A Framework for Defining Empirical Bulk Transfer Capacity Metrics
RFC 3148

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Network Working Group                                          M. Mathis
Request for Comments: 3148              Pittsburgh Supercomputing Center
Category: Informational                                        M. Allman
                                                          BBN/NASA Glenn
                                                               July 2001

   A Framework for Defining Empirical Bulk Transfer Capacity Metrics

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document defines a framework for standardizing multiple BTC
   (Bulk Transport Capacity) metrics that parallel the permitted
   transport diversity.

1   Introduction

   Bulk Transport Capacity (BTC) is a measure of a network's ability to
   transfer significant quantities of data with a single congestion-
   aware transport connection (e.g., TCP).  The intuitive definition of
   BTC is the expected long term average data rate (bits per second) of
   a single ideal TCP implementation over the path in question.
   However, there are many congestion control algorithms (and hence
   transport implementations) permitted by IETF standards.  This
   diversity in transport algorithms creates a difficulty for
   standardizing BTC metrics because the allowed diversity is sufficient
   to lead to situations where different implementations will yield
   non-comparable measures -- and potentially fail the formal tests for
   being a metric.

   Two approaches are used.  First, each BTC metric must be much more
   tightly specified than the typical IETF protocol.  Second, each BTC
   methodology is expected to collect some ancillary metrics which are
   potentially useful to support analytical models of BTC.

   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 [RFC2119].  Although

Mathis, et al.               Informational                      [Page 1]
RFC 3148      Framework for Defining Empirical BTC Metrics     July 2001

   [RFC2119] was written with protocols in mind, the key words are used
   in this document for similar reasons.  They are used to ensure that
   each BTC methodology defined contains specific pieces of information.

   Bulk Transport Capacity (BTC) is a measure of a network's ability to
   transfer significant quantities of data with a single congestion-
   aware transport connection (e.g., TCP).  For many applications the
   BTC of the underlying network dominates the overall elapsed time for
   the application to run and thus dominates the performance as
   perceived by a user.  Examples of such applications include FTP, and
   the world wide web when delivering large images or documents.  The
   intuitive definition of BTC is the expected long term average data
   rate (bits per second) of a single ideal TCP implementation over the
   path in question.  The specific definition of the bulk transfer
   capacity that MUST be reported by a BTC tool is:

      BTC = data_sent / elapsed_time

   where "data_sent" represents the unique "data" bits transfered (i.e.,
   not including header bits or emulated header bits).  Also note that
   the amount of data sent should only include the unique number of bits
   transmitted (i.e., if a particular packet is retransmitted the data
   it contains should be counted only once).

   Central to the notion of bulk transport capacity is the idea that all
   transport protocols should have similar responses to congestion in
   the Internet.  Indeed the only form of equity significantly deployed
   in the Internet today is that the vast majority of all traffic is
   carried by TCP implementations sharing common congestion control
   algorithms largely due to a shared developmental heritage.

   [RFC2581] specifies the standard congestion control algorithms used
   by TCP implementations.  Even though this document is a (proposed)
   standard, it permits considerable latitude in implementation.  This
   latitude is by design, to encourage ongoing evolution in congestion
   control algorithms.

   This legal diversity in congestion control algorithms creates a
   difficulty for standardizing BTC metrics because the allowed
   diversity is sufficient to lead to situations where different
   implementations will yield non-comparable measures -- and potentially
   fail the formal tests for being a metric.

   There is also evidence that most TCP implementations exhibit non-
   linear performance over some portion of their operating region.  It
   is possible to construct simple simulation examples where incremental
   improvements to a path (such as raising the link data rate) results
   in lower overall TCP throughput (or BTC) [Mat98].