diff --git a/src/internet-stack/ipv4-l3-protocol.h b/src/internet-stack/ipv4-l3-protocol.h
index a22425034..006434d46 100644
--- a/src/internet-stack/ipv4-l3-protocol.h
+++ b/src/internet-stack/ipv4-l3-protocol.h
@@ -53,6 +53,16 @@ class Icmpv4L4Protocol;
  * 
  * 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 --git a/src/internet-stack/ipv6-l3-protocol.h b/src/internet-stack/ipv6-l3-protocol.h
index ecbef39c1..c11961c72 100644
--- a/src/internet-stack/ipv6-l3-protocol.h
+++ b/src/internet-stack/ipv6-l3-protocol.h
@@ -44,6 +44,16 @@ class Ipv6AutoconfiguredPrefix;
 /**
  * \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
 {