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

Change-Id: I2d20e4194617b05b6b46d6fba5df1f6c12ae2793
Reviewed-on: https://go-review.googlesource.com/30574
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>