IP-Multicast and the MBONE

The Internet Multimedia Backbone (the MBONE) is a recent introduction in Multimedia networking on the internet. The Mbone can be considered the equivalent of radio and television on the Internet:

• Used for Audio and Video conferencing on the Internet.

  The MBONE today has over 20,000 users, 1500 routers, 30 countries.

• Based on IP-multicast technology (Fig 25.18):
  

  IP-multicast Technology

  • Technology for IP-multicast was built in 1988

    In March, 1992, Internet Engineering Task Force meeting was broadcast (audio only) in the Internet.

  • IP-multicast supports multicast - good for video conferencing, mailing lists, bulletin board, updates to replicated databases

  • IP-multicast has anonymous membership. Source multicasts to special IP address, doesn't know who will receive.

  • FDDI and Ethernet have hardware multicast (Fig. 25.19).
    

    Harware Multicast

Tunneling is used to support MBONE multicasting:

• MBONE uses a network of routers (mrouters) that support multicast.

• Augmented by tunneling - encapsulate multicast packets inside regular IP packets, so that they can be sent among the islands of the MBONE that do not support IP-multicast (Fig. 25.20).
  

  Tunneling Network

The details of IP-multicast are summarised below:

• Recall: IP addresses are 32 bits. If first 4 bits are "1110", then message is an IP-multicast message.

  Covers IP address range 224.0.0.0 to 239.255.255.255

• Host software maps IP-group addresses into list of recipients, then either hardware multicasts (if available), or sends multiple unicasts to next node on tree.

• IP has time-to-live (TTL) field: each router decrements, discards message when zero. Used to limit packet lifetime.

• Problem: How to do/maintain mapping? Solution: Internet Group Management Protocol (IGMP)
  • 2 Message types: Query and Report

    Query: multicast to special group (224.0.0.1) by routers to all local host to inquire group membership. Set TTL = 1

    Report: used to respond to query, and to join groups.

  • On receiving query, members wait a random time before responding. (Seed for random number generator is IP address of machine)

  • If a member hears another response, then it stops its timer.

  • Routers periodically query group membership, and declare themselves group members if it gets a response to at least one query. If no responses after a while, they declare themselves as non-member.