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The End- to- End Effects of Internet Path Selection Stefan Savage, Andy Collins, Eric Hoffman, John Snell, Tom Anderson Department of Computer Science and Engineering University of Washington. Rahul Mangharam. Motivation.

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Rahul mangharam

The End- to- End Effects ofInternet Path SelectionStefan Savage, Andy Collins, Eric Hoffman, John Snell, Tom AndersonDepartment of Computer Science and EngineeringUniversity of Washington

Rahul Mangharam


  • An optimal routing system would always choose the best available path between any two points

  • Best path?

    • Minimum average Latency

    • Minimum average Loss rate

    • Maximum average Bandwidth

  • Goal: Quantify and understand the impact of path selection on end- to- end performance

  • Focus on how “good” current routing is and not on alternative routing policies.

Metrics measurements
Metrics & Measurements

  • Measure paths between pairs of hosts

  • Generate synthetic topology – full N2 mesh

  • Find best alternate path through this graph

Rtt latency performance
RTT Latency Performance

Trans-Atlantic link

Metric: CDF of difference between the mean RTT and the mean value derived for the best alternate path

30%- 55% of default paths have longer round- trip times

Loss rate
Loss Rate

  • 75%- 85% of default paths have higher loss rates


BW calculated

from TCP RTT

and loss rate

  • Optimistic : Max. loss rate of any component of synthetic path.

  • Pessimistic: Loss rates are independent

  • 70%- 80% of default paths have lower bandwidth

Room for improvement
Room for Improvement !

  • The default path is usually not the best

    • True for latency, loss rate, and bandwidth

  • RTT:Alternate path performance over default

  • Loss Rate:Alternate path performance

95% confidence interval

Time of day variation
Time of day Variation

  • Alternate paths are better during peak hours

  • Reason: Routing instability and congestion

Congestion versus propagation delay
Congestion versus Propagation Delay

  • Estimate propagation delay (10th percentile)

  • Queuing delay = RTT – propagation delay

  • Congestion contribution: Difference between prop. Delay for each path and the best prop. Delay of alt path. CDF of mean RTT.

  • Congestion and prop. Delay are equally dominant.

Influence of popular hosts and as s
Influence of popular hosts and AS’s

  • RTT: Marginal difference in performance upon removing 10 hosts that caused the largest difference in CDF.

  • Contribution: CDF of number of paths in which a host appears

  • AS’s appear almost with the same frequency in both default and alternate paths

Potential sources of routing inefficiency
Potential sources of Routing Inefficiency

  • Poor Routing Metrics

    • Minimize number of AS traversed

    • Exchange only connectivity information

  • Restrictive Routing Policies

    • Co-operative or contractual?

    • Early-exit policy (could be in opp. geographic dir)

    • DiffServs and “accelerators” (starvation)

  • Manual Load Balancing

  • Single Path Routing

Transport inefficiencies
Transport Inefficiencies

  • Effective data-rate is a small fraction of Available data-rate

  • Prevention of “congestion collapse” – Adaptive Routing

    • Slow start (the learning curve)

    • Congestion avoidance

    • Timeouts (conservative) and fast retransmit

Transport inefficiencies1
Transport Inefficiencies

  • 10KB web page over a 10Mbps, 70ms RTT link

  • Ideal : BW < WIN/RTT (7.5 Mbps)

  • Long Flows : BW < (MSS/RTT) p-0.5 [Mathais97]

    • P is the probability that a packet is dropped (1.2 Mbps)

  • Short Flows : BW <TransferSize

    RTT.[log1.5(TransferSize/2.MSS) + 1]

    • Connection setup (ack), timeout, connection loss

  • Effective BW is reduced to 75 Kbps

  • Detour

    • Host needs more network status information

    • Detour routers measure and exchange latency, drop rate and bandwidth status

    • Adaptive routing over long time scale

    • Dynamic multi-path routing + load balancing

    • TCP adaptation: Cannot treat network as black box

      • RTT, timeouts, WIN, back pressure (early drop)

    How good is my routing

    How good is my Routing?

    CALL: 1-800-SAVAGE