[RRG] MPLS with different types of mapping

[Iljitsch van Beijnum <iljitsch@muada.com>]

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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