Chapter 3 Network Programming and Applications

Chapter 3

Network Programming and

Applications

Topics Covered• Client-Server Computing • Communication Paradigm • An Example Application Program Interface • Definition Of The API • An Echo Application • A Chat Application • A Web Application• Managing Multiple Connections With The Select

Function

Network Communication

• Applications us a network in pairs to exchange messages

• Example– A distributed database service that allows remote

users to access a central database• One application runs on a remote computer to

send request• The other application on the computer that has

the databases to send a response • Only the two applications understand the

message format and meaning

Client-Server Computing (1)

• One application starts first and waits for the other application to contact it

• The second application must know the location where the first application is waiting

• This is known as client-server computing • The program that waits for contact is called a

server• The program that initiates contact is known as a

client who must know where the server is running, and must specify the location

Client-Server Computing (2)

How does a client specify the location of a server?• In the Internet, a location is given by a pair of

identifiers– (computer, application)– computer identifies the computer on which the server

is running– application identifies a particular application program

on that computer• Humans enter the names (i.e. echo), and

software translates each name to a corresponding binary value automatically because application software use binary value

Communication Paradigm

• Two applications – establish communication, – exchange messages back and forth, – and then terminate

• The steps (in details) are:– The server starts first, and waits for contact from a client– The client specifies the server's location and requests a

connection be established– Once a connection is in place, the client and server use the

connection to exchange messages– After they finish sending data, the client and server each send

an end-of-file (EOF) and the connection is terminated

An Example Application Program Interface

• Application Program Interface (API) is used to describe the set of operations available to a programmer

• Seven functions that an application can call

An Intuitive Look At The API• A server begins by calling await_contact to wait for contact from a client• The client begins by calling make_contact to establish contact• Once the client has contacted the server, the two can exchange messages with

send and recv• Order of send or receive must be known by server and client, if both sides try to

receive without sending, they will block forever• An application calls send_eof to terminate the communication• On the other side, recv returns a value of zero to indicate the termination

Definition Of The API

• Define 3 data types to keep our API independent of particular OS and NW software

Functions• Await_Contact Function: connection await_contact(appnum a)

– A server calls function await_contact to wait for contact from a client – One argument specifies a number that identifies the server application

and a client must specify the same number when contacting the server. – The server uses the return value (type connection ) to transfer data

• Make_Contact Function: connection make_contact (computer c, appnum a) – client calls function make_contact to establish contact with a server– Two arguments to identify a computer of the server application number– The client uses the return value which is of type connection to transfer

data

Functions

• appnum appname_to_appnum(char *a)– Clients and servers both use it to translate from a

human-readable service name for a service to an internal binary value

– The call takes one argument and returns an equivalent binary value of type appnum

• computer cname_to_comp (char *c)– Clients call it to convert from a human-readable

computer name to the internal binary value– The call takes one argument and returns an

equivalent binary value of type computer

Functions• int send (connection con, char *buffer, int length, int flags)

– Both clients and servers use send to transfer data across the network– Four arguments specifies a connection previously established with await_contact or

make_contact; the address of a buffer containing data to send; the length of the data in bytes (octets); the fourth argument is zero for normal transfer

– Send returns the number of bytes transferred, or a negative value if an error occurred• int recv (connection con, char *buffer, int length, int flags)

– Both clients and servers use recv to access data that arrives– Four arguments specifies: a connection with await_contact or make_contact; the

address of a buffer into which the data to be placed; the size of the buffer in bytes; and the fourth is zero for normal transfer

– recv returns the number of bytes that were placed in the buffer– zero to indicate that EOF has been reached or a negative value to indicate that an

error occurred• int recvln (connection con, char *buffer, int length)

– function recvln that repeatedly calls recv until an entire line of text has been received.

Functions

• Both the client and server must use send_eof after sending data – to inform the other side that no further transmission will occur

• On the other side, the recv function returns zero when it receives the EOF

int send_eof(connection con)• Argument specifies a connection previously established

with await_contact or make_contact• The function returns a negative value

– to indicate that an error occurred, and a non-negative value otherwise

Code For An Echo Application (1)

• The client repeatedly – prompts the user for a line of input, – sends the line to the server, – and then displays whatever the server sends back.

• Like all the applications described in this chapter, – the echo application operates across a NW– That is, the client and server programs can run on

separate computers

Code For An Echo Application (2)

• Ex: suppose someone using computer lancelot.cs.purdue.edu chooses 20000 as the application number

• The server is invoked by the command:echoserver 20000

• If some other application is using number 20000, – the server emits an appropriate error message and exits– the user must choose another number.

• Once the server has been invoked, the client is invoked:echoclient lancelot.cs.purdue.edu 20000

Code For A Chat Application

• A simplified version of chat that works between a single pair of users

• One user begins by choosing an application number and running the server

• Ex: suppose a user on excalibur.cs.purdue.edu runs the server:

chatserver 25000• A user on another computer can invoke the client:

chatclient excalibur.cs.purdue.edu 25000• To keep the code as small as possible

– the scheme requires users to take turns entering text– Users alternate entering text until one of them sends an EOF

Code for Web Application

Browser Interface

• The World Wide Web (WWW) is the large-scale, online repository of information

• A browser is an interactive program used to access the Web

• Web is a “distributed” “hypermedia” system that supports interactive access because a browser can display both text and graphics.

Document Representation

• Web uses a standard representation known as the HyperText Markup Language (HTML)

• HTML is classified as a “markup language”– because it only gives general guidelines for display– but does not include detailed formatting instructions

• Markup language is important because it allows a browser to adapt the page to the underlying display: Thus, the same page can be formatted for – a large or small computer monitor– a high resolution or low resolution display– or a handheld device such as a PDA

HTML Format And Representation

• Each HTML document is divided into – a head that contains

information about the document

– a body that contains the information to be displayed

• HTML tags provide structure for the document as well as formatting

Other Markup Languages

Other markup languages have been created for special purposes.

• Voice Extensible Markup Language (VoiceXML) or VXML – specifies communication in human speech

• Extensible Markup Language (XML) – Unlike HTML, XML does not specify layout– Instead, XML gives an internal representation that

provides names for each field in a data item

Web and HTTP

First some jargon• Web page consists of objects• Object can be HTML file, JPEG image, Java

applet, audio file,…• Web page consists of base HTML-file which

includes several referenced objects• Each object is addressable by a URL• Example URL:

www.someschool.edu/someDept/pic.gif

host name path name

HTTP overview

HTTP: hypertext transfer protocol

• Web’s application layer protocol

• client/server model– client: browser that

requests, receives, “displays” Web objects

– server: Web server sends objects in response to requests

• HTTP 1.0: RFC 1945• HTTP 1.1: RFC 2068

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

HTTP overview (continued)

Uses TCP:• client initiates TCP

connection (creates socket) to server, port 80

• server accepts TCP connection from client

• HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)

• TCP connection closed

HTTP is “stateless”• server maintains no

information about past client requests

Protocols that maintain “state” are complex!

• past history (state) must be maintained

• if server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside

HTTP connections

Nonpersistent HTTP• At most one object is

sent over a TCP connection.

• HTTP/1.0 uses nonpersistent HTTP

Persistent HTTP• Multiple objects can

be sent over single TCP connection between client and server.

• HTTP/1.1 uses persistent connections in default mode

Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index

1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket

time

(contains text, references to 10

jpeg images)

Nonpersistent HTTP (cont.)

5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects

6. Steps 1-5 repeated for each of 10 jpeg objects

4. HTTP server closes TCP connection.

time

Response time modeling

Definition of RRT: time to send a small packet to travel from client to server and back.

Response time:• one RTT to initiate TCP

connection• one RTT for HTTP

request and first few bytes of HTTP response to return

• file transmission timetotal = 2RTT+transmit time

time to transmit file

initiate TCPconnection

RTT

requestfile

RTT

filereceived

time time

Persistent HTTP

Nonpersistent HTTP issues:• requires 2 RTTs per object• OS must work and allocate

host resources for each TCP connection

• but browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP• server leaves connection

open after sending response• subsequent HTTP messages

between same client/server are sent over connection

Persistent without pipelining:• client issues new request

only when previous response has been received

• one RTT for each referenced object

Persistent with pipelining:• default in HTTP/1.1• client sends requests as

soon as it encounters a referenced object

• as little as one RTT for all the referenced objects

HTTP request message

• two types of HTTP messages: request, response• HTTP request message:

– ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

HTTP request message: general format

Uploading form input

Post method:• Web page often

includes form input• Input is uploaded to

server in entity body

URL method:• Uses GET method• Input is uploaded in

URL field of request line:

www.somesite.com/animalsearch?monkeys&banana

Method types

HTTP/1.0• GET requests a specified

item from the server• POST sends data to the

server– The server appends the

data to a specified item

• HEAD requests status information about an item– The server returns the

status without returning a copy of the item itself

HTTP/1.1• GET, POST, HEAD• PUT sends data to the

server– The server uses the data to

replace a specified item .– It is seldom used; most

Web pages that send data use POST instead

• DELETE– deletes file specified in the

URL field

HTTP response message

HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ...

status line(protocol

status codestatus phrase)

header lines

data, e.g., requestedHTML file

HTTP response status codes

200 OK– request succeeded, requested object later in this message

301 Moved Permanently– requested object moved, new location specified later in

this message (Location:)

400 Bad Request– request message not understood by server

404 Not Found– requested document not found on this server

505 HTTP Version Not Supported

In first line in server->client response message.

A few sample codes:

Trying out HTTP (client side) for yourself

1. Telnet to your favorite Web server:

Opens TCP connection to port 80(default HTTP server port) at cis.poly.edu.Anything typed in sent to port 80 at cis.poly.edu

telnet cis.poly.edu 80

2. Type in a GET HTTP request:

GET /~ross/ HTTP/1.1Host: cis.poly.edu

By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server

3. Look at response message sent by HTTP server!

35.18 Alternative Transfer Protocols

• Several extensions and alternatives have been proposed to enhance or replace HTTP

• HTTPS – protocol provides for secure HTTP transfers

• (i.e., it uses encryption to guarantee confidentiality of the data being transferred in an HTTP session)

• HTTP version 1.1 introduces the concept of persistent connections– in which a single TCP connection is used for multiple transfers

• (i.e., multiple GET requests from the browser).

• HTTP-NG (Next-Generation of HTTP)– that will use binary encoding for headers instead of the current

ASCII representation

User-server state: cookies

Many major Web sites use cookies

Four components:1) cookie header line in the

HTTP response message

2) cookie header line in HTTP request message

3) cookie file kept on user’s host and managed by user’s browser

4) back-end database at Web site

Example:– Susan access Internet

always from same PC– She visits a specific e-

commerce site for first time

– When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID

Cookies: keeping “state” (cont.)

client server

usual http request msgusual http response

+Set-cookie: 1678

usual http request msg

cookie: 1678usual http response

msg

usual http request msg

cookie: 1678usual http response msg

cookie-specificaction

cookie-spectificaction

servercreates ID

1678 for user

entry in backend

database

access

acce

ss

Cookie file

amazon: 1678ebay: 8734

Cookie file

ebay: 8734

Cookie file

amazon: 1678ebay: 8734

one week later:

Cookies (continued)

What cookies can bring:

• authorization• shopping carts• recommendations• user session state

(Web e-mail)

Cookies and privacy:• cookies permit sites to

learn a lot about you• you may supply name

and e-mail to sites• search engines use

redirection & cookies to learn yet more

• advertising companies obtain info across sites

aside

Web caches (proxy server)

• user sets browser: Web accesses via cache

• browser sends all HTTP requests to cache– object in cache: cache

returns object – else cache requests

object from origin server, then returns object to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server

More about Web caching

• Cache acts as both client and server

• Typically cache is installed by ISP (university, company, residential ISP)

Why Web caching?• Reduce response time for

client request.• Reduce traffic on an

institution’s access link.• Internet dense with

caches enables “poor” content providers to effectively deliver content (but so does P2P file sharing)

Caching example Assumptions• average object size = 100,000

bits• avg. request rate from

institution’s browsers to origin servers = 15/sec

• delay from institutional router to any origin server and back to router = 2 sec

Consequences• utilization on LAN = 15%• utilization on access link = 100%• total delay = Internet delay +

access delay + LAN delay = 2 sec + minutes + milliseconds

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Caching example (cont)Possible solution• increase bandwidth of

access link to, say, 10 Mbps

Consequences• utilization on LAN = 15%• utilization on access link = 15%• Total delay = Internet delay +

access delay + LAN delay

= 2 sec + msecs + msecs• often a costly upgrade

originservers

public Internet


10 Mbps access link

institutionalcache

Caching example (cont)

Install cache• suppose hit rate is .4

Consequence• 40% requests will be satisfied

almost immediately• 60% requests satisfied by origin

server• utilization of access link reduced

to 60%, resulting in negligible delays (say 10 msec)

• total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + milliseconds < 1.4 secs

originservers

public Internet


1.5 Mbps access link

institutionalcache

Conditional GET

• Goal: don’t send object if cache has up-to-date cached version

• cache: specify date of cached copy in HTTP requestIf-modified-since: <date>

• server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified

cache server

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0

304 Not Modified

object not

modified

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0 200 OK

<data>

object modified

Code For A Web Application (1)

• To run the server, a user chooses an application number and invokes the server

• Ex: if a user on computer netbook.cs.purdue.edu chooses application number 27000, the server can be invoked with the command:

webserver 27000• The client specifies a computer, a path name, and an

application number:webclient netbook.cs.purdue.edu /index.html

27000• It is possible to use a conventional Web browser (such

as, Internet Explorer or Netscape) to access the serverhttp://netbook.cs.purdue.edu:27000/index.html


• The client code is extremely simple:– after establishing communication with the Web server,

it sends a request, which must have the form :

GET / path HTTP/1.0 CRLF CRLF CRLF GET

– where path denotes the name of an item such as index.html ,

– CRLF denotes the two characters carriage return and line feed.

• After sending the request, the client receives and prints output from the server

Code For A Web Application (3)• Web server may seem more complex than previous examples,

– complexity results from Web details rather than networking details• In addition to reading and parsing a request

– the server must send both a ``header'' and data in the response– The header consists of several lines of text that are terminated by CRLF

• The header lines are of the form: HTTP/1.0 status status_string CRLF Server: CNAI Demo Server CRLF Content-Length: datasize CRLF Content-Type: text/html CRLF CRLF

• where datasize denotes the size of the data that follows


• The code is also complicated by error handling – error messages must be sent in a form that a browser can

understand

• If a request is incorrectly formed, our server generates a 400 error message

• If the item specified in the request cannot be found404 error message

• The Web server differs from the previous examples in a significant way:– the server program does not exit after satisfying one request– Instead, it remains running, ready for additional requests

Managing Multiple Connections With The Select Function (1)

• Although our example API supports 1-to-1 interaction between a client and server, – the API does not support 1-to-many interaction

• To see why, consider multiple connections– To create such connections

• a single application must call make_contact multiple times • specifying a computer and appnum for each call

• Once the connections have been established– the application cannot know which of them will receive

a message first


• Many OS include a function named select that solves the problem of managing multiple connections– The select call checks a set of connections– The call blocks until at least one of the

specified connections has received data– The call then returns a value that tells which

of the connections have received data


• Ex: consider an application that must receive requests and send responses over two connections– Such an application can have the following general form:

Call make_contact to form connection1;Call make_contact to form connection2;

Repeat forever {Call select to determine which connection is readyIf (connection1 is ready) {

Call recv to read request from connection1;Compute response to request;Call send to send response over connection1;

} if (connection2 is ready) {Call recv to read request from connection2;Compute response to request;Call send to send response over connection2; } }

Documents

Chapter 3 Network Programming and Applications