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1Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
DualRTT: Enhancing TCP Performance During Delay Spikes
Mohammed Atiquzzaman, Ph.D.School of Computer Science
University of Oklahoma.Email: [email protected]
Web: www.cs.ou.edu/~atiq
Presentation at Tohoku University, Sendai, Japan.
Aug 6, 2002.
2Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Problem Statement
! TCP is a reliable transport protocol" Retransmits lost packets
! Non-receipt of acknowledgement within a certain time is treated as loss " Retransmission Timeout timer for measuring time
! Sudden long delays (delay spikes) confuse the timer and cause " unnecessary retransmission of packets and " throttling of packet transmission rate
! TCP throughput is seriously affected by delay spikes
3Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Causes of Delay Spike
! Handoff between cells" Delays due to channel allocation" Physical disconnection
! RLC layer recovery of errors" Packet error is a sudden phenomenon" Long time for retransmission at link layer
! Preemption of data traffic" Sudden arrival of voice call
4Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
TCP congestion control
! Slow Start" cwnd reduced to one
on timeout.! Fast Retransmit
" Three DupACKS" cwnd reduced to half.
0
5
10
15
20
25
0 3 6 9 12 15 20 22 25
Time (round trips)
Con
gest
ion
win
dow
(se
gmen
ts)
ssthresh = 8 ssthresh = 10
cwnd = 20
After timeout
5Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
TCP error recovery
! TCP timer mechanism" Packets are retransmitted on RTO expiry" Double RTO on every timeout
! RTO updates are slow" RTO can not adapt to sudden changes of RTT
! Karn’s algorithm solves retransmission ambiguity" Ignore RTT measurements from retransmitted packets.
Packettransmitted
Time-out occursbefore ack received,packet retransmitted
RTO doubledT1 T2 = 2 * T1
6Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Effect of Delay Spikes on TCP
! Retransmission ambiguity" TCP sender unable to distinguish between the acknowledgement
of original and retransmitted packet.! Spurious Timeout and Spurious Retransmission
Spurious timeout: Timeout which would not have happened if the sender waited long enough.
Link bandwidth = 46.8KbpsLink delay=1.4sStart of delay spike = 28sLength of delay spike = 12sSpurious Fast Retransmission:
Occurs when segments get re-ordered beyond the DUPACK-threshold in the network before reaching the receiver
7Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Spurious Timeout and Retransmission
Long delay
Pack
ets
131-
150
#131 spuriously retransmittedand delayed by delay spike
Fast retransmit of #151
Pack
ets
retra
nsm
itted
by
Slo
w S
tart
Segments received by receiver:131, 132, … 150, 131, 132….,150,… 151
8Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Congestion Window
9Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Illustration of RTO and RTT
RTTRTOTime
2.93.852.7043.34.047.7802.915.244.5792.915.244.4812.915.243.9882.915.243.8902.915.243.7912.915.243.1022.915.243.0042.915.242.9052.93.830.8402.94.617.545
Karn’s Algorithm: RTT measurements from retransmitted segments are ineligible for updating RTO
10Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Eifel
! Main idea of Eifel:" TCP timestamp option to solve retransmission ambiguity" Every packet is timestamped (12-bytes of header)" If the ACK’s timestamp is “smaller” than sending time, previous
timeout and retransmissions were spurious." Action on detection of spurious timeout:
# cwnd and ssthresh are restored to their values before spurious timeout.
# Sender sends new packets instead of going thru go-back-N.! Advantages:
" Enhances TCP throughput during delay spikes.! Disadvantages
" Deployment issues: Requires receivers to implement timestampoption# Only 15% of Internet hosts implement timestamp
" Bandwidth wastage: Extra header field# Not desirable in wireless networks, for which Eifel has been
developed !!!
11Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
DualRTT Objectives
! Wireless friendly" Need to be bandwidth efficient" No extra header fields
! No change at receiver or Internet infrastructure" All changes should be at the sender side only
12Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Principle of DualRTT
! Based on the dynamics of packet arrivals during delay spike" Segments are consecutively queued at the wireless sender during
delay spikes" Segments arrive at the receiver back to back." Interval between arrival of consecutively delayed packets is small
! Two new variables" NRTT: time between the "most recent retransmission" and the
"arrival of acknowledgement" of the corresponding segment at the sender.
" MinRTT: minimum value of RTT observed so far.! Spurious timeout detection criteria:
" NRTT < Threshold * MinRTT
13Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Detection of Spurious Timeout by DualRTT
Rcv A132Spurious timeout detected
14Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Illustration of NRTT and MinRTT
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
MinRTT
Recv A132Spurious timeout detected
Recv. A131, Snd S132, S133
NRTTRTTRTOTime
2.92.93.852.704
3.33.34.047.780
0.42.915.244.579
0.42.915.244.481
0.32.915.243.988
0.32.915.243.890
0.22.915.243.791
0.12.915.243.102
0.12.915.243.004
13.92.915.242.905
2.92.93.830.840
2.92.94.617.545
RTT measurements from retransmitted segments are ineligible for updating RTO
15Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Eifel and DualRTT: Spurious Timeout Detection
DualRTTEifel
16Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Performance of DualRTT
Throughput drops due to congestion losses during delay spikes
17Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Comparison: Throughput Improvement
Comparison of Throughput
Throughput = Number of packets received by the receiver during a period of time.
Size of payloadLink Goodput = ----------------------
Size of packet
Comparison of Link Goodput
18Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Conclusions
! DualRTT enhances TCP throughput during delay spikes! DualRTT
" is wireless friendly" requires changes only at the sender $ easy to deploy" does not require the receiver to implement the timestamp option
! Performance of DualRTT comparable or better than Eifel
19Mohammed Atiquzzaman, University of Oklahoma, USA.
Presentation at Tohoku University, JapanAug 6, 2002.
Further information
! Acknowledgements" National Aeronatics and Space Administration (NASA)" Shaojian Fu
! Further InformationDr. Mohammed [email protected], (405) 325 8077
! These slides are available atwww.cs.ou.edu/~atiq
