Link Layer Switching

1. Link Layer Switching • Connecting local networks • Bridges • Repeaters, Hubs, Bridges, Switches, Routers, Gateways • Virtual LANs

2. Ethernet • 50  thick: 500 m • 50  thinn: 185 m • max 4 repeaters • traffic on one segment means traffic on all other segments

3. CSMA/CD (IEEE 802.3) Logical Link Control Link A-MAC Phys. A B-MAC Phys. B C-MAC Phys. C Physical

4. Bridges • Connection on link layer: • forwarding based on MAC addresses • self-learning bridges • operation • Advantages and limitations • Spanning-tree bridges • operation • Advantages and limitations

5. LLC MAC_2 Phys_2 LLC MAC_1 Phys_1 Self-learning Bridge Bridge routing table Forwarder MAC_1 Phys_1 MAC_2 Phys_2 Network 1 Network 2

6. Self-learning Bridge Routing table Interface MAC-adr . . Learning time & Mac-1 2 - - routing - - - - - - - - - Mac-2 3 - - Driver interface 1 . Driver interface 2 . Driver interfaec 3 . LAN 2 LAN 1 LAN 3

7. Receiver known? Self-learning Bridge Learning phase Forwarding phase Start Extract Sender and receiver MAC-adresser Look up in Routing table Look up in Routing table Yes No Sender known? New entry MAC-addr interface # and timer Broadcast frame, except on receiving interface Update interface # and timer Put frame into correct outgoing queue End

8. Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

9. Bridge from 802.x to 802.y IEEE 802 frame formats

10. Bridges from 802.x to 802.y Operation of a LAN bridge from 802.11 to 802.3.

11. Local “Internetworking” A configuration with four LANs and two bridges.

12. Problem with standard bridge Two parallel transparent bridges.

13. Spanning tree • Goal: each bridge should identify the interfaes for forwarding traffic • Build a spanning tree • From on root node • Self-configuring • To all nodes • Only these interfaces in the spanning tree can forward traffic • Provides the shortest path for all traffic

14. Spanning Tree Algorithm • Configuration phase: • Each nodes sends out: • Its own identity (ID) (MAC-address) • ID to the root-bridge • Number of hops to root-bridge • In this way, building up a spanning tree, bridge with lowest ID become root node • Start forwarding frames

15. Spanning Tree Bridges (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.

16. Remote Bridges Bridges can be used to connection physically distant local networks

17. Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers.

18. Hub Hub Hub Hub (Nav) < 100 m Transceiver

19. Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) A hub. (b) A bridge. (c) a switch.

20. Switch Server Server Switched Ethernet • Switch: • Switches on MAC-addr • Buffers frames, therefor no collision • Competition only for switch capacity 10, 100, 1000 Mb/s

21. Central server Central server Group server Group server Gigabit Ethernet Gigabit switch 1000 Mb/s 100 Mb/s 100/1000 100/1000 Switch Switch Working group 1 Working group 2

22. Virtual LANs A building with centralized wiring using hubs and a switch.

23. 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.

24. The IEEE 802.1Q Standard Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.

25. The IEEE 802.1Q Standard (2) The 802.3 (legacy) and 802.1Q Ethernet frame formats.

26. Conclusion • Bridges: • efficient connection alternative • Limits/isolates collision domains • Can be used for traffic isolation • Do not consume IP addresses • Switches: • High use degree, no danger of collisions • Used for establishing virtual LANs

27. Routing and Packet Switching • Goal • Overview of how routing fits into the Internet architecture • Principles for typical routing protocols • Strengths and weaknesses • Structure • Primary tasks of the network layer • Datagram and virtual line • Some performance considerations • Routing and forwarding

28. Network layer Server Client Disk Disk link

29. Tasks of the Network Layer • Responsible for end-to-end transport • Addressing of machines • Forwarding • Connectionless • datagram; no fixed path through the network • Connection-oriented (e.g. MPLS or ATM) • Three phases: connection establishment, data transmission, teardown • Fixed path through the network • Relatively reliable and ordered transmission • Flow control

30. Forwarding R A R B LAN-A LAN-B

31. Routing and lookup • Mail: griff@ifi.uio.no • Name to address conversion: • ifi.uio.no til IP address: 129.240.64.2 • Find MAC-address to router and send packet(s) • Forward through the network w.r.t. the network address • Based on lookup in routing tables • At the destination router • Convert machines IP address to a MAC address • Send packet to the receiving machine

32. Place of Routing in the architecture • Structured • Network dimensioning • Where should lines be established? • Capacity of lines • Traffic directioning • Mapping of connections down to paths through the net • Routing to choose paths • Routing of individual packets • Best effort • Routers choose the next hops separately for each packet

33. Routing • Routing tables can be computed based on state information about the network • Data exchanged between nodes: • Between neighbour nodes (distance vector routing; RIP) • Between all nodes in the network (link state routing; OSPF, IS-IS)

34. Routing types • Static vs. dynamic • Dynamic with error handling, new links, changes of the load • Centralized vs. distributed • Distributed when routes are computed at all nodes • Global vs. local topology knowledge • Source routing vs. routing • Kilde ruting vs. ruting • In source routing the source chooses the routing • In routing each router choose the next hop

35. Routing Parameters • Performance parameters • Number of hops • Price • Delay • capacity • Sources of routing information • None • Local to the node • Neighbour nodes • Nodes along the path • All nodes in the network • Routing decisions made • In each node (distributed) • In a central node (routing center) • At the sender (source routing) • Update interval • Continously • Periodic • In case of large load variations • In case of topology changes

36. Routing hierarchy • In large networks • Hierarchically structured • Link state • Open Shortest Path (OSPF) • Intermediate System to Intermediate System (IS-IS) • On campus or in companies • Distance vector, RIP • Static routing • Ad-hoc networks, stationary or mobile wireless networks • Many different protocols depending on scenarios

37. Router model Routing prosess Route 1 1 computation Topology database Routing table 2 2 Pre- process 3 3 Forwarding process In Out e Principle structure of a router with three incoming and three outgoing connections

38. Routing alternatives • Flooding • Static routing • Adaptive routing should handle • Loss of a link (error, e.g. cable is broken) • Loss of a node (error, e.g. power loss, OS crash) • High traffic load (persistant of transient congestion, bottleneck) • Disadvantages • Complex, distributed, and not always correct • Adaptivity must be balanced against additional overhead • Can lead to oscillations (route flapping) if reactions are too fast • Can be unattractive if reactions are too slow

39. Demands on a routing strategy • Shall give correct routes • Shall demand minimal load on nodes • Shall be stable and converge quickly • Fair towards different data streams • Provide optimal routes • Scale with the size of the network • Size with the number of destinations

40. Plug-and-play capabilities • Find neighbour nodes and routers • Detect when neighbours go up and down • Detect capacity of own links • Send and receive topology information • Send after timer or major changes to the network

41. Distance vector characteristics • Nodes exchange a vector with their shortest distance to all destinations • Periodic exchange • Convergence is ensured • Advantage • Simple • Disadvantages • Vulnerable to errors • Slow dissemination in case of problems

42. si1 . . siN di1 . . diN Di == Si == Distance Vector 5 3 C 5 B 2 9 A 2 1 F 2 1 D E Distance vector Next node vector Dest. delay Next node A 0 - B 2 B C 5 C D 1 D E 6 C F 8 C Node A before change

43. Router model Routing prosess Route 1 1 computation Topology database Routing table 2 2 Pre- process 3 3 Forwarding process In Out e Principle structure of a router with three incoming and three outgoing connections

44. Link state • Routing database • Routing table • Periodical and in case of changes • Nodes flood their state onto the link to all other nodes • At start, new nodes downlink the database from a neighbour • Different kinds of link • Point-to-point • Point-to-multipoint • Broadcast • Each node calculates the best route to all other nodes • Checkpoints • Voting av entire database for link state at a sequence number

45. LS routing protocol architecture change route lookup change Protocol for handling of changes Routing table Routing algorithm Link state database

46. Flooding of link state • Statistically reliable • Each node forwards on all interfaces • All incoming link state packets • If sequence number of large than earlier sequence numbers • Will most probably reach all node in the network • Content • Sequence number • Avoid broadcast storms • Node ID of the source • Topology • Identify bi-directional links • List of all direct neighbour nodes with a cost function • Time-to-live

47. 5 3 C 5 B 2 9 A 2 1 F 2 1 D E Link state, LSA Routin database i D A B C D E F A 2 5 1 B 2 2 C 5 D 1 2 9 E F

48. Link state problems/strengths • Problems • Selection of a node that reports for a shared medium • Flooding does not scale for large networks • Division into hierarchical networks to limit flooding • Strengths • All nodes have full topology knowledge • Error have only local relevance

49. Link state problem a area 1 area 2 b • We have two problems with the link state method • Static cost factor • Can be the source of congestion, all traffic is routing through a single link • Oscillation effects in forwarding traffic • At one point in time a is the preferred router between areas • Then routing information is exchange • New tables are computed and b becomes the preferred router

50. Route center Router and Routecenter Routers do not have to participate in a routing protocol Routing center receives status reports from routers Transfers forwarding table to routers Network with router center