35-http.pptx

Sockets: send, recv

Network Applications: HTTP

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Client-Server Model

server client

  Server:

  Creates a socket to listen for incoming

connections.

  Must listen on a specific protocol/port.

TCP/80

Client-Server Model

server client

  Client:

  Creates a socket to connect to a remote

computer.

TCP/80

Client-Server Model

server client

  Client:

  Requests a connection to TCP port 80 on

74.125.225.70.

TCP/80

Client-Server Model

server client

  Server:

  Accepts the connection.

TCP/80

Client-Server Model

server client

  Server:

  Spawns a new socket to communicate

directly with the newly connected client.

  Allows other clients to connect.

TCP/80

Two way

communications

Creating a “Server Socket”

socket(): Creates a new socket for a

specific protocol (eg: TCP)

bind(): Binds the socket to a specific

port (eg: 80)

listen(): Moves the socket into a state

of listening for incoming

connections.

accept(): Accepts an incoming

connection.

Creating a “Client Socket”

socket(): Creates a new socket for a

specific protocol (eg: TCP)

connect():

Makes a network connection

to a specified IP address and

port.

Functions: accept

  Notes

  After accept() returns a new socket

descriptor, I/O can be done using read() and

write()

  Why does accept() need to return a new

descriptor?

Sending and Receiving Data

int send(int sockfd, const void * buf,

size_t nbytes, int flags);

  Write data to a stream (TCP) or “connected”

datagram (UDP) socket.

  Returns number of bytes written or -1.

int recv(int sockfd, void *buf, size_t

nbytes, int flags);

  Read data from a stream (TCP) or “connected”

datagram (UDP) socket.

  Returns number of bytes read or -1.

Functions: send

int send(int sockfd, const void * buf, size_t

nbytes, int flags);

  Send data un a stream (TCP) or “connected”

datagram (UDP) socket

  Returns number of bytes written or -1 and sets errno on

failure

  sockfd: socket file descriptor (returned from socket)

  buf: data buffer

  nbytes: number of bytes to try to write

  flags: control flags

  MSG_PEEK: get data from the beginning of the receive queue without

removing that data from the queue

Functions: send

int send(int sockfd, const void * buf, size_t

nbytes, int flags);

  Example

len = strlen(msg);

bytes_sent = send(sockfd, msg, len, 0);

Functions: recv

int recv(int sockfd, void *buf, size_t nbytes,

int flags);

  Read data from a stream (TCP) or “connected”

datagram (UDP) socket

  Returns number of bytes read or -1, sets errno on failure

  Returns 0 if socket closed

  sockfd: socket file descriptor (returned from socket)

  buf: data buffer

  nbytes: number of bytes to try to read

  flags: see man page for details; typically use 0

Functions: recv

int recv(int sockfd, char* buf, size_t nbytes);

  Notes

  read blocks waiting for data from the client but does not

guarantee that sizeof(buf) is read

  Example

if((r = read(newfd, buf, sizeof(buf))) < 0) {

perror(“read”); exit(1);

}

Sending and Receiving Data

  Datagram sockets aren't connected to a

remote host

  What piece of information do we need to give

before we send a packet?

  The destination/source address!

Sending and Receiving Data

int sendto (int sockfd, char* buf,

size_t nbytes, int flags, struct

sockaddr* destaddr, int addrlen);

  Send a datagram to another UDP socket.

  Returns number of bytes written or -1.

int recvfrom (int sockfd, char* buf,

size_t nbytes, int flags, struct

sockaddr* srcaddr, int* addrlen);

  Read a datagram from a UDP socket.

  Returns number of bytes read or -1.

Functions: sendto

int sendto (int sockfd, char* buf, size_t nbytes,

int flags, struct sockaddr* destaddr, int

addrlen);

  Send a datagram to another UDP socket

  Returns number of bytes written or -1 and sets errno on failure

  sockfd: socket file descriptor (returned from socket)

  buf: data buffer

  nbytes: number of bytes to try to read

  flags: see man page for details; typically use 0

  destaddr: IP address and port number of destination socket

  addrlen: length of address structure

  = sizeof (struct sockaddr_in)

Functions: sendto

int sendto (int sockfd, char* buf, size_t nbytes,

int flags, struct sockaddr* destaddr, int

addrlen);

  Example

n = sendto(sock, buf, sizeof(buf), 0,(struct

sockaddr *) &from,fromlen);

if (n < 0)

perror("sendto");

exit(1);

}

Functions: recvfrom

int recvfrom (int sockfd, char* buf, size_t

nbytes, int flags, struct sockaddr* srcaddr,

int* addrlen);

  Read a datagram from a UDP socket.

  Returns number of bytes read (0 is valid) or -1 and sets errno

on failure

  sockfd: socket file descriptor (returned from socket)

  buf: data buffer

  nbytes: number of bytes to try to read

  flags: see man page for details; typically use 0

  srcaddr: IP address and port number of sending socket

(returned from call)

  addrlen: length of address structure = pointer to int set to

sizeof (struct sockaddr_in)

Functions: recvfrom

int recvfrom (int sockfd, char* buf, size_t

nbytes, int flags, struct sockaddr* srcaddr,

int* addrlen);

  Example

n = recvfrom(sock, buf, 1024, 0, (struct sockaddr

*)&from,&fromlen);

if (n < 0) {

perror("recvfrom");

exit(1);

}

Tearing Down a Connection

int close (int sockfd);

  Close a socket.

  Returns 0 on success, -1 and sets errno on failure.

int shutdown (int sockfd, int howto);

  Force termination of communication across a socket in

one or both directions.

  Returns 0 on success, -1 and sets errno on failure.

Functions: close

int close (int sockfd);

  Close a socket

  Returns 0 on success, -1 and sets errno on failure

  sockfd: socket file descriptor (returned from socket)

  Closes communication on socket in both directions

  All data sent before close are delivered to other side

(although this aspect can be overridden)

  After close, sockfd is not valid for reading or

writing

Functions: shutdown

int shutdown (int sockfd, int howto);

  Force termination of communication across a socket in one or

both directions

  Returns 0 on success, -1 and sets errno on failure

  sockfd: socket file descriptor (returned from socket)

  howto:

  SHUT_RD to stop reading

  SHUT_WR to stop writing

  SHUT_RDWR to stop both

  shutdown overrides the usual rules regarding duplicated

sockets, in which TCP teardown does not occur until all copies

have closed the socket

Note on close vs. shutdown

  close(): closes the socket but the connection is

still open for processes that shares this socket

  The connection stays opened both for read and write

  shutdown(): breaks the connection for all

processes sharing the socket

  A read will detect EOF, and a write will receive SIGPIPE

  shutdown() has a second argument how to close the

connection:

  0 means to disable further reading

  1 to disable writing

  2 disables both

Application Layer

Networked Applications

  All networked applications use

“application level” protocols to

communicate

  Examples

  HTTP

  FTP

  SMTP

  …

Web and HTTP

  Web pages consist of

  Objects

  HTML files, JPEG images, Java applets, audio files,…

  Base HTML-file

  Includes several referenced objects

  Each object is addressable by a URL

  Example URL:

www.someschool.edu/someDept/pic.gif

host name

path name

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

PC running

Explorer

Server

running

Apache Web

server

Mac running

Chrome

CS 241

HTTP

  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

  Stateless

  Server maintains no information about past client requests

HTTP Connections

  Nonpersistent

HTTP

  At most one object

is sent over a TCP

connection

  Persistent HTTP

  Multiple objects can

be sent over single

TCP connection

between client and

server

Nonpersistent HTTP

  User enters URL

  Text plus references to 10 jpeg images

www.someschool.edu/someDepartment/home.index

32

time

1a. HTTP client initiates TCP

connection to HTTP server at

www.someschool.edu

on port 80

1b. HTTP server at host

www.someschool.edu waiting

for TCP connection at port 80.

“accepts” connection, notifying

client

2. HTTP client sends HTTP

request message (containing

URL) into TCP socket. Message

indicates that client wants object

someDepartment/

home.index

3. HTTP server receives request

message, forms response

message containing requested

object, and sends message into

its socket

Nonpersistent HTTP

33

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.

3. HTTP server receives request

message, forms response

message containing requested

object, and sends message into

its socket

time

Response Time: First request

  RTT

  Time for 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 time

=  2RTT+transmit time

34

time to

transmit

file

initiate TCP

connection

RTT

request

file

RTT

file

received

time

Response time for

whole web page

  Nonpersistent HTTP

  Requires 2 RTTs per

object

  OS overhead for each

TCP connection

  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

sent over open connection

  Client sends requests as soon

as it encounters a referenced

object

  As little as one RTT total for

all the referenced objects

  See “HTTP pipelining”

Aside:

Do a few RTTs matter?

  Collective experiment

ping your_favorite_domain.foo!

HTTP Request Message

  Two types of HTTP messages: request, response

  ASCII (human-readable format)

  HTTP request message:

37

GET /somedir/page.html HTTP/1.1

Host: www.someschool.edu

User-agent: Mozilla/4.0

Connection: 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

Method Types

  HTTP/1.0

  GET

  POST

  HEAD

  Asks server to

leave requested

object out of

response

  HTTP/1.1

  GET, POST, HEAD

  PUT

  Uploads file in entity

body to path

specified in URL

field

  DELETE

  Deletes file

specified in the URL

field

HTTP Request Message:

General Format

39

method sp URL sp version cr lf

header field name : value cr lf

…

cr lf

entity body

Request

line

Header

lines

GET /somedir/page.html HTTP/1.1

Host: www.someschool.edu

Uploading Form Input

  Post method

  Web page often includes form of 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

HTTP Response Message

41

HTTP/1.1 200 OK

Connection close

Date: 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 code

status phrase)

header

lines

data, e.g.,

requested

HTML file

HTTP response status codes

  In first line in server->client response message

  A few sample codes

42

200 OK request succeeded, requested object later in

this message

301 Moved

Permanently

requested object moved, new location

specified later in this message (Location:),

client automatically retrieves new URL

400 Bad Request request message not understood by server

404 Not Found requested document not found on this server

505 HTTP Version

Not Supported

HTTP response status codes

  In first line in server->client response message

  A few sample codes

  More in the illustrated guide...

  http://tinyurl.com/cvyepwt

Trying out HTTP (client side)

For Yourself

1. Telnet to your favorite Web server

telnet www.cs.illinois.edu 80

2. Type in a GET HTTP request

GET /class/sp12/cs241/index.html

HTTP/1.0

3. Look at response message sent by

HTTP server!

44

Opens TCP connection to port 80

(default HTTP server port) at

www.cs.illinois.edu.

Anything typed in sent

to port 80 at cs.illinois.edu

By typing this in (hit carriage

return twice), you send

this minimal (but complete)

GET request to HTTP server

User-server State: Cookies

  Many major Web sites

use cookies

  Four components

1.  Cookie header line of

HTTP response message

2.  Cookie header line in

HTTP request message

3.  Cookie file kept on user’s

host, managed by user’s

browser

4.  Back-end database at

Web site

  Example

  Alice always accesses

Internet from PC

  Visits specific e-

commerce site for first

time

  When initial HTTP

requests arrives at site,

site creates:

  unique ID

  entry in backend

database for ID

Cookies

  What cookies can bring

  Authorization

  Shopping carts

  Recommendations

  User session state (Web e-

mail)

  How to keep “state”

  Protocol endpoints: maintain

state at sender/receiver over

multiple transactions

  cookies: http messages

carry state

  Cookies and

privacy

  Cookies permit

sites to learn a lot

about you

  You may supply

name and e-mail

to sites

Cookies: Keeping “State”

47

client server

usual http response msg

cookie file

one week later:

usual http request msg

cookie: 1678

cookie-

specific

action

access

ebay 8734

usual http request msg

Amazon server

creates ID

1678 for user

create

entry

usual http response

Set-cookie: 1678

ebay 8734

amazon 1678

usual http request msg

cookie: 1678

cookie-

specific

action

access

ebay 8734

amazon 1678

backend

database