1: introduction last modified: 10/6/2015 8:30:39 pm
TRANSCRIPT
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1: Introduction
Last Modified: 04/21/23 04:39 AM
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Why study computer networks? They are engineering marvels!
Scalability, layered protocols, lots of subtleties- worthy of study
They are all around us! Understanding the nuts and bolts behind a
technology you use every day is exciting They are changing the world!
Profound societal changes taking place in our lifetime
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How much do you know already?
How much do you know about what happens when you use networking applications?
What do you hope to learn in this class? Write out what happens when you fetch
a web page in as much technical detail as you can
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Goals of this class
Understand both basic computer networking concepts and their instantiation in the current Internet
Question why the current Internet is the way it is Appreciate good Understand limitations and consider
solutions Gain practical skills (network analysis,
network programming)
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Network Trace Analysis?
Network Trace Analysis - what happens on our local network when use a network application?
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What can you tell from local network traces?
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Example: Web browsing
Trace a common network operation like web browsing
URL like http://www.google.comspecifies - protocol (http) - location (www.google.com)
What network traffic results? Request page Send page What else? Lots….
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DNS traffic
www.google.com is the name of a computer (and, implicitly, of a file in that computer)
To find the address, the application uses a hierarchical directory service called theDomain Name System to translate human readable names to IP addresses
local
com
host
www.google..com?
www.google.com?IP = a.b.c.d
IP = a.b.c.d
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How do you get human readable names?
How do you get IP addresses?
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TCP Connection Establishment The protocol (http) sets up a connection
(another protocol, tcp) between the host and www.google.com to transfer the page
SYN, SYNACK, ACK
Host www.google.com
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HTTP request/response
GET /foo.html HTTP/1.1 Server responds sending page
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TCP Connection
Each HTTP request/response can be made up of many TCP segments
Stream of packets regulated and controlled by both ends: retransmission of
erroneous or missing bytes Pacing, sender not
overwhelming the receiver (flow control)
Pacing, Sender not overwhelming the network (congestion control)
The end hosts worry about errors and pacing: Destination sends ACKs Source checks losses
Host A Host B
Seq=42, ACK=79, data = ‘C’
Seq=79, ACK=43, data = ‘C’
Seq=43, ACK=80
Usertypes
‘C’
host ACKsreceipt
of echoed‘C’
host ACKsreceipt of
‘C’, echoesback ‘C’
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2: Application Layer 13
QUICK LOOK AHEAD: TCP vs UDP
TCP service: connection-oriented: setup
required between client, server
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when nework overloaded
does not providing: 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
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IP Datagrams
Datagrams labeled with where they are going and where they are from – no directions
Source IP address Destination IP address
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Ethernet Frames
Data sent over a physical medium That equipment is not aware of the meaning
of the bits
01011...011...110
Transm itte r P hysica l M edium R ece iver
01011...011...110
O ptica lC opperW ire less
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Path through the Internet
Network trace analysis very useful but it gave us very little hint that between our machine and the remote machine there are many hops
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Traceroute/tracert
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Graphical Traceroute (plus DNS information )
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How are paths established?
Packets flows from end to end across many links
Routers receive packet and try to forward one step closer to destination (routing)
The packets contain all information necessary to indicate destination (addressing) and are handled individually Example: Caravan of cars on
a road trip Packets may take different
paths
C
HOST: B
GOOGLE.COM: A
A | B | # , CRC | bytes
B: toC
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Internet Topology?
Traceroute gave us one slice through the Internet
What does that tell us about the routes in the entire Internet? The full Internet map?
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Internet Map
What does the Internet really look like? That is a actually a hard question to answer Internet Atlas Project
• http://www.caida.org/projects/internetatlas/• Techniques, software, and protocols for mapping
the Internet, focusing on Internet topology, performance, workload, and routing data
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CAIDA: Layout showing Major ISPs
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CAIDA’s skitter plotHighly connected
Few connections
Location (longitude)
Skitter data
16 monitors probing approximately 400,000 destinations
626,773 IP addresses
1,007.723 IP links
48,302 (52%) of globally routable network prefixes
Europe
North America
Asia
Top 15 ASes are in North America (14 in US, 1 in Canada)Many links US to Asia and Europe; few direct Asia/Europe Links
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Internet growth and change
Maps not even a complete “snapshot” Even if they were, what about change over
time? Growth trends (years)? Cycles through months/weeks/days? Failures? Stability?
What about summarizing the dynamic Internet?
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The Internet around 1990
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The Internet in 1997 (Post NSFNET)
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New clients, mobile networks
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wirelesslaptop
cellularhandheld
wiredlinks
accesspoints
Introduction 1-
27
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Internet Domain SurveyNumber of Internet Hosts
"Source: Internet Software Consortium (http://www.isc.org/)".
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Periodic Cycles
Cycles of a day People come to work in one part of the
globe, go home in another Week
Weekends vs weekdays Holidays …
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Stability?
Despite any growth trends or periodic cycle, we also never have the Internet as a whole
Machines disconnecting/connecting Stability of routes? Viruses? Attacks?
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Internet principles?
Besides the details of protocols, topology and growth trends, are there fundamental “timeless” principles of the Internet?
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Packet Switching
We’ve already seen this Packets indicate their destination No predetermined path for a packet to
take Each intermediate note routes the
packet closer to its destination
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Protocol
If two entities are going to communicate, they must agree on the expected order and meaning of messages they exchange. Hi …Hi…Got the time?….two oclock
SUCCESSFUL PROTOCOL EXCHANGE Hi…Don’t bother meXX ABORTED
PROTOCOL Allo…Hello..Quelle heuere a’til …..XX<blank
stare> PROTOCOL MISMATCH
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Protocol
A protocol defines the format and the order of messages exchanged between communicating entities as well as the actions taken on the receipt or the transmission of a message.
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Layered Architectures
Human beings are able to handle lots of complexity in their protocol processing. Ambiguously defined protocols Many protocols all at once
How do computers manage complex protocol processing? Specify well defined protocols to enact. Decompose complicated jobs into layers
that each have a well defined task
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Layered Architectures
Break-up design problem intro smaller, more manageable problems.
Modular design: easy to extend/modify. Difficult to implement (careful with
interaction of layers for efficiency).
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Protocol stack
e-mail client
TCP server
IP server
ethernetdriver/card
user X
SMTP
TCP
IP
e-mail server
TCP server
IP server
ethernetdriver/card
user Y
IEEE 802.3 standard
electric signals
English
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Protocol encapsulation
e-mail client
TCP server
IP server
ethernetdriver/card
user X
e-mail server
TCP server
IP server
ethernetdriver/card
user Y“Hello”
“Hello”
“Hello”
“Hello”
“Hello”
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Physical Mail
Secretary X
Mail Room X
Postman X
Post office X
CEO X
Secretary Y
Mail Room Y
Postman Y
Post office Y
CEO Y“Lunch?”
“Lunch?”
“Lunch?”
“Lunch?”
“Lunch?”
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Protocol interfaces
e-mail client
TCP server
IP server
ethernetdriver/card
user X
e-mail server
TCP server
IP server
ethernetdriver/card
user Y
s = open_socket();socket_write(s, buffer);…
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Protocol stack:packet forwarding
HTTP
TCP
IP
ethernet
Host A
IP
ethernet
Router R
link
HTTP
TCP
IP
ethernet
Router W
Host B
IP
ethernetlink
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Internet future?
If we could figure out what the Internet is, where is the Internet going?
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How is Internet “governed”?
“We reject kings, presidents, and voting. We believe in rough consensus and running code”
Internet Engineering Task Force (IETF) Working groups Internet Engineering Steering Group
(IESG) Internet Assigned Numbers Authority
(IANA)
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Internet Standards Process
Internet Draft - anyone RFC - at discretion of RFC Editor Internet Standard Maturity Levels
Proposed - IESG Draft - 2 independent, implementations Standard
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Other forces
Proprietary technologies? Government funding and direction?
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Problems To Be Solved
Security? Vulnerabilities to attack? Reliability? Enough for mission critical
apps? Quality of service? Streaming media? Exhaustion of resources? IP address
space? Spam mail? Social problems? Equal access? Privacy?
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Internet?
Quick tour of the Internet Introduced more questions that it
answered? Hopefully Rest of semester is a detailed top-down tour
of the Internet
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Outtakes
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Online Resources
ISOC ACM CAIDA Internet History W3C
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Air travel
Ticket (purchase)
Baggage (check)
Gates (load)
Runway (take off)
Passenger Origin
Ticket (complain)
Baggage (claim)
Gates (unload)
Runway (landing)
Passenger Destination
Airplane routing
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A small Internet
A
V
R
B
W
r1,e1r2,e2
r3
a,e3
w,e5 b,e4Scenario:A wants to send datato B.
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Distributed Control
Where ever possible decompose the problem
Examples: No one central name to IP address data
base - Domain Name System No one global routing table - Hierarchical
network of networks – handle routing within small autonomous systems
Essential to Scalability
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Open System
Basic Internet protocols are published as open standards
Standards freely and readily available Ideal candidate for study
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