CS 5565 Network Architecture and Protocols

1. CS 5565Network Architecture and Protocols Godmar Back Lecture 41

2. Announcements • Project 2B due May 3 • Reading Assignment Chapter 5 • Review for final next Wednesday + course evaluation CS 5565 Spring 2006

3. Ethernet

4. Star Topology • Bus topology popular through mid 90s • Now star topology prevails • Connection choices: switch or hub • Hubs rarely used nowadays hub or switch CS 5565 Spring 2006

5. Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • used to synchronize receiver, sender clock rates  7   6   6  2  -----------  4 <05DC 802.3 Length 0800 IP 0806 ARP 809B Appletalk … CS 5565 Spring 2006

6. Hubs • Hubs are essentially physical-layer repeaters: • bits coming from one link go out all other links • at the same rate • no frame buffering • no CSMA/CD at hub: adapters detect collisions • provides net management functionality • Single collision domain CS 5565 Spring 2006

7. Switches • Link layer device • stores and forwards Ethernet frames • examines frame header and selectively forwards frame based on MAC dest address • when frame is to be forwarded on segment, uses CSMA/CD to access segment • transparent • hosts are unaware of presence of switches • plug-and-play, self-learning • switches do not need to be configured CS 5565 Spring 2006

8. switch hub hub hub Forwarding 1 3 2 • How do determine onto which LAN segment to forward frame? • Looks like a routing problem... • NB: Switches are called bridges if #interfaces == 2 CS 5565 Spring 2006

9. Self Learning • A switch has a switch table • entry in switch table: • (MAC Address, Interface, Time Stamp) • stale entries in table dropped (TTL can be 60 min) • switchlearns which hosts can be reached through which interface • when frame received, switch “learns” location of sender: incoming LAN segment • records sender/location pair in switch table CS 5565 Spring 2006

10. Switch Example Suppose C sends frame to D address interface switch 1 1 1 2 3 A B E G 3 2 hub hub hub A I F D G B C H E • Switch receives frame from C • notes in bridge table that C is on interface 1 • because D is not in table, switch forwards frame into interfaces 2 and 3 • frame received by D CS 5565 Spring 2006

11. Switch Example (2) Suppose D replies back with frame to C. address interface switch 1 1 2 3 1 A B E G C hub hub hub A I F D G B C H E • Switch receives frame from from D • notes in bridge table that D is on interface 2 • because C is in table, switch forwards frame only to interface 1 • frame received by C CS 5565 Spring 2006

12. switch hub hub hub Switch: Traffic Isolation • switch installation breaks subnet into LAN segments • switch filters packets: • same-LAN-segment frames not usually forwarded onto other LAN segments • segments become separate collision domains • Often nowadays: switch to host connection -> no collisions collision domain collision domain collision domain CS 5565 Spring 2006

13. Switches: Dedicated Access A • Switch with many interfaces • Hosts have direct connection to switch • No collisions; full duplex • A-to-E and B-to-F simultaneously • cut-through switching: frame forwarded from input to output port without first collecting entire frame • slight reduction in latency C’ B switch D F E This is the dominant technology today CS 5565 Spring 2006

14. Switches: Flow Control • What if A=100Mbps, E=10Mbps? • host sends frames faster than switch can send them out of outgoing port • undetected losses due to overflow could occur • IEEE 802.3 provides LLC layer that has flow control facilities • “pause” frames to get sender to stop CS 5565 Spring 2006

15. Switches vs Routers mail server to external network web server router switch IP subnet switch switch hub switch CS 5565 Spring 2006

16. Spanning Tree Bridges • Switches self-configure into spanning tree CS 5565 Spring 2006

17. 802.1D • Based on [Perlman 1985] • Spanning Tree algorithm • Root selection by lowest MAC ID • Link-State approach: “HELLO” messages • Links are in states: BACKUP (inactive) or FORWARDING (active) and in-between states • Breaks loops fast, recovery takes up to 1min • Disadvantage: • Slow convergence after breakage • 802.1w: RSTP (Rapid STP) • Lack of multipath capability • Not aware of VLANs (discussed next) CS 5565 Spring 2006

18. Virtual LANs CS 5565 Spring 2006

19. Virtual LANs (2) (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches. CS 5565 Spring 2006

20. The IEEE 802.1Q Standard Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not. CS 5565 Spring 2006

21. The IEEE 802.1Q Standard (2) 802.1Q Ethernet frame format VLAN Association may be done • Via port • Via MAC address • Via Layer 3 information (IP address) CS 5565 Spring 2006

22. Switches vs. Routers • both store-and-forward devices • routers: network layer devices (examine network layer headers) • switches are link layer devices • routers maintain routing tables, implement routing algorithms • switches maintain switch tables, implement filtering, learning algorithms + spanning tree algorithms CS 5565 Spring 2006

23. Hubs vs. Routers vs. Switches CS 5565 Spring 2006