Internet Multicasting

1. Internet Multicasting Chapter 16

2. Hardware Broadcast • Many HW technologies support sending packets to multi destinations concurrently • Broadcasting: most common form • Copy of packet delivered to each destination • Easy on bus technologies (Ethernet) • Done with single packet transmission • Others: SW must forward copies of the packet

3. Broadcast Address • Reserved destination address • Specifies broadcast delivery • Ex. – Ethernet: address of all 1’s • HW at each machine accepts packet for: • The machine’s address • The broadcast address • Chief disadvantage of broadcasting • Demand on resources • Network bandwidth • Computational resources on all machines

4. Hardware Origins of Multicast • Multicasting • Less common form of multi-point delivery • Also supported by hardware technologies • Allows each system to choose if it wants to participate in a given multicast • Large set of addresses reserved for multicast • One address used for each group of machines that want to communicate

5. Multicast is a generalization of all addressing • Unicast: multicast with one computer in the group • Broadcast: multicast with all computers in the group • Multicast: arbitrary computers in the group • Multicast cannot replace conventional addressing • Difference in underlying mechanisms • Forwarding and delivery done differently • Unicast and broadcast • Forwarding depends on network topology • Multicast • Forwarding must send packet to all segments • Conclusion: • Multicast may be a generalization of addressing schemes • However, underlying forwarding and delivery mechanisms make it less efficient

6. Ethernet Multicast • ½ of Ethernet addresses reserved for multicast • Low-order bit of high-order byte distinguishes • 0 for unicast; 1 for multicast • Multicast in dotted hexadecimal notation: 01.00.00.00.00.0016 • If interface configured for Ethernet multicast: 01.00.5E.00.00.0116 • Will accept any packet sent to computer’s unicast address, broadcast address, or above multicast address

7. IP Multicast • IP Multicasting • Internet abstraction of hardware multicasting • Allows transmission to subset of hosts • Generalizes subset • Can spread across arbitrary physical networks • Anywhere throughout the internet • Given subset is called a multicast group

8. IP multicasting has following characteristics: • Group address • Each multicast group has unique class D address • Some groups permanent; some temporary • Number of groups • Up to 228 simultaneous multicast groups • Number limited by constraints on routing table size • Dynamic group membership • Host can join or leave anytime • Host can be member of arbitrary number of groups • Use of hardware • If underlying network HW supports multicast; IP uses it • If not; IP uses broadcast or unicast to deliver IP multicast

9. Inter-network forwarding • Members of IP multicast group can attach to multiple nets • Multicast routers are required to forward IP multicast • Capability usually added to conventional routers • Delivery semantics • Uses same best-effort delivery as other IP datagram delivery • Multicast datagrams can be lost, delayed, duplicated, etc. • Membership and transmission • Arbitrary host can send datagrams to any multicast group • Membership only used to determine who receives

10. Conceptual Pieces • Three conceptual pieces required • Multicast addressing scheme • Effective notification & delivery mechanism • Efficient internetwork forwarding facility • Many goals, details, and constraints present challenges for an overall design

11. Addressing scheme has conflicting goals • Allow local autonomy in assigning addresses • Define addresses that have global meaning • Notification and delivery has same problem • Make effective use of hardware when available • Allow IP multicast over networks w/o HW support • Forwarding facility presents biggest challenge • Want both efficient and dynamic scheme • Route packets along shortest path • Not send copy on path not leading to member of the group • Allow hosts to join and leave a group at any time • IP multicast includes all three aspects • Rest of chapter considers each in more detail

12. IP Multicast Addresses • Permanently assigned addresses • Called well-known • Used for major services on global Internet • Other groups created when needed • Transient multicast groups • Discarded when group member count = 0 • Class D addresses reserved for multicast

13. Figure 16.1 • No identification information in the group bits • Not identify the origin or owner of a group • No info about whether all members on one physical network • Range: 224.0.0.0 through 239.255.255.255 • Lowest address reserved • Others up through 224.0.0.255: routing & group maintenance • Figure 16.2 shows examples of permanently assigned addresses

14. Figure 16.2

15. Multicast Address Semantics • Multicast treated differently than unicast • Multicast address can only be destination • No ICMP messages for multicast datagrams • Ping sent to multicast address never answered • Time-to-live field is honored • Reaches zero; datagram discarded • No ICMP message sent

16. Mapping IP Multicast to Ethernet Multicast • IP multicast standard does not cover all types of network hardware • Does specify mapping to Ethernet multicast • Efficient and easy • Place low-order 23 bits of IP multicast address into the low-order 23 bits of the special Ethernet multicast address 01.00.5E.00.00.0016 • Example: • 224.0.0.2 becomes 01.00.5E.00.00.0216 11101010 00000000 00000000 00000010

17. Mapping is not unique • IP multicast uses 28 significant bits • More than one IP multicast group may map to same Ethernet multicast address 11100000 0xxxxxxx xxxxxxxx xxxxxxxx 11101111 1xxxxxxx xxxxxxxx xxxxxxxx • Scheme chosen as a compromise • Uses 23 of the 28 bits • Chances are small that two groups will be the same • Using fixed part of Ethernet address helps • Makes debugging easier • Eliminates interference between IP and other protocols • Consequences: host may get msg in error • IP software must check incoming datagrams . . . . . .

18. Hosts and Multicast Delivery • IP multicast on single physical network • Host uses HW multicast address • Receiver always listening to it • Multicast throughout the internet • Special multicast routers forward the datagrams • Host must send datagram to multicast router • Does via the hardware, like in local multicast • Diff between local & non-local in the routers

19. Multicast Scope • Scope of multicast group • Refers to dispersion of group members • Perhaps restricted to a network or organization • Scope of multicast datagram • Set of networks over which datagram will be sent • Informally, datagram’s scope is called its range • IP uses two techniques to control scope • Time-to-live field • Administrative scoping

20. TTL field control • Set to small value, host can limit distance it’s routed • Control messages have TTL of 1 (host-router comm) • Applications on single host can use IP multicast • Set TTL value to 0; never leave host • Can configure site routers such that a certain TTL is needed or else the datagram will never leave the site • TTL field gives course-grain control over scope • Administrative scoping • Reserve parts of address space • Do for local groups or groups part of a single org • Routers forbidden from forwarding datagrams with addresses from this space

21. Extending Host SW to Handle Multicasting • Host participates in IP multicast in 1 of 3 levels (1) Host can neither send nor receive IP multicast (2) Host can send but not receive IP multicast (3) Host can both send and receive IP multicast • Modifications for host sending are not difficult • IP SW allows application to specify multicast as destination • Network interface SW must be able to map IP multicast address into HW multicast address

22. Extending host to receive is more complex • Host IP SW must have API allowing programs to join & leave multicast groups • If multiple applications join, must pass all a copy • If all leave, host must know it no longer participates • Host must inform multicast routers of membership • Hosts join specific IP groups on specific networks • Host may have multiple network connections • May join group on one network and not on another • Keep group membership associated with networks • Allows multicast use with local machines on one net • SW must keep separate lists of multicast addresses for each network • Application must specify a particular network when joining or leaving a multicast group

23. Internet Group Management Protocol • To participate in IP multicast, host must: • Local network multicast: • Have SW allowing send/receive multicast datagrams • Multicast that spans physical networks • Must inform local multicast routers • Multicast routers must know memberships • Use IGMP to communicate group membership • Current version is 3; knows as IGMPv3

24. IGMP is like ICMP • Uses IP datagrams to carry messages • IGMP provides a service used by IP • Think of as integral part of IP, not separate protocol • IGMP is a standard for TCP/IP • Required on all machines receiving IP multicast • Conceptually, IGMP has two phases • Host joins group; sends message • Routers get and establish routing • Routers poll hosts to see if still members • If none report, router stops advertising membership

25. IGMP Implementation • IGMP designed to avoid adding overhead • May have multiple multicast routers on a net • May have hosts participating, too • Must avoid having all participants generate control traffic • Several way IGMP minimizes effect on network

26. All host/router communication uses IP multicast • IP destination address is a multicast address • Datagrams with IGMP messages transmitted via HW multicast • Hosts not using IP multicast never receive IGMP messages • When polling, single query sent about all groups • Not send a separate query for each group • Default polling rate is 125 seconds • Single polling router used • Even if multiple multicast routers attach to same network • Hosts respond to queries at different times • Query contains value N • Hosts pick random number between 0 and N to wait • Have multiple groups; pick different number for each • Reports for multiple group memberships can be sent in a single packet

27. Group Membership State Transitions • IGMP must remember status of each multicast group to which host belongs • Keeps a table to record group membership • When host joins, allocates entry • Keeps group reference counter; initializes to 1 • Another application joins; increment counter • Application terminates execution or drops out; decrement • Counter reaches zero; informs multicast router

28. The three possible states of an entry in a host’s multicast group table and transitions among them, where each transition is labeled with an event and an action. The state transitions do not show messages sent when joining and leaving a group. Figure 16.4

29. IGMP Message Format • Skip: • 16.16: IGMP Membership Query Message Format • 16.17: IGMP Membership Report Message Format

30. Multicast Forwarding & Routing Information • Unanswered questions • How do routers exchange membership info? • How do routers ensure datagrams get to all group members? • No single standard for propagation • No real agreement on an overall plan • Protocols differ in goals and basic approach

31. Why is multicast routing so difficult? • Multicast routing differs from conventional routing because multicast forwarding differs Figure 16.9

32. Need dynamic routing • No “dots” on net 2; not send packets there • But, host can join at any time • Unicast routing: changes when topology changes • Multicast: change when application leaves/joins group • Insufficiency of destination routing • F & E can send to cross group • Same destination; different actions • A send to cross group: still another action • Multicast forwarding requires a router to look at more than destination address

33. Arbitrary senders • Any host can send to the group; not just members • G can send to dotted group • Not a member of the group • No members on G’s network • Datagram may pass through other networks with no members of the group • So: • Multicast datagram may originate on a computer that is not part of the group • May be forwarded across networks that do not have any group members attached

34. Basic Multicast Forwarding Paradigms • Routers must use more than destination • What exactly do they use? • Multicast destination represents a set of computers • Want to reach all members of the set • Do not want to route over same network twice • Partial solution: do not send back on arriving interface • Will not prevent problem if set of routers form loop • Rely on datagram’s source address to avoid loops

35. One idea: Reverse Path Forwarding • Uses source address to prevent loop • Multicast router must have conventional table • Have shortest path to all destinations • Datagram arrives • Looks up in table; finds interface I which leads to source • If it arrived on I; forward copy to all other interfaces • Otherwise, discard the copy • Each multicast datagram goes to every network • Every host in multicast group will receive it • RPF alone not used – wastes bandwidth • Transmits to nets with no members & lead to no members

36. Truncated Reverse Path Forwarding • Modified version of RPF • Avoids sending datagrams where not needed • Multicast router needs to have: • Conventional routing table • List of multicast groups reachable thru each NI • When a multicast datagram arrives: • Router first applies RPF rule • If should discard, does so • If not, checks to see if one or more members in the destination are reachable over each interface • Truncates sending when no more members lie along path • Uses both source and destination addresses in decision

37. Consequences of TRPF • Guarantees each member gets datagram • Has two surprising consequences • Delivers an extra copy to some networks • Delivery depends on the datagram’s source

38. If A is source, Net 4 & B will get duplicate copies Figure 16.10

39. Behavior of delivery depends on the source Figure 16.11

40. Multicast Trees • Use graph theory to describe a tree • Set of paths from a given source to all members • Graph is tree if no cycles appear • That is, router is on no more than one path • Called forwarding tree or delivery tree • Each multicast router is a node in the tree • Network that connects two routers is an edge • Last router along each path from source is a leaf • Network hanging off leaf router is a leaf network

41. Tree with root X • Leaves R3, R4, R5, and R6

42. Technically not a tree • R3 lies along two paths • Informally, referred to as a tree anyway

43. Important principle • Forwarding tree is defined as set of paths through multicast routers from source to all members of a multicast group • For a given multicast group, each possible source can determine a different forwarding tree • An immediate consequence concerns the size of tables used for forwarding

44. Each entry in multicast table is a pair: (multicast group, source) • Conceptually, source identifies a single host that can send datagram • In practice, do not keep separate entry for each host • All forwarding trees defined by all hosts on a single network are identical • To save space, use a network prefix as source • Router defines on forwarding entry for all hosts on same net • Aggregating entries by net prefix reduces table size • Multicast tables can still be much larger than conventional • Conventional: size proportional to number of networks • Multicast: proportional to product of number of networks and number of multicast groups

45. Essence of Multicast Routing • Inconsistency between IP multicast & TRPF • TRPF not forward datagram to a network unless that network leads to a member • Multicast router must know about group membership • IP allows host to leave/join at any time • Leads to rapid changes • Group membership must be propagated • Since membership does not follow local scope

46. Membership issue central to routing • All multicast routing schemes propagate info • In addition to using info for forwarding • Rapid changes may make router info imperfect • Routing updates may lag changes • Multicast design represents a tradeoff • Routing traffic vs. inefficient data transmission • Must propagate group membership info or routers will not forward datagrams efficiently • If scheme communicates with every member, resulting traffic will overwhelm an internet • Every design is a compromise

47. Reverse Path Multicasting • Derived from TRPF • Extensions make it more dynamic • Three underlying assumptions: • Receipt by every member more important than eliminating unnecessary transmissions • Multicast routers have conventional tables with correct information • Multicast routing should improve efficiency when possible

48. RPM uses two step process • At start, uses RPF broadcast scheme • Ensures all networks get copy of datagram • At same time, RPM has routers inform one another about paths not leading to members • Routers stop forwarding along such paths • How do routers learn location of members? • RPM propagates information bottom-up • Starts with hosts that join or leave groups • Sends info via IGMP to local router • Thus, routers know about local members, but not distant

49. Leaf routers can decide if forward to leaf networks • Does not forward if no members of the group • Leaf router informs next router along path back to source • Whenever router learns no members lie beyond it • Stops forwarding; informs next router back toward root • Called pruning a path from the forwarding tree • RPM called broadcast and prune • Router broadcasts using RPF • Until get information that allows a path to be pruned • RPM system also called data-driven • Router does not send membership info to any other routers until datagrams arrive for the group

50. Data-driven model must also handle case of host joining after path has been pruned • RPM handles joins bottom-up • Host informs local router it has joined • Router consults its record of the group • Obtains address of router previously sent prune msg to • Send new message that undoes prune • Known as graftrequests