Application Layer-Introduction - detail Things are included

Chapter 2
Application Layer
Computer Networking:
A Top Down Approach,
5th
edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
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All material copyright 1996-2010
J.F Kurose and K.W. Ross, All Rights Reserved
Application 2-1

Chapter 2: Application Layer
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
 programming network
applications
 socket API
Application 2-3

Some network apps
 e-mail
 web
 instant messaging
 remote login
 P2P file sharing
 multi-user network
games
 streaming stored video
(YouTube)
 voice over IP
 real-time video
conferencing
 cloud computing
 …
 …

Application 2-4

Creating a network app
write programs that
 run on (different) end
systems
 communicate over network
 e.g., web server software
communicates with
browser software
No need to write software
for network-core devices
 network-core devices do
not run user applications
 applications on end systems
allows for rapid app
development, propagation
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
Application 2-5

Chapter 2: Application layer
2.1 Principles of network
applications
2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 Socket programming
with TCP
2.8 Socket programming
with UDP
Application 2-6

Application architectures
 client-server
 peer-to-peer (P2P)
 hybrid of client-server and P2P
Application 2-7

Client-server architecture
server:
 always-on host
 permanent IP address
 server farms for scaling
clients:
 communicate with server
 may be intermittently
connected
 may have dynamic IP
addresses
 do not communicate directly
with each other
client/server
Application 2-8

Pure P2P architecture
 no always-on server
 arbitrary end systems
directly communicate
 peers are intermittently
connected and change IP
addresses
highly scalable but
difficult to manage
peer-peer
Application 2-9

Hybrid of client-server and P2P
Skype
 voice-over-IP P2P application
 centralized server: finding address of remote
party:
 client-client connection: direct (not through
server)
Instant messaging
 chatting between two users is P2P
 centralized service: client presence
detection/location
• user registers its IP address with central
server when it comes online
• user contacts central server to find IP
addresses of buddies
Application 2-10

Processes communicating
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
Application 2-11

Sockets
 process sends/receives
messages to/from its
socket
 socket analogous to door
 sending process shoves
message out door
 sending process relies on
transport infrastructure
on other side of door which
brings message to socket
at receiving process
process
TCP with
buffers,
variables
socket
host or
server
process
TCP with
buffers,
variables
socket
host or
server
Internet
controlled
by OS
controlled by
app developer
 API: (1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
Application 2-12

Addressing processes
 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 suffice for
identifying the process?
Application 2-13

Addressing processes
 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 suffice 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 gaia.cs.umass.edu web
server:
 IP address: 128.119.245.12
 Port number: 80
 more shortly…
Application 2-14

App-layer protocol defines
 types of messages
exchanged,
 e.g., request, response
 message syntax:
 what fields in messages &
how fields are delineated
 message semantics
 meaning of information in
fields
 rules for when and how
processes send &
respond to messages
public-domain protocols:
 defined in RFCs
 allows for
interoperability
 e.g., HTTP, SMTP
proprietary protocols:
 e.g., Skype
Application 2-15

What transport service does an app need?
Data loss
 some apps (e.g., audio) can
tolerate some loss
 other apps (e.g., file
transfer, telnet) require
100% reliable data
transfer
Timing
 some apps (e.g.,
Internet telephony,
interactive games)
require low delay to be
“effective”
Throughput
 some apps (e.g.,
multimedia) require
minimum amount of
throughput to be
“effective”
 other apps (“elastic apps”)
make use of whatever
throughput they get
Security
 encryption, data integrity,
…
Application 2-16

Transport service requirements of common apps
Application
file transfer
e-mail
Web documents
real-time audio/video
stored audio/video
interactive games
instant messaging
Data loss
no loss
no loss
no loss
loss-tolerant
loss-tolerant
loss-tolerant
no loss
Throughput
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Time Sensitive
no
no
no
yes, 100’s msec
yes, few secs
yes, 100’s msec
yes and no
Application 2-17

Internet transport protocols services
TCP service:
 connection-oriented: setup
required between client and
server processes
 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
guarantees, security
UDP service:
 unreliable data transfer
between sending and
receiving process
 does not provide:
connection setup,
reliability, flow control,
congestion control, timing,
throughput guarantee, or
security
Q: why bother? Why is
there a UDP?
Application 2-18

Internet apps: application, transport protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
Application
layer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
HTTP (e.g., YouTube),
RTP [RFC 1889]
SIP, RTP, proprietary
(e.g., Skype)
Underlying
transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
typically UDP
Application 2-19

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