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IP Multicast

IP Multicast. G53ACC. Chris Greenhalgh School of Computer Science. Contents. What is Multicasting Applications Application Semantics Implementation Multicast addresses and scopes Research Issues Example: DIVE Book: COMER, ch 9.6, 9.7, 18.5, 27.15. What is Multicasting?.

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IP Multicast

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  1. IP Multicast G53ACC Chris Greenhalgh School of Computer Science

  2. Contents • What is Multicasting • Applications • Application Semantics • Implementation • Multicast addresses and scopes • Research Issues • Example: DIVE • Book: COMER, ch 9.6, 9.7, 18.5, 27.15

  3. What is Multicasting? • Defines “multicast groups”… • like a host (destination) address • but identifies a logical destination or group • Any number of hosts can • sends packets to a multicast group • join a multicast group • => receive packets sent to that group • Plus: • each packet crosses any network at most once • packets are filtered in the network and host NICs for interest (joined)

  4. Multicasting example Host Host Host P Host P P P P P Router Host LAN LAN P Forwards P if required joined G, sees P Host P P P Joined G, sees P Host Host Sends P to multicast group G Network interface ignores P

  5. One-to-many A/V distribution, push media, file distribution, cacheing, announcements, monitoring (e.g. stocks). Many-to-many A/V conferencing, synchronized resources (e.g. distributed database), concurrent processing, collaboration, distance learning, chat groups, Distributed Interaction Simulation (DIS), multi-played games, jam sessions. One-to-any / many-to-one (“any-cast”) resource discovery, data collection, auctions Multicast Applications

  6. Application Semantics • For UDP only (unreliable datagrams). • Uses socket interface: java.net.MulticastSocket • See over • Send multicast packets from a normal (unicast) UDP socket, just give a class D destination IP address • or use a multicast socket, if TTL must be specified. • Receive multicast packets only on a MulticastSocket. • joinGroup(addr) performs a JOIN operation, • leaveGroup(addr) or destroying a socket performs a LEAVE operation. • (More complex options with multi-homed machines, using java.net.NetworkInterface)

  7. Java API: java.net.MulticastSocket • public class MulticastSocket extends java.net.DatagramSocket { public MulticastSocket() throws IOException; public MulticastSocket(int port) throws IOException; public void joinGroup(InetAddress mcastaddr) throws IOException; public void leaveGroup(InetAddress mcastaddr) throws IOException; public void setTimeToLive(int ttl) throws IOException;}

  8. ReverseServerMulticast.java excerpts • ...int port = Integer.parseInt(args[1]);InetAddress group = InetAddress.getByName(args[0]);MulticastSocket socket = new MulticastSocket(port);socket.joinGroup(group);…socket.receive(request);… // as per unicast server

  9. Defining Packets/Protocols • E.g. Real-Time Protocol (RTP) (RFC-1889, etc.) • Or make them up… • Remember UDP semantics • Unreliable • Packet-preserving • Unordered • Also consider • Time to join a group (start-up delay) • Time to leave a group (extra traffic) • Locally available number of groups (router state) • Tens rather than thousands! • Scope (see later slides...)

  10. Implementation (1) • Multicast group = Class D destination IP address • starts 11102 • = 224.0.0.0/4 • = 224.0.0.0 with netmask 240.0.0.0 • = 111000000000000000000000000000002 mask 111100000000000000000000000000002 • Mapped to underlying multicast • not ARP! • (LAN) Ethernet multicast addresses • Takes bits from IP address, use in Ether. addr. • But not all 28 variable bits! (24 bits only)

  11. Implementation (2) • Hosts use Internet Group Management Protocol (IGMP) (v.2 RFC-2236, v.3 RFC-3376) • tells network router(s) about groups joined/left • Routers use multicast routing protocol(s) (e.g. DVMRP, MOSPF) • forward a single packet on multiple interfaces • Many routers do NOT do multicast routing and will drop packets • Routers may “tunnel” multicast packets in unicast packets across non-multicast regions • E.g. MBone = Multicast BackBone (historical) • = Multicast “overlay” network, using unicast wide-area

  12. Choosing Multicast Addresses (1): Statically allocated Global addresses • RFC-1700 / IANA (www.iana.org) • e.g. 224.2.0.0-224.2.127.253 Multimedia Conference Calls • Apply to IANA for an allocation • Allocate locally within Autonomous Unit (RFC-3180, “GLOP addressing in 233/8”)

  13. Choosing Multicast Addresses (2): Administrative Scoped Multicast Addresses (RFC-2365) • Configured in network • Link-local scope (i.e. LAN) • 224.0.0.0/24 • i.e. first 24 bits count, like a netmask 255.255.255.0 • Local scope (e.g. department) • 239.255.0.0/16 • Organisation-local scope (e.g. UoN) • 239.192.0.0/14

  14. Choosing Multicast Addresses (3): choosing an address in a range • Make one up and hope (check w. RFCs & scopes) • See also http://www.29west.com/docs/THPM/multicast-address-assignment.html • Hash to an address range and hope • Optionally augment with a monitoring process which detects duplicate use and negotiates a change to a new address • As in the SDR MBone tool

  15. Choosing Multicast Addresses (4) • Source-specific multicast address (if supported) • RFC-4607 [1-to-many applications only] • Dynamically allocated using Multicast Address Allocation Service • E.g. SDR MBone tool for MM conferences or IETF malloc working group outputs???

  16. TTL Scope (How Far do Packets Travel?) • Use IP packet TTL (Time To Live) to determine scope of sending: • TTL 0 = on the same machine • iff “loopback” is on for sending socket and (on Windows) an Ethernet interface is “up”. • TTL 1 = on the same LAN (esp. Ethernet) • TTL >1 = internetwork • on a multicast-capable WAN; • on the MBone (Multicast Backbone). • E.g. • 31 = campus? 63 = country? • 127 = continent? 255 = whole world?

  17. Multicast Research & Deployment Issues • Reliable delivery • N.B. only a subset of receivers may have missed a packet. • Flow and congestion control • N.B. only a subset (one?) of receivers may be experiencing congestion or buffer overflow.=> lowest common denominator?? • Deployment • MBone, PIM, native (ISPs??) • Application-specific bridges and reflectors

  18. World X Example: DIVE System Outline(Swedish Institute of Computer Science, www.sics.se/dive) Process (Agent) Process (Agent) World X Join: TCP state transfer World Y World multicast group: updates, audio, video. Data uses Scalable Reliable Multicast (shared NAcks).

  19. Example: DIVE System Bootstrapping (1) Agent locates diveserver (m/c). (2) Diveserver finds World X’s multicast group. (3) Agent pings multicast group. (4) Agent downloads world definition. (5) Agent creates world locally. DIVE mgmt. multicast group Diveserver World  multicast group First Agent (1) (2) World X... HTTP Server (3) World X definition (5) World X multicast group (files) (4) HTTP transfer (c.f. state transfer)

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