Nick McKeown CS244 Lecture 6 Packet Switches. What you said The very premise of the paper was a bit...
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Transcript of Nick McKeown CS244 Lecture 6 Packet Switches. What you said The very premise of the paper was a bit...
![Page 1: Nick McKeown CS244 Lecture 6 Packet Switches. What you said The very premise of the paper was a bit of an eye- opener for me, for previously I had never.](https://reader030.fdocuments.in/reader030/viewer/2022032800/56649d355503460f94a0cdf8/html5/thumbnails/1.jpg)
Nick McKeown
CS244 Lecture 6
Packet Switches
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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.
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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
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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
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Head of Line Blocking
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Virtual Output Queues
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0% 20% 40% 60% 80% 100%Load
Delay
Output Queued Packet Switch
The best that any queueing system
can achieve.
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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.
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0% 20% 40% 60% 80% 100%Load
Delay
Input Queued Packet SwitchHead of Line Blocking
OQ Switch
2 2 58%
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Input Queued Packet SwitchWith Virtual Output Queues
OQ Switch
VOQs
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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
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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.
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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”
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Questions
1. What is a virtual output queue (VOQ)?
2. How does a VOQ help?
3. What does the scheduler/arbiter do?
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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
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3
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1
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uar selection
1
2
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4
1
2
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#1
#2
Itera
tion:
1
2
3
4
1
2
3
4
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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?
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Parallel Iterative Matching
Simulation16-port switch
Uniform iid traffic
FIFO
MaximumSize
OutputQueued
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Parallel Iterative MatchingPIM with
one iteration
Simulation16-port switch
Uniform iid traffic
FIFO
MaximumSize
OutputQueued
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Parallel Iterative MatchingPIM with
one iteration
Simulation16-port switch
Uniform iid traffic
PIM with four iterations
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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
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Virtual Output Queues
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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
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Uniform traffic
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= 1/N1 1 … 11 1 … 1… … …1 1 … 1
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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.
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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)
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Questions
1. Why does the PIM paper talk about TDM scheduled traffic?
2. What about multicast?
3. Multiple priorities?
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Question
What else does a router need to do apart from switching packets?
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