svn commit: r281024 - head/share/man/man4

[Hans Petter Selasky]

Author: hselasky
Date: Fri Apr  3 14:00:08 2015
New Revision: 281024
URL: https://svnweb.freebsd.org/changeset/base/281024

Log:
  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

Modified:
  head/share/man/man4/inet.4

Modified: head/share/man/man4/inet.4
==============================================================================
--- head/share/man/man4/inet.4	Fri Apr  3 13:57:14 2015	(r281023)
+++ head/share/man/man4/inet.4	Fri Apr  3 14:00:08 2015	(r281024)
@@ -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.