netdev - Re: [PATCH net 0/2] lwtunnel: make it really work, for IPv4

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Date:	Thu, 24 Sep 2015 01:08:08 +0200
From:	Thomas Graf <tgraf@...g.ch>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Jiri Benc <jbenc@...hat.com>, netdev@...r.kernel.org,
	Roopa Prabhu <roopa@...ulusnetworks.com>
Subject: Re: [PATCH net 0/2] lwtunnel: make it really work, for IPv4

On 09/23/15 at 04:09pm, Eric W. Biederman wrote:

[...]

> *Blink* You were targeting net.git with a feature enhancement????
> I will just ignore that.

The point of this series is to not expose the src and dst port Netlink
bits to user space in a released kernel because the ABI is not set in
stone yet. Hence targeting net.

If patch 1 is regarded unacceptable we should at least pull in patch 2
to not expose these bits until this has been worked out to leave the
option proposed here on the table.

> What I was observing is that in general the only tunneled packets that
> need an ingress metadata dst for a tunneled medium ethernet like medium
> are arp and ndisc packets.  In other cases if you aren't doing something
> exceptional like openvswitch the normal routing should be sufficient.
> 
> Which means a ndo_reply_dst method could remove the need in many cases
> for an ingress metadata dst to need to be allocated.

The tunnel RX metadata collected is used to associate packets matching
a particular tunnel id with the appropriate virtual networks by forwarding
them to a separate netns, separate VRF device or a separate bridge.

More sophisticated hypervisors may run multiple tunnel endpoints on
the same host using different host addresses and differentiate packets
based on the underlay destination IP as well.

> Regardless a netdevice operation that digs into the packet and figures
> out what is necessary for a reply seems like the clean way to make this
> work for both arp and neighbour discovery.

I'm not disagreeing entirely although I disagree that you can do the
NDO without looking at the original metadata dst. Even a full fib
lookup based on the requested IP in the ARP header is somewhat error
prone. I fully agree though that once we support additional types
besides IP tunneling then such an NDO might in fact make sense.
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