IP Multicasting
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Implementation of multicast communication in internet Individual hosts are configured as members of different multicast groups One particular user may a member of many multicast groups For a one multicast can be few members/nodes IP Multicast group is identified by Class D address (224.0.0.0 – 239.255.255.255) Every IP datagram send to a multicast group is transferred to all members of group
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IP Multicasting
- 1. “ ” IP Multicasting Background knowledge, What is IP Multicasting Applications and Architecture IGMP Multicast routing Reverse Path Forwarding (RPF) Multicast distribution PIM and Mapping and Rendezvous point Address translation
- 2. IP Multicasting “Implementation of multicast communication in internet” Individual hosts are configured as members of different multicast groups One particular user may a member of many multicast groups For a one multicast can be few members/nodes IP Multicast group is identified by Class D address (224.0.0.0 – 239.255.255.255) Every IP datagram send to a multicast group is transferred to all members of group
- 3. Applications Situations where multiple end points need to receive identical info at same time News/Sports/Stocks/Weather updates Teleconferencing (Audio, Video, White board etc…) Distance learning Data replication Stream video Issues Complexity Early days routers are not configured to do multicasting in proper manner
- 4. IP Multicasting components IP multicast addressing (how we address IP Group management Multicast routing Multicasting is UDP based Best effort delivery No congestion avoidance Added complexity IP Multicasting considerations
- 6. IP Multicast protocol stack
- 7. Multicast Address translation In Ethernet Mac addresses a multicast address is identified by setting the lowest bit of the most left byte First 4 bytes -> Class D address Special Class D addresses 224.0.0.1 really means all systems on this subnet 224.0.0.2 means all routers on this subnet 224.0.1.1 is for NTP (Network Time Protocol) used for synchronizing machines 224.0.0.9 is for RIP-2 (a routing protocol).
- 8. Internet Group Management Protocol (IGMP) Use by host to notify the local router that it wishes to receive (or stop receiving) multicast traffic for given destination node or group IGMP operates on physical network Eg: single Ethernet segment IGMP supports Joining a multicast group Query membership Send membership reports IGMP snooping Send multicast only to needed ports
- 9. Multicast Routing Source can determine who can be the member of group and who will not be Required mechanism Packet forwarding can send multiple copies of same packet Multicast routing algorithm which builds spanning tree Two types of trees Source Tree Shared Tree Single copy replicate in network Routing tables are different in the two trees
- 10. Protocol Independent Multicast (PIM) Router to router signal protocol Use by a router to notify an upstream router that it wishes to receive (or stop receiving) multicast traffic Three main classifications of PIM Dense Sparse Sparse dense Source specific Bidirectional
- 11. Reverse Path Forwarding (RPF)
- 12. PIM-SM Rendezvous Point (RP) PIM-SM uses a router called Rendezvous Point The purpose of RP to allow, the first hop router to find out the IP address of the source for a particular group. The receiver don’t know the source address A RP is mandatory for PIM Sparse Mode PIM-SM Rendezvous Point Discovery Static RP configuration Dynamic RP configuration
- 13. Multicast advantages Enhanced scalability Network utilization is independent of the number of receivers Network handle replication Reduce resource utilization Controls network bandwidth and reduces server and router load Deterministic performance Subscriber number 1 and subscriber number 10000 have identical experience
- 14. Summery IP Multicasting is somewhat similar to “spreading of virus” Replication at each router Functions are layered IGMP to handle host to router PIM to inter router signaling Different message formats for IP Multicasting Scalability accommodate more functions and features
- 15. References [1] YouTube. “Lecture - 32 IP Multicasting”. Youtube.com. [Online]. Available https://www.youtube.com/watch?v=TApIo_BiX6U [Accessed: 3 Aug2014]. [2] YouTube. “Cisco Live 2014: IP Multicast Concepts, Design and Troubleshooting (Melbourne)”. Youtube.com. [Online]. Available https://www.youtube.com/watch?v=l9RJdrdjDU8 [Accessed: 3 Aug2014].
Editor's Notes
- The flow of the presentation mentioned in this slide. At the end of the presentation there will be two slides stating a “summery” and “references”.
- 3 modes of operations Unicast : One sender to one receiver Broadcast: One sender to all receivers Multicast: One /Many sender/senders to many receivers in the group There for “Multicasting” is “1 to Many” or “Many to Many” IP Multicasting more concern on from where the packets come from In IP Multicasting, packet replicating happens inside the network
- Developing multicast-enabled applications is ostensibly simple. Having datagram access allows any application to send to a multicast address. A multicast application need only increase the Internet Protocol (IP) time-to-live (TTL) value to more than 1 (the default value) to allow outgoing datagrams to traverse routers. To receive a multicast datagram, applications join the multicast group, which transparently generates an [IGMPv2, IGMPv3] group membership report. This apparent simplicity is deceptive, however. Enabling multicast support in applications and protocols that can scale well on a heterogeneous network is a significant challenge. Specifically, sending constant bit rate data streams, reliable data delivery, security, and managing many-to-many communications all require special consideration. Some solutions are available, but many of these services are still active research areas.
- IP Multicast considerations in detail Multicast is UDP based No flow control Sequencing Error correction Retransmission Best effort delivery Sender has no idea if all the subscribers have received the data. Subscribers don’t know if they have missed a packet. Applications should handled missed packets No congestion avoidance Slow start hence might result in network congestion Added complexity If we have the bandwidth available then unicast delivery model may be a simple option. Adding protocol mean adding complexity
- Right hand side, source pours multicast to network Left hand side, nodes that receive the multicast No control plan from receiver to sender A device can be multicast sender/receiver at same time Layer 2: Host to Router protocol : IGMP Layer 3: Multicast Routing protocol
- IP Multicast only support UDP as higher layer (Connectionless datagram oriented protocol) So use UDP as transport layer protocol “IP” takes part in normal routing protocol “IP Multicast” takes care of multicast routing protocol
- Mapping process Suppose in this 1110, the first 4 bits and suppose this is the class D address and we are looking at the first byte of that address and the first four bits 1110 identifies that this is a class D address. Then this bit is actually ignored and then we have a 23 bit address. This 23 bit address comes straight to the Ethernet address. So these 7 bits, these 8 bits and these 8 bits are matched straight to the last 3 bytes of the Ethernet address. For the first three bytes of the Ethernet address we have a one here showing that this is multicast. Actually the Ethernet address with 01, 00, 5e in the first 3 bytes are reserved for IP multicast. So 01, 00 and this is 101 is 5e and 1110 is e. So, this is 01, 00, 5e and this is first 3 bytes, this is reserved for multicast and this part comes straight away.
- IGMP : Joining group Receiver sends IGMP membership report to router Once received, send it to LAN and to host Second receiver asks for same That report is redundant ( since router is already doing what it suppose to do) IGMP : Maintaining group A host does not want multicast any more It sends IGMP leave message to Router Router check if is there any one still need it If still need, still forward (Switch also knows) IGMP : Leaving Last host on the network say that it doesn’t need multicasting anymore Router turnoff packet sending Advantages of IGMP snooping Hosts only receive MC traffic that they request Fast
- Multicast routing protocol This is an additional service by router since unicast is already there Building spanning tree between all members of multicast group Source tree Packet goes directly from source to receiver Receiver needs knowledge of receiver Traffic travel from source to receiver Packets are replicated at branches Provide optimal routing Representation (S,G) Shared tree Packets don’t go directly from source to receiver Flows through an arbitrary point : “Rendezvous point” Root is common Less state required
- PIM-SM Router-Router Signaling Each PIM router forms neighbor relationship with adjacent PM router at every 30 seconds When PIM router wants to receive a multicast stream it sends a “PIM Join” message When want to stop receiving send “Prune” message
- Tree formed from destination Each potential destination reach up-to source For each recipient minimize path cost RPF forward: forward only if receive from neighbor
- PIM-SM operation Advantages of Source Specific Multicasting Easy to configure and maintain Efficient network usage Enforce security PIM-SSM mapping Use PIM-SSM in the network when we have hosts that only supports IGMP v2 Mapping can used as an interim measure until IGMP v3 is supposed on all hosts Two ways to map * PIM-SSM is static mapping * PIM-SSM dynamic mapping (DNS)
- Following drawbacks can be identified Issues with multicast source Issues with receiver Underlying network issues MC network misconfiguration