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Latency Equalization:A Programmable Routing Service Primitive

Minlan Yuminlanyu@cs.princeton.edu

Joint work with Marina Thottan, Li Li at Bell Labs

PRESTO’08

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Motivation Latency EQualization (LEQ) service

Online interactive applications require equalized delay among multiple users

Online gaming Players vote to exclude players with higher lag

Distributed online music concert Delay difference among musicians at different

places degrade music quality Online trading

Unfair advantage to shopping agents with lower delay

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Latency Compensation Techniques

Server side solutions Buffer packets till all clients response arrive Expensive due to processing overhead Limits on number of clients

Client side solutions Buffer the packets to wait for other clients Requires coordination among clients Easy to cheat

We need network service for LEQ

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LEQ Service Reducing latency difference

Difference of Maximum delay and minimum delay

client1

client2

server

client3

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LEQ Architecture - Basic Idea

Avoid changing every router Place a few hub routers in the network Add customized logic to hub routers Redirect traffic through hub routers

serverclient1

client2 client3

hub

hub

Easy to Deploy Require only a few programmable routers Allow incremental deployment

With one hub in the network, we can reduce delay difference by 40% on average compared with OSPF

No modification of underlying routing protocols Can be implemented as an overlay

LEQ Architecture Architecture

Select a set of hubs for each client Set up tunnels between clients and hubs,

hubs and servers. MPLS tunnels or packet encapsulation

Hub routers redirect packets to servers

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Hub Router Implementation

Packet classifier Identify application from src, dst, port Identify class of packets within an

application Initial game setup packets: shortest path routing Interactive event packets: LEQ routing FIB

Packet classifier

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Hub Router Implementation

Customize routing for each application Different applications may have different

max delay bound; different client, server location

LEQ routing for gaming, live concert, trading

OSPF routing for other applicationsControl Plane

LEQ routing for gaming

OSPFLEQ routingFor concert

LEQ routingFor trading

FIB FIBFIB FIB

Other benefits of hubs Add service-specific logic to a few enhanced

routers in the network Application level packet processing

Gaming: Update aggregation, packet inspection Concert: Echo cancellation

Multicast Server multicast to hubs Hubs then multicast to their clients

Load balancing among servers Hubs can select the least-loaded server for the

client10

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Hub Placement Problem Input

Location of client/server edge routers Maximum number of hub routers (M) Number of hub routers per client (m)

Output: A set of m hubs for each client

Goal: Minimize delay difference among clients

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Problem Complexity We proved it is NP-hard and inapproximable Reduced to set cover problem

Greedy hub placement algorithm Based on multi-set cover algorithm See details in the paper

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Evaluation Static analysis

Use Rocketfuel Data with different ISPs Focus on lightly loaded network

propagation delay Result

LEQ achieve 80% reduction of latency difference compared with shortest path routing

Reduce from 35ms to 5ms Only need to place 5 hubs in the network Similar maximum delay of LEQ and OSPF

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Evaluation (cont.) Dynamic Analysis

Under dynamic traffic condition Focus on congested network

Consider both propagation delay and queuing delay

Result LEQ routing can get around congestion

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Alternative Network-based Solutions

OSPF No delay difference consideration Tune weights for each application Computationally hard

Source routing Clients need to know the global network

condition Require collaboration among clients Hard to compute and implement

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Conclusion Interactive online applications

New requirement on delay difference Latency Equalization service

Place a few hubs to provide latency-equalized, reliable paths

Deployment on programmable routers Easy to implement and deploy

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Acknowledgement Grenville Armitage, Swinburne University of

Technology Wu-Chang Feng, Washington State

University Jennifer Rexford, Princeton University Thomas Woo, Bell Labs

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Thanks! Q&A?

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Static Analysis Telstra network

80%

5 hubs are sufficient

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Static Analysis AT&T network

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Premise of LEQ: Trading delay for delay difference