From c6a796675d51cba3a9d858652e7d6b52815769b3 Mon Sep 17 00:00:00 2001
From: Matthias Radestock <matthias.radestock@gmail.com>
Date: Tue, 26 Aug 2014 21:05:13 +0100
Subject: [PATCH] pay attention to EMSGSIZE errors when sending pmtu
 verification packets and drop the pmtu accordingly This way, when we actually
 *do* get pmtu info from the network/kernel we can discover the correct pmtu
 much more quickly. Especially in the absence of other traffic, forwarding of
 which was previously the only place where we'd check for EMSGSIZE.

Fixes #12
---
 forwarder.go | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/forwarder.go b/forwarder.go
index c00631e4..c57d58e4 100644
--- a/forwarder.go
+++ b/forwarder.go
@@ -70,15 +70,29 @@ func (conn *LocalConnection) forwarderLoop(forwardCh <-chan *ForwardedFrame, sto
 	var pmtuVerifyTick <-chan time.Time
 	var clientPMTU int
 	maxPayload := pmtu - UDPOverhead
+	calculateClientPMTU := func(ftbe FrameTooBigError) {
+		maxPayload = ftbe.PMTU - UDPOverhead
+		clientPMTU = maxPayload - enc.PacketOverhead() - enc.FrameOverhead() - EthernetOverhead
+	}
 	updateClientPMTU := func() {
-		conn.setClientPMTU(clientPMTU)
-		pmtuVerifyFrame := &ForwardedFrame{
-			srcPeer: conn.local,
-			dstPeer: conn.remote,
-			frame:   make([]byte, clientPMTU+EthernetOverhead)}
-		for cnt := 0; cnt < 10; cnt += 1 {
-			enc.AppendFrame(pmtuVerifyFrame)
-			udpSender.Send(enc.Bytes())
+		for gotNewPMTU := true; gotNewPMTU; {
+			conn.setClientPMTU(clientPMTU)
+			pmtuVerifyFrame := &ForwardedFrame{
+				srcPeer: conn.local,
+				dstPeer: conn.remote,
+				frame:   make([]byte, clientPMTU+EthernetOverhead)}
+			gotNewPMTU = false
+			for cnt := 0; cnt < 10; cnt += 1 {
+				enc.AppendFrame(pmtuVerifyFrame)
+				err := udpSender.Send(enc.Bytes())
+				if err != nil {
+					if ftbe, ok := err.(FrameTooBigError); ok {
+						calculateClientPMTU(ftbe)
+						gotNewPMTU = true
+						break
+					}
+				}
+			}
 		}
 		pmtuVerifyTick = time.After(PMTUVerifyTimeout)
 	}
@@ -94,8 +108,7 @@ func (conn *LocalConnection) forwarderLoop(forwardCh <-chan *ForwardedFrame, sto
 		err := udpSender.Send(enc.Bytes())
 		if err != nil {
 			if ftbe, ok := err.(FrameTooBigError); ok {
-				maxPayload = ftbe.PMTU - UDPOverhead
-				clientPMTU = maxPayload - enc.PacketOverhead() - enc.FrameOverhead() - EthernetOverhead
+				calculateClientPMTU(ftbe)
 				updateClientPMTU()
 			} else if PosixError(err) == syscall.ENOBUFS {
 				// TODO handle this better