CMPE 150 Fall 2005 Lecture 24

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CMPE 150 Fall 2005Lecture 24

• Introduction to Computer Networks

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Announcements

• Homework 4 due on Wed.,11.23.05.
• No class on Friday, 11.25.05.
• We will have a real lab this week.
• Routing with RIP.
• Print lab description before going to your lab
  session.
• Midterm statistics
• Average 54.07
• Std. deviation 18.21

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Last Class

• Finished routing.
• Internetworking.
• Interconnecting networks.
• Heterogeneity.
• Different approaches to internetworking.
• Translating versus gluing.
• Tunneling.

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Today

• Internetworking (contd).
• IP.

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Internetworking
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Internetwork Routing

• Inherently hierarchical.
• Routing within each network interior gateway
  protocol (IGP).
• Routing between networks exterior gateway
  protocol (EGP).
• Within each network, different routing algorithms
  can be used.
• Each network is autonomously managed and
  independent of others autonomous system (AS).

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Internetwork Routing Example

• (a) An internetwork. (b) A graph of the
  internetwork.

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Internetwork Routing (Contd)

• Typically, packet starts in its LAN. Gateway
  receives it (broadcast on LAN to unknown
  destination).
• Gateway sends packet to gateway on the
  destination network using its routing table. If
  it can use the packets native protocol, sends
  packet directly. Otherwise, tunnels it.

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Fragmentation

• Happens when internetworking.
• Network-specific maximum packet size.
• Width of TDM slot.
• OS buffer limitations.
• Protocol (number of bits in packet length field).
• Maximum payloads range from 48 bytes (ATM cells)
  to 64Kbytes (IP packets).

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Problem

• What happens when large packet wants to travel
  through network with smaller maximum packet size?
  Fragmentation.
• Gateways break packets into fragments each sent
  as separate packet.
• Gateway on the other side have to reassemble
  fragments into original packet.
• 2 kinds of fragmentation transparent and
  non-transparent.

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Types of Fragmentation

• (a) Transparent fragmentation. (b)
  Nontransparent fragmentation.

Transparent Fragmentation
Non-Transparent Fragmentation
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Transparent Fragmentation

• Small-packet network transparent to other
  subsequent networks.
• Fragments of a packet addressed to the same exit
  gateway, where packet is reassembled.
• OK for concatenated VC internetworking.
• Subsequent networks are not aware fragmentation
  occurred.
• ATM networks (through special hardware) provide
  transparent fragmentation.

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Problems with Transparent Fragmentation

• Exit gateway must know when it received all the
  pieces.
• Fragment counter or end of packet bit.
• Some performance penalty but requiring all
  fragments to go through same gateway.
• May have to repeatedly fragment and reassemble
  through series of small-packet networks.

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Non-Transparent Fragmentation

• Only reassemble at destination host.
• Each fragment becomes a separate packet.
• Thus routed independently.
• Problems
• Hosts must reassemble.
• Every fragment must carry header until it reaches
  destination host.

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Keeping Track of Fragments

• Fragments must be numbered so that original data
  stream can be reconstructed.
• Tree-structured numbering scheme
• Packet 0 generates fragments 0.0, 0.1, 0.2,
• If these fragments need to be fragmented later
  on, then 0.0.0, 0.0.1, , 0.1.0, 0.1.1,
• But, too much overhead in terms of number of
  fields needed.
• Also, if fragments are lost, retransmissions can
  take alternate routes and get fragmented
  differently.

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Keeping Track of Fragments (Contd)

• Another way is to define elementary fragment size
  that can pass through every network.
• When packet fragmented, all pieces equal to
  elementary fragment size, except last one (may be
  smaller).
• Packet may contain several fragments.

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Keeping Track of Fragments

• Header contains packet number, number of first
  fragment in the packet, and last-fragment bit.

1 byte
Last-fragment bit
27 0 1 A B C D E F G
H I J
(a) Original packet with 10 data bytes.
Number of first fragment
Packet number
27 0 0 A B C D E F G
H
27 8 1 I J
(b) Fragments after passing through network with
maximum packet size 8 bytes.
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The Internet
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Design Principles for Internet

• Keep it simple.
• Exploit modularity.
• Expect heterogeneity.
• Think robustness.
• Avoid static options and parameters.
• Think about scalability.
• Consider performance and cost.

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Internet as Collection of Subnetworks
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IP (Internet Protocol)

• Glues Internet together.
• Common network-layer protocol spoken by all
  Internet participating networks.
• Best effort datagram service
• No reliability guarantees.
• No ordering guarantees.

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IP

• Transport layer breaks data streams into
  datagrams fragments transmitted over Internet,
  possibly being fragmented.
• When all packet fragments arrive at destination,
  reassembled by network layer and delivered to
  transport layer at destination host.

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IP Versions

• IPv4 IP version 4.
• Current, predominant version.
• 32-bit long addresses.
• IPv6 IP version 6 (aka, IPng).
• Evolution of IPv4.
• Longer addresses (16-byte long).

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IP Datagram Format

• IP datagram consists of header and data (or
  payload).
• Header
• 20-byte fixed (mandatory) part.
• Variable length optional part.

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The IP v4 Header
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IP Options
5-54
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IP Addresses

• IP address formats.

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IP Addresses (Contd)

• Class A 128 networks with 16M hosts each.
• Class B 16,384 networks with 64K hosts each.
• Class C 2M networks with 256 hosts each.
• More than 500K networks connected to the
  Internet.
• Network numbers centrally administered by ICANN.

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IP Addresses (Contd)

• Special IP addresses.

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Scalability of IP Addresses

• Problem a single A, B, or C address refers to a
  single network.
• As organizations grow, what happens?

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Example A Campus Network
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Solution

• Subnetting divide the organizations address
  space into multiple subnets.
• How? Use part of the host number bits as the
  subnet number.
• Example Consider a university with 35
  departments.
• With a class B IP address, use 6-bit subnet
  number and 10-bit host number.
• This allows for up to 64 subnets each with 1024
  hosts.

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Subnets

• A class B network subnetted into 64 subnets.

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Subnet Mask

• Indicates the split between network and subnet
  number host number.

Subnet Mask 255.255.252.0 or
/22 (network subnet part)
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Subnetting Observations

• Subnets are not visible to the outside world.
• Thus, subnetting (and how) is a decision made by
  local network admin.

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Subnet Example

• Subnet 1 10000010 00110010 00000100 00000001
• 130.50.4.1
• Subnet 2 10000010 00110010 00001000 00000001
• 130.50.8.1
• Subnet 3 10000010 00110010 00001100 00000001
• 130.50.12.1