partly fixed bug #25882: TCP hangs on MSS > pcb->snd_wnd (by not creating segments bigger than half the window)

CHANGELOG

@@ -76,6 +76,10 @@ HISTORY
 
  ++ Bugfixes:
 
+  2012-02-12: Simon Goldschmidt
+  * tcp.h, tcp_in.c, tcp_out.c: partly fixed bug #25882: TCP hangs on
+    MSS > pcb->snd_wnd (by not creating segments bigger than half the window)
+
   2012-02-11: Simon Goldschmidt
   * tcp.c: fixed bug #35435: No pcb state check before adding it to time-wait
     queue while closing

src/core/tcp_in.c

@@ -501,6 +501,7 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
     npcb->rcv_nxt = seqno + 1;
     npcb->rcv_ann_right_edge = npcb->rcv_nxt;
     npcb->snd_wnd = tcphdr->wnd;
+    npcb->snd_wnd_max = tcphdr->wnd;
     npcb->ssthresh = npcb->snd_wnd;
     npcb->snd_wl1 = seqno - 1;/* initialise to seqno-1 to force window update */
     npcb->callback_arg = pcb->callback_arg;
@@ -653,6 +654,7 @@ tcp_process(struct tcp_pcb *pcb)
       pcb->rcv_ann_right_edge = pcb->rcv_nxt;
       pcb->lastack = ackno;
       pcb->snd_wnd = tcphdr->wnd;
+      pcb->snd_wnd_max = tcphdr->wnd;
       pcb->snd_wl1 = seqno - 1; /* initialise to seqno - 1 to force window update */
       pcb->state = ESTABLISHED;
 
@@ -853,7 +855,7 @@ tcp_oos_insert_segment(struct tcp_seg *cseg, struct tcp_seg *next)
  * data, and if so frees the memory of the buffered data. Next, is places the
  * segment on any of the receive queues (pcb->recved or pcb->ooseq). If the segment
  * is buffered, the pbuf is referenced by pbuf_ref so that it will not be freed until
- * i it has been removed from the buffer.
+ * it has been removed from the buffer.
  *
  * If the incoming segment constitutes an ACK for a segment that was used for RTT
  * estimation, the RTT is estimated here as well.
@@ -888,6 +890,11 @@ tcp_receive(struct tcp_pcb *pcb)
        (pcb->snd_wl1 == seqno && TCP_SEQ_LT(pcb->snd_wl2, ackno)) ||
        (pcb->snd_wl2 == ackno && tcphdr->wnd > pcb->snd_wnd)) {
       pcb->snd_wnd = tcphdr->wnd;
+      /* keep track of the biggest window announced by the remote host to calculate
+         the maximum segment size */
+      if (pcb->snd_wnd_max < tcphdr->wnd) {
+        pcb->snd_wnd_max = tcphdr->wnd;
+      }
       pcb->snd_wl1 = seqno;
       pcb->snd_wl2 = ackno;
       if (pcb->snd_wnd == 0) {

src/core/tcp_out.c

@@ -232,7 +232,7 @@ tcp_pbuf_prealloc(pbuf_layer layer, u16_t length, u16_t max_length,
   LWIP_UNUSED_ARG(apiflags);
   LWIP_UNUSED_ARG(first_seg);
   /* always create MSS-sized pbufs */
-  alloc = pcb->mss;
+  alloc = max_length;
 #else /* LWIP_NETIF_TX_SINGLE_PBUF */
   if (length < max_length) {
     /* Should we allocate an oversized pbuf, or just the minimum
@@ -372,6 +372,8 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
   u16_t concat_chksummed = 0;
 #endif /* TCP_CHECKSUM_ON_COPY */
   err_t err;
+  /* don't allocate segments bigger than half the maximum window we ever received */
+  u16_t mss_local = LWIP_MIN(pcb->mss, pcb->snd_wnd_max/2);
 
 #if LWIP_NETIF_TX_SINGLE_PBUF
   /* Always copy to try to create single pbufs for TX */
@@ -430,7 +432,7 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
 
     /* Usable space at the end of the last unsent segment */
     unsent_optlen = LWIP_TCP_OPT_LENGTH(last_unsent->flags);
-    space = pcb->mss - (last_unsent->len + unsent_optlen);
+    space = mss_local - (last_unsent->len + unsent_optlen);
 
     /*
      * Phase 1: Copy data directly into an oversized pbuf.
@@ -523,7 +525,7 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
   while (pos < len) {
     struct pbuf *p;
     u16_t left = len - pos;
-    u16_t max_len = pcb->mss - optlen;
+    u16_t max_len = mss_local - optlen;
     u16_t seglen = left > max_len ? max_len : left;
 #if TCP_CHECKSUM_ON_COPY
     u16_t chksum = 0;
@@ -533,7 +535,7 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
     if (apiflags & TCP_WRITE_FLAG_COPY) {
       /* If copy is set, memory should be allocated and data copied
        * into pbuf */
-      if ((p = tcp_pbuf_prealloc(PBUF_TRANSPORT, seglen + optlen, pcb->mss, &oversize, pcb, apiflags, queue == NULL)) == NULL) {
+      if ((p = tcp_pbuf_prealloc(PBUF_TRANSPORT, seglen + optlen, mss_local, &oversize, pcb, apiflags, queue == NULL)) == NULL) {
         LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_write : could not allocate memory for pbuf copy size %"U16_F"\n", seglen));
         goto memerr;
       }
@@ -1067,7 +1069,6 @@ tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb)
 
   /* Add any requested options.  NB MSS option is only set on SYN
      packets, so ignore it here */
-  //LWIP_ASSERT("seg->tcphdr not aligned", ((mem_ptr_t)seg->tcphdr % MEM_ALIGNMENT) == 0);
   opts = (u32_t *)(void *)(seg->tcphdr + 1);
   if (seg->flags & TF_SEG_OPTS_MSS) {
     *opts = TCP_BUILD_MSS_OPTION(pcb->mss);

src/include/lwip/tcp.h

@@ -227,6 +227,7 @@ struct tcp_pcb {
                              window update. */
   u32_t snd_lbb;       /* Sequence number of next byte to be buffered. */
   u16_t snd_wnd;   /* sender window */
+  u16_t snd_wnd_max; /* the maximum sender window announced by the remote host */
 
   u16_t acked;