To match up with other packages that work with IP protocol stack.

Change-Id: I1f4eb76051964766113914c9698233022e73d5eb
Reviewed-on: https://go-review.googlesource.com/30575
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>