CMPE 150 – Winter 2009

1. CMPE 150 – Winter 2009 Lecture 10 February 10, 2009 P.E. Mantey

2. CMPE 150 -- Introduction to Computer Networks • Instructor: Patrick Mantey mantey@soe.ucsc.edu http://www.soe.ucsc.edu/~mantey/ • Office: Engr. 2 Room 595J • Office hours: Tues 3-5 PM, Mon 5-6 PM* • TA: Anselm Kia akia@soe.ucsc.edu • Web site: http://www.soe.ucsc.edu/classes/cmpe150/Winter09/ • Text: Tannenbaum: Computer Networks (4th edition – available in bookstore, etc. )

3. Syllabus

4. Reading Assignment • Thursday, February 12 Chapter 5, sections 5.3, 5.4.1-5.4.3, 5.5

5. Today’s Agenda • Link Layer • Bridge Routing • Network Layer • Functions / IP • Routing Text: Chapter 5, sections 5.1, 5.2.1-5.2.8 (pp. 343-370)

6. Internet Layering Level 5 -- Application Layer (rlogin, ftp, SMTP, POP3, IMAP, HTTP..) Level 4 -- Transport Layer(a.k.a Host-to-Host) (TCP, UDP, ARP, ICMP, etc.) Level 3 -- Network Layer (a.k.a. Internet) (IP) Level 2 -- (Data)Link Layer / MAC sub-layer (a.k.a. Network Interface or Network Access Layer) Level 1 -- Physical Layer

7. Bridging (802.x) • Fixed Route Bridging • Source Route Bridging • Transparent Bridging • “Plug ‘n Play”: • Flooding algorithm • Backward Learning • Spanning Tree (avoids loops) Ref: http://en.wikipedia.org/wiki/Bridging_%28networking%29

8. Spanning Tree Approach- Uses: • IEEE 802.1 • Frame Forwarding • Each bridge maintains Forwarding Table • List of stations on the “side” of each port • Forwards to port and on to LAN corresponding to Table • Unless blocked • Floods those whose MAC address not in Table • Address Learning • Spanning Tree Algorithm

9. Address Learning • When frame arrives on a port, table records source address as being on that port. (Backward learning) • Timer set for each entry • Timer expires, entry is deleted • Timer reset if new frame gives same info.

10. Spanning Tree Algorithm • Algorithm to avoid Loops • (not needed if topology is a “tree”) • Root of tree is lowest serial number (all bridges broadcast their serial number) • Tree constructed from root to every bridge with shortest path • Algorithm runs “continually” to detect topology changes (801.1D)

11. http://www.cisco.com/warp/public/473/lan-switch-transparent.swfhttp://www.cisco.com/warp/public/473/lan-switch-transparent.swf

12. Network Layer Topics • Network Layer (Layer 3) • Services for the Transport Layer • Uses the Link Layer • Datagrams (Packets) • Internet Protocol (IP) • Addressing (IP Address)

13. Communication challenges • Analog - continuously varying value • Digital - sequence of symbols from a finite set • Network challenge - transmission of information across interconnected channels • Circuit - think telephone call • Datagram - think telegram Information Audio Video Text Data Images Signals Analog Digital Channels Analog Digital Networks Circuit Datagram

14. First networks • Which came first, analog or digital? • Digital(!) • Signal fires - 1200 BC • Optical telegraph - 1791 • Electronic telegraph - 1836 • First analog telecommunication was telephone - 1875

15. Telephone network • Originally all analog • Internal links shared with FDM • Internal links replaced with digital in 1962 • Allowed sharing with more efficient TDM • Circuit-switched • Simple, hierarchical (static) topology and routing • First done by human operators… • …then automatic relays • Provide guaranteed Quality-of-Service (QoS) • Dedicate resources to each connection

16. Packet-switched networks • Defining characteristics • Data and addressing in packets • Statistical multiplexing • Dynamic topology and routing • Virtual-circuit • X.25 (1974), Frame Relay (1984), ATM (1988) • Datagram • ARPAnet (1966), Internet (1977)

17. Virtual-circuit networks • Generalization of telephone model • “Smart net” architecture • Compute route on-demand at flow setup • Per-flow state in routers • Assumes need of resource reservation for some flows

18. Datagram networks • Three inventors • Kleinrock (1961) - efficiency • Baran (1962) - survivability • Davies (1964) - commercial service • Controversy over who invented • Defining characteristics • Pre-compute routes to all destinations • No per-flow state • Assumes best-effort will be good enough

19. ARPAnet • First datagram network • 1966 - project proposed • September, 1969 - first “IMP” installed • Modified “smart net” architecture • Built on reliable link technology • Hop-by-hop reliability, congestion and flow control • Some end-to-end flow control (“RFNM”) • Much redundant functionality • E.g. reliability and flow control • Led to the “end-to-end argument”

20. End-to-end argument If a function can completely and correctly be implemented only with the knowledge and help of the application end-points of a communication system, then the function should not be implemented in the communication system itself (although sometimes it may be useful to implement an incomplete version of the function in the communication system as a performance enhancement). J. Saltzer, D. Reed and D. Clark: “End-to-end arguments in system design”, ACM Trans. Comp. Sys., 2(4):277-88, Nov. 1984

21. The Internet • Goal - encourage development of new network technologies • Make minimal assumptions about links • Attach many computers • Transport datagrams • Quickly send datagrams to different destinations • Best-effort delivery (drop or out of order) • “Smart host” architecture • End-to-end argument

22. Network layer summary • Communication across interconnected channels • Two styles: circuit vs packet-switched • Packet-switching characteristics • Data and addressing in packets • Statistical multiplexing • Dynamic topology and routing • Datagram - pre-compute routes, no flow state • VC - on-demand routing, per flow state

23. Datagram vs VC

24. Network Layer Design Issues • Store-and-Forward Packet Switching • Providing Services to the Transport Layer • Implementation of Connectionless Service • Implementation of Connection-Oriented Service • Comparison of Virtual-Circuit and Datagram Subnets

25. Routers (or Level 3 Switches) • Determing route • Run routing algorithms to build table • Routing tables determine packet path = output line • Forwarding packets

26. Routing Algorithms Attributes of Good algorithms for routing • Correct • Fair • Simple • Robust • Stable • Optimal

27. Routing “Flavors” • Non-Adaptive • “static” • Adaptive • “dynamic”

28. Static Algorithms • (Non-Adaptive) • Shortest-path routing. • Flooding.

29. Non-adaptive Routing • Fixed routing, static routing. • Do not take current state of the network (e.g., load, topology) into decision • Routes are computed in advance, off-line, and downloaded to routers when booted.

30. Flooding Algorithm (Static) • Every incoming packet forwarded on every outgoing link except the one it arrived on. • Problem: duplicates. • Constraining the flood: • Hop count. • Keep track of packets that have been flooded. • Robust, shortest delay (picks shortest path as one of the paths).

31. Flooding • Stallings Figure 12.4 (hop-count=3)

32. Adaptive Routing • Routes change dynamically as function of current state of network. • Algorithms vary on how they get routing information, metrics used, and when they change routes.

33. Adaptive Routing Dynamic Routing Algorithms • Distance vector routing. Bellman – Ford (or Ford-Fulkerson) • Link state routing.

34. Adaptive routing • Needed in both datagram and VC networks • When run in different architectures? • Based on Shortest-Path algorithms • Find minimum cost of paths between src and dst • Dijkstra - search paths by increasing cost • Bellman-Ford - search paths by increasing hop count • Dijkstra used for Link-State • Bellman-Ford basis of Distance-Vector

35. Optimality Principle • General statement about optimal routes (topology, routing algorithm independent). • If router J is on optimal path between I and K, then the optimal path from J to K also falls along the same route. • Proof by contradiction.

36. Optimality Principle • Corollary: • Set of optimal routes from all sources to destination form a tree rooted at destination. • Sink tree.

37. Shortest Path Routing 1 • Dijkstra (1959). • Network represented by graph G(V, E), where V is set of nodes and E is set of links connecting nodes. • What is “shortest”? • Different metrics. • Example: number of hops (static), geographic distance (static), delay, bandwidth (raw versus available), combination of a subset of these.

38. Dijkstra’s Shortest Path • Nodes labeled with distance to source through best known path. • At start, no known paths so all nodes labeled with infinity. • As algorithm progresses, nodes are labeled; “tentative” labels may change, while “permanent” labels don’t change. • Label made permanent when it’s known to be in the shortest path to source.

39. Dijkstra algorithm • Network represented by graph: G(V,E) • V contains vertices i,j,… • E contains edges (i,j), … • Algorithm data structures • “s” - source vertex • “dij” - cost of the edge (i,j); dij = ∞ if (i,j)  E • “Di” - cost of the shortest path from s to i • “P” - set of routes with final (“permanent”) labels

40. Dijkstra algorithm P = {s} Ds = 0 Dj = dsjfor all {j  V | j  s} Do Di = Min {Djfor all j  P} P = P  {i} For each {j | (i,j)  E, j  P} Dj = Min {Dj, Di+dij} Until (|P| = |V|)

41. B(2,A) B(2,A) 7 7 (9,B) C C 2 3 2 3 3 3 (4,B) 2 2 2 2 A D A D 2 1 E F 1 E F 2 6 2 6 2 4 4 G(6,A) H H (6,A) G (2,A) (9,B) (2,A) B 7 (9,B) C B 7 C 3 2 3 3 2 (4,B) 2 3 (4,B) 2 2 2 (6,E) A D A (6,E) D E F 1 E F 1 2 2 6 2 6 2 4 (5,E) G 4 H G(5,E) H(9,G) (2,A) (9,B) (2,A) B 7 (9,B) C B 7 C 2 3 3 3 2 (4,B) 2 3 (4,B) 2 2 2 A (6,E) D A (6,E) D(10,H) E F 1 E F 1 2 2 6 2 6 2 4 H(8,F) H(8,F) G(5,E) 4 G(5,E) Dijkstra illustrated

42. Distance-Vector protocol Initialize routing table with local links Flood routing table to all routers Do Compute local routing table from graph Wait for update or link cost change or timer Update network graph If link cost change Flood updated link to all routers Else if timer expired Flood routing table to all routers Forever

43. Distance Vector Routing • (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.