Optical/IP Networks

MPLS Spurs Metro Ethernet Debate

Last week the Metro Ethernet Forum met in Boston to hash out a draft of technical specifications for deploying Ethernet in the metropolitan area network. The forum addressed areas that are key to making a more heavy-duty version of Ethernet for metropolitan networks: Ethernet Service Definition, Metro Ethernet Reference Model, Ethernet QoS framework, and Ethernet Protection (see MEF Touts Progress).

The issue of "protection" or network resiliency is fast becoming a major focus of debate.

What's surprised some analysts is the extent to which the proposal goes beyond simply aiming for traffic restoration within 50 milliseconds, the standard for legacy Sonet gear. Instead, the draft proposed by the MEF specifically states that MPLS (multiprotocol label switching) should be used to meet this goal. Analysts following the developments of metro Ethernet find this troublesome.

"I thought that their purpose was to define service levels so that carriers could create service-level agreements,” says Scott Clavenna, director of research at Light Reading and president of PointEast Research LLC. "I don’t think their role is to develop a method for doing protection. That’s the job of standards bodies like the IETF and the IEEE."

The meat of the controversy lies in the fact that for Ethernet to be accepted by service providers as a replacement for Sonet, it must provide the same quality and meet all the same standards as Sonet. This is especially critical if Ethernet ever hopes to replace Sonet for carrying voice traffic, for which 50 millisecond recovery times are a must. But this is easier said than done, because Ethernet was designed for local area networks, not large metropolitan networks.

Some service providers argue that MPLS is too complicated to be used for resiliency. They say that protection should be done at lower layers of the ISO stack to reduce complexity.

“I think that using MPLS for protection could be stretching its use,” says Eugene Pisman, a founder and the chief network architect at IntelliSpace, a metro service provider offering Ethernet data services. "It costs hard dollars to provision and buy the hardware to do MPLS. For example, from Riverstone or Extreme, you’d need to buy special blades Then sometimes they are implementing different versions of MPLS. You may get them to interoperate, but it will take work and will probably cost a lot to do.”

Clavenna and Pisman suggest other ways for handling resiliency. Resilent packet ring (RPR) technology is one way. The Resilient Packet Ring Alliance has been working with the IEEE 802.17 working group since last spring to come up with a standard for this technology (see RPR Divided). RPR, which has not been finalized yet, plans to develop a new MAC layer that will provide protection for all ring-based transport, including Ethernet.

Clavenna says that introducing a new signaling protocol like MPLS into the network could cause more problems than developing a new MAC to handle restoration.

But Nan Chen, chairman of the MEF and director of product marketing for Atrica Inc., an Ethernet equipment provider, argues that RPR would require service providers to buy new boxes, which he says will be expensive. He also says that disagreements within that standards body have made it difficult to predict the outcome of RPR (see RPR: RIP?).

"RPR is great for the future," says Chen. "When it’s complete we can take a look at it. Right now, no one is sure if it will be designed for voice traffic. Cisco’s version of DPT isn’t designed for that. Carriers want something they can implement now.”

Pisman agrees that RPR may be too expensive in some instances. He suggests using two technologies: RPR for the metro core, and rapid spanning tree for the access network. Rapid spanning tree is derived from the spanning tree protocol that has been used in local area networks for years. The new, enhanced protocol, which is currently going through the standards process in the IEEE, reduces recovery time to between one and five seconds, down from 30 seconds. While this is still not good enough for voice traffic, it is a huge improvement for service providers looking to provide real-time data services to customers.

Some Ethernet equipment companies are even improving the new rapid spanning tree protocol with one that they say can restore traffic even faster. Riverstone Networks has developed a technology it calls Rapid Ring Spanning Tree, which reduces recovery time to under 400 milliseconds (see Riverstone Rings Rapidly).

Riverstone announced today that France-based CompleTel Europe and South Korean-based Dacom are deploying Riverstone routers with rapid ring spanning tree turned on, to expand their metro Ethernet service offerings.

But the fact of the matter is that neither rapid spanning tree nor rapid ring spanning tree have gotten recovery times down to 50ms, the standard recovery time of Sonet-based voice networks.

“Right now there is nothing available that gives service providers 50ms restoration for Ethernet,” says Chen. “But using the MEF framework they will be able to.”

Still, other companies that are members of both the MEF and RPR Alliance say there is room for all of these technologies.

"MPLS wont solve every problem over night," says Gady Rosenfeld, director of strategic marketing for Corrigent Systems. “There are good things about it, and it is good to use for certain things like providing protection at higher layers, like between ring meshes. But RPR can be easier, cheaper, and simpler than MPLS for protection at lower layers. We plan to use both.” — Marguerite Reardon, Senior Editor, Light Reading
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metroman 12/4/2012 | 7:37:49 PM
re: MPLS Spurs Metro Ethernet Debate You should not take the view that MPLS is just for redundancy. It is designed for a great many service orientated solutions that will add value for SPs and drive costs lower over Ethernet.

The issue here is to compare the Metro networks of today with the incumbent providers who are offering Frame and ATM services as their predominant offering. If you create a network topology that has the cost advantages of Ethernet and overlay ot with a common control protocol such as MPLS you have a perfect match. MPLS will act as an enhancement and a substitute for some of Ethernet's failings. Enhancing VLANs by the creation of Draft-Martini services and replacing spanning tree (and some IGP convergence issues for that matter) with hot standby and fast re-route LSPs. Draft Martini tunnels can also be engineered for carrying ATM and Frame traffic over the underlying Ethernet topology. All of this means that you have a single network to operate and a network deployed based on low cost components. (even the additional MPLS hardware is cheap compared to having 3 networks.)

So to say that MPLS is just there for protection is not true, it is there to deliver a number of other services while enhancing the way Ethernet deals with failures. If you rely on the lower layers to protect your data services you will more than likely create cost and cause IGP issues which will negate the 50ms nirvana.

RPR is so split that I think by the time it arrive's in a standard form it's speeds will be too slow and will only be useful as an access technology and only if you run MPLS over it so you have an end to end control protocol.

Rapid Spanning Tree in the access and MPLS in the core is great, but have you tried to deploy it? It makes more sense to have a common control protocol so you can fully control opex and SLAs.

tintin 12/4/2012 | 7:37:48 PM
re: MPLS Spurs Metro Ethernet Debate As one of my colleague would put it (with a lot of, obvious, humor): "MPLS will save the world"..., it might not save the world, but it does simplify carrier architectures significantly.
The reason is that it is medium-independent.
This is also why it is in-line with MEF's positioning statement which leaves physical media out of its realm.

My $0.02.

gladysnight 12/4/2012 | 7:37:48 PM
re: MPLS Spurs Metro Ethernet Debate You make some good points metroman.

My reservations about this whole discussion are more about having seen other candidate technologies for "silver bullet" and having watched them regularly fail to deliver networking nirvana.

In spite of the sense of much of your post, I believe that there remains an element of the "one ring to rule them all, one ring to bind them" mentality in much of these marketecture efforts.

The whole reason we have a 7 layer model is that horizontal layered architectures fundamentally work, they make networking easier, they aid interoperability, and they ease migration of specific layers.

I believe that trying to collapse functions of multiple layers into a single technology is more or less doomed to failure.

From my pov protection switching is a layer 2 function and should stay there. In SONet/SDH we already have a working link-level protection mechanism, and it works for pretty much all traffic types. RPR has a long way to go to achieve even sub-second times, and MPLS still isn't anywhere as interoperable as ethernet, which it would need to be to make an ethernet based end-to-end MPLS solution workable. Even when it gets there, there remains doubt in my mind that it will work as advertised, but I am sure that a lot of operators will spend a lot of money finding that out.

Just as they did to discover that ATM is not networking nirvana, IP is not packet paradise, and so on and so on and etc . . . . .

I'd prefer to see development of a simple and cost effective SONet/SDH blade for metro scale (GigE and 10GigE) ethernet switches. Possibly a v.short haul implementation using lowest cost components to achieve 3 to 5 kilometre spans at a maximum.

This would interface directly into existing transport networks, require few if any new skills, and ideally would be managable from existing management applications.

For many operators that would be Christmas in July.

gea 12/4/2012 | 7:37:46 PM
re: MPLS Spurs Metro Ethernet Debate Ah, Poster...you ask a loaded question!

Why wouldn't anyone NOT use G.709? My canned answer is, "because we already have a perfectly fine digital wrapper. It's called SONET."

Surely you must be thinking that this is a preposterous statement, but if you look at the digitial wrapper standard and compare it to SONET, I think it makes some sense.

When I say "SONET", I don't mean circuit switching, I mean SONET framing, possibly in the form of large,concatenated (or virtually concatenated) payloads. Increasingly, I think we'll see lots of optical layer formats grow a SONET-framed version, and 10GbE is only the first.

Although I don't have time to drill down into lots of details, let's just say that the use of G.709 REQUIRES an "output" transponder before handoff to the client layer, which is uneccessary if the client is SONET frame-friendly. Sure, G.709 has got FEC, but I really only need that for long haul networks, or 10Gig in the metro which I can often avoid by using the in-band (i, within the SONET overhead) OC-192 FEC.

In addition, G.709 will often limit you to optical channel-type protection...this is probably inadequate for the flow-level multiplexing done in MPLS networks.

Finally, G.709 does nothing for provisioning. The MPLS/GMPLS vision is to allow control plane protocols to set up and tear down data/optical/TDM paths, and possibly also protection. With G.709 I still need the control plane protocols unless i want to do manual provisioning.

That said, there are things I like about G.709, particularly in networks that run lots of different rates and protocols. I'm also starting to like the various flavors of FEC that can be utilized. Finally, G.709+GFP (Generic Framing Procedure) might make a potent little AIDS cocktail for some of what ails the network.

So don't ya' go gettin' too religious on us Poster.
gea 12/4/2012 | 7:37:46 PM
re: MPLS Spurs Metro Ethernet Debate GladysNight: Great quote, and very funny.

The fact that the Metro Ethernet forum has "agreed" on using MPLS probably reflects the fact that, in one way or another, the MPLS control plane protocols are going to play a very important part in a lot of networks in the future, either through GMPLS, MPlamdaS, or MPLS (in the context of layer 3 or here, layer 2).

I think there's enough work going on collectively on these protocols that there will be a lot of nutritious "scraps" falling from those tables, so that a coherent protection approach in the context of Ethernet should be straightforward. I think it likely the MEF pretty muich understands this, so decided to latch on the this wave of work rather than start from scratch. Sorry--sound like a smart decision to me.
poster 12/4/2012 | 7:37:46 PM
re: MPLS Spurs Metro Ethernet Debate which transport technology will win in the metro? none. it's a moot point. MPLS is a traffic engineering protocol, not a physical layer transport method. RPR is great for packet rings, not for circuits/voice. same for 10GbE. additionally, aggregate ring bandwidth for both of these top out your highest rate interface (10Gbps for RPR, maybe faster in the future - 10Gbps tops for 10GbE)

why would a carrier standardize on transport protocol that's optimized them to one traffic type, in this case data, puts a ceiling on capacity, and lacks the required protection failover? why do so many people now want an alternative to a strictly SONET-based network? for the same reason.

who wins? digital wrapper-based DWDM transport platforms. all of the protection capabilities of SONET combined with mappings for all data protocols. G.709 DW exceeds SONET PM capability as well. no ring bandwidth limitations. designed for physical layer transport. carriers will build RPR and/or MPLS and/or 10GbE packet *AND* SONET rings on top of G.709 DWDM networks.

and guess what? the technology is standardized and available on chips NOW. someone please explain why they think this WOULDN'T happen. (aside from the fact that some soon-to-be defunct carriers only plan to offer data services)

jhakaas 12/4/2012 | 7:37:45 PM
re: MPLS Spurs Metro Ethernet Debate > Finally, G.709+GFP (Generic Framing Procedure) > might make a potent little AIDS cocktail for
> some of what ails the network.

Can you explain the above?

poster 12/4/2012 | 7:37:45 PM
re: MPLS Spurs Metro Ethernet Debate good points gea. but SONET does have serious limitations. no TCM = no carrier-to-carrier handoffs. no PM across multiple carrier networks. DW has 8 layers of PM in TCM.

and SONET was only designed with one wavelength in mind. pretty simplistic an incapable of managing anything but a circuit based, single-wavelength ring. given that DWDM is the de-facto transport technology now in fiber networks, I see a limited lifetime for SONET as the underlying transport protocol.

turns out the real advantage of FEC is not distance, but OSNR. significant difference.

the way I see it you are not limited at all by G.709 to OCh protection only. it just becomes an option if you choose. any higher-layer packet or SONET protection method can still be used if desired. just disable the OCh-level protetion. I strongly beleive SONET, along with MPLS, RPR, 10GbE will all transition to be service layer protocols, not transport. that is carriers will build these rings on top of G.709 wavelengths.

SONET/SDH will now be used for circuit services, basically muxing/grooming/switching anything < OC-192. the packet protocols will be used for flow-level muxing/grooming switching any packet-based services < 10Gbps. and with packet-over-G.709, now you have all of the lacking transport-layer functions like optical protection at 50msec if you so choose. an guess what they both operate on a single G.709 transport network. becuase it offers the highest level of transport control, exceeding SONET/SDH, it becomes the lowest common denominator.

yes you are 100% correct about provisioning. G.709, like SONET, really makes no effort to automate provisioning in and of itself. MPLS/G-MPLS will be incorporated to do that. the key difference is, unlike SONET, it has been designed with that in mind.

don't worry I'm not religious, but this just seems so simple.

dark_light 12/4/2012 | 7:37:44 PM
re: MPLS Spurs Metro Ethernet Debate The problem is MPLS itself is not a smart protocol. Its takes a real routing protocol like RSVP to make MPLS a viable solution for the MAN. MPLS doesn't set up secondary routes and such, its the job of another protocol. There is no inherent way in mpls for it to know a tunnel is down other than some form of ping. So to have such fast fail-over times such a protocol needs to be send tons of hello messages and clog up the network in general. This is clearly the main problem with mpls, and worsens as the network gets more and more complicated. Other than that MPLS is ideal as its highly flexible.

Do you want a fail over protocol that has to message every 50ms?
skeptic 12/4/2012 | 7:37:44 PM
re: MPLS Spurs Metro Ethernet Debate
The problem with the approach is that its based
on the idea that overlays (MPLS) solve all
the problems in the network. You end up trying
to solve physical problems at higher layers
in networking. And in the long run, that isn't
going to solve the problem.

If you can't do 50ms restoration on ethernet
without MPLS, its not really credible to think
that MPLS is going to solve the problem in
software. Your building soft restoration on
top of a physical media that doesn't support
restoration well in the first place. And if
it can be done with MPLS, there is nothing
to prevent other non-MPLS solutions solutions
to the problem.

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