程序代写代做代考 dns Transport Layer

Transport Layer

George Parisis
School of Engineering and Informatics

University of Sussex

Transport Layer 3-2

Transport Layer
objectives:
v  understand principles

behind transport layer
services:
§ multiplexing,

demultiplexing
§  reliable data transfer
§  flow control
§  congestion control

v  learn about Internet
transport layer protocols:
§ UDP: connectionless

transport
§ TCP: connection-oriented

reliable transport
§ TCP congestion control

Transport Layer 3-3

Outline
v  transport-layer services
v  multiplexing and demultiplexing
v  connectionless transport: UDP
v  principles of reliable data transfer
v  connection-oriented transport: TCP

§  segment structure
§  reliable data transfer
§  flow control
§  connection management

v  principles of congestion control
v  TCP congestion control

Transport Layer 3-4

Transport services and protocols
v  provide logical

communication between app
processes running on
different hosts

v  transport protocols run in
end systems
§  send side: breaks app

messages into segments,
passes to network layer

§  rcv side: reassembles
segments into messages,
passes to app layer

v  more than one transport
protocol available to apps
§  Internet: TCP and UDP

application
transport
network
data link
physical

Transport Layer 3-5

Transport vs. network layer

v  network layer: logical communication between
hosts

v  transport layer: logical communication between
processes
§  relies on, enhances, network layer services

Transport Layer 3-6

Internet transport-layer protocols
v  reliable, in-order

delivery (TCP)
§  congestion control
§  flow control
§  connection setup

v  unreliable, unordered
delivery: UDP
§ no-frills extension of

“best-effort” IP
v  services not

available:
§ delay guarantees
§ bandwidth guarantees

application
transport
network
data link
physical

network
data link
physical

Transport Layer 3-7

Outline
v  transport-layer services
v  multiplexing and demultiplexing
v  connectionless transport: UDP
v  principles of reliable data transfer
v  connection-oriented transport: TCP

§  segment structure
§  reliable data transfer
§  flow control
§  connection management

v  principles of congestion control
v  TCP congestion control

Transport Layer 3-8

Multiplexing/demultiplexing

process

socket

use header info to deliver
received segments to correct
socket

demultiplexing at receiver: handle data from multiple
sockets, add transport header
(later used for demultiplexing)

multiplexing at sender:

transport

application

physical

link

network

P2 P1

transport

application

physical

link

network

transport

application

physical

link

network

Transport Layer 3-9

How demultiplexing works

v  host receives IP datagrams
§  each datagram has source IP

address, destination IP address
§  each datagram carries one

transport-layer segment
§  each segment has source,

destination port number
v  host uses IP addresses &

port numbers to direct
segment to appropriate
socket

source port # dest port #
32 bits

application
data

(payload)

other header fields

TCP/UDP segment format

Transport Layer 3-10

Connectionless demultiplexing
v  recall: created socket has

host-local port #:
DatagramSocket mySocket1
= new DatagramSocket(12534);

v  when host receives
UDP segment:
§  checks destination port

# in segment
§ directs UDP segment to

socket with that port #

v  recall: when creating
datagram to send into
UDP socket, must
specify
§ destination IP address
§ destination port #
IP datagrams with
same dest. port #, but
different source IP
addresses and/or
source port numbers
will be directed to same
socket at dest

Transport Layer 3-11

Connectionless demux: example
DatagramSocket
serverSocket = new
DatagramSocket

(6428);

transport

application

physical

link

network

P3
transport

application

physical

link

network

transport

application

physical

link

network

DatagramSocket
mySocket1 = new
DatagramSocket
(5775);

DatagramSocket
mySocket2 = new
DatagramSocket
(9157);

source port: 9157
dest port: 6428

source port: 6428
dest port: 9157

source port: ?
dest port: ?

Transport Layer 3-12

Connection-oriented demux
v  TCP socket identified

by 4-tuple:
§  source IP address
§  source port number
§ dest IP address
§ dest port number

v  demux: receiver uses
all four values to
direct segment to
appropriate socket

v  server host may
support many
simultaneous TCP
sockets:
§ each socket identified

by its own 4-tuple
v  servers have different

sockets for each
connecting client

Transport Layer 3-13

Connection-oriented demux: example

transport

application

physical

link

network

P3
transport

application

physical

link

transport

application

physical

link

network

source IP,port: A,9157
dest IP, port: B,80

source IP,port: B,80
dest IP,port: A,9157

host: IP
address

network

P6 P5
P3

source IP,port: C,5775
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

server:
IP

address
B

Transport Layer 3-14

Connection-oriented demux: example

transport

application

physical

link

network

P3
transport

application

physical

link

transport

application

physical

link

network

source IP,port: A,9157
dest IP, port: B,80

source IP,port: B,80
dest IP,port: A,9157

host: IP
address

server:
IP

address
B

network

source IP,port: C,5775
dest IP,port: B,80

source IP,port: C,9157
dest IP,port: B,80

threaded server

Transport Layer 3-15

Outline
v  transport-layer services
v  multiplexing and demultiplexing
v  connectionless transport: UDP
v  principles of reliable data transfer
v  connection-oriented transport: TCP

§  segment structure
§  reliable data transfer
§  flow control
§  connection management

v  principles of congestion control
v  TCP congestion control

Transport Layer 3-16

UDP: User Datagram Protocol [RFC 768]

v  “no frills,” “bare bones”
Internet transport protocol

v  “best effort” service, UDP
segments may be:
§  lost
§  delivered out-of-order to app

v  connectionless:
§  no handshaking between

UDP sender, receiver
§  each UDP segment handled

independently of others

v  UDP use:
§  streaming multimedia apps

(loss tolerant, rate sensitive)
§  DNS
§  SNMP

v  reliable transfer over UDP:
§  add reliability at application

layer
§  application-specific error

recovery!

Transport Layer 3-17

UDP: segment header

source port # dest port #
32 bits

application
data

(payload)

UDP segment format

length checksum

length, in bytes of
UDP segment,

including header

v  no connection
establishment (which
can add delay)

v  simple: no connection
state at sender, receiver

v  small header size
v  no congestion control:

UDP can blast away as
fast as desired

why is there a UDP?

Transport Layer 3-18

UDP checksum

sender:
v  treat segment contents,

including header fields, as
sequence of 16-bit integers

v  checksum: addition (one’s
complement sum) of
segment contents

v  sender puts checksum
value into UDP checksum
field

receiver:
v  compute checksum of received

segment
v  check if computed checksum

equals checksum field value:
§  NO – error detected
§  YES – no error detected.
§  But maybe errors

nonetheless?

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

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

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

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

wraparound

sum
checksum

Note: when adding numbers, a carryout from the most
significant bit needs to be added to the result

Transport Layer 3-20

Summary
v  principles behind transport layer services:

§ multiplexing, demultiplexing
§ reliable data transfer

v  UDP

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