Fixed MTU method for linux TUN interface

Updated the TUN interface
Added the "MTU" method for the linux implementation of the TUN interface

src/constants.go

@@ -4,15 +4,17 @@ import (
 	"time"
 )
 
+/* Specification constants */
+
 const (
 	RekeyAfterMessages      = (1 << 64) - (1 << 16) - 1
+	RejectAfterMessages     = (1 << 64) - (1 << 4) - 1
 	RekeyAfterTime          = time.Second * 120
 	RekeyAttemptTime        = time.Second * 90
-	RekeyTimeout            = time.Second * 5 // TODO: Exponential backoff
+	RekeyTimeout            = time.Second * 5
 	RejectAfterTime         = time.Second * 180
-	RejectAfterMessages     = (1 << 64) - (1 << 4) - 1
 	KeepaliveTimeout        = time.Second * 10
-	CookieRefreshTime       = time.Minute * 2
+	CookieRefreshTime       = time.Second * 120
 	MaxHandshakeAttemptTime = time.Second * 90
 )
 
@@ -20,11 +22,13 @@ const (
 	RekeyAfterTimeReceiving = RekeyAfterTime - KeepaliveTimeout - RekeyTimeout
 )
 
+/* Implementation specific constants */
+
 const (
 	QueueOutboundSize      = 1024
 	QueueInboundSize       = 1024
 	QueueHandshakeSize     = 1024
 	QueueHandshakeBusySize = QueueHandshakeSize / 8
 	MinMessageSize         = MessageTransportSize // keep-alive
-	MaxMessageSize         = 4096
+	MaxMessageSize         = 4096                 // TODO: make depend on the MTU?
 )