PowerPoint Presentation
Application Layer
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
George Parisis
School of Engineering and Informatics
University of Sussex
Application Layer
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Outline
Principles of network applications
Electronic mail
SMTP, POP3, IMAP
Web and HTTP
DNS
socket programming with UDP and TCP
Application Layer
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Web and HTTP
First, a review…
web page consists of objects
object can be HTML file, CSS, JavaScript, 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, e.g.,
www.someschool.edu/someDept/pic.gif
host name
path name
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HTTP overview
HTTP: hypertext transfer protocol
Web’s application layer protocol
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
HTTP request
HTTP response
HTTP request
HTTP response
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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
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HTTP – TCP connections
non-persistent HTTP
at most one object sent over TCP connection
connection then closed
downloading multiple objects required multiple connections
persistent HTTP
multiple objects can be sent over single TCP connection between client, server
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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
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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
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Non-persistent HTTP: response time
RTT (definition): time for a small packet to travel from client to server and back
HTTP 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
non-persistent HTTP response time =
2RTT+ file transmission time
time to
transmit
file
initiate TCP
connection
RTT
request
file
RTT
file
received
time
time
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Persistent HTTP
non-persistent HTTP issues:
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
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HTTP request message
two types of HTTP messages: request, response
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\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\r\n
Accept: text/html,application/xhtml+xml\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n
Keep-Alive: 115\r\n
Connection: keep-alive\r\n
\r\n
carriage return character
line-feed character
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HTTP request message: general format
request
line
header
lines
body
method
sp
sp
cr
lf
version
URL
entity body
cr
lf
value
header field name
cr
lf
value
header field name
~
~
~
~
cr
lf
~
~
~
~
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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
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Uploading form input
POST method:
web page often includes form input
input is uploaded to server in entity body
URL:
uses GET method
input is uploaded in URL field of request line:
www.somesite.com/animalsearch?monkeys&banana
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HTTP response message
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
HTML file
HTTP/1.1 200 OK\r\n
Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n
Server: Apache/2.0.52 (CentOS)\r\n
Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT\r\n
ETag: “17dc6-a5c-bf716880″\r\n
Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n
Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO-8859-1\r\n
\r\n
data data data data data …
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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
status code appears in 1st line in server-to-client response message.
some sample codes:
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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
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Styling slide, for reference only.
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
HTTP v2.0
Derived from SPDY (Google)
Main aim: decrease latency to improve page load speed in web browsers
data compression of HTTP headers
HTTP/2 Server Push
fixing the head-of-line blocking problem in HTTP 1.x
multiplexing multiple requests over a single TCP connection
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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)
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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:
Susan always access 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
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Cookies: keeping “state” (cont.)
client
server
cookie file
one week later:
backend
database
usual http response msg
usual http response msg
access
usual http request msg
cookie: 1678
cookie-
specific
action
ebay 8734
create
entry
usual http request msg
Amazon server
creates ID
1678 for user
ebay 8734
amazon 1678
usual http response
set-cookie: 1678
usual http request msg
cookie: 1678
cookie-
specific
action
access
ebay 8734
amazon 1678
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Cookies (continued)
what cookies can be used for:
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
aside
how to keep“state”:
protocol endpoints: maintain state at sender/receiver over multiple transactions
cookies: HTTP messages carry state
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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
proxy
server
client
origin
server
origin
server
HTTP request
HTTP response
HTTP request
HTTP request
HTTP response
HTTP response
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More about Web caching
cache acts as both client and server
server for original requesting client
client to origin 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 content providers to effectively deliver content (so too does P2P file sharing)
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Caching example:
origin
servers
public
Internet
institutional
network
1 Gbps LAN
15 Mbps access link
assumptions:
avg object size: 1Mbits
avg request rate from browsers to origin servers:15 requests/sec
avg data rate to browsers: 15 Mbps
RTT from institutional router to any origin server: 2 sec
access link rate: 15 Mbps
consequences:
LAN utilization: 1.5%
access link utilization = 100%
total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + usecs
problem!
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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:
avg object size: 1Mbits
avg request rate from browsers to origin servers:15 requests/sec
avg data rate to browsers: 15 Mbps
RTT from institutional router to any origin server: 2 sec
access link rate: 150 Mbps
consequences:
LAN utilization: 1.5%
access link utilization = 10%
total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs+ usecs
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institutional
network
1 Gbps LAN
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Caching example: install local cache
origin
servers
15 Mbps access link
How to compute link
utilization, delay?
Cost: web cache (cheap!)
public
Internet
assumptions:
avg object size: 1Mbits
avg request rate from browsers to origin servers:15 requests/sec
avg data rate to browsers: 15 Mbps
RTT from institutional router to any origin server: 2 sec
access link rate: 15 Mbps
consequences:
LAN utilization: 1.5%
access link utilization =
total delay = Internet delay + access delay + LAN delay
=
local web
cache
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Caching example: install local cache
Calculating access link utilization, delay with cache:
suppose cache hit rate is 0.4
40% requests satisfied at cache
60% requests satisfied at origin
origin
servers
access link utilization:
60% of requests use access link
data rate to browsers over access link = 0.6 * 15 Mbps = 9 Mbps
utilization = 9/15 = 0.6
total delay
= 0.6 * (delay from origin servers) + 0.4 * (delay when satisfied at cache)
less than with 150 Mbps link (and cheaper too!)
public
Internet
institutional
network
15 Mbps access link
1 Gbps LAN
local web
cache
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Conditional GET
Goal: don’t send object if cache has up-to-date cached version
no object transmission delay
lower link utilization
Last-modified
cache: specify date of cached copy in HTTP request
If-modified-since:
server: response contains no object if cached copy is up-to-date:
HTTP/1.0 304 Not Modified
HTTP request msg
If-modified-since:
object
not
modified
before
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0 200 OK
object
modified
after
client
server
HTTP response
HTTP/1.0
304 Not Modified
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Summary
HTTP
Web Caching
Application Layer