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Rice Networks Grouphttp://www.ece.rice.edu/networks
Aleksandar Kuzmanovic &
Edward W. Knightly
TCP-LP: A Distributed Algorithm for Low Priority
Data Transfer
A. Kuzmanovic and E. W. Knightly
Motivation
Traditional view of service differentiation:– High priority: real-time service– Best-effort: everything else
What’s missing?– Low-priority (receiving only excess bandwidth)– Lower than best-effort!
Non-interactive apps, bulk download Speeds up best-effort service Inference of available bandwidth for resource selection
Routers could achieve via a low (strict) priority queue
Objective: realize low-priority via end-point control– Premise: routers will not help
A. Kuzmanovic and E. W. Knightly
Applications for Low Priority Service
LP vs. rate-limiting:– P2P file sharing
Often rate limited Isolation vs. sharing
LP vs. fair-share:– Bulk downloads
Improve my other
applications Data-base replication
across the Internet
T im e
R a te lim it
C a pa c ityA va ila bleba ndw idth
G a in
AB
A. Kuzmanovic and E. W. Knightly
Problem Formulation & Design Objectives
Low-priority service objectives– Utilize the “excess/available” capacity
What no other flows are using
– TCP-transparency (non-intrusiveness)– Inter-LP flow fairness (fair-share of the available
bandwidth)
Design end- host- based transm ission protocol that em ulates the low- priority service
A. Kuzmanovic and E. W. Knightly
Origins of the Available Bandwidth
Why is excess bandwidth available when TCP is greedy?
1. TCP is imperfect Cross-traffic burstiness Delayed ACKs due to reverse traffic frees up available
bandwidth
2. Short-lived flows Majority of traffic consists of short-lived flows (web
browsing) Bandwidth gaps between short lived-flows
A. Kuzmanovic and E. W. Knightly
Illustration of TCP Transparency
LP flow utilizes only
excess bandwidth– Does not reduce the
throughput of TCP flows
A B
T C P
LP T im e
F ull C ap ac ity
T C P d e m a n d
T im e
F ull C ap ac ity
L P d e m a n d
T im e
F ull C ap ac ity
B a n d w id th s h a re
L P flo w u tilize sa v a ila b le b a n d w id th
T C P -tra n s p a re n c y
A. Kuzmanovic and E. W. Knightly
How Is This Different from TCP?
In presence of TCP
cross-traffic:– TCP achieves fairness– LP achieves
TCP-transparency
A B
T C P 1
T C P 2 T im e
F ull C ap ac ityT C P 1 d e m a n d
T im e
F ull C ap ac ityT C P 2 d e m a n d
T im e
B a n d w id th s h a re
In ter-T C P -fairn es s
A. Kuzmanovic and E. W. Knightly
Fairness Among LP Flows
Inter-LP-fairness is
essential for
simultaneous– file transfers– estimates of available
bandwidth
A B
T C P
LP 1LP 2 T im e
F ull C ap ac ityT C P d e m a n d
T im e
F ull C ap ac ityL P 1 a n d L P 2 d e m a n d s
T im e
B a n d w id th s h a re
In ter-L P -fairn es s
A. Kuzmanovic and E. W. Knightly
TCP-LP:A Congestion Control Protocol
Key concepts– Early congestion indication
One-way delay thresholds
– Modified congestion avoidance policy Less aggressive than TCP
Implication: Sender-side modification of TCP incrementally deployable and easy to
implement
A. Kuzmanovic and E. W. Knightly
Early Congestion Indication
For transparency, TCP-LP must know of congestion before TCP
Idealized objective: buffer threshold indication
Endpoint inference: one-way delay threshold
– RFC1323 Source - destination time stamping Synchronized clocks not needed
– Eliminates bias due to reverse traffic
b uffer thres ho ld
)( minmaxmin dddsdi
A. Kuzmanovic and E. W. Knightly
TCP-LP Congestion Avoidance
Objectives: LP-flow fairness and TCP transparency
LP-flow fairness– AIMD with early congestion indication
Transparency– Early congestion indication– Inference phase goals:
Infer the cross-traffic Improve dynamic properties “MD” not conservative enough
A. Kuzmanovic and E. W. Knightly
TCP-LP Timeline IllustrationC
onge
stio
n W
indo
w
T im e
- Send 1 pkt/RTT- Ensure available x bandwidth > 0
A. Kuzmanovic and E. W. Knightly
Con
gest
ion
Win
dow
T im e
TCP-LP Timeline Illustration
- AI phase- CWND/2 upon __early congestion xxindication- Inference phase
A. Kuzmanovic and E. W. Knightly
TCP-LP Timeline IllustrationC
onge
stio
n W
indo
w
T im e
-2nd CI => CWND=1- Inference phase
A. Kuzmanovic and E. W. Knightly
TCP-LP Timeline IllustrationC
onge
stio
n W
indo
w
T im e
A. Kuzmanovic and E. W. Knightly
Low-Aggregation Regime
Hypothesis: TCP cannot attain 1.5 Mb/s throughput due to reverse cross-traffic
How much capacity remains and can TCP-LP utilize it?
R 1 R 2
TC P-L P
TC P
C = 1 .5 M b/s
cro s s - t ra f f ic
A. Kuzmanovic and E. W. Knightly
TCP-LP in Action
TCP alone 745.5 Kb/s TCP vs. 739.5 Kb/s TCP-LP 109.5 Kb/s
TCP-LP is invisible to TCP traffic!
R 1 R 2
TC P-L P
TC P
C = 1 .5 M b/s
cro s s - t ra f f ic
A. Kuzmanovic and E. W. Knightly
High-Aggregation Regime with Short-Lived Flows
Bulk FTP flow using TCP-LP vs. TCP Explore delay improvement to web traffic Explore throughput penalty to FTP/TCP-LP flow
R 1 R 2... ...
... ...
F ile T ra ns fe r
C lie n t Po o lS e rv e r Po o l
re qu e s t
re s po n s e
A. Kuzmanovic and E. W. Knightly
TCP Background Bulk Data Transfer
Web response times
are normalizedR 1 R 2... ...
... ...
T C P F ile T ra ns fe r
C lie n t Po o lS e rv e r Po o l
re qu e s t
re s po n s e
A. Kuzmanovic and E. W. Knightly
TCP-LP Background Bulk Data Transfer
Web response times improved
3-5 times FTP throughput: TCP: 58.2%
TCP-LP: 55.1%
R 1 R 2... ...
... ...
T C P -L P F T P
C lie n t Po o lS e rv e r Po o l
re qu e s t
re s po n s e
A. Kuzmanovic and E. W. Knightly
Conclusions
TCP-LP adds a new service to the Internet– General low priority service (compared to “best-effort”)
TCP-LP is easy to deploy and use– Sender side modification of TCP without changes to routers
TCP-LP is attractive for many applications: ftp, web updates, overlay networks, P2P
Significant benefits for best effort traffic, minimal throughput loss for bulk flows
http://www.ece.rice.edu/networks/TCP-LP
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