you might just run out of RAM in the 
heap (MEM_SIZE).
> Well the above helped and I was able to send more small packets but 
> after 32 instead of 20 it again
>
> It stopped sending and I got ERR_MEM.
>
> The most important thing is when I check wireshark I see that the 
> stack is sending all my data in one
>
> Frame ???
That's a little weired. Which version of lwIP are you using, anyway?
> Now can someone explain what is going on ?
>
> For every call to tcp_write the enqueue mechanism is advancing 
> snd_queulen by one !
Of course it does. tcp_write cannot know in advance that the next thing 
you do is to call tcp_output. snd_queuelen is always incremented in 
tcp_write and decremented in tcp_output. The only thing changed by 
disabling nagle is that tcp_output always sends everything that is enqueued.
> I see two problems here. If the stack eventually sends all these in 
> one TCP packet why is snd_queulen incremented at all ??
Because every call to tcp_write creates a new pbuf. Although there is 
only one segment enqueued, this segments consists of multiple pbufs 
(unless you queue the data at application layer and pass it to tcp_write 
with one single call).
> Secondly if I use pcb->flags |= TF_NODELAY why is the stack adding all 
> the data into one packet ??
Dunno.

Simon