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3.2.- MPLS and TE  

First, the concept of MPLS Traffic Trunk (TT) is explained. A TT is an aggregation of traffic flows of the same class which are placed inside an MPLS LSP. It is useful to view TTs as objects that can be routed; that is, the LSP which a TT traverses can be changed. Then, it is very important to emphasize that there is a fundamental distinction between a TT and the LSP through which it traverses.
Attractiveness of MPLS for TE can be attributed to the following factors:
  1. Explicit LSPs which are not constrained by the destination based forwarding paradigm can be easily created through manual administrative action or through automated action by the underlying protocols.
     
  2. LSPs can potentially be efficiently maintained.
     
  3. TTs can be instantiated and mapped onto LSPs.
     
  4. A set of attributes can be associated with TTs which modulate their behavioral characteristics.
     
  5. A set of attributes can be associated with resources which constraint the placement of LSPs and TTs across them.
     
  6. MPLS allows for both traffic aggregation and disaggregation whereas classical destination only based IP forwarding permits only aggregation.
     
  7. It is relatively easy to integrate a "constraint-based routing" framework with MPLS.
     
  8. A good implementation of MPLS can offer significantly lower overhead than competing alternatives for TE.
Induced MPLS Graph
 
An induced MPLS graph consists of a set of LSRs which comprise the nodes of the graph and a set of LSPs which provide logical point to point connectivety between the LSRs, and hence serve as the link of the induced graph.
 
Induced MPLS graphs are important because the basic problem of bandwidth management in an MPLS domain is the issue of how to efficiently map an induced MPLS graph onto the physical network topology.
 
Let G = (V,E,c) be a capacitated graph depicting the physical topology of the network. V is the set of nodes, E is the set of links; i.e., for each pair v,w e V, the object (v,w) e E, this means, v and w are directly connected under G. c is the set of capacity and other constraint associated with V and E. G is the "base" network topology.
 
   

 
Let H = (U,F,d) be the induced MPLS graph. U is the subset of V representing the set of LSRs that are endpoint of at least one LSP. F is the set of LSPs, so that for x and y in U, the object (x,y) is in F. d is the set of demands and restrictions associated with F. H is a directed graph and it depends on the transitivity characteristics of G.
Fundamental Problems of TE over MPLS
There are three fundamental problems to be addressed:
  1. How to map packets onto FECs.
     
  2. How to map FECs onto TTs.
     
  3. How to map TTs onto the physical network topology using MPLS.
This document focuses on the third of these problems. This is really the problem of mapping an induced MPLS graph (H) onto the "base" network topology (G).
Capabilities for TE over MPLS
Functional capabilities required to fully support TE over MPLS consist of:
  1. A set of attributes associated with TTs which colectively specify their behavioral characteristics.
     
  2. A set of attributes associated with resources which constraint the placement of TTs through them. These can be viewed as topology attribute constraints.
     
  3. A "constraint-based routing" framework which is used to select paths for TTs subject to constraints imposed by items 1 and 2 above.
Attributes associated with TTs and resources and parameters associated with routing, represent the control variables which can be modified through administrative actions or through automated agents to drive the network to a desire state.
   

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