1. 1 ELEN Lecture 13 LAN Bridges Routers, Switches, Gateways Network layer -IP Reading: 6.7, 8.1-8.3

2. 2 Switches A multi-input multi-output device that transfers packets from one input to an output The number of ports on a switch determine the number of hosts that can be connected. Larger # of hosts can be interconnected by interconnecting switches Point-to-point links can be used to extend the geographic reach Adding a new host does not limit or reduce the performance of other hosts

3. 3 Scalable Networks 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) Input ports T3 STS-1 T3 STS-1 Switch Output ports

4. 4 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

5. 5 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

6. 6 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.

7. 7 Datagram Model There is no round trip time 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.

8. 8 Bridges, switches, routers, gateways Devices used to interconnect multiple networks Bridge: device interconnecting two or more networks at MAC layer Router: device interconnecting two or more networks at the network layer Gateway: device interconnecting two or more networks at a higher layer

9. 9 A Bridged LAN

10. 10 Bridge Network Physical Network LLC Physical LLC MAC Interconnection by a Bridge

11. 11 B1 S1S2 B2 S3S4 S5 Address Port port 1port 2port 1port 2 LAN1LAN2LAN3 Example Configuration

12. 12 B1 S1S2 B2 S3S4 S5 Address Port port 1port 2port 1port 2 LAN1LAN2LAN3 S1 S5 S11 1 S1 sends a frame to S5

13. 13 B1 S1S2 B2 S3S4 S5 Address Port port 1port 2port 1port 2 LAN1LAN2LAN3 S3 S2 S11 1 S32 1 S3 sends a frame to S2

14. 14 B1 S1S2 B2 S3S4 S5 Address Port port 1port 2port 1port 2 LAN1LAN2LAN3 S4 S3 S11 1 S32 1 S42 2 S4 Sends a frame to S3

15. 15 Bridge1 S1S2 Bridge 2 S3S4 S5 Address Port port 1port 2port 1port 2 LAN1LAN2LAN3 S2 S1 S11 1 S32 1 S42 2 S21 S2 sends a frame to S1

16. 16 (1) (2) (3) Sample Topology

17. 17 Spanning Tree

18. 18 Routing Control Route-1 Designator Route-2 Designator Route-m Designator Destination Address Source Address Routing Information Data FCS 2 bytes Frame Format for Source Routing

19. 19 LAN interconnection with source routing bridges

20. 20 LAN1B1 B3 B4 LAN3B6LAN5 LAN4 Routes followed by single-route broadcast frames

21. 21 LAN5 B6 B7 LAN3 LAN4 B2 B3 B5 LAN1 B1LAN2 B3 B4 LAN4 B5 B7 LAN2 B1 B4 LAN1 B2 LAN4 B5 B7 LAN4 B4 B7 LAN2 B1 B3 LAN1 B2 B4 B5 LAN2 B1 B3 LAN3 B2 B5 B6 LAN1 B1 LAN1 B2LAN3 B3 B5 B6 LAN3 B3 B2 B6 LAN1 LAN2 B1LAN2 B3 B4 B1 B4 LAN1 B2 Routes of all-routes broadcast frames

22. 22 IP Internet Concatenation of Networks Protocol Stack R2 R1 H4 H5 H3 H2 H1 Network 2 (Ethernet) Network 1 (Ethernet) H6 Network 3 (FDDI) Network 4 (point-to-point) H7R3H8 R1 ETH FDDI IP ETH TCP R2 FDDI PPP IP R3 PPP ETH IP H1 IP ETH TCP H8

23. 23 Service Model Connectionless (datagram-based) Best-effort delivery (unreliable service) –packets are lost –packets are delivered out of order –duplicate copies of a packet are delivered –packets can be delayed for a long time Datagram format VersionHLen TOSLength IdentFlagsOffset TTLProtocolChecksum SourceAddr DestinationAddr Options (variable) Pad (variable) 048161931 Data

24. 24 Fragmentation and Reassembly Each network has some MTU Strategy –fragment when necessary (MTU < Datagram) –try to avoid fragmentation at source host –re-fragmentation is possible –fragments are self-contained datagrams –use CS-PDU (not cells) for ATM –delay reassembly until destination host –do not recover from lost fragments

25. 25 Example Ident = xOffset = 0 Start of header 0 Rest of header 1400 data bytes Ident = xOffset = 0 Start of header 1 Rest of header 512 data bytes Ident = xOffset = 512 Start of header 1 Rest of header 512 data bytes Ident = xOffset = 1024 Start of header 0 Rest of header 376 data bytes

26. 26 Global Addresses Properties –globally unique –hierarchical: network + host Dot Notation –10.3.2.4 –128.96.33.81 –192.12.69.77 NetworkHost 724 0 A: NetworkHost 1416 10 B: NetworkHost 218 110 C:

27. 27 Datagram Forwarding Strategy –every datagram contains destination’s address –if directly connected to destination network, then forward to host –if not directly connected to destination network, then forward to some router –forwarding table maps network number into next hop –each host has a default router –each router maintains a forwarding table Example (R2) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0

28. 28 Address Translation Map IP addresses into physical addresses –destination host –next hop router Techniques –encode physical address in host part of IP address –table-based ARP –table of IP to physical address bindings –broadcast request if IP address not in table –target machine responds with its physical address –table entries are discarded if not refreshed