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CS 471G Wednesday, 16 January 2013
Introduction 2-1
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Announcements, Recap
v No Class Monday! (MLK Holiday) v No Calvert Office Hours
today v Monday’s attendance quiz recap:
§ What is packet switching? • Sharing transmission channels by
multiplexing packets • What UPS does, only with data
§ Name two kinds of access network • Cable, DSL, [WiFi, WiMax,
Cellular, Ethernet, ...]
§ How long to transmit a 1Mbit packet from A to B via one
switch, where links are 1Mbps
• First link: 1M bits/1Mbits/sec = 1 sec • Second link: ditto
Total: 2 sec
Introduction 2-2
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Introduction
Packet Switching: queueing delay, loss
A
B
C R = 100 Mb/s
R = 1.5 Mb/s D
E queue of packets waiting for output link
1-3
queuing and loss: v If arrival rate (in bits) to link exceeds
transmission rate of
link for a period of time: § packets will queue, wait to be
transmitted on link § packets can be dropped (lost) if memory
(buffer) fills up
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Network Layer 4-4
Two key network-core functions forwarding: move packets from
router’s input to appropriate router output
routing: determines source-destination route taken by
packets
§ routing algorithms
routing algorithm
local forwarding table header value output link
0100 0101 0111 1001
3 2 2 1
1
2 3
dest address in arriving packet’s header
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Introduction
Alternative core: circuit switching end-end resources
allocated
to, reserved for “call” between source & dest:
v In diagram, each link has four circuits. § call gets 2nd
circuit in top
link and 1st circuit in right link.
v dedicated resources: no sharing § circuit-like
(guaranteed)
performance v circuit segment idle if not used
by call (no sharing) v Commonly used in traditional
telephone networks 1-5
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Introduction
Circuit switching: FDM versus TDM
FDM
frequency
time TDM
frequency
time
4 users Example:
1-6
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Introduction
Packet switching versus circuit switching
example: § 1 Mb/s link § each user:
• 100 kb/s when “active” • active 10% of time
v circuit-switching: § 10 users
v packet switching: § with 35 users, probability >
10 active at same time is less than .0004 *
packet switching allows more users to use network!
N users
1 Mbps link
Q: how did we get value 0.0004?
Q: what happens if > 35 users ?
…..
1-7 * Check out the online interactive exercises for more
examples
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Introduction
v great for bursty data § resource sharing § simpler, no call
setup
v excessive congestion possible: packet delay and loss §
protocols needed for reliable data transfer, congestion
control v Q: How to provide circuit-like behavior?
§ bandwidth guarantees needed for audio/video apps § still an
unsolved problem (chapter 7)
is packet switching a “slam dunk winner?”
Q: human analogies of reserved resources (circuit switching)
versus on-demand allocation (packet-switching)?
Packet switching versus circuit switching
1-8
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Internet structure: network of networks
v End systems connect to Internet via access ISPs (Internet
Service Providers) § Residential, company and university ISPs
v Access ISPs in turn must be interconnected. v So that any
two hosts can send packets to each other
v Resulting network of networks is very complex v Evolution
was driven by economics and national policies
v Let’s take a stepwise approach to describe current Internet
structure
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Internet structure: network of networks Question: given millions
of access ISPs, how to connect them together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
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Internet structure: network of networks Option: connect each
access ISP to every other access ISP?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
…
…
connecting each access ISP to each other directly doesn’t
scale: O(N2) connections.
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
Option: connect each access ISP to a global transit ISP?
Customer and provider ISPs have economic agreement.
global ISP
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
But if one global ISP is viable business, there will be
competitors ….
ISP B
ISP A
ISP C
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
But if one global ISP is viable business, there will be
competitors …. which must be interconnected
ISP B
ISP A
ISP C
IXP
IXP
peering link
Internet exchange point
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
… and regional networks may arise to connect access nets to
ISPS
ISP B
ISP A
ISP C
IXP
IXP
regional net
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
… and content providers (e.g., Google, Microsoft, Akamai ) may
run their own network, to bring services, content close to end
users
ISP B
ISP A
ISP B
IXP
IXP
regional net
Content provider network
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Introduction
Internet structure: network of networks
v at center: small # of well-connected large networks §
“tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT),
national &
international coverage § content provider network (e.g,
Google): private network that connects
it data centers to Internet, often bypassing tier-1, regional
ISPs
1-17
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
Regional ISP Regional ISP
IXP IXP
Tier 1 ISP Tier 1 ISP Google
IXP
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Introduction
Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
…
………
POP: point-of-presence
1-18
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Traceroute to cs.utexas.edu 1 128.163.146.2 (128.163.146.2)
0.986 ms 0.280 ms 0.280 ms 2 pks2-04-pop2-new.net.uky.edu
(128.163.221.33) 0.447 ms 0.353 ms 0.330 ms 3
uk-i2-kyron.net.uky.edu (128.163.221.90) 0.487 ms 0.505 ms 0.464 ms
4 i2-v501.lou.ky-ron.net (216.249.136.197) 3.124 ms 2.974 ms 2.839
ms 5 i2-v501.i2.ky-ron.net (216.249.136.198) 13.255 ms 15.375 ms
13.299 ms 6 xe-1-0-0.0.rtr.hous.net.internet2.edu (64.57.28.112)
36.714 ms 36.636 ms 36.670 ms 7
rt1-hardy-hstn-xe-0-1-0-3018.tx-learn.net (74.200.187.6) 37.622 ms
37.268 ms 79.213 ms 8 tx-bb-i2-hstn.tx-learn.net (74.200.187.26)
37.293 ms 37.384 ms 37.278 ms 9
aust-utnoc-core-ge-5-0-0-706.tx-bb.net (192.88.12.50) 40.769 ms
40.666 ms 40.772 ms 10 192.88.12.26 (192.88.12.26) 40.740 ms 40.702
ms 40.690 ms 11 nocb10-n7k-e1-2-noca2.gw.utexas.edu (146.6.10.205)
41.108 ms 40.876 ms 40.768 ms 12 cs-nocb10-v690.gw.utexas.edu
(146.6.10.34) 41.017 ms 40.824 ms 40.846 ms 13
cs65k-cs45k-po1-p2p.aces.utexas.edu (128.83.37.66) 40.847 ms 40.988
ms 40.831 ms 14 cs.utexas.edu (128.83.120.11) 40.807 ms 40.786 ms
40.811 ms
Introduction 2-19
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Traceroute to my house... 1 dazzler.netlab.uky.edu
(128.163.140.1) 1.052 ms 1.094 ms 1.142 ms 2 128.163.142.33
(128.163.142.33) 0.407 ms 0.455 ms 0.524 ms 3
pks2-04-pop2-new.net.uky.edu (128.163.221.33) 0.499 ms 0.616 ms
0.628 ms 4 128.163.221.142 (128.163.221.142) 0.576 ms 0.574 ms
0.554 ms 5 64.57.21.106 (64.57.21.106) 2.921 ms 2.890 ms 2.885 ms 6
64.57.21.105 (64.57.21.105) 26.591 ms 26.655 ms 26.609 ms 7
107.14.16.81 (107.14.16.81) 24.819 ms 24.984 ms 24.847 ms 8
ae-2-0.cr0.dca20.tbone.rr.com (66.109.6.164) 30.037 ms
107.14.19.134 (107.14.19.134) 27.542 ms 25.828 ms 9
ae15.tr00.clevohek.mwrtn.rr.com (66.109.6.71) 27.937 ms
ae14.tr00.clevohek.mwrtn.rr.com (107.14.19.15) 28.282 ms 27.282 ms
10 network-065-189-140-167.mwrtn.rr.com (65.189.140.167) 30.583 ms
30.002 ms
29.986 ms 11 74.128.11.10 (74.128.11.10) 29.734 ms 29.880 ms
29.713 ms 12 * * * 13 74-131-52-155.dhcp.insightbb.com
(74.131.52.155) 42.942 ms 42.920 ms 42.912 ms
Introduction 2-20
