From 7ea81ca23758656396fdc50e5efc0c17d028891f Mon Sep 17 00:00:00 2001
From: Keith Winstein <keithw@mit.edu>
Date: Wed, 3 Aug 2011 21:20:44 -0400
Subject: [PATCH] Path MTU

---
 dos_assert.hpp |  2 +-
 network.cpp    | 30 +++++++++++++++++++++++++++++-
 network.hpp    |  7 +++++--
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/dos_assert.hpp b/dos_assert.hpp
index 3b1acf2..f28ecf9 100644
--- a/dos_assert.hpp
+++ b/dos_assert.hpp
@@ -6,7 +6,7 @@
 
 static void dos_detected( const char *expression, const char *file, int line, const char *function )
 {
-  fprintf( stderr, "Illegal counterparty input (possible DOS) in function %s at %s:%d, failed test: %s\n",
+  fprintf( stderr, "Illegal counterparty input (possible denial of service) in function %s at %s:%d, failed test: %s\n",
 	   function, file, line, expression );
   exit( 1 );
 }
diff --git a/network.cpp b/network.cpp
index 3667995..b5c71ab 100644
--- a/network.cpp
+++ b/network.cpp
@@ -60,7 +60,8 @@ Connection<Outgoing, Incoming>::Connection( bool s_server )
     sock( -1 ),
     remote_addr(),
     server( s_server ),
-    attached( false )
+    attached( false ),
+    MTU( RECEIVE_MTU )
 {
 
   /* create socket */
@@ -80,6 +81,31 @@ Connection<Outgoing, Incoming>::Connection( bool s_server )
     perror( "bind" );
     exit( 1 );
   }
+
+  /* Enable path MTU discovery */
+  char flag = IP_PMTUDISC_DO;
+  socklen_t optlen = sizeof( flag );
+  if ( setsockopt( sock, IPPROTO_IP, IP_MTU_DISCOVER, &flag, optlen ) < 0 ) {
+    perror( "setsockopt" );
+    exit( 1 );
+  }
+}
+
+template <class Outgoing, class Incoming>
+void Connection<Outgoing, Incoming>::update_MTU( void )
+{
+  socklen_t optlen = sizeof( MTU );
+  if ( getsockopt( sock, IPPROTO_IP, IP_MTU, &MTU, &optlen ) < 0 ) {
+    perror( "getsockopt" );
+    exit( 1 );
+  }
+
+  if ( optlen != sizeof( MTU ) ) {
+    fprintf( stderr, "Error getting path MTU.\n" );
+    exit( 1 );
+  }
+
+  fprintf( stderr, "Path MTU: %d\n", MTU );  
 }
 
 template <class Outgoing, class Incoming>
@@ -115,6 +141,8 @@ void Connection<Outgoing, Incoming>::send( Outgoing &s )
     perror( "sendto" );
     exit( 1 );
   }
+
+  update_MTU();
 }
 
 template <class Outgoing, class Incoming>
diff --git a/network.hpp b/network.hpp
index f74403d..26b7e78 100644
--- a/network.hpp
+++ b/network.hpp
@@ -49,10 +49,9 @@ namespace Network {
 
     uint64_t next_seq;
     const Direction direction;
-    int MTU;
 
     Flow( Direction s_direction )
-      : next_seq( 0 ), direction( s_direction ), MTU( 2048 )
+      : next_seq( 0 ), direction( s_direction )
     {}
 
     Packet new_packet( Payload &s_payload );
@@ -71,6 +70,10 @@ namespace Network {
     bool server;
     bool attached;
 
+    int MTU;
+
+    void update_MTU( void );
+
   public:
     Connection( bool s_server );