How do transport protocols affect applications

1
How do transport protocols affect
applicationsThe relative importance of
different protocol propertiesPanel Discussion
Richard Hughes-Jones The University of
Manchester www.hep.man.ac.uk/rich/ then
Talks
2
Panellists

• Pascale Primet
• INREA, France
• Ralph Niederberger
• Research Center Juelich, Germany
• Tim Sheppard
• Katsushi Kobayashi
• National Institute Adv. Industrial Science
  Technology, Japan
• Michael Welzl
• University of Innsbruck, Austria

3
Some Areas for Discussion

• What is the interaction between Application and
  Transport Protocol?
• What is the relative importance of fairness vs
  throughput?
• rtt fairness (OK what is fairness?)
• mtu fairness
• TCP friendliness
• How to AIMD rate fluctuations relate to
  stability sharing?
• Stability of Achievable Throughput
• Does provable stability of protocols matter?
• Is the computational complexity of a protocol
  important?
• What is the relative importance of convergence
  time?
• Link utilisation (by this flow or all flows)
• Should there be a bias towards "mice?
  Applications
• Is conceptual simplicity of the protocol
  important?

4

• Action of the transport protocol -
• help or hindrance to the application ?

5
Remote Compute Farms Application Req-Resp

• CERN-Manc
• Round trip time 20 ms
• Web100 hooks for TCP status
• 64 byte Request green1 Mbyte Response blue
• TCP in slow start
• 1st event takes 19 rtt or 380 ms

6
VLBI Application Protocol

• VLBI signal wave front

• Data wave front send to Correlator

7
Visualising CBR/TCP
Stephen Kershaw

• When packet loss is detected TCP
• Reduces Cwnd
• Halves the sending rate
• Expect a delay in the message arrival time

Arrival time
Message number / Time
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CBR/TCP UKLight JBO-JIVE-Manc

• Message size 1448 Bytes
• Wait time 22 us
• Data Rate 525 Mbit/s
• RouteJB-UKLight-JIVE-UKLight-Man
• RTT 27 ms
• TCP buffer 32M bytes
• BDP _at_512Mbit 1.8Mbyte
• Estimate catch-up possible if loss lt 1 in 1.24M

Timely data arrival
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• And now for the protocols

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SC2004 Disk-Disk bbftp

• bbftp file transfer program uses TCP/IP
• UKLight Path- London-Chicago-London PCs-
  Supermicro 3Ware RAID0
• MTU 1500 bytes Socket size 22 Mbytes rtt 177ms
  SACK off
• Move a 2 GByte file
• Web100 plots
• Standard TCP
• Average 825 Mbit/s
• (bbcp 670 Mbit/s)
• Scalable TCP
• Average 875 Mbit/s
• (bbcp 701 Mbit/s4.5s of overhead)
• Disk-TCP-Disk at 1Gbit/s

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Transport Protocols

• TCP
• Reno HS-TCP Scalable H-TCP C-TCP BIC CUBIC
  LCTP
• XCP
• UDP
• Some applications NEED this form of delivery
• RTP / RTSP
• Lots of streaming applications available now
• DCCP
• multicast

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DCCP Datagram Congestion Control Protocol

• Unreliable
• No re-transmissions
• Has modular congestion control
• Can detect congestion and take avoiding action
• Different algorithms can be selected ccid
• TCP-like
• TCP Friendly Rate Control
• DCCP is like UDP with congestion control
• DCCP is like TCP without reliability
• Application uses
• Multi-media send new data instead of re-sending
  useless old data
• Applications that can choose data encoding
  transmission rate
• e-VLBI discussing a special ccid
• RFCs 4340, CCIDs RFC 4341 4342
• e-VLBI considering a ccid UDP with congestion
  detection API extension
• Detect potential problems with other network
  users unexpected route changes

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Fairness and Throughput
Smaller RTT is faster !
Larger MTU is faster !
14
Rate fluctuations, Stability SharingTCP Reno
single stream
Les Cottrell PFLDnet 2005

• Low performance on fast long distance paths
• AIMD (add a1 pkt to cwnd / RTT, decrease cwnd by
  factor b0.5 in congestion)
• Net effect recovers slowly, does not effectively
  use available bandwidth, so poor throughput
• Unequal sharing

SLAC to CERN
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• Which Protocol for my Network

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Transports for LightPaths

• Host to host Lightpath
• One Application
• No congestion
• Lightweight framing

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Transports for Academic Networks

• High Bandwidth Backbones
• But care needed with Access links Countries and
  Campus
• Many Application flows
• Note the Digital Divide
• Roles for Advanced TCP stack and other
  transports.

Transports for Global Internet

• Many different technologies often low
  Bandwidths
• Cautious/conservative Transport Protocols
• Standard TCP
• Linux BIC
• Microsoft C-TCP

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Summary Some Areas for Discussion

• What is the interaction between Application and
  Transport Protocol?
• What is the relative importance of fairness vs
  throughput?
• rtt fairness (OK what is fairness?)
• mtu fairness
• TCP friendliness
• How to AIMD rate fluctuations relate to
  stability sharing?
• Stability of Achievable Throughput
• Does provable stability of protocols matter?
• Is the computational complexity of a protocol
  important?
• What is the relative importance of convergence
  time?
• Link utilisation (by this flow or all flows)
• Should there be a bias towards "mice?
  Applications
• Is conceptual simplicity of the protocol
  important?

19
Thanks to the Panellists

• Pascale Primet
• INREA, France
• Ralph Niederberger
• Research Center Juelich, Germany
• Tim Sheppard
• Katsushi Kobayashi
• National Institute Adv. Industrial Science
  Technology, Japan
• Michael Welzl
• University of Innsbruck, Austria

20
CBR/TCP Catch-up?
Stephen Kershaw

• If Throughput NOT limited by TCP buffer size /
  Cwnd maybe we can re-sync with CBR arrival
  times.
• Need to store CBR messages during the Cwind drop
  in the TCP buffer
• Then transmit Faster than the CBR rate to catch
  up

Arrival time
Message number / Time