Held Wednesday, March 15, 2000
Today, we move on the next layer and begin to consider issues
relevant to packet switching.
- Don't forget that assignment 5
is due today.
- This is an assignment that I don't plan to grade. The primary goal
was to ensure that you thought carefully about how to implement
some part of the data-link layer.
- I found an interesting article on networking today,
- Packet switching issues
- Techniques of addressing packets
- Some problems
- Right now, we know how to move frames around at the
data link layer
- With two nodes connected by a single link
- With n nodes on a bus or ring
- What happens when we want other topologies, or want
- We start linking networks together. A node connected
two two networks serves as a switch or bridge
between those two networks.
- Recall that there are two main models for communication
in a complex topology:
- In datagram switching, we treat each packet
separately; we call this a connectionless protocol
- In circuit switching, we set up a path from
sender to receiver and use that path again and again
and ....; we call this a connection-oriented protocol
- Why use one instead of the other?
- Datagrams can make better use of the network
- Datagrams better recover from errors, such as dead nodes or
links (just route around the error)
- Datagrams can lead to some errors (e.g., if a
datagram comes around a loop in the network)
- Datagrams may require more addressing information
- Virtual circuits can guarantee performance
- Virtual circuits have a startup time
- How do we address the packets we send?
- We can rely on the switches to keep track of ``approximate''
locations of nodes
- When a packet arrives addressed for a particular node, the
switch forwards it along to the proper network, if it knows of one.
- If the switch doesn't know where to send it, it forwards it
- We can include the full path from sender to receiver in the
address. (Source routing)
- We can include a circuit number (as long as we're using virtual
- How do switches and nodes learn about the locations of other
- Switches can pay attention to where packets come from.
- Are there other techniques?
- How does a switch learn where another packet is for source
- One technique: use a special "trying to find X" message.
Routers must add their name on the end and rebroadcast.
- If there's a loop in the graph, we may have problems.
- What kinds of problems? Think about them.
- How might we resolve those problems?
Thursday, 20 January 2000
- Created as a blank outline.
Wednesday, 15 March 2000
- Filled in the details, many of which had been prepared for
Monday's class. Updated slightly.
Back to Homework 4, Discussed.
On to Bridges and Switches.