MPLOT: A Transport Protocol Exploiting Multipath Diversity using Erasure Codes

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MPLOT A Transport Protocol Exploiting
Multi-path Diversity using Erasure Codes

• Vicky Sharma,Vijay Subramanian
• Shiv Kalyanaraman,, K. K. Ramakrishnan,
• Koushik Kar
• (Rensselaer Polytechnic Institute), (ATT),
• (IBM India Research Laboratory, India)

This work is supported by AFOSR ESC Hanscom and
MIT Lincoln Laboratory.
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Multiple Path - Advantages
Low capacity, High delay and loss
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Wireless Networks and Multiple Paths

• Wireless channels suffer from-
• High Packet Loss-rates
• As high as 50
• Variable Bandwidth
• Depending on link strength and environment
• High delay Jitter

Conventional Transport Protocols cannot work
Efficiently in such cases
Can use multiple paths to suppress the volatile
behavior of wireless networks
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MPLOT Features
MPLOT has several features aggregate bandwidths
from Multiple paths and exploit diversity
Forward Error Correction (FEC) coding to counter
high loss rates
Map pkts?paths intelligently based upon Rank(pi,
RTTi, wi)
Socket Buffer
Congestion control (per path)
Reliability _at_ aggregate, across paths (FEC block
weighted sum of windows, PFEC based upon
weighted average loss rate)
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MPLOT FEC coding(1)

• MPLOT transmits data as encoded blocks
• An encoded block of B packets is composed of F
  data packets and K FEC packets
• The K packets are called Proactive FEC (PFEC)
  packets
• Any more FEC packets sent are called Reactive
  FEC (RFEC) packets
• K/B depends on aggregate loss-rate(µagg) and
  standard-deviation(sagg). MPLOT uses
• K (µagg sagg)B

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MPLOT FEC coding(2)

• Choice of Block size, PFEC and RFEC packets
  affects the performance of MPLOT in different
  ways
• Select block size and the number of PFEC, RFEC
  packets to set a trade-off between goodput and
  latency

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MPLOT Reliability Congestion Control

• Reliable delivery is ensured across paths
• Acknowledgement is sent back across all paths
  reduces effective RTT of longer paths
• The acknowledgement is structured like SACK
• Congestion control is independent for each path
• Same as TCP
• Ensures that MPLOT shares bandwidth equally on
  each path with other users (MPLOT or conventional
  TCP)
• Explicit Congestion Notification (ECN) is used to
  respond to congestion

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MPLOT Packet Mapping(1)

• MPLOTs packet mapping policy has 2 important
  features-
• Adaptive Mapping A packet is not mapped to a
  path until and unless a path can transmit a
  packet and is more likely to deliver the packet
  when it would be required
• Makes MPLOT adaptive to network conditions
• Out of order transmission
• Recover earlier blocks quickly

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MPLOT Packet Mapping(2)
High RANK
Path rank
Low RANK
Blocks
Earlier (unrecovered) blocks, RFEC mapped to
good paths. Later block packets (goodput
enhancing) mapped to longer, lossy paths Reduce
delay variance improve timeliness/usefulness at
receiver recover early blocks quickly.
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MPLOT Packet Mapping(3)

• Each path has a different RTT, window w and loss
  rate µ
• Each path is assigned a rank R by MPLOT
• Rank R w(1 - µ)RTTmax/RTT
• Rank is high for a path with large window, low
  loss rate and low RTT
• A path is classified as GOOD if rank of the path
  is greater than median rank
• Else it classified as BAD
• A GOOD path can transmit a packet faster than BAD
  path with a higher chance of successful delivery
  to destination

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Bandwidth Aggregation and more
Each path has 50 average loss rate and a
bandwidth of 10Mb/s
Diversity Gain
Simple Bandwidth Aggregation
MPLOT gains more goodput with paths than is
possible with simple bandwidth aggregation
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MPLOT Diversity Gain(1)

• Diversity is due to multiple paths, it can be
  classified as-
• Loss Diversity
• Losses on different paths are independent or only
  partially correlated
• Delay Diversity
• Different paths have different delays. Can use
  the shortest reverse path for feedback

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Loss Diversity(1) Gain from Multiple Paths
Aggregate error with 1 path, Avg. loss rate 20
Aggregate error with 5 paths, avg. loss rate 20
Loss-Rate()
Loss-Rate()
time (sec)
time (sec)
Single paths exhibit a bursty error process
Multiple paths experience a smoother error rate
at aggregate level
A smoother aggregate error rate allows sender to
provision RFEC better as well as reduce RFEC
traffic
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Loss Diversity(2) Goodput Gains
Each path has an average 50 loss-rate and the
total aggregate bandwidth is kept constant at
10Mb/s
Goodput increases as paths increase
For the same bottleneck bandwidth, the goodput
increases with paths when loss-rates are
independent
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Loss Diversity(2) Goodput Gains
Each path exhibits loss-rates of 50 with small
probability of outage/breakdown
MPLOT
First come first serve mapping
If path rank is not used, goodput reduces with
increase in paths
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Delay Diversity
Delay diversity gain
Average goodput per-path increases as number of
paths increases. The shortest reverse path
effectively reduces the RTT experienced by any
path
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MPLOT vs PTCP
Goodput when each path has loss-rate of 30
Goodput when each path has loss-rate of 10
MPLOT
PTCP
PTCP
MPLOT
Goodput (Mb/s)
Goodput (Mb/s)
Number of Paths
Number of Paths
Total bottleneck bandwidth is kept constant a
10Mb/s in all cases
PTCP is not able to utilize the diversity in
paths as effectively As MPLOT as loss-rate
increases
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Summary

• MPLOT achieves significant goodput gains by-
• Transferring data intelligently over multiple
  paths
• Using FEC coding to counter high packet-loss
  rates
• trading off Goodput and Latency
• MPLOT uses diversity across paths to-
• Reduce variance in aggregate loss-rates
• Reduce effective RTT of long paths
• Diversity gain is reflected as increase in
  goodput with number of paths while the aggregate
  bandwidth remains constant

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Thank You!
Questions?
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MPLOT Flow Control(2) Fairness
MPLOT is proportionally fair with TCP-SACK on a
per-path basis, even When operating over multiple
paths
The path windows of TCP and MPLOT show the same
behavior in a loss-less case.
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MPLOT Packet Mapping(4)
The mapping policy is near-optimal across a wide
range of packet loss-rates.
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Wireless Networks The Future
Investment in wireless multi-hop networks

• Multi-hop wireless networks are being
  increasingly deployed in domains traditionally
  dominated by wired networks
• Tremendous growth in such networks in the last 5
  years
• Serious performance issues remain

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Wireless Networks Possible Solution

• Wireless networks do have one significant
  advantage-
• Multiple Paths
• Can establish new links and paths without
  additional investment in infrastructure (dont
  have to lay more wires)

Use this inherent path diversity to counter the
adverse effects of a single unpredictable path by
transmitting data over multiple paths
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Using Multiple Paths Prior Work

• Several transport protocols have been proposed to
  operate over multiple paths-
• mTCP
• Objective is to avoid route failure in the
  forward direction.
• Maintains only a single reverse path
• pTCP
• Uses multiple reverse paths, but ack for a packet
  is sent on the same path from which it was
  received
• Limits on packet redundancy limits its
  performance on lossy networks

We present MPLOT (Multi Path Loss Tolerant)
transport protocol for the purpose
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MPLOT Packet Mapping(4)
MPLOT achieves near optimal goodput values across
a wide range of packet loss rates
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Bandwidth Aggregation and more
Each path has 50 average loss rate
MPLOT gains more goodput with paths than is
possible with simple bandwidth aggregation
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Loss Diversity(2) Goodput Gains
Goodput (Mb/s)
If path rank is not used, goodput reduces with
increase in pathss