Subject: Asymmetrical Bi-directional LSPs

From: Adrian Farrel <[email-masked]>
Date: Wed, 27 Sep 2000 22:33:57 +0100

Hi all,

Lou Berger suggested I poll the list to see what interest there is in being
able to set up asymmetrical bi-directional LSPs without the need for an
exterior signaling protocol.

In summary the initiator determines the need for a bi-directional LSP to the
responder.  The TE component determines that (for various reasons - see
below) the two directions of the LSP should follow different paths through
the network.  This may involve different LSRs or simply different links
between the LSRs.  How should the initiator set up the LSP?

Options seem to be
1. signal the forward path and use some external 
   signaling method (such as SNMP) to request that
   the responder signals the reverse path
2. over-load the MPLS signaling protocol to convey 
   the request for the reverse path and its ERO to
   the responder
3. enhance the signaling protocol to allow the 
   two directions to be signaled from the initiator.

Obviously, 1 is simple (although perhaps a cop out - after all, we could use
SNMP in place of RSVP-TE for uni-directional signaling!).  Option 2 has
interworking implications.  Option 3 should only be considered if there is a
real need.

A quick digression into motivation...

Call blocking can be a considerable problem when trying to set up
bi-directional LSPs.  Simulations on various topologies of fully meshed
optical switches show that call blocking is dramatically reduced when the
selection of asymmetric bi-directional paths are allowed rather than only
symmetric bi-directional paths.

Asymmetric optical paths can follow the same node hops in both directions
(but using different links) or different node hops for each direction.

The general case of asymmetric bi-directional paths would have the initiator
"set up" two separate LSPs and binding the two LSPs into one logical
bi-directional path. The forward path would clearly be forward signaled
(from the initiator). The reverse path would either be reverse signaled
(from the responded) or forward signaled from the initiator, but in either
case, the ERO would be calculated at the initiator.

A special case of asymmetric bi-directional path setup is when the forward
and reverse node hops are identical, but the Links/Data Channels are
different in each direction. It would obviously speed path establishment
would be accomplished if both paths could be set up using one signaling
exchange.

All response gladly received.

Regards,
Adrian
--
Adrian Farrel  mailto:[email-masked]
Network Convergence Group
Data Connection Ltd., Chester, UK
http://www.datcon.co.uk/
Tel: +44 (0) 1244 313440  Fax: +44 (0) 1244 312422

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From: Bora Akyol <[email-masked]>
Date: Wed, 27 Sep 2000 14:58:05 -0700

Although I don't like the idea, if this comes to happen, option 3 would be my
preference.

On the other hand, if optical switches have call blocking problems, maybe they
should solve that internally instead of mangling once again a protocol that has
been deployed for the past 2 years and is pretty much considered stable.

Bora

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From: Curtis Villamizar <[email-masked]>
Date: Wed, 27 Sep 2000 21:48:27 -0400

In message <[email-masked]>, Adrian F
> symmetric bi-directional paths.

Are you sure about this?  Is this the case when lights are out in one
direction but not the other in parts of the topology?

> All response gladly received.

A down side may be that asymmetric bi-directional paths will increase
the number of SRLGs that apply to a primary path and make it harder to
efficiently allocate backup paths.

Curtis