Add more documentation about the "net.inet.ip.random_id" sysctl knob

and how it can affect information flow between observers.

MFC after:	1 week

1 files changed, 15 insertions, 3 deletions

diff --git a/share/man/man4/inet.4 b/share/man/man4/inet.4
index 3a2bf8a..60a382c 100644
--- a/share/man/man4/inet.4
+++ b/share/man/man4/inet.4
@@ -28,7 +28,7 @@
 .\"     From: @(#)inet.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd April 2, 2015
+.Dd April 3, 2015
 .Dt INET 4
 .Os
 .Sh NAME
@@ -244,10 +244,22 @@ IP datagrams (or all IP datagrams, if
 .Va ip.rfc6864
 is disabled) to be randomized instead of incremented by 1 with each packet
 generated.
-This closes a minor information leak which allows remote observers to
+This prevents information exchange between any combination of two or
+more inside and/or outside observers using packet frequency
+modulation, PFM.
+An outside observer can ping the outside facing port at a fixed rate
+sampling the returned counter.
+An inside observer can ping the inside facing port sampling the same
+counter.
+Even though packets don't flow directly between any of the observers
+any single observer can influence the data rate the other observer(s)
+is or are sampling.
+This is done by sending more or less ping packets towards the gateway
+per measured interval.
+Setting this sysctl also prevents the remote and internal observers to
 determine the rate of packet generation on the machine by watching the
 counter.
-In the same time, on high-speed links, it can decrease the ID reuse
+At the same time, on high-speed links, it can decrease the ID reuse
 cycle greatly.
 Default is 0 (sequential IP IDs).
 IPv6 flow IDs and fragment IDs are always random.