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Lecture-3 1
Administrative Things Textbook in the bookstore ? Lecture notes on the web ?? Any questions and suggestions so
far ??? …
Lecture-3 2
Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched
networks1.7 Protocol layers, service models1.8 History1.8 History
Lecture-3 3
A Review of Last Lecture network edge:
applications and hosts connection-oriented and connectionless services Internet protocols: ??? and ???
network core: routers, network of networks data switching: circuit switching and packet
switching comparison of the two switching approaches virtual circuit networks and datagram networks
access networks, physical media Dail-up, DSL, Cable modem, LAN, wireless LAN, 3G, Satellite Twisted pair copper, Coaxial cable, fiber optics, ratio
Lecture-3 4
Internet structure: network of networks
roughly hierarchical at center: “tier-1” ISPs
e.g., UUNet, BBN/Genuity, Sprint, AT&T national/international coverage
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
NAP
Tier-1 providers also interconnect at public network access points (NAPs)
Lecture-3 5
Internet structure: network of networks
“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISPTier-2 ISP
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider
Tier-2 ISPs also peer privately with each other, interconnect at NAP
Lecture-3 6
Internet structure: network of networks
“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISPTier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet
Lecture-3 7
A packet passes through many networks
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
NAP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISPTier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
Lecture-3 8
Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.2 Network edge1.3 Network core1.3 Network core1.4 Network access and physical media1.4 Network access and physical media1.5 Internet structure and ISPs1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched
networks1.7 Protocol layers, service models1.8 History1.8 History
Lecture-3 9
How do loss and delay occur?packets queue in router buffers, wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers: arriving packets dropped (loss) if no free buffers
Lecture-3 10
Four sources of packet delay 1. nodal processing:
check bit errors determine output link
A
B
propagationtransmission
nodalprocessing queueing
2. queueing time waiting at output
link for transmission depends on congestion
level of router
Lecture-3 11
Delay in packet-switched networks3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into
link = L/R
4. Propagation delay: d = length of physical
link s = propagation speed in
medium (~2x108 m/sec) propagation delay = d/s
A
B
propagationtransmission
nodalprocessing queueing
Note: s and R are very different quantities!
Lecture-3 12
Caravan analogy
Cars “propagate” at 100 km/hr
Toll booth takes 12 sec to service a car (transmission time)
car~bit; caravan ~ packet Q: How long until caravan
is lined up before 2nd toll booth?
Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec
Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr
A: 62 minutes
toll booth
toll booth
ten-car caravan
100 km
100 km
Lecture-3 13
Caravan analogy (more)
Cars now “propagate” at 1000 km/hr
Toll booth now takes 1 min to service a car
Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?
Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.
1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!
toll booth
toll booth
ten-car caravan
100 km
100 km
Lecture-3 14
Nodal delay
dproc = processing delay typically a few microsecs or less
dqueue = queuing delay depends on congestion
dtrans = transmission delay = L/R, significant for low-speed links
dprop = propagation delay a few microsecs to hundreds of msecs
proptransqueueprocnodal ddddd
Lecture-3 15
Queueing delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet
arrival rate
traffic intensity = La/R
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!
Lecture-3 16
“Real” Internet delays and routes What do “real” Internet delay & loss look like? Traceroute program: provides delay
measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path
towards destination router i will return packets to sender sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Lecture-3 17
“Real” Internet delays and routes
traceroute to www.sun.com (64.124.140.199), 30 hops max, 40 byte packets 1 137.99.15.129 (137.99.15.129) 5.902 ms 1.946 ms 4.148 ms 2 10.12.250.1 (10.12.250.1) 0.682 ms 0.618 ms 0.578 ms 3 137.99.24.193 (137.99.24.193) 0.891 ms 0.871 ms 0.852 ms 4 uconn-edge.net.uconn.edu (137.99.22.41) 1.478 ms 1.866 ms 2.736 ms 5 159.247.232.129 (159.247.232.129) 2.162 ms 1.983 ms 2.068 ms 6 65.115.97.193 (65.115.97.193) 6.237 ms 6.785 ms 6.199 ms 7 205.171.28.29 (205.171.28.29) 6.399 ms 8.517 ms 6.036 ms 8 205.171.8.19 (205.171.8.19) 11.142 ms 10.784 ms 11.431 ms 9 205.171.30.14 (205.171.30.14) 11.137 ms 11.752 ms 10.963 ms10 pos4-1.pr1.lga1.us.mfnx.net (208.185.156.25) 13.200 ms 11.095 ms 11.963 ms11 so-3-0-0.cr1.lga1.us.mfnx.net (208.185.0.237) 11.542 ms 12.008 ms 12.055 ms12 so-1-0-0.cr1.iad1.us.mfnx.net (208.184.233.61) 15.587 ms 16.120 ms 16.459 ms13 so-1-0-0.cr1.dca2.us.mfnx.net (208.184.233.125) 17.613 ms 17.086 ms 16.744 ms14 so-3-0-0.mpr3.sjc2.us.mfnx.net (208.184.233.133) 80.789 ms 82.137 ms 82.055 ms15 so-0-0-0.cr1.sjc3.us.mfnx.net (208.185.175.153) 82.030 ms 82.679 ms 82.160 ms16 pos0-0.er2a.sjc3.us.mfnx.net (208.185.175.190) 81.658 ms 81.977 ms 80.668 ms17 alt1-1.java.sun.com (64.124.128.212) 83.412 ms 82.077 ms 87.823 ms
traceroute: fester.engr.uconn.edu to www.sun.comThree delay measurements from Fester.engr.uconn.edu to www.sun.com
Lecture-3 18
Packet loss queue (aka buffer) preceding link in
buffer has finite capacity when packet arrives to full queue,
packet is dropped (aka lost) lost packet may be retransmitted by
previous node, by source end system, or not retransmitted at all
Lecture-3 19
Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.2 Network edge1.3 Network core1.3 Network core1.4 Network access and physical media1.4 Network access and physical media1.5 Internet structure and ISPs1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched
networks1.7 Protocol layers, service models1.8 History1.8 History
Lecture-3 20
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?
Lecture-3 21
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 routingairplane routing
Lecture-3 22
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 routingairplane routing
Lecture-3 23
Layered air travel: servicesCounter-to-counter delivery of person+bags
baggage-claim-to-baggage-claim delivery
people transfer: loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
Lecture-3 24
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
Depa
rting
ai
rpor
t
arriv
ing
airp
ort
intermediate air traffic sitesairplane routing airplane routing
Lecture-3 25
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?
Lecture-3 26
Internet protocol stack application: supporting network
applications FTP, SMTP, STTP
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
Lecture-3 27
Layering: logical communication applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical application
transportnetwork
linkphysical
applicationtransportnetwork
linkphysical
networklink
physical
Each layer: distributed “entities”
implement layer functions at each node
entities perform actions, exchange messages with peers
Lecture-3 28
Layering: logical communication applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical application
transportnetwork
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
Lecture-3 29
Layering: physical communication applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical application
transportnetwork
linkphysical
applicationtransportnetwork
linkphysical
networklink
physical
data
data
Lecture-3 30
Protocol layering and dataEach 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 destinationMMMM
HtHtHnHtHnHl
MMMM
HtHtHnHtHnHl
messagesegmentdatagramframe
Lecture-3 31
Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.2 Network edge1.3 Network core1.3 Network core1.4 Network access and physical media1.4 Network access and physical media1.5 ISPs and Internet backbones1.5 ISPs and Internet backbones1.6 Delay & loss in packet-switched
networks1.7 Protocol layers, service models1.8 History1.8 History
Lecture-3 32
Homeworks Reading assignments
Chapter 1: a good overview of computer networks
Written assignments Part of 1st written assignment Chapter 1: Problem #5, 10, 16 (a, b, c) More (& due date) will be posted on the web
Try traceroute program again and do some delay analysis choose whatever destination you like