Re: [lwip-users] lwip_close

[Ashutosh Srivastava]

Hi Atte,

> I think I've seen this question before in this list. First I'd like to ask
> that why would you want to wait for the ACK after you have closed the
> connection?

Thanks a lot for the reply. Yes, you are correct, I myself had posted
this question before but no one answered to that :)

Okay, I have understood the fact that TCPIP stack internally handles the
FIN ACK and FIN from the server.

Now, let me describe the problem I am facing because of this. The server
I am connecting to is a shoutcast Internet radio server which streams MP3
data over HTTP. When I close the connection, lwip sends a FIN segment
and comes out. I am not sure why, but the server sends the FIN ACK after
a long time and keeps on sending some more MP3 packets. So, in between
if want to connect to the same server again, it fails because the server is 
still
sending over the MP3 data for the previous connection and does not get the
SYN ack. Another observation is that, the server itself does not send the 
FIN
segment, but the lwip timesout itself and deallocates the connection.

That's why I thought that connection should be closed only when the ACK is
received and a FIN is sent from the server.

Is there an alternative method to handle this?

Thanks,
Ashu

----- Original Message ----- 
From: "Atte Kojo" <address@hidden>
To: <address@hidden>
Sent: Thursday, August 25, 2005 12:11 PM
Subject: Re: [lwip-users] lwip_close


> On Wednesday 24 August 2005 16:41, Ashutosh Srivastava wrote:
> > My question is, for a lwip_close call, why netconn_delete () function 
> > does
> > not wait for the ACK from server for the FIN segment sent?
>
> I think I've seen this question before in this list. First I'd like to ask
> that why would you want to wait for the ACK after you have closed the
> connection?
>
> Also, a TCP connection isn't closed after the ACK for the first FIN 
> segment
> arrives. The connection is closed when both ends have sent a FIN and 
> received
> an ACK for it. i.e.
>
> local remote
> FIN --------------->
> <-------------- ACK
> <-------------- FIN
> ACK ------------->
>
> or
>
> local remote
> FIN --------------->
> <--------------- ACK,FIN
> ACK ------------->
>
> > Ideally, I thought the lwip_close () implementation should be like the
> > lwip_connect () function call where a callback function is called when a
> > SYN ACK is received from the server.
>
> The lwip_close call just tells the stack that the application no longer 
> needs
> the socket. From the application's point of view the connection exists no
> more. The stack takes care of all the necessary housekeeping in closing 
> the
> TCP connection from this point forward (i.e. sending and receiving the
> necessary FINs and ACKs and freeing the resources that were reserved for 
> the
> connection).
>
> You should remember that conceptually the socket layer has nothing to do 
> with
> TCP or UDP or any other protocol. It is just a very simple API with which 
> one
> can do almost any kind of I/O.
>
> I hope this helps.
> - Atte
>
>
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