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Transport Layer Concurrent Multipath Transfer
(CMT)
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What is CMT ?

• As per current SCTP
• if peer is multihomed, sender can transmit only
  to one destination (primary destination)
• new data not sent to multiple destinations
  concurrently
• Proposal Enable sending concurrently to multiple
  destinations over multiple paths

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What is CMT ?

• Definition of Concurrent Multipath Transfer
  (CMT) Simultaneous transfer of new data from
  source to destination via two or more separate
  paths
• Why use CMT at transport layer ?
• Exploit all network resources visible at
  transport layer
• Avoid replication of work and coarse-grain load
  balancing in applications

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Experiment Send new data concurrently between a
dual-homed sender and a dual-homed receiver -
Available bandwidth Path1 0.3Mbps Path2
1Mbps - End-to-end delay Path1 Path2 35ms -
Loss in network zero
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Research on CMT at PEL

• CMT should increase net throughput, but
• not clear if it will
• side-effects unknown
• more research needed
• Identified three negative side-effects of CMT
  with SCTP
• Unnecessary fast retransmissions
• Reduced cwnd growth due to fewer cwnd updates
• Increased ack traffic due to fewer delayed acks
• three algorithms proposed to eliminate them
• Currently investigating other performance hurdles
• due to shared receiver buffer, shared bottleneck,
  etc.

J. Iyengar, K. Shah, P. Amer, R. Stewart.
Concurrent Multipath Transfer Using SCTP
Multihoming. Technical Report, CIS Department,
June 2003.
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Mapping HTTP over SCTP
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Problems using TCP for Web transfers

• Typical web pages have multiple components
• 1 TCP connection for all components head of
  line blocking
• 1 TCP connection per component slow!
• Desired transport properties
• Low connection establishment overhead
• Concurrent transfer of components
• TCP-friendly congestion control applied to
  aggregate flow

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HTTP over SCTP design

• FTP-like design
• Use stream 0 for command messages
• Use other streams for data delivery

Web Server
Web Client/Browser
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Why SCTP?

• SCTP be modified to
• Exploit network resources visible at transport
  layer
• Perform fine-grain load balancing at transport
  layer
• Avoid replication of work and coarse-grain load
  balancing in applications

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Analytical Results
min packets outstanding on old path during
changeover
new path bandwidth (kbps)
changeover time (ms)

• Using
• Old path avail bw 500 kbps
• Old path end-to-end delay for negligible sized
  packets 50 ms
• New path end-to-end delay for negligible sized
  packets 50 ms