netdev - A question about hh

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Message-ID: <147a89290712300531m4a8845betd23ef71f6d944799@mail.gmail.com>
Date:	Sun, 30 Dec 2007 15:31:53 +0200
From:	"Andy Johnson" <johnsonzjo@...il.com>
To:	netdev@...r.kernel.org
Subject: A question about hh_cache

Hi,

struct neighbour has a member named hh_cache; this
hh_cache caches link layer headers to speed up transmission
(instead filling them again and again for each packet).

The hh_cache struct is defined in linux/netdev.h.
Its first member is hh_next, which is a pointer to
hh_cache.
I tried to think of scenarios where this hh_next pointer is used,
and could not think of anything.

Each neighbour has one ip address (the primary key), so it has one
dev member (which is the net_device on which that primary key is configured),
so it has one mac address (which is the dev_addr of net_device).
So there should be only one entry in that hh_cache ; it should be
the entry which its hh_data (the ethernet header) is composed from the
dev_addr, the source MAC address, and the type.

Can anybody give an example when hh_cache of a neighbour instance is a
list with more than
one entry ?

Regards,
Johnson
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