The MPLS WG Archive

[Mark Stewart <Mstewart@nexen.com>]

Hi Dave

The concept of jointly routing primary and protection paths has been
well accepted by Bell heads looking at optimizing their networks for a
long time. Part of the reason for this is the assumption that protection
path(s) must also be conformant to the same SLA as the primary path, and
joint routing is the most likely to achieve this.

This does not of course address your concerns about race conditions at
connection establishment. But joint routing is guaranteed to produce a
solution not worse than independent routing, and results in a lower
commitment of network resources.

ciao

mark






David Allan wrote:

>
>
> Angela:
>
> Jointly routing the paths intuitively does not strike me as optimal.
> Not unless we are periodically performing path maintenance on the
> entire set of network resources. I would have to assume that the
> overall configuration of the network occurred incrementally, and with
> a desire to minimize service interruption of the already established
> paths.
>
> With that in mind, I would assume the routing of the primary path
> should always be chosen as the optimal route. The backup path is then
> required to be diverse (node, fiber, conduit, trench) with the optimal
> primary path and would frequently be less optimal as the physical
> routing would frequently be in the form of a longer path.
>
> I do not understand how whether this is done sequentially or
> simultaneously affects these basics, except in the possible deadlock
> scenario where the optimal routing of the primary path precludes a
> viable backup. In the meantime, I would assume sequential
> establishment of primary then backup would stand an overall greater
> chance of success, especially if the path computing node does not have
> a comprehensive and authoritative view of the network state. It
> strikes me that routing the backup should have the ability to
> intelligently crank back with knowledge of what to avoid to maintain
> diversity with the primary path.
>
> regards
> Dave
>
>      -----Original Message-----
>      From:   Angela Chiu [SMTP:alchiu@research.att.com]
>      Sent:   Wednesday, October 25, 2000 10:03 AM
>      To:     'Yakov Rekhter'
>      Cc:     ip-optical; mpls; sc; xuyg; yxue
>      Subject:        RE: [IP-Optical] RE: Optical link bundling. Was
>      Re: DraftMinutes From  Pittsburgh
>
>      Yakov,
>
>      Yes, you are right. Routing the primary and backup paths jointly
>      is always
>      more optimal than fixing the path for primary first then routing
>      the backup
>      accordingly. The same argument can be applied to comparing
>      centralized
>      routing with distributed routing. Even distributed routing is
>      less optimal,
>      it is the trend today. I think the real issue is at what cost the
>      additional
>      optimality is gained, and how much the additional optimality is
>      in a typical
>      network setting. In this case the cost is all the topological
>      information
>      including SRLG information as well as physical impairment
>      constraints in the
>      optical network that routers need to obtain in order to make
>      proper routing
>      decision.
>
>      I have an idea, this will be a valid master/PhD thesis for some
>      graduate
>      students who would like to work on real world problems.
>
>      Regards,
>
>      Angela
>
>      -----Original Message-----
>      From: ip-optical-admin@lists.bell-labs.com
>      [mailto:ip-optical-admin@lists.bell-labs.com]On Behalf Of Yakov
>      Rekhter
>      Sent: Wednesday, October 25, 2000 7:35 AM
>      To: alchiu@research.att.com
>      Cc: ip-optical@lists.bell-labs.com; mpls@UU.NET; sc@tellium.com;
>      xuyg@lucent.com; yxue@UU.NET
>      Subject: Re: [IP-Optical] RE: Optical link bundling. Was Re:
>      DraftMinutes From Pittsburgh
>
>      Angela,
>
>      > Some followup discussions in line.
>
>      more in line...
>
>      > Regards,
>      > Angela
>      >
>      > -----Original Message-----
>      > From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of
>      Kireeti
>      > Kompella
>      > Sent: Monday, October 23, 2000 1:58 PM
>      > To: kireeti@juniper.net; sc@tellium.com; xuyg@lucent.com;
>      yxue@UU.NET
>      > Cc: ip-optical@lists.bell-labs.com; mpls@UU.NET
>      > Subject: RE: [IP-Optical] RE: Optical link bundling. Was Re:
>      > DraftMinutes From Pittsburgh
>      >
>      >
>      > > I don't see why TE and protection require the routers to
>      specify
>      explicit
>      > > routes.
>      > > The routers can simply specify to the optical layer what type
>      of optical
>      > > layer protection
>      > > it requires.
>      >
>      > Suppose router A wants to get to router B, and wants to take
>      two
>      > different ingress and egress points in the optical domain, X->Y
>
>      > for the primary LSP, and W->Z for the backup.  A does not
>      require
>      > optical protection for the X->Y path, nor for the W->Z path.  A
>
>      > *does* require that the X->Y path and the W->Z path do not
>      share
>      > common links.  How is this to be done?
>      >
>      > If A did the full path computation, this is simplicity itself.
>      >
>      > [AC] I think you have a good point here. I also heard the same
>      kind if
>      > reasoning (i.e., have a layer-3 like protection switching) for
>      supporting
>      > the peer model. But after discussing with others, it seems that
>      overlay
>      > model should be able to provide the same capability.
>
>      Not really... for more on this see below...
>
>      > Normally, the primary
>      > LSP X->Y is set up first, and becomes a forwarding adjacency
>      (FA)
>      according
>      > to your LSP Hierarchy draft. Then the associated information of
>      the FA
>      X->Y
>      > including its exact path and SRLG information should be
>      propagated via IGP
>      > extensions, same as with any other link in the network. Thus if
>      router A
>      > sends a request to OXC W to set up a backup lightpath from W->Z
>      to be
>      > diversely router from the existing FA X->Y, OXC W should
>      already have the
>      > right information to perform proper routing.
>
>      It is a known fact that for computing disjoint paths the approach
>
>      you outlined above may result in a situation where no backup
>      path will be found, despite the fact that that it is possible
>      (using some other approach) to find two disjoint paths.
>
>      > Comparing with the peer model solution where routers need to
>      know all the
>      > SRLG information of the optical domain as well as all relevant
>      physical
>      > impairments in the optical signal in the case of transparent
>      optical
>      > network, it is still not clear to me which one is simpler.
>      >
>      > I think it is very good to have this kind of technical
>      discussion openly
>      on
>      > the list. Hope others can provide more technical and business
>      (after all
>      > carriers need to pay for these features) evidences for the need
>      of each
>      > model. Some other reasoning I heard includes that peer model
>      can improve
>      the
>      > IGP scalability in terms of the number of neighbors a router
>      needs to peer
>      > with. But since large ISPs today seem to cope well with the IGP
>
>      scalability
>      > today, I don't see why the problem will get significant worst
>      when optical
>      > networks come into play.
>
>      In the end it is not the discussion on this list, but the
>      competition
>      in the marketplace that will determine the viability of different
>
>      models.
>
>      Yakov.
>
>      _______________________________________________
>      IP-Optical mailing list
>      IP-Optical@lists.bell-labs.com
>      http://lists.bell-labs.com/mailman/listinfo/ip-optical
>