Network Working Group G. Almes
Request for Comments: 2679 S. Kalidindi
Category: Standards Track M. Zekauskas
Advanced Network & Services
September 1999
A One-way Delay Metric for IPPM
1. 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.
2. Introduction
This memo defines a metric for one-way delay of packets across
Internet paths. It builds on notions introduced and discussed in the
IPPM Framework document, RFC 2330 [1]; the reader is assumed to be
familiar with that document.
This memo is intended to be parallel in structure to a companion
document for Packet Loss ("A One-way Packet Loss Metric for IPPM")
[2].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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.
The structure of the memo is as follows:
+ A 'singleton' analytic metric, called Type-P-One-way-Delay, will
be introduced to measure a single observation of one-way delay.
Almes, et al. Standards Track [Page 1]RFC 2679 A One-way Delay Metric for IPPM September 1999
+ Using this singleton metric, a 'sample', called Type-P-One-way-
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.
2.1. Motivation:
One-way 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 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.
The measurement of one-way delay instead of round-trip delay is
motivated by the following factors:
+ In today's Internet, the path from a source to a destination may
be different than 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. Measuring each path independently highlights the
performance difference between the two paths which may traverse
Almes, et al. Standards Track [Page 2]RFC 2679 A One-way Delay Metric for IPPM September 1999
different Internet service providers, and even radically different
types of networks (for example, research versus commodity
networks, or ATM versus packet-over-SONET).
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