CSCI 5273 Computer Networks Internetworking

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CSCI 5273Computer NetworksInternetworking
AddressingStevens 1.3-1.17

• Dirk GrunwaldAssoc. ProfessorDept. of Computer
  ScienceUniversity of Colorado, Boulder

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Review

• IP (Internet protocol) is designed to connect
  networks that are
• Possibly managed by organizations / people
• May have different physical connections
• May be connected via a sequence of arbitrary
  intermediaries
• A layered approach is used simplify application
  protocol design

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Protocol Layering
FTP
FTP
TCP
TCP
IP
IP
Ethernet
Token Ring
IP
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Review

• The link layer deals with the actual transport of
  bits across a physical medium.
• The network layer abstracts the characteristics
  of the different link layers to a common layer
  (e.g. IP) and provides management functions at
  that layer.
• The transport layer adds various features
• Reliable communication (tcp)
• Arbitrary message sizes (udp)
• The application layer is the API provided to the
  programmer. Protocols are defined above that.

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Problems to identify solve

• Addressing
• How do we name applications?
• How do we name connections?
• How do we name computers?
• For humans
• Across networks
• Within a physical network
• How do we deal with a decentralized organization?
• Who arbitrates decisions?
• Who defines standards?
• How do we deal with a plurality of physical
  networks?

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Naming Addresses

• Addresses are defined across three layers
• Physical / link level
• Medium Access Control (MAC)
• Network/IP level
• IP address
• Transport/application level
• Ports

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Ethernet 101

• AlohaNet
• Developed _at_ University of Hawaii in 1975
• Radio with a single channel
• Users transmit whenever they have data
• They also listen at the same time
• If theres a collision, retransmit

T
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Detecting a Collision
A
B
T

• A starts transmitting a frame
• The first bit arrives in T seconds.
• The last at TL seconds
• B starts transmitting when the last bit just
  arrives
• A must continue to listen for 2TL seconds to
  detect B.

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CSMA CSMA/CD

• CSMA - Carrier Sense, Multiple Access
• Listen.If idle, transmit.Listen during
  transmission.If theres a possibility of
  conflict, retransmit
• CSMA/CD - CSMA/Collision Detection
• Listen. If idle, transmit.Listen during
  transmission.If theres a collision,
  then jam the channel Wait for 2T period to
  insure everyone heard you Wait a random period
  of time retransmitWhen done, wait 2T to insure
  your last bit wasnt corrupted

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Physical problems

• Signal attenuation requires repeaters
• This is still a single collision domain

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Consequences

• Large T leads long delay between messages
• Limiting T means limiting the physical size of a
  physical media
• T must be defined for the maximum possible length
• Large frame size leads to better bandwidth
• Less time spent contending
• Its easier to detect collisions if theres a
  minimum frame size. Frames must take at least 2T
  time to insure that contention is detected.
• For 802.3 specification, at 10Mb/s with a maximum
  length of 2500 meters w/4 repeaters, T51.2
  microseconds
• Or, minimum frame length of 64 bytes

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Further Consequences

• As transmission speed increases, either
• Minimum frame size increases
• Or, network length decreases

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Hubs Switches

• A hub is a single collision domain, although it
  has a physical hub and spoke topology
• A switch is a set of distinct collision
  domains.Frames destined for another collision
  domain are switched from one domain to another

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Addressing at the physical layer

• Ethernet (or 802.3) networks specify a 48-bit
  physical MAC address
• 00-00-f8-75-5b-a6 -- Unique identifier for the
  network interface card (NIC)
• Address ranges are assigned to specific
  vendorsE.g., 00-00 is Digital Equipment corp.
• Certain MAC addresses mean broadcast

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Addressing at the physical/link layer

• Frames are delivered to NICs with that
  specific MAC address (or all w/broadcast)
• A hub presents each frame to all NICs
• A switch moves frames from one collision domain
  to another based on the MAC address
• A table is maintained that specifies which MAC
  addresses are on which collision domain.
• Frames destined for an unknown MAC address are
  broadcast to all collision domains

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The reality of the world today

• A 10-BaseT ethernet NIC runs 9 for a cheapo
  PCI/ISA10-BaseT via USB is 40. 100BaseT via
  PCI is 30.Gigabit NIC is 350.
• A 4-port hub costs 40. Switches are gt70.
  Gigabit is much more (gt2000).

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More Realities

• Single nodes on switches allow you to use duplex
  communication
• Send receive concurrently
• You need to use high-qualitycabling (Cat5)
  for100 Mb/s networks
• Gigabit networks currently require fiber, but
  cable standard now available.
• Modest network bandwidth contention is a
  problem you throw money at, not brains.

NIC
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TokenRing / FDDI

• A token circulates amoung all computers.You
  can only transmit if you have the token.
• Variations More than one tokenbased on length
  or e.g. FDM.

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Coming to a home near you!

• Cable modems Ethernet
• Home Phone Networking Alliance (HomePNA)
• www.homepna.org
• 1-Mbit/s over your existing phonelines
• Typically combined with 10-BaseT connection(for
  cable modem)
• 16-Mbit/s standard coming in 1H00
• AMD, Intel make chipsets
• Tut Systems, Broadcom are active players
• 80Mbit/s is realizable

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More Addressing

• So, at the physical layer, Ethernet/802.3 uses a
  MAC address
• Can locate computers within a single physical
  network
• You want to limit network size - broadcast
  packets still affect full network.
• How do you address at the network and transport
  level?

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IP Addressing

• Each host in the internet has a unique 32-bit
  address
• Im lying
• There are three address types
• Unicast communication -- destined for a single
  host
• Broadcast communication -- destined for all hosts
  on a network
• Multicast communication -- destined for a set of
  hosts that belong to a multicast group.
• Note the use of network and host
• Network IDs are assigned by the InterNIC

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IP Addressing
0
netid/7
hostid/24
Class A
1
netid/14
hostid/16
0
Class B
1
0
1
netid/21
hostid/8
Class C
1
1
1
multicast group/28
0
Class E
1
1
1
1
multicast group/28
Class F
Class Range (as dotted quad)A 0.0.0.0 to 127.25
5.255.255B 128.0.0.0 to 191.255.255.255C 192.0.0
.0 to 223.255.255.255D 224.0.0.0 to 239.255.255.2
55E 240.0.0.0 to 255.255.255.255
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Problems Subnets

• A few companies got class A networks(e.g.,
  Digital, Xerox)
• Many educational institutions got class B
  networksE.g., my primary computer is
  128.138.241.78
• Most people get class C networks. E.g., my cable
  modem in Palo Alto was 208.166.41.96
• Allegedly, broadcasts would go to an entire
  network
• Obviously impractical for a Class A
  network.Thats 16,777,216 hosts
• Well discuss subnetting and routing later

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Mapping names to numbers

• Obviously, its hard to remember that
  128.138.241.78 is my computer
• But, numbers are more useful when actually
  switching messages
• The Domain Naming System maps names to IP
  addresses
• A tree-structured distributed database and naming
  scheme
• Each separately administered subtree is a zone
• Network Solutions handles registration of each
  top level domain (e.g., colorado.edu).
• Sub-domains are then administered by individual
  groups
• cs.colorado.edu
• Well discuss how names are resolved later

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Transport Level Naming

• Each NIC receives messages for a number of
  applications
• How do we differentiate the data intended for
  different apps?
• Each IP connection has an associated 16-bit port
  number.
• Port numbers are contained in each TCP UDP
  packet
• Some port numbers are well known services
• E.g., telnet is always port number 23
• Port numbers from 0..1023 are for well known
  services.Those port numbers are assigned by the
  Internet Assigned Numbers Authority (IANA)

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Transport Naming in Unix

• Unix uses reserved ports for security
• Only the superuser can create ports in the range
  of 0..1023.
• This is used for simplistic authentication
• On most unix systems, /etc/services lists the
  reserved ports

systat 11/tcp users daytime 13/tcp daytime 13/
udp netstat 15/tcp qotd 17/tcp
quote text chargen 19/tcp ttytst
source chargen 19/udp ttytst source ftp-data 20/
tcp ftp 21/tcp ssh 22/tcp SSH Remote Login
Server ssh 22/udp SSH Remote Login Server
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Representing TCP UP

• UDP is a datagram or message oriented
  protocol
• Maps well to Ethernet, etc
• TCP is a stream oriented
• Appears to be an infinite stream of bytes
• This maps to frames by packetization

IP Packet
IP Packet
IP Packet
IP Packet
IP Packet
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Encapsulation

• Application level communication typically has
  three levels of addressing
• Application information (e.g., HTML headers)
• Transport information (port)
• Network information (IP address)
• Link information (MAC address)
• Each layer is encapsulated in the preceding
  layer.
• We mux or encapsulate the message when its
  sent
• We demultiplex the message when it arrives
• Leads to layered software design

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Encapsulation as it goes down the protocol stack
User Data
App.
User Data
App Hdr
User Data
App Hdr
TCP Hdr
TCP
User Data
App Hdr
TCP Hdr
IP Hdr
IP
User Data
App Hdr
TCP Hdr
IP Hdr
EthernetHeader
Ethernettrailer
Ethernet
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20
20
4
46-1500 bytes
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Demultiplexing
Ethernetdriver
EthernetFrame
ARP
IP
RARP
Other
IPHeader
TCP
UDP
ICMP
IGMP
TCP/UDPHeader
App
App
App
App
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Standards Bodies

• Lots of arbitrary constants here!
• Naming, IP assignment, protocol header formats,
  etc
• Largely volunteer organization
• Internet Society
• Internet Architecture Board (IAB) - technical
  oversight coordination body
• Internet Engineering Task Force (IETF) -
  near-term, standards-oriented. Develops
  specifications that become internet standards
• Internet Research Task Force (IRTF) - RD arm

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Standards are embodied by RFCs

• Request for Comment (RFC)
• Unique monotoniclly assigned numbers. RFCs can
  not be revised, only re-issued.
• All RFCs are available on-line