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 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 Switch Lookup Address Update Header Forwarding Table Forwarding Table Lookup Address Update Header Forwarding Table Forwarding Table Lookup Address Update Header Forwarding Table Forwarding Table Queue Packet Buffer Memory Buffer Memory Queue Packet Buffer Memory Buffer Memory Queue Packet Buffer Memory Buffer Memory DataH H H

Lookup Address Update Header Forwarding Table Forwarding Table Queue Packet Buffer Memory Buffer Memory Lookup Address Update Header Forwarding Table Forwarding Table Queue Packet Buffer Memory Buffer Memory Lookup Address Update Header Forwarding Table Forwarding Table Queue Packet Buffer Memory Buffer Memory Input Queued Packet Switch DataH H H

Head of Line Blocking

Virtual Output Queues

7 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 Input Queued Packet Switch Head of Line Blocking OQ Switch

Input Queued Packet Switch With 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 Request Grant Accept uar selection #1 #2 Iteration :

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 Simulation 16-port switch Uniform iid traffic FIFO Maximum Size Output Queued

Parallel Iterative Matching PIM with one iteration Simulation 16-port switch Uniform iid traffic FIFO Maximum Size Output Queued

Parallel Iterative Matching PIM with one iteration Simulation 16-port switch Uniform iid traffic PIM with four iterations

Parallel Iterative Matching Number of iterations Consider the n requests to output j Requesting inputs receiving no other grants Requesting inputs receiving other grants k n-k j

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/ N 1 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