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Lecture 2 - Weeks 2 3.pptx
Lecture_2_-_Weeks_2___3.pptx
Showing 17-22 out of 70
Lecture 2 - Weeks 2 3.pptx-Data Communication an...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Lecture 2 - Weeks 2 3.pptx-Data C...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Page 17
Application Layer A 2-17
Services
not
provided by Internet transport
protocols
The services:
1) reliability, 2) throughput,
3) timing,
4) security
❖
Which of these services are provided by TCP and
UDP?
▪
1) Reliability (by TCP),
▪
4) Security (by SSL-enhanced-TCP)
▪
What about throughput and timing?!!!
❖
NO! these services are not provided by today’s
Internet transport protocols.
❖
Then, we cannot run time-sensitive applications
(i.e. Internet telephony) over the Internet.
Correct?
❖
No; that is incorrect! How then!
Page 18
Application Layer A 2-18
Application-layer protocols
❖
application-layer protocols define how
application processes pass messages to each
other
❖
in particular, they define:
▪
Types of exchanged messages: i.e.
request
and
response
messages
▪
Syntax of message types: i.e. what the
different fields in the message are
▪
Semantics of the fields: the meaning of the
information in the fields
▪
Rules of communication: i.e. when and how a
process can send a message (or respond to a
message)
❖
some public application-layer protocols are
specified in RFCs; i.e. RFC 2616 for HTTP
▪
for instance, if a browser developer follows the rules of
RFC2616, the browser
will be able to retrieve web
pages from any server that followed the rules of HTTP
Page 19
Application Layer A 2-19
Network Applications
❖
there are MANY network applications (possibly
new ones are being written as you read these
slides!)
❖
we will here look at 5 of the most important and
pervasive network applications:
▪
Web and HTTP
▪
FTP
▪
Electronic Mail
▪
DNS
▪
P2P
Page 20
Application Layer A 2-20
Web and HTTP
First, a review…
❖
web page
consists of
objects
❖
object can be HTML file, 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
Page 21
Application Layer A 2-21
HTTP overview
HTTP: hypertext
transfer protocol
❖
the 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
iphone running
Safari browser
HTTP request
HTTP response
HTTP request
HTT
P re
sponse
Page 22
Application Layer A 2-22
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
asi
de
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