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Re: [RRG] MPLS with different types of mapping



Hi Iljitsch,

I must admit that I was thinking of using interdomain encapsulation to be MPLS and in fact I have one idea not related to RRG but to just labeled BGP RFC3107 but I would like to clarify few of the points you make below:

> - ITR only gets to choose destination ETR, not alternative ways to get
> there

No matter if you use IP or MPLS encapsulation you are following IP routing table. Of course I am assuming you are not suggesting inter-domain internet wide RSVP-TE LSPs, but just label distribution with LDP/labeled-BGP combination.

> the possibility to specify paths rather than just exit points.

That would be possible with path building ... see above.

> Because
> MPLS uses a label rather than a tunnel endpoint address, it's much
> easier to decapsulate packets at arbitrary locations rather than at
> one exactly specified point.

I would say this is just opposite. Label is allocated by a given router/ETR. There is no way unless to synchronize the meaning of a label between decapsulators for the label to mean the same in N nodes. In fact this is why there was not one hero today to manage to come with anycasting in MPLS :)

> Next step would be something along the lines of distributing labels in
> eBGP between consenting neighboring ASes, so that AS A can tell AS B
> to put label L on packets towards prefix X, so that B's ingress router
> doesn't have to do a full table lookup, either. Fairly boring stuff.

That's already done today for host routes .. RFC3107.

> it's useful to make these packets go through the core faster, so it

Note that MPLS marketing used to say back in 1996 that MPLS will make the packets go faster ... Not true ... IP lookup and MPLS lookups with corresponding rewrites in today's routers are of the same speed.

To conclude I do not see it feasible for hosts to support MPLS ... further I do not see this src (host or router) based path selection flexibility you are describing and last I fail to see the reduction of global routing table size in your proposal.

Cheers,
R.

Hi all,

I'm a bit behind on the recent discussions, but I want to throw out a new idea. Please let me know whether you would like to see this worked out in detail. (Yes/no in private mail is acceptable, as is discussion on the list, of course.)

LISP is prominently on the table along with a variety of mapping mechanisms. However, LISP suffers from a number of issues:

- requires new code and new ways of doing things
- incremental deployment looks hard
- ITR only gets to choose destination ETR, not alternative ways to get there
- packet overhead is relatively large

What I propose is use MPLS instead. The MPLS equivalent of the ITR/ETR functions are widely available in implementations today. MPLS has the potential to have relatively little packet overhead, and it allows for the possibility to specify paths rather than just exit points. Because MPLS uses a label rather than a tunnel endpoint address, it's much easier to decapsulate packets at arbitrary locations rather than at one exactly specified point.

It's fairly easy to deploy MPLS today and put labels on packets when they enter an ISP network, tunnel them through the core and pop the label at a border router and egress the packet from the AS.

Next step would be something along the lines of distributing labels in eBGP between consenting neighboring ASes, so that AS A can tell AS B to put label L on packets towards prefix X, so that B's ingress router doesn't have to do a full table lookup, either. Fairly boring stuff.

But it gets more interesting if we give sources (could be site exit routers or middleboxes, but ideally source hosts) the ability to put a label stack on packets. That would work like this: at some point, a source decides that it's sending so many packets to a destination that it's useful to make these packets go through the core faster, so it sends out a bubble packet. This is an IP packet with as its destination address the destination in question, preceded by an MPLS header with a special "bubble" label value. If the ISP doesn't support this mechanism, the packet is simply dropped and nothing happens. (Remember, this packet is not part of the ongoing communication with the destination.) If the ISP supports the mechanism, it returns to the source a set of label stacks with priorities. The source now encapsulates packets to the destination in one of the label stacks. It monitors return traffic to see if the path works, and switches to a different one if it doesn't. (So this requires state, want this to happen close to the source.) The source can then send a packet that has one of the thusly discovered label stacks followed by a bubble to discover label stacks for paths through the next AS. And so on until it knows a full path or possibly a number of full paths.

This mechanism allows for multihoming and network (even host?) mobility by having a destination declare that it's currently residing at an alternative address. The source then sends a bubble for that address, but when it has a full path, it encapslates packets with the original destination address using the MPLS stack for the path towards the care of address.

The thing that I really like about this is that it allows for a source to find different paths towards the same destination, which gives the host ways to optimize its communication that don't exist today.

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