Re: [lwip-users] Retransmit too quick?

[Xun Chen]

Hi Sergio,

Thanks again for your kind reply.

I am not an expert in lwip, but I did several projects on other TCP stack

The receiver's ACK is taken care inside lwip, not by the callback of 
tcp_sent

and tcp_sndbuff tells the size of available tx buffer

I agree with you about the timer may have some problem in the port

On 7/10/2015 10:20 AM, address@hidden wrote:
> ------------------------------
>
> Message: 2
> Date: Fri, 10 Jul 2015 09:22:14 -0300
> From: "Sergio R. Caprile"<address@hidden>
> To:address@hidden
> Subject: Re: [lwip-users] Retransmit too quick?
> Message-ID:<address@hidden>
> Content-Type: text/plain; charset=utf-8
>
> > >    So I don't understand why calling sys_check_timeouts without any
> > >delay yields a poorer performance than calling it or tcp_tmr at 250ms
> > >pace.
> Neither do I.
> Either your port is broken or your code is not properly written and this
> condition masks that out.
>
> > >    I think tcp_sent callback is only an option, not a requirement, in
> > >lwip.
> You are free to think whatever you want. However, that doesn't
> necessarily need to be correct.
> Do you actually know TCP ? If you understand TCP, you'll understand why
> tcp_sent() is not an option, unless you know exactly what you are doing.
> Now, in my opinion, that is the reason why you are observing that
> strange timer dependent performance change.
>
> > >    So I think it is OK to use the following flow:
> > >
> > >    use tcp_sndbuf to check available buffer
> > >    use tcp_write to write data to queue
> > >    finally use tcp_output to send out data asap.
> > >
> > >    Is my understanding correct?
> No, afaik it is not.
> TCP can't send more data than the receiver allows for, so if you don't
> pay attention to the receiver's ACKs, you will end up with a full
> buffer, won't be able to send anything, and will be wasting precious
> time calling functions that can't do anything, hogging your CPU. Then,
> automagically, you will adjust your timers and suddenly it somehow
> approaches the receiver's ACK speed and you see an increase in performance.
> Comer is your friend, Stallings too, be a good boy and read them.