Am 23.02.2019 um 10:20 schrieb Giuseppe Modugno:
I know, this is a hot topic and many times this was explaied in the list
and in doc/rawapi.txt, however now I'm in trouble understanding this.
tcp_recved() is related to the TCP window size of the receiver: "The
purpose is to advertise a larger window when the data has been
processed.". Isn't the TCP window size fixed to TCP_WND macro? Does it
mean that the window size increases every time tcp_recved() is called
from the application? I don't think, otherwise the window size increases
continuously during a connection.
The window has a more or less fixed total size. But it changes with each
segment sent: for every byte sent (and ACKed), it decrease. The receiver
is responsible to "reopen" the window by sending an appropriate window
size with its ACK segments. lwIP does this for you, but you have to tell
it when you are ready to accept more data. This is what 'tcp_recved()' does.
What are the situations when it should be better to not call
tcp_recved() in recv() callback()? Here the application has the received
data, can process and free the pbufs or can avoid freeing the pbufs to
wait for additional data. In both cases I think the application should
call tcp_recved().
If you need to process the received data further at slower speed, it's
better to not call tcp_recved() right away. For example if you're
programming into slow flash, calling tcp_recved after the bytes are
programmed ensures the client does not send faster than you can program.
Regards,
Simon
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