CHAPTER 2
Creating a network app
write programs that
application
transport
network
data link
physical
– 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 2-2
Application architectures
• client-server
• peer-to-peer (P2P)
• hybrid of client-server and P2P
Application 2-3
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
Application 2-4
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
Application 2-5
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 2-6
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 2-7
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 2-8
Addressing processes
• to receive messages,
process must have
identifier
• host device has unique 32bit 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 2-9
App-layer protocol defines
• types of messages
exchanged,
– e.g., request, response
• message syntax:
– what fields in messages &
how fields are delineated
• message semantics
public-domain protocols:
• defined in RFCs
• allows for interoperability
• e.g., HTTP, SMTP
proprietary protocols:
• e.g., Skype
– meaning of information in
fields
• rules for when and how
processes send & respond
to messages
Application 2-10
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
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, …
Timing
• some apps (e.g., Internet
telephony, interactive
games) require low delay
to be “effective”
Application 2-11
Transport service requirements of common apps
Data loss
Throughput
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
no loss
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
instant messaging
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Application
yes, few secs
yes, 100’s msec
yes and no
Application 2-12
Internet transport protocols services
TCP service:
UDP 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
• 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 2-13
Cookies (continued)
aside
what cookies can bring:
• authorization
• shopping carts
• recommendations
• user session state (Web email)
cookies and privacy:
 cookies permit sites to learn a lot
about you
 you may supply name and e-mail
to sites
how to keep “state”:
 protocol endpoints: maintain state at sender/receiver
over multiple transactions
 cookies: http messages carry state
Application 2-14
Web caches (proxy server)
Goal: satisfy client request without involving origin server
• user sets browser: Web
accesses via cache
• browser sends all HTTP
requests to cache
– object in cache: cache
returns object
– else cache requests object
from origin server, then
returns object to client
origin
server
Proxy
server
client
client
origin
server
Application 2-15
More about Web caching
• cache acts as both client
and server
• typically cache is installed
by ISP (university, company,
residential ISP)
why Web caching?
• reduce response time for
client request
• reduce traffic on an
institution’s access link.
• Internet dense with caches:
enables “poor” content
providers to effectively
deliver content (but so does
P2P file sharing)
Application 2-16
Caching example
origin
servers
assumptions
• average object size = 100,000 bits
• avg. request rate from institution’s
browsers to origin servers = 15/sec
• delay from institutional router to
any origin server and back to
router = 2 sec
consequences
•
•
•
utilization on LAN = 15%
utilization on access link = 100%
total delay = Internet delay + access
delay + LAN delay
= 2 sec + minutes + milliseconds
public
Internet
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Application 2-17
Caching example (cont)
possible solution
• increase bandwidth of access link
to, say, 10 Mbps
consequence
• utilization on LAN = 15%
• utilization on access link = 15%
• Total delay = Internet delay +
access delay + LAN delay
= 2 sec + msecs + msecs
• often a costly upgrade
origin
servers
public
Internet
10 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Application 2-18
Caching example (cont)
origin
servers
possible solution:
• install cache
consequence
public
Internet
• suppose hit rate is 0.4
– 40% requests will be satisfied
almost immediately
– 60% requests satisfied by origin
server
• utilization of access link reduced
to 60%, resulting in negligible
delays (say 10 msec)
• total avg delay = Internet delay
+ access delay + LAN delay =
.6*(2.01) secs + .4*milliseconds
< 1.4 secs
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Application 2-19
FTP: the file transfer protocol
FTP
user
interface
user
at host
FTP
client
local file
system
file transfer
FTP
server
remote file
system
• transfer file to/from remote host
• client/server model
– client: side that initiates transfer (either to/from remote)
– server: remote host
• ftp: RFC 959
• ftp server: port 21
Application 2-20
FTP: separate control, data connections
• FTP client contacts FTP server at
port 21, TCP is transport protocol
• client authorized over control
connection
• client browses remote directory by
sending commands over control
connection.
• when server receives file transfer
command, server opens 2nd TCP
connection (for file) to client
• after transferring one file, server
closes data connection.
TCP control connection,
server port 21
FTP
client



TCP data connection,
server port 20
FTP
server
server opens another TCP data
connection to transfer another
file.
control connection: “out of band”
FTP server maintains “state”:
current directory, earlier
authentication
Application 2-21
outgoing
message queue
Electronic Mail
user mailbox
Three major components:
• user agents
• mail servers
• simple mail transfer protocol:
SMTP
User Agent
• a.k.a. “mail reader”
• composing, editing, reading mail
messages
• e.g., Outlook, elm, Mozilla
Thunderbird, iPhone mail client
• outgoing, incoming messages
stored on server
user
agent
mail
server
user
agent
SMTP
SMTP
mail
server
user
agent
SMTP
user
agent
mail
server
user
agent
user
agent
Application 2-22
Electronic Mail: mail servers
user
agent
Mail Servers
• mailbox contains incoming
messages for user
• message queue of outgoing (to
be sent) mail messages
• SMTP protocol between mail
servers to send email messages
– client: sending mail server
– “server”: receiving mail
server
mail
server
user
agent
SMTP
SMTP
mail
server
user
agent
SMTP
user
agent
mail
server
user
agent
user
agent
Application 2-23
DNS: Domain Name System
people: many identifiers:
– SSN, name, passport #
Internet hosts, routers:
– IP address (32 bit) - used
for addressing datagrams
– “name”, e.g.,
www.yahoo.com - used by
humans
Q: map between IP address
and name, and vice versa
?
Domain Name System:
• distributed database implemented
in hierarchy of many name servers
• application-layer protocol host,
routers, name servers to
communicate to resolve names
(address/name translation)
– note: core Internet function,
implemented as applicationlayer protocol
– complexity at network’s “edge”
Application 2-24
DNS
DNS services
• hostname to IP address
translation
• host aliasing
– Canonical, alias names
• mail server aliasing
• load distribution
– replicated Web servers: set
of IP addresses for one
canonical name
Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance
doesn’t scale!
Application 2-25
Formulas
•
•
•
•
•
Dcs = max { NR/Us , F/dmin }
N = Number of clients
Us= Upload capacity of server
F = file size
Dmin = minimum distribution time.
• Dp2p = max { F / Us , F / dmin, NF / us + i=1 to n ui}
•
Problem 1
• P1: Consider distributing a file of F=10 Gbits to N
peers. The server has an upload rate of Us = 20
Mbps and each peer has a download rate of di = 1
Mbps and upload rate of u. for N = 10, 100 and
1000 and u = 200 Kbps, 600 kbps and 1 Mbs,
prepare a chart giving the minimum distribution
time for each of the combination of N and u for
both client server distribution and P2P distribution.