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Lecture 2 - Weeks 2 3.pptx
Lecture_2_-_Weeks_2___3.pptx
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Lecture 2 - Weeks 2 3.pptx-Data Communication an...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
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Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Page 7
Application Layer A 2-7
P2P architecture
❖
Cost-effective
▪
no need for server infrastructure
and server bandwidth
❖
However, future P2P
applications face 3 major
challenges:
▪
ISP-Friendly
: residential ISP
(DSL, Cable, …) are
asymmetric
▪
Security
: a challenge, since
P2P apps are highly
distributed and naturally
open
▪
Incentive
: users must
volunteer bandwidth, storage
space and computational
peer-
peer
Page 8
Application Layer A 2-8
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
clients,
servers
Page 9
Application Layer A 2-9
Sockets
❖
process sends/receives messages to/from its
socket
(a software interface; an
API
)
❖
socket analogous to door
▪
sending process shoves 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
proce
ss
transport
application
physical
link
network
proce
ss
sock
et
controlled
by OS
controlled by
app developer
Page 10
Application Layer A 2-10
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
receiving process
(more precisely, the
receiving socket)?
❖
identifier
includes both
IP address
and
port
numbers
associated
with process on host.
❖
example port numbers:
▪
HTTP (web) server: 80
▪
SMTP (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…
▪
A:
no,
many
network processes
can be running on
same host
Page 11
Application Layer A 2-11
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
open protocols:
❖
defined in RFCs
❖
allows for
interoperability
❖
e.g., HTTP, SMTP
proprietary protocols:
❖
e.g., Skype
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