Re: [lwip-users] LWIP and sliding window

[Yann Suisini]

Here's the way for sending data with TCPnet :
(we guess the connection is established)

First I get a tcp buffer with tcp_get_buf function.
(  http://www.keil.com/support/man/docs/rlarm/rlarm_tcp_get_buf.htm
http://www.keil.com/support/man/docs/rlarm/rlarm_tcp_get_buf.htm  )
The max size I can request is MSS (max 1500 bytes).

After I send the data with tcp_send(...)
http://www.keil.com/support/man/docs/rlarm/rlarm_tcp_send.htm
http://www.keil.com/support/man/docs/rlarm/rlarm_tcp_send.htm 

Before I can send data again I have to be sure the previous data was ACK
with tcp_check_send(...)
http://www.keil.com/support/man/docs/rlarm/rlarm_tcp_check_send.htm
http://www.keil.com/support/man/docs/rlarm/rlarm_tcp_check_send.htm 

so, the TCP stack only allows to send one frame one to one . the stack uses
a little trick to force the ACK from remote to avoid delayed ACK  :
IF you want to send 1000 bytes of data , the stack send a first frame of 996
bytes , immediately followed by another frame with the last 4 bytes. This is
the only optimization for the bandwidth ... :,( 



Kieran Mansley wrote:
> 
> On Wed, 2009-09-30 at 10:57 -0700, Yann Suisini wrote:
>> ello ,
>> 
>> 
>> Currently I'm using the KEIL TCPnet stack on my system (ARM LPC23xx) .
>> It's working well on a LAN with low latency (about 650KB/s on the
>> embedded
>> ftp server with SD card access).
>> But I can only send frames one at one , waiting for an ACK before I can
>> send
>> another frame .
>> So with high ping about 300ms on a intercontinental connection , I  get
>> this
>> :
>> 
>> 1500 bytes sent
>> 300ms before ack
>> 1500 bytes sent
>> 300ms for ACK...
>> 
>> So the max I get in this case is about 3.5 KB/s  !!!
>> 
>> I would replace this stack by Lwip but I would know if I cand send more
>> than
>> one frames before waiting for an ACK
>> in order to optimize use of the bandwith. So , does Lwip manges tcp
>> windowing ?
> 
> Yes lwIP can do this, but I find it very hard to believe that there is
> any TCP stack that doesn't do TCP windowing.  Are you sure the KEIL
> TCPnet stack can't do this?  You may just need to change the way it is
> configured.
> 
> Kieran
> 
> 
> 
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> 

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