Subject: mpl(amda)s questions

From: "Dmitri Krioukov" <[email-masked]>
Date: Sun, 9 Jul 2000 18:45:42 -0400

in the context of the (augmented) ip over mpl(amda)s
model, advertising lsp/lp state information as link
state information into the igp becomes virtually unavoidable.
doesn't this again pose the question about the fundamental
problem of running routing protocols derived from the static
graph theory (stable, fixed topology) on essentially dynamic
(being dynamically modified by the te control plane) l2
topologies? what is the fundamental difference between
this approach and having, say, an ls protocol with weights
depending on load (with all its well-known downsides
(oscillations, instability, cpu load, etc.))?

on the other hand, although it's well understood
that *highly* dynamic, data driven lsp setups are
not permitted, some manageable and *relatively* dynamic
(ie, "stable") te control systems are required for te
purposes (like congestion avoidance). any references
to these systems? do they already exist or are they
under research? are any statistical prediction mechanisms
(like markov chains) used here?

thanks,
--
dima.

---

From: Alex Zinin <[email-masked]>
Date: Sun, 9 Jul 2000 18:22:19 -0700

Dmitry,

Several points here. Others will add more.

1. The only part in link-state routing protocols that is really
   using static graph theory is the Dijkstra algorithm.
   Everything else (topology abstraction, continuous LSDB
   synchronization, etc.) is really dynamic and, in fact,
   comes from the theory of distributed algorithms.

2. When we say an LSP should be announced into an IGP,
   it does not mean it is announced as a native IGP adjacency,
   but instead it is announced as one of TE attributes.
   At the extremum of this technique, each announced TE
   can be augmented with its level. In this case, IGP adjacencies
   are used to perform level-0 topology abstraction (i.e.,
   abstraction of the physical topology) and announced TE LSPs
   are used to abstract higher-level topology (topology of the
   LSPs) so that they can be used to tunnel other, newly
   established LSPs.

3. As for comparing this method to dynamically calculated link
   costs, it seems to be very important to me that LSP announcement
   should not cause immediate recalculation of already established
   LSPs, while dynamic link costs immediately affect paths of forwarded
   packets, and thus cause circular feedback dependency (Curtis
   Vilamizar's OMP addresses this problem). Also, even if already
   established LSPs are rerouted, proper signaling and resource
   reservation should prevent LSP bouncing. The main problem with
   dynamic link costs in current IGPs is that when routers converge
   on a new SPT, traffic trajectories are changed and so are link load
   and link costs, so current best paths become not quite best and IGP
   reroutes again. This causes constant route bouncing.
   In the case of LSPs, when a new LSP is announced, some LSRs may
   reroute their already established LSPs through the new pseudo-link.
   When all resources are allocated, the network will converge on
   a finite set of LSPs tunneled over the new LSP within some time,
   other LSPs will stay on their previous paths (LSPs being rerouted
   should not be torn down until the signaling procedure is complete
   for the new path). Note, however, that exhaustion of resources of
   this tunnel LSP should not lead to rerouting of tunneled LSPs,
   since resource requirements are met for them.

   In short, I'd say MPLS networks inherit a lot from the VC-oriented
   network concept and this allows us to avoid some screaming problems
   we have in datagram networks, if we're careful, of course.

-- 
Alex Zinin