Re: [lwip-devel] tcp_output does not force seg sent out

[Simon Goldschmidt]

narke <address@hidden> wrote:
> I found it's not always that the tcp_output() call can make a segment
> to be sent out.   Sometimes it still accumulated data until TCP_MSS
> reached before output a segment.

That's the nagle algorithm (-> google). In short, it tries to limit the 
overhead of ACK segments by sending as little segments as possible by combining 
multiple calls to write() into a single segment.

> In what situation that tcp_output()
> does not output segment?

A segment is sent tcp_do_output_nagle() results in 1:

#define tcp_do_output_nagle(tpcb) ((((tpcb)->unacked == NULL) || \
                            ((tpcb)->flags & (TF_NODELAY | TF_INFR)) || \
                            (((tpcb)->unsent != NULL) && (((tpcb)->unsent->next 
!= NULL) || \
                              ((tpcb)->unsent->len >= (tpcb)->mss))) || \
                            ((tcp_sndbuf(tpcb) == 0) || (tcp_sndqueuelen(tpcb) 
>= TCP_SND_QUEUELEN)) \
                            ) ? 1 : 0)

In other words, if there is unacked data, we wait for it to be acked or to send 
a full segment (unless we are in fast retransmit or ran out of lwIP buffers).

> And, how do I ensure the output behavior
> really happen?

Call tcp_nagle_enable(pcb).


Simon
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