Internetworking

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• Internetworking

2G1316Data Communications and Computer Networks
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Illustrations in this material are collected from
Behrouz A Forouzan, Data Communications and
Networking, 3rd edition, McGraw-Hill.
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Todays Lecture

• Introduction
• The Internet architecture
• IP addressing, address resolution
• The Internet Protocol (IP)
• Internet routing
• Control and error reporting (ICMP)

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Internetworking

• Interconnection of networks
• Different network technologies
• LANs, point-to-point links, etc.
• Different address formats
• Data link level scope
• Build an internetwork by connecting networks
• Layer 3 packet switchesrouters
• Global addressing
• The Internet
• Router-based internetwork
• TCP/IP protocol suite

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Network Layer
LAN1
LAN2
Host 1
Host 2
Router
TCP
TCP
IP
IP
IP
MAC1
MAC1
MAC2
MAC2
PHY
PHY
PHY
PHY
Packet Frame
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IP Addressing

• Identify each node on the IP network
• A common addressing scheme
• Independent from the data link layer
• IP address
• addressing on network level
• unique 32-bit number

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IP Address Notation
10000010
11101101
11010100
00011001
130.237.212.25

• Written as four decimal numbers with dots .
  between
• Each decimal number represents the binary value
  of one byte
• Dotted-decimal notation
• dot address, dotted quad address,

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IP Address Structure

• An address has two purposes
• Uniquely identify a host (identifier)
• Give the location of the host (locator)
• For that purpose, an address has two parts
• Network (prefix, netid)
• Host (suffix, hostid)
• Network identifies a network
• Host identifies a node on that network
• Network and Host together identify a node on the
  Internet

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Original IP Address Classes
Byte 1
Byte 2
Byte 3
Byte 4
Class A
0.0.0.0-127.0.0.0
Class B
128.0.0.0-191.0.0.0
Class C
192.0.0.0-223.0.0.0
Class D
224.0.0.0-239.0.0.0
1
1
1
0
Multicast addresses
Class E
240.0.0.0-255.0.0.0
1
1
1
1
Reserved for future use

• The address class is identified by the first few
  bits
• Boundary between network and host parts is at a
  fixed location (for each class)

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IP Addressing Example

• Size of cloud number of hosts determines the
  class of address

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Classful Addressing Allocation
B
16 384 networks with 64536 hosts each
A
128 networks with 16 777 216 hosts each
C
2 097 152 networks with 256 hosts each
D
E
268 435 456 multicast groups
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Number of Networks and Hosts
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Address Space Depletion

• Class A too large for most organizations
• No address class suitable for mid-size
  organisations
• C too small
• B too large
• Fear to run out of Class B addresses in early
  90s
• Still only using about 5 of the total address
  space!

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Classless Inter-Domain Routing (CIDR)

• No fixed boundaries between network and host
• IP network addresses represented as a prefix
• Address and a prefix length
• Written as 192.16.30.0/20
• Prefix length can take any value
• Between 0 and 32
• More flexibility in network size

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Address Mask

• A network address can be specified as address and
  a bit mask
• For example 192.16.30.0/20
• 192.16.30.0 mask 255.255.240.0 (or mask FFFFF000)
• The network address can be computed by applying
  the mask to any address within the subnet
  (bitwise AND)

130.237.16.18

Mask
255.255.128.0

Network
130.237.0.0
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Router Addresses

• No specific IP suffix for router interfaces

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Next-hop Routing

• Routing table gives next node on the path to
  destination

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IP and MAC Addresses

• In order to forward a packet, a router needs data
  link layer address (MAC address) of next hop
• At a router
• Next hop IP address determined from the routing
  table
• Next hop IP address translated to next hop MAC
  address
• Packet transferred using next hop MAC address

MAC
IP
MAC
IP
h
f
g
a
e
b
g
a
R
IP aMAC b
IP gMAC h
IP2 dMAC2 f
IP1 cMAC1 e
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Address Resolution

• Mapping between an IP and a MAC address
• within the same physical network
• Different alternatives
• table lookup
• all mappings stored in a table in memory
• e.g. X.25 network
• closed form computation
• translation using simple Boolean operations
• e.g. token ring
• message exchange
• e.g. Ethernet Address Resolution Protocol (ARP)

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Address Resolution Protocol (ARP)
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ARP (contd)

• Address mappings are stored in a cache
• Reduces ARP message exchanges
• oldest one out
• not used out
• A node answering an ARP request can store the
  senders addresses in the cache
• Mappings are removed after a certain period of
  time
• Needs to be refreshed

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IP Datagram Forwarding

• Datagram transmission
• Connectionless
• Non-confirmed delivery
• No acknowledgement that the packet has been
  received
• Best-effort service
• no dedicated transmission capacity
• Responsibility of higher layer to guarantee
  delivery

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IP Header

• Version (VER) 4
• Header length (HLEN) (32-bit units)
• Header is 20-60 bytes
• Typically HLEN5 (20 bytes, no options)
• Service type (DS)
• Quality of service purposes
• Total length (bytes)
• Header data, max 65 536 bytes
• Identification, flags, offset
• For fragmentation
• Time to live
• Max number of routers to pass
• Decremented at each router hop
• Dropped if zero
• Prevent errors due to loops
• Protocol
• TCP, UDP, etc
• Header checksum
• 16-bit checksum of all fields in header

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IP Header

• Version (VER) 4
• Header length (HLEN) (32-bit units)
• Header is 20-60 bytes
• Typically HLEN5 (20 bytes, no options)
• Service type (DS)
• Quality of service purposes
• Total length (bytes)
• Header data, max 65 536 bytes
• Identification, flags, offset
• For fragmentation
• Time to live
• Max number of routers to pass
• Decremented at each router hop
• Dropped if zero
• Prevent errors due to loops
• Protocol
• TCP, UDP, ICMP, etc
• Header checksum
• 16-bit checksum of all fields in header

4-bit VER
4-bit HLEN
16-bit total length (bytes)
8-bit type of service
3-bit flags
16-bit identification
13-bit fragmentation offset
8-bit time to live (TTL)
8-bit protocol
16-bit header checksum
20 bytes
32-bit source IP address
32-bit destination IP address
Options
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Fragmentation

• MTUMaximum Transmission Unit
• Any router may fragment a packet
• Packet is reassemblied at the receiver

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ICMP (Internet Control Message Protocol)

• Indicated by protocol type ICMP in IP header
• Error messages
• Destination unreachable
• No route, fragmentation needed, port does not
  exists, ...
• Time exceeded
• TTL, Packet reassembly
• Parameter problem
• Bad IP header
• Redirect
• Use other router
• (Source quenchflow/congestion control)
• Query messages
• Echo request/reply (Ping)
• (Time stamp, address mask, router solicitation)

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Structure of the Internet

• Internet Service Provider
• Provide Internet access
• Carriers
• Interconnect ISPs
• Peering agreements between carriers

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Autonomous Systems

• Group of networks and routers under same
  administrative control
• Interior routing protocolwithin an autonomous
  system
• Exterior routing protocolbetween autonomous
  systems

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Distance Vector Routing

• Bellman-Ford algorithm
• Router keeps list of minimum hop counts (or
  costs)
• List is sent to all neighbor routers
• Periodical updates
• Routing Information Protocol (RIP)
• Internal routing

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RIP Updates
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Link State Routing

• Dijkstras algorithm
• Link state information
• State of neighborhood
• Flooding
• Each router sends link information to its
  neighbors
• The neighbors forward the information to their
  neighbors, and so on
• Interior routing protocols
• Open Shortest Path First (OSPF)
• Intermediate System-Intermediate System (IS-IS)

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OSPF Areas

• Routers inside an area flood the area
• Area border routers summarize to other areas

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Exterior Routing
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BGP (Border Gateway Protocol)

• Path vector routing
• Routing based on policies
• Administrative rules
• Shortest path not always sufficient
• Some autonomous systems may be preferred to
  others
• Security, reliability, etc
• Internal structure kept private
• Receives routing information from interior
  routing protocols

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IP Version 6 Addresses


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IPv6 Header
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IPv6 Tunnels over IPv4 Networks
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Summary

• Internetworking, router-based networking
• IP addressing, address resolution
• IP protocol datagram forwarding
• routing, fragmentation, unreliable service
• IP header format
• Routing
• Autonomous Systems
• Interior routing RIP, OSPF, IS-IS
• Exterior routing BGP
• ICMP for query and error reporting

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Reading Instructions

• Behrouz A. Forouzan, Data Communications and
  Networking, third edition
• 19 Internetworking
• 19.2 Addressing
• 19.3 Routing
• 20 Network Layer Protocols ...
• 20.1 ARP
• 20.2 IP
• 20.3 ICMP
• 21 Unicast and Multicast Routing Routing
  Protocols
• 21.1 Unicast routing
• 21.2 Unicast routing protocols