chapter 2, slide: 1 ece/cs 372 – introduction to computer networks lecture 5 announcements: r lab1...
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Chapter 2, slide: 1
ECE/CS 372 – introduction to computer networks
Lecture 5
Announcements: Lab1 is due today
Lab2 is posted and is due next Tuesday
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 2, slide: 2
Chapter 1: recapBy now, you should know:
the Internet and its components
circuit-switching networks vs. packet-switching networks
different network access technologies
the three Tiers 1, 2, and 3
layered architecture of networks
types of delays and throughput analysis
Chapter 2, slide: 3
Chapter 2: Application LayerOur goals: aspects of network
application protocols transport-layer
service models client-server
paradigm peer-to-peer
paradigm
learn about protocols by examining popular application-level protocol: HTTP
Chapter 2, slide: 4
Some network apps
e-mail web instant messaging remote login P2P file sharing multi-user network
games streaming stored
video clips
voice over IP real-time video
conferencing grid computing
Chapter 2, slide: 5
Creating a network app
write programs that run on (different) end
systems communicate over
network e.g., web server software
communicates with browser software
little software written for devices in network core network core devices do
not run user applications
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Chapter 2, slide: 6
Chapter 2: Application layer
Principles of network applications app architectures app requirements
Web and HTTP
P2P file sharing
Chapter 2, slide: 7
Application architectures
There are 3 types of architectures:
Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P
Chapter 2, slide: 8
Client-server architectureserver:
always-on fixed/known IP address serves many clients at same time
clients: communicate with server only may be intermittently connected may have dynamic IP addresses do not communicate directly with
each other
E.g., of client-server archit.: Google, Amazon, MySpace,
YouTube,
client/server
Chapter 2, slide: 9
Pure P2P architecture no always-on server arbitrary end systems
directly communicate peers are intermittently
connected and change IP addresses
example: BitTorrent
Pros and cons: scalable and distributive difficult to manage not secure
peer-peer
Chapter 2, slide: 10
Hybrid of client-server and P2PSkype
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 location
• user registers its IP address with central server when it comes online
• user contacts central server to find IP addresses of buddies
Chapter 2, slide: 11
Processes communicating
Process: is program running within a host.
processes in same host communicate using inter-process communication (managed 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
Chapter 2, slide: 12
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
controlledby OS
process
TCP withbuffers,variables
socket
host orserver
process
TCP withbuffers,variables
socket
host orserver
Internet
controlled byapp developer
App Program Interface (API): (1) choice of transport protocol; (2) ability to fix a few parameters
Chapter 2, slide: 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 to identify the process?
identifier consists of: IP address (host) port numbers (process)
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…
A: No, many processes can be running on same host
Chapter 2, slide: 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, SMTPProprietary protocols: e.g., Skype
Question: why do we need an “App-layer protocol” ?
Chapter 2, slide: 15
What transport service does an app need?Data loss/reliability 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”
Bandwidth some apps (e.g.,
multimedia) require minimum amount of bandwidth to be “effective”
other apps (“elastic apps”) make use of whatever bandwidth they get
Security what about it !!!
Chapter 2, slide: 16
Transport service requirements of common apps
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive gamesinstant messaging
Data loss
no lossno lossno lossloss-tolerant
loss-tolerantloss-tolerantno loss
Bandwidth
elasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps upelastic
Time Sensitive
nononoyes, 100’s msec
yes, few secsyes, 100’s msecyes and no
Chapter 2, slide: 17
What services do Internet transport protocols provide?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 bandwidth guarantees
UDP service: unreliable data transfer
between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee
Q: why bother? Why is there a UDP?
Chapter 2, slide: 18
Internet apps: application, transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
Internet telephony
Applicationlayer protocol
SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]proprietary(e.g. RealNetworks)proprietary(e.g., Vonage, Skype)
Underlyingtransport protocol
TCPTCPTCPTCPTCP or UDP
typically UDP
Chapter 2, slide: 19
Chapter 2: Application layer
Principles of network applications app architectures app requirements
Web and HTTP
P2P file sharing
Chapter 2, slide: 20
Web and HTTPFirst some terminologies:
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 Example URL (Uniform Resource Locator):
www.someschool.edu/someDept/pic.gif
host name path name
Chapter 2, slide: 21
HTTP overview: app architecture
HTTP: hypertext transfer protocol
Web’s appl-layer protocol client/server model
client: browser that requests, receives, “displays” Web objects
server: Web server sends objects in response to requests
HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP response
Chapter 2, slide: 22
HTTP overview (continued)
Uses TCP: client initiates TCP
connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP 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
Chapter 2, slide: 23
HTTP connections
Nonpersistent HTTP At most one object is
sent over a TCP connection.
HTTP/1.0 uses nonpersistent HTTP
Persistent HTTP Multiple objects can
be sent over single TCP connection between client and server.
HTTP/1.1 uses persistent connections in default mode
Chapter 2, slide: 24
Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index
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.index
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)
Chapter 2, slide: 25
Nonpersistent HTTP (cont.)
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.
time
Chapter 2, slide: 26
Non-Persistent HTTP: Response timeDefinition of RTT (round trip
time): time to send a small packet to travel from client to server and back.
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 = 2RTT + transmit time
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
Chapter 2, slide: 27
Non-Persistent HTTP: issues
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
Nonpersistent HTTP: Name some issues?? requires 2 RTTs per
object
E.g., a 10-object page needs ~ 20 RTTs
Open/close TCP connection for each object => OS overhead
Any ideas for improvement?
Chapter 2, slide: 28
Persistent HTTP
Persistent HTTP server leaves
connection open after sending response
subsequent HTTP messages between same client/server sent over open connection
reduces response time
Persistent without pipelining:
client issues new request only when previous response has been received
one RTT for each referenced object
Persistent with pipelining: default in HTTP/1.1 client sends requests as
soon as it encounters a referenced object
as little as one RTT for all the referenced objects
Chapter 2, slide: 29
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
A HTTP request consists of: 1 basic html object 2 referenced JPEG objects Each object is of size = 106bits
RTT = 1 second Transmission rate = 1Mbps Consider transmission delay of
objects only
Question: how long it takes to receive the entire page:a) Non-persistent connectionb) Persistent without pipeliningc) Persistent with pipelining
Web and HTTP: Review Question
Chapter 2, slide: 30
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
A HTTP request consists of: 1 basic html object 2 referenced JPEG objects Each object is of size = 106bits
RTT = 1 second Transmission rate = 1Mbps Consider transmission delay of
objects only
Answer: (transmit time = 1 sec)a) 3+3+3=9 sec
(initiate + request + transmit) for each of all 3
b) 1+2+2+2=7 sec initiate + (request + transmit) for each of all 3
c) 1+2+3=6 sec initiate + (request + transmit for basic) + (one request for 2 + two transmits, one for each of the 2 objects)
Web and HTTP: Review Question
Chapter 2, slide: 31
ECE/CS 372 – introduction to computer networks
Lecture 6
Announcements: Lab2 is due next Tuesday
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 2, slide: 32
Web caches (or proxy server)
If page is needed, browser requests it from the Web cache
Q: what if object not in cache??
cache requests object from origin server, then returns object to client
Goal: satisfy client request without involving origin server
client
Proxyserver
client
HTTP request
HTTP response
HTTP request HTTP request
origin server
origin server
HTTP response HTTP response
Chapter 2, slide: 33
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.
Chapter 2, slide: 34
Caching example
Assumptions avg. object size =
0.1x106bits avg. request rate from
institution to origin servers = 10/sec
Internet delay = 2 sec
Consequences utilization on LAN = 10%
(LAN: local area network) utilization on access link =
100% total delay = Internet delay +
access delay + LAN delay = 2 sec + sec + milliseconds unacceptable delay!
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1 Mbps access link
institutionalcache
Chapter 2, slide: 35
Caching example (cont)
possible solution increase bandwidth of
access link to, say, 10 Mbps
consequence utilization on LAN = 10% utilization on access link =
10% Total delay = Internet delay +
access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade total delay still dominated by
Internet delay
originservers
public Internet
institutionalnetwork 10 Mbps LAN
10 Mbps access link
institutionalcache
Chapter 2, slide: 36
Caching example (cont)2nd possible sol: web cache suppose hit rate is 0.4 (typically, between 0.3 & 0.7)
consequence 40% requests will be satisfied
almost immediately and 60% requests satisfied by origin server
utilization of access link reduced by 40%, giving an access delay in the order of milliseconds; say 10 millisec
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1 Mbps access link
institutionalcache
Total delay = 0.4x(0.1) (LAN) + 0.6x(0.1+0.1+2) (LAN + access + Internet) = (about) 1.3
second
total avg delay reduced by about 40%
Chapter 2, slide: 37
Web cache (cont)
Advantages are obvious: Reduce response time Reduce internet traffic
Any problems with caches??
Local cache copies of web pages may not be up-to-date??
What do we do then?
Solution Upon receiving a web
request, a cache must consult origin server to check whether the requested page is up-to-date
Conditional GET method What: Sent by cache to origin
server: check page status
When:For each new request: client checks with cache
Chapter 2, slide: 38
Conditional GET
Goal: don’t send object if cache has up-to-date version
How: cache specifies date of cached copy in HTTP requestIf-modified-since: <date>
Server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified
cache server
HTTP request msgIf-modified-since: <date>
HTTP responseHTTP/1.0
304 Not Modified
object not
modified
HTTP request msgIf-modified-since: <date>
HTTP responseHTTP/1.0 200 OK
<data>
object modified
Chapter 2, slide: 39
Chapter 2: Application layer
Principles of network applications app architectures app requirements
Web and HTTP
P2P file sharing
Chapter 2, slide: 40
File sharing approaches
There are 2 approaches
Centralized: Client-server architecture
Distributed: P2P architecture (e.g., BitTorrent)
obtain listof peers
trading chunks
peer
server
server
peers
Alice
Bob
Chapter 2, slide: 41
File sharing: P2P vs. client-server architectures
Robustness to failure
Scalability
Security
Performance
Client-Server
Single point of failure
Not scalable
More secure
Bottleneck
P2P
Fault-tolerant
Scalable
Less secure
Better
Chapter 2, slide: 42
Comparing Client-Server, P2P architecturesQuestion : What is the file distribution time:
from one server to N hosts?
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
File, size F
us: server upload bandwidth
ui: client/peer i upload bandwidth
di: client/peer i download bandwidth
Chapter 2, slide: 43
Client-server: file distribution time
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
F server
sequentially sends N copies: NF/us time
client i takes F/di
time to download
= dcs > max { NF/us, F/min(di) }i
Time to distribute F to N clients using
client/server approach
increases linearly in N(for large N)
Chapter 2, slide: 44
Client-server: file distribution time
dcs = max { NF/us, F/min(di) }i
We now show that the distribution time is actually equal to max{NF/us, F/min(di) }
See board notes
uN NF bits must be downloaded- NF bits must be uploaded- Fastest possible upload rate (assuming all nodes sending file chunks to same peer) is:
us + ui
Chapter 2, slide: 45
P2P: file distribution time
us
u2d1 d2u1
dN
Server
Network (with abundant bandwidth)
F
server must send one copy: F/us time
client i takes F/di time to download
i=1,N
dP2P > max { F/us, F/min(di) , NF/(us + ui) }i=1,N
Chapter 2, slide: 46
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
Min
imum
Dis
trib
utio
n T
ime P2P
Client-Server
Comparing Client-server, P2P architectures
Chapter 2, slide: 47
Chapter 2: Summary
application architectures client-server, P2P
application service requirements: reliability, bandwidth, delay
Web and HTTP Non-Persistent, persistent, web
cache
Distribution time Client-server, P2P
We covered general concepts, like: