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QoS Not Needed
Ben TeitelbaumInternet2 VoIP SIG
September, 2006
Outline
QoS dreams Mechanisms Problems
Theoretic Practical Economic
Alternatives Non-History Non-Future
The Holy Grail of computer networking is to design a network that has the flexibility and low cost of the Internet, yet offers the end-to-end quality-of-service guarantees of the telephone network.
- S. Keshav
What QoS Is Not
It is not a synonym for “good performance”It is not about local rationing
E.g. Packeteer, Vonage ATA
And, in this talk, is not taken to include non-elevated services E.g. ABE, QBone Scavenger
What QoS Is
Differentiated network service to provide better-than-default (BE) service
WLOG, assume hereafter a Van Jacobson, virtual leased line, Premium Service
Note that QoS is about removing only one factor that can cause a networked transaction to fail
Mechanisms: Classification
Policing Queuing / AQMClassification
?
Mechanisms: Policing
Queuing / AQMClassification Policing
Mechanisms: Queuing / AQM
Policing Queuing / AQMClassification
Some Problems with QoS
TheoreticPracticalEconomic
GigaPoPA
CampusA
CampusC
CampusD
Backbone
CampusB
GigaPoPB
Theoretic Problems How do edge-to-edge
“virtual trunks” concatenate to form an e2e service?
What exactly are the policers and shapers at inter-domain boundaries?
Practical Problems
Requires all-or-nothing network upgrades (e.g. all access interfaces must police)
Dramatic changes to network operations, peering arrangements, and business models
Practical Problems (cont.)
In a well-provisioned network Premium is indistinguishable from BE
How can a user (or even a provider) verify service?
What happens to Premium service in the face of a determined adversary?
Economic Problems
Router costsOperational costsBilling costsSupport costs
Some Alternatives to QoS
Overprovisioning Cheapest way to provide fabulous service to important
apps, is to provide it to all apps Pricing
Congestion pricing Nice theoretic properties But not practical
Usage-based pricing Would help a lot Business access is increasingly metered Could provide differentiated services (e.g. Paris Metro
Pricing)
A History of Non-Deployment
QoS Wasn’t Needed (1997-2001)QoS Isn’t Needed (2002-2006)QoS Shouldn’t Be Needed (2007-)
QoS Wasn’t Needed (1997-2001)
Ambitious QoS program (QBone) Many hard-won lessons Negative outcome not at all a foregone
conclusion
Naïve codecs ported from ISDN world wouldn’t tolerate packet loss
Few users of real-time applications anyway
QoS Isn’t Needed (2002-2006)
Adaptive, loss-tolerant codecsMany users of real-time applications
(Vonage, Skype, Internet2 videoconf)Generous provisioning ensures that real-
time apps just work
Hang On a Second!
●~104 hosts with nothing slower than switched 100Mbps Ethernet between them●~25 of these could congest the 2.4 Gbps Abilene backbone (or 100 the 10 Gbps)●90% of traffic is TCP●TCP is designed to congest●Yet, the backbone is lightly loaded●What’s going on?!
http://netflow.internet2.edu/weekly/20060501/
The Terrible Truth(overprovisioning works, because TCP doesn’t)
QoS Shouldn’t Be Needed (2007-)
Either TCP will stay broken or be replacedNew transport protocols (e.g. XCP,
MaxNet, PCP) don’t build huge queuesEven better packet loss concealment
through improved codecs
Summary
QoS is interestingQoS is expensiveScarcity should become scarcerQoS has not been needed thus farQoS should not be needed for the
foreseeable future