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A Receiver-Driven Transport Protocol
for the Web
Rajarshi Gupta, Mike Chen, Steven McCanne, Jean Walrand
UC Berkeley EECS
INFORMS 2000 3/6/00Boca Raton, Florida
WebTP Project @ EECS-UC B
World Wide Web today is vast and vital
Mostly runs request-response HTTP over byte-stream TCP
WebTP is ALF-based, multi-layeredIncludes the User into the transport
control loopWe present here a proposed transport
layer that is receiver-driven
INFORMS 2000 3/6/00Boca Raton, Florida
Why Receiver-Driven
Incorporating User Control
Application Controlled Retransmissions
Reduced Server State
Using processing power at Clients
INFORMS 2000 3/6/00Boca Raton, Florida
Protocol Design Goals
A receiver-based schemeAmenable to ALFSits astride the IP layerInteracts well with TCP, and with other
WebTP flowsSeamless integration with higher
layers
INFORMS 2000 3/6/00Boca Raton, Florida
Protocol Components
Sender-Receiver InteractionCalculating RTT at the ReceiverSender ActionsReceiver Window ControlReceiver Rate ControlRetransmission (receiver controlled)
Timeout Out-of-Order Arrivals
INFORMS 2000 3/6/00Boca Raton, Florida
Sender-Receiver Interaction
Receiver is the controlling entityNo sense of connection Flow initiated by Receiver by
requesting first data objectALF-based naming mechanismFlow terminated by Receiver by not
requesting for any more objects“Soft-State” at Sender
INFORMS 2000 3/6/00Boca Raton, Florida
RTT Calculation at Receiver
Sender-Based Receiver-based
S R
Packet / Ack
Timestamp
S R
heldtime
RTTs
RTTr + heldtime
SRTTnew = α SRTTold + (1 - α) RTT
heldtime2
RTTr + heldtime2
INFORMS 2000 3/6/00Boca Raton, Florida
Flow Control at Receiver
WINDOW-BASED
Limits maximum amount of data in transition
Slow-StartAdditive Increase/
Multiplicative DecreaseRate of increase will be
biased by RTT
RATE-BASED
Additive Increase/ Multiplicative Decrease
Rate control based on window size rate = cwnd / srtt
Rate Control independent of cwnd rate = f (oldrate)
INFORMS 2000 3/6/00Boca Raton, Florida
Sender (rate-based)
Sender Object Queue
Processor
<n,n+4>,r1 <n+5,n+6>,r2
[n][n+1]
[n+2] [n+4] [n+5][n+3]
1/r1 1/r1 1/r1 1/r2 1/r2
Sender
Receiver
[n+5 [n+6]
1/r2
INFORMS 2000 3/6/00Boca Raton, Florida
Retransmission (timeout)
Occurs when no packet is received for a long period (w.r.t Current Rate)
timeout = 1/r + M. r = current rate new = old + (1-).|iat - 1/r |
M = tolerance parameter
Action: Cut window, Cut rateRequest again for first object in queue
INFORMS 2000 3/6/00Boca Raton, Florida
Retransmission (out-of-order)
abcdefghijkl
hijkcdl
abcdefghijkl
Expected Object Queue (initial)
Expected Object Queue (after queue reordering)
Expected Object Queue (after out-of-order arrivals)Arrival Order
baefgRequested
Retransmissions
Retransmitted packets movedto Expected Positions
SRTT rate = 4
Expected Object Queue emulates the queue at the Sender
INFORMS 2000 3/6/00Boca Raton, Florida
Experimental Setup
Used the ns network simulator
WebTP Message Agents implemented in ns
All calculations and control at receiver
Built-in TCP agents WebTP
WebTPTCP
TCP
TCP
TCP
WebTP
WebTP
Network
INFORMS 2000 3/6/00Boca Raton, Florida
Single WebTP Flow
Details1.5 Mbps linkDropTail Queue
(queue limit 10)ObservationsEfficientStableNetwork-friendlyRate Plot for One WebTP Flow
INFORMS 2000 3/6/00Boca Raton, Florida
Three WebTP Flows
*Efficiency* *Fairness*
Rate Plot Fairness Plot
INFORMS 2000 3/6/00Boca Raton, Florida
One WebTP and One TCP Flow
*Efficiency* *Stability* *Fairness*
Rate Plot Fairness Plot
INFORMS 2000 3/6/00Boca Raton, Florida
Conclusions
Motivation for WebTPArgued desirability of Receiver-driven
protocolDemonstrated a working solution to the
design problem outlinedEfficient, Stable and Fair flows - work
well when WebTP pkts are isolatedAcceptable interaction with TCP
INFORMS 2000 3/6/00Boca Raton, Florida
Future Work
Need to tune various design parameters
Address performance issues across asymmetric connections
Hazards relating to server securityDynamically generated contentsExtensive testing of WebTP flow
control over Internet
INFORMS 2000 3/6/00Boca Raton, Florida
Questions :-)
WebTP Home Page:http://webtp.eecs.berkeley.edu