Note 7: Local Area Networks

1. Note 7: Local Area Networks LAN Bridges

2. Hub Hub Two Twisted Pairs Station Station Station Hubs, Bridges & Routers • Hub: Active central element in a star topology • Twisted Pair: inexpensive, easy to insall • Simple repeater in Ethernet LANs • “Intelligent hub”: fault isolation, net configuration, statistics • Requirements that arise: User community grows, need to interconnect hubs Hubs are for different types of LANs ? Hub Two Twisted Pairs Station Station Station

3. Hub Hub Two Twisted Pairs Two Twisted Pairs Station Station Station Station Station Station Hubs, Bridges, Routers & Gateways • Interconnecting Hubs • At the physical layer • Repeater • At the MAC or data link layer • Bridges • At the network layer • Router • At even higher layers • Gateway Higher Scalability ?

4. General Bridge Issues • Operation at data link level implies capability to work with multiple network layers • However, must deal with • Difference in MAC formats • Difference in data rates; buffering; timers • Difference in maximum frame length Network Network LLC LLC MAC 802.5 MAC 802.3 802.3 802.5 802.3 802.5 802.3 802.5 PHY PHY 802.5 802.3 Token Ring CSMA/CD

5. Network Network Bridge LLC LLC MAC MAC MAC MAC Physical Physical Physical Physical Bridges of Same Type • Common case involves LANs of same type • Bridging is done at MAC level

6. S1 S2 S3 LAN1 Bridge LAN2 S6 S4 S5 Transparent Bridges • Interconnection of IEEE LANs with complete transparency • Use table lookup, and • discard frame, if source & destination in same LAN • forward frame, if source & destination in different LAN • use flooding, if destination unknown • Use backward learning to build table • observe source address of arriving frames • handle topology changes by removing old entries

7. Address Port Address Port S5 S1 S2 S3 S4 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2

8. S1→S5 S5 S1 S2 S3 S4 S1 to S5 S1 to S5 S1 to S5 S1 to S5 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1

9. S3→S2 S5 S1 S2 S3 S4 S3S2 S3S2 S3S2 S3S2 S3S2 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 S3 2 S3 1

10. S4S3 S5 S1 S2 S3 S4 S4 S3 S4S3 S4S3 LAN1 LAN2 LAN3 S4S3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port Address Port S1 1 S1 1 S3 2 S3 1 2 2 S4 S4

11. Address Port S1 1 S3 1 2 S4 S2S1 S5 S1 S2 S3 S4 S2S1 S2S1 LAN1 LAN2 LAN3 B1 B2 Port 1 Port 2 Port 1 Port 2 Address Port S1 1 S3 2 2 S4 1 S2

12. Adaptive Learning • In a static network, tables eventually store all addresses & learning stops • In practice, stations are added & moved all the time • Introduce timer (minutes) to age each entry & force it to be relearned periodically • If frame arrives on port that differs from frame address & port in table, update immediately

13. LAN1 (1) (1) B1 B2 (2) B3 LAN2 B4 LAN3 B5 LAN4 Avoiding Loops

14. Spanning Tree Algorithm • Select a root bridge among all the bridges. • root bridge = the lowest bridge ID. • Determine the root port for each bridge except the root bridge • root port = port with the least-cost path to the root bridge • Select a designated bridge for each LAN • designated bridge = bridge has least-cost path from the LAN to the root bridge. • designated port connects the LAN and the designated bridge • All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state.

15. LAN1 (1) (1) B1 B2 (1) (2) (2) (3) B3 LAN2 (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4

16. LAN1 (1) (1) Bridge 1 selected as root bridge B1 B2 (1) (2) (2) (3) B3 LAN2 (2) (1) B4 (2) LAN3 (1) B5 (2) LAN4

17. LAN1 R (1) (1) Root port selected for every bridge except root bridge B1 B2 R (1) (2) (2) (3) B3 LAN2 R (2) (1) B4 (2) R LAN3 (1) B5 (2) LAN4

18. LAN1 D R (1) (1) Select designated bridge for each LAN B1 B2 R (1) (2) (2) (3) D B3 LAN2 R (2) (1) D D B4 (2) R LAN3 (1) B5 (2) LAN4

19. LAN1 D R (1) (1) All root ports & designated ports put in forwarding state B1 B2 R (1) (2) (2) (3) D B3 LAN2 R (2) (1) D D B4 (2) R LAN3 (1) B5 (2) LAN4

20. Routing Route 1 Route m Route 2 control designator designator designator 2 bytes 2 bytes 2 bytes 2 bytes Destination Routing Source Data FCS address information address Source Routing Bridges • To interconnect IEEE 802.5 token rings • Each source station determines route to destination • Routing information inserted in frame

21. Route Discovery • To discover route to a destination each station broadcasts a single-route broadcast frame • Frame visits every LAN once & eventually reaches destination • Destination sends all-routes broadcast frame which generates all routes back to source • Source collects routes & picks best

22. Bridges must be configured to form a spanning tree Source sends single-route frame without route designator field Bridges in first LAN add incoming LAN #, its bridge #, outgoing LAN # into frame & forwards frame Each subsequent bridge attaches its bridge # and outgoing LAN # Eventually, one single-route frame arrives at destination When destination receives single-route broadcast frame it responds with all-routes broadcast frame with no route designator field Bridge at first hop inserts incoming LAN #, its bridge #, and outgoing LAN # and forwards to outgoing LAN Subsequent bridges insert their bridge # and outgoing LAN # and forward Before forwarding bridge checks to see if outgoing LAN already in designator field Source eventually receives all routes to destination station Detailed Route Discovery

23. LAN 2 LAN 4 B4 S1 B1 S2 B3 LAN3 B6 LAN5 LAN 1 B5 B3 B7 LAN1 B1 LAN2 B2 S3 B4 LAN4 B6 LAN 3 LAN 5 Find routes from S1 to S3

24. B3 LAN1 B2 B1 LAN2 B5 LAN4 B4 B7 LAN 2 LAN 4 B4 LAN1 B1 B2 LAN3 B6 B3 LAN2 S1 B5 B4 LAN4 B1 S2 B7 LAN1 B1 B2 B4 LAN2 LAN4 B5 LAN 1 B3 LAN5 B5 B3 B7 B7 B3 B5 LAN1 B1 B2 LAN3 B6 B4 LAN2 LAN1 B1 B2 B2 B3 LAN3 S3 B5 B7 LAN4 B6 B6 LAN 3 LAN 5 B3 LAN1 B2 B1 LAN2 B4 LAN1 LAN3 B1 B2 B5 B3 LAN2 B4 B6

25. Virtual LAN VLAN 2 VLAN 3 VLAN 1 S3 S6 S9 Floor n + 1 Physical partition S2 S5 S8 Floor n 2 3 4 5 6 1 S1 S4 7 S7 Bridge or switch 8 9 Floor n – 1 Logical partition

26. Per-Port VLANs VLAN 2 VLAN 3 VLAN 1 S3 S6 S9 Floor n + 1 S2 S5 S8 Floor n 2 3 4 5 6 1 S1 S4 7 S7 Bridge or switch 8 9 Floor n – 1 Logical partition Bridge only forwards frames to outgoing ports associated with same VLAN

27. Tagged VLANs • More flexible than Port-based VLANs • Insert VLAN tag after source MAC address in each frame • VLAN protocol ID + tag • VLAN-aware bridge forwards frames to outgoing ports according to VLAN ID • VLAN ID can be associated with a port statically through configuration or dynamically through bridge learning • IEEE 802.1q

28. ECE 683Computer Network Design & Analysis Note 8: Packet Switching Networks (Network Layer Protocols)

29. Outline • Network Services and Internal Network Operation • Packet Network Topology • Datagrams and Virtual Circuits • Routing in Packet Networks • Shortest Path Routing

30. Network Layer • Network Layer: the most complex layer • Requires the coordinated actions of multiple, geographically distributed network elements (switches & routers) • Must be able to deal with very large scales • Billions of users (people & communicating devices) • Biggest Challenges • Addressing: where should information be directed to? • Routing: what path should be used to get information there?

31. t1 t0 Network Packet Switching • Transfer of information as payload in data packets • Packets undergo random delays & possible loss • Different applications impose differing requirements on the transfer of information

32. Messages Messages Segments Transport layer Transport layer Network service Network service Network layer Network layer Network layer Network layer Data link layer Data link layer Data link layer Data link layer End system β End system α Physical layer Physical layer Physical layer Physical layer Network Service • Network layer can offer a variety of services to transport layer • Connection-oriented service or connectionless service • Best-effort or delay/loss guarantees

33. Network Service Connectionless Datagram Transfer Connection-Oriented Reliable and possibly constant bit rate transfer Internal Network Operation Connectionless IP Connection-Oriented ATM Network Service vs. Operation • Various combinations are possible • Connection-oriented service over Connectionless operation • Connectionless service over Connection-Oriented operation • Context & requirements determine what makes sense

34. C 3 2 1 2 1 2 2 1 1 End system α End system β 1 1 1 2 2 2 3 3 3 2 4 1 4 2 2 2 1 2 1 2 1 1 1 3 Medium 2 1 B A Network 3 1 Physical layer entity Network layer entity 3 Network layer entity Data link layer entity 2 4 Transport layer entity Complexity at the Edge or in the Core?

35. The End-to-End Argument for System Design • An end-to-end function is best implemented at a higher level than at a lower level • End-to-end service requires all intermediate components to work properly • Higher-level better positioned to ensure correct operation • Example: stream transfer service • Establishing an explicit connection for each stream across network requires all network elements (NEs) to be aware of connection; All NEs have to be involved in re-establishment of connections in case of network fault • In connectionless network operation, NEs do not deal with each explicit connection and hence are much simpler in design

36. Network Layer Functions Essential • Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements • Forwarding: transfer of packets from NE inputs to outputs • Priority & Scheduling: determining order of packet transmission in each NE Optional: congestion control, segmentation & reassembly, security

37. Note 8: Packet Switching Networks Packet Network Topology

38. End-to-End Packet Network • Packet networks very different from telephone networks • Individual packet streams are highly bursty • Statistical multiplexing is used to concentrate streams • User demand can undergo dramatic change • Peer-to-peer applications stimulated huge growth in traffic volumes • Internet structure highly decentralized • Paths traversed by packets can go through many networks controlled by different organizations • No single entity responsible for end-to-end service

39. Access MUX To packet network Access Multiplexing • Packet traffic from users multiplexed at access to network into aggregated streams • DSL traffic multiplexed at DSL Access Mux • Cable modem traffic multiplexed at Cable Modem Termination System

40. r r • • • • • • nc r Nc Nr Oversubscription • Access Multiplexer • N subscribers connected @ c bps to mux • Each subscriber active r/c of time • Mux has C=nc bps to network • Oversubscription rate: N/n • Find n so that at most 1% overflow probability Feasible oversubscription rate increases with size

41. Home LANs • Home Router • LAN Access using Ethernet or WiFi (IEEE 802.11) • Private IP addresses in Home (192.168.0.x) using Network Address Translation (NAT) • Single global IP address from ISP issued using Dynamic Host Configuration Protocol (DHCP) WiFi Ethernet Home Router To packet network

42.             LAN Concentration Switch / Router • LAN Hubs and switches in the access network also aggregate packet streams that flows into switches and routers

43. Campus Network Organization Servers Servers have redundant connectivity to backbone To Internet or wide area network s s Gateway Backbone R R R S S S R Departmental Server R R s s s High-speed campus backbone net connects dept routers Only outgoing packets leave LAN through router s s s s s s

44. Connecting to Internet Service Provider Internet service provider Border routers Campus Network Border routers Interdomain level Autonomous system or domain Intradomain level s LAN network administered by single organization s s

45. National Service Provider A National Service Provider B NAP NAP Private peering National Service Provider C Internet Backbone • Network Access Points: set up during original commercialization of Internet to facilitate exchange of traffic • Private Peering Points: two-party inter-ISP agreements to exchange traffic

46. National Service Provider A (a) National Service Provider B NAP NAP Private peering National Service Provider C (b) NAP RA RB Route Server LAN RC

47. Key Role of Routing How to get packet from here to there? • Decentralized nature of Internet makes routing a major challenge • Interior gateway protocols (IGPs) are used to determine routes within a domain • Exterior gateway protocols (EGPs) are used to determine routes across domains • Routes must be consistent & produce stable flows • Scalability required to accommodate growth • Hierarchical structure of IP addresses essential to keeping size of routing tables manageable

48. Note 8: Packet Switching Networks Datagrams and Virtual Circuits

49. Packet switching network Transfers packets between users Transmission lines + packet switches (routers) Origin in message switching Two modes of operation: Connectionless Virtual Circuit User Transmission line Network Packet switch Packet Switching Network

50. Message Message Message Source Message Switches Destination Message Switching • Message switching invented for telegraphy • Entire messages multiplexed onto shared lines, stored & forwarded • Headers for source & destination addresses • Routing at message switches • Connectionless