Well,
I am using lwip 1.3.2
I do understand that on every call to tcp_write the snd_queuelen is
incremented but I do not understand why it is not
decremented when I call tcp_output. Also take into consideration that I have
closed NGLE so it should have worked ??
Thanks,
Noam.
-Original Message-
From: lwip-users-bounces+noam=silrd@nongnu.org
[mailto:lwip-users-bounces+noam=silrd@nongnu.org] On Behalf Of
goldsi...@gmx.de
Sent: א 27 מרץ 2011 15:22
To: Mailing list for lwIP users
Subject: Re: [lwip-users] enqueing problem
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
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