A Round-trip Delay Metric for IPPM
RFC 2681

Document Type RFC - Proposed Standard (September 1999; Errata)
Authors Sunil Kalidindi  , Matthew Zekauskas  , Guy Almes 
Last updated 2013-03-02
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Network Working Group                                           G. Almes
Request for Comments: 2681                                  S. Kalidindi
Category: Standards Track                                   M. Zekauskas
                                             Advanced Network & Services
                                                          September 1999

                   A Round-trip Delay Metric for IPPM

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Introduction

   This memo defines a metric for round-trip delay of packets across
   Internet paths.  It builds on notions introduced and discussed in the
   IPPM Framework document, RFC 2330 [1], and follows closely the
   corresponding metric for One-way Delay ("A One-way Delay Metric for
   IPPM") [2]; the reader is assumed to be familiar with those

   The memo was largely written by copying material from the One-way
   Delay metric.  The intention is that, where the two metrics are
   similar, they will be described with similar or identical text, and
   that where the two metrics differ, new or modified text will be used.

   This memo is intended to be parallel in structure to a future
   companion document for Packet Loss.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [6].
   Although RFC 2119 was written with protocols in mind, the key words
   are used in this document for similar reasons.  They are used to
   ensure the results of measurements from two different implementations
   are comparable, and to note instances when an implementation could
   perturb the network.

Almes, et al.               Standards Track                     [Page 1]
RFC 2681          Round-trip for Delay Metric for IPPM    September 1999

   The structure of the memo is as follows:

   +  A 'singleton' analytic metric, called Type-P-Round-trip-Delay,
      will be introduced to measure a single observation of round-trip

   +  Using this singleton metric, a 'sample', called Type-P-Round-trip-
      Delay-Poisson-Stream, will be introduced to measure a sequence of
      singleton delays measured at times taken from a Poisson process.

   +  Using this sample, several 'statistics' of the sample will be
      defined and discussed.

   This progression from singleton to sample to statistics, with clear
   separation among them, is important.

   Whenever a technical term from the IPPM Framework document is first
   used in this memo, it will be tagged with a trailing asterisk.  For
   example, "term*" indicates that "term" is defined in the Framework.

1.1. Motivation

   Round-trip delay of a Type-P* packet from a source host* to a
   destination host is useful for several reasons:

   +  Some applications do not perform well (or at all) if end-to-end
      delay between hosts is large relative to some threshold value.

   +  Erratic variation in delay makes it difficult (or impossible) to
      support many interactive real-time applications.

   +  The larger the value of delay, the more difficult it is for
      transport-layer protocols to sustain high bandwidths.

   +  The minimum value of this metric provides an indication of the
      delay due only to propagation and transmission delay.

   +  The minimum value of this metric provides an indication of the
      delay that will likely be experienced when the path* traversed is
      lightly loaded.

   +  Values of this metric above the minimum provide an indication of
      the congestion present in the path.

Almes, et al.               Standards Track                     [Page 2]
RFC 2681          Round-trip for Delay Metric for IPPM    September 1999

   The measurement of round-trip delay instead of one-way delay has
   several weaknesses, summarized here:

   +  The Internet path from a source to a destination may differ from
      the path from the destination back to the source ("asymmetric
      paths"), such that different sequences of routers are used for the
      forward and reverse paths.  Therefore round-trip measurements
      actually measure the performance of two distinct paths together.

   +  Even when the two paths are symmetric, they may have radically
      different performance characteristics due to asymmetric queueing.

   +  Performance of an application may depend mostly on the performance
      in one direction.

   +  In quality-of-service (QoS) enabled networks, provisioning in one
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