Internet Networking Spring 2002

1. Internet Networking Spring 2002 Tutorial 7 Multicast Routing Protocols DVMRP + MOSPF

2. Introduction • When we discuss multicast routing protocols we should learn 3 things: • How hosts can join to a multicast group (IGMP) • How routers distribute between them information about registered multicast subscriptions • How a router performs routing of a multicast packet

3. RPF – Reverse Path Forwarding • Simplest algorithm for multicast routing is flooding packets from a source to all ASs. Disadvantages: • Large amount of unnecessary packets sending • Need to remember already received packets to prevent duplications • Better idea is to build some kind of spanning tree ( What is the problem in Distance Vector routing ?) • RPF algorithm: • When a multicast packet is received from source (S) on interface (I) at router R • If I belongs to the shortest path from R to S, forward it to all interfaces except I; Otherwise, refuse the packet • Assumes symmetric routing : distance(R→S) = distance(S→ R)

4. RPF Properties • RPF properties: • Generates Shortest Path spanning tree • Different spanning tree for each source • But unnecessary forward packets to all networks, even not subscribed to the multicast group • RPF+ (improvement - look one step further) • A router forwards packets only to the neighbors that it is their “father” on the shortest path from a source

5. RPF Pruning • Idea: • There is no sense to forward a packet if there is no group member down the tree • Algorithm Flood & Prune: • The first multicast packet from a source S is propagated to all the network nodes (flooding) • When a leaf router (at the specific tree) receives a multicast message and doesn’t have group members for it, it sends prune message to the father node • When an intermediate router gets prune messages through all the outgoing interfaces then it sends prune message by self to its father node

6. RPFPruning Properties • If a router wants to join a group after pruning, it sends join request to its father node at the tree (the next node on the shortest path to S: again symmetry is assumed). • After predefined time exceeding joining to the group is renewed • The algorithm is robust for topology changes • However, it has 2 drawbacks: • Packets are flooded to the whole network on the periodic basis • All routers must keep state on a per-group and per-source basis

7. DVMRP Protocol • Protocol for multicast routing inside of ASs that use Distance Vector Routing (i.e. RIP), defined in RFC 1075. • DVMRP uses IGMP messages of a new type (not usual QUERY or REPORT messages) for exchanging multicast information between routers • Based on RPF algorithm • Uses its own routing tables (because of the tunnels and forward-path metric)

8. DVMRP Protocol Forwarding algorithm: • When a router receives a multicast packet from an interface (or a tunnel) it checks in the DVRMP routing table whether it is on the shortest path from the source • If this is not a case the packet is dropped; otherwise, it is forwarded to all multicast-capable interfaces and tunnels for which the router is the previous hop from the source • 2 additional checks are applied at this stage: a verification of the packet TTL and a check for group membership

9. DVMRP Tunneling net 1 net 2 Internet with no support for multicast R1 R2 Host on net 1 wants to send a multicast message to a host on net 2 Problem: Sometimes, some of intermediate routers required for sending a multicast message are not supporting DVMRP

10. DVMRP Tunneling • Solution: Sending a multicast packet inside of an usual unicast packet from one DVMRP router R1 to another DVRMP router R2 through intermediate “usual” routers. Virtual connection (tunnel) defined between R1 and R2. Source:R1 Dest:R2 Protocol: IP in IP Source:S Dest:G Protocol: UDP UDP header and data IP header (1) IP header (2) R1 R2 S G

11. MBONE • Experimental Multicast Backbone on the Internet (’92) • Based on the virtual connection between multicast routers (manually configured: What problem is caused when joining the net?) • Uses DVMRP as multicast protocol

12. MOSPF • MOSPF - enhancements to OSPF to support IP multicast routing (RFC 1584) • The OSPF link state database provides a complete description of AS topology. • MOSPF complements the database with a new type of link state records – group membership • A special flag is added in the database for supporting MOSPF • Therefore MOSPF routers actually can perform RPF and prune computation “in memory” (Locally computed: How can symmetry be broken?)

13. MOSPF – Designated Router: Reminder • A router responsible for registering and propagation of multicast group membership information for a certain network called Designated Router • When a host sends a request to the Designated Router to be a member of some multicast group (by IGMP protocol) then the last performs 2 things: • Update Local Group Database • Propagate update in the form of Group Membership - LSP in its area (LSA, Link State Advertisement, in the RFC)

14. MOSPF – Local Group Database • The local group database keeps track of the group membership of the router's directly attached networks. • Each entry in the local group database is a pair of: [group, attached network] which indicates that the attached network has one or more hosts belonging to the multicast destination group. • This information is then used by the router when deciding which directly attached networks to forward a received IP multicast datagram onto, in order to complete delivery of the datagram to local, group members.

15. Local Group Database - Example • If Router RT3 has been elected Designated Router for Network N3, then the Table: lists the local group database for the routers RT1-RT4. Router local group database RT1 [Group B, N1] RT2 [Group A, N2], [Group B, N2] RT3 [Group B, N3] RT4 None

16. Group Membership LSP • The Designated Router for some network propagates group membership information of its network with this new type of LSP. Its distribution intended for all the area. • The Designated Router propagates information only about networks assigned to this router • The LSP includes records. Their structure is: a network and a multicast group for which it wants to belong.

17. MOSPF Routing • MOSPF routing is influenced by source and by the multicast destination • When a router receives a multicast packet it generates Shortest Path Tree with a packet sender as the root and destination networks as the leafs • After the router computed the tree it finds its place in the tree and decides about next targets for packet sending • To prevent repeated generation of the same tree, the results of the computations are saved in the Forwarding Cache, including: source address, multicast group, upstream and downstream interfaces (explained in the lecture)

18. MOSPF – Inter Area Routing • HW2 q 2. • If Group-Membership-LSP distribution is limited to an area, then how multicast between different areas of the same AS is executed ? • A part of Area-Border-Routers are defined to function also as a Inter Area Multicast Forwarders • In this role, they summarize Group-Membership-LSP for their area and advertise it over the Backbone. The summary will contain all the groups for which at least one member exists in the area. • The router will get all outside messages for the advertised groups from Backbone and forward them to its area.

19. MOSPF – Inter Area Routing • To prevent explosion of data external routers don’t advertise in their areas all the groups that have been defined in other areas. • How is it possible to send multicast packets from one area to another? • For this purpose Multicast Wildcard Receivers are defined (all Inter Area Multicast Forwarders perform this functionality): The Intra-Area routers believe that the Inter Area router is a part of all the multicast groups. • These routers get all the multicast traffic of their area and then through the Backbone, the datagram can be forwarded to all group members in other areas