Subject: MPLS and fast reroute.

From: Juan Diego Otero <[email-masked]>
Date: Thu, 08 Jun 2000 12:20:14 +0200

Hi,

In IEEE Communications Magazine (December 1999), there is an article
written
by Tony Li called "MPLS and the Evolving Internet Architecture". In page
41,
talking about fast reroute, Tony says: "...MPLS can also be used in a
manner very similar to synchronous optical network (SONET), where no
signaling is necessary to perform the protection switching of the LSP.
This
results in restoration times competitive with SONET." My question is:

Can anybody explain me how MPLS can be used as SONET in
switching a LSP after a faliure, with no signaling?

Thanks a lot.

Diego Otero
UPC (Universitat Politècnica de Catalunya)

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From: Yangguang Xu <[email-masked]>
Date: Thu, 08 Jun 2000 09:25:04 -0400

Diego Otero,

For the most simple protection, unidirectional 1+1, neither MPLS nor SONET
requires signaling. The trade of is that you may have to dedicate 100% network
resource for protection. The protection switching is sure very fast. 

For more complicated SONET protection switching mechanisms (BLSR for example),
they do have signaling, however not message-based signaling but within SONET
overhead bytes with hardware facilitation. So in order to have MPLS to achieve
SONET like performance, not only protocol but also the infrastructure has to be
there. For example, at least QoS has to be guaranteed to differentiate
protection switching messages from other traffic. 

MPLS rerouting has to be both fast and deterministic in order to be competitive
with SONET. Actually, they don't need to compete with each other. They can
absolutely work together well.

Yangguang

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From: Curtis Villamizar <[email-masked]>
Date: Thu, 08 Jun 2000 11:01:24 -0400

> UPC (Universitat Politècnica de Catalunya)

See draft-ietf-mpls-rsvp-lsp-tunnel-05.txt.  The feature is known as
local-protect in MPLS.

At each potential point of failure a backup path is created ahead of
time.  On the LSR the spare outsegment is programmed (the hardware is
set up) but the insegment points to the primary outsegment.  When the
failure is detected by the interface egressing the LSR, a message is
sent to all other interfaces.  Each interface reprograms the insegment
(makes a table entry change) that points the insegment at the backup
path.  This can be accomplished in about the same time required to do
SONET restoration.

There is more information in draft-swallow-rsvp-bypass-label-00.txt,
though this provides a means of using one LSP to backup multiple LSP
that take the same recovery path.  It does provide additional insight
into how this works.  You can also check the mailing list archives for
discussion of "local protect" or "fast reroute".

Curtis

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From: Curtis Villamizar <[email-masked]>
Date: Thu, 08 Jun 2000 16:18:04 -0400

In message <[email-masked]>, Yangguang Xu writes:
> 
> For example, at least QoS has to be guaranteed to differentiate

The granularity for MPLS protection is the LSP while the granularity
for SONET is the link itself.  It is therefore much easier to put
higher priority trraffic on different LSPs from lower priority traffic
and provide local-protect for the former and require the latter to
reroute from the ingress.

Curtis

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From: "Brijesh Kumar" <[email-masked]>
Date: Thu, 8 Jun 2000 17:34:59 -0400

> -----Original Message-----

I would like to add a bit to Curtis's message.

I don't see a point in comparing essentially two altogether dis-joint
technologies. Moreover, it is meaningless to talk about QoS in the
context of APS in SONET as all services are being shifted on the
alternate path (including all MPLS LSPs). MPLS over SONET
automatically inherits all the virtues of SONET reliability.

MPLS local protection mechanisms (over non-SONET network) are purely
software "possibilities" at this point, but I hope some day techniques
imagined above will indeed be available. But we have to go from MPLS
(1) to MPLS with Diffserv (2) to MPLS with Diffserv with local-protect
(3). That sounds lot of coding work for lot of s/w persons;-).

SONET APS is available now, and there is more to SONET APS such as a
number of options can be considered in the deployment of SONET
architectures, including:

1) What is being protected: SONET has mechanisms to protect at either
the line or path level.

2) Which direction the signals flow in a ring: Signals can all route
in the same direction (unidirectional) or in opposite directions
(bidirectional).

3) How many fibers are being used in each link: Either one- or
two-pair (two- or four-fiber) configurations are selected.

The number of combinations in which these options can be employed
leads to a variety of SONET protection schemes. Some of the more
frequently used abbreviations of the SONET protection architectures
are Bidirectional line-switched ring (BLSR), Bidirectional
path-switched ring (BPSR), Unidirectional line-switched ring (ULSR),
Unidirectional path-switched ring (UPSR).

Whether MPLS can emulate such a wide variety of Protection options
over other networks can't be definitively answered yet. There is a
long way to go, before we reach there.

Cheers,

--brijesh
Ennovate Networks Inc.

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From: Yangguang Xu <[email-masked]>
Date: Thu, 08 Jun 2000 21:47:15 -0400

> automatically inherits all the virtues of SONET reliability.

I guess my point is misunderstood. SONET protection switching uses dedicated
overhead bytes for signaling and end-to-end coordination. This is why SONET
protection is fast and deterministic. MPLS uses message-based signaling to
coordinate protection switching. With only best effort IP, there is no way you
can guarantee to deliver these signaling messages on time. So there has to be
infrastructure to treat these signaling messages specially. This is why I
mentioned QoS.

> long way to go, before we reach there.

MPLS doesn't need to mimic SONET protection, which so far are almost ring-based.
MPLS can use these ring protection ideas, but it should focus on mesh-based.

Yangguang

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From: "Brijesh Kumar" <[email-masked]>
Date: Fri, 9 Jun 2000 11:05:15 -0400

In his previous mail, Yanhguang writes,

> infrastructure to treat these signaling messages specially.

It appears that we are perfectly in sync. You raised a very valid
point regarding the need to accommodate QoS in MPLS protection
switching. The point I was trying make was that even without QoS, it
is very hard to obtain SONET like reliability in the system with the
mpls local protect mechanisms. There are many more issues such as
provisioning for fall back capacity, and ability to survive
multi-point failures.

Cheers,

--brijesh
Ennovate Networks Inc.