bug #50476: initialize ssthresh to TCP_SND_BUF

This commit changes ssthresh to be the largest effective congestion
window (amount of in-flight data). This follows the guidance of RFC
5681 which recommends setting ssthresh arbitrarily high.

LwIP was previously using the receive window value at the end of the
3-way handshake and in the case of an active open where the receiver
used window scaling and/or window auto-tuning, this resulted in a very
small ssthresh value even though the window ramped up once the connection
was established

src/core/tcp.c

@@ -980,7 +980,6 @@ tcp_connect(struct tcp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port,
   pcb->mss = tcp_eff_send_mss_netif(pcb->mss, netif, &pcb->remote_ip);
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
   pcb->cwnd = 1;
-  pcb->ssthresh = TCP_WND;
 #if LWIP_CALLBACK_API
   pcb->connected = connected;
 #else /* LWIP_CALLBACK_API */
@@ -1647,6 +1646,14 @@ tcp_alloc(u8_t prio)
     pcb->tmr = tcp_ticks;
     pcb->last_timer = tcp_timer_ctr;
 
+    /* RFC 5681 recommends setting ssthresh abritrarily high and gives an example
+    of using the largest advertised receive window.  We've seen complications with
+    receiving TCPs that use window scaling and/or window auto-tuning where the
+    initial advertised window is very small and then grows rapidly once the
+    connection is established. To avoid these complications, we set ssthresh to the
+    largest effective cwnd (amount of in-flight data) that the sender can have. */
+    pcb->ssthresh = TCP_SND_BUF;
+
 #if LWIP_CALLBACK_API
     pcb->recv = tcp_recv_null;
 #endif /* LWIP_CALLBACK_API */

src/core/tcp_in.c

@@ -61,8 +61,6 @@
 
 /** Initial CWND calculation as defined RFC 2581 */
 #define LWIP_TCP_CALC_INITIAL_CWND(mss) LWIP_MIN((4U * (mss)), LWIP_MAX((2U * (mss)), 4380U));
-/** Initial slow start threshold value: we use the full window */
-#define LWIP_TCP_INITIAL_SSTHRESH(pcb)  ((pcb)->snd_wnd)
 
 /* These variables are global to all functions involved in the input
    processing of TCP segments. They are set by the tcp_input()
@@ -615,7 +613,6 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
     tcp_parseopt(npcb);
     npcb->snd_wnd = tcphdr->wnd;
     npcb->snd_wnd_max = npcb->snd_wnd;
-    npcb->ssthresh = LWIP_TCP_INITIAL_SSTHRESH(npcb);
 
 #if TCP_CALCULATE_EFF_SEND_MSS
     npcb->mss = tcp_eff_send_mss(npcb->mss, &npcb->local_ip, &npcb->remote_ip);
@@ -771,9 +768,6 @@ tcp_process(struct tcp_pcb *pcb)
       pcb->mss = tcp_eff_send_mss(pcb->mss, &pcb->local_ip, &pcb->remote_ip);
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
 
-      /* Set ssthresh again after changing 'mss' and 'snd_wnd' */
-      pcb->ssthresh = LWIP_TCP_INITIAL_SSTHRESH(pcb);
-
       pcb->cwnd = LWIP_TCP_CALC_INITIAL_CWND(pcb->mss);
       LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_process (SENT): cwnd %"TCPWNDSIZE_F
                                    " ssthresh %"TCPWNDSIZE_F"\n",
@@ -854,11 +848,6 @@ tcp_process(struct tcp_pcb *pcb)
          * we'd better pass it on to the application as well. */
         tcp_receive(pcb);
 
-        /* passive open: update initial ssthresh now that the correct window is
-           known: if the remote side supports window scaling, the window sent
-           with the initial SYN can be smaller than the one used later */
-        pcb->ssthresh = LWIP_TCP_INITIAL_SSTHRESH(pcb);
-
         /* Prevent ACK for SYN to generate a sent event */
         if (recv_acked != 0) {
           recv_acked--;