--- a/src/internet-stack/ipv4-l3-protocol.h	Tue Oct 06 22:59:55 2009 -0700
+++ b/src/internet-stack/ipv4-l3-protocol.h	Tue Oct 06 23:00:21 2009 -0700
@@ -249,6 +249,7 @@
   TracedCallback<const Ipv4Header &, Ptr<const Packet>, uint32_t> m_unicastForwardTrace;
   TracedCallback<const Ipv4Header &, Ptr<const Packet>, uint32_t> m_localDeliverTrace;
 
+  // The following two traces pass a packet with an IP header
   TracedCallback<Ptr<const Packet>, uint32_t> m_txTrace;
   TracedCallback<Ptr<const Packet>, uint32_t> m_rxTrace;
   // <ip-header, payload, reason, ifindex> (ifindex not valid if reason is DROP_NO_ROUTE)