RTT (milliseconds)

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Transport Layer 3-1

Chapter 3 Transport Layer

Computer Networking:

A Top Down Approach Featuring the Internet, 3^rd edition.

Jim Kurose, Keith Ross Addison-Wesley, July 2004.

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Pipelined protocols

Pipelining: sender allows multiple, “in-flight”, yet-to- be-acknowledged pkts

range of sequence numbers must be increased

buffering at sender and/or receiver

Two generic forms of pipelined protocols: go-Back-N,

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Transport Layer 3-3

Pipelining: increased utilization

first packet bit transmitted, t = 0

sender receiver

RTT last bit transmitted, t = L / R

first packet bit arrives

last packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

last bit of 2^nd packet arrives, send ACK last bit of 3^rd packet arrives, send ACK

U _sender= .024

30.008 = ^0.0008 3 * L / R

RTT + L / R =

Increase utilization by a factor of 3!

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Go-Back-N

Sender:

k-bit seq # in pkt header

“window” of up to N, consecutive unack’ed pkts allowed

ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”

A single timer

timeout(n): retransmit pkt n and all higher seq # pkts in window

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Transport Layer 3-5

GBN: sender extended FSM

Wait start_timer

udt_send(sndpkt[base]) udt_send(sndpkt[base+1])

…

udt_send(sndpkt[nextseqnum-1]) timeout

rdt_send(data)

if (nextseqnum < base+N) {

sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum])

if (base == nextseqnum) start_timer

nextseqnum++

} else

refuse_data(data)

base = getacknum(rcvpkt)+1 If (base == nextseqnum)

stop_timer else

start_timer

rdt_rcv(rcvpkt) &&

notcorrupt(rcvpkt) base=1

nextseqnum=1

rdt_rcv(rcvpkt)

&& corrupt(rcvpkt)

Λ

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GBN: receiver extended FSM

ACK-only: always send ACK for correctly-received pkt with highest in-order seq #

may generate duplicate ACKs

need only remember expectedseqnum

out-of-order pkt:

Wait

udt_send(sndpkt) default

rdt_rcv(rcvpkt)

&& notcurrupt(rcvpkt)

&& hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data)

deliver_data(data)

sndpkt = make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpkt)

expectedseqnum++

expectedseqnum=1 sndpkt =

make_pkt(expectedseqnum,ACK,chksum)

Λ

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Transport Layer 3-7

GBN in

action

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Selective Repeat

receiver individually acknowledges all correctly received pkts

buffers pkts, as needed, for eventual in-order delivery to upper layer

sender only resends pkts for which ACK not received

sender timer for each unACKed pkt

sender window

N consecutive seq #’s

again limits # of sent, unACKed pkts

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Transport Layer 3-9

Selective repeat: sender, receiver windows

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Selective repeat

data from above :

if next available seq # in window, send pkt

timeout(n):

resend pkt n, restart timer

ACK(n)

ⁱⁿ[sendbase,sendbase+N]:

mark pkt n as received

if n is the smallest unACKed pkt, advance window base to next unACKed seq #

sender

pkt n in

[rcvbase, rcvbase+N-1]

send ACK(n)

out-of-order: buffer

in-order: deliver (also

deliver buffered, in-order pkts), advance window to next not-yet-received pkt

pkt n in

[rcvbase-N,rcvbase-1]

Send ACK(n)

otherwise:

ignore

receiver

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Transport Layer 3-11

Selective repeat in action

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Selective repeat:

dilemma

Example:

seq #’s: 0, 1, 2, 3

window size=3

receiver sees no difference in two scenarios!

incorrectly passes duplicate data as new in (a)

Q: what relationship

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

segment structure

reliable data transfer

flow control

connection management

3.6 Principles of congestion control

3.7 TCP congestion

control

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TCP: Overview

RFCs: 793, 1122, 1323, 2018, 2581

full duplex data:

bi-directional data flow in same connection

MSS: maximum segment size

connection-oriented:

handshaking (exchange of control msgs) init’s sender, receiver state before data exchange

flow controlled:

point-to-point:

one sender, one receiver

reliable, in-order byte steam:

no “message boundaries”

pipelined:

TCP congestion and flow control set window size

send & receive buffers

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TCP segment structure

source port # dest port # 32 bits

application data

(variable length) sequence number

acknowledgement number Receive window

Urg data pnter checksum

F S P R A

head U

len not used

Options (variable length) URG: urgent data

(generally not used) ACK: ACK #

valid PSH: push data now (generally not used) RST, SYN, FIN:

connection estab (setup, teardown commands)

# bytes rcvr willing to accept for flow control

Internet checksum (as in UDP)

counting by bytes of data

(not segments!)

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TCP seq. #’s and ACKs

Seq. #’s:

byte stream

“number” of first byte in segment’s data

ACKs:

seq # of next byte expected from

other side

cumulative ACK Q: how receiver handles

out-of-order segments

A: TCP spec doesn’t

Host A Host B

Seq=42, ACK=79, data = ‘C’

Seq=79, ACK=43, data = ‘C’

Seq=43, ACK=80

typesUser

‘C’

host ACKs receipt of echoed

‘C’

host ACKs receipt of

‘C’, echoes back ‘C’

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TCP Round Trip Time and Timeout

Q: how to set TCP timeout value?

longer than RTT

but RTT varies

too short: premature timeout

unnecessary retransmissions

too long: slow reaction to segment loss

Q: how to estimate RTT?

SampleRTT: measured time from segment transmission until ACK receipt

ignore retransmissions

SampleRTT will vary, want estimated RTT “smoother”

average several recent measurements, not just current SampleRTT

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TCP Round Trip Time and Timeout

EstimatedRTT = (1- α)*EstimatedRTT + α*SampleRTT

Exponential weighted moving average

influence of past sample decreases exponentially fast

typical value: α = 0.125

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Example RTT estimation:

RTT: gaia.cs.umass.edu to fantasia.eurecom.fr

100 150 200 250 300 350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106

time (seconnds)

RTT (milliseconds)

SampleRTT Estimated RTT

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TCP Round Trip Time and Timeout

Setting the timeout

EstimtedRTT plus “safety margin”

large variation in EstimatedRTT -> larger safety margin

first estimate of how much SampleRTT deviates from EstimatedRTT:

DevRTT = (1-β)*DevRTT +

β*|SampleRTT-EstimatedRTT|

(typically, β = 0.25) Then set timeout interval:

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Chapter 3 outline

3.1 Transport-layer services

3.2 Multiplexing and demultiplexing

3.3 Connectionless transport: UDP

3.4 Principles of

reliable data transfer

3.5 Connection-oriented transport: TCP

segment structure

reliable data transfer

flow control

connection management

3.6 Principles of congestion control

3.7 TCP congestion

control

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TCP reliable data transfer

TCP creates rdt

service on top of IP’s unreliable service

Pipelined segments

Cumulative acks

TCP uses single

retransmission timer

Retransmissions are triggered by:

timeout events

duplicate acks

Initially consider

simplified TCP sender:

ignore duplicate acks

ignore flow control, congestion control

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TCP sender events:

data rcvd from app:

Create segment with seq #

seq # is byte-stream number of first data byte in segment

start timer if not

already running (think of timer as for oldest unacked segment)

expiration interval:

TimeOutInterval

timeout:

retransmit segment that caused timeout

restart timer Ack rcvd:

If acknowledges previously unacked segments

update what is known to be acked

start timer if there are outstanding segments

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TCP sender

(simplified)

NextSeqNum = InitialSeqNum SendBase = InitialSeqNum

loop (forever) { switch(event)

event: data received from application above

create TCP segment with sequence number NextSeqNum if (timer currently not running)

start timer

pass segment to IP

NextSeqNum = NextSeqNum + length(data) event: timer timeout

retransmit not-yet-acknowledged segment with smallest sequence number

start timer

event: ACK received, with ACK field value of y if (y > SendBase) {

SendBase = y

if (there are currently not-yet-acknowledged segments)

Comment:

• SendBase-1: last cumulatively

ack’ed byte Example:

• SendBase-1 = 71;

y= 73, so the rcvr wants 73+ ;

y > SendBase, so that new data is acked

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TCP: retransmission scenarios

Host A

Seq=100, 20 bytes data

ACK=100

time premature timeout

Host B

Seq=92, 8 bytes data ACK=120 Seq=92, 8 bytes data

Seq=92 timeout

ACK=120

Host A

ACK=100

timeout loss

lost ACK scenario

Host B

X

ACK=100

time

Seq=92 timeout

SendBase

= 100

SendBase

= 120

SendBase

= 120 Sendbase

= 100

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TCP retransmission scenarios (more)

Host A

Seq=92, 8 bytes data ACK=100

timeout loss

Host B

X

Seq=100

, 20 bytes data

ACK=120

time

SendBase

= 120

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TCP ACK generation [RFC 1122, RFC 2581]

Event at Receiver

Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. # . Gap detected

Arrival of segment that

partially or completely fills gap

TCP Receiver action

Delayed ACK. Wait up to 500ms

for next segment. If no next segment, send ACK

Immediately send single cumulative ACK, ACKing both in-order segments

Immediately send duplicate ACK,

indicating seq. # of next expected byte

Immediate send ACK, provided that segment starts at lower end of gap

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Fast Retransmit

Time-out period often relatively long:

long delay before resending lost packet

Detect lost segments via duplicate ACKs.

Sender often sends

many segments back-to- back

If segment is lost,

there will likely be many duplicate ACKs.

If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost:

fast retransmit: resend segment before timer expires

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event: ACK received, with ACK field value of y if (y > SendBase) {

SendBase = y

if (there are currently not-yet-acknowledged segments) start timer

} else {

increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) {

resend segment with sequence number y }

Fast retransmit algorithm:

a duplicate ACK for

already ACKed segment fast retransmit