server client01

8/12/2019 Server Client01

1/23

HTTP Client & Server Interaction

Bob Dickerson

Dept. of Computer Science, University of Hertfordshire

HTTP Client & Server Interaction p.1/25


2/23

What happens when a browser contacts a server

this example shows the statements executed in a browserand a server, and also what they do,

this example assumes that the server is running on a host

with address3.3.3.3, the server program is using port8888,

the file request istiny.html

therefore the request URL is:

http://3.3.3.3:8888/tiny.html

but this is, of course, not sent in this form. . .



3/23

Initial request

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

SERVER

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

f=open(name..);write(s,hdr...);

hdr

tiny.html

This..Ex.

parag..

Request is entered into browser(but server hasnt started)

server-client01a



4/23

Browser prepares request

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

SERVER

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){


hdr

tiny.html

This..Ex.

parag..

Browser prepares to send request to HTTP server,(but server still hasnt started)

server-client01b



5/23

Connection fails

connect port 8888s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

SERVER

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

ERROR no such port


hdr

tiny.html

This..Ex.

parag..

Client cannot make a connection,server isnt listening.

server-client02a



6/23

Browser tells user of failure

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

SERVER

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

f=open(name..);

write(s,hdr...);

hdr

tiny.html

This..

Ex.

parag..

Browser announces failure to connect

server-client02b



7/23

Server binds to 8888

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listeningports:8888>

SERVER

server


hdr

tiny.html

This..Ex.

parag..

Server tells its operating system thatit will accept connections to port 8888

server-client03



8/23

Server executesacceptand waits

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


hdr

tiny.html

This..Ex.

parag..

Server executes accept and awaits a connection from a client

server-client04



9/23

Browser does request again

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


hdr

tiny.html

This..Ex.

parag..

Browser requests page again

server-client05a



10/23

Browsers operating systems connects

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

serverconnect port 8888


hdr

tiny.html

This..Ex.

parag..

Browser (the client) asks its operating system to connect toport 8888 on server 3.3.3.3

server-client05b



11/23

Servers operating system accepts

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server

OK connected


hdr

tiny.html

This..Ex.

parag..

The server programs operating system replies OK

because port 8888 exists and a program is listening

server-client06



12/23

Connected socket is created

Connectedsockets:server s>

browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server

Connectedsockets:browser s>

server s


hdr

tiny.html

This..Ex.

parag..

Both operating systems create a connected socket and

return descriptors to their programs (the browser and the server)

server-client07



13/23

Browser builds request


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s

GET /tiny.html HTT

P/1.1\r\nHost: 3...


hdr

tiny.html

This..Ex.

parag..

The browser constructs a request string:GET /tiny.html HTTP/1.1\r\n....

server-client08



14/23

Browser sends request, server reads it


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s

write(s,hdr...);f=open(name..);

hdr

tiny.html

GET /tiny.html HTT

GET /tiny.html HTT

P/1.1\r\nHost: 3...

P/1.1\r\nHost: 3...

GET /tiny.html HTTP/1.1\r\nHost: 3...

This..Ex.

parag..

The browserwrites the request to the socket andit is sent to the HTTP server where it isreadintoreq

server-client09



15/23

Example of a full browser request


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s

write(s,hdr...);f=open(name..);

hdr

tiny.html

GET /tiny.html HTT

GET /tiny.html HTT

P/1.1\r\nHost: 3...

P/1.1\r\nHost: 3...


This..Ex.

parag..

The only really essential bits are: the first request line, and the blank line at theend. Most of the other lines are optional and vary between browsers.

server-client09b



16/23

Server analyses and checks request


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s


hdr

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...


This..Ex.

parag..

The server analyses the request: it checks if the fileexists and the request is legal, it is ok so. . .

server-client10



17/23

Server opens file


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s


Open files:f>tiny.html

hdr

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...


This..Ex.

parag..

The server opens the file

server-client11



18/23

Server sends OK header


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

read(s,resp..);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s



hdr

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...


HTTP/1.1 200 OK\r\n

Server: Apache..

HTTP/1.1 200 OK\r\nServer: Apache..


This..Ex.

parag..

BUT before it can send the file it mustsend the correctHTTP response lines:HTTP/1.1 200 OK\r\nServer:...

server-client12



19/23

Server reads file


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s



read(s,resp..);

read(s,resp+..);

buf

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...



Example Page \r\nThis is the

This..Ex.

parag..

The server now reads the requested file from discinto a buffer (character array) calledbuf

server-client13



20/23

Server writes the file contents to the socket


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s



read(s,resp..);

read(s,resp+..);

buf

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...

GET /tiny.html HTTP/1.1\r\nHost: 3...HTTP/1.1 200 OK\r\nServer: Apache..




This..Ex.

parag..

The server writes the contents ofbufto the socket and it is read by the browser

server-client14



21/23

Example of possible full response


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s



read(s,resp..);

read(s,resp+..);

buf

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...





This..Ex.

parag..

Possible response including required header line, some optional lines, a blank lineand the file.

server-client14b



22/23

How the full response is sent

No server should ever just send a file (HTML page, orimage), it must send a response code and header lines first,

the header lines and file are separated by a blank line,

clearly the server code might have separatewritestatements to the socket for the response lines (constructedas strings in memory) and the file (which is read from disc),

however because this will be faster than the network speedthe client might see it all as one stream of bytes,

whether the browser reads it all in onereador repeated

reads in a loop is a separate issue, it should use theContent-Length: line OR detect the

socket closing, to decide when to stop.

server-client14c



23/23

Browser checks response and shows file


browser s

s=Socket(a,8888);

req="GET.....";

write(s,req...);

req

resp

p=8888;a="3.3.3.3";

BROWSER

req

ss=ServerSocket(p);

s=Accept(ss);

read(s,req...);

// analyse req

read(f,buf..);

write(s,buf..);

while(! eof){

Listening

ports:8888>

SERVER

server


server s



read(s,resp..);read(s,resp+..);

buf

tiny.html

GET /tiny.html HTT

P/1.1\r\nHost: 3...




This..Ex.

parag..

The browser checks the response header and because its OKit displays the contents of the file following the header

server-client15


server client01

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