Held Friday, March 17, 2000
Today we continue our discussion of packet switching by visiting
some topological issues and ATM.
- Have a great break.
- No new assignment until after break.
- Distributed spanning tree algorithm
- Limitations of bridges
- Cell switching and ATM
- Most of our addressing discussions have had to do with
- What if we want to broadcast to all the nodes in the extended
- In a datagtam model, if the addresses is clearly a broadcast address
(depends on the
protocol), forward it to all active output channgels.
- If it's so simple, why do we visit this problem? Because in
other architectures, it's not so simple.
- What do we do when there's a loop in our LAN graph?
- We can design our protocols to be smart about loops
- We can temporarily ignore some of the connections, turning
our tree into a graph.
- Some of you may have seen a spanning tree algorithm in
CSC152 or CSC153 (or MAT218).
- The standard spanning tree algorithm assumes complete knowledge
of the graph. What do you do with partial knowledge?
- You create a distributed spanning tree graph.
- One bridge gets designated as the root of the spanning tree.
- This is typically the bridge with the smallest value.
- When two bridges are connected to the same subnet, the one closest
to the root forwards messages from that subnet.
- That is, the other one may still forward messages from other
subnets and to other subnets.
- If two are the same distance, the one with the smaller number
- How does everyone figure this out?
- Initially: broadcast your bridge number and distance.
- At each step: consider the packets you've received.
- If you received the name of a smaller bridge, you know
that you're not the root.
- If you received a smaller path to the root along one
port, stop forwarding along that port.
- We'll do an example to make sure we understand.
- What are the advantages and disadvantages of this technique?
- Why are bridges not enough?
- Bridges typically are between homogeneous networks.
- Broadcast becomes unwieldy.
- The algorithm above is linear in the number of bridges.
- Basically, bridges do not scale.
Thursday, 20 January 2000
- Created as a blank outline.
Friday, 17 March 2000
Monday, 3 April 2000
- Removed uncovered section on ATM.
Back to Switching and Forwarding.
On to IP.