Communication networks

1. Communication networks • Instractor: Dr. Yuval Shavitt, • Office hours: room 030, Mon 17:00-18:00 • Requiresments (דרישות קדם): • Introduction to computer communications (TAU, Technion, BGU) • Expectations from students: • Queueing theory basics • Graph theory • Good C/C++ programming skills Communication Networks

2. Introduction to switching, router types HOL analysis Matching algorithms and their analysis CLOS networks: non-blocking theorem, routing algorithms and their analysis Scheduling algorithms: WFQ, W2FQ, priorities Event simulators – introduction Programming tasks: Single queue HOL blocking (cells) iSLIP algorithm (cells & packets) …. Course Syllabus (tentative) Communication Networks

3. Source books • D. Bertsekas and R. Gallager. Data Networks, 2nd Ed., 1992. P-H. • S. Keshav. An Engineering Approach to Computer Networking. 1997. E-W • J.F. Kurose and K.W. Ross. Computer Networking. 2000, E-W. • L. Kleinrock. Queueing Systems, Vol. 1. 1975. Wiley • J.Y.Hui, Switching and Traffic Theory for Integrated Broadband Networks, Kluwer 1990 • A.M. Law and W.D. Kelton. Simulation Modeling & Analysis, 2nd Ed., 1991,M-H Communication Networks

4. Switching • S.Keshav, An Engineering Approach to Computer Networks, A-W, 1997 • M. Karol, M. Hluchyj, and S. Morgan, "Input Versus Output Queueing on a Space-Division Packet Switch," IEEE Trans. on Communications, 35(12):1347-1356, Dec. 1987.

5. What is it all about? • How do we move traffic from one part of the network to another? • Connect end-systems to switches, and switches to each other • Data arriving to an input port of a switch have to be moved to one or more of the output ports Communication Networks

6. Outline • switching - general • Packet switching • General • Type of switches • Switch generations • Buffer placement • Port mappers • Buffer Placement • Dropping policies Communication Networks

7. Types of switching elements • Telephone switches • switch samples • Datagram routers • switch datagrams • ATM switches • switch ATM cells Communication Networks

8. Classification • Packet vs. circuit switches • packets have headers and samples don’t • Connectionless vs. connection oriented • connection oriented switches need a call setup • setup is handled in control plane by switch controller • connectionless switches deal with self-contained datagrams Communication Networks

9. Other switching element functions • Participate in routing algorithms • to build routing tables • Resolve contention for output trunks • scheduling • Admission control • to guarantee resources to certain streams Communication Networks

10. Requirements • Capacity of switch is the maximum rate at which it can move information, assuming all data paths are simultaneously active • Primary goal: maximize capacity • subject to cost and reliability constraints • Circuit switch must reject call if can’t find a path for samples from input to output • goal: minimize call blocking • Packet switch must reject a packet if it can’t find a buffer to store it awaiting access to output trunk • goal: minimize packet loss • Don’t reorder packets Communication Networks

11. Outline • switching - general • Packet switching • General • Type of switches • Switch generations • Buffer placement • Port mappers • Buffer Placement • Dropping policies Communication Networks

12. Packet switching • In a circuit switch, path of a sample is determined at time of connection establishment • No need for a sample header--position in frame is enough • In a packet switch, packets carry a destination field • Need to look up destination port on-the-fly • Datagram • lookup based on entire destination address • Cell • lookup based on VCI • Other than that, very similar Communication Networks

13. Blocking in packet switches • Can have both internal and output blocking • Internal • no path to output • Output • trunk unavailable • Unlike a circuit switch, cannot predict if packets will block (why?) • If packet is blocked, must either buffer or drop it Communication Networks

14. Dealing with blocking • Overprovisioning • internal links much faster than inputs (speedup) • Buffers • at input or output (or both) • Backpressure • if switch fabric doesn’t have buffers, prevent packet from entering until path is available • Parallel switch fabrics • increases effective switching capacity Communication Networks

15. Repeaters, bridges, routers, and gateways • Repeaters: at physical level • Bridges: at datalink level (based on MAC addresses) (L2) • discover attached stations by listening • Routers: at network level (L3) • participate in routing protocols • Application level gateways: at application level (L7) • treat entire network as a single hop • e.g mail gateways and transcoders • Gain functionality at the expense of forwarding speed • for best performance, push functionality as low as possible Communication Networks

16. Outline • switching - general • Packet switching • General • Type of switches • Switch generations • Buffer placement • Port mappers • Buffer Placement • Dropping policies Communication Networks

17. Three generations of packet switches • Different trade-offs between cost and performance • Represent evolution in switching capacity, rather than in technology • With same technology, a later generation switch achieves greater capacity, but at greater cost • All three generations are represented in current products Communication Networks

18. linecard linecard First generation switch • Most Ethernet switches and cheap packet routers • S/w router, e.g., Linux/FreeBSD boxes • Bottleneck can be CPU, host-adaptor or I/O bus, depending computer CPU queues in memory linecard Communication Networks

19. Second generation switch • Port mapping intelligence in line cards • ATM switch guarantees hit in lookup cache computer bus front end processors or line cards Communication Networks

20. ILC ILC ILC Third generation switches • Bottleneck in second generation switch is the bus (or ring) • Third generation switch provides parallel paths (fabric) OLC NxN packet switch fabric OUT OLC IN OLC Communication Networks

21. Third generation (contd.) • Features • self-routing fabric • output buffer is a point of contention • unless we arbitrate access to fabric • potential for unlimited scaling, as long as we can resolve contention for output buffer Communication Networks

22. Outline • switching - general • Packet switching • General • Type of switches • Switch generations • Port mappers • Buffer Placement • Dropping policies Communication Networks

23. Port mappers • Look up output port based on destination address • Easy for VCI: just use a table • Harder for datagrams: • need to find longest prefix match • e.g. packet with address 128.32.1.20 • entries: (128.32.*, 3), (128.32.1.*, 4), (128.32.1.20, 2) • A standard solution: trie Communication Networks

24. Tries • Some ways to improve performance • cache recently used addresses in a CAM • move common entries up to a higher level (match longer strings) root (10.*) 10 128 32 (32.*) 54 32 4 1 (128.54.4.*) 25 (128.32.25.*) 120 100 (128.32.1.120) (128.32.1.100) Communication Networks

25. Outline • switching - general • Packet switching • General • Type of switches • Switch generations • Port mappers • Buffer Placement • Dropping policies Communication Networks

26. Buffering • All packet switches need buffers to match input rate to service rate • or cause heavy packet loses • Where should we place buffers? • input • output • in the fabric Communication Networks

27. buffer control buffer control buffer control queues queues queues Input buffering (input queueing) • No speedup in buffers or trunks (unlike output queued switch) • Needs arbiter • Problem: head of line blocking • with randomly distributed packets, utilization at most 58.6% NxN switch outputs inputs arbitrator Communication Networks

28. head of line blocking – simple upper bound • Assume nxn switch with uniform distribution of destination • Probability for an output port not to be selected is • Capacity is bounded by 1-1/e = 0.63 • For 2x2 switch the max capacity is 0.75 (tight bound) Communication Networks

29. head of line blocking – alternative calculation • The success probability of an input port selection: Communication Networks

30. Dealing with HOL blocking • Per-output queues at inputs (VOQ) • Arbiter must choose one of the input ports for each output port • How to select? • Parallel Iterated Matching • inputs tell arbiter which outputs they are interested in • output selects one of the inputs • some inputs may get more than one grant, others may get none • if >1 grant, input picks one at random, and tells output • losing inputs and outputs try again • Used in DEC Autonet 2 switch, McKeown’s iSLIP, andmore. Communication Networks

31. Output queueing • Don’t suffer from head-of-line blocking • But output buffers need to run much faster than trunk speed • Can reduce some of the cost by using the knockout principle • unlikely that all N inputs will have packets for the same output • drop extra packets, fairly distributing losses among inputs NxN switch fabric inputs outputs Communication Networks

32. Buffered fabric • Buffers in each switch element • Pros • Speed up is only as much as fan-in • Hardware backpressure reduces buffer requirements • Cons • costly (unless using single-chip switches) • scheduling is hard Communication Networks

33. Buffered crossbar • What happens if packets at two inputs both want to go to same output? • Can defer one at an input buffer • Or, buffer crosspoints Communication Networks

34. Hybrid solutions • Buffers at more than one point • Becomes hard to analyze and manage • But common in practice Communication Networks

35. Multicasting • Useful to do this in hardware • Assume portmapper knows list of outputs • Incoming packet must be copied to these output ports • Two subproblems • generating and distributing copies • VCI translation for the copies Communication Networks

36. Generating and distributing copies • Either implicit or explicit • Implicit • suitable for bus-based, ring-based, crossbar, or broadcast switches • multiple outputs enabled after placing packet on shared bus • used in Paris and Datapath switches • Explicit • need to copy a packet at switch elements • use a copy network • place # of copies in tag • element copies to both outputs and decrements count on one of them • collect copies at outputs • Both schemes increase blocking probability Communication Networks

37. Outline • switching - general • Packet switching • General • Type of switches • Switch generations • Buffer placement • Port mappers • Buffer Placement • Dropping policies Communication Networks

38. Packet dropping • Packets that cannot be served immediately are buffered • Full buffers => packet drop strategy • Packet losses happen almost always from best-effort connections (why?) • Shouldn’t drop packets unless imperative? • packet drop wastes resources (why?) Communication Networks

39. Classification of drop strategies 1. Degree of aggregation 2. Drop priorities 3. Early or late 4. Drop position Communication Networks

40. 1. Degree of aggregation • Degree of discrimination in selecting a packet to drop • E.g. in vanilla FIFO, all packets are in the same class • Instead, can classify packets and drop packets selectively • The finer the classification the better the protection Communication Networks

41. 2. Drop priorities • Drop lower-priority packets first • How to choose? • endpoint marks packets • regulator marks packets • congestion loss priority (CLP) bit in packet header Communication Networks

42. CLP bit: pros and cons • Pros • if network has spare capacity, all traffic is carried • during congestion, load is automatically shed • Cons • separating priorities within a single connection is hard • what prevents all packets being marked as high priority? Communication Networks

43. 3. Early vs. late drop • Early drop => drop even if space is available • signals endpoints to reduce rate • cooperative sources get lower overall delays, uncooperative sources get severe packet loss • Early random drop • drop arriving packet with fixed drop probability if queue length exceeds threshold • intuition: misbehaving sources more likely to send packets and see packet losses Communication Networks

44. 3. Early vs. late drop: RED • Random early detection (RED) makes three improvements • Metric is moving average of queue lengths • small bursts pass through unharmed • only affects sustained overloads • Packet drop probability is a function of mean queue length • prevents severe reaction to mild overload • Can mark packets instead of dropping them • allows sources to detect network state without losses • RED improves performance of a network of cooperating TCP sources • No bias against bursty sources • Controls queue length regardless of endpoint cooperation Communication Networks

45. 4. Drop position • Can drop a packet from head, tail, or random position in the queue • Tail • easy • default approach • Head • harder • lets source detect loss earlier Communication Networks

46. 4. Drop position (contd.) • Random • hardest • if no aggregation, hurts hogs most • unlikely to make it to real routers Communication Networks