Sunday, January 27, 2013

MPLS Traffic Engineering

Traffic on a network is always sent over a least cost path computed by the routing protocols. On the bellow figure, traffic, either from A to D or from A to G,  will always flow over   A->B->C  portion   because it is on  is the least cost path. Even if the preferred path is oversubscribed, no packet will flow over the portion (A->E->F->C). Clearly, this is not an optimal way to do.




Traffic Engineering (TE) is the ability to send traffic over a non least cost path (steering traffic) 

Traffic Engineering (TE) is an MPLS feature, it is a powerful tool used to spread traffic more efficiently throughout a network, avoiding oversubscription or under subscription. Implementing TE on the previous infrastructure, one can make traffic from A to D to flow on A->B->C->D path, and traffic from A to G to flow on A->E->F->C->G path. 




The path calculated by TE is called an LSP Tunnel. The tunnel is unidirectional (A->D tunnel and D-> A Tunnel), it is always maintained by the Head-End router (Router A in the previous example)

The TE Tunnel is calculated based on constraints (Bandwidth, Latency...). The Bandwidth constraint is the one mostly used; a tunnel is built on path that satisfies a certain amount bandwidth

In order to build an TE Tunnel based on bandwidth constraint, the information on current available bandwidth on each link must be distributed to all the routers that performs path computation, this is done by TE-specific extension made to Link state routing protocols (OSPF and ISIS). In this way each node has knowledge of the current available bandwidth on all links in the network. This information is stored in TED (Traffic Engineering Database) on each router


TE makes use of 2 protocols
  1. CSPF (Constrained Shortest Path First): This is the protocol that  compute the shortest path based on constraint (Bandwidth).The path is computed much like SPF do, it uses the information stored in the TED.
  2. RSVP -TE (Ressource ReserVation Protocol): After the path was computed, RSVP-TE is used to set up the path and to lock (reserve) the requested resources on each node.
    It is important to note that this reservation is done on the control plane only, RSVP is not aware about what is going on on the data plane. If, for example, 10 Mbps was reserved for A->D tunnel, RSVP can’t prevent A from, actually, sending more traffic. 


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