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Passive Calibration of an Active Measurement System

Stephen Donnelly*, Ian Graham*, Rene Wilhelm**
* The University of Waikato ** RIPE NCC

Measuring QoS in the internet today is difficult as notions of what constitutes QoS vary. Service Level Agreements between customers and network service providers are often poorly defined. The most workable SLAs currently are defined in terms of how long packets may take to travel from some host to another host, and what acceptable levels of packet loss are.

This paper presents a direct comparison of active One-way-Delay measurements with an independent passive measurement system over an intercontinental distance.

The IETF's IP Performance Metrics Working Group have codified metrics for One-way-Delay[1], One-way-Loss[2], and Round-trip-Delay[3], which should be useful to people writing SLAs. The IPPM WG also discuss practical methods for making measurements of their metrics[4]. This involves actively sending a periodic or poisson stream of test packets from one host to another, and observing the arrivals at their destination.

There are at least two implementations of measurement systems to collect these metrics. One is by ANS[5], and the other by RIPE NCC[6].

Since 1997, RIPE NCC has been running an experimental system called 'Test Traffic Measurement', which collects Type-P-One-way-Delay, Type-P-One-way-Loss, and routing vectors across a full mesh of their 'testboxes'. There are currently approximately 50 testboxes distributed mostly amongst memeber ISPs, with a few outside Europe, including the USA, and one in New Zealand.

The testbox is a PC running FreeBSD, with a conventional Ethernet NIC. Each testbox also contains a GPS receiver, which is used for timekeeping. This global source of synchronised timing information allows the measurement of one way metrics, by placing timestamps inside packets at the source, and sending them to a destination machine. At the destination the timestamps can be compared to the synchronised clock.

This approach should lead to a very accurate and stable measurement platform. It is however quite difficult to characterise the error distribution of such a system over a large distance. Previous work[7] compared the RIPE NCC system with Surveyor, but as both are active systems, and do not necessarily probe the network at the same time, only a general agreement between the systems could be inferred.

The WAND group DAG monitor[8] is a hardware and software system that passively receives and timestamps network traffic in hardware. The DAG includes a hardware clock that is used for timestamping which is constantly and smoothly adjusted using a GPS reference to within half a microsecond of UTC.

Two of these systems at two RIPE NCC testbox sites have passively collected and timestamped the testbox active probes, providing an independent measure the One-way-Delay of the packets from the time they leave one testbox until they arrive at their destination. The passive measurements at each site provide a very accurate measure of the difference between the testbox source timestamp, and the time at which the packet actually appears on the source network. This time difference is referred to as the transmission latency. The difference between the packet arriving at the destination network and the testboxes timestamp of the event can be similarly found. This is referred to as reception latency, and quantifies the end host packet reception behaviour. Finally the passive timestamps are compared to produce a 'wire One-way-Delay' measurement which is compared to the corresponding testbox One-way-Delay result.

A high level of agreement was found between the active and passive systems in this study. An explanation of the distribution of differences between the systems results is offered.

It is hoped that these results will help researchers making active measurements to understand error contributions from end-point equipment when interpreting their results.

  1. "A One-way Delay Metric for IPPM" G. Almes, S. Kalidindi, M. Zekauskas RFC2679, September 1999.
  2. "A One-way Packet Loss Metric for IPPM" G. Almes, S. Kalidindi, M. Zekauskas RFC2680, September 1999.
  3. "A Round-trip Delay Metric for IPPM" G. Almes, S. Kalidindi, M. Zekauskas RFC2681, September 1999.
  4. "Network performance measurement for periodic streams" V. Raisanen, G. Grotefeld Internet Draft , July 2000.
  5. "Surveyor: An Infrastructure for Internet Performance Measurements" Sunil Kalidindi, Matthew J. Zekauskas ANS Proc. INET'99 , June 1999.
  6. "Internet Delay Measurements using Test Traffic" Henk Uijterwaal, Olaf Kolkman RIPE NCC Document: RIPE-158, May 1997.
  7. "Comparing two Implementations of the IETF IPPM One-way Delay and Loss Metrics" Sunil Kalidindi, Matt Zekauskas and Henk Uijterwaal Proc. PAM2000, April 2000.
  8. "Nonintrusive and Accurate Measurement of Unidirectional Delay and Delay Variation on the Internet" I. D. Graham, S. F. Donnelly, S. Martin, J. Martens, and J. G. Cleary Proc. INET'98, July 1998.