Switching

1
Switching

• An Engineering Approach to Computer Networking

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

3
Outline

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

4
Types of switching elements

• Telephone switches
• switch samples
• Datagram routers
• switch datagrams
• ATM switches
• switch ATM cells

5
Classification

• Packet vs. circuit switches
• packets have headers and samples dont
• 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

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

7
Requirements

• Capacity of 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 cant find a
  path for samples from input to output
• goal minimize call blocking
• Packet switch must reject a packet if it cant
  find a buffer to store it awaiting access to
  output trunk
• goal minimize packet loss
• Dont reorder packets

8
Outline

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

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
• Datagram
• lookup based on entire destination address
• Cell
• lookup based on VCI
• Other than that, very similar

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

11
Dealing with blocking

• Overprovisioning
• internal links much faster than inputs
• Buffers
• at input or output
• Backpressure
• if switch fabric doesnt have buffers, prevent
  packet from entering until path is available
• Parallel switch fabrics
• increases effective switching capacity

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

13
Outline

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

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

15
First generation switch
computer
CPU
queues in memory
linecard

• Most Ethernet switches and cheap packet routers
• Bottleneck can be CPU, host-adaptor or I/O bus,
  depending

16
Second generation switch
computer
bus
front end processors or line cards

• Port mapping intelligence in line cards
• ATM switch guarantees hit in lookup cache

17
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
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

19
Outline

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

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

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.)
32
128
(32.)
54
32
4
1
(128.54.4.)
25
(128.32.25.)
120
100
(128.32.1.120)
(128.32.1.100)
22
Tries

• Level hashing
• Hash entries in each level
• Number of memory access log (depth)

root
10
(10.)
32
128
(32.)
54
32
4
(128.54.4.)
25
1
(128.32.25.)
120
100
(128.32.1.120)
(128.32.1.100)
23
Outline

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

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

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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
26
Dealing with HOL blocking

• Per-output queues at inputs
• 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 gt1 grant, input picks one at random, and tells
  output
• losing inputs and outputs try again
• Used in DEC Autonet 2 switch

27
Output queueing
NxN switch fabric
inputs
outputs

• Dont 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

28
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

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

30
Hybrid solutions

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

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
• generating and distributing copes
• VCI translation for the copies

32
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

33
Outline

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

34
Packet dropping

• Packets that cannot be served immediately are
  buffered
• Full buffers gt packet drop strategy
• Packet losses happen almost always from
  best-effort connections (why?)
• Shouldnt drop packets unless imperative
• packet drop wastes resources (why?)

35
Classification of drop strategies

• 1. Degree of aggregation
• 2. Drop priorities
• 3. Early or late
• 4. Drop position

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

37
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

38
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?

39
3. Early vs. late drop

• Early drop gt 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

40
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

41
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

42
4. Drop position (contd.)

• Random
• hardest
• if no aggregation, hurts hogs most
• unlikely to make it to real routers