SMUCSE 8344 Constraint-Based Routing in MPLS

SMUCSE 8344 Constraint Based Routing (CBR) What is CBR –Each link a collection of attributes (performance, administrative) –Path built to satisfy/not to violate some constraints, at the same time optimizing some scalar metrics Applications –Traffic Engineering –Fast Re-route –QoS support How different from IP routing –CBR is source based whereas IP routing is distributed –CBR needs explicit routing –Support for distribution of link attributes

SMUCSE 8344 Traffic Engineering Objectives Traffic Engineering (TE) concerned with performance optimization The key performance objectives –traffic oriented e.g. minimization of packet loss –resource oriented - optimization of resource utilization e.g. efficient management of bandwidth

SMUCSE 8344 Objectives (cont’d) Minimizing congestion is a major traffic and resource oriented performance objective Congestion manifest under two scenarios –Network resources insufficient or inadequate Solved by capacity expansion or classical congestion control techniques –Inefficient mapping of traffic streams onto available resources Reduced by adopting load balancing policies

SMUCSE 8344 MPLS and Traffic Engineering Main components used –Traffic Trunk - aggregation of traffic flows of the same class which are placed inside a Label Switched Path –Induced MPLS Graph Analogous to a virtual topology Logically mapped onto the physical network Set of LSRs as nodes of the graph Set of LSPs providing logical point to point connectivity between LSRs as edges

SMUCSE 8344 CBR Components Mechanism for source based path computing Mechanism to collect necessary information –Constraints (local), attributes, topology Support forwarding along the computed paths Notification of residual resources after allocation

SMUCSE 8344 Constrain-Based SPF

SMUCSE 8344 CSPF Uses the following inputs –Link attributes –Topology state information –Path constraints Basic approach –Prune resources that do not meet the constraints –Run a shortest path algorithm on the residual graph

SMUCSE 8344 MPLS for Forwarding Ideal to use MPLS explicit routing capability Once the path is computed –Need to establish forwarding state along the path –Reserve resources along the path Two approaches –RSVP extensions –CR-LDP

SMUCSE 8344 CBR - Forwarding RSVP extensions –How to send PATH messages on explicit routes? Introduce new object ERO (Explicit route object) similar to source routing –Use RESV message for label binding CR-LDP –In addition to using label_request and label_mapping messages, use ER message similar to ERO

SMUCSE 8344 CBR (cont’d) Comparison of RSVP and CR-LDP –Scalability –Signaling mechanism –Qos Models

SMUCSE 8344 Fish Network R8 R1 R5 R2 R3 R4 R7R6 150

SMUCSE 8344 Is Plain IP Enough? R8 R1 R5 R2 R3 R4 R7R6 150 Under utilized

SMUCSE 8344 Why IP Routing Fails Based only on metric optimization –Shortest path –Administrative optimization –Split paths Per link constraints not taken into consideration

SMUCSE 8344 TE in MPLS Using CBR Define traffic trunks –Collection of micro-flows that share same path and class of service –These are not end-to-end paths, rather paths within a single service provider No. of trunks dependent only on the topology Forwarding table does not grow with the traffic Rerouting –RSVP, CR-LDP, or IGP

SMUCSE 8344 Fast Rerouting Total restoration time after failure –Failure detection time –Propagation –Computation of new path Usually the 2 nd and 3 rd steps are significantly slow

SMUCSE 8344 Is FR possible with IP? R1 R4 R3 R2 R5 Even if the traffic is rerouted to R3, it will send that back to R1 since R3 is not aware of the failure X

SMUCSE 8344 FR using CBR Compute protection LSP for every link When a failure happens –Traffic rerouted to the protection LSP –Use label stacking for the transit within the protection LSP –Beyond the end-nodes labels original labels remain in tact