internet: (fixes #2741) IPv4 fragmentation fails when re-fragmenting a previous last fragment.

RELEASE_NOTES

@@ -103,6 +103,7 @@ Bugs fixed
 - Bug 2671 - ArpCache::Entry::SetMacAddress is misspelled
 - Bug 2717 - Fix mask generation for Ipv4RoutingTableEntry::CreateDefaultRoute
 - Bug 2722 - 802.11g sends DSSS spectrum signals using CreateOfdmTxPowerSpectralDensity
+- Bug 2741 - IPv4 fragmentation fails when last fragment have to be re-fragmented.
 
 Known issues
 ------------

src/internet/model/ipv4-l3-protocol.cc

@@ -1422,16 +1422,10 @@ Ipv4L3Protocol::DoFragmentation (Ptr<Packet> packet, const Ipv4Header & ipv4Head
 
   uint16_t offset = 0;
   bool moreFragment = true;
-  uint16_t originalOffset = 0;
-  bool alreadyFragmented = false;
+  uint16_t originalOffset = ipv4Header.GetFragmentOffset();
+  bool isLastFragment = ipv4Header.IsLastFragment();
   uint32_t currentFragmentablePartSize = 0;
 
-  if (!ipv4Header.IsLastFragment())
-    {
-      alreadyFragmented = true;
-      originalOffset = ipv4Header.GetFragmentOffset();
-    }
-
   // IPv4 fragments are all 8 bytes aligned but the last.
   // The IP payload size is:
   // floor( ( outIfaceMtu - ipv4Header.GetSerializedSize() ) /8 ) *8
@@ -1453,7 +1447,7 @@ Ipv4L3Protocol::DoFragmentation (Ptr<Packet> packet, const Ipv4Header & ipv4Head
         {
           moreFragment = false;
           currentFragmentablePartSize = p->GetSize () - offset;
-          if (alreadyFragmented)
+          if (!isLastFragment)
             {
               fragmentHeader.SetMoreFragments ();
             }