Network Routing: Link Metrics and NonTraditional Routing

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Network Routing Link Metrics and
Non-Traditional Routing

• Y. Richard Yang
• 2/26/2009

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Admin.

• Homework 3
• Project proposal
• March 6 by email to yry_at_cs.yale.edu and
  richard.alimi_at_yale.edu

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Recap Routing Protocols

• Proactive protocols
• distance vector
• e.g., DSDV
• link state
• link reversal
• e.g., partial link reversal, TORA
• Reactive (on-demand) protocols
• DSR
• AODV

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Recap ETX

• ETX The predicted number of data transmissions
  required to successfully transmit a packet over a
  link
• Link loss rate p
• Expected number of transmissions

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ETX Performance
DSDV
DSR
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Problems of ETX

• ETX does not handle multirate 802.11 networks
• ETX does not work out well when nodes have
  multiple radios that can operate at different
  channels

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Extending ETX Multirate

• In a multirate environment, need to consider link
  bandwidth
• packet size S, Link bandwidth B
• each transmission lasts for S/B

Routing in Multi-radio, Multi-hop Wireless Mesh
Network, Richard Draves, Jitendra Padhye, and
Brian Zill. Mobicom 2004.
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Extending ETX Multirate

• Add ETTs of all links on the path
• Use the sum as path metric
• Interpretation pick a path with the lowest total
  network occupation time
• Q under what condition is SETT the network
  occupation time?

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Problem of SETT

• In networks with multiple channels/radios, SETT
  does not consider channel reuse

2.66 ms
3 Mbps
2.66 ms
6 Mbps
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Observation

• Interference reduces throughput
• throughput of a path is lower if many links are
  on the same channel
• path metric should be worse for non-diverse paths

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Extending SETT for Multiple Channels

• Group links on a path according to channel
• assumes links on the same channel interfere with
  one another
• pessimistic for long paths
• Add ETTs of links in each group
• Find the group with largest sum (BG-ETT)
• this is the bottleneck group
• too many links, or links with high ETT (poor
  quality links)
• Use this largest sum as the path metric
• Lower value implies better path

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BG-ETT Example
0
5.33 ms
5.33 ms
1.5 Mbps
1.33 ms
4 ms
4 ms
2 Mbps
2.66 ms
2.66 ms
2.66 ms
3 Mbps
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BG-ETT May Select Long Paths
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Path Metric Putting it all together

• SETT favors short paths
• BG-ETT favors channel diverse paths
• ß is a tunable parameter
• Higher value more preference to channel
  diversity
• Lower value more preference to shorter paths

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Implementation and such

• Implemented in a source-routed, link-state
  protocol, Multi-Radio Link Quality Source Routing
  (MR-LQSR)
• Nodes discover links to its neighbors, measure
  quality of those links
• link information floods through the network
• each node has full knowledge of the topology
• sender selects best path
• packets are source routed using this path
• Measure loss rate and bandwidth
• loss rate measured using broadcast probes similar
  to ETX
• updated every second
• bandwidth estimated using periodic packet-pairs
• updated every 5 minutes

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Evaluations
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Median Throughput (Baseline, single radio)
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Median Throughput (Baseline, two radios)
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Impact of ß value
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Summary

• Link metrics are still an active research area,
  in particular, due to interactions with (channel,
  spatial) diversity

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Summary Traditional Routing

• So far, all routing protocols in the framework of
  traditional wireline routing
• a graph representation of underlying network
• point-to-point graph, edges with costs
• select a lowest-cost route for a src-dest pair
• commit to a specific route before forwarding
• each node forwards a received packet as it is to
  next hop
• Problems dont fully exploit path (spatial)
  diversity and wireless broadcast opportunities

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Motivating Scenario I
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Motivating Scenario II
Motivating question can we take advantage of
transmissions that reach unexpectedly far or
unexpectedly short?

• Traditional routing picks a single route, e.g.,
  src -gt B -gt D -gt dst
• Packets received off path are useless

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Motivating Scenario III

• Src A sends 1 packet to dst B src B sends packet
  3 to dst A
• The network needs to transmit 4 packets
• Motivating question can we do better?

A
B
R
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Motivating Scenario III

• If R has both packets 1 and 3, it can combine
  them and explore coding and broadcast nature of
  wireless

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Outline

• Admin.
• Link metrics
• Non-traditional routing
• motivation
• network coding exploiting network broadcast

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Network Coding

• We have covered source coding (FEC, compression)
• The new approach uses opportunistic network
  coding
• goal increase the amount of information that is
  transported

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Opportunistic Coding Basic Idea

• Each node looks at the packets available in its
  buffer, and those its neighbors buffers
• It selects a set of packets, computes the XOR of
  the selected packets, and broadcasts the XOR

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Opportunistic Coding
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Outline

• Admin.
• Link metrics
• Non-traditional routing
• motivation
• network coding exploiting network broadcast
• opportunistic routing

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Key Issue in Opportunistic Routing
Key Issue opportunistic forwarding may lead to
duplicates.
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Extreme Opportunistic Routing (ExOR)

• Basic idea avoid duplicates by scheduling
• Instead of choosing a fix sequential path (e.g.,
  src-gtB-gtD-gtdst), the source chooses a list of
  forwarders (a forwarder list in the packets)
  using ETX-like metric
• a background process collects ETX information via
  periodic link-state flooding
• Forwarders are prioritized by ETX-like metric to
  the destination

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ExOR Forwarding

• Group packets into batches
• The highest priority forwarder transmits when the
  batch ends
• The remaining forwarders transmit in prioritized
  order
• each forwarder forwards packets it receives yet
  not received by higher priority forwarders
• status collected by batch map

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Batch Map

• Batch map indicates, for each packet in a batch,
  the highest-priority node known to have received
  a copy of that packet

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ExOR Example
N2
N0
N3
N1
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ExOR Stopping Rule

• A nodes stops sending the remaining packets in
  the batch if its batch map indicates over 90 of
  this batch has been received by higher priority
  nodes
• the remaining packets transferred with
  traditional routing

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Evaluations

• 65 Node pairs
• 1.0MByte file transfer
• 1 Mbit/s 802.11 bit rate
• 1 KByte packets
• EXOR bacth size 100

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Evaluation 2x Overall Improvement
1.0
0.8
0.6
Cumulative Fraction of Node Pairs
0.4
0.2
ExOR
Traditional
0
0
200
400
600
800
Throughput (Kbits/sec)

• Median throughputs 240 Kbits/sec for ExOR,
• 121 Kbits/sec for Traditional

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OR uses links in parallel

• ExOR
• 7 forwarders
• 18 links

• Traditional Routing
• 3 forwarders
• 4 links

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OR moves packets farther
0.6
ExOR
Traditional Routing
Fraction of Transmissions
0.2
0.1
0
0
100
200
300
400
500
600
700
800
900
1000
Distance (meters)

• ExOR average 422 meters/transmission
• Traditional Routing average 205 meters/tx

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Comments ExOR

• Pros
• takes advantage of link diversity (the
  probabilistic reception) to increase the
  throughput
• does not require changes in the MAC layer
• can cope well with unreliable wireless medium
• Cons
• scheduling is hard to scale in large networks
• overhead in packet header (batch info)
• batches increase delay

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Outline

• Admin.
• Link metrics
• Non-traditional routing
• motivation
• network coding exploiting network broadcast
• opportunistic routing
• ExOR
• MORE

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MORE MAC-independentOpportunistic Routing
Encoding

• Basic idea
• Replace node coordination with network coding
• Trading structured scheduler for random packets
  combination
• Previous network coding technique is for
  inter-flow
• MORE is for intra-flow network coding

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Basic Idea Source

• Chooses a list of forwarders (e.g., using ETX)
• Breaks up file into K packets (p1, p2, , pK)
• Generate random packets
• MORE header includes the code vector cj1, cj2,
  cjK for coded packet pj

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Basic Idea Source
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Basic Idea Forwarder

• Check if in the list of forwarders
• Check if linearly independent of new packet with
  existing packet
• Re-coding and forward

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Basic Idea Destination

• Decode
• Send ACK back to src if success

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Key Practical Question How many packets does a
forwarder send?

• Compute zi the expected number of times that
  forwarder i should forward each packet

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Computes zs
?ij loss probability of the link between i and j
Compute zs so that at least one forwarder that is
closer to destination is expected to have
received the packet
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Compute zj for forwarder j

• Only need to forward packets that are
• received by j
• sent by forwarders who are further from
  destination
• not received by any forwarder who is closer to
  destination

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Compute zj for forwarder j

• To guarantee at least one forwarder closer to d
  receives the packet

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Evaluations

• 20 nodes distributed in a indoor building
• Path between nodes are 1 5 hops in length
• Loss rate is 0 60 average 27

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Throughput
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Problem of MORE?
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Mesh Networks API So Far
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Motivation
10-3 BER
0
D
S
0
10-3 BER
? Packet loss of 99
570 bytes 1 bit in 1000 incorrect
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Implication
99 (10-3 BER)
Loss
0
D
S
Loss
0
99 (10-3 BER)
Opportunistic Routing ? 50 transmissions
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Outline

• Admin.
• Link metrics
• Non-traditional routing
• motivation
• network coding exploiting network broadcast
• opportunistic routing
• ExOR
• MORE
• MIXIT

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New API
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What Should Each Router Forward?
D
S
P1
P2
P1
P2
P1
P2
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What Should Each Router Forward?
D
S
P1
P2
P1
P2
P1
P2

• Forward everything ? Inefficient
• Coordinate ? Unscalable

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Symbol Level Network Coding
D
S
P1
P2
P1
P2
P1
P2
Forward random combinations of correct symbols
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Symbol Level Network Coding
Routers create random combinations of correct
symbols
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Symbol Level Network Coding
Solve 2 equations
Destination decodes by solving linear equations
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Symbol Level Network Coding
Routers create random combinations of correct
symbols
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Symbol Level Network Coding
Solve 2 equations
Destination decodes by solving linear equations
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Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
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Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
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Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
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Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
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Destination needs to know which combinations it
received
Use run length encoding
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Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
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Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
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Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
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Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
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Evaluation

• Implementation on GNURadio SDR and USRP
• Zigbee (IEEE 802.15.4) link layer
• 25 node indoor testbed, random flows
• Compared to
• Shortest path routing based on ETX
• MORE Packet-level opportunistic routing

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Throughput Comparison
CDF
2.1x
MIXIT
3x
MORE
Shortest Path
Throughput (Kbps)
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Backup Slides
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Motivation for a Better Metric
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Implementation and such

• Modify DSDV or DSR
• Example evaluation
• in DSDV w/ ETX, route table is a snapshot taken
  at end of 90 second warm-up period
• in DSR w/ ETX, source waits additional 15 sec
  before initiating the route request

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Where do the gains come from?
CDF
MIXIT without concurrency
1.5x
MORE
Shortest Path
Throughput (Kbps)
Take concurrency away from MIXIT
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Where do the gains come from?
CDF
MIXIT
MORE
Shortest Path
Throughput (Kbps)
Take concurrency away from MIXIT