Re: [lwip-users] tcp_sent() callback: Called once per sent packet or for multiple packets at once?

[address@hidden]

Am 03.02.2022 um 15:51 schrieb Matthias Paul:
> Hello all,
>
> I'm using lwip 2.1.2 in non-OS / non-threaded mode (main loop mode) with
> raw API to send/receive TCP packets.
>
> When sending packets with tcp_write() I'd like to use zero copy
> mechanism. Therefore I need to keep the pbuf objects until the buffer
> has been sent and acknowledged by the recipient. When acknowledged, my
> callback registered by tcp_sent() is called:
>
> static err_t
>
> TCPSERVER_sentCallback(
>
>          void                        * arg,
>
>          struct tcp_pcb                * tcpPcb,
>
>          uint16_t                    length) {
>
>     // remove pbuf which are reported as "sent"
>     ...
> }
>
> Is "length" always matching a single pbuf length or could it happen that
> TCPSERVER_sentCallback() reports multiple pbuf objects at once?
>
> Also, can I assume, that TCPSERVER_sentCallback() is called in the order
> the packets have been sent?

You have to understand the tcp protocol to understand the sent callback:
a host acknowledges the bytes received from a the other side. It is free
to send an ACK for an arbitrary byte count between 1 and the maxiumum
outstanding bytes. But since tcp is a streaming protocol, you'll always
know when to free the next buffer from the from of your list when you
sum up the 'length' parameter of all 'sent' calls.

Regards,
Simon