程序代写代做代考 scheme database dns Java cache javascript Application Layer

Application Layer

George Parisis
School of Engineering and Informatics

University of Sussex

Application Layer 2-2

Outline

v  Principles of network applications
v  Electronic mail

§  SMTP, POP3, IMAP
v  Web and HTTP
v  DNS
v  socket programming with UDP and TCP

Application Layer 2-3

Web and HTTP
First, a review…
v  web page consists of objects
v  object can be HTML file, CSS, JavaScript,

JPEG image, Java applet, audio file,…
v  web page consists of base HTML-file which

includes several referenced objects
v  each object is addressable by a URL, e.g.,
www.someschool.edu/someDept/pic.gif

host name path name

Application Layer 2-4

HTTP overview
HTTP: hypertext transfer

protocol
v  Web’s application layer

protocol
v  client/server model

§  client: browser that
requests, receives,
(using HTTP protocol)
and “displays” Web
objects

§  server: Web server
sends (using HTTP
protocol) objects in
response to requests

PC running
Firefox browser

server
running

Apache Web
server

mobile device

Application Layer 2-5

HTTP overview (continued)
uses TCP:
v  client initiates TCP

connection (creates
socket) to server, port
80

v  server accepts TCP
connection from client

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

v  TCP connection closed

HTTP is
“stateless”

v  server maintains no
information about
past client requests

protocols that maintain
“state” are complex!

v  past history (state) must be
maintained

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

aside

Application Layer 2-6

HTTP – TCP connections
non-persistent HTTP
v  at most one object

sent over TCP
connection
§  connection then

closed
v  downloading

multiple objects
required multiple
connections

persistent HTTP
v  multiple objects can

be sent over single
TCP connection
between client,
server

Application Layer 2-7

Non-persistent HTTP
suppose user enters URL:

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)
www.someSchool.edu/someDepartment/home.index

Application Layer 2-8

Non-persistent 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

Application Layer 2-9

Non-persistent HTTP: response time

RTT (definition): time for a
small packet to travel from
client to server and back

HTTP response time:
v  one RTT to initiate TCP

connection
v  one RTT for HTTP

request and first few bytes
of HTTP response to
return

v  file transmission time
v  non-persistent HTTP

response time =
2RTT+ file

transmission time

time to
transmit
file

initiate TCP
connection

RTT

request
file

RTT

file
received

time time

Application Layer 2-10

Persistent HTTP

non-persistent HTTP
issues:

v  requires 2 RTTs per
object

v  OS overhead for each
TCP connection

v  browsers often open
parallel TCP connections
to fetch referenced
objects

persistent HTTP:
v  server leaves

connection open after
sending response

v  subsequent HTTP
messages between
same client/server sent
over open connection

v  client sends requests as
soon as it encounters a
referenced object

v  as little as one RTT for
all the referenced
objects

Application Layer 2-11

HTTP request message

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

§  ASCII (human-readable format)

request line
(GET, POST,
HEAD commands)

header
lines

carriage return,
line feed at start
of line indicates
end of header lines

GET /index.html HTTP/1.1

Host: www-net.cs.umass.edu

User-Agent: Firefox/3.6.10

Accept: text/html,application/xhtml+xml

Accept-Language: en-us,en;q=0.5

Accept-Encoding: gzip,deflate

Accept-Charset: ISO-8859-1,utf-8;q=0.7

Keep-Alive: 115

Connection: keep-alive

carriage return character
line-feed character

Application Layer 2-12

HTTP request message: general format

request
line

header
lines

body

method sp sp cr lf version URL

cr lf value header field name

~ ~ ~ ~

cr lf

entity body ~ ~ ~ ~

Application Layer 2-13

Method types

HTTP/1.0:
v  GET
v  POST
v  HEAD

§  asks server to leave
requested object out
of response

HTTP/1.1:
v  GET, POST, HEAD
v  PUT

§  uploads file in entity
body to path
specified in URL field

v  DELETE
§  deletes file specified

in the URL field

Application Layer 2-14

Uploading form input
POST method:
v  web page often includes

form input
v  input is uploaded to

server in entity body

URL:
v  uses GET method
v  input is uploaded in

URL field of request
line:

www.somesite.com/animalsearch?monkeys&banana

Application Layer 2-15

HTTP response message
status line
(protocol
status code
status phrase)

header
lines

data, e.g.,
requested
HTML file

HTTP/1.1 200 OK

Date: Sun, 26 Sep 2010 20:09:20 GMT

Server: Apache/2.0.52 (CentOS)

Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT

ETag: “17dc6-a5c-bf716880″

Accept-Ranges: bytes

Content-Length: 2652

Keep-Alive: timeout=10, max=100

Connection: Keep-Alive

Content-Type: text/html;

charset=ISO-8859-1

data data data data data …

Application Layer 2-16

HTTP response status codes

200 OK
§  request succeeded, requested object later in this msg

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

msg (Location:)
400 Bad Request

§  request msg not understood by server
404 Not Found

§  requested document not found on this server
505 HTTP Version Not Supported

v  status code appears in 1st line in server-to-
client response message.

v  some sample codes:

Common Request Header Fields
Accept-Charset Character sets that are acceptable Accept-Charset: utf-8

Accept-Encoding List of acceptable encodings Accept-Encoding: gzip, deflate
Accept-Language List of acceptable human languages Accept-Language: en-US

Authorization Authentication credentials Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

Cookie
A cookie previously sent by the server
with Set-Cookie

Cookie: $Version=1; Skin=new;

Content-Length The length of the request body in octets Content-Length: 348

Content-Type The MIME type of the body of the request Content-Type: application/x-www-form-urlencoded

Date
The date and time that the message was
sent

Date: Tue, 15 Nov 1994
08:12:31 GMT

Host
The domain name of the server, and the
listening TCP Port number

Host: en.wikipedia.org:8080
Host: en.wikipedia.org

If-Modified-Since
Allows a 304 Not Modified to be returned
if content is unchanged

If-Modified-Since: Sat, 29
Oct 1994 19:43:31 GMT

If-Unmodified-
Since

Only send the response if the entity has not
been modified since a specific time

If-Unmodified-Since: Sat, 29
Oct 1994 19:43:31 GMT

User-Agent The user agent User-Agent: Mozilla/5.0
Upgrade Ask the server to upgrade to other protocol Upgrade: websocket

Common Response Header Fields
Content-Encoding The type of encoding used on the data Content-Encoding: gzip
Content-Language The language the content is in Content-Language: da
Content-Length The length of the response body in octets Content-Length: 348
Content-Location Alternate location for the returned data Content-Location:/index.htm

Content-Type MIME Type of this content Content-Type: text/html;charset=utf-8
Date Date and time that the message was sent Date:Tue,15 Nov 1994 08:12

Expires When the response is considered stale Expires: Thu, 01 Dec 1994 16:00:00 GMT
Last-Modified Last modified date for the object Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

Location
Used in redirection, or when a new
resource has been created (PUT)

Location: www.w3.org/pub/WWW/
p.html

Server A name for the server Server: Apache/2.4.1 (Unix)

Set-Cookie An HTTP Cookie Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1
Upgrade Ask the client to upgrade to other protocol Upgrade: websocket
WWW-Authenticate Authentication scheme that should be used WWW-Authenticate: Basic

Application Layer 2-19

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/1.1
Host: cis.poly.edu

by typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to HTTP server

3. look at response message sent by HTTP server!

(or use Wireshark to look at captured HTTP request/response)

Application Layer 2-20

User-server state: cookies
many Web sites use cookies
four components:
1) cookie header line of HTTP response message
2) cookie header line in next HTTP request message
3) cookie file kept on user’s host, managed by user’s browser
4) back-end ‘database’ at Web site

example:
v  Susan always access Internet from PC
v  visits specific e-commerce site for first time
v  when initial HTTP requests arrives at site, site creates:

§  unique ID
§  entry in backend database for ID

Application Layer 2-21

Cookies: keeping “state” (cont.)
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

Application Layer 2-22

Cookies (continued)
what cookies can be used for:
v  authorization
v  shopping carts
v  recommendations
v  user session state (Web e-

mail)

cookies and privacy:
v  cookies permit sites to learn

a lot about you
v  you may supply name and e-

mail to sites

aside

how to keep“state”:
v  protocol endpoints: maintain state at

sender/receiver over multiple
transactions

v  cookies: HTTP messages carry state

Application Layer 2-23

Web caches (proxy server)

v  user sets browser: Web
accesses via cache

v  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

proxy
server

client origin
server

origin
server

Application Layer 2-24

More about Web caching

v  cache acts as both
client and server
§  server for original

requesting client
§  client to origin server

v  typically cache is
installed by ISP
(university,
company, residential
ISP)

why Web caching?
v  reduce response time

for client request
v  reduce traffic on an

institution’s access
link

v  Internet dense with
caches: enables
“poor” content
providers to effectively
deliver content (so too
does P2P file sharing)

Application Layer 2-25

Caching example:

origin
servers

public
Internet

institutional
network

1 Gbps LAN

15 Mbps access link

assumptions:
v  avg object size: 1Mbits
v  avg request rate from browsers to

origin servers:15 requests/sec
v  avg data rate to browsers: 15 Mbps
v  RTT from institutional router to any

origin server: 2 sec
v  access link rate: 15 Mbps

consequences:
v  LAN utilization: 1.5%
v  access link utilization = 100%
v  total delay = Internet delay + access

delay + LAN delay
= 2 sec + minutes + usecs

problem!

Application Layer 2-26

Caching example: fatter access link

origin
servers

150 Mbps access link

Cost: increased access link speed (not cheap!)

public
Internet

institutional
network

1 Gbps LAN

assumptions:
v  avg object size: 1Mbits
v  avg request rate from browsers to

origin servers:15 requests/sec
v  avg data rate to browsers: 15 Mbps
v  RTT from institutional router to any

origin server: 2 sec
v  access link rate: 150 Mbps

consequences:
v  LAN utilization: 1.5%
v  access link utilization = 10%
v  total delay = Internet delay + access

delay + LAN delay
= 2 sec + msecs+ usecs

institutional
network 1 Gbps LAN

Application Layer 2-27

Caching example: install local cache

origin
servers

15 Mbps access link

local web
cache How to compute link

utilization, delay?
Cost: web cache (cheap!)

public
Internet

assumptions:
v  avg object size: 1Mbits
v  avg request rate from browsers to

origin servers:15 requests/sec
v  avg data rate to browsers: 15 Mbps
v  RTT from institutional router to any

origin server: 2 sec
v  access link rate: 15 Mbps

consequences:
v  LAN utilization: 1.5%
v  access link utilization =
v  total delay = Internet delay + access

delay + LAN delay
=

Application Layer 2-28

Caching example: install local cache
Calculating access link

utilization, delay with cache:
v suppose cache hit rate is 0.4

§  40% requests satisfied at cache
§  60% requests satisfied at origin

origin
servers

v access link utilization:
§  60% of requests use access link

v  data rate to browsers over access link
= 0.6 * 15 Mbps = 9 Mbps
§  utilization = 9/15 = 0.6

v total delay
§  = 0.6 * (delay from origin servers) +

0.4 * (delay when satisfied at cache)
§  less than with 154 Mbps link (and

cheaper too!)

public
Internet

institutional
network

local web
cache

15 Mbps access link

1 Gbps LAN

Application Layer 2-29

Conditional GET

v  Goal: don’t send object if cache
has up-to-date cached version
§  no object transmission delay
§  lower link utilization

v  Last-modified
v  cache: specify date of cached

copy in HTTP request
If-modified-since:

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

HTTP request msg
If-modified-since:

HTTP response
HTTP/1.0

304 Not Modified

object
not

modified
before

HTTP request msg
If-modified-since:

HTTP response
HTTP/1.0 200 OK

object
modified

after

client server

Application Layer 2-30

Summary

v  HTTP
v  Web Caching

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