Responding to Spurious Timeouts in TCP Andrei Gurtov University of - - PowerPoint PPT Presentation

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Mar 15, 2024 302 likes •465 views

Responding to Spurious Timeouts in TCP Andrei Gurtov University of Helsinki Reiner Ludwig Ericsson Research Outline Motivation Spurious Timeouts in TCP Robustness to Packet Losses Undoing Congestion Control Adapting

SLIDE 1

Responding to Spurious Timeouts in TCP

Andrei Gurtov

University of Helsinki

Reiner Ludwig

Ericsson Research

SLIDE 2

2/15

Outline

Motivation
Spurious Timeouts in TCP
Robustness to Packet Losses
Undoing Congestion Control
Adapting the Retransmit Timer
Performance Evaluation
Conclusions

SLIDE 3

3/15

Motivation

Delay variation in wireless

networks

– Cell reselections in GPRS last 3-15 sec – Bandwidth oscillation in CDMA2000 – Link-layer persistent error recovery

Deployed aggressive

retransmission timers

– 10 ms granularity and 200 ms minimum in Linux TCP – Solaris

5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 500 510 520 530 540 550 560 570 580 Tim e of D ay (s) Sequence Number (B) Sn d_D ata Sn d_Ack

A TCP trace shows spurious retransmissions caused by two cell reselections in a live GPRS network.

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Spurious Timeouts in TCP

Spurious timeouts hurt

TCP performance

– Unnecessary retransmissions during go- back-N – Disrupted congestion control

In a short run, slow start

causes congestion

In a long run,

underutilization due to reduced ssthresh

SLIDE 5

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Spurious Timeouts in TCP

Detecting spurious

timeouts

– Eifel, F-RTO, etc…

Response after

detecting a spurious timeout

– Our focus

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Main Issues w.r.t. Response

1. Robustness to packet losses

– Danger of genuine timeouts

2. How to restore the

congestion control state

– Does a full restore of cwnd and ssthresh cause a burst? – Do partial restore options perform well?

3. How to adapt the

retransmit timer

– To avoid clogging the network with unnecessary retransmissions in the future

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1. Robustness to Packet

Losses

State-of-the-art TCP is often

sufficient

– Fast Retransmit+ Sack+Limited Transmit

Heavy losses do trigger genuine

timeouts

– TCP gets low throughput – Cannot adapt RTO to a more conservative level

Solutions

– FACK works well but not with reordering – NewReno+Sack works almost as well and appears safe for the Internet

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2. Undoing Congestion

Control

Full undo – too aggressive?

– Appropriate, no bursts

bserved
Partial undo sets the sender idle

for a while

– The flight size is higher than the reduced cwnd – The ACK clock is can be lost

A new proposal: use the ACK

clock but in congestion avoidance

– Ssthresh=cwnd_old, cwnd=ssthresh

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TCP RTO

TCP does not take samples

from delayed segments (Karn algorithm)

TCP with timestamps can do

that

– RTO is more conservative but decays quite fast

TCP with Eifel uses timestamps

– Already more conservative than the standard TCP – Also, maintains a larger window that results into higher RTT and higher RTO

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3. Adapting RTO
Upon a spurious timeout

– Reseed: initialize SRTT and RTTVAR with new sample (history discard) – Back-off: keep the exponential back-off count – Min++: increase the minimum RTO (by 1 sec)

Reset to the standard timer upon a geniune

timeout

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Performance Evaluation

ns2, dumbbell with TCP and CBR sources

– 3G or satellite link: 2 Mbps, 400 ms RTT – Periodic delay spikes

250 % gain in throughput when delay spikes occur on

uncongested path (without CBR)

TCP fairness does not suffer because response to packet

losses is unchanged

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Robustness to Packet Losses: TCPs with CBR

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 std eifel std eifel std eifel Reno-SACK NewReno-SACK FACK N

rmalized

Download Time Segments Sent Spurious RTOs Genuine RTOs

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Undo of Congestion Control: TCP FACK with CBR

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Download Time Segments Sent Spurious RTOs Genuine RTOs Normalized Full Partial None New partial

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Adopting RTO: TCP FACK with CBR

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Download Time Segments Sent Spurious RTOs Genuine RTOs Normalized Std Reseed Back-off Min++

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Summary

An update of TCP sender improving performance over

paths with variable delays

Up to 250% throughput gain on links with a high

bandwidth-delay product

Adequate performance on congested paths

– NewReno-SACK is robust to packet losses

Full restore of cong. control after a spurious timeout is ok
Using back-offs or increasing the min RTO can reduce the

number of spurious RTOs by 40% with only slightly lower throughput

We have some real measurements for 2.5G links