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29 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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1

Latency Equalization:

A Programmable Routing Service Primitive

Minlan Yu

minlanyu@cs.princeton.edu


Joint work with Marina Thottan, Li Li at Bell Labs

PRESTO’08

2

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


3

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

4

LEQ Service


Reducing latency difference


Difference of Maximum delay and minimum delay

client1

client2

server

client3

5

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

server

client1

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



8

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

9

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 gaming

OSPF

LEQ routing

For concert

LEQ routing

For trading

FIB

FIB

FIB

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 client

10

11

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

12

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

13

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

14

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

15

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



16

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



17

Acknowledgement


Grenville Armitage, Swinburne University of
Technology


Wu
-
Chang Feng, Washington State University


Jennifer Rexford, Princeton University


Thomas Woo, Bell Labs

18

Thanks!


Q&A?

19

Static Analysis


Telstra network

80%

5 hubs are sufficient

20

Static Analysis


AT&T network

21

Premise of LEQ: Trading delay
for delay difference