Nick McKeown

CS244 Lecture 6

Packet Switches

What you saidThe very premise of the paper was a bit of an eye-opener for me, for previously I had never even considered the role of switching technology in overall network throughput. I had assumed that link throughput was the key determinant, so reading this paper made me realize how high-level improvements in network performance can be contingent upon advancements in several different areas of the networking stack.

2

Output Queued Packet SwitchLookupAddress

UpdateHeader

ForwardingTable

ForwardingTable

LookupAddress

UpdateHeader

ForwardingTable

ForwardingTable

LookupAddress

UpdateHeader

ForwardingTable

ForwardingTable

QueuePacket

BufferMemory

BufferMemory

QueuePacket

BufferMemory

BufferMemory

QueuePacket

BufferMemory

BufferMemory

Data H

Data H

Data H

LookupAddress

UpdateHeader

ForwardingTable

ForwardingTable

QueuePacket

BufferMemory

BufferMemory

LookupAddress

UpdateHeader

ForwardingTable

ForwardingTable

QueuePacket

BufferMemory

BufferMemory

LookupAddress

UpdateHeader

ForwardingTable

ForwardingTable

QueuePacket

BufferMemory

BufferMemory

Input Queued Packet SwitchData H

Data H

Data H

Head of Line Blocking

Virtual Output Queues

7

0% 20% 40% 60% 80% 100%Load

Delay

Output Queued Packet Switch

The best that any queueing system

can achieve.

Properties of OQ switches

1. They are “work conserving”.

2. Throughput is maximized.

3. Expected delay is minimized.

4. We can control packet delay.

Broadly speaking: When possible, use an OQ design.

8

9

0% 20% 40% 60% 80% 100%Load

Delay

Input Queued Packet SwitchHead of Line Blocking

OQ Switch

2 2 58%

Input Queued Packet SwitchWith Virtual Output Queues

OQ Switch

VOQs

What you said

"... It seems like the paper assumes that, outside of overflowing buffers, no packets will ever be lost. I'd like to know where this assumption comes from. I feel like there are always random drops or packet corruption, so I have a hard time believing that these delay guarantees are 100% valid.”

- Anonymous

11

Properties of OQ switches

1. They are “work conserving”.

2. Throughput is maximized.

3. Expected delay is minimized.

4. We can control packet delay.

Broadly speaking: When possible, use an OQ design.

12

Practical Goal

Problem: Memory bandwidth

Therefore: Try to approximate OQ.

In this paper, we are just looking at those switches that attempt to match “Property 2: Maximize throughput”

13

Questions

1. What is a virtual output queue (VOQ)?

2. How does a VOQ help?

3. What does the scheduler/arbiter do?

14

Parallel Iterative Matching

1

2

3

4

1

2

3

4

Request

1

2

3

4

1

2

3

4Grant

1

2

3

4

1

2

3

4Accept

uar selection

1

2

3

4

1

2

3

4

uar selection

1

2

3

4

1

2

3

4

#1

#2

Itera

tion:

1

2

3

4

1

2

3

4

PIM Properties

1. Guaranteed to find a maximal match in at most N iterations.

2. Inputs and outputs make decisions independently and in parallel.

3. In general, will converge to a maximal match in < N iterations.

4. How many iterations should we run?

Parallel Iterative Matching

Simulation16-port switch

Uniform iid traffic

FIFO

MaximumSize

OutputQueued

Parallel Iterative MatchingPIM with

one iteration

Simulation16-port switch

Uniform iid traffic

FIFO

MaximumSize

OutputQueued

Parallel Iterative MatchingPIM with

one iteration

Simulation16-port switch

Uniform iid traffic

PIM with four iterations

Parallel Iterative MatchingNumber of iterations

Consider the n requests to output j

Requesting inputs

receiving no other grants

Requesting inputs

receiving other grants

k

n-kj

Virtual Output Queues

Throughput

“Maximize throughput” is equivalent to “queues don’t grow without bound for all non-oversubscribing traffic matrices”i.e. ≤for every queue in the system.

Observations: 1. Burstiness of arrivals does not affect

throughput

2. When traffic is uniform, solution is trivial

22

Uniform traffic

23

= 1/N1 1 … 11 1 … 1… … …1 1 … 1

Throughput for uniform traffic

100% throughput is easy for uniform traffic:1.Serve every queue at rate 1/N in fixed round-robin schedule

2.Pick a permutation each time uniformly and at random from all possible N! permutations

3.Or, from among N round-robin permutations

4.Wait until all VOQs are non-empty, then pick any algorithm above.

24

With non-uniform traffic

100% throughput is now known to be theoretically possible with:- Input queued switch, with VOQs, and- An arbiter to pick a permutation to maximize

the total matching weight (e.g. weight is VOQ occupancy or packet waiting time)

It is practically possible with:- IQ switch, VOQs, all running twice as fast- An arbiter running a maximal match (e.g. PIM)

25

Questions

1. Why does the PIM paper talk about TDM scheduled traffic?

2. What about multicast?

3. Multiple priorities?

26

Question

What else does a router need to do apart from switching packets?

27