Principles of Communication Networks

Principles of Communication Networks
Switching Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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

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

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

First generation switch
computer CPU linecard linecard queues in memory 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 Oct 2004 Principles of Communication Networks

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

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

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 Oct 2004 Principles of Communication Networks

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

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 entries: ( *, 3), ( *, 4), ( , 2) A standard solution: trie Oct 2004 Principles of Communication Networks

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 1 4 ( *) 25 120 ( *) 100 ( ) ( ) Oct 2004 Principles of Communication Networks

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

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 Oct 2004 Principles of Communication Networks

Input buffering (input queueing)
control queues 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 buffer control queues buffer control queues arbitrator Oct 2004 Principles of Communication Networks

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. Oct 2004 Principles of Communication Networks

Output queueing NxN switch fabric inputs 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 unlikely that all N inputs will have packets for the same output drop extra packets, fairly distributing losses among inputs Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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?) Oct 2004 Principles of Communication Networks

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

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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? Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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 Oct 2004 Principles of Communication Networks

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

Principles of Communication Networks

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