Switching and Forwarding

1. Switching and Forwarding Sections 3.1

2. Connecting More Than Two Hosts • Multi-access link: Ethernet, wireless • Single physical link, shared by multiple nodes • Limitations on distance and number of nodes • Point-to-point links: fiber-optic cable • Only two nodes (separate link per pair of nodes) • Limitations on the number of adapters per node point-to-point links multi-access link

3. Beyond Directly-Connected Networks • Switched network • End hosts at the edge • Network nodes switch traffic • Links between the nodes • Multiplexing • Many end hosts communicate over the network • Traffic shares access to the same links

4. What is a Switch • A mechanism to interconnect links to form a larger network • Multi-input, multi-output • Transfers packets from an input to one or more outputs • Adds star topology network to • point-to-point • bus (Ethernet) • ring (802.5 and FDDI) • ad hoc

5. Properties of Star Topology • Large geographic scope networks can be built by interconnecting a number of switches • connect switches to each other and to hosts using point-to-point links • A new host connected to a switched network does not necessarily degrade performance of already connected hosts • Every host has its own link to the switch • Hosts can transmit at full link bandwidth • Switch can be designed to handle aggregate bandwidth • In contrast, all hosts on an Ethernet cannot transmit continuously at 10Mbps (share medium)

6. Circuit Switching (e.g., Phone Network) • Source establishes connection to destination • Nodes along the path store connection info • Nodes may reserve resources for the connection • Source sends data over the connection • No destination address, since nodes know path • Source tears down connection when done

7. Time-division Each circuit allocated certain time slots (in the figure 6 circuits) Frequency-division Each circuit allocated certain frequencies Circuit Switching: Multiplexing a Link frequency time time

8. Virtual Circuit Switching • Explicit connection setup (and tear-down) phase • Subsequence packets follow same circuit • Sometimes called connection-oriented model • Each switch maintains a VC table • Analogy: phone call

9. Connection Setup Phase Establish “connection state” in each of the switches between the source and destination hosts • an entry in a “VC table” in each switch through which the connection passes. Fields of each entry are: • a virtual circuit identifier (VCI) that uniquely identifies the connection at this switch and that will be carried inside the header of incoming packets that belong to this connection • incoming interface on which packets for this VC arrive • outgoing interface in which packets for this VC leave • VCI that will be used for outgoing packets (a potentially different from incoming VCI)

10. Virtual Circuit Table (switch 1, port 2) 0 0 0 1 3 11 3 1 3 1 2 Switch 1 Switch 2 2 2 2 5 0 7 Switch 3 3 1 Host B Host A 4 Example Table

11. 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.

12. Advantages of Circuit Switching • Guaranteed bandwidth • Predictable communication performance • Not “best-effort” delivery with no real guarantees • Simple abstraction • Reliable communication channel between hosts • No worries about lost or out-of-order packets • Simple forwarding • Forwarding based on time slot or frequency • No need to inspect a packet header • Low per-packet overhead • Forwarding based on time slot or frequency • No IP (and TCP/UDP) header on each packet

13. Disadvantages of Circuit Switching • Wasted bandwidth • Bursty traffic leads to idle connection during silent period • Unable to achieve gains from statistical multiplexing • Blocked connections • Connection refused when resources are not sufficient • Unable to offer “okay” service to everybody • Connection set-up delay • No communication until the connection is set up • Unable to avoid extra latency for small data transfers • Network state • Network nodes must store per-connection information • Unable to avoid per-connection storage and state

14. Packet Switching (e.g., Internet) • Packet Switching: statistical multiplexing of links • Data traffic divided into packets • Each packet contains a header (with address) • Packets travel separately through network • Packet forwarding based on the header • Network nodes may store packets temporarily • Destination reconstructs the message

15. Packet Switching (cont. I) • No connection setup phase • Each packet forwarded independently • Sometimes called connectionless model • Analogy: postal system • Each switch maintains a forwarding (routing) table

16. Datagram Switching (cont. II) • Forwarding Table (switch 1)

17. Datagram Model vs. Virtual Circuit Model • There is no round trip delay waiting 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.

18. Routing Alternative: Source Routing • All information required to switch a packet across the network is provided by the source host • an ordered list of switch ports in header • in every switch on the route, rotate the list so that next switch output port is always at the front of the list