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PRESTO’08 Latency Equalization: A Programmable Routing Service Primitive Minlan Yu minlanyu@cs.princeton.edu Joint work with Marina Thottan, Li Li at Bell Labs Motivation Latency EQualization (LEQ) service Online interactive applications require equalized delay among multiple users

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latency equalization a programmable routing service primitive

PRESTO’08

Latency Equalization:A Programmable Routing Service Primitive

Minlan Yu

minlanyu@cs.princeton.edu

Joint work with Marina Thottan, Li Li at Bell Labs

motivation
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
latency compensation techniques
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
leq service
Reducing latency difference

Difference of Maximum delay and minimum delay

server

client1

client2

client3

LEQ Service
leq architecture basic idea
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

hub

server

client1

hub

client2

client3

easy to deploy
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
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
hub router implementation
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

hub router implementation9
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 applications

Control Plane

LEQ routing

For trading

LEQ routing

for gaming

OSPF

LEQ routing

For concert

FIB

FIB

FIB

FIB

other benefits of hubs
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 client

10

hub placement problem
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
problem complexity
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
evaluation
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
evaluation cont
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
alternative network based solutions
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

15

conclusion
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
acknowledgement
Acknowledgement
  • Grenville Armitage, Swinburne University of Technology
  • Wu-Chang Feng, Washington State University
  • Jennifer Rexford, Princeton University
  • Thomas Woo, Bell Labs
static analysis
Static Analysis

Telstra network

5 hubs are sufficient

80%

19

static analysis20
Static Analysis
  • AT&T network