COSC 465: Networking

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COSC 465 Networking

• Week 1

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Applications

• P2P
• napster, gnutella, bit torrent
• email
• Web (HTTP)
• FTP
• Skype
• streaming video

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Hosts on the internet

• Computers
• mac / PC / unix
• Devices
• PDAs
• Vending machines
• Toasters
• Why is this possible (or even desirable)?

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Protocols

• Speakeasies in the 1920s
• banana pastry
• Hello / Hi / How are you? / Just fine
• Hello / Hi / How are you? / Bite me
• Answering the phone
• Hello
• (checks caller ID) Fred, quit calling me!s

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Switching (draw picture on the board)

• Circuit-switched network
• Dedicated connection
• Telephone
• Packet-switched network
• Break transmission into packets
• Internet
• Snail-mail

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Circuit-switched network

• Dedicated connection between 2 endpoints
• Telephone
• landlines
• Trains
• Overhead required to establish and maintain the
  dedicated resources for the connection

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Multiplexing on circuit-switched networks

• Necessary because we cant have dedicated wires
  between every possible pair of endpoints
• Frequency-Division Multiplexing (FDM)
• FM radio
• Bus lane / HOV lane
• Time-Division Multiplexing (TDM)
• Train tracks

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Inefficiencies of circuit-switched network?
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Inefficiencies of circuit-switched network?

• During a pause in telephone conversation
• Resources are still dedicated
• Establishing and maintaining dedicated
  connections requires complex software

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How long does it take

• 1,600,000 bit file
• 3.2 Mbps connection
• 16 slots TDM
• 1 second to establish the connection
• (figure out how long it takes)
• (now how long would it take if we didnt use TDM)

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Packet-switched network

• Break messages up into pieces
• Packets
• Send packets along links
• Store-and-forward
• Must receive entire packet before the first bit
  can be sent out
• Effects of packet size?

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Packet switched delays

• Store and forward delay
• L bits
• R bps
• L/R store-and-forward-delay
• Queuing delays
• routers need to buffer packets while links are
  busy
• If the buffer gets full while the link is
  saturated
• packets get dropped!
• Unpredictable

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Disadvantages of packet-switching

• variable and unpredictable end-to-end delays
• Telephones
• video-conferencing

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Comparison of circuit and packet switched networks

• 1 Mbps pipe
• 10 users
• each is active only 10 of time (0.10)
• Circuit-switching
• 100 kbps per user
• maximum of 10 users

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Haha! These look like future exam questions

• Very possible
• But the more important issue here is being
  comfortable doing back-of-the-envelope
  calculations to guesstimate stuff
• Very important skill for a scientist
• Helps prevent me from making outrageous claims

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Packet-switching

• 35 users
• 0.0004 chance of 11 or more being active at the
  same time
• Math involved in computing this
• If we get 11 or more users, performance degrades
  until we get back under 1 Mbps total usage

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Suppose one user generates a lot of data

• circuit switching
• they just have to wait
• packet switching
• if others are inactive, then they can use more
  bandwidth

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Statistical Multiplexing

• Taking advantage of how often people use the
  system
• Efficiently use the network
• Better tolerance for heavier bursts of data

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Take-home message here

• Packet-switching is the model predominantly used
  by the internet
• Important to understand the tradeoffs
• And that there are other ways of building a
  network

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Net Neutrality

• ISPs want to give preference to certain types of
  traffic
• higher-paying customers get preference for their
  traffic
• video conferencing
• virtual private network (VPN)

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Datagram vs Virtual Circuit (VC) networks

• VC networks maintain state in the routers
• Similar to circuit-switched networks
• X.25
• frame relay
• asynchronous transfer mode (ATM)
• Datagram networks forward packets based only on
  the addresses
• The internet

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Datagram networks are stateless inside the network

• Switches forward packets based on destination
  address only
• Analogy
• Snail mail through the post office
• Advantages?
• Disadvantages?

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Residential access

• Modem
• Does anyone remember these?
• DSL
• FDM of the telephone wire
• Reserve more space for downstream
• Cable modem
• hybrid fiber coaxial cable (HFC)
• also reserves more bandwidth for downstream
• broadcasts all data from neighborhood access
  points

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Tradeoffs between DSL/HFC

• Both are always on
• advantage or disadvantage?
• Cable is a shared broadcast technology

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Local Area Network

• College campus
• Company
• Big edge router
• Lots of other machines/devices connected to that
• You can do interesting things on a LAN
• iTunes for sharing music
• networked video games
• LANs use ethernet
• broadcast
• switched

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Wireless

• cell phones
• radio waves
• WAP (wireless access protocol)
• Wireless broadband access is better in Japan than
  in the US

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Wireless access in the developing world

• Easier to put up cell towers than to wire houses
• Same for internet
• Business model
• long tail
• The World is Flat

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Physical layer

• Two media
• twisted-pair copper wire
• fiber and coaxial cable
• bits get sent over these media
• electromagnetic waves
• optical pulses

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Internet backbone

• The internet is really a meta-network
• complicated picture
• details arent terribly important
• tier 1
• international and interconnected
• exclusive club
• not really regulated
• Not all ISPs are created equally
• tier 2 ISP ! tier 278 ISP

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Whos in charge of the internet?

• China does various things to censor internet
  traffic
• Dont route any packets through Canada
• IP addresses are not distributed equally

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Internet Corporation For Assigned Names and
Numbers (ICANN)

• ICANN is responsible for the global coordination
  of the Internet's system of unique identifiers.
  These include domain names (like .org, .museum
  and country codes like .UK), as well as the
  addresses used in a variety of Internet
  protocols. Computers use these identifiers to
  reach each other over the Internet. Careful
  management of these resources is vital to the
  Internet's operation, so ICANN's global
  stakeholders meet regularly to develop policies
  that ensure the Internet's ongoing security and
  stability
• ICANN is a California non-profit
• Function used to be done by the US government
• ICANN exists under a remit from the US Dept. of
  Commerce
• ICANN lacks teeth to prevent commercial
  organizations from having .org

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Total Nodal Delay

• processing delay
• queuing delay
• transmission delay
• propagation delay

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processing delay

• Time to read the packets headers
• Possibly perform checksums or other
  error-detection algorithms
• typically very, very fast on modern routers
• microseconds

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queuing delay

• unpredictable
• based on traffic at that router at any given
  moment
• depends on what everyone else is doing

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transmission delay

• all bits must arrive before the first bit can be
  sent out
• L bit packet
• R bits/second link
• L/R

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propagation delay

• depends on the speed of the physical medium
• fiber, twisted-pair, copper-wire, etc
• 2 108 to 3 108 m/sec
• close to the speed of light
• 2.99 108 m/sec
• distance between routers / propagation speed
• d / s
• millis for Wide-Area Networks (WANs)

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transmission vs propagation

• transmission is the time to receive and forward
  the whole packet
• function of packets length and the speed of the
  outgoing link
• propagation delay is how long it takes for the
  packet to travel between the routers
• takes into account distance between routers

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queuing delays revisited

• Very active research area
• a is packets/sec (arrival rate)
• R is bits/sec (transmission rate)
• L is packet size (assume all packets are uniform
  size)
• La/R is the traffic intensity

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Traffic intensity

• La/R 1
• packets will be buffered
• buffers are finite, thus packets will be lost
• La/R
• What does this mean?
• Think about how the traffic arrives

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La/R

If packets arrive in bursts, there can be
significant queuing delays

Suppose N packets arrive simultaneously every
(L/RN) seconds

first packet?

second packet?

in general?

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Traffic intensity conclusions

• La/R is helpful, but is not the full story
• pattern of packet arrival effects queuing delays
• La/R is a good general guideline
• 1
• system will eventually fall down
• close to 1
• high traffic intensity
• packet loss
• close to 0
• very low queuing delays
• not much packet loss

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Who should handle a lost packet?

• (try to elicit class participation)

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Total End-to-End delays

• N -1 routers between source and destination
• N hops
• d-proc processing delay
• d-trans transmission delay
• d-prop propagation delay

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Total End-to-End delays

• N -1 routers between source and destination
• N hops
• d-proc processing delay
• d-trans transmission delay
• d-prop propagation delay
• N -1 ( d-proc d-trans d-prop)
• Other delays?

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www.traceroute.org
Route from Colgate to MIT 1
W92-RTR-1-W92SRV21.MIT.EDU (18.7.21.1) 0.817 ms
0.273 ms 0.257 ms 2 EXTERNAL-RTR-1-BACKBONE.MIT
.EDU (18.168.0.18) 112.438 ms 7.047 ms 1.233
ms 3 ge-6-23.car2.Boston1.Level3.net
(4.79.2.1) 0.525 ms 0.739 ms 0.508 ms 4
ae-5-5.ebr1.NewYork1.Level3.net (4.69.132.250)
5.242 ms 16.197 ms 5 ae-1-100.ebr2.NewYork1
.Level3.net (4.69.132.26) 9.440 ms 17.718 ms 6
ae-24-52.car4.NewYork1.Level3.net (4.68.97.51)
5.535 ms ae-24-54.car4.NewYork1.Level3.net
(4.68.97.115) 5.755 ms ae-24-56.car4.NewYork1.Lev
el3.net (4.68.97.179) 5.558 ms 7
ROADRUNNER.car4.NewYork1.Level3.net (4.78.188.2)
18.827 ms 4.78.166.234 (4.78.166.234) 12.138 ms
4.78.166.238 (4.78.166.238) 12.282 ms 8
pos5-0.syrcnyspp-rtr02.nyroc.rr.com (24.24.7.17)
12.423 ms 18.396 ms 12.236 ms 9
rdc-24-24-7-86.nyroc.rr.com (24.24.7.86) 12.713
ms 18.542 ms 12.094 ms 10 srp2-0.syrcnyrmh-rtr0
1.nyroc.rr.com (24.92.229.200) 13.812 ms 13.402
ms 13.853 ms 11 fas0-1.syrcnyhml-swt03.nyroc.rr.
com (24.92.225.174) 14.832 ms 21.988 ms 15.089
ms 12 rrcs-72-43-89-2.nys.biz.rr.com
(72.43.89.2) 15.125 ms 16.403 ms 15.526 ms 13

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Layered architecture

• Example in the book, page 47
• draw picture on the board

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Layered architecture of the internet
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Application

• HTTP
• FTP
• Email (SMTP)
• AIM

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Transport

• TCP
• Transmission Control Protocol
• Connection-oriented
• UDP
• User Datagram Protocol
• connectionless

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Question

• I thought that the internet doesnt store any
  state?
• How is TCP able to keep track of connections?

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Network

• Datagrams
• Different from UDP
• Internet Protocol layer
• IP
• Everything on the internet uses IP
• Well, there are routing protocols, but it all
  uses IP

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Link Layer

• routes IP datagrams from the level above
• Ethernet
• PPP
• link layer packets are called frames

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Physical layer

• moves bits along the physical medium
• twisted pair, coaxial cable, fiber optic,
  satellite, smoke signals, whatever

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How this works at the endpoints
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Switches VS Routers

• I was a little loose with my terminology last
  week
• switches and router are different
• switches forward Link-layer packets
• routers forward packets Link-layer or
  Network-layer packets
• Draw picture from page 51 of KR textbook

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(ethereal demo, frame 9)