IP Multicasting in Networking

1. COMS/CSEE 4140 Networking LaboratoryLecture 11 Salman Abdul Baset Spring 2008

2. Announcements • Prelab 10 and Lab report 9 due next week before your lab slot • Weekly project reports due before Friday 5pm

3. Agenda • Multicast • why multicast? • multicast groups • terminology • (S,G) notation • Addressing • IGMP • Source and core-based trees • PIM • PIM-DM • PIM-SM

4. Applications with Multiple Receivers • Many applications transmit the same data at one time to multiple receivers • Broadcasts of radio or video (streaming) • Video conferencing (interactive video) • Shared applications (data, file transfer) • A network “must” have mechanisms to support such applications in an efficient manner

5. Multicasting • Multicast communications refers to one-to-many or many-to-many communications. Unicast Broadcast Multicast IP Multicasting refers to the implementation of multicast communication in the Internet Multicast is driven by receivers: Receivers indicate interest in receiving data

6. Multiple copies of the same message is transmitted on the same link Multicasting over a Packet Network • Without support for multicast at the network layer:

7. Multicasting over a Packet Network • With support for multicast at the network layer: • Requires a set of mechanisms: (1) Packet forwarding can send multiple copies of same packet(2) Multicast routing algorithm which builds a spanning tree (dynamically)

8. Why IP Multicast? • Cost-efficient distribution of data • Timely distribution of data • Robust distribution of data “Data” here could be • Files • Streamed Audio or Video Source: Internet2 workshop

9. Cost Efficient Data Distribution • This is the core of the streaming case. • Unicast streaming is just too expensive. • This is true either on the commodity Internet or on the Intranet. • Multicast is especially compelling for video. • Question: Is multicast a good model for • YouTube? • Live video streaming? Source: Internet2 workshop

10. Timely Distribution of Data • An argument for multicast in financial services. • Though reliability is an issue. • In unicast, it takes time to send packets separately to each receiver. • Even if cost is not a problem, time may be. • 100 Mb/s link, takes 22 hours to distribute a 100 MB file t0 10,000 machines. With multicast, it will take eight seconds. • Alternate solution is to have huge upstream bw • But… • Multicasting is compelling here if timely distribution is important. Source: Internet2 workshop

11. Robust Distribution of Data • Handle sudden large increases in load • streaming • data distribution • Multicast was designed to handle sudden large increases in load. Source: Internet2 workshop

12. Multicast Groups • The set of receivers for a multicast transmission is called a multicast group • A multicast group is identified by a multicast address • A user that wants to receive multicast transmissions joins the corresponding multicast group, and becomes a member of that group • After a user joins, the network builds the necessary routing paths so that the user receives the data sent to the multicast group

13. IP Multicasting: Key questions • What IP address / Ethernet address to send multicast packets to? • How to manage groups? • How to route multicast packets?

14. Socket Layer Stream Sockets Datagram Sockets Multicast Sockets TCP UDP IP IP Multicast The IP Protocol Stack • IP Multicasting only supports UDP as higher layer • There is no multicast TCP ! User Layer Network Interface

15. e.g., video server Essential Multicast Terminology IP source = IP unicast addr Ethernet source = MAC addr IP destination = IP multicast addr Ethernet dest = MAC addr receivers source Multicast stream source = origin of multicast stream multicastaddress = an IP address in the Class D range (224.0.0.0 – 239.255.255.255), used to refer to multiple recipients.A multicast address is also called a multicast group or channel. multicaststream = stream of IP packets with multicast address for IP destination address. All multicast uses UDP packets. receiver(s) = recipient(s) of multicast stream tree= the path taken by multicast data. Routing loops are not allowed, so there is always a unique series of branches between the root of the tree and the receivers. Source: Internet2 workshop

16. (S,G) notation • For every multicast source there must be two pieces of information: the source IP address, S, and the group address, G. • These correspond to the sender and receiver addresses in unicast. • This is generally expressed as (S,G). • Also commonly used is (*,G) - every source for a particular group. Source: Internet2 workshop

17. Multicast Protocols • Receivers • Group Management Protocol - enables hosts to dynamically join/leave multicast groups. Receivers send group membership reports to the nearest router. • Multicast Routing Protocol - enables routers to build a delivery tree between the sender(s) and receivers of a multicast group. • Delivery tree • Senders Multicast Routing Protocol (e.g. PIM-SM) Group Management Protocol (e.g. IGMP)

18. Agenda • Multicast • why multicast? • multicast groups • terminology • (S,G) notation • Addressing • IGMP • Source and core-based trees • PIM • PIM-DM • PIM-SM

19. Multicast Addressing • Multicast address • IP addresses in the range 224.0.0.0 - 239.255.255.255 (class D address) • Multicast group: same as multicast address • Every host (interface) can join and leave a multicast group dynamically • no access control • Every IP datagram send to a multicast group is transmitted to all members of the group • no security, no “floor control” • Sender does not need to be a member of the group • The IP Multicast service is unreliable

20. Multicast Addressing • All Class D addresses are multicast addresses: • Multicast addresses are dynamically assigned. • An IP datagram sent to a multicast address is forwarded to everyone who has joined the multicast group • If an application is terminated, the multicast address is (implicitly) released.

21. Types of Multicast Addresses • RFC 3171 • http://www.iana.org/assignments/multicast-addresses • Examples: • 224.0.0.0 - 224.0.0.255 (224.0.0/24) - reserved & not forwarded • 224.0.0.1 - All local hosts • 224.0.0.2 - All local routers • 224.0.0.4 - DVMRP • 224.0.0.5 - OSPF • 224.0.0.9 - RIP2 • 224.0.0.13 – PIM • 224.0.0.22 – IGMP • 232.0.0.0 - 232.255.255.255 (232/8) - SSM • 239.0.0.0 - 239.255.255.255 (239/8) - Administrative Scoping (RFC 2365) • Site-local scope: 239.253.0.0/16 • Organization-local scope: 239.192.0.0/14 • “Ordinary” multicasts don’t have to request a multicast address from IANA.

22. Multicast Address Translation • In Ethernet MAC addresses, a multicast address is identified by setting the lowest bit of the “most left byte” Not all Ethernet cards can filter multicast addresses in hardware - Then: Filtering is done in software by device driver.

23. Multicast Address Mapping

24. Agenda • Multicast • why multicast? • multicast groups • terminology • (S,G) notation • Addressing • IGMP • Source and core-based trees • PIM • PIM-DM • PIM-SM

25. Multicast Protocols • Receivers • Group Management Protocol - enables hosts to dynamically join/leave multicast groups. Receivers send group membership reports to the nearest router. • Multicast Routing Protocol - enables routers to build a delivery tree between the sender(s) and receivers of a multicast group. • Delivery tree • Senders Multicast Routing Protocol (e.g. PIM-SM) Group Management Protocol (e.g. IGMP)

26. IGMP (RFC 1112) • The Internet Group Management Protocol (IGMP) is a simple protocol for the support of IP multicast. • IGMP operates on a physical network (e.g., single Ethernet Segment. • IGMP is used by multicast routers to keep track of membership in a multicast group. • Support for: • Joining a multicast group • Query membership • Send membership reports

27. IGMP Protocol • A host sends an IGMP report when it joins a multicast group • Multiple processes on a host can join. • A report is sent only for the first process. • No report is sent when a process leaves a group • A multicast router regularly multicasts an IGMP query to all hosts (group address is set to 0.0.0.0). • A host responds to an IGMP query with an IGMP report. • Multicast router keeps a table of the multicast groups that have joined hosts. The router only forwards a packet, if there is a host still joined.

28. IGMP Packet Format • IGMP messages are only 8 bytes long IGMPv20x11 Membership query (group addr: 0.0.0.0)0x16 Membership report0x17 Leave group

29. IGMP Protocol

30. IGMP Protocol

31. Only one router responds to IGMP queries (Querier) Router with smallest IP address becomes the querier on a network. One router forwards multicast packets to the network (Forwarder). Networks with multiple multicast routers

32. Agenda • Multicast • why multicast? • multicast groups • terminology • (S,G) notation • Addressing • IGMP • Source and core-based trees • PIM • PIM-DM • PIM-SM

33. Multicast Protocols • Receivers • Group Management Protocol - enables hosts to dynamically join/leave multicast groups. Receivers send group membership reports to the nearest router. • Multicast Routing Protocol - enables routers to build a delivery tree between the sender(s) and receivers of a multicast group. • Delivery tree • Senders Multicast Routing Protocol (e.g. PIM-SM) Group Management Protocol (e.g. IGMP)

34. Multicast Routing Protocols • Goal: Build a spanning tree between all members of a multicast group

35. Multicast routing as a graph problem • Problem: Embed a tree such that all multicast group members are connected by the tree

36. Multicast routing as a graph problem • Problem: Embed a tree such that all multicast group members are connected by the tree • Solution 1: Shortest Path Tree or source-based treeBuild a tree that 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

37. Multicast routing as a graph problem • Problem: Embed a tree such that all multicast group members are connected by the tree • Solution 2: Minimum-Cost Tree Build a tree thatminimizes the total cost of the edges • Good solution if there are multiple senders • Very expensive to compute (not practical for more than 30 nodes)

38. Multicast routing in practice • Routing Protocols implement one of two approaches: • Source Based Tree: • Essentially implements Solution 1. • Builds one shortest path tree for each sender • Tree is built from receiver to the sender  reverse shortest path / reverse path forwarding (RPF) • Core-based Tree: • Build a single distribution tree that is shared by all senders • Does not use Solution 2 (because it is too expensive) • Selects one router as a “core” (also called “rendezvous point”) • All receivers build a shortest path to the core  reverse shortest path / reverse path forwarding

39. Reverse Path Forwarding (RPF) • RPF builds a shortest path tree in a distributed fashion by taking advantage of the unicast routing tables. • Main concept: Given the address of the root of the tree (e.g., the sending host), a router selects as its upstream neighbor in the tree the router which is the next-hop neighbor for forwarding unicast packets to the root. • This concept leads to a reverse shortest path from any router to the sending host. The union of reverse shortest paths builds a reverse shortest path tree. RPF Forwarding: Forward a packet only if it is receives from an RPF neighbor

40. Multicast Routing table • Routing table entries for source-based trees and for core-based trees are different • Source-based tree: (Source, Group) or (S, G) entry. • Core-based tree: (*, G) entry.

41. Multicast routing in practice • Routing algorithms in practice implement one of two approaches: • Source Based Tree Tree: • Establish a reverse path to the source • Core-based Tree: • Establish a reverse path to the core router

42. Building a source-based tree • Set routing tables according to RPF forwarding • Flood-and-Prune IGMP

43. Building a source-based tree • Set routing tables according to RPF forwarding • Flood-and-Prune Flood= Forward packets that arrive on RPF interface on all non-RPF interfaces

44. Building a source-based tree • Set routing tables according to RPF forwarding • Flood-and-Prune Flood= Forward packets on all non-RPF interfaces Receiver drops packets not received on RPF interface

45. Building a source-based tree • Set routing tables according to RPF forwarding • Flood-and-Prune Prune= Send a prune message when a packet is received on a non-RPF interface or when there are no receivers downstream Prune message disables routing table entry

46. Pruning • Prune message temporarily disables a routing table entry • Effect: Removes a link from the multicast tree • No multicast messages are sent on a pruned link • Prune message is sent in response to a multicast packet • Question: Why is routing table only temporarily disabled? • Who sends prune messages? • A router with no group members in its local network and no connection to other routers (sent on RPF interface) • A router with no group members in its local network which has received a prune message on all non-RPF interfaces (sent on RPF interface) • A router with group members which has received a packet from a non-RPF neighbor (to non-RPF neighbor)

47. Building a source-based tree • When a receiver joins, one needs to re-activate a pruned routing table entry • Grafting Sending a Graft message disables prune, and re-activates routing table entry.

48. Alternative method for building a source-based tree • This only works if the receiver knows the source • Explicit-Join • Receiver sends a Join message to RPF neighbor • Join message creates (S,G) routing table entry • Join message is passed on

49. Building a core-based tree • One route is the core • Receiver sends a Join message to RPF neighbor with respect to core • Join message creates (*, G) routing table entry

50. Building a core-based tree • Source sends data to the core • Core forwards data according to routing table entry