WB-RTO: A Window-Based Retransmission Timeout

Ioannis Psaras

Demokritos University of Thrace, Xanthi, Greece

Outline of the presentation

 Contributions of this work  The current Retransmission Timeout Algorithm  Recent Related Work on the subject  The proposed algorithm: WB-RTO  The algorithm  Expected Behavior  Evaluation Plan  Experimental Results COMputer NETworks Group (COMNET)

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Contributions of this work

 Our perspective:  When contention increases, the timeout becomes the

scheduler

the link.

for  Our observations:  TCP-RTO should not be solely based on RTT estimations.

 Congestion events cause retransmission synchronization.

 Wireless burst errors should not be interpreted as congestion events.

 Our solutions:  Approximation of the current level of network contention  Estimation of the contribution of each flow to congestion  Allowance for asynchronous retransmissions when timeout happens.

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COMputer NETworks Group (COMNET)

The current Retransmission Timeout Algorithm

 Upon each ACK arrival, the sender:  Calculates the RTT Variation:  Updates the expected RTT prior to calculating the timeout:  Calculates the Retransmission Timeout value:

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COMputer NETworks Group (COMNET)

Recent Related Work on the subject



Eifel RTO

Algorithm  Uses the timestamp option to detect a spurious timeout.



Forward RTO

Algorithm  Uses the first 3 ACKs after the timeout to decide if the timeout was spurious or not.



Peak-Hopper RTO

Algorithm  Uses 2 timers: one is aggressive and one is conservative. Each time it decides which one to follow.



CA-RTO

: A Contention-Adaptive RTO  Integrates a contention-adaptive parameter and introduces retransmission randomness.

COMputer NETworks Group (COMNET)

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Window-Based RTO (1/4): Proportional Timeout

 Estimation of the contribution of the flow to congestion:

c = f(cwnd , max cwnd )

 Compare the current

cwnd_

with the

max_cwnd_

:  If

cwnd_

<

max_cwnd_ / 2

, 

c = 1

: minimal charge  If

max_cwnd_ / 2

<

cwnd_ < (3/4)* max_cwnd_

, 

c = 1,5

: medium charge  If

(3/4)* max_cwnd_ < cwnd_ < max_cwnd_

, 

c = 2

: major charge

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COMputer NETworks Group (COMNET)

Window-Based RTO (2/4): Contention Estimation

 Flow classification according to its

cwnd_

history (

awnd_

):

ai = g(awnd , Thresholdi)

where

Thresholds 1 to 4

represent different levels of network contention: 

Threshold 1

corresponds to very high contention, 

Threshold 4

corresponds to low contention.

1. awnd_ < 5: a1 = 10 2. 5 < awnd_ < 10: a2 = 5 3. 10 < awnd_ < 30: a3 = 3 4. 30 < awnd_ < 50: a4 = 2

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COMputer NETworks Group (COMNET)

Window-Based RTO (3/4): Timeout Adjustment

 Calculation of the Window-Based RTO:

WB − RTO = random(rtt, c × ai)

or

WB − RTO = random(rtt, f(cwnd , max cwnd ) × g(awnd , Thresholdi))

1.

2.

3.

4.

rtt

, avoids timeout expiration prior to the estimated RTT measurement Parameter

c

captures the contribution of the flow to congestion Parameter

a

approximates the current level of flow contention Randomization guarantees asynchronous retransmission attempts

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COMputer NETworks Group (COMNET)

Window-Based RTO (4/4): Expected Behavior

 High penalties result in high timeout values.

 As

awnd_

increases timeout settles to smaller values.

but  Large windows do not always mean large timeout values.

WB-RTO vs awnd_

COMputer NETworks Group (COMNET)

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Performance Evaluation Plan

  WB-RTO is implemented in TCP-Reno Topologies used:  Evaluation Scenarios  Simple Wired Scenario  Interaction with Active Queue Management (i.e. RED)  Satellite Environment  Traffic Diversity (Mice with Elephants) COMputer NETworks Group (COMNET)

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An Important Note…

 WB-RTO does not improve the Goodput performance of TCP significantly, but  here we focus on the

Retransmission

Timeout Algorithm of TCP hence  we pay more attention on the combination of the

retransmission

effort and the Goodput performance, rather than on the Goodput performance alone.

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COMputer NETworks Group (COMNET)

Simple Wired Scenario (1)

      

Topology: Dumbbell Queuing Policy: DropTail DBP = Buffer Size = 10 pkts 5 participating flows 1500 sec total simulation time Flows ideal rate = 2 pkts/wnd We trace: Seqno progress, RTT, RTO

TCP-RTO

COMputer NETworks Group (COMNET)

WB-RTO 12

Simple Wired Scenario (2)

 For the TCP-RTO performance, we observe:  RTT stabilization  Similar timeout values  50% more retransmitted pkts  Less Goodput

RTT (in secs)

COMputer NETworks Group (COMNET)

RTO (in secs) 13

Interaction with AQM (i.e. RED) (1)

     

Topology: Dumbbell Queuing Policy: RED DBP = Buffer Size = 40 pkts min_thresh = 4 pkts max_thresh = 12 pkts 1500 sec total simulation time

Goodput (in B/s) Number of Timeouts Retransmitted Packets

COMputer NETworks Group (COMNET)

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Interaction with AQM (i.e. RED) (2)

 We observe:  Similar Goodput  Significant difference in Retransmission Effort (50%)  WB-RTO results in 66% less timeout expirations  TCP-RTO causes inefficient queue utilization  The average queue length always overcomes the

max_thresh

, when using TCP RTO

TCP-RTO WB-RTO

COMputer NETworks Group (COMNET)

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Satellite Scenario (1)

          

Topology: Cross-Traffic Bottleneck Queuing Policy: RED The rest of the buffers use DT bw_bottleneck = 20Mbps bw_delay = 300ms Buffer Size = 200 pkts min_thresh = 20 pkts max_thresh = 60 pkts 150 sec total simulation time PER = 0,0001 3 blackouts on the backbone link

No blackout

COMputer NETworks Group (COMNET)

After 3 blackouts 16

Satellite Scenario (2)

 We observe that:  TCP-RTO interprets the second timeout as a congestion signal  WB-RTO does not extend the timeout, due to low contention and hence exploits bandwidth faster  TCP-RTO still waits for the extended timeout to occur, while  WB-RTO resumes transmission immediately

TCP-RTO

COMputer NETworks Group (COMNET)

WB-RTO 17

Traffic Diversity (Mice and Elephants) (1)

    

Topology: Dumbbell Bottleneck Queuing Policy: RED bw_bottleneck = 10Mbps bw_delay = 30ms Buffer Size = 40 pkts

Goodput (KB/s) Goodput per flow (KB/s) Retransmitted Packets

COMputer NETworks Group (COMNET)

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Traffic Diversity (Mice and Elephants) (2)

  We observe:  Simultaneous timeout events for TCP-RTO  All flows timeout during the Slow-Start    Flows 7-9 timeout simultaneously 10 times during the experiment Short flows: 83 vs 50 timeouts Long flows: 43 vs 12 timeouts We conclude that:  most of the timeouts are spurious  WB-RTO achieves an important goal: it reduces the number of timeouts

TCP-RTO WB-RTO

COMputer NETworks Group (COMNET)

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Conclusions

 RTT measurements cannot always reflect the level of network contention  TCP-RTO should not be solely based on RTT samples  A contention-aware RTO proves to be more efficient, since it is aware of current network conditions.

 A randomization factor in the RTO schedules retransmissions in a fairer manner .

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COMputer NETworks Group (COMNET)

WB-RTO: A Window-Based Retransmission Timeout

Thank you!!



Presented by Ioannis Psaras e-mail: [email protected]

URL: utopia.duth.gr/~ipsaras COMputer NETworks Group (COMNET)

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