Socket Intro aca

8/13/2019 Socket Intro aca

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NJIT

Introduction to Sockets

and TCP/IP Protocols


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Introduction to Sockets

A socket is one of the most fundamentaltechnologies of computer networking.

The socket is the BSD method for accomplishing

interprocess communication (IPC). What this means is a socket is used to allow one

process to speak to another, very much like thetelephone is used to allow one person to speak

to another. Many of today's most popular software

packages -- including Web Browsers, InstantMessaging and File Sharing -- rely on sockets.


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The Socket Interface

Funded by ARPA (Advanced Research

Projects Agency) in 1980.

Developed at UC Berkeley Objective: to transport TCP/IP software to

UNIX

The socket interface has become a defacto standard.


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History of Sockets

Sockets were introduced in 1981 as the

Unix BSD 4.2 generic interface for Unix

to Unix communications over networks. In 1985, SunOS introduced NFS and

RPC over sockets.

In 1986, AT&T introduced the TransportLayer Interface (TLI) with socket-likefunctionality but more networkindependent.


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TCP/IP Network Standard

The Windows socket API, Winsock, is a multi-vendor specification to standardize the use ofTCP/IP under Windows. It is based on the

Berkeley sockets interface.

In BSD Unix, Sockets are part of the kerneland provide standalone and networked IPCservices.

MS-DOS, Windows, Mac OS, and OS/2provide sockets in the form of libraries.


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3 Types of Socket

Stream sockets interface to the TCP

(transmission control protocol). Datagram sockets interface to the UDP

(user datagram protocol).

Raw sockets interface to the IP (Internetprotocol).


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TCP vs. UDP

TCP used for services with a large data capacity, and apersistent connection, while UDP is more commonly used forquick lookups, and single use query-reply actions.

Some common examples of TCP and UDP with their default

ports:DNS lookup UDP 53

FTP TCP 21

HTTP TCP 80

POP3 TCP 110

Windows shared printer UDP 137

name lookup

Telnet TCP 23


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IPv4 and IPv6

In 1978-1982, when the TCP/IP protocols weredeveloped, provisions were made for 232(about 4billion) hosts. The address protocol, IPv4, has

proven inadequate due to the unexpected rapidgrowth of the internet and inefficient use ofaddress space.

IPv6 uses 16 byte (128 bit) addresses allowing

2128addressable entities. This is roughly 1,000 IPaddresses for each square meter of the surfaceof the earth, including the oceans.


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Addresses and Headers

Try to avoid confusion between an IP addressand an IP header. An IP header usuallyincludes the address and port number of boththe source and destination nodes, along withother information, and has attached data. Theaddress is just an identifier for a networklocation.


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IPv4 addresses

The 32 bits of an IPv4 address are broken into 4octets, or 8 bit fields. In decimal notation, an 8bit number can be represented by the values 0-255.

For networks of different size, the first one (forlarge networks) to three (for small networks)

octets can be used to identify the network, whilethe rest of the octets can be used to identify thenode on the network.


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Class A, B, C, D, E Addresses

Using reserved values for the first octet, networkaddresses are broken into classes:

Class Avery large networks (up to 224hosts)

Class Blarge networks (up to 216hosts)

Class Csmall networks (up to 255 hosts)

Class Dmulti-cast messages to multiple hosts

Class Eaddresses not allocated and reserved. This addressing scheme is shown graphically on

the following slides.


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Figure 3.15 IP addresses (bits)

7 24

Class A : 0 Netw ork ID Host ID

14 16

Class B: 1 0 Netw ork ID Host ID

21 8

Class C: 1 1 0 Netw ork ID Host ID

28

Class D (multicast): 1 1 1 0 Multicast address

27

Class E (reserved): 1 1 1 1 unused0

Coulouris et al


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Figure 3.16 IP addresses(decimal)

octet 1 octet 2 octet 3

Class A: 1 to 127

0 to 255 0 to 255 1 to 254

Class B: 128 to 191

Class C: 192 to 223

224 to 239Class D (multicast):

Network ID

Network ID

Network ID

Host ID

Host ID

Host ID

Multicast address

0 to 255 0 to 255 1 to 254

0 to 255 0 to 255 0 to 255

0 to 255 0 to 255 0 to 255

Multicast address

0 to 255 0 to 255 1 to 254240 to 255Class E (reserved):

1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to

239.255.255.255

240.0.0.0 to255.255.255.255

Range of addresses

Coulouris et al


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Socket Address (IPv4)

A socket address on the TCP/IP internet consists oftwo parts:

An internet(IP) address, a 32 bit number usually

represented by 4 decimal number separated by dots.It is a unique identifier for a network interface cardwithin an administered AF_INET domain. A TCP/IPHost may have as many addresses as it has network

interfaces. (Newer IP addresses have 6 decimal numbers) A 16 bit port number, which is an entry point to anapplication that resides on a host. Port define entrypoints for services provided by server applications.Important commercial applications such as Oraclehave their own well known ports.


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IP Protocol Approach

Define functions that support network

communications in general, and use

parameters to make TCP/IPcommunication a special case.

Socket calls refer to all TCP/IP protocols

as a single protocol family.


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IP Protocol

The IP protocol transmits datagrams from onehost to another with unreliable or best-effortsemantics. Delivery is not guaranteed.

The IP layer puts datagrams into packetssuitable for transmission in the underlyingnetwork, such as Ethernet.

It must also inform the underlying network of theaddress of the message destination usingaddress resolution.


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Address Resolution

The address resolution module must convert aninternet address so that it can be understood bythe underlying network. For example, the 32 bit

IPv4 address has to be converted to a 48 bitEthernet address on an Ethernet network.

This process is specific for each network, andnetwork addressing schemes do not correlate

directly to one another. Typically, known addressresolutions will be cached, while new addressesare found by querying each node on the network.


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Classless InterdomainRouting (CIDR)

In 1996, due largely to the allocation of class Bnetwork addresses to small networks, the Internetbegan to run out of addresses. Network

administrators who could not be certain that theirnetwork would not grow past 255 nodes used classB addresses instead of Class C.

The CIDR scheme was developed to allow a series

of contiguous class C addresses to be used for asubnet requiring more than 255 addresses. This alsoallowed existing Class B addresses to be subdivided.


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CIDR Routing Tables

CIDR required redesign of the routing tables toavoid inefficiency, since a former class Bnetwork address might now represent manywidely separated CIDR networks.

The solution was to add a mask field to a routingtable. The mask is used to select the portion of

the IP address that is to be used to select thenetwork identifier as opposed to the nodeidentifier.


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Unregistered Addresses

All of the computers and devices that access theInternet do not need globally unique IP addresses.Computers that are attached to a local network and

access the Internet through a router can use therouter to redirect packets to the correct computer. For example, the instructors home network is

connected through a router to a cable modem to an

Internet provider. The single globally unique IPaddress provided by the Internet service is theaddress of the cable modem, and is shared by thefour computers on the home network.


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Network Address Translation

Unregistered internal Internet enabled devices areassigned addresses, usually by the Dynamic HostConfiguration Protocol (DHCP). Normally, small

networks are assigned addresses on the192.168.1.x class C subnet, while larger networksuse either the 10.z.y.x. class A subnet or the172.16.y.x Class B subnet.

NAT enabled routers maintain an addresstranslation table and use available source anddestination port numbers to assign packets to localnodes.


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Figure 3.18 A Home Network83.215.152.95

Ethernet switch

Modem / f irewa ll / router (NAT enabled)

printer

DSL or Cableconnection to ISP

192.168.1.xx subnet

PC 1

WiFi base station/access point 192.168.1.10

192.168.1.5

192.168.1.2

192.168.1.1

192.168.1.104 PC 2192.168.1.101

Laptop

192.168.1.105

Game box

192.168.1.106

Media hub

TV m onitor

Bluetoothadapter

Bluetoothprinter

CameraCoulouris

et al


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IPv6

In 1994, IPv6 was adopted as a more permanentsolution to the shortage of IP addresses andmigration to it over a period of time was

recommended. IPv6 contains not only a much larger address space,

but also provisions desired by large Internet serviceproviders. Some of these are controversial, such as

the ability to assign classes to packets, so a providercan give a higher quality of service to its ownsubscribers than to transient traffic on its network.


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New IPv6 Provisions

Larger address space

Partitioned address space

Reduced header complexity for faster routing Traffic class and flow label headers to identify

traffic for special handling, such as a multimediastream

The IPv6 header format is shown on the nextslide.


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Figure 3.19 IPv6 Header

Coulouris et al

Source address(128 bits)

Destination address(128 bits)

Version (4 bits) Traffic class (8 bits) Flow label (20 bits)

Payload length (16 bits) Hop limit (8 bits)Next header (8 bits)

C ti O i t d


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Connection OrientedProtocols

Also known as session-based protocols, virtualcircuits, or sequenced packet exchanges.

Provide reliable two-way connection service over asession.

Packets are given unique sequence numbers.

Delivered packets are individually acknowledged. Duplicated packets are detected and discarded.


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Connection OrientedProtocols

Connection-oriented protocols operate in threephases. The first phase is the connection setupphase, during

which the corresponding entities establish theconnection and negotiate the parameters defining theconnection.

The second phase is the data transferphase, duringwhich the corresponding entities exchange messagesunder the auspices of the connection.

Finally, the connection releasephase is when thecorrespondents "tear down" the connection because it

is no longer needed.


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TCP/IP

TCP/IP is a family of protocols.

TCP/IP is built on "connectionless" technology.Information is transferred as a sequence of

"datagrams". Generally, TCP/IP applications use 4 layers:

An application protocol such as mail .

A protocol such as TCP that provides services need by

many applications. IP, which provides the basic service of getting datagrams

to their destination .

The protocols needed to manage a specific physicalmedium, such as Ethernet or a point to point line.


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Cost of Session Oriented

Reliable service has an overhead cost.

You must create and manage the session.

A lost session must be reestablished byone of the parties, a problem for faulttolerant servers that switch automatically

to backup. Sessions are a two party affair, and not

well suited to broadcasting.


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Basic I/O Functions in UNIX

Sockets extend these basis I/O functions:

open

close

read (see alsorecv andrecvfrom)

write (see alsosend andsendto)

lseek

ioctl


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Using I/O in UNIX

int desc;

...

desc = open(file, O_RDWR, 0);read(desc, buffer, 128);

close(desc);


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Using UNIX I/O with TCP/IP

They extended the conventional UNIX I/O

facilities

It became possible to use file descriptorsfor network communication

Extended the readand writesystem calls

so they work with the new networkdescriptors.


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Descriptor Table

...

0

1

2

Internal data structure

for file 0


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...

0

1

2

Internal data structure

for file 0

Family: PF_INET

...

Service: SOCK_STREAM

Local IP:Remote IP:

Local Port:

Remote Port:


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Passive/Active Socket

Apassivesocket is used by a server to

wait for an incoming connection.

An activesocket is used by a client toinitiate a connection.


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Sockets

When a socket is created it does not

contain information about how it will be

used. TCP/IP protocols define a communication

endpoint to consist of an IP address and a

protocol port number.


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Sockets

Figure A

Figure B


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socket()

bind()

listen()

accept()

read()

write()

procees request

get a blocked client

Server Process

TCP UDP

socket()

connect()

write()

read()

Client Process

socket()

bind()

sendto()

recvfrom()

Client Process

1

2

3

socket()

bind()

recvfrom()

sendto()

Server Process


process request

UNIX

version


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socket()

bind()

listen()

accept()

recv()

send()

process request


Server Process

TCP UDP

socket()

connect()

send()

recv()

Client Process

socket()

bind()

sendto()

recvfrom()

Client Process

1

2

3

socket()

bind()

recvfrom()

sendto()

Server Process


process request

Winsock

or Unix

version


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TCP vs. UDP

TCP (Transmission Control Protocol)

Connection-oriented

Reliability in delivery of messages Splitting messages into datagrams

keep track of order (or sequence)

Use checksums for detecting errors


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TCP vs. UDP (Contd)

UDP (User Datagram Protocols)

Connectionless

No attempt to fragment messages No reassembly and synchronization

In case of error, message is retransmitted

No acknowledgment


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Datagrams

Also known as connectionlessor transmit andprayprotocols.

Simple, but unreliable. They are not trackedby sequence number or acknowledged.

LAN Server and some others haveacknowledged datagrams.


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Datagrams

A datagram, often called apacket, is much moreatomic in nature.

A datagramis an independent, self-contained

message sent over the network whose arrival,arrival time, and content are not guaranteed.

All data sent over the channel is received in thesame order in which it was sent. This is

guaranteed by the channel.

In modern data networking, it is important todistinguish between datagramsand streams.


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Selecting UDP

Remote procedures are idempotent*

Server and client messsages fit

completely within a packet. The server handles multiple clients (UDP

is stateless)

*a mathematical operation that always produces the sameresult


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Selecting TCP

Procedures are not idempotent

Reliability is a must

Messages exceed UDP packet size


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IP (Raw) Socket

To use RAW sockets in Unix it is mandatorythat one have root authority. To create aRAW socket write:s=socket(AF_INET,SOCK_RAW,[protocol])

Then you can sending or receive over it.Raw sockets are used to generate / receive

packets of a type that the kernel doesn'texplicitly support.


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IP Socket example

A familiar example is PING. Ping works bysending out an ICMP (internet controlmessage protocol - another IP protocol

distinct from TCP or UDP) echo packet.The kernel has built-in code to respond toecho/ping packets. It doesn't have code togenerate these packets, because it isn't

required. The "ping packet generator" is aprogram in user space. It formats an ICMPecho packet and sends it out over aSOCK_RAW, waiting for a response.


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OSI Layers vs. TCP/IP

Network

Hardware

Interface

IP

TCP UDP

User

Application5-7. Session

4. Transport

3. Network

1-2. Data Link/

Physical


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Four Types of Servers

Iterative

Connectionless

Iterative

Connection-

Oriented

Concurrent

Connectionless

Concurrent

Connection-

Oriented


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NJIT

Summary

Algorithms for TCP and UDP

Clients and Servers


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TCP Client AlgorithmComer and Stevens, Algorithm 6.1

Find IP address and protocol port number onserver

Allocate a socket Allow TCP to allocate an arbitrary local port Connect the socket to the server Send requests and receive replies Close the connection


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TCP Iterative Server AlgorithmComer and Stevens, Algorithm 8.1

Create a socket and bind to the well knownaddress for the service offered

Place socket in passive mode Accept next connection request and obtain a

new socket Repeatedly receive requests and send replies When client is done, close the connection and

return to waiting for connection requests


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TCP Concurrent Server AlgorithmComer and Stevens, Algorithm 8.4

Master: Create a socket and bind to the well known address

for the service offered. Leave socket unconnected

Place socket in passive mode Repeatedly call acceptto get requests and create a

new slave thread

Slave:

Receive connection request and socket Receive requests and send responses to client

Close connection and exit

UDP Cli Al i h


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UDP Client AlgorithmComer and Stevens, Algorithm 6.2

Find IP address and protocol port number onserver

Allocate a socket Allow UDP to allocate an arbitrary local port Specify the server Send requests and receive replies Close the socket


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UDP Iterative Server AlgorithmComer and Stevens, Algorithm 8.2

Create a socket and bind to the well knownaddress for the service offered

Repeatedly receive requests and send replies


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UDP Concurrent Server AlgorithmComer and Stevens, Algorithm 8.3

Master: Create a socket and bind to the well known address

for the service offered. Leave socket unconnected

Repeatedly call recvfromto get requests and create anew slave thread

Slave: Receive request and access to socket

Form reply and send to client with sendto Exit


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References

Robert Orfali, Dan Harkey, Jeri Edwards, ClientServer Survival Guide, Third Edition, Wiley, 1999.

Douglas E. Comer and David L. Stevens,

Internetworking With TCP/IP, Volume III, PrenticeHall, multiple editions and dates.

George Coularis, Jean Dollimore and Tim Kindberg,Distributed Systems, Concepts and Design, Addison

Wesley, Fourth Edition, 2005 Figures from the Coulouris text are from the

instructors guide and are copyrighted by PearsonEducation 2005

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