1. Fall, 2001CS 6401 Switching and Routing Outline Routing overview Store-and-Forward switches Virtual circuits vs. Datagram switching

2. Fall, 2001CS 6402 Internet Structure Backbone service provider Peering point Peering point Large corporation Small corporation “ Consumer ” ISP “Consumer” ISP “ Consumer” ISP

3. Fall, 2001CS 6403 Routing Overview Up to now we have considered applications and the transport layer and assumed the network knows how to get packets from host A to host B We’re now moving down to the network layer of the protocol stack Routing is the task of determining how to get packets from host A to host B through a series of hops We will consider routing from two perspectives –Intra-domain OSPF, RIP –Inter-domain BGP

4. Fall, 2001CS 6404 Switch –forwards packets from input port to output port –port selected based on address in packet header Advantages –cover large geographic area (tolerate latency) –support large numbers of hosts (scalable bandwidth) How does the switch decide which port to place packets? Scalable Networks Input ports T3 STS-1 T3 STS-1 Switch Output ports

5. Fall, 2001CS 6405 General Routing Methods Datagram or connectionless –Make sure each packet has enough info to enable switching decision Virtual circuit or connection-oriented –Circuit is constructed before packets are sent Source routing –Less common but simple –All data about route is provided by source host Assume all end nodes have addresses Assume ports on switches are either have a number or name of switch to which it is attached

6. Fall, 2001CS 6406 Source Routing

7. Fall, 2001CS 6407 Virtual Circuit Switching Explicit connection setup (and tear-down) phase Subsequence packets follow same circuit Sometimes called connection-oriented model 0 13 2 0 13 2 0 13 2 5 11 4 7 Switch 3 Host B Switch 2 Host A Switch 1 Analogy: phone call Each switch maintains a VC table Virtual circuit identifier

8. Fall, 2001CS 6408 Datagram Switching No connection setup phase Each packet forwarded independently Sometimes called connectionless model 0 13 2 0 13 2 0 13 2 Switch 3 Host B Switch 2 Host A Switch 1 Host C Host D Host E Host F Host G Host H Analogy: postal system Each switch maintains a forwarding (routing) table

9. Fall, 2001CS 6409 Virtual Circuit Model Typically wait full RTT for connection setup before sending first data packet. While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small. If a switch or a link in a connection fails, the connection is broken and a new one needs to be established. Connection setup provides an opportunity to reserve resources.

10. Fall, 2001CS 64010 Datagram Model There is no round trip time delay waitint for connection setup; a host can send data as soon as it is ready. Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up. Since packets are treated independently, it is possible to route around link and node failures. Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.

11. Fall, 2001CS 64011 Scalable Routing Forwarding vs Routing –forwarding: to select an output port based on destination address and routing table –routing: process by which routing table is built Network as a Graph Problem: Find lowest cost path between two nodes Factors –static: topology –dynamic: load