Lec 6(Application Layer)

Computer Networks – CSE331
Lecture 6
2 Hrs

CIDR: Classless InterDomain Routing
 subnet portion of address of arbitrary length
 address format: a.b.c.d/x, where x is # bits in subnet portion
of address
 Purchase what you need
11001000 00010111 00010000 00000000
subnet
part
host
part
200.23.16.0/23

206.0.64.0/18
204.188.0.0/15
209.88.232.0/21Internet Backbone
ISP K owns:
Company X :
206.0.68.0/22
ISP Y :
209.88.237.0/24
Organization Z1 :
209.88.237.192/26
Organization Z2 :
209.88.237.0/26
Backbone sends everything
which matches the prefixes
206.0.64.0/18, 204.188.0.0/15,
209.88.232.0/21 to ISP K.
ISP K sends everything which
matches the prefix:
206.0.68.0/22 to Company X,
209.88.237.0/24 to ISP Y
Backbone routers do not know
anything about Company X, ISP
Y, or Organizations Z1, Z2.
ISP K does not know about
Organizations Z1, Z2. ISP Y sends everything which matches
the prefix:
209.88.237.192/26 to Organizations Z1
209.88.237.0/26 to Organizations Z2

our goals:
 conceptual,
implementation aspects
of network application
protocols
 transport-layer service
models
 client-server paradigm
 peer-to-peer paradigm
 learn about protocols by
examining popular
application-level
protocols
 HTTP
 FTP
 SMTP / POP3 / IMAP
 DNS
 creating network
applications
 socket API

 e-mail
 web
 text messaging
 remote login
 P2P file sharing
 multi-user network games
 streaming stored video
(YouTube, dailymotion)
 voice over IP (e.g.,
Skype)
 real-time video
conferencing
 social networking
 search
 …
 …

write programs that:
 run on (different) end
systems
 communicate over network
 e.g., web server software
communicates with browser
software
Two models
 Client Server
 Peer to Peer (P2P)
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical

process: program running
within a host
 within same host, two
processes communicate
using inter-process
communication (defined by
OS)
 processes in different hosts
communicate by
exchanging messages
client process: process that
initiates communication
server process: process that
waits to be contacted
 aside: applications with P2P
architectures have client
processes & server
processes
clients, servers

server:
 always-on host
 permanent IP address
 data centers for scaling
clients:
 communicate with server
 may be intermittently connected
 may have dynamic IP addresses
 do not communicate directly with
each other
client/server

 no always-on server
 arbitrary end systems directly
communicate
 peers request service from
other peers, provide service in
return to other peers
 self scalability – new peers
bring new service capacity,
as well as new service
demands
 peers are intermittently
connected and change IP
addresses
 complex management
peer-peer

 to receive messages, process
must have identifier
 host device has unique 32-bit
IP address
 Q: does IP address of host on
which process runs sufficient
for identifying the process?
 A: no, many processes can be
running on same host
 identifier includes both IP
address and port numbers
associated with process on
host.
 example port numbers:
 HTTP server: 80
 mail server: 25
 to send HTTP message to
uet.edu.pk web server:
 IP address: 111.68.102.6
 port number: 80
 more shortly…

 process sends/receives messages to/from its socket
 socket analogous to door
 sending process send message out door
 sending process relies on transport infrastructure on other
side of door to deliver message to socket at receiving process
Internet
controlled
by OS
controlled by
app developer
transport
application
physical
link
network
process
transport
application
physical
link
network
processsocket

 Suppose Gifts have to be
delivered to cousins
 Two houses
 House 1
 Ali, Amna
 House 2
 Rehman, Fatima
 Ali want to send to
Rehman & Amna want to
send Fatima
host
host
process
process
Every process associated with a socket – socket has a 16bit number
Complete address is IP address + socket no

TCP service:
 reliable transport between sending
and receiving process
 flow control: sender won’t
overwhelm receiver
 congestion control: throttle sender
when network overloaded
 does not provide: timing, minimum
throughput guarantee, security
 connection-oriented: setup required
between client and server processes
UDP service:
 unreliable data transfer
between sending and receiving
process
 does not provide: reliability,
flow control, congestion control,
timing, throughput guarantee,
security, or connection setup,
Q: why bother? Why is there a
UDP?

Application Application Layer
Protocol
Transport
protocol
Email SMTP [RFC 2821] TCP
Remote terminal access Telnet [RFC 854] TCP
Web HTTP [RFC 2616] TCP
File transfer FTP [RFC 959] TCP
Streaming multimedia HTTP, RTP [RFC 1889] TCP or UDP
Internet telephony SIP, RTP, proprietary (e.g.
skype)
TCP or UDP

TCP & UDP
 no encryption
 cleartext passwords sent into
socket traverse Internet in
cleartext
SSL
 provides encrypted TCP
connection
 data integrity
 end-point authentication
SSL is at app layer
 Apps use SSL libraries, which
“talk” to TCP
SSL socket API
 cleartext passwords sent
into socket traverse Internet
encrypted
 More in Chapter 7

First, a review…
 Until 1990s universities, researchers => remote loging, file
transfer, email
 World wide web (www) in 1994 Tim Berners Lee
 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

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
iphone running
Safari browser

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

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

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

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 timeIf 1 RTT is 500ms file size is 100KB link
speed is 0.5MB what is total time?
1.2sec If 1 RTT is 500ms file size is 100MB link
speed is 0.5MB what is total time?
201sec

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
 as little as one RTT for all
the referenced objects
Demo – Persistent & non-persistent HTTP

 Quiz upto this Lecture on next Monday
 Ross book 2.1 to 2.2.2
 Forouzan book chapter 17
 List of standard internet protocols
 http://www.iana.org
 For those who want to learn Java
 Lecture 3, 4 (CS-2) on Tuesday 13 Feb 2017
 Bring your laptops with you

Lec 6(Application Layer)

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