Re: [lwip-users] TCP retransmissions although packet has been recieved

[Inderjit Singh]

So,

I have updated reception code (from the contrib examples httpd_raw) and still the behaviour is the same (code at bottom).

I did a quick and ugly test by calling tcp_ack_now and tcp_output at the end of the function and then the behaviour dramatically improved as ACKs are being sent back and there is no RST from server (as expected).

I might also add that each http request I am sending is a blocking call (and awaits a response) with timeout at application level. So practically I am only handling one full transaction at a time. The transactions are deterministic and small in size.

So this for me boils down to TCP_WND setting. The http requests I'm sending is roughly at 200 bytes and reception is in equal side so they are fairly small. Is there a guidance on how to set LWIPOPTS variables regarding TCP for this kind of behaviour? Right now my settings are:

LWIPOPTS.H

#define TCP_MSS                            1460

/**
 * TCP_WND: The size of a TCP window.  This must be at least
 * (2 * TCP_MSS) for things to work well
 */
#define TCP_WND                            (2 * TCP_MSS)

/**
 * TCP_SND_BUF: TCP sender buffer space (bytes).
 * To achieve good performance, this should be at least 2 * TCP_MSS.
 */
#define TCP_SND_BUF                        (2 * TCP_MSS)

Reception Code:

err_t tcp_tx_resp_cb(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err)
{
    eth_ctx_t* ctx = (eth_ctx_t*)arg;
    uint8_t* data;

    if ((err != ERR_OK) || (p == NULL)) {

        DBG_W(EINVAL, "Ignoring response err '%d'.", err);

        /* error or closed by other side? */
        if (p != NULL) {
            /* Inform TCP that we have taken the data. */
            tcp_recved(tpcb, p->tot_len);
            pbuf_free(p);
        }

        tcp_close(tpcb);
        return ERR_OK;
    }

    if (p->len != p->tot_len) {
        DBG_E(ECOMM, "Received message is incomplete %d<%d.", p->len, p->tot_len);
    }

    /* Acknowledge that we have received the packet */
    tcp_recved(pcb_tcp, p->tot_len);

    if (ctx->recv_buf == NULL){
        DBG_W(EINVAL, "Cannot transfer received buffer. No buffer given.");

        return ERR_OK;
    }

    /*** Parse data ***/

    if(p != NULL) {
        pbuf_free(p);
    }
   
    return ERR_OK;
}