Simple Hit-Metering and Usage-Limiting for HTTP
RFC 2227

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Network Working Group                                           J. Mogul
Request for Comments: 2227                                        DECWRL
Category: Standards Track                                       P. Leach
                                                               Microsoft
                                                            October 1997

            Simple Hit-Metering and Usage-Limiting for HTTP

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 (1997).  All Rights Reserved.

ABSTRACT

   This document proposes a simple extension to HTTP, using a new
   "Meter" header, which permits a limited form of demographic
   information (colloquially called "hit-counts") to be reported by
   caches to origin servers, in a more efficient manner than the
   "cache-busting" techniques currently used.  It also permits an origin
   server to control the number of times a cache uses a cached response,
   and outlines a technique that origin servers can use to capture
   referral information without "cache-busting."

TABLE OF CONTENTS

   1 Introduction                                                      2
        1.1 Goals, non-goals, and limitations                          3
        1.2 Brief summary of the design                                4
        1.3 Terminology                                                5
   2 Overview                                                          5
        2.1 Discussion                                                 7
   3 Design concepts                                                   8
        3.1 Implementation of the "metering subtree"                   8
        3.2 Format of the Meter header                                10
        3.3 Negotiation of hit-metering and usage-limiting            10
        3.4 Transmission of usage reports                             14
        3.5 When to send usage reports                                15
        3.6 Subdivision of usage-limits                               16

Mogul & Leach               Standards Track                     [Page 1]
RFC 2227            Hit-Metering and Usage-Limiting         October 1997

   4 Analysis                                                         17
        4.1 Approximation accuracy for counting users                 18
        4.2 What about "Network Computers"?                           19
        4.3 Critical-path delay analysis                              19
   5 Specification                                                    20
        5.1 Specification of Meter header and directives              20
        5.2 Abbreviations for Meter directives                        23
        5.3 Counting rules                                            24
             5.3.1 Counting rules for hit-metering                    24
             5.3.2 Counting rules for usage-limiting                  25
             5.3.3 Equivalent algorithms are allowed                  26
        5.4 Counting rules: interaction with Range requests           27
        5.5 Implementation by non-caching proxies                     27
        5.6 Implementation by cooperating caches                      28
   6 Examples                                                         28
        6.1 Example of a complete set of exchanges                    28
        6.2 Protecting against HTTP/1.0 proxies                       30
        6.3 More elaborate examples                                   30
   7 Interactions with content negotiation                            31
        7.1 Treatment of responses carrying a Vary header             31
        7.2 Interaction with Transparent Content Negotiation          32
   8 A Note on Capturing Referrals                                    32
   9 Alternative proposals                                            33
   10 Security Considerations                                         34
   11 Acknowledgments                                                 35
   12 References                                                      35
   13 Authors' Addresses                                              36
   14 Full Copyright Statement                                        37

1 Introduction

   For a variety of reasons, content providers want to be able to
   collect information on the frequency with which their content is
   accessed. This desire leads to some of the "cache-busting" done by
   existing servers.  ("Cache-busting" is the use by servers of
   techniques intended to prevent caching of responses; it is unknown
   exactly how common this is.)  This kind of cache-busting is done not