CSC383 Lecture notes Ch 1 2 3

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CSC383 Computer Networks

Dr. Allen M. Johnson, Jr.

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History…

 11stst networks designed to share large-scale networks designed to share large-scale

computational power

 11stst commercially available network system commercially available network system

was IBM’s PROFS system. (mainframes)

 Late 1960s, ARPA started investigating data Late 1960s, ARPA started investigating data

networking.

– _{Idea of networking computers of different}_{Idea of networking computers of different}

architectures

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…

History…

 In the late 1970s inexpensive In the late 1970s inexpensive

minicomputers provided time-sharing

facilities to people within a department.

facilities to people within a department.  Organizations installed Local Area Organizations installed Local Area

Networks (LANs) to interconnect these

machines together.

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…

History…

 ARPA chose to investigate a revolutionary ARPA chose to investigate a revolutionary

approach, known as

approach, known as packet switchingpacket switching..

 They hired people to work on network They hired people to work on network

research and contractors to turn the designs

into a working system called the

into a working system called the ARPANETARPANET..

 This was so successful that ARPA continued This was so successful that ARPA continued

funding research into a networking

technology called

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…

History

 By the 1970s, internetworking was the By the 1970s, internetworking was the

focus of ARPA research and the

focus of ARPA research and the InternetInternet

had emerged.

 ARPA continued funding research into the ARPA continued funding research into the

1980s,

 Internet became a commercial success in Internet became a commercial success in

the 1990s.

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Growth of Internet

 The Internet has grown from an early research The Internet has grown from an early research

prototype into a global communications system

that connects every country.

 By 1999, the Internet was growing so fast that a By 1999, the Internet was growing so fast that a

new computer was added to the Internet every

second.

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…

Growth of Internet

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…

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Protocols and Layering

 Computer networks are complex systems Computer networks are complex systems

including both hardware and software.

including both hardware and software.  Designers divided the communication Designers divided the communication

problem into subparts, called

problem into subparts, called layerslayers, with the , with the interfaces between the layers defined by

interfaces between the layers defined by

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TCP/IP Layered Model

 physical layer corresponds to basic network hardware.physical layer corresponds to basic network hardware.

 Network interface layer specifies how data is divided into Network interface layer specifies how data is divided into frames for transmission over a network.

frames for transmission over a network.

 Internet layer protocols specify the format of packets sent Internet layer protocols specify the format of packets sent

across an internet as well as the mechanisms used to forward across an internet as well as the mechanisms used to forward

packets from a computer through one or more routers to a packets from a computer through one or more routers to a

final destination. final destination.

 Transport layer specifies how to provide reliable transfer and Transport layer specifies how to provide reliable transfer and the

the Transmission Control ProtocolTransmission Control Protocol (TCP) software also (TCP) software also

provides additional facilities required by higher-level provides additional facilities required by higher-level

applications. applications.

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Probing the Internet

 Simplest probing tool is the Simplest probing tool is the Packet InterNet Packet InterNet

Groper

Groper, or , or ping_ping. .

 According to the Internet requirements According to the Internet requirements

document

document [RFC 1122] [RFC 1122] , every TCP/IP stack , every TCP/IP stack should implement such a program.

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PING Command Example

penguin(101)% ping -c5 www.netbook.cs.purdue.edu PING lucan.cs.purdue.edu (128.10.19.20) from 193.61.29.127: 56(84) bytes of data 64 bytes from lucan.cs.purdue.edu (128.10.19.20): icmp_seq=1 ttl=235 time=131 ms 64 bytes from

lucan.cs.purdue.edu (128.10.19.20): icmp_seq=2 ttl=235 time=131 ms 64 bytes from lucan.cs.purdue.edu (128.10.19.20): icmp_seq=3 ttl=235 time=131 ms 64 bytes from lucan.cs.purdue.edu

(128.10.19.20): icmp_seq=4 ttl=235 time=131 ms 64 bytes from lucan.cs.purdue.edu (128.10.19.20): icmp_seq=5 ttl=235 time=131 ms --- lucan.cs.purdue.edu ping statistics --- 5 packets transmitted, 5 received, 0% loss, time 4008ms rtt min/avg/max/mdev =

131.394/131.742/131.978/0.317 ms penguin(102)%

• -c5 instructs ping to stop after sending five probes

• summary on the first line indicates an alternative name for the target is

lucan.cs.purdue.edu, that its IP address is 128.10.19.20, and it’s sending 56 bytes of data in an 84-byte IP datagram.

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What Good Is PING

 ConsCons

– _{Little info for average user}_{Little info for average user} – _{Not good as debug tool}_{Not good as debug tool}

» Output occurs only when computer responds_{Output occurs only when computer responds} » _{Fails if network is congested}_{Fails if network is congested}

» PING cannot determine cause of problem_{PING cannot determine cause of problem}

» _{Some companies configure systems to reject pings}_{Some companies configure systems to reject pings}

 ProsPros

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Traceroute

 Determine intermediate computers along Determine intermediate computers along

the way

 Uses same arguments as PING (remote Uses same arguments as PING (remote

computers name or address)

 Shows how the hops between source and Shows how the hops between source and

destination.

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[image:16.720.73.640.206.451.2]

Figure 2.6

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Chapter 3 Introduction

 Use of a network requires programs (and Use of a network requires programs (and systems) that are “network-aware.”

systems) that are “network-aware.”

 Some applications are usually provided with Some applications are usually provided with machines (e.g. web clients, e-mail clients, file

machines (e.g. web clients, e-mail clients, file

sharing).

 New custom applications must use the interface New custom applications must use the interface (API) to the network facilities provided by the

(API) to the network facilities provided by the

system.

 We will look at a simple API and three sample We will look at a simple API and three sample applications (both the client and the server).

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Network Communication

 Networks are the mechanisms that transmit data Networks are the mechanisms that transmit data

from one point to another.

 Logically, networks are (or should be) passive, Logically, networks are (or should be) passive,

and do not understand, act on, or modify the data

being moved.

 There are two ends of a network communication, There are two ends of a network communication,

each associated with an application program that

understands the generation and interpretation of

the data being moved across the network.

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Client-Server Computing

 The two applications that communicate using a The two applications that communicate using a network must locate each other.

network must locate each other.

 Typically, one application (the server) starts and Typically, one application (the server) starts and then waits for the other application (the client) to

then waits for the other application (the client) to

contact it.

 This arrangement is usually called the This arrangement is usually called the client- client-server paradigm

server paradigm, or just , or just client-server computingclient-server computing..  The client must, of course, be able to locate the The client must, of course, be able to locate the

server for this arrangement to work.

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Addressing Internet Applications

 Each network server application has a two-part address of the Each network server application has a two-part address of the form (

form (computercomputer,,applicationapplication).).

 The The computercomputer part of the address identifies the particular part of the address identifies the particular computer on which the application is running. This can be

computer on which the application is running. This can be

provided in symbolic (e.g. apollo.unomaha.edu) or numeric

(e.g. 137.48.1.12) form.

 The application part of the address identifies the particular The application part of the address identifies the particular application. It, too, can be provided symbolically or

application. It, too, can be provided symbolically or

numerically.

 When transmitted on a network, the (_{When transmitted on a network, the (}computer_computer,_,application_application) ₎ address is always sent in binary.

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Communication Paradigm

 Most Internet applications follow this basic Most Internet applications follow this basic

sequence of operations for communication: sequence of operations for communication:

– _{The server starts first, and waits for contact from a}_{The server starts first, and waits for contact from a}

client.

– _{The client contacts the server by specifying its location}_{The client contacts the server by specifying its location}

and requesting communication.

– _{The client and the server exchange messages.}_{The client and the server exchange messages.}

– _{After they finish, the client and server each send an}_{After they finish, the client and server each send an}

end-of-file

end-of-file to terminate communication. to terminate communication.

 Functions are provided in the class library on Functions are provided in the class library on

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An Example Application Program Interface

 The term “application program interface,” or The term “application program interface,” or API, is used to describe the set of operations

API, is used to describe the set of operations

available (in a particular programming

domain) to a programmer.

 The simplified API presented in the textbook The simplified API presented in the textbook hides much of the complexity in network

hides much of the complexity in network

programming, but also doesn’t provide the

richness of the usual networking API. We will

see more details in later chapters.

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Example API Overview

Operation Meaning

await_contact used by server to wait for a contact make_contact used by client to contact a server cname_to_comp translate a computer name to an

equivalent internal binary value appname_to_appnum translate a program name to an

equivalent internal binary value

send used by client or server to send data recv used by client or server to receive data send_eof used by client or server after all data

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An Intuitive Look at the API

 The application actions usually follow this pattern:The application actions usually follow this pattern:

– _{The server calls}_{The server calls}_{await_contact}_{await_contact}_{to wait for contact from a}_{to wait for contact from a}

client. The client calls

client. The client calls make_contactmake_contact to establish the to establish the connection. [The server is said to be

connection. [The server is said to be blockedblocked until the until the contact is made.]

contact is made.]

– Once contact is made, the applications use _{Once contact is made, the applications use}sendsend and and recvrecv to exchange data. [The

to exchange data. [The protocolprotocol dictates when each dictates when each application sends or receives.]

application sends or receives.]

– Once the data exchange is complete, the client or the _{Once the data exchange is complete, the client or the}

server terminates the connection by calling

server terminates the connection by calling send_eofsend_eof. . The other application detects the termination of the

The other application detects the termination of the

communication when

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A Trivial Example

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API Definition – Data Types

 There are three data types (in addition to the usual There are three data types (in addition to the usual

integer, character, and real data types) used in the

API:

–_appnum_appnum: a binary value used to identify an : a binary value used to identify an application [this is really a

application [this is really a port numberport number].].

–_computer_computer: a binary value used to identify a : a binary value used to identify a computer [this is really an

computer [this is really an Internet Protocol addressInternet Protocol address, , or

or IP addressIP address.].]

–_connection_connection: a value used to identify one : a value used to identify one

endpoint of a connection between a client and a server endpoint of a connection between a client and a server [normally this identifies a

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API Definition –

await_contact

 A server calls A server calls _{await_contact}_{await_contact} to wait to wait

for a contact from a client. The function

expects one argument of type

expects one argument of type _appnum_appnum, and , and returns a

returns a _connection_connection which is then used which is then used in

in _send_send, _,_recv_recv, and _{, and}_{send_eof}_{send_eof} functions._functions.

connection await_contact connection await_contact

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API Definition –

make_contact

 A client calls A client calls make_contact_{make_contact} to establish to establish contact with a server. The arguments identify

contact with a server. The arguments identify

the

the _computer_computer on which the server is running, on which the server is running, and the application number to be contacted.

and the application number to be contacted.

The function returns a connection used for

send

send, , _recv_recv, and , and _{send_eof}_{send_eof} by the client. by the client.

connection make_contact (computer

connection make_contact (computer

c, appnum a)

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API Definition –

appname_to_appnum

 This function is used to translate from a This function is used to translate from a

predefined human-readable name [character

string] to a binary application number. Standard

application names are defined in the /etc/services

file on a UNIX system. It takes a string argument

and returns the binary application number.

appnum appname_to_appnum (char appnum appname_to_appnum (char

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API Definition –

cname_to_comp

 This function translates a symbolic This function translates a symbolic

computer name (

computer name (_cname_cname) to the equivalent ) to the equivalent binary form (which is returned) of type

binary form (which is returned) of type computer

computer..

computer cname_to_comp (char computer cname_to_comp (char

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API Definition –

send

 Send_Send arranges to transmit data (a sequence of arranges to transmit data (a sequence of

n

n bytes in a char array bytes in a char array _buf_buf) over an established ) over an established connection

connection _c_c, returning the number of bytes , returning the number of bytes actually transmitted. The last argument is

actually transmitted. The last argument is

always

always ₀₀ in this simplified API. in this simplified API.

int send (connection c, char int send (connection c, char

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API Definition –

recv

 _Recv_Recv receives at most receives at most _n_n bytes of data from a bytes of data from a

connection

connection _c_c, placing it in a char array , placing it in a char array _buf_buf, and , and returning the number of bytes received and stored. returning the number of bytes received and stored.

Recv

Recv will return 0 only when the end of file is will return 0 only when the end of file is reached, and a negative value when an error

reached, and a negative value when an error occurred (e.g.

occurred (e.g. _recv_recv without a valid connection). without a valid connection).

int recv (connection c, char buf, int recv (connection c, char buf,

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API Definition –

recvln

 _Recvln_Recvln reads one line of data (terminated by an reads one line of data (terminated by an end of line character) from a connection

end of line character) from a connection _c_c into a into a char array

char array _buf_buf; no more than ; no more than _n_n-1 bytes will be -1 bytes will be read (allowing one byte for string termination).

read (allowing one byte for string termination).

Recvln

Recvln uses uses _recv_recv to read one byte at a time. to read one byte at a time.

int recvln (connection c, char

*buf,

int n)

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API Definition –

send_eof

 _{Send_eof}_{Send_eof} terminates the connection terminates the connection _c_c, ,

returning a negative number if an error

occurred.

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The ECHO Application

 The ECHO application server merely resends each line it The ECHO application server merely resends each line it

receives from a client. The server is started (from a

command line) by typing the program name and a selected

application number (usually in the range 1025 to 32767).

 The client application is started by typing its name, the The client application is started by typing its name, the

name of the computer on which the server is running, and

the same application number.

 Lines are then read by the client, echoed by the server, and Lines are then read by the client, echoed by the server, and

displayed by the client, until an end of file (control-D on

UNIX, or control-Z on Windows) is entered.

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An Important Observation

 If you examine the code for the echo client, you If you examine the code for the echo client, you

will find that it does not immediately read

another line from the keyboard after reading data

from the connection. This is because recv only

returns as much data as it has actually read.

 The reason for this is that the underlying network The reason for this is that the underlying network

may send a group of data bytes in several

different packets. Thus the received data may

arrive in pieces with sizes different from those

used when the data was sent.

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The CHAT Application

 The chat application is similar to the echo The chat application is similar to the echo

application. Here, however, the server displays

the received data line and waits for a response

from the user on that machine, which is then

sent back to the client for display.

 Either the user at the server machine or the user Either the user at the server machine or the user

at the client machine may terminate the chat

session by entering an end of file.

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The WEB Application

 Our final example is a simple text-mode web server _{Our final example is a simple text-mode web server} and browser. Only two web pages are provided by and browser. Only two web pages are provided by the server: “/” and “/time”. It is easy, however, to the server: “/” and “/time”. It is easy, however, to extend this application to deliver arbitrary pages extend this application to deliver arbitrary pages stored as files on the server machine.

stored as files on the server machine.

 The server and client both use the standard HTTP The server and client both use the standard HTTP protocol, and the server delivers HTML documents protocol, and the server delivers HTML documents with appropriate headers. Thus a commercial web with appropriate headers. Thus a commercial web browser could be used to contact our server.

CSC383 Lecture notes Ch 1 2 3

CSC383 Computer Networks

History…

History…

History

Growth of Internet

Growth of Internet

TCP/IP Layered Model

Probing the Internet

What Good Is PING

Traceroute

Chapter 3 Introduction

Network Communication

Client-Server Computing

Addressing Internet Applications

Communication Paradigm

An Example Application Program Interface

Example API Overview

An Intuitive Look at the API

A Trivial Example

API Definition –

connection await_contact connection await_contact

API Definition –

connection make_contact (computer

API Definition –

appnum appname_to_appnum (char appnum appname_to_appnum (char

API Definition –

computer cname_to_comp (char computer cname_to_comp (char

API Definition –

int send (connection c, char int send (connection c, char

API Definition –

int recv (connection c, char *buf, int recv (connection c, char *buf,

API Definition –

Recvln

API Definition –

An Important Observation

The CHAT Application

The WEB Application

int recv (connection c, char buf, int recv (connection c, char buf,