A TCP Friendly Traffic Marker for IP Differentiated Services

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A TCP Friendly Traffic Marker for IP
Differentiated Services

• Feroz Azeem, Shiv Kalyanaraman, Amit Rao
• Rensselaer Polytechnic Institute
• shivkuma_at_ecse.rpi.edu
• http//www.ecse.rpi.edu/Homepages/shivkuma

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Overview

• TCP performance over Differentiated Services
• What are TCP-friendly building blocks ?
• TCP-friendly traffic conditioners
• Sample performance results

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Diff-serv Model
End system
End system
Interior Router
Egress Edge Router
Ingress Edge Router

• Congestion traffic jams of the Internet
• Congestion control (CC) requires end-system
  support
• Traffic management providing bandwidth services
• TM requires some support from all network
  components
• Problem providing better-than-best-effort
  services for TCP
• Lies at the intersection of the TM and CC problems

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TCP service performance
Metrics a) User metrics b) Provider Metrics
Parameters a) Workload b) Configuration
components and protocols
System

• User metrics
• Average per-flow goodput
• Coefficient of variation (spread) of per-flow
  goodput
• Timeouts
• Provider metrics
• Bottleneck Utilization
• Queue length (avg/max)
• Packet loss rate

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TCP performance issues

• User-perceived performance can be bad even if
  provider-perceived performance is good.
• Key problem second-order effects of packet-loss
• Per-connection burst loss of packets is bad for
  TCP Reno
• Even isolated loss of packets is bad for a TCP
  connection with a small window. Happens with
• high degrees of muxing or,
• slower bottleneck rates or buffer sizes
• Probability of burst loss high when a number of
  TCP connections in slow start phase (eg WWW)
• Hypothesis problems can be alleviated by using
  TCP-friendly building blocks (possible
  violation of layering)

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TCP-friendly building blocks

• Goals
• Reduce probability of burst loss and TCP
  synchronization
• Convert aggregate burst loss into per-connection
  isolated packet loss. Also minimize
  per-connection loss instances.
• Protect small window connections from packet loss
• Sample Methods
• Random early dropping packet drop scheme like
  RED
• Control TCP rate or dampen TCP rate increases
• Control left, right edges of TCP window (rcv_wnd
  and ack ) and/or rate of release of acks
• Reduce TCP burstiness
• Interleave packets of multiple flows
• Protect small window flows from loss

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TCP-friendly Marker

• Problem Allocate a limited aggregate pool of
  tokens among the active TCP flows to maximize
  performance as measured by user and provider
  best-effort metrics.
• Method
• Estimate window sizes of flows W(i).
• Assume maximum token requirement for flow i
  W(i)
• First allocate and satisfy IN token requirements
  for all small-window flows, I.e., W(i) lt 4.
• Divide the remaining tokens in a max-min fair way
  among the rest of the flows.
• Provide maximum equal spacing between packets
  marked OUT.
• Carry over of tokens across intervals limited by
  policy

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Configuration
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Sample Results

• In all cases, the performance improvement is
  consistent, as measured by provider and user
  metrics.
• Performance improvement is marked with higher
  speed links and larger number of flows.
• EgWith 100 flows, 150 Mbps bottleneck
• TCPFM 1261 timeouts, 240 losses/s, 194kbps
• TBM 4254 timeouts, 311 losses/s, 134kbps
• No marker2954 timeouts, 322 losses/s, 151kbps
• Validates Ibanez et als observations in the IETF
  (March 1999) about the unpredictability of TBM

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Sample Results (contd)

• Results also extend to TCP SACK.
• But the total number of timeouts in all cases is
  smaller.
• Better-than-best-effort service
• The token and capacity over-provisioning required
  to provide assured service also reduces with the
  TCP-friendly marker
• But the second-order effects of loss on TCP are
  not totally compensated for.
• Later experimental work (submitted to ICNP2000)
  also protects retransmissions which leads to
  order-of-magnitude performance gains, validating
  the approach.

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Summary

• TCP-friendly refers to components which promote
  better TCP performance, esp timeout and
  service-variance reduction.
• A simple TCP-friendly marker which leverages the
  dimension of network-based packet marking in
  conjunction with the assured service. Effects
• Reduce user-perceived service variance (user
  benefit)
• Reduce token provisioning requirements (provider
  benefit)