1. Engineering Workshops 40 Multicast on the LAN

2. Engineering Workshops 41 Multicast Addressing at Layer 2 An IPv4 multicast address is 32 bits, of which the first 4 bits are always the same, leaving 28 bits. A MAC multicast address is 48 bits, of which the first 24 bits are always the same (01-00-5E). One of the remaining bits is reserved, leaving 23 bits. So, one multicast MAC address maps to 32 multicast IP addresses. See Interdomain Multicast Routing, p. 18.

3. Engineering Workshops 42 000000010000000001011110 0831 0 32-bit IP address 48-bit Ethernet address 0 24 0 = Internet multicast 1 = Reserved for other use IANA owns 01-00-5E vendor address block; half of it is assigned for IP multicast. IEEE Ethernet m ulticast bit 1110 ignored, leaving 28 bits 47 Ethernet Multicast Addressing 23 bits 01-00-5E- Class D address 00-00-00 thru 7F-FF-FF

4. Engineering Workshops 43 11101001111111110000000111111111 233.255.1.255 11111110000000111111111 011111110000000111111111 011111110000000111111111000000010000000001011110 0 0 1 1 5 5 E E 0 0 0 0 01:00:5e:7f:01:ff 233 255 1 1 32 bits binary 23 bits binary IP Multicast Address 48 bit Ethernet address Ethernet Multicast Addressing

5. Engineering Workshops 44 IGMP Internet Group Management Protocol - how hosts tell routers about group membership Routers also solicit group membership from directly connected hosts RFC 1112 specifies version 1 of IGMP –Supported on Windows 95 RFC 2236 specifies version 2 of IGMP –Supported on current Windows releases, and most UNIX systems RFC 3376 specifies version 3 of IGMP –Provides source include-list capabilities (SSM!) –Included in Linux kernel 2.6 and later –Supported by Windows XP –Not supported by MacOS X

6. Engineering Workshops 45 IGMPv2 Router: –sends Membership Query messages to All Hosts (224.0.0.1) default query-interval = 125 seconds –router with lowest IP address is Querier (rest non-queriers) –If lower-IP address query heard, back off to non-querier state Other Querier Present Interval default: (robust-count x query- interval) + (0.5 x query-response-interval) = 255 seconds –listens for reports (whether querier or not) and adds group to membership list for that interface default query-response-interval = 10 seconds –timeout (Group member interval) default: (robust-count x query-interval) + (1 x query-response-interval) = 260 seconds –robust-count - provides fine-tuning to allow for expected packet loss on a subnet. Default = 2 (tunable from 2-10)

7. Engineering Workshops 46 IGMPv2 Host: –responds to router query with Membership Report messages to groups it is a member of (e.g.224.10.8.5) waits 0-10 sec (default; specified in Query) Hosts listen to other host reports Only 1 host responds. Others become “idle-members.” –sends unsolicited Membership Reports (i.e., Join Messages) to group address (e.g. 224.10.8.5) –sends Leave messages to All Routers (224.0.0.2) –reports group membership ONLY – no sources. –Only the existence of local group members is known, not the actual members themselves (due to idle-member state).

8. Engineering Workshops 47 IGMP Protocol Flow - Join a Group Router triggers group membership request to PIM. Hosts can send unsolicited Join membership messages – called reports in the RFC (usually more than 1) Or hosts can join by responding to periodic query from router I want 230.0.0.1 230.0.0.1 Forwards stream Router adds group I want to JOIN! 230.0.0.1

9. Engineering Workshops 48 IGMP Protocol Flow - Querier Hosts respond to query to indicate (new or continued) interest in group(s) –only one host should respond per group Hosts fall into idle-member state when same-group report heard. After 260 sec with no response, router times out group. 224.0.0.1 Still interested? (general query ) 224.0.0.1 125 sec I want 230.0.0.1 230.0.0.1 0-10 sec 230.0.0.1 group Yes, me!

10. Engineering Workshops 49 IGMP Protocol Flow - Leave a Group Hosts that support IGMPv2 send Leave messages to all-routers group indicating group they’re leaving. –Router follows up with 2 group-specific query messages. IGMPv1 hosts leave by not responding to queries (260 sec timeout). I don’t want 230.0.0.1 anymore 224.0.0.2 230.0.0.1 group I want to leave! Anyone still want this group? 230.0.0.1 230.0.0.1 1 sec (re-transmit timer)

11. Engineering Workshops 50 Soft State Say I set up an active multicast group, say by issuing a membership report. What happens if my computer goes down and never directly leaves the group? This is fixed with “Soft State” –Everything has a timer, and if not periodically reinitiated the timer will expire and the state will be removed. –So there is no danger of some rogue group lasting forever.

12. Engineering Workshops 51 Source = 1.1.1.1 Group = 232.1.1.1 H1 - Member of 232.1.1.1 R1 R3 R2 Source = 2.2.2.2 Group = 232.1.1.1 IGMPv3: MODE_IS_INCLUDE Join 1.1.1.1, 232.1.1.1 IGMPv3 H1 wants to receive from S = 1.1.1.1 but not from S = 2.2.2.2 With IGMPv3, specific sources can be included or excluded. In this case S = 1.1.1.1 is included. Specified in RFC 3376 Enables hosts to listen only to a specified subset of the sources sending to the group Video Server

13. Engineering Workshops 52 IGMPv3 Differences Group-Source Report message is defined. Enables hosts to specify which senders it can receive data from. This maps directly to a PIM (S,G) Join for SSM. Group-Source Leave message is defined. Enables host to specify the specific IP addresses of a (source,group) that it wishes to leave. Other source filtering is supported, such as requesting traffic from all sources except those specified (blocking). This does not map directly to PIM (S,G) Join.

14. Engineering Workshops 53 IGMPv3 Differences (cont’d) Membership reports (Joins, Leaves) are sent to all IGMP listeners (224.0.0.22) instead of to. The idle-member state is eliminated. Hosts always send group membership reports to 224.0.0.22. Membership report packet now can contain a list of group records including, for example, multiple group memberships. Examples follow...

15. Engineering Workshops 54 IGMPv2 Frame 2 (46 bytes on wire, 46 bytes captured) Ethernet II, Src: 00:00:86:51:bd:b2, Dst: 01:00:5e:05:06:07 Internet Protocol, Src Addr: 128.59.7.10 (128.59.7.10), Dst Addr: 224.5.6.7 (224.5.6.7) Internet Group Management Protocol IGMP Version: 2 Type: Membership Report (0x16) Max Response Time: 0.0 sec (0x00) Header checksum: 0x03f3 (correct) Multicast Address: 224.5.6.7 (224.5.6.7)

16. Engineering Workshops 55 IGMPv3 Frame 10 (62 bytes on wire, 62 bytes captured) Ethernet II, Src: 00:00:86:51:bd:b2, Dst: 01:00:5e:00:00:16 Internet Protocol, Src Addr: 128.59.7.10 (128.59.7.10), Dst Addr: 224.0.0.22 (224.0.0.22) Internet Group Management Protocol IGMP Version: 3 Type: Membership Report (0x22) Header checksum: 0x0de3 (correct) Num Group Records: 2 Group Record : 224.5.6.8 Mode Is Exclude Group Record : 224.5.6.7 Mode Is Exclude

17. Engineering Workshops 56 Switches and Snooping IGMP host reports (Joins) tell the router to start sending multicast traffic to the LAN, since one or more hosts on the LAN are members of the group. In a conventional shared broadcast LAN using switches that have no multicast smarts, the traffic is flooded to all hosts. With multiple high bandwidth multicast sources (e.g. video at 5 Mbps), this does not scale. There are a few techniques used to deal with this...

18. Engineering Workshops 57 IGMP Snooping Implemented by several vendors. Support for IGMPv2 is common; support for IGMPv3 is becoming more common. What happens at the MAC layer: –IGMP snoopers add a bridge table entry for each multicast group destination address (GDA) to each switch port that has the interested member's unicast source address (USA) already on it. (Remember that there are likely to be hubs or switches downstream of a given switch port, so more than one USA can be on a single port.) –When an IGMP Leave is received, the GDA entries are pruned.

19. Engineering Workshops 58 Why IGMP snooping is harder than it looks The IGMP membership reports have to be captured from each host and suppressed to other hosts to prevent the others from going into idle-member state. Every interested host has to be spoofed into thinking it is the only member of the group, so that it actively sends membership reports. The IGMP snooper then forwards one of these membership reports up to the router or makes up a fake membership report coming from one of: –the host –the switch’s management IP address, or –0.0.0.0

20. Engineering Workshops 59 Why IGMP snooping is harder than it looks, continued Since multiple USAs can be on a port (via downstream switch), the switch has to actually do the IGMP membership query/timeout before pruning a port. Since membership reports are sent to the same GDA as the (possibly high-bandwidth) multicast traffic, there is a potential for heavy loading of the switch CPU, unless you use more expensive ASICs that can separate the IGMP protocol messages from general traffic and route only the IGMP messages to the CPU. The switch has to know which is the multicast router port. It does this by snooping for IGMP queries.

21. Engineering Workshops 60 Join without IGMP snooping Switch 230.0.0.1 I want 230.0.0.1 1. Host A sends membership report. 2. Switch floods it to all ports. 3. Router sends traffic (floods). 4. Host B wants to join. No IGMP message needed (idle-member).

22. Engineering Workshops 61 Join with IGMP snooping Switch 230.0.0.1 I want 230.0.0.1 230.0.0.1 1. Host A sends membership report. 2. Switch forwards it to router. 3. Router sends traffic. 4. Host B sends membership report. Switch suppresses it and adds port to bridge table.

23. Engineering Workshops 62 Maintaining state w/IGMP snooping Switch 230.0.0.1 224.0.0.1 General Query 224.0.0.1 ? 230.0.0.1 1. Router sends general query. 2. A&B both respond w/membership report (no idle member). 3. Switch sends one to router and suppresses one.

24. Engineering Workshops 63 Leave with IGMP snooping Switch 224.0.0.22 230.0.0.1 done 230.0.0.1 ? 1. Host A sends Leave. 2. Switch spoofs G-specific query. 3. No reply, switch prunes port. (Nothing sent to router.)

25. Engineering Workshops 64 5. Router sends 2 G-specific queries, gets no response, and prunes the group. (Queries may [not] be suppressed) Leave with IGMP snooping, cont’d Switch 224.0.0.22 230.0.0.1 done 230.0.0.1 ? 224.0.0.22 230.0.0.1 ? 1. Host B sends Leave. 2. Switch spoofs G-specific query. 3. No reply; switch prunes port. 4. Switch sends Leave to router.

26. Engineering Workshops 65 Sourcing Multicast: conventional switch Switch 230.0.0.1 Video Server Multicast is just like broadcast: Flooded out all ports.

27. Engineering Workshops 66 Sourcing with multicast-aware switch Switch 230.0.0.1 Video Server Multicast traffic is forwarded only to mrouter ports (learned by snooping for IGMP queriers). Exception: flood 224.0.0.0/24

28. Engineering Workshops 67 CGMP The proprietary Cisco Group Management Protocol puts the bulk of the Layer 3 logic in Layer 3 devices rather than cramming it into Layer 2 devices like IGMP snooping does. The router sends CGMP Joins and Leaves to the switch, specifying the USA and GDA. On receipt of an IGMP Membership Report, the router sends the switch a CGMP Join. On receipt of an IGMP Leave, the router sends the switch a CGMP Leave. IGMP membership reports still have to be suppressed so that hosts don't go into idle-member state.

29. Engineering Workshops 68 CGMP, continued CGMP does not work correctly with IGMPv3. –Leaves are not fully implemented, resulting in channel surfers causing a multicast flooding DoS on their subnet. –See http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/ 122cgcr/fipr_c/ipcpt3/1cfmulti.htm#1046127. Alan Crosswell notes that while CGMP is poorly documented, the hundreds of CGMP switches in his network at Columbia generally work pretty well for IGMPv2. Newer switch models (running IOS rather than CatOS) seem to have abandoned CGMP.

30. Engineering Workshops 69 PIM Snooping and RGMP For Layer 2 networks with routers but no hosts (transit LANs). PIM, not IGMP, is spoken among routers, so IGMP snooping does not work in this case. PIM snooping and the Cisco-proprietary Router Group Management Protocol (RGMP) are used by the Layer 2 switch to send only the multicast flows that the router needs to the router's port. These work analogously to IGMP snooping (smarts in the switch) and CGMP (smarts in the router). PIM snooping is still mostly experimental. Some Foundry Networks switches support it. Cisco RGMP appears to only work for non-trunked interfaces (on Cat 6500 MSFC/2 IOS 12.1).

31. Engineering Workshops 70 Problems with Multicast on the LAN In general, multicast on the LAN is not as well understood as multicast on the WAN. Switch behaviors are not standardized. But see RFC 4541, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", at http://www1.ietf.org/html.charters/magma-charter.html Problems with switches: –when snooping is enabled and CPU load is high, they may drop packets that shouldn’t be dropped. –even without snooping, sometimes they step outside their bailiwick, trying to do non-Layer-2 tasks.

32. Engineering Workshops 71 Observed problems w/snooping switches See www.columbia.edu/~alan/igmp/ Incomplete/nonexistent IGMPv3 implementations, including among vendors who claim IGMPv3 support. Join works, Leave doesn’t, sometimes leading to flooding when the switch’s soft-state times out before the router’s. Flooding of sourced (non-224.0.0.0/24) traffic to other switch ports. Implementations vary across hardware/software versions of “same” vendor platform (e.g Catalyst 4500 Sup2 vs. Sup2+).

33. Engineering Workshops 72 Case Study A few months ago I converted all our interfaces over to IGMP Version 3. Then I started getting complaints from our lab/classroom support group that Norton Ghost was failing for them. It would hang after about 3 minutes. So far the fix, without understanding why it works, has been to revert the interfaces to IGMP version 2. The switches downstream from these interfaces are running CGMP and CGMP LEAVE (which is actually a form of IGMP snooping/spoofing for IGMP Leaves sent to 224.0.0.2). I suspect that the fact that these switches are actually looking at IGMP packets may have something to do with the problem that reverting to v2 fixed... — Alan Crosswell

34. Engineering Workshops 73 Case Study This author traveled to Los Alamos, New Mexico to help debug a multicast problem that had everyone stumped. Everyone was assuming the only known router on the subnet was also acting as the multicast gateway. Unfortunately, this wasn’t the case. A nominally Layer 2 switch on the subnet was accidentally configured with PIM active, and won the PIM Designated Router election. Of course, this Layer 2 switch had no upstream to anywhere. — Bill Nickless

35. Engineering Workshops 74 More generally... Switches and snooping may be evils, but they are necessary evils. Learn to cope with them. www.cisco.com/warp/public/473/22.html is a good place to start.

36. Engineering Workshops 75 Lab 1 Multicast on the LAN Time: Approx. 1 hour

37. Engineering Workshops 76 Multicast Routing: PIM-SM

38. Engineering Workshops 77 PIM-SM Protocol Independent Multicast - Sparse Mode The core multicast protocol: builds and tears down multicast trees. Documented in RFC 4601 “Protocol Independent” means independent of the protocol used to build the reachability table, not independent of IP. (More on reachability in a moment.) “Sparse Mode” refers to the explicit join approach taken by PIM-SM — the protocol assumes that not everyone wants the data. PIM also has a Dense Mode, which starts with the assumption that everyone does want the data. This is also known as a flood-and-prune approach. Not recommended. Cisco offers a proprietary Sparse-Dense Mode, which is used for RP discovery.

39. Engineering Workshops 78 Multicast routing can be thought of as the reverse of unicast forwarding. –Unicast forwarding is concerned with where the packet is going. –Multicast routing is concerned with where the packet will be coming from. Multicast paths to receivers form a “tree”. The tree is built (or torn down) from the receiver back toward the source. This is easy to forget, but very important to remember. Multicast Routing

40. Engineering Workshops 79 Multicast Routing PIM-SM uses an externally-provided reachability table to build forwarding topology. The unicast forwarding table and the reachability table contain the same kind of information — unicast routes, or reachability information — and may in fact be the same table. (The point of having separate tables is to enable separate policies and paths for unicast forwarding and multicast routing. You need MBGP, IS-IS, or static mroutes to do this.)

41. Engineering Workshops 80 Multicast Routing Multicast forwarding topology is stored in outgoing interface lists (OILs). On each router, PIM-SM maintains an OIL for each group for which it has downstream listeners. Multicast packets received from a given source for a given group are sent out only on the interfaces specified in the appropriate OIL.

42. Engineering Workshops 81 Multicast Routing When a unicast packet shows up on a router interface, the destination address is looked up in the unicast forwarding table to determine where the router should send the packet next. When a multicast (S,G) Join shows up on a router interface, the source address, S, is looked up in the reachability table to determine which of the router's interfaces offers the best way to reach S. This is called a reverse-path lookup. The router adds the interface on which the (S,G) Join was received to the appropriate OIL, and sends an (S,G) Join to the next upstream router, as determined by the reverse-path lookup.

43. Engineering Workshops 82 Multicast Routing The process of doing reverse-path lookups, making OIL entries, and sending Joins continues hop-by-hop until it reaches a) a router that already has the necessary state, or b) the source's first-hop router. This process is called reverse-path forwarding (RPF); the reachability table is also called an RPF table. Once the multicast distribution tree is built, multicast forwarding works similarly to unicast forwarding — but instead of using unicast forwarding tables to send packets out single interfaces, routers use OILs to send packets out multiple interfaces.

44. Engineering Workshops 83 Multicast Distribution Trees A shortest path tree (SPT) is a tree rooted in a multicast source. An SPT is sometimes called a source tree. A rendezvous point tree (RPT) is a tree rooted in a multicast rendezvous point (RP). An RPT is sometimes called a shared tree.

45. Engineering Workshops 84 Multicast Distribution Trees In the original multicast service model, a connection between a source and a receiver is first set up by building an RPT from the receiver back to the RP, and an SPT from the RP back to the source. Once data starts flowing to the receiver, an SPT is built directly from the receiver back to the source.

46. Engineering Workshops 85 Shortest Path Tree Group Member 1 Source Group Member 2 State Information: (S, G) S = Source G = Group

47. Engineering Workshops 86 Rendezvous Point Tree Group Member 1 Source 1 Group Member 2 State Information: (*, G) * = Any Source G = Group Source 2 Rendezvous Point RP Tree Shortest Path Trees

48. Engineering Workshops 87 Shortest Path Tree –More resource-intensive; requires more state (of order n(S x G)) –You get optimal paths from source to all receivers, which minimizes delay –Best for one-to-many distribution Rendezvous Point Tree –Uses less resources; requires less state (of order n(G)) –You may get suboptimal paths from source to all receivers, depending on topology –The RP itself and its location may affect performance –Best for many-to-many distribution –Necessary for in-band source discovery Multicast Distribution Trees Compared

49. Engineering Workshops 88 SSM

50. Engineering Workshops 89 ASM and SSM: Two ways to use PIM-SM ASM: Any-Source Multicast. Traditional multicast – data and joins are forwarded to an RP. –All routers in a PIM domain must have RP mapping. –When load exceeds threshold, forwarding switches to an SPT. The default threshold is one packet; in this case, the sole purpose of the RPT is to learn which sources are active. (With IGMPv2, the receiver can only specify the group, not specific sources.) –State increases (not everywhere) as number of sources and number of groups increase. –SPT state is refreshed when data is forwarded and with Join/Prune control messages. SSM: Source-Specific Multicast. PIM-SM without RPs – instead, the source is learned out-of-band, and the SPT is built directly to it.

51. Engineering Workshops 90 SSM Source-Specific Multicast (SSM) is a subset of ASM, so –SSM concepts apply directly to ASM, but –SSM is a lot simpler than ASM. For these reasons, we cover SSM first in this workshop. 232/8 is assigned to SSM as an address space. Other address ranges can also be set up for SSM — this is primarily a function of the receiving network. Source activity and IP addresses are assumed known. IGMPv3 allows for “Include” lists of (S,G) pairs.

52. Engineering Workshops 91 SSM RFC 4607 232/8 – IANA assigned No RPTs Guarantees ONE source on any delivery tree –Content security – no unwanted sources Reduced protocol dependence – more later... Solves address allocation issues for inter-domain one-to-many –tree address is 64 bits – S,G Host must learn source address out-of-band (e.g, from a web page) Host-to-router join request specifies source as well as group –requires IGMPv3 for include-source list SSM behavior in 232/8 by default –Configurable to expand range

53. Engineering Workshops 92 SSM in Action Each (S,G) pair listed in the IGMPv3 include list generates a (S,G) Join directly towards the source. That’s it. It’s very simple. All you need to implement is: –Edge routers need IGMPv3 –Interior routers need filters to prevent RP (*,G) Joins & other RP state for the SSM address block

54. Engineering Workshops 93 SSM Group Addresses 232/8 is assigned to SSM as an address space. –You don’t have to ask, you can just pick one and use it. How can this be ? –Note that all joins are unique as long as the combination of S and G are unique. Not only can one source support multiple groups, but if there are two sources using the same group address, everything works just fine.

55. Engineering Workshops 94 SSM Receiver RP Receiver announces desire to join group G AND source S with an IGMPv3 include-list. IGMPv3 host report (S, G) Join Last-hop router joins the SPT. Source Shortest Path Tree Traffic Flow

56. Engineering Workshops 95 SSM Receiver RP Data flows down the shortest path tree to the receiver. Source Shortest Path Tree Traffic Flow

57. Engineering Workshops 96 Lab 2 SSM Time: Approx. 1 hour