TCP and congestion control

8/13/2019 TCP and congestion control

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Introduction to TCP

Communication abstraction: Reliable

Ordered

Point-to-point

Byte-stream

Full duplex

Flow and congestion controlled

Protocol implemented entirely at the ends Fate sharing

Sliding window with cumulative acks Ack field contains last in-order packet received

Duplicate acks sent when out-of-order packet received


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Evolution of TCP

1975 1980 1985 1990

1982

TCP & IPRFC 793 & 791

1974

TCPdescribed by

Vint Cerfand Bob Kahn

In IEEE Trans Comm

1983

BSD Unix 4.2supports TCP/IP

1984

Nagels algorithm

to reduce overhead

of small packets;

predicts congestion

collapse

1987

Karns algorithm

to better estimate

round-trip time

1986Congestion

collapse

observed

1988Van Jacobsons

algorithms

congestion avoidance

and congestion control

(mostimplemented in

4.3BSD Tahoe)

1990

4.3BSD Reno

fast retransmit

delayed ACKs

1975

Three-way handshake

Raymond Tomlinson

In SIGCOMM 75


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TCP Through the 1990s

1993 1994 1996

1994ECN

(Floyd)

Explicit

Congestion

Notification

1993TCP Vegas

(Brakmo et al)

real congestion

avoidance

1994

T/TCP

(Braden)

Transaction

TCP

1996

SACK TCP

(Floyd et al)

Selective

Acknowledgement

1996Hoe

Improving TCP

startup

1996FACK TCP

(Mathis et al)

extension to SACK


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Timeout-based Recovery

Wait at least one RTT before retransmitting

Importance of accurate RTT estimators:

Low RTTunneeded retransmissions

High RTTpoor throughput

RTT estimator must adapt to change in RTT

But not too fast, or too slow!

Spurious timeouts Conservation of packets principle more than a

window worth of packets in flight


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Initial Round-trip Estimator

Round trip times exponentially averaged:

New RTT = a(old RTT) + (1 - a) (new

sample)

Recommended value for a: 0.8 - 0.9

0.875 for most TCPs

Retransmit timer set to bRTT, where b= 2

Every time timer expires, RTO exponentially backed-off

Like Ethernet

Not good at preventing spurious timeouts


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Jacobsons Retransmission

Timeout Key observation:

At high loads round trip variance is high

Solution: Base RTO on RTT and standard deviation orRRTT

rttvar = c* dev + (1- c)rttvar

dev = linear deviation

Inappropriately namedactually smoothed linear

deviation


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TCP Flavors

Tahoe, Reno, Vegasdiffer in data-

driven reliability

TCP Tahoe (distributed with 4.3BSD Unix)

Original implementation of Van Jacobsons

mechanisms (VJ paper)

Includes:

Slow start

Congestion avoidance

Fast retransmit


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Fast Retransmit

What are duplicate acks (dupacks)? Repeated acks for the same sequence

When can duplicate acks occur?

Loss Packet re-ordering

Window updateadvertisement of new flow controlwindow

Assume re-ordering is infrequent and not oflarge magnitude Use receipt of 3 or more duplicate acks as indication

of loss

Dont wait for timeout to retransmit packet


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Fast Retransmit

Time

Sequence NoDuplicate Acks

Retransmission

X


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Multiple Losses

Time

Sequence No Duplicate Acks

RetransmissionX

X

XX

Now what?


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Time

Sequence No

X

X

XX

Tahoe


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TCP Reno (1990)

All mechanisms in Tahoe

Addition of fast-recovery

Opening up congestion window after fast retransmit

Delayed acks

Header prediction

Implementation designed to improve performance

Has common case code inlined With multiple losses, Reno typically timeouts

because it does not receive enough duplicate

acknowledgements


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Reno

Time

Sequence No

X

X

XX

Now what?timeout


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NewReno

The ack that arrives after retransmission

(partial ack) should indicate that a second

loss occurred

When does NewReno timeout?

When there are fewer than three dupacks for

first loss

When partial ack is lost

How fast does it recover losses?

One per RTT


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NewReno

Time

Sequence No

X

X

XX

Now what?partial ack

recovery


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SACK

Basic problem is that cumulative acks

provide little information

Ack for just the packet received

What if acks are lost? carry cumulative also

Not used

Bitmask of packets received

Selective acknowledgement (SACK)

How to deal with reordering


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Congestion Collapse

Definition: Increase in network load results indecrease of useful work done

Many possible causes

Spurious retransmissions of packets still in flight

Classical congestion collapse

How can this happen with packet conservation

Solution: better timers and TCP congestion control

Undelivered packets

Packets consume resources and are dropped elsewhere innetwork

Solution: congestion control for ALL traffic


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Other Congestion Collapse

Causes Fragments

Mismatch of transmission and retransmission units

Solutions Make network drop all fragments of a packet (early packet

discard in ATM) Do path MTU discovery

Control traffic Large percentage of traffic is for control

Headers, routing messages, DNS, etc.

Stale or unwanted packets Packets that are delayed on long queues

Push data that is never used


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Where to Prevent Collapse?

Can end hosts prevent problem?

Yes, but must trust end hosts to do right thing

E.g., sending host must adjust amount of data

it puts in the network based on detectedcongestion

Can routers prevent collapse?

No, not all forms of collapse

Doesnt mean they cant help

Sending accurate congestion signals

Isolating well-behaved from ill-behaved

sources


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Congestion Control and

Avoidance A mechanism which:

Uses network resources efficiently

Preserves fair network resource allocation

Prevents or avoids collapse

Congestion collapse is not just a theory

Has been frequently observed in many

networks


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TCP Congestion Control

Motivated by ARPANET congestion collapse

Underlying design principle: packet conservation

At equilibrium, inject packet into network only when

one is removed Basis for stability of physical systems

Why was this not working?

Connection doesnt reach equilibrium

Spurious retransmissions

Resource limitations prevent equilibrium


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TCP Congestion Control -

Solutions Reaching equilibrium

Slow start

Eliminates spurious retransmissions

Accurate RTO estimation

Fast retransmit

Adapting to resource availability

Congestion avoidance


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TCP Congestion Control

Changes to TCP motivated by

ARPANET congestion collapse

Basic principlesAIMD

Packet conservation

Reaching steady state quickly

ACK clocking


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AIMD

Distributed, fair and efficient

Packet loss is seen as sign of congestion and

results in a multiplicative rate decrease

Factor of 2 TCP periodically probes for available bandwidth

by increasing its rate

Time

Rate


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Implementation Issue

Operating system timers are very coarsehow to pace

packets out smoothly?

Implemented using a congestion window that limits how

much data can be in the network.

TCP also keeps track of how much data is in transit

Data can only be sent when the amount of outstanding

data is less than the congestion window.

The amount of outstanding data is increased on a send and

decreased on ack (last sentlast acked) < congestion window

Window limited by both congestion and buffering

Senders maximum window = Min (advertised window, cwnd)


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Congestion Avoidance

If loss occurs when cwnd = W Network can handle 0.5W ~ W segments

Set cwnd to 0.5W (multiplicative decrease)

Upon receiving ACK Increase cwnd by (1 packet)/cwnd

What is 1 packet?1 MSS worth of bytes

After cwnd packets have passed byapproximately increase of 1 MSS

Implements AIMD


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Congestion Avoidance

Sequence Plot

Time

Sequence No

Packets

Acks


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Congestion Avoidance Behavior

Time

CongestionWindow

Packet loss+ Timeout

Grabbingback

Bandwidth

CutCongestion

Windowand Rate


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Packet Conservation

At equilibrium, inject packet into network only

when one is removed

Sliding window and not rate controlled

But still need to avoid sending burst of packets would overflow links

Need to carefully pace out packets

Helps provide stability

Need to eliminate spurious retransmissions Accurate RTO estimation

Better loss recovery techniques (e.g. fast retransmit)


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TCP Packet Pacing

Congestion window helps to pace the

transmission of data packets

In steady state, a packet is sent when an ack is

received Data transmission remains smooth, once it is smooth

Self-clocking behavior

Pr

Pb

ArAb

ReceiverSender

As


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Reaching Steady State

Doing AIMD is fine in steady state but

slow

How does TCP know what is a good initial

rate to start with?

Should work both for a CDPD (10s of Kbps or

less) and for supercomputer links (10 Gbps

and growing) Quick initial phase to help get up to speed

(slow start)


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Slow Start Packet Pacing

How do we get thisclocking behavior tostart? Initialize cwnd = 1

Upon receipt of everyack, cwnd = cwnd + 1

Implications Window actually

increases to W in RTT *log2(W)

Can overshoot windowand cause packet loss


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TCP Saw Tooth

Time

CongestionWindow

InitialSlowstart

FastRetransmit

and Recovery

Slowstartto pacepackets

Timeoutsmay still

occur


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TCP Modeling

Given the congestion behavior of TCP can we

predict what type of performance we should get?

What are the important factors

Loss rate Affects how often window is reduced

RTT

Affects increase rate and relates BW to window

RTO Affects performance during loss recovery

MSS

Affects increase rate


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Overall TCP Behavior

Time

Window

Lets concentrate on steady state behaviorwith no timeouts and perfect loss recovery


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Simple TCP Model

Some additional assumptions

Fixed RTT

No delayed ACKs

In steady state, TCP losses packet eachtime window reaches W packets

Window drops to W/2 packets

Each RTT window increases by 1packetW/2 * RTT before next loss

BW = MSS * avg window/RTT = MSS * (W +W/2)/(2 * RTT) = .75 * MSS * W / RTT


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Simple Loss Model

What was the loss rate?

Packets transferred = (.75 W/RTT) * (W/2 *

RTT) = 3W2/8

1 packet lostloss rate = p = 8/3W2

W = sqrt( 8 / (3 * loss rate))

BW = .75 * MSS * W / RTT

BW = MSS / (RTT * sqrt (2/3p))


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TCP Vegas Slow Start

ssthresh estimation via packet pair

Only increase every other RTT

Tests new window size before increasing


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Packet Pair

What would happen if a source transmitted

a pair of packets back-to-back?

Spacing of these packets would be

determined by bottleneck link

Basis for ack clocking in TCP

What type of bottleneck router behavior

would affect this spacing

Queuing scheduling


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Packet Pair in Practice

Most Internet routers are FIFO/Drop-Tail

Easy to measure link bandwidths

Bprobe, pathchar, pchar, nettimer, etc.

How can this be used? NewReno and Vegas use it to initialize

ssthresh

Prevents large overshoot of availablebandwidth

Want a high estimateotherwise will take along time in linear growth to reach desired

bandwidth

TCP V C ti


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TCP Vegas Congestion

Avoidance Only reduce cwnd if packet sent after last

such action

Reaction per congestion episode not per loss

Congestion avoidance vs. control

Use change in observed end-to-end delay to

detect onset of congestion

Compare expected to actual throughput Expected = window size / round trip time

Actual = acks / round trip time


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TCP Vegas

Fine grain timers Check RTO every time a dupack is received or for

partial ack

If RTO expired, then re-xmit packet

Standard Reno only checks at 500ms

Allows packets to be retransmitted earlier Not the real source of performance gain

Allows retransmission of packet that would havetimed-out Small windows/loss of most of window

Real source of performance gain

Shouldnt comparison be against NewReno/SACK


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TCP Vegas

Flaws

Sensitivity to delay variation

Paper did not do great job of explaining where

performance gains came from

Some ideas have been incorporated into

more recent implementations

Overall Some very intriguing ideas

Controversies killed it

TCP and congestion control

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