Date:	Thu, 05 Mar 2015 08:00:34 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Venkatraman <vivek@...ulusnetworks.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	roopa <roopa@...ulusnetworks.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	santiago@...reenet.org
Subject: Re: [PATCH net-next 8/8] ipmpls: Basic device for injecting packets into an mpls tunnel

Vivek Venkatraman <vivek@...ulusnetworks.com> writes:

> It is great to see an MPLS data plane implementation make it into the
> kernel. I have a couple of questions on this patch.
>
> On Wed, Feb 25, 2015 at 9:18 AM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>>
>>
>> Allow creating an mpls tunnel endpoint with
>>
>> ip link add type ipmpls.
>>
>> This tunnel has an mpls label for it's link layer address, and by
>> default sends all ingress packets over loopback to the local MPLS
>> forwarding logic which performs all of the work.
>>
>
> Is it correct that to achieve IPoMPLS, each LSP has to be installed as
> a link/netdevice?

This is still a bit in flux.  The ingress logic is not yet merged.  When
I resent the patches I did not resend this one as I am less happy with
it than I am about the others and the problem is orthogonal.

> If ingress packets loopback with the label associated with the link to
> hit the MPLS forwarding logic, how does it work if each packet has to
> be then forwarded with a different label stack? One use case is a
> common IP/MPLS application such as L3VPNs (RFC 4364) where multiple
> VPNs may reside over the same LSP, each having its own VPN (inner)
> label.

If we continue using this approach (which I picked because it was simple
for bootstrapping and testing) the way it would work is that you have a
local label that when you forward packets with that label all of the
other needed labels are pushed.

That said I think the approach I chose has a lot going for it.

Fundamentally I think the ingress to an mpls tunnel fundamentally needs
the same knobs and parameters as struct mpls_route.  Aka which machine
do we forward the packets to, and which labels do we push.

The extra decrement of the hop count on ingress is not my favorite
thing.

The question in my mind is how do we select which mpls route to use.
Spending a local label for that purpose does not seem particularly
unreasonable.

Using one network device per tunnel it a bit more questionable.  I keep
playing with ideas that would allow a single device to serve multiple
mpls tunnels.

For going from normal ip routing to mpls routing somewhere we need the 
the destination ip prefix to mpls tunnel mapping. There are a couple of 
possible ways this could be solved.
- One ingress network device per mpls tunnel.
- One ingress network device and with with a a configurable routing
  prefix to mpls mapping.  Possibly loaded on the fly.  net/atm/clip.c
  does something like this for ATM virtual circuits.
- One ingress network device that looks at IP_ROUTE_CLASSID and
  use that to select the mpls labels to use.
- Teach the IP network stack how to insert packets in tunnels without
  needing a magic netdevice.

None of the ideas I have thought of so far feels just right.

At the same time I don't think there is a lot of wiggle room in the
fundmanetals.  Mapping ip routes to mpls tunnels in a way that software
can process quickly and efficiently and the code is maintainable, does
not leave a lot of wiggle room at the end of the day.

Eric
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