Re: [lwip-users] TCP_WRITE_FLAG_MORE, tcp_write(), tcp_output() and real output TCP segment

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Giuseppe Modugno wrote:

I'm trying to understand the mechanisms under TCP_WRITE_FLAG_MORE, tcp_write() and tcp_output(). I'm using raw API.

First of all, I couldn't understand if tcp_output() is useful or not.

Why do you think all the world would call tcp_output() if it's not useful?

I understood tcp_output() *really* outputs unsent data, pushing all data to netif driver. Here it's not clear:

• if unacked data (data already sent but not yet acknlowedged) is sent too;

No, it's not. Unacked data is only retransmitted for a 'rexmit' event. To do that, it is moved back to 'unsent' and tcp_output() is called.

• what happens if data in the output buffer is bigger than what netif driver could accept in its sending function

Then it is not sent. :-)
TCP re-triggers tcp_output() if data is ACKed or new data is received or in a timer, so eventually, more data is sent.

• (maybe a well written application checks tcp_sndbuf() before writing output data, so this event never happens).

No, tcp_sndbuf() does not know anything about your netif driver.

For example:

tcp_write(...)
tcp_write(...)
tcp_write(...)
tcp_output()

Is tcp_output() really needed?

Yes, of course! If not, I would have deleted it already ;-)

It seems data are send even without calling tcp_output().

It is, but only at some time later (by tcp_tmr). This drops your throughput, of course.

Another question is the correspondence between tcp_write() calls and output TCP segments, IP packets and Ethernet frames. Are three TCP segments, three IP packets and three Ethernet frames generated? Does lwip try to compact output data in a single TCP segment, IP packet and Ethernet frame?

Yes, data is appended, to unsent segments. However, it's not easy to cite the rules here, as there are many optimizations in this path. In the end, don't try to rely on something here!

Another question is about TCP_WRITE_FLAG_MORE flag. I understood this flag should be set in all tcp_write() except the last. However it seems it works with and without that flag in sending data.

Of course it works, but pbuf allocation and tcp header flags differ. If it doesn't make a difference for you, you can just set the flag to what you want.

Simon