Lecture 6: Multicast

Transcription

1 Lecture 6: Multicast Challene: how do we efficiently send messaes to a roup of machines? Need to revisit all aspects of networkin Routin Autonomous systems and admin control Address allocation Conestion control Reliable delivery Ordered delivery

2 Why send data to multiple machines? How much traffic on Internet today is multicast? In five years?

3 Multicast Motivation Send data to multiple receivers at once broadcastin, narrowcastin telecollaboration software update Hierarchical web caches Send question to unknown receiver peer-to-peer file sharin (nutella, freenet) resource discovery distributed database anonymous directory services

4 Multicast Efficiency Send data only once down link shared by multiple receivers Sender R R R R

5 Native IP Multicast Service Model Provided by (inter) network with help from LAN Delivery best effort (unreliable, unordered, ) packets addressed to roup roup address allocated from special rane in IP TTL for scope control (ex: send one hop)

6 Native IP Multicast Service Model Receivers zero, one or many receivers dynamic -- anyone can join, leave host can belon to any number of roups Senders Anyone can send; need to know address Any number of senders just send packet to roup address May or may not be in roup

7 Is this a ood model?

8 Holbrook and Cheriton IP multicast Can t chare for it Can t know who is listenin Easy to DoS Hard to do dynamic address allocation Doesn t scale Which of these aruments is the real problem?

9 How multicast works today How do we provide multicast services without router/isp support? Virtual networks Application level multicast, tunnelin over Internet Routin can be adaptive to Internet performance Limit tunnel bw to avoid interferin with TCP Widespread use RealNetworks, Internet radio, CDNs, web caches,

10 LAN Multicast Routin Directly supported on shared media LANs App subscribes to roup IP layer notifies Ethernet card to listen to packets with roup address What about switched LANs? ARP requires broadcast support!

11 How do you do broadcast on aswitchedlan?

12 Simple Approach: Floodin If haven t seen a packet before forward it on every link but incomin requires switches to remember each pkt! Sender R R R

13 Forward usin Spannin Tree Ensures every host ets one copy (retransmission needed for drops)

14 Computin a Spannin Tree One (simple) solution Elect a leader (ex: node with lowest id) Spannin tree is shortest path to leader Re-compute if topoloy chanes Another (simple) approach distribute topoloy everywhere, compute in parallel (aka link state)

15 How would you implement multicast on a switched LAN?

16 Switched LAN Multicast Routin Use spannin tree What if only few receivers? Data sent needlessly over some links => prune links if no receivers What if lare scale LAN? No sinle tree efficient for all conditions => spannin tree per roup => spannin tree per source

17 How would you implement multicast on the Internet?

18 Internet Multicast Routin How do we distribute packets across thousands of LANs? Each router responsible for its attached LAN Reduces to: How do we forward packets to all interested routers? (DVMRP, M-OSPF, MBone) How do hosts declare interest to their routers? (IGMP)

19 Basic Approaches Broadcast data and prune where there are no members based on distance vector routin (DVMRP) Broadcast membership and compute forwardin tables based on link state routin (MOSPF) Broadcast rendezvous for roup addresses independent of unicast routin (PIM)

20 Distance Vector Multicast Intuition: unicast routin tables form inverse tree from senders to destination why not use backwards for multicast? Various refinements to eliminate useless transfers DVMRP (Distance Vector Multicast Routin Protocol)

21 Reverse Path Floodin (RPF) Router forwards packet from S iff packet came via shortest path back to S S s R s R R

22 Redundant Sends RPF will forward packet to router, even if it will discard each router ets pkt on all of its input links! Each router connected to LAN will broadcast packet Ethernet

23 Reverse Path Broadcast (RPB) With distance vector, neihbors exchane routin tables Only send to neihbor if on its shortest path back to source Only send on LAN if have shortest path back to source break ties arbitrarily

24 Truncated RPB End hosts tell routers if interested Routers forward on LAN iff there are receivers Challenes: robust to router/host failures avoid overloadin LAN with announcements

25 Internet Group Manaement Protocol (IGMP) To join, host multicasts join requests to hosts in roup + routers on LAN, TTL=1 Routers periodically query hosts for roup membership stop forwardin if receiver crashes Hosts set random timer When expire, multicast join Other hosts in roup disable timer

26 IGMP (step 1) Elect one router to periodically query about all roups (TTL=1) Q

27 IGMP (step 2) One roup member responds to roup with TTL=1, everyone else defers Q G G G

28 Reverse Path Multicast (RPM) Forward packets only to those areas of the network with receivers Broadcast and prune UseIGMPtotellifLANifnomembers If no children are members, propaate prune to parent in tree Assume membership and prune if wron vs. assume non-membership and explicit join

29 How does a receiver join? Graft (prune cancellation) Routers remember where they sent prunes where multicast traffic came from If child joins, send raft in same direction(s) Requires ARQ what if raft is dropped? What if prune is dropped?

30 Phase 1: Truncated Broadcast s

31 Phase 2: Prunin prune(s,) s prune(s,)

32 Phase 3: Graftin raft(s,) report s raft(s,)

33 Phase 4: Steady State s

34 RPM Mechanics Data-driven -- prune only when packet arrives Why? Periodically time out prunes Why? Are loops possible in routin tree?

35 Link State Multicast MOSPF (Multicast OSPF) Use IGMP to determine LAN members Flood topoloy/roup chanes Each router ets complete topoloy, roup membership compute shortest path spannin tree recompute tree every time topoloy chanes add/delete links if membership chanes

36 Link State Mechanics Unicast link state precompute routes for all destinations all-to-all shortest path Use CIDR to reduce table size Multicast link state precompute routes for all source-roup pairs? compute on demand when packet arrives? Are loops possible in routin tables?

37 PIM Motivation PIM = Protocol Independent Routin What about sparse roups -- roups that involve only a few members? Ex: narrowcastin -- chat rooms, virtual corporations, => could be common use Distance vector + link state => periodic broadcast to every router in Internet!

38 PIM Approach Every multicast roup has a set of predefined rendezvous points (RP) need multiple for failure redundancy Receiver join: send packet to one RP Source join: send to all RP s If infrequent traffic, use tree per roup rooted at rendezvous points Add/delete links via reference countin

39 What if frequent traffic? Want to specialize with tree per source All routers below RP observe traffic If lots of packets from one source router sends join to source source sends traffic to router in addition to RPs once router ets traffic from source, prune

40 PIM Shared Tree s RP

41 PIM Receiver Join s RP Join *, Report What if join here?

42 PIM Shared Tree After Join s RP

43 PIM Source Based Tree s RP Join s,

44 PIM Source Based Tree What if source? s RP What if join s,?

45 PIM Protocol Independence? Packets sent to specific tarets usin unicast routin services Receiver join: send to RP Source join: send to RPs Source based tree creation: send to source Multicast packets: send to RPs, routers Multiaccess links confuse reference countin: all routers listen for add/prune

46 How do we decide on RP s? Need consensus of which RP to use for each roup, or nothin works Replicated bootstrap manaer RP candidates reister with manaer Manaer broadcasts candidate RP list Hosts use consistent hash to select RP

47 Hierarchical Routin Motivation Decompose routin problem into subproblems Allow each oranization to choose their own alorithm Reduce table size by areatin addresses

48 Hierarchical Broadcast and Prune Reverse Path Floodin Discard incomin packet if not from reverse path Multicast incomin packet to all borders Reverse Path Multicast For each neihbor AS, compute if we re on reverse path to source Multicast incomin packets to useful borders Propaate prunes across AS back to source

49 Hierarchical Link State Broadcast topoloy, roup membership within each AS Broadcast AS topoloy, roup membership, to all AS s Compute multicast tree across AS s identify incomin/outoin borders Compute multicast tree within each AS

50 Hierarchical PIM Explicit sender and receiver joins Protocol independence => border routers propaate joins across boundaries Hard to interoperate PIM with broadcast&prune

51 Hierarchical Routin Mechanics What if each AS uses its own alorithm? Incomin multicast packet may arrive on any border router How do we detect routin loops? Propaate explicit joins in PIM How do we reduce table size usin areates? Combine nearby sources? similar roups?

52 Express Sinle sender; relay for multisender RPF-based tree Authenticated send/join Enforced by network Countin tree implemented in network Pros? Cons?