Transcript Traffic Engineering in MPLS - Lyle School of Engineering

Constraint-Based Routing in MPLS
SMU
CSE 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
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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
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CSE 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
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CSE 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
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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
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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
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Constrain-Based SPF
7
2
45
4
150
1
150
150
150
5
3
150
6
150
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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
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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
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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
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CBR (cont’d)
• Comparison of RSVP and CR-LDP
– Scalability
– Signaling mechanism
– Qos Models
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Fish Network
R8
R3
150
R4
150
R5
R2
150
R1
150
R6
R7
150
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Is Plain IP Enough?
R8
R3
150
R4
150
R5
R2
150
R1
150
R6
Under utilized
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R7
150
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Why IP Routing Fails
• Based only on metric optimization
– Shortest path
– Administrative optimization
– Split paths
• Per link constraints not taken into
consideration
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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
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Fast Rerouting
• Total restoration time after failure
– Failure detection time
– Propagation
– Computation of new path
• Usually the 2nd and 3rd steps are
significantly slow
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Is FR possible with IP?
R1
R2
X
R3
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R5
R4
Even if the traffic is rerouted to R3, it will send
that back to R1 since R3 is not aware of the
failure
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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
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CSE 8344