Multicast IP and the MBONE
LESSON 17

Broadcasting:

• Broadcasting:
• Network delivers a copy to all destinations using a special reserved broadcast address.
  • Ethernet broadcast address: FF.FF.FF.FF.FF.FF
  • TCP/IP broadcast address: 255.255.255.255
• Disadvantage:
  • Consumes resources on all machines for every broadcast.

Hardware Multicasting:

• Hardware Multicasting:
  • Allows each machine to choose whether it wants to participate in the multicast (also know as selective broadcasting).
• Multicast address:
  • Decided upon and used by all member machines of the multicast group.
• Network Interface Cards (NIC):
  • Must be configured to receive hardware multicast addresses.
• Advantage:
  • Only machine resources of those wishing to receive the selective broadcast are consumed.

Software Multicasting (IP):

• IP Multicasting:
  • The internet abstraction (equivalent) of hardware multicasting, provided through software.
• Multicast groups are formed in software to determine which nodes receive the selective broadcast.
• Membership in a multicast group is dynamic.
• Multicast addresses are Class D IP addresses, where each group must use a unique one either:
  • Permanently - Internet Authority assigns well-known addresses.
  • Temporarily - Corresponding to transient multicast groups.
• Multicast gateways must be used to forward multicast datagrams.
• The TCP/IP Multicasting standard defines:
  1. IP multicast addressing.
  2. Specifies how hosts send and receive multicast datagrams.
  3. Describes the gateway protocol that is used to determine the multicast group membership on a network.

Multicast Addresses:

• Multicast address range:
  • 224.0.0.0 - 239.255.255.255
  • 224.0.0.0 - Reserved
  • 224.0.0.1 - Permanently assigned to the All Hosts Group
• Multicast addresses:
  • Can only be used as destination addresses.
  • Can never appear in the source field of a datagram.
  • Can never appear in the Source Route or Record Route options.
  • No ICMP error messages can be generated for them.
• IP multicast addresses must be mapped to hardware multicast addresses.
• Broadcast addresses are used if multicast support is not available.
• A host participates in the IP multicast at one of three status levels.

• It must run a protocol that informs the local multicast gateways of it’s group membership status.
• Hosts join specific IP multicast groups of specific networks.

Internet Group Management Protocol (IGMP):

• Internet Group Management Protocol (IGMP):
  • Used by hosts and gateways that implement multicast to communicate group membership information.
• ICMP (a TCP/IP standard) is required on all machines that participate in IP Multicast at level 2.
• IGMP is similar in service to ICMP.
• Two phases to IGMP’s functionality.
• Phase One:
  • Host joins a new group and send an IGMP message to the "all hosts" group (224.0.0.1).
  • Messages are received and membership information gets propagated to other multicast gateways.
• Phase Two:
  • Local multicast gateways poll hosts periodically to determine if it is still in each multicast group.
  • If yes, it continues advertising it’s membership.
  • If no, it stops and deletes that membership entry from the gateway.
• IGMP must remember the status of each multicast group to which a host belongs.
• It keeps a table of membership and each entry can have one of three possible states (levels):
  • Not used
  • Timer Active
  • Member

IGMP Message Format:

• VERS (4-bits):
  Current version of IGMP.
• TYPE (4-bits):
  • Query sent by gateway = 1
  • Response sent by host = 2
• GROUP ADDR (32-bits):
  • Host reports membership in particular multicast group.

Propagating Route Information:

• No standard currently exists for propagating routing information among multicast routers.
• Current multicast routers are adopting an experimental routing protocol to exchange information
• Distance Vector Multicast Routing Protocol (DVMRP):
  • Uses IGMP messages to carry information and exchange group membership status.
• Other routing protocols proposed for use with multicast networks are:
  • MOSPF (Multicast Open Shortest Path First) - OSPF routing with multicast extensions.
  • PIM (Protocol Independent Multicast) - proposed by the (IDMR) Inter-Domain Multicast Routing working group, and has two modes:
    • Dense and Sparse mode
• Most mrouters (used in the MBONE) employ DVMRP which is commonly considered inadequate for rapidly changing network topologies because routing information propagates too slowly.
• The multicast extension to the Open Shortest Path (MOSPF) link-state protocol has been proposed by John Moy of Proteon Inc. to address this problem.

Multicast Routers (Mrouters):

• Mrouters (IP Multicast Routers):
  • Assist or replace the functionality of host/gateway participation in distributing and replication of the multicast datastream.
• Mrouters:
  • Provide "efficient" distribution of packets
  • Use (in part) multicast tunnels to achieve it.
  • Once properly established, they need little or no attention.

Multicast Tunneling:

• Multicast Tunnels:
  • Mrouters append additional headers to packets which are routed through non-multicast gateways as normal packets.
  • The headers are then stripped off when received by an mrouter at the other end of the tunnel.

• Tunneling Scheme:
  • A scheme to forward multicast packets through Internet IP routers which typically do not support IP multicast.
  • The tunneling scheme encapsulates multicast streams into unicast packets which can be passed as regular Internet protocol packets.
  • Packets are sent along a virtual network of dedicated multicast routers (mrouters) until they reach the various destination local area networks.
• The use of dedicated mrouters segregates multicast packet delivery, protecting standard network communications (mail, ftp, telnet) from real-time applications.

Virtual Internet Backbone for Multicast-IP (MBONE):

• What is the MBONE?
  • MBONE is a virtual network layered on top of portions of the physical Internet.
  • MBONE stands for Multicast Backbone, a virtual network that has been in existence for over three years.
  • MBONE is currently used for multicast real-time applications.
• The network originated from an effort to multicast audio and video from the IETF meetings.
• MBONE today is used by several hundred researchers for protocols and applications development and for group communication.
• Multicast is used because it provides one-to-many and many-to-many simultaneous network delivery services for real-time applications such as videoconferencing and audio.

MBONE:
Virtual Network

• The reason that MBONE is a virtual network:
  • It shares the same physical media as the Internet
  • It uses a parallel system of routers that can support multicast, augmented with IP tunnels.
• Usually the routers are dedicated workstations running with modified kernels and multiple network interfaces.

MBONE:
Bandwidth Considerations

• The reason why a multicast stream is bandwidth-efficient is that one packet can be sent to all workstations on a network.
• Example:
  • 125 Kbps video stream (1 frame/second) uses the same bandwidth whether it is received by one workstation or twenty.

MBONE:
Networking Details

• When a host on an MBONE-equipped subnet establishes or joins a group it announces that event using IGMP.
• The multicast router on that subnet forwards the group information to other routers in the network.
• MBONE sessions use the session directory (sd) tool (developed by Van Jacobson of Lawrence Berkeley Laboratories) to display the announcements by multicast groups.
• SD is also used for:
  • Launching multicast applications.
  • Automatically selecting an unused address for any new groups.
• Groups are disestablished when all members leave, freeing-up that IP multicast address for reuse.
• The MBONE routers occasionally poll hosts on the subnets to determine if any are still group members.
• If there is no reply by a host, the router stops advertising that hosts group membership to the other multicast routers.
• This is an exact implementation of IGMP functionality.

MBONE:
Topology and Scheduling

• "The MBONE topology and the scheduling of multicast sessions must be actively managed by the MBONE community to minimize congestion."
• The topology consists of the backbone and regional (midlevel) networks linked by a mesh of IP tunnels and mrouters.

• Currently, over 20 countries participate in the MBONE project and use over 1000 multicast routers.
• Approximately 400 sites worldwide are currently MBONE members.

MBONE:
Real Time Protocol (RTP)

• In addition to the multicast protocols, MBONE applications are using the Real Time Protocol (RTP) on top of User Datagram Protocol (UDP) and IP.
• RTP is being developed by the Audio-Video Transport Working Group within the IETF.
• RTP provides timing and sequencing services; which allows applications to adapt and smooth out network-induced latencies and errors.
• The end result:
  • Participants using real time audio normally perceive conversations as if they are in real-time even though there is a small buffering delay to synchronize and sequence the arriving voice packets.

MBONE:
Application Tools

• Video, audio, and a shared drawing whiteboard are the principal applications used on the MBONE.
• Specific software packages which implement them are:
  • nv (net video): Ron Frederick (Xerox PARC)
  • vat (visual audio tool): Van Jacobson
  • wb (whiteboard): Van Jacobson (Berkeley Lab)
• Additional tools are also available or under development such as:
  • imm (Image Multicaster Client):
    • A low-bandwidth image server by Winston Dang (Univ. Hawaii)
    • It is used to provide live images of Earth from geostationary satellites at half-hour intervals.
  • Distributed Interactive Simulation: (Mike Macedonia)
    • Ported from IEEE Distributed Interactive Simulation (DIS) protocol.
    • Used to enable live interaction between virtual worlds as an MBONE communications tool.

MBONE:
Events

• NASA Select, the NASA in-house cable channel broadcast during space shuttle missions.
• Some conferences include:
  • Supercomputing
  • Internet Engineering Task Force
  • Scientific visualization
• Lectures by distinguished speakers (Xerox PARC)
• Internet Talk Radio (Carl Malamud) "geek of the week"

MBONE:
Debugging Tools

Tcpdump	Mcollect
Traceroute	Mdump
Mtrace	Map-Mbone
Etherfind	Rtpqual
Netstat	Slack
Ifconfig	Mrmap
Mrdebug	Etherman
Mrinfo	Interman

MBONE:
Resource Acknowledgments

• Many thanks to Mike Macedonia and Don Brutzman of the Naval Postgraduate School for the information resources on the MBONE.
• Their guide - MBONE - An Overview, (M.Macedonia & D.Brutzman) can be found at:

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1995 Illusion Industries Inc.