[lwip-devel] Re: tcp_input: order of processing question/comment

[Jakob Stoklund Olesen]

"bill" <address@hidden> writes:

> The questions are: Why would we want an error returned by the receive callback
> to prevent us from sending any pending data?  Wouldn?t we still want to send
> queued ACKs or queued data?

It could also be an error from the tcp_sent callback. tcp_output is only
called if everything succeeds. I suppose in an attempt not to propagate
errors.

Is your receive callback returning errors? It should only do that is
serious cases like running out of memory.

> The comment is: It would seem to be better to call tcp_output *before* calling
> the tcp_recv callback.  This would allow the other end of the connection to
> process any data that may be sent in that call while the lwIP side goes off 
> and
> processes its data in the receive callback. In my bandwidth testing I do see
> slightly better performance by adding a tcp_output call before the receive
> callback.

The tcp_recv callback is likely to call tcp_write with a reply to the
data received. Calling tcp_output after the callback allows the ACK to
piggyback on the reply. If you call tcp_output first, it may send an
empty ACK segment only to be followed immediately by the reply.

The same goes for the tcp_sent callback.