1. Oct 2004 Principles of Communication Networks 1 Source books D. Bertsekas and R. Gallager. Data Networks, 2 nd 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, 2 nd Ed., 1991,M-H

2. Switching n S. Keshav, An Engineering Approach to Computer Networks, A-W, 1997 n 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.

3. Oct 2004 Principles of Communication Networks 3 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

4. Oct 2004 Principles of Communication Networks 4 Outline switching - general Packet switching  General  Type of switches  Switch generations  Buffer placement Port mappers Buffer Placement Dropping policies

5. Oct 2004 Principles of Communication Networks 5 Types of switching elements Telephone switches  switch samples Datagram routers  switch datagrams ATM switches  switch ATM cells

6. Oct 2004 Principles of Communication Networks 6 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

7. Oct 2004 Principles of Communication Networks 7 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

8. Oct 2004 Principles of Communication Networks 8 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

9. Oct 2004 Principles of Communication Networks 9 Outline switching - general Packet switching  General  Type of switches  Switch generations  Buffer placement Port mappers Buffer Placement Dropping policies

10. Oct 2004 Principles of Communication Networks 10 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

11. Oct 2004 Principles of Communication Networks 11 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

12. Oct 2004 Principles of Communication Networks 12 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

13. Oct 2004 Principles of Communication Networks 13 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

14. Oct 2004 Principles of Communication Networks 14 Outline switching - general Packet switching  General  Type of switches  Switch generations  Buffer placement Port mappers Buffer Placement Dropping policies

15. Oct 2004 Principles of Communication Networks 15 Three generations of packet switches n Different trade-offs between cost and performance n Represent evolution in switching capacity, rather than in technology u With same technology, a later generation switch achieves greater capacity, but at greater cost All three generations are represented in current products

16. Oct 2004 Principles of Communication Networks 16 First generation switch Old 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 queues in memory CPU linecard

17. Oct 2004 Principles of Communication Networks 17 Second generation switch Port mapping intelligence in line cards ATM switch guarantees hit in lookup cache bus computer front end processors or line cards

18. Oct 2004 Principles of Communication Networks 18 Third generation switches Bottleneck in second generation switch is the bus (or ring) Third generation switch provides parallel paths (fabric) NxN packet switch fabric OLC IN ILC OUT

19. Oct 2004 Principles of Communication Networks 19 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

20. Oct 2004 Principles of Communication Networks 20 Outline switching - general Packet switching  General  Type of switches  Switch generations Port mappers Buffer Placement Dropping policies

21. Oct 2004 Principles of Communication Networks 21 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

22. Oct 2004 Principles of Communication Networks 22 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 32128 54 4 (128.54.4.*) (32.*) (10.*) 32 25 (128.32.25.*) 1 120 100 (128.32.1.100) (128.32.1.120)

23. Oct 2004 Principles of Communication Networks 23 Outline switching - general Packet switching  General  Type of switches  Switch generations Port mappers Buffer Placement Dropping policies

24. Oct 2004 Principles of Communication Networks 24 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

25. Oct 2004 Principles of Communication Networks 25 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% buffer control queues buffer control queues buffer control queues arbitrator NxN switch outputs inputs

26. Oct 2004 Principles of Communication Networks 26 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)

27. Oct 2004 Principles of Communication Networks 27 head of line blocking – alternative calculation The success probability of an input port selection:

28. Oct 2004 Principles of Communication Networks 28 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, and more.

29. Oct 2004 Principles of Communication Networks 29 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 inputsoutputs NxN switch fabric

30. Oct 2004 Principles of Communication Networks 30 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

31. Oct 2004 Principles of Communication Networks 31 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

32. Oct 2004 Principles of Communication Networks 32 Hybrid solutions Buffers at more than one point Becomes hard to analyze and manage But common in practice

33. Oct 2004 Principles of Communication Networks 33 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

34. Oct 2004 Principles of Communication Networks 34 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

35. Oct 2004 Principles of Communication Networks 35 Outline switching - general Packet switching  General  Type of switches  Switch generations  Buffer placement Port mappers Buffer Placement Dropping policies

36. Oct 2004 Principles of Communication Networks 36 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?)

37. Oct 2004 Principles of Communication Networks 37 Classification of drop strategies 1. Degree of aggregation 2. Drop priorities 3. Early or late 4. Drop position

38. Oct 2004 Principles of Communication Networks 38 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

39. Oct 2004 Principles of Communication Networks 39 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

40. Oct 2004 Principles of Communication Networks 40 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?

41. Oct 2004 Principles of Communication Networks 41 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

42. Oct 2004 Principles of Communication Networks 42 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

43. Oct 2004 Principles of Communication Networks 43 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

44. Oct 2004 Principles of Communication Networks 44 4. Drop position (contd.) Random  hardest  if no aggregation, hurts hogs most  unlikely to make it to real routers