diff -rU3 man-pages-2.43-a/man7/ip.7 man-pages-2.43-b/man7/ip.7
--- man-pages-2.43-a/man7/ip.7	2006-09-26 09:54:29.000000000 -0400
+++ man-pages-2.43-b/man7/ip.7	2007-03-27 15:46:18.000000000 -0400
@@ -515,6 +515,7 @@
 IP_PMTUDISC_WANT:Use per-route settings.
 IP_PMTUDISC_DONT:Never do Path MTU Discovery.
 IP_PMTUDISC_DO:Always do Path MTU Discovery. 
+IP_PMTUDISC_PROBE:Set DF but ignore Path MTU.
 .TE   
 
 When PMTU discovery is enabled the kernel automatically keeps track of
@@ -550,6 +551,15 @@
 with the
 .B IP_MTU
 option.     
+
+It is possible to implement RFC 4821 MTU probing with
+.B SOCK_DGRAM
+of
+.B SOCK_RAW
+sockets by setting a value of IP_PMTUDISC_PROBE.  This is also particularly
+useful for diagnostic tools such as
+.BR tracepath (8)
+that wish to deliberately send probe packets larger than the observed Path MTU.
 .TP
 .B IP_MTU
 Retrieve the current known path MTU of the current socket.