Document route caching, how it can be enabled, and its consequences.

Wording by:	cperciva

1 files changed, 12 insertions, 1 deletions

diff --git a/share/man/man4/gif.4 b/share/man/man4/gif.4
index 287bc15..19ad835 100644
--- a/share/man/man4/gif.4
+++ b/share/man/man4/gif.4
@@ -150,9 +150,20 @@ performs both martian and ingress filtering against the outer source address
 on egress.
 Note that martian/ingress filters are in no way complete.
 You may want to secure your node by using packet filters.
-Ingress filtering can be turned off by
+Ingress filtering can break tunnel operation in an asymmetrically
+routed network.
+It can be turned off by
 .Dv IFF_LINK2
 bit.
+.Ss Route caching
+Processing each packet requires two route lookups: first on the
+packet itself, and second on the tunnel destination.
+This second route can be cached, increasing tunnel performance.
+However, in a dynamically routed network, the tunnel will stick
+to the cached route, ignoring routing table updates.
+Route caching can be enabled with the
+.Dv IFF_LINK0
+flag.
 .\"
 .Ss Miscellaneous
 By default,