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Advanced Networking
Wickus Nienaber
Daniel Beech
Today’s Topics
• Routing Algorithms (Dan)
– Up/Down routing (Wickus)
– L-turn routing (Wickus)
– Descending layers (Dan)
– Path Selection (Dan)
• Autonet (both)
Routing Algorithms
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Select a “path” between two machines
Minimal Cost (least amount of hops)
Balance network traffic
Avoid Congestion
Routing in High Performance
Networks
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Usually local area networks
High bandwidth, low latency
Wormhole or Virtual Cut Routing is used
Deadlock free
Used in high performance computing or SANs
Strict/High network requirements
Internet or Ethernet routing fails
Regular vs. Irregular
Topologies
• Regular
– Network has graph structure (ring, meshes,
hypercube, etc)
– Easier to optimize
• Irregular
– Abnormal shape
– Discovering topology is a challenge
Up/Down Routing
• Prevents Deadlock
• Assigns a direction to links
• Use of spanning tree
Up/Down Routing (cont)
• Non-minimal paths
• Leads to congestion
L-turn Routing
• Attempts to solve unbalanced traffic issue
• Adaptive Routing
• Used in Irregular networks
Building a L-R tree
• Build a BFS
spanning Tree
– Like up/down
• Assign width to
every node
– Increasing number is
order of visit (width)
– Width is horizontal
distance from root
Building L-R (cont)
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Assign horizontal directions to
channels
– Left Direction: the channel with
the largest width
– Right Direction to the rest
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Assign vertical directions to
channels
– Up down direction (vertical
direction). Done based on
distance from root.
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Channels that are not in the
spanning tree are added to the
L-R tree (dotted lines)
L-R routing
• Channel that faces left is called LEFT
• Channel that faces right is called RIGHT
• Don’t use the LEFT channel after using RIGHT channel
– Deadlock free
– Guarantees any path between any pair of nodes
No Root Traversal
• Node 3 – 8:
– L-R (3  7 6  8)
4 hops
– Up/Down (3  1  0 2 
6  8)
6 hops
• Node 7 – 2:
– Up/Down (7  6  2)
3 hops
– L-R (7  1  3  0  2) or
(7  3  1 4  2)
4 hops
L-turn routing
• LU/LD/RU/RD
– Left up/down is channels
facing up/down in L-R tree
– Right up/down is channels
facing up/down in L-R tree
– Same depth channels: the
right hand side node is
assumed closer to the root.
Restrictions in L-turn routing
• No left-up channel after using the
channel except left up channel.
• Cyclic dependencies not including any
left-up channels, does not allow the turn
from left-down channel to the right
direction in the cyclic channel
Cyclic Dependence detection
• Algorithm:
– Nodes with:
• a. Two or more right-up
channels
• b. One or more right up
channels and one or more
right down channel exists
– Search these nodes for
cycles and mark the
channels prohibited
– Algorithm runs in 0(n2)
Analysis
• Deadlock free
• Guarantees any path between any pair on
nodes
Descending Layer Routing
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For use in SANs
Implements deadlock free routing
Reduces non-minimal paths
Reduces traffic congestion
Descending Layer Routing
(cont)
• Divide target
network in layers
• Impose deadlock
avoidance
conditions
• Select deterministic
paths
Descending Layer Routing (cont)
• Deadlock Avoidance Methods
– (UD)* Up down scheme (down to up not allowed)
– (UD-DU)* if sub network is even down to up not
allowed, if odd up to down not allowed
– UD-(DU)* In network 0 down to up not allowed on
all other networks up to down not allowed
Path Selection Algorithm
• Pick the paths between nodes
• Choose a path among set of valid paths
• Goal is to select the optimal path
• Random Selection
• Smaller port-UID selection (low port first)
• Sancho’s traffic balancing algorithm
Path Selection (cont)
• Selection of deterministic path
• Algorithms use Virtual Channels
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High virtual-channel first
High physical-channel first
Low virtual-channel first
Low physical-channel first
AutoNet
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100 Mbits LAN
Packet Switched Network
Cut-through forwarding
30 switches , 100 hosts
Low latency, 2 microseconds per switch
Autonet (cont)
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Point to point links
Distributed Spanning Tree
UP/Down routing
Flow control via “start/stop” commands
“Idhy” (I don’t hear you) and “Panic”