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 Freceives one or tworedundant 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 tothe 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 fromall senders.

Constructs a multicast routing tree for eachsource in the multicast group.

Reverse Path Forwarding is used to construct amulticast forwarding tree.

Pruning to solve the problem with unwanted multicast packets.