The MPLS WG Archive

[David Charlap <David.Charlap@marconi.com>]

Erblichs wrote:
> 
> 	I would like to remind people that MPLS can not
> 	be used without IP, however IP can be used without
> 	MPLS.

This is not entirely true.  Although MPLS is almost always used in 
conjunction with IP, it doesn't have to be.  For instance:

- LSPs can be configured instead of signaled (much like how PVCs are
   configured on ATM switches.)

- Routing/topology information may be distributed using a non-IP routing
   protocol, like IS-IS.  (Although IS-IS is usually used to distribute
   IP topology information, it uses ISO addresses, not IP addresses.)

- Although today's MPLS signaling protocols (LDP and RSVP-TE) are both
   tied to IP addressing, there is no reason why a non-IP signaling
   protocol couldn't be developed for setting up LSPs.  (For that matter,
   RSVP-TE could even be extended to allow signaling over non-IP networks
   (it would only require new C-Types for the SESSION, SENDER_TEMPLATE
   and FILTER_SPEC objects and a few new subobjects for the
   EXPLICIT_ROUTE and RECORD_ROUTE classes.)

- Non-IP traffic may be (and sometimes is already) carried in LSPs.  In
   RSVP-TE, the layer-3 protocol is signaled in the SESSION_ATTRIBUTES
   object.  For layer-2 LSPs, an extra label (Martini or PWE3) may be
   used to identify the traffic.

> 	IPvX is a end to end forwarding environment where
> 	MPLS is only a router to router forwarding
> 	environment.

Conventionally, yes.  But there's no technical reason why a host 
couldn't drive LSP creation.  It isn't done because there isn't any need 
for this, not because of any technical problems.

> 	MPLS, is normally associated within a domain
> 	so that the edge routers need specialized
> 	features; i.e. translate IP headers into lables.

Domain?  What exactly do you mean by this?  In the IP/internet world, a 
domain is a DNS term that has no applicability to routing/signaling.

In the ATM world, it refers to a group of routers that share routing 
information together.  The closest IP analog to an ATM domain would be 
an autonomous system (AS).

Although MPLS networks typically do not span across ASs, they can, in 
theory.  There is ongoing work to develop practical ways to make this a 
practical reality.

You are correct, however, that LERs (LSP edge routers) need the 
capability to push/pop labels onto/from unlabeled packets.

> 	Since routers within the MPLS doamin don't need the
> 	edge router capability, I would expect that this
> 	non feature could minimally reduce the cost.

This was a concern in the not-too-recent past.  Especially when using 
MPLS software to retrofit legacy ATM gear (interfaces on ATM switches 
typically don't have the capability to push/pop labels.)  Today, 
however, this is less of a concern, since all the major MPLS players are 
making hardware with this capability.

> 	IMO, a still important item that has been overlooked
> 	is the convergence time of some of the larger link-state
> 	environments and/or the newer wireless paths. I think
> 	that the label distribution mechanisms within MPLS
> 	probably are more efficient and can set up the
> 	paths quicker. This relates to fewer delays and
> 	lost pkts while routing environments are somewhat 
> 	chaotic.

MPLS, in a signaled environment, relies on an IGP.

LDP allocates and distributes labels as routes are learned.

RSVP-TE can't set up an LSP until routing converges.  Either the ingress 
node must compute a path to the destination or all the transit nodes 
must be able to forward the Path messages to the destination using local 
route tables.

You can, of course, pre-configure LSPs to get around this, that 
undermines many of the benefits of MPLS.

> 	Also, the link state protocols (ISIS and OSPF)
> 	assume that ONE path is preferred, and this translates
> 	to adding single entries for a dst in the forwarding
> 	table.

Not necessarily.  Equal-cost multipath extensions are common in the 
non-MPLS world.  Providing alternate routing paths where they're not all 
equal cost is uglier, of course.

> 	With MPLS, it is possible to have numerous
> 	paths to a dst where non-equal paths exist. This
> 	would allow a set of equal dst pkts (FEC) to traverse
> 	different paths based on different characteristics.
> 	These characteristics could be different ISP
> 	origination src addrs, bandwidth, delay, priority,
> 	SLAs, etc.

You can do this without MPLS, but it requires all your routers to 
classify all the packets and classify them according to the same set of 
rules.  It's inefficient (and probably will limit your throughput) but 
it can be done.

The big deal about MPLS isn't that you can do anything new, but that all 
of the work in classifying the packets is focussed entirely in the edge 
router (where bandwidths are probably lower) so that the core (where 
bandwidth is higher) doesn't have to do this work.

-- David

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