Basic Dynamic Scheduling for Multiple Path Routing

1
Basic Dynamic Scheduling for Multiple Path
Routing

• Joseph A LaConte
• CS 526
• May 5, 2005

2
OVERVIEW

• Goals
• Purpose
• Dynamic Scheduler
• Design Issues
• TCP
• Slow Start Algorithm
• Proposed Solution Model
• Benefits
• Penalties
• Future Work

3
GOALS

• Review some traits of TCP.
• Discuss some of the design issues associated with
  a dynamic scheduler in multiple path routing.
• Propose a connection-based dynamic scheduler for
  TCP utilizing multiple paths.

4
PURPOSE

• What is multiple path routing?
• Set of Proxy servers with indirect routing.
• Overlay network.
• Wireless ad hoc networks.
• Multihoming.
• How does a dynamic scheduler relate to multiple
  path routing?

5
DYNAMIC SCHEDULER

• The dynamic scheduler should
• Increase performance.
• Have the capability to diagnose bandwidth (within
  some degree).
• Reduce network congestion (efficiency).
• How would you implement a dynamic scheduler for
  TCP using multipaths?

6
DESIGN ISSUES

• Network Layer (IP) versus Transport Layer.
• Concurrent Multipaths
• Arrival order is NOT guaranteed.
• Bandwidth fluctuates.
• TCP
• NOT required to send an immediate ACK.
• NOT required to use NACK SACK.
• Network Congestion

7
TCP

• Brief Review
• Sliding Window?
• Congestion Control Window?
• Slow Start Algorithm.
• Fast Retransmit.

8
SLOW START ALGORITHM

• Controls congestion window.
• Starts small, but grows exponentially.
• Continues until a retransmission timeout.
• Loop
• Restarts using ½ old window size as a threshold.
• When threshold is reached grows linearly.
• Continues until a retransmission timeout.

9

• Tanenbaum p. 550

10
TCP Congestion Control
Time-out
Packet loss
Window size
Congestion Avoidance
Reach initial ssthresh switch to CA mode
Fast retransmit Fast recovery
Slow Start
Slow Start
1
Time (seconds)
Cai, Yu slide 26
11
PROPOSED SOLUTION MODEL

• Use congestion control as a rough diagnostic tool
  for each path.
• Iterate each path until restart, storing the
  threshold for each path.
• Handle retransmission queue RTO timers (initial
  solution by retransmission of all not yet
  acknowledged segments in buffer through next
  path).
• Upon diagnostic completion
• Find weights using LCD on thresholds approx.
• Calculate new threshold based on
• ß( sum(thresholds) ) where 0 lt ß lt 1
• RTO Retransmission TimeOut
• LCD Least (lowest) Common Denominator

12
BENEFITS OF MODEL

• Gives a relatively quick estimate of
  bandwidth/reliability.
• Spends less time on slow paths (and more time on
  less congested paths).
• Inherently avoids and reduces network congestion.
• ß provides for limited network fluctuations.

13
PENALTIES FOR MODEL

• Poor solution for multiple wireless paths.
• Multiple paths are not exploited during
  diagnostics.
• Overreaching may occur in each window containing
  a timeout.
• Shorter TCP sessions benefit the least.

14
FUTURE WORK

• Implementation and testing.
• Make enhancements to model
• Experiment with caps on congestion window size
  based on receivers advertised window size to
  reduce cost penalties of overreach/timeout during
  diagnostic phase.
• Incorporate other TCP bookkeeping variables (RTO,
  RTTM, etc).
• Refine mechanism for re-entering diagnostic
  phase.
• Detect/Decide when to drop a path.
• Allow for cap on number of paths (ie. use best m
  of n paths).
• Implement global mechanism in network layer that
  allows transport layer and application layer (if
  specific transport layer allows) to implement
  dynamic schedulers.
• Look at fast retransmit/broadcasting.
• Experiment with timeout values.
• RTTM Round Trip Time Measured

15
ADDITIONAL RESOURCES

• Multiple Path Routing
• Watson, Frank E. 2005. Enhanced TCP
  Performance with Multiple Path Routing.
  Masters thesis, University of Colorado at
  Colorado Springs.
• Cai, Yu. 2005. On the Proxy Server based
  Multipath Connection. PhD Dissertation Defense,
  University of Colorado at Colorado Springs.
• http//cs.uccs.edu/chow/pub/master/ycai/doc/phd
  _thesis_defense_yu_cai.ppt
• Gerla, M., Lee, S. S., Pau, G. 2002. TCP
  Westwood Performance Over Multiple Paths.
  http//www.cs.ucla.edu/NRL/hpi/papers/2002-tr-0.pd
  f
• TCP
• Tanenbaum, Andrew S. 2003. Computer Networks,
  4th ed. Prentice Hall PTR, Upper Saddle River,
  NJ.
• Sarolahti, Pasi. 2002. Linux TCP. Seminar on
  Linux Kernel. http//www.cs.helsinki.fi/u/kraatik
  a/Courses/sem02a/Linux-TCP.pdf
• Casetti, C., Gerla, M., Lee S. S., Mascolo, S.,
  Sanadidi, M. 2000. TCP with Faster Recovery.
  http//www.cs.ucla.edu/NRL/hpi/tcpw/tcpw_papers/20
  00-milcom-0.pdf
• Sacerdoti, Federico D. 2004. Tcphealth TCP
  Connection Monitoring in Linux.
  http//heron.ucsd.edu/tcphealth/
• TCP From PSH to ACK. 2005. Maintained by
  Rafael Stekolshchik.
• http//cities.lk.net/tcp.html