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Re: iMPLS benefit

  • From: W. Mark Townsley
  • Date: Mon Mar 15 13:19:37 2004 

Please see inline.

Yakov Rekhter wrote:


Mark,
i heard there is a way to run MPLS for layer3 VPN(2547)
service without needing to run label switching in the
core(LDP/TDP/RSVP) but straight IP (aka iMPLS). 
	ftp://ftp.ietf.org/internet-drafts/draft-townsley-l2tpv3-mpls-01.txt

	See also Mark's talk from the last NANOG

	http://nanog.org/mtg-0402/townsley.html

That requires to run L2TP. An alternative is to run GRE (or even plain
IP). The latter (GRE) is implemented by quite a few vendors (and is
known to be interoperable among multiple vendors).
The only multi-vendor interoperable mode of GRE that I am aware of requires manual provisioning of point-to-point GRE tunnels between MPLS networks and to each and every IP-only reachable PE.

I guess you are *not* aware of the Redback implementation of 2547
over GRE, as this implementation is (a) available today, (b)
interoperable with other implementations of 2547 over GRE, and (c)
does *not* require manual provisioning of point-to-point GRE tunnels
between MPLS networks and to each and every IP-only reachable PE.

And, just for the record, (stating the obvious) I don't work for Redback.
Are you referring to draft-raggarwa-ppvpn-tunnel-encap-sig-03.txt? (Just guessing as the principal author used to work for Redback). Thanks for the update, I was in fact not aware that companies other than Redback had implemented it. You didn't name those companies, but I will happily stand corrected here.

In any case, the point I was trying to make was that there is more than one way to do "MPLS over GRE" and that they are not all necessarily interoperable as you seemed to imply in your message. You have helped to make that quite clear.

The BGP extension defined in the draft below allows "iMPLS" for 2547 VPN support without requiring any manually provisioned tunnels (and works for "mGRE" or L2TPv3).

http://www.watersprings.org/pub/id/draft-nalawade-kapoor-tunnel-safi-01.txt
The question to ask is whether the extension you mentioned above is
truly necessary for supporting 2547 over GRE. The Redback implementation
I mentioned above is an existence proof that the extension is *not*
necessary for 2547 over GRE that does *not* involve manually provisioned
GRE tunnels.
Both draft-nalawade-kapoor-tunnel-safi-01.txt and draft-raggarwa-ppvpn-tunnel-encap-sig-03.txt use BGP to advertise capabilities for carrying MPLS-labeled packets over various encapsulation types. Proof of one is essentially proof for the other, though the existence of both definitions highlights an unfortunate interoperability concern (particularly so for GRE, since each identify slightly different ways to advertise MPLS over GRE).

If you are not advertising capabilities at all, then you either have to maintain a list of which PEs can and will perform 2547 over GRE (and we are back to manual provisioning of tunnels), or you have to assume that ALL will in precisely the same way (eliminating all native MPLS processing for PEs that are reachable via MPLS directly).

Note that "mGRE" (multipoint GRE) is *not* the same as the point-to-point GRE
method that Yakov is referring to. Same header, different usage.

Enabling MPLS over any type of IP tunnel changes the security characteristics
of your 2547 deployment, in particular with respect to packet spoofing attacks. The L2TPv3 encapsulation used with the extension defined above provides anti-spoofing protection for blind attacks (e.g., the kind that a script kiddie could launch fairly easily) with miniscule operational overhead vs. GRE which relies on IPsec.

GRE relies on IPSec in *some*, but *not all* cases. Another alternative
is to use packet filtering. Quoting from the 2547 over GRE spec:

Protection against spoofed IP packets requires having all the
boundary routers perform filtering; either filtering out packets
from "outside" which are addressed to PE routers, or filtering out
packets from "outside" which have source addresses that belong
"inside" and filtering on each PE all packets which have source addresses that belong "outside".
And the same paragraph goes on to say:

"The maintenance of these filter lists can be
management-intensive, and the their use at all border routers can
affect the performance seen by all traffic entering the SP's network."

So, 2547 over GRE without IPsec relies upon a valid source/dest IP check for each packet at all boundary points in the network. Rather than rely only on this, the 2547 over L2TPv3 encapsulation defines its own (randomly selected and advertised along with the tunnel capabilities for that PE) 64-bit value to be validated before processing a packet at the router which is actually performing the 2547 VPN service.

Both of these methods are filtering on cleartext header information in order to avoid the complexity and overhead of IPsec while inhibiting "script-kiddie-like" IP spoofing attacks attempting to infiltrate a VPN, but 2547 over L2TPv3 gets around the concerns with 2547 over GRE identified above as there are no filter lists to be manually maintained, and the filtering is performed only on the routers actually participating in 2547 over L2TPv3 service.

- Mark