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CSCI 5273 Computer Networks Internetworking

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Title: CSCI 5273 Computer Networks Internetworking


1
CSCI 5273Computer NetworksInternetworking
AddressingStevens 1.3-1.17
  • Dirk GrunwaldAssoc. ProfessorDept. of Computer
    ScienceUniversity of Colorado, Boulder

2
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

3
Protocol Layering
FTP
FTP
TCP
TCP
IP
IP
Ethernet
Token Ring
IP
4
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.

5
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?

6
Naming Addresses
  • Addresses are defined across three layers
  • Physical / link level
  • Medium Access Control (MAC)
  • Network/IP level
  • IP address
  • Transport/application level
  • Ports

7
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
8
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.

9
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

10
Physical problems
  • Signal attenuation requires repeaters
  • This is still a single collision domain

11
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

12
Further Consequences
  • As transmission speed increases, either
  • Minimum frame size increases
  • Or, network length decreases

13
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

14
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

15
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

16
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).

17
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
18
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.

19
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

20
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?

21
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

22
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
23
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

24
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

25
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)

26
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
27
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
28
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

29
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
14
20
20
4
46-1500 bytes
30
Demultiplexing
Ethernetdriver
EthernetFrame
ARP
IP
RARP
Other
IPHeader
TCP
UDP
ICMP
IGMP
TCP/UDPHeader
App
App
App
App
31
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

32
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
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