1 Overview of Computer Networking Goal of this lecture: get context, overview, “feel” of networking more depth, detail later in course approach: descriptive use Internet as example Contents: what’s the Internet what’s a protocol? network edge network core access net, physical media performance: loss, delay protocol layers, service models backbones, NAPs, ISPs Internet history ATM network
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1
Overview of Computer Networking
Goal of this lecture: get context,
overview, “feel” of networking
more depth, detail later in course
approach: descriptive use Internet as
example
Contents: what’s the Internet what’s a protocol? network edge network core access net, physical media performance: loss, delay protocol layers, service
models backbones, NAPs, ISPs Internet history ATM network
2
What’s the Internet: “nuts and bolts” view
millions of connected computing devices: hosts, end-systems pc’s workstations, servers PDA’s phones, toasters
running network apps communication links
fiber, copper, radio, satellite
routers: forward packets (chunks) of data thru network
local ISP
enterprisenetwork
regional ISP
router workstation
servermobile
3
What’s the Internet: “nuts and bolts” view protocols: control sending,
receiving of msgs e.g., TCP, IP, HTTP, FTP
Internet: “network of networks” loosely hierarchical public Internet versus
private intranet
Internet standards RFC: Request for Comments IETF: Internet Engineering
Task Force
local ISP
enterprisenetwork
regional ISP
router workstation
servermobile
4
What’s the Internet: a service view
communication infrastructure enables distributed applications: WWW, email, games, e-
commerce, database., voting
What else?
communication services provided: connectionless connection-oriented
cyberspace [Gibson]:“a consensual hallucination
experienced daily by billions of operators, in every nation, ...."
5
Some useful Web sites Internet Engineering Task Force (IETF)
http://www.ietf.org World Wide Web Consortium (W3C)
http://www.w3c.org Association for Computing Machinery (ACM)
http://www.acm.org”Special interest group in Data Communications”
Institute of Electrical and Electronics Engineers (IEEE)http://www.comsoc.org ”Communications Society” http://www.computer.org/ ”Computer Society”
Connected: An Internet Encyclopediahttp://www.FreeSoft.org/CIE/index.htm
6
What’s a Protocol?human protocols: “hello” – “hello” “could you tell me
the time please?”
… specific msgs sent… specific actions
taken when msgs received, or other events
network protocols: For machines rather
than humans all communication
activities in Internet governed by protocols
protocols define msg format,order of msgs sent and received among network entities, and actions taken onmsg transmission & receipt
7
What’s a protocol?a human protocol and a computer network protocol:
Hi
Hi
Could you tell me what time it is?
2:00
TCP connection req.
TCP connectionreply.
Get http://dpnm.postech.ac.kr/cs702/index.html
<file>time
8
A closer look at network structure: network edge:
applications and hosts network core:
routers network of networks
access networks, physical media: communication links
9
The network edge: end systems (hosts):
run application programs e.g., WWW, email at “edge of network”
LA/R ~ 0: average queueing delay small LA/R -> 1: delays become large LA/R > 1: more “work” arriving than can
be serviced, average delay infinite!
34
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?
35
Organization of air travel
a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
36
Organization of air travel: a different view
Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
37
Layered air travel: services
Counter-to-counter delivery of person+bags
baggage-check-to-baggage-claim delivery
people transfer: loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
38
Distributed implementation of layer functionality
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Dep
art
ing
air
port
arr
ivin
g
air
port
intermediate air traffic sites
airplane routing airplane routing
39
Let’s talk about Network Protocols Organized into layers to reduce complexity Each protocol belongs to a layer “n” Layer n protocol is distributed among end systems and packet switches
communicating by exchanging messages “n-PDU” Put together, the protocols of various layers are called “protocol stack”
Layer nLayer n
Layer nLayer n-1
Layer nLayer n
Layer nLayer n-1
HOST A HOST Bn-PDU
n-PDU n-PDU(n-1)-PDU
Layer n is said to rely on layer n-1 to deliver its n-PDUs Layer n-1 is said to offer “services” to layer n, e.g., guaranteeing a timely delivery without errors, or with no assurances.
40
Example of a 4 layers Protocol Stack
source destination
M
M1
M1
M1
H3
H3H2
H3H2H1
Originalmessage
M2
M2
M2
H3
H3H2
H3H2H1
M
M1
M1
M1
H3
H3H2
H3H2H1
M2
M2
M2
H3
H3H2
H3H2H1
3-PDU
2-PDU
1-PDU
41
Interoperation between layers Interoperation between layers achieved through
standard interfaces Each layer may perform one or more generic tasks:
Error Control, to make logical channel between 2 layers reliable
Flow Control, to avoid overwhelming a slower peer with PDUs Segmentation, to divide large data chunks into smaller pieces
at transmitting side Reassembly, to reassemble the smaller pieces into original
large chunk at receiving side Multiplexing, to allow several higher-level sessions to share a
single lower-level connection Connection setup, to provide handshaking with a peer
42
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?
43
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
44
Layering: logical communication
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
networklink
physical
Each layer: distributed “entities”
implement layer functions at each node
entities perform actions, exchange messages with peers
45
Layering: logical communication
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
networklink
physical
data
dataE.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
transport
transport
ack
46
Layering: physical communication
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
networklink
physical
data
data
47
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
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
source destination
M
M
M
M
Ht
HtHn
HtHnHl
M
M
M
M
Ht
HtHn
HtHnHl
message
segment
datagram
frame
48
Internet structure: network of networks
roughly hierarchical national/international
backbone providers (NBPs) e.g. BBN/GTE, Sprint,
AT&T, UUNet, KT interconnect (peer) with
each other privately, or at public Network Access Point (NAPs)
regional ISPs connect into NBPs
local ISP, company connect into regional ISPs
NBP A
NBP B
NAP NAP
regional ISP
regional ISP
localISP
localISP
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National Backbone Providere.g. BBN/GTE US backbone network
50
Internet History
1961: Kleinrock - queueing theory shows effectiveness of packet-switching
1964: Baran - packet-switching in military nets
1967: ARPAnet conceived by Advanced Research Projects Agency
1969: first ARPAnet node operational
1972: ARPAnet
demonstrated publicly
NCP (Network Control Protocol) first host-host protocol
first e-mail program
15 nodes in ARPAnet
1961-1972: Early packet-switching principles
51
Internet History
1970: ALOHAnet satellite network in Hawaii
1973: Metcalfe’s PhD thesis proposes Ethernet
1974: Cerf and Kahn - architecture for interconnecting networks