CprE 458/558: Real-Time Systems (G. Manimaran) 1

CprE 458/558: Real-Time Systems

Real-Time Networks – WAN

Channel establishment

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QoS metrics: types of constraint

Metrics and their nature• End-to-end delay: additive metric, path metric

• Delay jitter: additive metric, path metric

• Bandwidth: convex (min), link metric

• Packet loss: multiplicative, path metric

Theorem: A problem with two or more path constraints is NP-Complete, assuming the constraints are independent and the values are real.

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QoS routing problem instances

Given a graph G(V,E), with each edge labeled with (cost, delay, available bw).

• Bandwidth-constrained least-cost routing– Polynomial time problem– Solution: Remove edges that don’t satisfy BW

constraint and then run shortest path algorithm on the reduced graph.

• Delay-constrained, BW-constrained, least-cost routing– NP-complete problem– Heuristics exists

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Channel establishment – QoS routing

• Source Routing– Source computes the routing path– Obtaining global state is difficult– Not scalable

• Distributed Routing (hop-by-hop)– Each node decides what is the next hop– Lack of global knowledge inferior paths– Scalable

• Hierarchical Routing– Based on aggregate state information– Scalable (works well for inter-domain routing)– Compromise between source and distributed routings

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Source Routing

S

D

Connection 11

•The source has the entire network state information (cost, delay, etc.. of each edge)

Choose the entire path

from source to destination

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Distributed / Hop-by-hop Routing

S

D

Connection 11

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Hierarchical Routing

S

D

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Hierarchical Routing

S

D

Routingbased on precise state

Routing based on precise state

Routing based on aggregate

state

1

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QoS Source Routing

S

D

Connection 1

With some QoS

requirements

1

•The source has the entire network state information (cost, delay, etc.. of each edge)

Choose the path from source to destination that

satisfies the QoS requirements based on some heuristic function

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QoS Hop-by-hop Connection setup

S

D

Connection 1

With some QoS

requirements

1

No outgoing edge satisfies the required

QoSTherefore, back off

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Channel establishment – Resource reservation

• Once a feasible path is found, resources such as bandwidth, buffers are reserved along the path

• Resource Reservation Protocol (RSVP)– Receiver-driven protocol– Supports multicast as well as unicast– Provides different type of reservation styles