Syslinux - Jul 2016 - [PATCH] core/lwip: Avoid 3-second delay for a half-open connection

If lwIP encounters a half-open connection (e.g. due to a restarted
application reusing the same port numbers) it will correctly send a
RST but will not resend the SYN until one retransmission timeout later
(approximately three seconds).  This can increase the time taken by
lpxelinux.0 to fetch its configuration file from a few milliseconds to
around 30 seconds.

Fix by immediately retransmitting the SYN whenever a half-open
connection is detected.

Signed-off-by: Michael Brown <mcb30 at ipxe.org>
---
 core/lwip/src/core/tcp_in.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/core/lwip/src/core/tcp_in.c b/core/lwip/src/core/tcp_in.c
index 9095264..d410dab 100644
--- a/core/lwip/src/core/tcp_in.c
+++ b/core/lwip/src/core/tcp_in.c
@@ -675,6 +675,9 @@ tcp_process(struct tcp_pcb *pcb)
       /* send a RST to bring the other side in a non-synchronized state. */
       tcp_rst(ackno, seqno + tcplen, ip_current_dest_addr(),
ip_current_src_addr(),
         tcphdr->dest, tcphdr->src);
+      /* Resend SYN immediately to establish connection */
+      pcb->rtime = 0;
+      tcp_rexmit_rto(pcb);
     }
     break;
   case SYN_RCVD:
-- 
2.3.8

On 04/07/16 08:23, Michael Brown wrote:
> If lwIP encounters a half-open connection (e.g. due to a restarted
> application reusing the same port numbers) it will correctly send a
> RST but will not resend the SYN until one retransmission timeout later
> (approximately three seconds).  This can increase the time taken by
> lpxelinux.0 to fetch its configuration file from a few milliseconds to
> around 30 seconds.
>
> Fix by immediately retransmitting the SYN whenever a half-open
> connection is detected.
FYI: this patch has been accepted by upstream lwIP:

   https://savannah.nongnu.org/patch/?9039

   http://git.savannah.gnu.org/cgit/lwip.git/commit/?id=2e26fc9

Michael