Switching

1. Switching Principles of Communication Networks

2. 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 Principles of Communication Networks

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

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

5. 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 Principles of Communication Networks

6. 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 Principles of Communication Networks

7. Requirements • Capacity of a 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 Principles of Communication Networks

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

9. 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 • IP Datagram • lookup based on entire destination address • ATM Cell, MPLS frame • lookup based on VCI/VPI or MPLS label • Other than that, very similar Principles of Communication Networks

10. 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 Principles of Communication Networks

11. 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 Principles of Communication Networks

12. 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 Principles of Communication Networks

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

14. 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 Principles of Communication Networks

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

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

17. 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 Principles of Communication Networks

18. 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 Principles of Communication Networks

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

20. 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 Principles of Communication Networks

21. 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) Principles of Communication Networks

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

23. 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 Principles of Communication Networks

24. 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 Principles of Communication Networks

25. 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. Principles of Communication Networks

26. 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 Principles of Communication Networks

27. 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 Principles of Communication Networks

28. 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 Principles of Communication Networks

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

30. 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 • ATM VCI/MPLS label translation for the copies Principles of Communication Networks

31. 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 Principles of Communication Networks

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

33. 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?) Principles of Communication Networks

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

35. 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 Principles of Communication Networks

36. 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 Principles of Communication Networks

37. 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? Principles of Communication Networks

38. 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 Principles of Communication Networks

39. 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 Principles of Communication Networks

40. 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 Principles of Communication Networks

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