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Switching 2004Oct Principles of Communication Networks 1 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 2004Oct Principles of Communication Networks 2 Outline switching - general Packet switching General Type of switches Switch generations Buffer placement Port mappers Buffer Placement Dropping policies 2004Oct Principles of Communication Networks 3 Types of switching elements Telephone switches Datagram routers switch datagrams ATM switches 2004Oct switch samples switch ATM cells Principles of Communication Networks 4 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 Packet switch Circuit switch 2004Oct Connectionless (router) Internet router Connection-oriented (switching system) ATM switching system Telephone switching system Principles of Communication Networks 5 Other switching element functions Participate in routing algorithms Resolve contention for output trunks scheduling Admission control 2004Oct to build routing tables to guarantee resources to certain streams Principles of Communication Networks 6 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 Circuit switch must reject call if can’t find a path for samples from input to output 2004Oct 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 subject to cost and reliability constraints goal: minimize packet loss Don’t reorder packets Principles of Communication Networks 7 Outline switching - general Packet switching General Type of switches Switch generations Buffer placement Port mappers Buffer Placement Dropping policies 2004Oct Principles of Communication Networks 8 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 ATM Cell, MPLS frame 2004Oct lookup based on entire destination address lookup based on VCI/VPI or MPLS label Other than that, very similar Principles of Communication Networks 9 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 2004Oct Principles of Communication Networks 10 Dealing with blocking Overprovisioning Buffers if switch fabric doesn’t have buffers, prevent packet from entering until path is available Parallel switch fabrics 2004Oct at input or output (or both) Backpressure internal links much faster than inputs (speedup) increases effective switching capacity Principles of Communication Networks 11 Repeaters, bridges, routers, and gateways Repeaters: at physical level Bridges: at datalink level (based on MAC addresses) (L2) Routers: at network level (L3) treat entire network as a single hop e.g., mail gateways and transcoders Gain functionality at the expense of forwarding speed 2004Oct participate in routing protocols Application level gateways: at application level (L7) discover attached stations by listening for best performance, push functionality as low as possible Principles of Communication Networks 12 Outline switching - general Packet switching General Type of switches Switch generations Buffer placement Port mappers Buffer Placement Dropping policies 2004Oct Principles of Communication Networks 13 Three generations of packet switches Different trade-offs between cost and performance Represent evolution in switching capacity, rather than in technology 2004Oct 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 14 First generation switch computer CPU queues in memory linecard linecard linecard 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 2004Oct Principles of Communication Networks 15 Second generation switch computer bus front end processors or line cards Port mapping intelligence in line cards ATM switch guarantees hit in lookup cache 2004Oct Principles of Communication Networks 16 Third generation switches Bottleneck in second generation switch is the bus (or ring) Third generation switch provides parallel paths (fabric) OLC ILC IN ILC ILC 2004Oct NxN packet switch fabric OLC OUT OLC Principles of Communication Networks 17 Third generation (contd.) Features 2004Oct 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 18 Outline switching - general Packet switching General Type of switches Switch generations Port mappers Buffer Placement Dropping policies 2004Oct Principles of Communication Networks 19 Port mappers Look up output port based on destination address Easy for VCI: just use a table Harder for datagrams: 2004Oct 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 20 Tries root (10.*) 10 128 32 54 32 4 1 120 (32.*) 25 (128.32.25.*) (128.54.4.*) 100 (128.32.1.120) (128.32.1.100) Some ways to improve performance 2004Oct cache recently used addresses in a CAM move common entries up to a higher level (match longer strings) Principles of Communication Networks 21 Outline switching - general Packet switching General Type of switches Switch generations Port mappers Buffer Placement Dropping policies 2004Oct Principles of Communication Networks 22 Buffering All packet switches need buffers to match input rate to service rate Where should we place buffers? 2004Oct or cause heavy packet loses input output in the fabric Principles of Communication Networks 23 Input buffering (input queueing) queues buffer control NxN switch inputs queues buffer control queues buffer control outputs arbitrator No speedup in buffers or trunks (unlike output queued switch) Needs arbiter Problem: head of line blocking 2004Oct with randomly distributed packets, utilization at most 58.6% Principles of Communication Networks 24 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. 2004Oct Principles of Communication Networks 27 Output queueing inputs NxN switch fabric outputs 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 2004Oct unlikely that all N inputs will have packets for the same output drop extra packets, fairly distributing losses among inputs Principles of Communication Networks 28 Buffered fabric Buffers in each switch element Pros Cons 2004Oct Speed up is only as much as fan-in Hardware backpressure reduces buffer requirements costly (unless using single-chip switches) scheduling is hard Principles of Communication Networks 29 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 2004Oct Principles of Communication Networks 30 Hybrid solutions Buffers at more than one point Becomes hard to analyze and manage But common in practice 2004Oct Principles of Communication Networks 31 Multicasting Useful to do this in hardware Assume portmapper knows list of outputs Incoming packet must be copied to these output ports Two subproblems 2004Oct generating and distributing copies ATM VCI/MPLS label translation for the copies Principles of Communication Networks 32 Generating and distributing copies Either implicit or explicit Implicit Explicit 2004Oct 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 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 33 Outline switching - general Packet switching General Type of switches Switch generations Buffer placement Port mappers Buffer Placement Dropping policies 2004Oct Principles of Communication Networks 34 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? 2004Oct packet drop wastes resources (why?) Principles of Communication Networks 35 Classification of drop strategies 1. Degree of aggregation 2. Drop priorities 3. Early or late 4. Drop position 2004Oct Principles of Communication Networks 36 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 2004Oct Principles of Communication Networks 37 2. Drop priorities Drop lower-priority packets first How to choose? 2004Oct endpoint marks packets regulator marks packets congestion loss priority (CLP) bit in packet header Principles of Communication Networks 38 CLP bit: pros and cons Pros Cons 2004Oct if network has spare capacity, all traffic is carried during congestion, load is automatically shed separating priorities within a single connection is hard what prevents all packets being marked as high priority? Principles of Communication Networks 39 3. Early vs. late drop Early drop => drop even if space is available Early random drop 2004Oct signals endpoints to reduce rate cooperative sources get lower overall delays, uncooperative sources get severe packet loss 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 40 3. Early vs. late drop: RED Random early detection (RED) makes three improvements Metric is moving average of queue lengths Packet drop probability is a function of mean queue length small bursts pass through unharmed only affects sustained overloads 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 2004Oct Principles of Communication Networks 41 4. Drop position Can drop a packet from head, tail, or random position in the queue Tail Head 2004Oct easy default approach harder lets source detect loss earlier Principles of Communication Networks 42 4. Drop position (contd.) Random 2004Oct hardest if no aggregation, hurts hogs most unlikely to make it to real routers Principles of Communication Networks 43
