[tcp] Truncate TCP window to prevent future packet discards

Whenever memory pressure causes a queued packet to be discarded (and
so retransmitted), reduce the maximum TCP window to a size that would
have prevented the discard.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/net/tcp.c b/src/net/tcp.c
index e98d8b17d256a2f486e9255f4b3f9890b42d5bc3..938edd577ddcfe9bf8c9af98924c99478f3bf3cd 100644 (file)
--- a/src/net/tcp.c
+++ b/src/net/tcp.c
@@ -97,6 +97,8 @@ struct tcp_connection {
         * Equivalent to Rcv.Wind.Scale in RFC 1323 terminology
         */
        uint8_t rcv_win_scale;
+       /** Maximum receive window */
+       uint32_t max_rcv_win;
 
        /** Transmit queue */
        struct list_head tx_queue;
@@ -295,6 +297,7 @@ static int tcp_open ( struct interface *xfer, struct sockaddr *peer,
        tcp->tcp_state = TCP_STATE_SENT ( TCP_SYN );
        tcp_dump_state ( tcp );
        tcp->snd_seq = random();
+       tcp->max_rcv_win = TCP_MAX_WINDOW_SIZE;
        INIT_LIST_HEAD ( &tcp->tx_queue );
        INIT_LIST_HEAD ( &tcp->rx_queue );
        memcpy ( &tcp->peer, st_peer, sizeof ( tcp->peer ) );
@@ -557,9 +560,7 @@ static int tcp_xmit ( struct tcp_connection *tcp ) {
        tcp_process_tx_queue ( tcp, len, iobuf, 0 );
 
        /* Expand receive window if possible */
-       max_rcv_win = ( ( freemem * 3 ) / 4 );
-       if ( max_rcv_win > TCP_MAX_WINDOW_SIZE )
-               max_rcv_win = TCP_MAX_WINDOW_SIZE;
+       max_rcv_win = tcp->max_rcv_win;
        app_win = xfer_window ( &tcp->xfer );
        if ( max_rcv_win > app_win )
                max_rcv_win = app_win;
@@ -1299,13 +1300,29 @@ struct tcpip_protocol tcp_protocol __tcpip_protocol = {
 static unsigned int tcp_discard ( void ) {
        struct tcp_connection *tcp;
        struct io_buffer *iobuf;
+       struct tcp_rx_queued_header *tcpqhdr;
+       uint32_t max_win;
        unsigned int discarded = 0;
 
        /* Try to drop one queued RX packet from each connection */
        list_for_each_entry ( tcp, &tcp_conns, list ) {
                list_for_each_entry_reverse ( iobuf, &tcp->rx_queue, list ) {
+
+                       /* Limit window to prevent future discards */
+                       tcpqhdr = iobuf->data;
+                       max_win = ( tcpqhdr->seq - tcp->rcv_ack );
+                       if ( max_win < tcp->max_rcv_win ) {
+                               DBGC ( tcp, "TCP %p reducing maximum window "
+                                      "from %d to %d\n",
+                                      tcp, tcp->max_rcv_win, max_win );
+                               tcp->max_rcv_win = max_win;
+                       }
+
+                       /* Remove packet from queue */
                        list_del ( &iobuf->list );
                        free_iob ( iobuf );
+
+                       /* Report discard */
                        discarded++;
                        break;
                }