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13. Broadcast, Multicast and Discovery

24/03/23

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IP Multcast and broadcast​

IP Broadcast​

  • IP Broadcast allows a single UDP datagram to be sent to all of the hosts on a physical network
  • Datagram is addressed to an IP broadcast address
    • Effectively represents ALL hosts on a network
  • Address 255.255.255.255 = broadcast address for this network
    • Highest IP address on a subnet is the broadcast address for that subnet
  • On the destination network, the IP broadcast address is mapped to a link layer broadcast address
  • The physical network copies the broadcast frame to all hosts
  • All physical network interfaces accept frames addressed to the broadcast MAC address
  • Then the frame (IP datagram) is passed to the OS to handle

IP Multicast​

  • Multicast is a Class D IPv4 address
  • These addresses represent groups of machines rather than individual hosts (or networks)
    • Any host can choose to join any number of groups
    • OS then configures network cards to accept corresponding frames, and inform local routers/switches
  • Specific ranges of multicast addresses are assigned to specific purposes
    • Local network control block (224.0.0.0 to 224.0.0.224) for multicast traffic within a given local network
  • IP Multicast allows a single UDP datagram to be sent efficiently to many (not all) hosts
  • Datagram is addressed to an IP multicast address
  • This is mapped to a link layer multicast address
  • Physical network copies the frame to all relevant hosts
  • The physical network interface accepts the multicast frame

Multicast considerations​

  • Failure Model - Same as UDP, best effor
    • Any subset of hosts may receive a given message
    • Messages may be re-ordered across inter networks
  • Making multicast reliable is complicated
  • Wide-area support - Multicast works well on Ethernet and WiFi but is often NOT forward by routers

Applications​

  • Performance/network efficiency - single physical copy can reach all machines on (local) network
  • Fault tolerance - multiple servers can receive/handle a single multicast request
  • Service discovery - multicast request when server IP is unknown
  • Local Peer-to-Peer applications

Broadcast and Multicast Applications (including Discovery)​

DHCP​

  • Every host on the internet needs to know its IP address, network, gateway, DNS server
  • Often discovered when the machine connects to a local network by using DHCP
  • New machines broadcasts a DHCP request to the local network
  • Delivered to every machine on the network
  • Any DHCP server on the local network handles the request and provides the information

Network Discovery Service​

  • Discovery services are directory services that typically operate over local networks where devices and processes are initially unknown to one anohter
  • Various implementation approaches
    • Often use network multicast initally
    • Java JINI