Broadcasting, Multicasting and IGMP

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Broadcasting, Multicasting and IGMP
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Packet Filtering
UDP
discard
IP
discard
Device Driver
discard
Interface Card
discard
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Broadcasting AddressesLimited Broadcast

• Limited Broadcast 255.255.255.255
• Routers never forward it
• Useful when booting up as network address and IP,
  both may be unknown
• For multi-interface devices
• Some systems send on all interfaces
• Some send on one interface
• All subnets-directed broadcast
• HostID and SubnetID is all 1s

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Broadcasting Addresses

• Net-directed Broadcast
• Host ID of all 1 bits
• Example for Class A Netid.255.255.255
• Subnet-directed Broadcast
• Specific subnet ID, but host ID is all 1s
• Qs Assume destination address is 128.1.2.255
• Is it a broadcast address if mask is
  255.255.255.0?
• Is it a broadcast address if mask is
  255.255.254.0?

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Multicast Group ADdresses

• Multicast addresses 224.0.0.0 through
  239.255.255.255
• Some special addresses
• 224.0.0.1 All systems on the subnet
• 224.0.0.2 All routers on this subnet
• NTP uses 224.0.1.1

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1
1
0
Multicast group ID (28 bits)
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Multicast Addresses to Ethernet Addresses

• What is the need?
• 01005e000000 to 01005e7fffff
• 23 bits are available in the Ethernet address for
  mapping a multicast IP address
• Multicast addresses require 28 bits for
  uniqueness

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IGMP Internet Group Management Protocol

• Part of IP, just like ICMP

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IGMP Message Types

• Membership Query
• Membership Report

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IGMP

• When host joints, an IGMP report is sent
• When a host leaves there is no special message.
  Upon the next query, no response is sent if no
  process registers to that IP address
• Router sends periodic queries. Multicast hosts
  respond.
• Each host sends one IGMP report per multicast
  address

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Report Suppression

• Choose a random delay and schedule report
  transmission after that delay
• If another report heard before transmission of
  report, cancel report transmission
• Qs
• What is the destination IP address for IGMP
  query?
• What is the destination IP address for IGMP
  report?

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Some comments

• 224.0.0.0 through 224.0.0.255 reserved for
  applications that never need to multicast beyond
  one hop.
• Irrespective of TTL packets with these addresses
  are not forwarded by a router.
• Qs
• Should multicast routers generate ICMP error if
  TTL reaches 0?
• By increasing TTL an application can perform an
  expanding ring search for a server