Subject: Re: [RFC PATCH] ppp: add support for L2 multihop / tunnel switching

From: Benjamin LaHaise <[email-masked]>
Date: Mon, 9 Jul 2012 10:15:11 -0400

On Mon, Jul 09, 2012 at 12:52:15PM +0100, James Chapman wrote:
> As a mechanism for switching PPP interfaces together, this patch is
> good. For L2TP though, I prefer an approach that would be applicable for
> all L2TP traffic types, not just PPP.

*nod*  This seems like a reasonable consideration.

> L2TP supports many different pseudowire types, and this patch will only
> be useful for tunnel switching between PPP pseudowires. Whereas if we
> implement it within the L2TP core, rather than in the PPP code, we would
> get switching between all pseudowire types. If we add this patch and
> then subsequently add switching between other pseudowires in the L2TP
> core (which we're likely to want to do), then we're left with two
> different interfaces for doing L2TP tunnel switching in the kernel.

At least for ethernet pseudowires, it can already be implemented by using 
an ethernet bridge device.  Besides PPP and ethernet pseudowires, what 
other types are supported at present by the L2TP core?

> The L2TP core allows traffic to be passed directly into an L2TP session.
> In the case of PPPoE, for example, the PPP data can be  extracted from a
> PPPoE packet and passed into an L2TP tunnel/session, with no PPP
> interface(s) involved.
> 
> That said, your approach allows two PPP interfaces to be switched
> together, which has its own advantages.

I think the approach I'm using should be reasonably efficient for PPPoE 
to L2TP, although the locking overhead in the PPP core probably needs to 
be reduced to improve scaling.  I haven't yet done any benchmarking on this 
approach to see how much overhead there is compared to the other code I'd 
written which took a more direct approach (this wasn't on top of the 
ppp_generic core, but the old Babylon kernel modules which have had this 
functionality for a long time).

> > The reasoning behind using dev_queue_xmit() rather than outputting directly 
> > to another PPP channel is to enable the use of the traffic shaping and 
> > queuing features of the kernel on multihop sessions.
> 
> I'm not sure about using a pseudo packet type to do this. For L2TP, it
> would seem better to add netfilter/tc support for L2TP data packets,
> which would let people add rules for, say, traffic in L2TP tunnel x /
> session y. This would avoid the need for ETH_P_PPP and you could then
> output directly to the ppp channel.

The downside of an L2TP specific method is that all the mechanisms need to 
be duplicated, resulting in a much higher maintenance overhead for the 
code and functionality, not to mention all the tool changes to go along 
with that.

As for the pseudo packet type, it may indeed be better to avoid the pseudo 
packet type for known PPP packet types.  One of the benefits of going the 
network device route is that it makes it much easier to implement additional 
functionality like lawful intercept, which would be yet more functionality 
that would have to be implemented if the mechanism is L2TP specific.  The 
pseudo packet type would still be needed for forwarding PPP frames that the 
kernel doesn't know about (all the *CP packet types and MLPPP come to mind)

I had thought about doing the packet forwarding in a manner similar to the 
bridging code -- that is, as a pseudowire bridge in the network core that 
only works between 2 devices.  That approach might work better for L2TP, as 
it would be able to pass packets of any type between the 2 endpoints.

		-ben
-- 
"Thought is the essence of where you are now."