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Network Routing: Link Metrics and NonTraditional Routing

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ETX does not work out well when nodes have multiple radios that ... hop Wireless Mesh Network,' Richard Draves, Jitendra Padhye, and Brian Zill. Mobicom 2004. ... – PowerPoint PPT presentation

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Title: Network Routing: Link Metrics and NonTraditional Routing


1
Network Routing Link Metrics and
Non-Traditional Routing
  • Y. Richard Yang
  • 2/26/2009

2
Admin.
  • Homework 3
  • Project proposal
  • March 6 by email to yry_at_cs.yale.edu and
    richard.alimi_at_yale.edu

3
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

4
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

5
ETX Performance
DSDV
DSR
6
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

7
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.
8
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?

9
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
10
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

11
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

12
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
13
BG-ETT May Select Long Paths
14
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

15
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

16
Evaluations
17
Median Throughput (Baseline, single radio)
18
Median Throughput (Baseline, two radios)
19
Impact of ß value
20
Summary
  • Link metrics are still an active research area,
    in particular, due to interactions with (channel,
    spatial) diversity

21
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

22
Motivating Scenario I
23
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

24
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
25
Motivating Scenario III
  • If R has both packets 1 and 3, it can combine
    them and explore coding and broadcast nature of
    wireless

26
Outline
  • Admin.
  • Link metrics
  • Non-traditional routing
  • motivation
  • network coding exploiting network broadcast

27
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

28
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

29
Opportunistic Coding
30
Outline
  • Admin.
  • Link metrics
  • Non-traditional routing
  • motivation
  • network coding exploiting network broadcast
  • opportunistic routing

31
Key Issue in Opportunistic Routing
Key Issue opportunistic forwarding may lead to
duplicates.
32
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

33
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

34
Batch Map
  • Batch map indicates, for each packet in a batch,
    the highest-priority node known to have received
    a copy of that packet

35
ExOR Example
N2
N0
N3
N1
36
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

37
Evaluations
  • 65 Node pairs
  • 1.0MByte file transfer
  • 1 Mbit/s 802.11 bit rate
  • 1 KByte packets
  • EXOR bacth size 100

38
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

39
OR uses links in parallel
  • ExOR
  • 7 forwarders
  • 18 links
  • Traditional Routing
  • 3 forwarders
  • 4 links

40
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

41
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

42
Outline
  • Admin.
  • Link metrics
  • Non-traditional routing
  • motivation
  • network coding exploiting network broadcast
  • opportunistic routing
  • ExOR
  • MORE

43
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

44
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

45
Basic Idea Source
46
Basic Idea Forwarder
  • Check if in the list of forwarders
  • Check if linearly independent of new packet with
    existing packet
  • Re-coding and forward

47
Basic Idea Destination
  • Decode
  • Send ACK back to src if success

48
Key Practical Question How many packets does a
forwarder send?
  • Compute zi the expected number of times that
    forwarder i should forward each packet

49
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
50
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

51
Compute zj for forwarder j
  • To guarantee at least one forwarder closer to d
    receives the packet

52
Evaluations
  • 20 nodes distributed in a indoor building
  • Path between nodes are 1 5 hops in length
  • Loss rate is 0 60 average 27

53
Throughput
54
Problem of MORE?
55
Mesh Networks API So Far
56
Motivation
10-3 BER
0
D
S
0
10-3 BER
? Packet loss of 99
570 bytes 1 bit in 1000 incorrect
57
Implication
99 (10-3 BER)
Loss
0
D
S
Loss
0
99 (10-3 BER)
Opportunistic Routing ? 50 transmissions
58
Outline
  • Admin.
  • Link metrics
  • Non-traditional routing
  • motivation
  • network coding exploiting network broadcast
  • opportunistic routing
  • ExOR
  • MORE
  • MIXIT

59
New API
60
What Should Each Router Forward?
D
S
P1
P2
P1
P2
P1
P2
61
What Should Each Router Forward?
D
S
P1
P2
P1
P2
P1
P2
  • Forward everything ? Inefficient
  • Coordinate ? Unscalable

62
Symbol Level Network Coding
D
S
P1
P2
P1
P2
P1
P2
Forward random combinations of correct symbols
63
Symbol Level Network Coding
Routers create random combinations of correct
symbols
64
Symbol Level Network Coding
Solve 2 equations
Destination decodes by solving linear equations
65
Symbol Level Network Coding
Routers create random combinations of correct
symbols
66
Symbol Level Network Coding
Solve 2 equations
Destination decodes by solving linear equations
67
Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
68
Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
69
Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
70
Destination needs to know which combinations it
received
Use run length encoding
Coded Packet
Original Packets
71
Destination needs to know which combinations it
received
Use run length encoding
72
Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
73
Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
74
Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
75
Symbol-level Network Coding
Forward random combinations of correct symbols
Coded Packet
Original Packets
76
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

77
Throughput Comparison
CDF
2.1x
MIXIT
3x
MORE
Shortest Path
Throughput (Kbps)
78
Backup Slides
79
Motivation for a Better Metric
80
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

81
Where do the gains come from?
CDF
MIXIT without concurrency
1.5x
MORE
Shortest Path
Throughput (Kbps)
Take concurrency away from MIXIT
82
Where do the gains come from?
CDF
MIXIT
MORE
Shortest Path
Throughput (Kbps)
Take concurrency away from MIXIT
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