Leveraging Multiple Network Interfaces for Improved TCP Throughput

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Leveraging Multiple Network Interfaces for
Improved TCP Throughput

• Sridhar Machiraju
• SAHARA Retreat, June 10-12, 2002

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Motivation

• Proliferation of Wireless LANs, rollout of 3G and
  availability of devices with multiple network
  interfaces (MNIs)
• Can endhosts leverage multiple network interfaces
  for
• improved throughput (current focus)
• masking packet losses
• improving connectivity

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Contents

• What should u expect from this presentation?
• why TCP cannot take advantage of MNIs easily
• two simple routing policies (at end-hosts) that
  improve TCP throughput
• Work started recently feedback appreciated
  greatly

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Problem Statement
N3
2 Mbps, 50ms
2 Mbps, 50ms
(Sender) N1
N2 (Receiver)
5 Mbps, 50ms
5 Mbps, 100ms
N4

• FTP Source N1 has 2 NIs
• The 2 paths to N2 are using N3 (2Mbps, 100ms) and
  N4 (5Mbps, 150ms)
• Can N1 obtain a throughput of 7Mbps?

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A Simple RR Scheme

• Assume that the ratio of bottlenecks on both
  paths is known (25 in the example)
• Route 2 of every 7 packets through N3 and the
  rest through N4
• Future Work Deduce the above ratio dynamically

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Why it fails
Different RTTs on paths Packet reordering Duplicat
e ACKs False congestion signals
Throughput
Time
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Increase DupACK threshold

• Prevents TCP from reacting to duplicate ACKs due
  to reordering
• But, dupACKs due to losses are also neglected

Throughput
Time
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Discussion

• Packet reordering is inevitable
• A solution buffer packets at receiver IP layer
  Chebrolu et al., UCSD
• Another possibility buffer ACKs and eliminate
  unnecessary duplicates
• Our solution use multiple routes to the source
  to send ACKs

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DupACK routing policy
RP1
FP1
(Sender) N1
N2 (Receiver)
If DupACK, route through RP1 else use RP2
FP2
RP2

• Assume 2 paths are (delayFP1 lt delayFP2) such
  that (delayFP2delayRP1) lt (delayFP1delayRP2)
  (delay property)
• DupACKs received only after subsequent ACKs and
  are discarded

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Example
(Sender) N1
N2(Receiver)
p2
p1

• 2 Packets p1 and p2 sent one after another on
  different paths

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Example
(Sender) N1
N2(Receiver)
p2
p1

• p2 reaches before p1 since its forward path is
  shorter

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Example
(Sender) N1
N2(Receiver)
p1
ACK1
DupACK

• p2 generates a DupACK even though p1 is not
  dropped

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Example
ACK3
(Sender) N1
N2(Receiver)
ACK1
DupACK

• p1 would generate an ACK indicating receipt of p1
  and p2

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Example
ACK3
(Sender) N1
N2(Receiver)
ACK1
DupACK is discarded

• (delayFP2delayRP1) lt (delayFP1delayRP2) implies
  that the duplicate ACK is received later and
  discarded

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Discussion

• Single TCP flow is able to obtain better
  throughput 7 Mbps

Throughput
Time

• Cons
• 2 paths need to obey the delay property
• slower increase of congestion window

• Pros
• better throughput
• simple policy
• packets not buffered or dropped

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ACK Routing Policy
P1
(Sender) N1
N2 (Receiver)
Route ACKs through P1
P2

• What if DupACK policy cannot be used?
• Throughput can still be improved if
• delay(RP1) lt delay(RP2)
• capacity (RP1) gt ACK traffic
• Use high capacity path (P2) for data low delay
  path (P1) for ACKs

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Discussion

• Among competing TCP flows, flows with smaller RTT
  fare better
• Using our policy, RTT can be reduced

Throughput
Time
Time
Competing TCP Multipath TCP
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Future Work

• Investigate the effect of competing TCP flows on
  such policies
• Devise more mechanisms esp. for other transport
  protocols
• Infer path properties dynamically
• Investigate approaches to deployment
• receiver side modification
• use mobility servers such as home agents
• Finally, implement these