Computer Networking A Top-Down Approach Chapter 4.7

 When a packet is needed to be sent from a source node to ALL other nodes in the network.  Example: A company needs to upgrade a program in all computers at the office building. The network admin can send an update to all computers on the network using broadcast routing.

 N-way-unicast  Uncontrolled flooding  Controlled flooding  Spanning-tree broadcast

 The source node(R1) sends duplicates of a pkt to the N destinations (R3 & R4).  The network nodes (R2) should instead create duplicates to the final destinations (R3 & R4).  All destination addresses needs to be known: adds extra overhead and complexity.

 Flooding is the obvious technique of broadcast routing:  1. Source node sends copies of pkt to all its neighbours  2. Receiving node also sends copies to all neighbours(N) except its sender  Can send a pkt into an endless cycle!  Can create a broadcast storm if N>2

 Solution: Sending node should only flood neighbours that haven’t been flooded before.  There are two ways of doing this:  1. Sequence-number-controlled flooding  2. Reverse Path Forwarding (RPF)

 1. Source node puts its address and broadcast sequence no. into a broadcast pkt  2. Sends this pkt to all its neighbors  3. Each node keeps a list with above info about each broadcast pkt already received  4. If pkt already received, drop pkt. Otherwise duplicate and forward to neighbors

 Router only transmits pkt if it arrived on shortest unicast path back to the source B, C, D, E and F receives one or two redundant packets!

 Every node only receives one copy of pkt  Nodes only sends pkts to its neighbors in the spanning tree  A node only need to know which neighbors are in spanning tree

 Example: Center-based approach  1. A center node is defined  2. Nodes unicast tree-join messages addressed to center node  3. Msg forwarded until hits the spanning tree

 Enables a single source node to send a copy of a packet to a subset of the other network nodes.  Multiple receivers  Problems: ◦ identify receiver ◦ address a packet

 Used in Internet architecture  A multicast packet is addressed using this method.  Single identifier  Multicast group  Still one difficulty…

 Operates between a host and its directly attached router.  Provides operations for joining and leaving a group.  IGMP messages

 Only a subset of routers that joined to the multicast group actually needs to receive the multicast traffic.

 Goal to find a tree of links that connects all of the routers that have attached hosts belonging to the multicast group.  Two approaches: ◦ Multicast routing using a group-shaded tree. ◦ Multicast routing using a source-based tree.

 Constructs a single, shared routing tree to route packets from all senders.

 Constructs a multicast routing tree for each source in the multicast group.  Reverse Path Forwarding is used to construct a multicast forwarding tree.  Pruning to solve the problem with unwanted multicast packets.