How does IP multicast work

developing ip multicast networks volume 2 and ip multicast configuration example
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Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
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Multicast Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1Overview  Why multicast ? Multicast apps ...  Concepts: groups, scopes, trees  Multicast addresses, LAN multicast  Group management: IGMP  Multicast routing and forwarding: MBONE, PIM  Reliable Multicast Transport Protocols  Multicast Congestion Control  Deployment issues, Source-Specific Multicast (SSM), Application-level Multicast Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 2Multicast = Efficient Data Distribution Src Src Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 3Why Multicast ?  Need for efficient one-to-many delivery of same data  Applications:  News/sports/stock/weather updates  Distance learning  Configuration, routing updates, service location  Pointcast-type “push” apps  Teleconferencing (audio, video, shared whiteboard, text editor)  Distributed interactive gaming or simulations  Email distribution lists  Content distribution; Software distribution  Web-cache updates  Database replication Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 4Why Not Broadcast or Unicast?  Broadcast:  Send a copy to every machine on the net  Simple, but inefficient  All nodes must process packet even if they don’t care  Wastes more CPU cycles of slower machines (“broadcast radiation”)  Network loops lead to “broadcast storms”  Replicated Unicast:  Sender sends a copy to each receiver in turn  Receivers need to register or sender must be pre- configured  Sender is focal point of all control traffic  Reliability = per-receiver state, separate sessions/processes at sender Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 5Why IP multicast ?  Application-layer relays:  A ―relay‖ node or set of nodes does the replicated unicast function instead of the source  Multiple relays can handle ―groups‖ of receivers and reduce number of packets per multicast = efficiency  Manager has to manually configure names of receivers in relays etc  App-level topology may be sub-optimal But bandwidth is becoming cheaper Becoming more popular in content distribution  IP Multicast: replication/multicast engine at the network layer Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 6Multicast Apps Characteristics  Number of (simultaneous) senders to the group  The size of the groups  Number of members (receivers)  Geographic extent or scope  Diameter of the group measured in router hops  The longevity of the group  Number of aggregate packets/second  The peak/average used by source  Level of human interactivity  Lecture mode vs interactive  Data-only (eg database replication) vs multimedia Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 7IP Multicast Architecture Service model Hosts Host-to-router protocol (IGMP) Routers Multicast routing protocols (various) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 8IP Multicast model: RFC 1112  Message sent to multicast “group” (of receivers) Senders need not be group members A group identified by a single “group address” Use “group address” instead of destination address in IP packet sent to group Groups can have any size; Group members can be located anywhere on the Internet Group membership is not explicitly known Receivers can join/leave at will Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 9IP Multicast Concepts (Continued)  Packets are not duplicated or delivered to destinations outside the group Distribution tree constructed for delivery of packets Packets forwarded “away” from the source No more than one copy of packet appears on any subnet Packets delivered only to “interested” receivers = multicast delivery tree changes dynamically Network has to actively discover paths between senders and receivers Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 10IP Multicast Addresses  Class D IP addresses  224.0.0.0 – 239.255.255.255 1 1 1 0 Group ID  Address allocation:  Well-known (reserved) multicast addresses, assigned by IANA: 224.0.0.x and 224.0.1.x Transient multicast addresses, assigned and reclaimed dynamically, e.g., by ―sdr‖ program  Each multicast address represents a group of arbitrary size, called a “host group”  There is no structure within class D address space like subnetting = flat address space Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 11IP Multicast Service — Sending  Uses normal IP-Send operation, with an IP multicast address specified as the destination  Must provide sending application a way to: Specify outgoing network interface, if 1 available Specify IP time-to-live (TTL) on outgoing packet Enable/disable loop-back if the sending host is/isnt a member of the destination group on the outgoing interface Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 12IP Multicast Service — Receiving  Two new operations Join-IP-Multicast-Group(group-address, interface) Leave-IP-Multicast-Group(group-address, interface)  Receive multicast packets for joined groups via normal IP-Receive operation Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 13Link-Layer Transmission/Reception  Transmission • IP multicast packet is transmitted as a link-layer multicast, on those links that support multicast • Link-layer destination address is determined by an algorithm specific to the type of link • Reception • Necessary steps are taken to receive desired multicasts on a particular link, such as modifying address reception filters on LAN interfaces • Multicast routers must be able to receive all IP multicasts on a link, without knowing in advance which groups will be used Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 14Using Link-Layer Multicast Addresses  Ethernet and other LANs using 802 addresses:  Direct mapping Simpler than unicast No ARP etc.  32 class D addrs may map to one MAC addr IP multicast address 1 1 1 0 28 bits Group bit 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 0 23 bits LAN multicast address  Special OUI for IETF: 0x01-00-5E.  No mapping needed for point-to-point links Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 15Multicast over LANs & Scoping  Multicasts are flooded across MAC-layer bridges along a spanning tree But flooding may steal sending opportunity for non-member stations which want to transmit Almost like broadcast  Scope: How far do transmissions propagate?  Implicit scoping: Reserved Mcast addresses = don’t leave subnet. Also called “link-local” addresses Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 16Scope of Multicast Forwarding  TTL-based scoping:  Multicast routers have a configured TTL threshold  Mcast datagram dropped if TTL = TTL threshold  Useful as a blanket parameter.  Administrative scoping:  Use a portion of class D address space (239.0.0.0 thru 239.255.255.255)  Truly local to admin domain; address reuse possible.  In IPv6, scoping is an internal attribute of an IPv6 multicast address Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 17Multicast Scope Control – Small TTLs  TTL expanding-ring search to reach or find a nearby subset of a group  Rings can be nested, but not overlapping s 1 2 3 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 18Multicast Scope Control  Administratively-Scoped Addresses (RFC 1112 )  Uses address range 239.0.0.0 — 239.255.255.255  Supports overlapping (not just nested) domains The rest of the Internet address boundary set on interfaces to these links An administrative domain Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 19IP Multicast Architecture Service model Hosts Host-to-router protocol (IGMP) Routers Multicast routing protocols (various) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 20