Routing in Multicast Communication
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The presentation describes the different ways of routing that takes place in Multicast Communication. The presentation was prepared in collaboration with Shubham Singhal at IIIT-Delhi for the course "Technical Communication".
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Routing in Multicast Communication
- 1. Routing in Multicast Communication Sahil Jain (2010071) Shubham Singhal (2010083)
- 2. Multicast Delivery of a message to a group simultaneously in a single transmission from the source One-to-many or many-to-many communications Unicast Broadcast Multicast 2
- 3. Why Multicast? Working • Better bandwidth utilization • Less host/router processing Applications • Video Conferencing • Data Sharing Applications • Multi-player Gaming 3
- 6. Multicast Model Senders transmit IP datagrams to a host group Host group identified by a class D IP address Members join and leave the group and indicate this to the routers Routers use multicast routing protocols to manage groups Every IP datagram send to a multicast group is transmitted to all members of the group Sender may not be a member of the group IP Multicast service is unreliable 6
- 8. Shortest Path Tree Minimizes the path cost from the source to each receiver • Good tree if there is a single sender • If there are multiple senders, need one tree per sender • Easy to compute S 8
- 9. Minimum Cost Tree S Minimizes the total cost of the edges • Good solution if there are multiple senders • Single tree for multiple senders • Very expensive to compute (not practical for more than 30 nodes) 9
- 10. Reverse Path Forwarding Builds shortest path tree using unicast routing tables A router selects its next hop neighbor(router) in the tree for forwarding unicast packets to the root Traces a reverse shortest path from any router to the sending host Union of reverse shortest paths builds a reverse shortest path tree. 10
- 11. Routing Approaches Source Based Tree One shortest path tree for each sender Tree is built from receiver to the sender [Reverse Path Forwarding (RPF)] Useful when there are small number of senders and large number of receivers • Core Based Tree • A single distribution tree that is shared by all senders • Selects one router as a “core” • All receivers build a shortest path to the core [Reverse Path Forwarding] 11
- 12. Source Based Tree Flood & Prune • Set routing tables according to RPF forwarding • Next hop neighbor of R5 is R2 • Next hop neighbor of R7 is R3 • And so on….. Image Source: www.cs.virginia.edu/~itlab/book/slides/module21-mcastv4.ppt 12
- 13. Each router in the network uses reverse path forwarding to forward the packets out every interface except the upstream interface. As a result, the multicast session initially reaches every router in the network. This is called flooding H2 H3 H5H4 H1 Source R1 R2 R6 R4 R5 R3 R8 R7 joined joined Source Based Tree Flood & Prune 13
- 14. Source Based Tree Flood & Prune H2 H3 H5H4 H1 Source R1 R2 R6 R4 R5 R3 R8 R7 joined joined Pruning = If no group members or downstream routers to which the traffic must be forwarded, router sends a poison-reverse message called a prune message to its upstream neighbor. This called pruning 14
- 15. H2 H3 H5H4 H1 Source R1 R2 R6 R4 R5 R3 R8 R7 Prune joined joined Prune Prune Prune Prune Prune Prune Prune Prune Prune Prune Prune Source Based Tree Flood & Prune That upstream neighbor then stops forwarding the session traffic to the pruned router. • Prune message temporarily disables routing table entry 15
- 16. Source Based Tree Grafting H2 H3 H5H4 H1 Source R1 R2 R6 R4 R5 R3 R8 R7 joined joined Graft Graft joined When a receiver joins, one needs to re-activate a pruned routing table entry Sending a Graft message disables prune, and re-activates routing table entry. 16
- 17. Core Based Tree Receiver 1 E BA D F Source C Receiver 2 S1 R R Shared Root (Core) S2 Source • Constructs a single delivery tree that is shared by all members of a group • Source sends data to the core • Core forwards data to the group members according to routing table entry 17
- 18. Challenges Difficult because: sources may join and leave dynamically need to dynamically update routing table entry How can a sender restrict who can receive? need authentication, authorization encryption of data key distribution This is still an active area of research 18
- 19. Summary Unicast and Broadcast not always efficient Applications need multicast for better bandwidth utilisation Source based tree Optimal path between sender and receiver but overall expensive One path for each receiver- more memory requirement Core based tree Less memory requirement Careful selection of the core required Scalable for many senders and receivers 19
- 20. References 1. Xiaola Lin and Lionel M. Ni, “Multicast Communication in Multicomputer Networks”, IEEE, VOL.4, NO. 10, OCTOBER 1993 2. Yuan Gao and Jennifer C. Hou, “Reliable Multicasts for Core-based Multicast Trees”, IEEE, 2000 3. Rick Graziani, (Fall 2006), “IP Multicasting” [Online], Available: alextm.no-ip.com/appunti/ccnp/lucidi/ccnp1-Multicasting.ppt 4. Radhika Rengaswamy, “IP Multicasting” [Online], Available: www.cse.buffalo.edu/~qiao/cse620/ip_multicasting.ppt 20
- 21. Thank You 21
Notas del editor
- Good morning
- But before moving to routing algorithms we should know about Multicast.It is the delivery of a message to members of a group simultaneously in a single transmission from the sender.It is One-to-many or many-to-many communication.The picture depicts the difference between unicast, broadcast and multicast.
- But why we need multicast?We use multicast because it gives better utilization of the bandwidth as only one copy of data is forwarded to the router which makes multiple copies as per the requirement. This reduces processing at each router. Multicast is used in video conferencing. multi player gaming and other applications where data is shared among the users
- Lets see the working of multicast:
- But in multicast system the sender sends only one copy of the data to the forwarding router and the router then makes multiple copies.
- In multicast communication the sender transmit IP datagrams to a group of receivers called host groupHost group is identified by a class D IP address which ranges from 224.0.0.0 to 239.255.255.255Membersjoin and leave the group informing the nearby routersRouters use multicast routing protocols to manage groupsEvery IP datagram send to a multicast group is transmitted to all members of the group where the sender may not be a member of the groupIP multicast uses UDP which is fast but unreliable.
- To understand the routing algorithms we need to learn a few concepts:Shortest Path treeWe have a blue box as the sender and 3 green boxes as the receivers.
- Shortest path tree minimizes the path cost from the source to each receiverConsidering the cost of each edge as 1-
- Minimum cost tree:Here we minimizes the total cost of the edgesIt is a better option if we have multiple senders.However it becomes very expensive to compute with increase in number of nodes.
- When a sender first initiates a session, each router in the internetwork uses reverse path broadcasting to forward the packets out every interface except the upstream interface. As a result, the multicast session initially reaches every router in the internetwork.
- If there are no group members in the subnet or downstream routers to which the traffic must be forwarded, the router sends a poison-reverse message called a prune message to its upstream neighbor. That upstream neighbor then stops forwarding the session traffic to the pruned router.