Bài giảng Mạng máy tính - Chapter 3: Transport Layer

Computer Networks

Lectured by:

Nguyen Le Duy Lai

(lai@hcmut.edu.vn)

Computer

Networking: A Top

Down Approach

7^thEdition, Global Edition

Jim Kurose, Keith Ross

Pearson

April 2016

Introduction 1-1

Chapter 3

Transport Layer

Computer

Networking: A Top

Down Approach

7^thEdition, Global Edition

Jim Kurose, Keith Ross

Pearson

April 2016

Transport Layer

2-2

Chapter 3: Transport Layer

our goals:

▪ understand

principles behind

transport layer

services:

▪ learn about Internet

transport layer protocols:

• UDP: connectionless

transport

• TCP: connection-oriented

• Multiplexing (mux),

demultiplexing

(demux)

reliable transport

• TCP flow control

• reliable data transfer

• TCP congestion control

(rdt)

• flow control

• congestion control

Transport Layer

3-3

Chapter 3: outline

3.5 connection-oriented

3.1 transport-layer

services

transport: TCP

• segment structure

• reliable data transfer

• flow control

3.2 multiplexing and

demultiplexing

3.3 connectionless

transport: UDP

3.4 principles of reliable

data transfer

• connection management

3.6 principles of congestion

control

3.7 TCP congestion control

Transport Layer

3-4

Transport services and protocols

application

transport

network

data link

physical

▪ provide logical communication

between app processes

running on different hosts

▪ transport protocols run in

end systems

• send side: breaks app

messages into segments,

passes to network layer

• receive side: reassembles

segments into messages,

passes to app layer

application

transport

network

data link

physical

▪ more than one transport

protocol available to apps

• Internet: TCP and UDP

Transport Layer

3-5

Transport vs. network layer

▪ network layer

household analogy:

▪ logical communication

between hosts

12 kids in Ann’s house sending

letters to 12 kids in Bill’s

house:

▪ transport layer

▪ logical communication

▪ hosts = houses

▪ processes = kids

between processes

▪ relies on, enhances,

▪ app messages = letters in

network layer services

envelopes

▪ transport protocol = Ann and

Bill who demux to in-house

siblings

▪ network-layer protocol =

postal service

Transport Layer

3-6

Internet transport-layer protocols

application

transport

network

data link

physical

▪ reliable, in-order delivery:

TCP

network

data link

physical

network

data link

physical

• connection setup

network

data link

physical

• flow control

• congestion control

network

data link

physical

▪ possibly unreliable,

network

data link

physical

unordered delivery: UDP

network

data link

physical

• no-frills extension of

application

transport

network

data link

physical

network

data link

physical

“best-effort” IP

▪ services not available:

• delay guarantees

• bandwidth guarantees

Transport Layer

3-7

Chapter 3 outline

3.5 connection-oriented

transport: TCP

3.1 transport-layer

services

• segment structure

• reliable data transfer

• flow control

3.2 multiplexing and

demultiplexing

3.3 connectionless

transport: UDP

3.4 principles of reliable

data transfer

• connection management

3.6 principles of congestion

control

3.7 TCP congestion control

Transport Layer

3-8

Multiplexing/demultiplexing

multiplexing at sender:

demultiplexing at receiver:

handle data from multiple

sockets, add transport header

(later used for demultiplexing)

use header info to deliver

received segments to correct

socket

application

P1

P2

application

socket

P4

P3

transport

process

network

link

transport

network

link

network

link

physical

Transport Layer

3-9

How demultiplexing works

▪ host receives IP datagrams

32 bits

source port # dest port #

• each datagram has source IP

address, destination IP address

• each datagram carries one

other header fields

transport-layer segment

• each segment has source,

destination port number

application data

(payload)

▪ host uses IP addresses &

port numbers to direct

segment to appropriate

socket

TCP/UDP segment format

Transport Layer

3-10

Connectionless demultiplexing

▪ recall: created socket has

▪ recall: when creating

datagram to send into

UDP socket, must specify

• destination IP address

• destination port #

host-local port #:

DatagramSocket mySocket1

= new

DatagramSocket(12534);

IP datagrams with same

dest. port #, but different

source IP addresses

▪ when host receives UDP

segment:

• checks destination port #

and/or source port

in segment

numbers will be directed

to same socket at dest

• directs UDP segment to

socket with that port #

Transport Layer

3-11

Connectionless demux: example

DatagramSocket

serverSocket = new

DatagramSocket

(6428);

DatagramSocket

mySocket2 = new

DatagramSocket

mySocket1 = new

DatagramSocket

(5775);

(9157);

application

P1

P3

P4

transport

network

link

network

link

network

link

physical

source port: 6428

dest port: 9157

source port: ?

dest port: ?

source port: ?

dest port: ?

source port: 9157

dest port: 6428

Transport Layer

3-12

Connection-oriented demux

▪ server host may support

▪ TCP socket identified

by 4-tuple:

many simultaneous TCP

sockets:

• source IP address

• source port number

• dest IP address

• each socket identified by its

own 4-tuple

▪ E.g., web servers have

different sockets for

each connecting client

• dest port number

▪ demux: receiver uses

all four values to direct

segment to appropriate

socket

• non-persistent HTTP will

have different socket for

each request

Transport Layer

3-13

Connection-oriented demux: example

application

P4

P5

P6

P3

P1

P2

transport

network

link

transport

network

link

network

link

physical

server: IP

address B

host: IP

address C

host: IP

address A

source IP,port: B,80

dest IP,port: A,9157

source IP,port: C,5775

dest IP,port: B,80

source IP,port: A,9157

dest IP, port: B,80

source IP,port: C,9157

dest IP,port: B,80

three segments, all destined to IP address: B,

dest port: 80 are demultiplexed to different sockets

Transport Layer

3-14

Connection-oriented demux: example

threaded server

application

P4

P3

P2

transport

network

link

transport

network

link

network

link

physical

server: IP

address B

host: IP

address C

host: IP

address A

source IP,port: B,80

dest IP,port: A,9157

source IP,port: C,5775

dest IP,port: B,80

source IP,port: A,9157

dest IP, port: B,80

source IP,port: C,9157

dest IP,port: B,80

Transport Layer

3-15

Chapter 3 outline

3.5 connection-oriented

transport: TCP

3.1 transport-layer

services

• segment structure

• reliable data transfer

• flow control

3.2 multiplexing and

demultiplexing

3.3 connectionless

transport: UDP

3.4 principles of reliable

data transfer

• connection management

3.6 principles of congestion

control

3.7 TCP congestion control

Transport Layer

3-16

UDP: User Datagram Protocol [RFC 768]

▪ UDP use:

▪ “no frills,” “bare bones”

Internet transport

protocol

▪ streaming multimedia

apps (loss tolerant, rate

sensitive)

▪ “best effort” service,

UDP segments may be:

▪ DNS

▪ SNMP

• lost

• delivered out-of-order

▪ reliable transfer over

to app layer

UDP:

▪ connectionless:

▪ add reliability at

• no handshaking

between UDP sender,

receiver

• each UDP segment

handled independently

of others

application layer

▪ application-specific error

recovery!

Transport Layer

3-17

UDP: segment header

length (in bytes) of

32 bits

UDP segment,

including header

source port #

length

dest port #

checksum

why is there a UDP?

▪ no connection establishment

(which can add delay)

Application data

▪ simple: no connection

(payload)

state at sender, receiver

▪ small header size

▪ no congestion control: UDP

can blast away as fast as

desired

UDP segment format

Transport Layer

3-18

UDP checksum

Goal: detect “errors” (e.g., flipped bits) in transmitted

segment

sender:

receiver:

▪ treat segment contents,

including header fields,

as sequence of 16-bit

integers

▪ compute checksum of

received segment

▪ check if computed

checksum equals checksum

field value:

▪ checksum: addition

(one’s complement sum)

of segment contents

• NO - error detected

• YES - no error detected.

But maybe errors

nonetheless? More later

….

▪ sender puts checksum

value into UDP

checksum field

Transport Layer

3-19

Internet checksum: example

example: add two 16-bit integers

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

wraparound

1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

sum

checksum

1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0

1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

Note: when adding numbers, a carryout from the most

significant bit needs to be added to the result

* Check out the online interactive exercises for more

examples: http://gaia.cs.umass.edu/kurose_ross/interactive/

Transport Layer

3-20

Bài giảng Mạng máy tính - Chapter 3: Transport Layer - Nguyễn Lê Duy Lai