Internet组播简介.ppt

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1、1,清华大学计算机系,Internet组播简介,2,主要内容,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议IPv6,3,主要内容,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议IPv6,4,Server,Router,Unicast,Server,Router,Multicast,单播和组播的比较,5,Example:Audio StreamingAll clients listening to the same 8 Kbps audio,0,0.2,0.4,0.6,0.8,T

2、raffic,Mbps,1,20,40,60,80,100,#Clients,Multicast,Unicast,组播的优势,Enhanced Efficiency:Controls network traffic and reduces server and CPU loadsOptimized Performance:Eliminates traffic redundancyDistributed Applications:Makes multipoint applications possible,6,组播带来的问题,Best Effort Delivery:Drops are to b

3、e expected.Multicast applications should not expect reliable delivery of data and should be designed accordingly.Reliable Multicast is still an area for much research No Congestion Avoidance:Lack of TCP windowing and“slow-start”mechanisms can result in network congestion.If possible,Multicast applic

4、ations should attempt to detect and avoid congestion conditions,组播是基于UDP的！,7,组播带来的问题,Duplicates:Some multicast protocol mechanisms(e.g.Asserts,Registers and SPT Transitions)result in the occasional generation of duplicate packetsOut of Order Delivery:Some protocol mechanisms may also result in out o

5、f order delivery of packets,8,组播的应用,MultimediaStreaming media,IPTVTraining,corporate communications Conferencingvideo/audioNet GameAny one-to-many data push applications,9,主要内容,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议IPv6,10,IPv4 Multicast Group Addresses 224.0.0.0239.255.255.255 Class“D

6、”Address Space High order bits of 1st Octet=“1110”Reserved Link-local Addresses 224.0.0.0224.0.0.255 Transmitted with TTL=1 Examples:224.0.0.1 All systems on this subnet224.0.0.2 All routers on this subnet224.0.0.4 DVMRP routers224.0.0.5 OSPF routers224.0.0.13 PIMv2 routers,组播地址,11,Administratively

7、Scoped Addresses239.0.0.0239.255.255.255Private address spaceSimilar to RFC1918 unicast addressesNot used for global Internet trafficUsed to limit“scope”of multicast trafficSame addresses may be in use at different locations for different multicast sessionsExamplesSite-local scope:239.253.0.0/16Orga

8、nization-local scope:239.192.0.0/14,组播地址,12,32 Bits,28 Bits,25 Bits,23 Bits,48 Bits,01-00-5e-7f-00-01,1110,5 BitsLost,组播地址,IP Multicast MAC Address Mapping(FDDI and Ethernet),239.255.0.1,13,224.1.1.1224.129.1.1225.1.1.1225.129.1.1.238.1.1.1238.129.1.1239.1.1.1239.129.1.1,0 x0100.5E01.0101,1-Multicas

9、t MAC Address(FDDI and Ethernet),32-IP Multicast Addresses,组播地址,Be Aware of the 32:1 Address Overlap,IP Multicast MAC Address Mapping(FDDI&Ethernet),14,组播地址,Dynamic Group Address AssignmentHistorically accomplished using SDR applicationSessions/groups announced over well-known multicast groupsAddres

10、s collisions detected and resolved at session creation timeHas problems scaling,15,组播地址,Future dynamic techniques under considerationMulticast Address Set-Claim(MASC)Hierarchical,dynamic address allocation schemeExtremely complex garbage-collection problem Long ways offMADCAPSimilar to DHCPNeed appl

11、ication and host stack support,16,组播地址,Static Group Address AssignmentTemporary method to meet immediate needsGroup range:233.0.0.0-233.255.255.255Your AS number is inserted in middle two octetsRemaining low-order octet used for group assignmentDefined in IETF RFC3180GLOP Addressing in 233/8,17,主要内容

12、,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议IPv6,18,Routers solicit group membership from directly connected hostsRFC 1112 specifies version 1 of IGMPRFC 2236 specifies version 2 of IGMPRFC 3376 specifies version 3 of IGMPSupported on latest service pack for Windows and most UNIX systems,Ho

13、w hosts tell routers about group membership,主机和路由器的交互：IGMP,19,H3,Host sends IGMP Report to join group,H1,H2,Joining a Group,主机和路由器的交互：IGMP,20,Router sends periodic Queries to 224.0.0.1,One member per group per subnet reports,Other members suppress reports,Maintaining a Group,主机和路由器的交互：IGMP,21,Host q

14、uietly leaves group,H1,H3,Router sends 3 General Queries(60 secs apart),No IGMP Report for the group is received,Group times out(Worst case delay=3 minutes),H2,Leaving a Group(IGMPv1),主机和路由器的交互：IGMP,22,Host sends Leave message to 224.0.0.2,H1,H3,Router sends Group specific query to 224.1.1.1,No IGMP

15、 Report is received within 3 seconds,Group 224.1.1.1 times out,H2,Leaving a Group(IGMPv2),主机和路由器的交互：IGMP,23,IGMPv3,RFC3376Enables hosts to listen only to a specified subset of the hosts sending to the group,24,Source=1.1.1.1Group=224.1.1.1,H1-Member of 224.1.1.1,R1,R3,R2,Source=2.2.2.2Group=224.1.1.

16、1,H1 wants to receive from S=1.1.1.1 but not from S=2.2.2.2With IGMP,specific sources can be pruned back-S=2.2.2.2 in this case,IGMPv3:Join 1.1.1.1,224.1.1.1Leave 2.2.2.2,224.1.1.1,IGMPv3,25,主要内容,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议IPv6,26,Shortest Path or Source Distribution Tree,Re

17、ceiver 1,B,E,A,D,F,Source 1,Notation:(S,G)S=Source G=Group,C,Receiver 2,Source 2,组播分发树,27,Receiver 1,B,E,A,D,F,Source 1,Notation:(S,G)S=Source G=Group,C,Receiver 2,Source 2,组播分发树,Shortest Path or Source Distribution Tree,28,组播分发树,Shared Distribution Tree,Receiver 1,B,E,A,D,F,Notation:(*,G)*=All Sour

18、ces G=Group,C,Receiver 2,(RP)PIM Rendezvous Point,(RP),29,组播分发树,Shared Distribution Tree,Receiver 1,B,E,A,F,Notation:(*,G)*=All Sources G=Group,C,Receiver 2,(RP)PIM Rendezvous Point,Shared Tree,D,(RP),30,组播分发树,Source or Shortest Path treesuses more memory O(S G)but you get optimal paths from source

19、to all receiversminimizes delay Shared treesuses less memory O(G)but you may get sub-optimal paths from source to all receiversmay introduce extra delay,Characteristics of Distribution Trees,31,主要内容,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议IPv6,32,组播转发,Multicast Forwarding is backwards fr

20、om Unicast ForwardingUnicast Forwarding is concerned about where the packet is goingMulticast Forwarding is concerned about where the packet came fromMulticast Forwarding uses“Reverse Path Forwarding”,33,组播转发,What is RPF?A router forwards a multicast datagram only if received on the up stream interf

21、ace to the source(i.e.it follows the distribution tree)The RPF Check The routing table used for multicasting is checked against the“source”IP address in the packet If the datagram arrived on the interface specified in the routing table for the source address;then the RPF check succeeds Otherwise,the

22、 RPF Check fails,Reverse Path Forwarding(RPF),34,组播转发,Source151.10.3.21,Example:RPF Checking,Mcast Packets,RPF Check Fails Packet arrived on wrong interface!,35,组播转发,RPF Check Fails!,Unicast Route TableNetwork Interface151.10.0.0/16S1198.14.32.0/24S0204.1.16.0/24E0,A closer look:RPF Check Fails,E0,S

23、1,S0,S2,S1,36,组播转发,A closer look:RPF Check Succeeds,RPF Check Succeeds!,Unicast Route TableNetwork Interface151.10.0.0/16S1198.14.32.0/24S0204.1.16.0/24E0,E0,S1,S0,S2,S1,Forward out all outgoing interfaces.(i.e.down the distribution tree),37,主要内容,为什么需要组播？组播地址主机和路由器的交互：IGMP 组播分发树组播转发域内组播路由协议域间组播路由协议I

24、Pv6,38,Multicast Routing is not unicast routing You have to think of it differently It is not like OSPF It is not like RIP It is not like anything you may be familiar with,组播路由和单播路由,39,组播路由协议的类型,Dense-modeUses“Push”ModelTraffic Flooded throughout networkPruned back where it is unwantedFlood&Prune be

25、havior(typically every 3 minutes)Sparse-modeUses“Pull”ModelTraffic sent only to where it is requestedExplicit Join behavior,40,域内组播路由协议概况,Currently,there are four multicast routing protocolsDVMRPv3(Internet-draft)DVMRPv1(RFC 1075)is obsolete and unused.A variant is currently implementedMOSPF(RFC 158

26、4)PIM-DM(Internet-draft)PIM-SM(RFC 2362-v2)Others(CBT,OCBT,QOSMIC,SM,etc.),41,Dense Mode ProtocolDistance vector-basedSimilar to RIPInfinity=32 hopsSubnet masks in route advertisementsDVMRP Routes usedFor RPF CheckTo build Truncated Broadcast Trees(TBTs)Uses special“Poison-Reverse”mechanism,DVMRP概况,

27、42,DVMRP概况,Dense Mode ProtocolUses Flood and Prune operationTraffic initially flooded down TBTsTBT branches are pruned where traffic is unwantedPrunes periodically time-out causing reflooding,43,DVMRPSource Trees,Route for source network of metric“n”,m,Source Network,E,X,Y,A,B,C,D,Poison reverse(met

28、ric+infinity)sent to upstream“parent”routerRouter depends on“parent”to receive traffic for this source,Truncated Broadcast Trees Are Built using Best DVMRP Metrics Back to Source NetworkLowest IP Address Used in Case of a Tie(Note:IP Address of D C B A),mrouted,mrouted,mrouted,mrouted,mrouted,Result

29、ing Truncated Broadcast Tree for Source Network,mrouted,mrouted,44,DVMRPSource Trees,Forwarding onto Multi-access Networks,Network X,A,B,C,2,2,1,1,mrouted,mrouted,mrouted,Route advertisement for network X of metric“n”,n,Both B and C have routes to network X.To avoid duplicates,only one router can be

30、“Designated Forwarder”for network X.Router with best metric is elected as the“Designated Forwarder”.Lowest IP address used as tie-breaker.Router C wins in this example.,(Note:IP Address of C B),45,E,X,Y,A,B,C,D,DVMRPSource Trees,Resulting Truncated Broadcast Tree for Source Network“S1”,Source Networ

31、k“S1”,S1 Source Tree,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,46,DVMRPSource Trees,Each Source Network has its Own Truncated Broadcast Tree,E,X,Y,A,B,C,D,Note:IP Address of D C B A,S2 Source Tree,Source“S2”,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,47,DVMRPFlood&Prune,So

32、urce“S”,Receiver 1(Group“G”),Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,Initial Flooding of(S,G)Multicast Packets Down Truncated Broadcast Tree,E,X,Y,A,B,C,D,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,48,DVMRPFlood&Prune,Routers C is a Leaf Node so

33、it sends an“(S,G)Prune”Message,Source“S”,Receiver 1(Group“G”),E,X,Y,A,B,C,D,mrouted,Router B Prunes interface.,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,49,DVMRPFlood&Prune,Routers X,and Y are also Leaf Nodesso th

34、ey send“Prune(S,G)”Messages,Prune,Prune,Source“S”,Receiver 1(Group“G”),E,X,Y,A,B,C,D,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,Router E prunes interface.,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,50,DVMRPFlood&Prune,Router E is now a Leaf Node;it

35、sends an(S,G)Prune message.,Prune,Source“S”,Receiver 1(Group“G”),E,X,Y,A,B,C,D,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,Router D prunes interface.,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,51,DVMRPFlood&Prune,Final Pruned State,Source“S”,Receiver

36、 1(Group“G”),E,X,Y,A,B,C,D,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,52,Receiver 2 joins Group“G”,Receiver 2(Group“G”),Router Y sends a“Graft(S,G)”Message,Graft,DVMRPGrafting,Source“S”,Receiver 1(Group“G”)

37、,E,X,Y,A,B,C,D,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,53,DVMRPGrafting,Router E Responds with a“Graft-Ack”,Sends its Own“Graft(S,G)Message,Receiver 2(Group“G”),Source“S”,Receiver 1(Group“G”),E,X,Y,A,B,C

38、,D,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,54,Receiver 2(Group“G”),Source“S”,Receiver 1(Group“G”),E,X,Y,A,B,C,D,mrouted,mrouted,mrouted,mrouted,mrouted,DVMRPGrafting,Router D Responds with a“Graft-Ack”,G

39、raft-Ack,Begins Forwarding(S,G)Packets,mrouted,mrouted,Truncated Broadcast Tree based on DVMRP route metrics,(S,G)Multicast Packet Flow,55,DVMRPEvaluation,Widely used on the MBONE(being phased out)Significant scaling problemsSlow ConvergenceRIP-like behaviorSignificant amount of multicast routing st

40、ate information stored in routers(S,G)everywhere No support for shared treesMaximum number of hops 32Not appropriate for large scale production networksDue to flood and prune behaviorDue to its poor scalability,56,MOSPF(RFC 1584),Extension to OSPF unicast routing protocolOSPF:Routers use link state

41、advertisements to understand all available links in the network(route messages along least-cost paths)MOSPF:Includes multicast information in OSPF link state advertisements to construct multicast distribution trees(each router maintains an up-to-date image of the topology of the entire network),57,M

42、OSPF(RFC 1584),Group membership LSAs are flooded throughout the OSPF routing domain so MOSPF routers can compute outgoing interface listsUses Dijkstra algorithm to compute shortest-path treeSeparate calculation is required for each(SNet,G)pair,58,Membership LSAs,Membership LSAs,Area 1,Area 2,MABR1,M

43、ABR2,Area 0,MOSPF Membership LSAs,59,Area 1,Area 2,MOSPF Intra-Area Traffic,Not receiving(S2,A)traffic,MABR1,MABR2,Area 0,60,MOSPF Inter-Area Traffic,Area 1,Area 2,Wildcard Receivers“pull”traffic from all sources in the area.,MABR1,MABR2,Area 0,61,MOSPF Inter-Area Traffic,Area 1,Area 2,MABR1,MABR2,A

44、rea 0,62,Area 1,Area 2,MABR1,MABR2,MOSPF Inter-Area Traffic,SummarizedMembership LSA,MABR routers inject Summary Membership LSAs into Area 0.,Area 0,Membership LSAs,Membership LSAs,63,Area 1,Area 2,MABR1,MABR2,MOSPF Inter-Area Traffic,Area 0,64,Area 1,Area 2,MABR1,MABR2,MOSPF Inter-Area Traffic,Unne

45、cessary traffic still flowing to the MABR Routers!,Area 0,65,Area 1,Area 2,MABR1,MABR2,MOSPF Inter-Domain Traffic,External AS,MASBR,Area 0,Membership LSAs,Membership LSAs,66,MOSPF Inter-Domain Traffic,External AS,Area 1,Area 2,MA,MABR1,MA,MB,MB,MA,MABR2,MASBR,Area 0,67,External AS,Area 1,Area 2,MABR

46、1,MABR2,MASBR,MOSPF Inter-Domain Traffic,Unnecessary traffic may flow all the way to the MASBR Router!,Area 0,68,MOSPFEvaluation,Appropriate for use within single routing domainFlood multicast traffic everywhere to create state,Uses LSAs and the link-state databaseProtocol dependentworks only in OSP

47、F-based networks,69,MOSPFEvaluation,Significant scaling problemsDijkstra algorithm run for EVERY multicast(SNet,G)pair!Dijkstra algorithm rerun when:Group Membership changesLine-flapsDoes not support shared-trees Not appropriate forGeneral purpose multicast networks where the number of senders may b

48、e quite largeIP/TV(Every IP/TV client is a multicast source),70,PIM-DM,Protocol IndependentSupports all underlying unicast routing protocols including:static,RIP,IGRP,EIGRP,IS-IS,BGP,and OSPFUses reverse path forwardingFloods network and prunes back based on multicast group membershipAssert mechanis

49、m used to prune off redundant flowsAppropriate for.Densely distributed receivers located in close proximity to sourceFew senders-to-many receivers(due to frequent flooding),71,PIM-DM Flood&Prune,Source,Initial Flooding,Receiver,(S,G)State created inevery router in the network!,72,PIM-DM Flood&Prune,

50、Source,Pruning Unwanted Traffic,Receiver,73,PIM-DM Flood&Prune,Results After Pruning,Source,Receiver,Flood&Prune processrepeats every 3 minutes!,(S,G)State still exists inevery router in the network!,74,PIM-DM Assert Mechanism,E0,Incoming Multicast Packet(Successful RPF Check),E0,S0,S0,Compare dista