CODEWITHSUNDEEP
tcpembeddedlwip
Kai Neumann
Kai Neumann

Reputation: 361

LWIP: How exactly does the TCP_INTERVAL relate to the reception of ACK Messages?

I am trying to implement a data transfer from an embedded board to a PC. For this, I need to use low latency communication and I am bound to use Ethernet with TCP/IP. Furthermore, I'm using the lwip stack.

First of all, I disabled nagle algorithm, because I have to send small packets of data (10 KB) and I want them to be sent as soon as possible, without waiting for intermediate ACKS. The Wireshark Log shows me that this is working quite fine (the whole data is being sent to the PC in about 1msec).

After that, the PC takes about 200msec to send the last ACK (because the last Segment is not maximum size).

The problem is now, that on the embedded processor, it takes a very long time, until the lwip gives my application the message, that all of the data has been ACKED. When I decrease the TCP_INTERVAL (to let's say 5), it speeds up greatly.

I am wondering, why lwip behaves like this? I would think that the Periodic-TCP-Tasks (which are being called according to the TCP_INTERVAL) have nothing to do with the Handling of the received frames (which is really another call in the main).

I hope I could state my problem somehow understandable, if not I would appreciate feedback, so I can improve my question!

Thanks!

EDIT:

After more debugging, I found out that the process of sending data results in the following function calls:

So now it is clear that if I reduce the TCP_INTERVAL, of course the data gets sent sooner, because the tcp_tmr() function is called more quickly.

but my question is still: Is this the normal behaviour? It seems a bit odd, that lwIP is waiting such a long time before actually sending the data.

Upvotes: 2

Views: 3423

Answers (1)

Sushant
Sushant

Reputation: 63

Since Youre doing this My main calls tcp_write(...) use tcp_output() immediately after tcp_write

or else use tcp_write() in tcp_recv callback

Upvotes: 0

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