Chapter 4 IP Multicast Professor Rick Han University of Colorado at Boulder

Prof. Rick Han, University of Colorado at Boulder Announcements Netstat portion of Homework #3 on Web, due March 12 (two weeks) Programming Assignment #2 coming Friday… Midterm March 14 Next, IP multicast

Prof. Rick Han, University of Colorado at Boulder Network Topology for Programming Assignment #2 B F A C E D Figure 1 1. Distance Vector Routing 2. Link State Routing

Prof. Rick Han, University of Colorado at Boulder Recap of Previous Lecture More on Hierarchy in the Internet BGP, IBGP OSPF Subnets and CIDR prefixes DHCP Dynamic IP address Local client broadcasts to DHCP Relay, which unicasts to a DHCP Server ICMP Reports IP packet delivery errors back to source Ping: ICMP echo and echo reply, “smurf” attack Traceroute: ICMP time expired Router advertisement and solicitation

Prof. Rick Han, University of Colorado at Boulder IP Multicast Unicast: one source to one destination Broadcast: one source to every destination Multicast: one source to N destinations –Variant: N sources to N destinations Application Scenarios: –Video conference –Audio concert –Interactive games Example: –C wants to multicast to D and F Multicast Receiver Multicast Receiver Multicast Sender

Prof. Rick Han, University of Colorado at Boulder IP Multicast (2) How do we multicast 1->N efficiently? –N unicast connections can waste bandwidth over shared links not scalable: sender would have to send 100,000 copies of the same packet to 100,000 subscribers –Hint: use the shortest path spanning tree to efficiently route multicast packets

Prof. Rick Han, University of Colorado at Boulder Link-State IP Multicast Dijkstra gives us the shortest path spanning tree –Root B of multicast tree sends packets only to nearest neighbors on tree, rather than to 100,000 members –Neighbors forward to their neighbors along shortest path tree, and so on… –Also called MOSPF

Prof. Rick Han, University of Colorado at Boulder Link-State IP Multicast (2) How does a router know an IP packet is a multicast packet, and not a unicast packet? –IP packets have a special multicast address –Class D IP addresses: top 4 bits are “1110” followed by 28 bits to identify the multicast “group” address G Not all nodes want to be part of the multicast tree –Participating nodes announce that they want to be part of the tree –A Designated Router on each LAN hears this and floods Link State Packets that this LAN has a multicast group member All routers learn multicast membership of entire network

Prof. Rick Han, University of Colorado at Boulder Link-State IP Multicast (3) Forwarding of packets: –When a router R receives a packet from S with destination multicast address G, it Performs Dijkstra calculation with S as root (not R) if not already in cache Finds itself (R ) in tree For each subtree from R with a group member, –Cache info for this subtree –Forward packet to subtree –Example: S=B, R=E Member Sender

Prof. Rick Han, University of Colorado at Boulder Distance Vector Multicast (DVMRP) How do we use distance vectors to achieve multicast? –Dijkstra’s shortest path tree was more intuitive –Distance vectors tell us next-hop path for shortest route to destination –Basic method: Flood, then Prune Flood: Reverse-Path Broadcast –When receiving a multicast packet to group G from source S Does packet arrive from the shortest-hop link to S? If yes, then multicast on all other outgoing links –This efficiently floods the network Any source can send to any node along a shortest hop tree

Prof. Rick Han, University of Colorado at Boulder DVMRP (2) Flooding Example : Reverse-Path Broadcast –When E receives a multicast packet to group G from source S=C, Does packet arrive from the shortest-hop link to B? Yes, it arrived on B-E link which is next hop using the spanning tree centered on E Since yes, then multicast on all other outgoing links –To A, F, and D –This floods the network Compare to LSP’s reliable flooding

Prof. Rick Han, University of Colorado at Boulder DVMRP (3) Problems with Reverse-Path Broadcast –Since we’re flooding on all outgoing links, then multiple routers on same Ethernet send copies of same packet Solution: assign a parent router on each link. For each source S, the parent router is the one closest to S. B-C link is Ethernet LAN Parent

Prof. Rick Han, University of Colorado at Boulder DVMRP (4) Problems with Reverse-Path Broadcast –Flooding the network reaches nodes who don’t want to be part of the multicast group –Solution: Prune using Reverse-Path Multicast Same “parent” router knows it’s a leaf when it is the only router in a network Each host that is a member of G periodically broadcasts its membership Parent router hears this, and will forward multicast packets to this LAN If parent hears no members, then sends “no members” up the shortest path tree

Prof. Rick Han, University of Colorado at Boulder DVMRP (5) Pruning Example: –B sends to multicast group G consisting of F and D This message floods E sends a request to its leaf nodes: is anyone part of group G? –F and D respond, A does not E prunes A (remembers not to forward to A when it receives a multicast dest addr G) C is also pruned from B Sender Member

Prof. Rick Han, University of Colorado at Boulder Protocol Independent Multicast (PIM) Problems with MOSPF and DVMRP –Each router has to have state –Flooding is costly in DVMRP Routers not part of multicast group are flooded, then prune via “No members” messages –When only a few “sparse” nodes in multicast tree, then flooding and state storage become excessive

Prof. Rick Han, University of Colorado at Boulder PIM (2) PIM “sparse” mode –Create a Rendezvous Point (RP) for each multicast group –From a multicast tree rooted from the well-known RP Reverse path tree formed from unicast Joins by leaf nodes –Senders unicast to RP, which then multicasts along tree Rendezvous Point

Prof. Rick Han, University of Colorado at Boulder PIM (3) PIM “sparse” mode –If a particular sender transmits frequently then Receiver sees many packets with Then, receiver sends a Join to the source Reverse path multicast tree is formed to the source, rather than to RP

Prof. Rick Han, University of Colorado at Boulder Internet Group Management Protocol (IGMP) Used to join and leave multicast groups A multicast-enabled router on a LAN sends membership_query IGMP message –“Is anyone part of multicast group G?” A multicast-enabled host replies with an IGMP membership_report –“Yes, I’m part of multicast group G” –Host can send unsolicited membership_report to explicitly join a multicast group –Router only needs to know that one host on LAN is a member of group G, not which host nor how many hosts Feedback suppression

Prof. Rick Han, University of Colorado at Boulder Other Multicasting Issues Source doesn’t have to be a member of a multicast tree! –Any malicious user can overwhelm a multicast tree Source doesn’t have to know in advance who is in multicast tree –Clean decoupling of senders and receivers Receiver-driven multicast model [Deering, 1990] –Any host can send an IGMP Join message to its attached IP multicast-enabled router No control over who joins an IP multicast group –How do you prevent someone from subscribing at the IP router level? –Once in a multicast group, can eavesdrop

Prof. Rick Han, University of Colorado at Boulder Other Multicasting Issues (2) Multicast is unreliable –Uses UDP datagrams over IP multicast datagrams How do we design a reliable multicast protocol? –Suppose B reliably multicasts, and F and D are both members of multicast tree –F and D both send ACK’s upstream –Source can get swamped with ACK’s –Solution: put ACK filters in intermediate routers Would have to understand TCP semantics Controversial