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Switching. 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. Outline. switching - general

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switching
Switching

Principles of Communication Networks

what is it all about
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

outline
Outline
  • switching - general
  • Packet switching
    • General
    • Type of switches
    • Switch generations
    • Buffer placement
  • Port mappers
  • Buffer Placement
  • Dropping policies

Principles of Communication Networks

types of switching elements
Types of switching elements
  • Telephone switches
    • switch samples
  • Datagram routers
    • switch datagrams
  • ATM switches
    • switch ATM cells

Principles of Communication Networks

classification
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

other switching element functions
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

requirements
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

outline1
Outline
  • switching - general
  • Packet switching
    • General
    • Type of switches
    • Switch generations
    • Buffer placement
  • Port mappers
  • Buffer Placement
  • Dropping policies

Principles of Communication Networks

packet switching
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

blocking in packet switches
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

dealing with blocking
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

repeaters bridges routers and gateways
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

outline2
Outline
  • switching - general
  • Packet switching
    • General
    • Type of switches
    • Switch generations
    • Buffer placement
  • Port mappers
  • Buffer Placement
  • Dropping policies

Principles of Communication Networks

three generations of packet switches
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

first generation switch

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

second generation switch
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

third generation switches

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

third generation contd
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

outline3
Outline
  • switching - general
  • Packet switching
    • General
    • Type of switches
    • Switch generations
  • Port mappers
  • Buffer Placement
  • Dropping policies

Principles of Communication Networks

port mappers
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

tries
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

outline4
Outline
  • switching - general
  • Packet switching
    • General
    • Type of switches
    • Switch generations
  • Port mappers
  • Buffer Placement
  • Dropping policies

Principles of Communication Networks

buffering
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

input buffering input queueing

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

dealing with hol blocking
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

output queueing
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

buffered fabric
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

buffered crossbar
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

hybrid solutions
Hybrid solutions
  • Buffers at more than one point
  • Becomes hard to analyze and manage
  • But common in practice

Principles of Communication Networks

multicasting
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

generating and distributing copies
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

outline5
Outline
  • switching - general
  • Packet switching
    • General
    • Type of switches
    • Switch generations
    • Buffer placement
  • Port mappers
  • Buffer Placement
  • Dropping policies

Principles of Communication Networks

packet dropping
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

classification of drop strategies
Classification of drop strategies

1. Degree of aggregation

2. Drop priorities

3. Early or late

4. Drop position

Principles of Communication Networks

1 degree of aggregation
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

2 drop priorities
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

clp bit pros and cons
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

3 early vs late drop
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

3 early vs late drop red
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

4 drop position
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

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

Principles of Communication Networks