Application Layer (Email, DNS, P2P)
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Transcript Application Layer (Email, DNS, P2P)
Throughput: Internet scenario
per-connection
end-end
throughput:
min(Rc,Rs,R/10)
in practice: Rc or
Rs is often
bottleneck
Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Protocol “Layers”
Networks are complex!
many “pieces”:
hosts
routers
links of various
media
applications
protocols
hardware,
software
Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion
of networks?
Why layering?
Dealing with complex systems:
Explicit structure allows identification, relationship
of complex system’s pieces
layered reference model for discussion
Modularization eases maintenance, updating of
system
change of implementation of layer’s service
transparent to rest of system
e.g., change in gate procedure doesn’t affect rest
of system
Layering considered harmful?
Internet protocol stack
application: supporting network applications
FTP, SMTP, HTTP
transport: host-host data transfer
TCP, UDP
network: routing of datagrams from source
to destination
IP, routing protocols
link: data transfer between neighboring
network elements
PPP, Ethernet
physical: bits “on the wire”
application
transport
network
link
physical
Layering: logical communication
Each layer:
distributed
“entities”
implement
layer functions
at each node
entities
perform
actions,
exchange
messages with
peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
Layering: logical communication
E.g.: transport
take data from app
add addressing,
reliability check
info to form
“datagram”
send datagram to
peer
wait for peer to
ack receipt
analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
Protocol layering and data
Each layer takes data from above
adds header information to create new data unit
passes new data unit to layer below
M
Ht M
Hn Ht M
Hl Hn Ht M
source
destination
application
transport
network
link
physical
application
transport
network
link
physical
M
message
Ht M
Hn Ht M
Hl Hn Ht M
segment
datagram
frame
Summary
Network access and physical media
Internet structure and ISPs
Delay & loss in packet-switched networks
Protocol layers, service models
More depth, detail to follow!
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
Some network apps
e-mail
voice over IP
web
real-time video
instant messaging
remote login
P2P file sharing
multi-user network
games
streaming stored video
(YouTube)
conferencing
cloud computing
…
…
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
Outline
Principles of network applications
App architectures
App requirements
Web and HTTP
FTP
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Application architectures
Client-server
Peer-to-peer (P2P)
Hybrid of client-server and P2P
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Client-server architecture
server:
always-on host
permanent IP address
server farms for
scaling
clients:
client/server
communicate with server
may be intermittently
connected
may have dynamic IP
addresses
do not communicate
directly with each other
Pure P2P architecture
no always-on server
arbitrary end systems
directly communicate peer-peer
peers are intermittently
connected and change IP
addresses
highly scalable but
difficult to manage
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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
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Outline
Principles of network applications
App architectures
App requirements
Web and HTTP
FTP
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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
Note: applications with
P2P architectures have
client processes &
server processes
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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
host or
server
host or
server
process
controlled by
app developer
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
API: (1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
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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?
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Addressing processes
to receive messages,
identifier includes both
process must have
IP address and port
identifier
numbers associated with
process on host.
host device has unique
32-bit IP address
example port numbers:
HTTP server: 80
Q: does IP address of
Mail server: 25
host on which process
runs suffice for
to send HTTP message
identifying the process?
to gaia.cs.umass.edu web
server:
A: No, many
IP address: 128.119.245.12
processes can be
Port number: 80
running on same host
more shortly…
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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
public-domain protocols:
defined in RFCs
allows for
interoperability
e.g., HTTP, SMTP
proprietary protocols:
e.g., Skype
rules for when and how
processes send &
respond to messages
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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,
…
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