diff -r a63513286aa0 src/internet-stack/ipv4-l3-protocol.h
--- a/src/internet-stack/ipv4-l3-protocol.h	Thu Oct 01 22:49:25 2009 -0700
+++ b/src/internet-stack/ipv4-l3-protocol.h	Fri Oct 02 14:30:05 2009 +0100
@@ -53,6 +53,16 @@
  * 
  * This is the actual implementation of IP.  It contains APIs to send and
  * receive packets at the IP layer, as well as APIs for IP routing.
+ *
+ * This class contains two distinct groups of trace sources.  The
+ * trace sources 'Rx' and 'Tx' are called, respectively, immediately
+ * after receiving from the NetDevice and immediately before sending
+ * to a NetDevice for transmitting a packet.  These are low level
+ * trace sources that include the Ipv4Header already serialized into
+ * the packet.  In contrast, the Drop, SendOutgoing, UnicastForward,
+ * and LocalDeliver trace sources are slightly higher-level and pass
+ * around the Ipv4Header as an explicit parameter and not as part of
+ * the packet.
  */
 class Ipv4L3Protocol : public Ipv4
 {
diff -r a63513286aa0 src/internet-stack/ipv6-l3-protocol.h
--- a/src/internet-stack/ipv6-l3-protocol.h	Thu Oct 01 22:49:25 2009 -0700
+++ b/src/internet-stack/ipv6-l3-protocol.h	Fri Oct 02 14:30:05 2009 +0100
@@ -44,6 +44,16 @@
 /**
  * \class Ipv6L3Protocol
  * \brief IPv6 layer implementation.
+ *
+ * This class contains two distinct groups of trace sources.  The
+ * trace sources 'Rx' and 'Tx' are called, respectively, immediately
+ * after receiving from the NetDevice and immediately before sending
+ * to a NetDevice for transmitting a packet.  These are low level
+ * trace sources that include the Ipv4Header already serialized into
+ * the packet.  In contrast, the Drop, SendOutgoing, UnicastForward,
+ * and LocalDeliver trace sources are slightly higher-level and pass
+ * around the Ipv4Header as an explicit parameter and not as part of
+ * the packet.
  */
 class Ipv6L3Protocol : public Ipv6
 {