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 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\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
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 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\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 …
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
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