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Network Working Group                                           J. MogulRequest for Comments: 2227                                        DECWRLCategory: Standards Track                                       P. Leach                                                               Microsoft                                                            October 1997Simple Hit-Metering and Usage-Limiting for HTTPStatus 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                               16Mogul & 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                                        371 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   for the purpose of maintaining transparency or security properties,   but simply to collect demographic information.  Some cache-busting is   also done to provide different advertising images to appear on the   same page (i.e., each retrieval of the page sees a different ad).   This proposal supports a model similar to that of publishers of   hard-copy publications: such publishers (try to) report to their   advertisers how many people read an issue of a publication at least   once; they don't (try to) report how many times a reader re-reads an   issue. They do this by counting copies published, and then try to   estimate, for their publication, on average how many people read aMogul & Leach               Standards Track                     [Page 2]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   single copy at least once. The key point is that the results aren't   exact, but are still useful. Another model is that of coding   inquiries in such a way that the advertiser can tell which   publication produced the inquiry.1.1 Goals, non-goals, and limitations   HTTP/1.1 already allows origin servers to prevent caching of   responses, and evidence exists [9] that at least some of the time,   this is being done for the sole purpose of collecting counts of the   number of accesses of specific pages.  Some of this evidence is   inferred from the study of proxy traces; some is based on explicit   statements of the intention of the operators of Web servers.   Information collected this way might or might not be of actual use to   the people who collect it; the fact is that they want to collect it,   or already do so.   The goal of this proposal is to provide an optional performance   optimization for this use of HTTP/1.1.   This specification is:      - Optional: no server or proxy is required to implement it.      - Proxy-centered: there is no involvement on the part of        end-client implementations.      - Solely a performance optimization: it provides no        information or functionality that is not already available        in HTTP/1.1.  The intent is to improve performance overall,        and reduce latency for almost all interactions; latency        might be increased for a small fraction of HTTP        interactions.      - Best-efforts: it does not guarantee the accuracy of the        reported information, although it does provide accurate        results in the absence of persistent network failures or        host crashes.      - Neutral with respect to privacy: it reveals to servers no        information about clients that is not already available        through the existing features of HTTP/1.1.   The goals of this specification do not include:      - Solving the entire problem of efficiently obtaining        extensive information about requests made via proxies.Mogul & Leach               Standards Track                     [Page 3]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      - Improving the protection of user privacy (although our        proposal may reduce the transfer of user-specific        information to servers, it does not prevent it).      - Preventing or encouraging the use of log-exchange        mechanisms.      - Avoiding all forms of "cache-busting", or even all        cache-busting done for gathering counts.   This design has certain potential limitations:      - If it is not deployed widely in both proxies and servers,        it will provide little benefit.      - It may, by partially solving the hit-counting problem,        reduce the pressure to adopt more complete solutions, if        any become available.      - Even if widely deployed, it might not be widely used, and        so might not significantly improve performance.   These potential limitations might not be problems in actual practice.1.2 Brief summary of the design   This section is included for people not wishing to read the entire   document; it is not a specification for the proposed design, and   over-simplifies many aspects of the design.   The goal of this design is to eliminate the need for origin servers   to use "cache-busting" techniques, when this is done just for the   purpose of counting the number of users of a resource.  (Cache-   busting includes techniques such as setting immediate Expiration   dates, or sending "Cache-control:  private" in each response.)   The design adds a new "Meter" header to HTTP; the header is always   protected by the "Connection" header, and so is always hop-by-hop.   This mechanism allows the construction of a "metering subtree", which   is a connected subtree of proxies, rooted at an origin server.  Only   those proxies that explicitly volunteer to join in the metering   subtree for a resource participate in hit-metering, but those proxies   that do volunteer are required to make their best effort to provide   accurate counts.  When a hit-metered response is forwarded outside of   the metering subtree, the forwarding proxy adds "Cache-control: s-   maxage=0", so that other proxies (outside the metering subtree) are   forced to forward all requests to a server in the metering subtree.Mogul & Leach               Standards Track                     [Page 4]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      NOTE: the HTTP/1.1 specification does not currently define a "s-      maxage" Cache-control directive.  The HTTP working group has      decided to add such a directive to the next revision of the      HTTP/1.1 specification [7].   The Meter header carries zero or more directives, similar to the way   that the Cache-control header carries directives.  Proxies may use   certain Meter directives to volunteer to do hit-metering for a   resource.  If a proxy does volunteer, the server may use certain   directives to require that a response be hit-metered.  Finally,   proxies use a "count" Meter directive to report the accumulated hit   counts.   The Meter mechanism can also be used by a server to limit the number   of uses that a cache may make of a cached response, before   revalidating it.   The full specification includes complete rules for counting "uses" of   a response (e.g., non-conditional GETs) and "reuses" (conditional   GETs).  These rules ensure that the results are entirely consistent   in all cases, except when systems or networks fail.1.3 Terminology   This document uses terms defined and explained in the HTTP/1.1   specification [4], including "origin server," "resource," "hop-by-   hop," "unconditional GET," and "conditional GET."  The reader is   expected to be familiar with the HTTP/1.1 specification and its   terminology.   The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", SHOULD NOT",   "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be   interpreted as described inRFC 2119 [1].2 Overview   The design described in this document introduces several new features   to HTTP:      - Hit-metering: allows an origin server to obtain reasonably        accurate counts of the number of clients using a resource        instance via a proxy cache, or a hierarchy of proxy caches.      - Usage-limiting: allows an origin server to control the        number of times a cached response may be used by a proxy        cache, or a hierarchy of proxy caches, before revalidation        with the origin server.Mogul & Leach               Standards Track                     [Page 5]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   These new non-mandatory features require minimal new protocol   support, no change in protocol version, relatively little overhead in   message headers.  The design adds no additional network round-trips   in any critical path that directly affects user-perceived latency   (seesection 4.3 for an analysis).   The primary goal of hit-metering and usage-limiting is to obviate the   need for an origin server to send "Cache-control: s-maxage=0" with   responses for resources whose value is not likely to change   immediately.  In other words, in cases where the only reason for   contacting the origin server on every request that might otherwise be   satisfied by a proxy cache entry is to allow the server to collect   demographic information or to control the number of times a cache   entry is used, the extension proposed here will avoid a significant   amount of unnecessary network traffic and latency.   This design introduces one new "Meter" header, which is used both in   HTTP request messages and HTTP response messages.  The Meter header   is used to transmit a number of directives and reports.  In   particular, all negotiation of the use of hit-metering and usage   limits is done using this header.  No other changes to the existing   HTTP/1.1 specification [4] are proposed in this document.   This design also introduces several new concepts:      1. The concepts of a "use" of a cache entry, which is when a         proxy returns its entity-body in response to a conditional         or non-conditional request, and the "reuse" of a cache         entry, which is when a proxy returns a 304 (Not Modified)         response to a conditional request which is satisfied by         that cache entry.      2. The concept of a hit-metered resource, for which proxy         caches make a best-effort attempt to report accurate         counts of uses and/or reuses to the origin server.      3. The concept of a usage-limited resource, for which the         origin server expects proxy caches to limit the number of         uses and/or reuses.   The new Meter directives and reports interact to allow proxy caches   and servers to cooperate in the collection of demographic data.  The   goal is a best-efforts approximation of the true number of uses   and/or reuses, not a guaranteed exact count.Mogul & Leach               Standards Track                     [Page 6]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   The new Meter directives also allow a server to bound the inaccuracy   of a particular hit-count, by bounding the number of uses between   reports.  It can also, for example, bound the number of times the   same ad is shown because of caching.Section 7.1 describes a way to use server-driven content negotiation   (the Vary header) that allows an HTTP origin server to flexibly   separate requests into categories and count requests by category.   Implementation of such a categorized hit counting is likely to be a   very small modification to most implementations of Vary; some   implementations may not require any modification at all.2.1 Discussion   Mapping this onto the publishing model, a proxy cache would increment   the use-count for a cache entry once for each unconditional GET done   for the entry, and once for each conditional GET that results in   sending a copy of the entry to update a client's invalid cached copy.   Conditional GETs that result in 304 (Not Modified) are not included   in the use-count, because they do not result in a new user seeing the   page, but instead signify a repeat view by a user that had seen it   before.  However, 304 responses are counted in the reuse-count.   HEADs are not counted at all, because their responses do not contain   an entity-body.   The Meter directives apply only to shared proxy caches, not to end-   client (or other single-user) caches.  Single user caches should not   use Meter, because their hits will be automatically counted as a   result of the unconditional GET with which they first fetch the page,   from either the origin-server or from a proxy cache.  Their   subsequent conditional GETs do not result in a new user seeing the   page.   The mechanism specified here counts GETs; other methods either do not   result in a page for the user to read, aren't cached, or are   "written-through" and so can be directly counted by the origin   server. (If, in the future, a "cachable POST" came into existence,   whereby the entity-body in the POST request was used to select a   cached response, then such POSTs would have to be treated just like   GETs.)  The applicability of hit-metering to any new HTTP methods   that might be defined in the future is currently unspecifiable.   In the case of multiple caches along a path, a proxy cache does the   obvious summation when it receives a use-count or reuse-count in a   request from another cache.Mogul & Leach               Standards Track                     [Page 7]

RFC 2227            Hit-Metering and Usage-Limiting         October 19973 Design concepts   In order to allow the introduction of hit-metering and usage-limiting   without requiring a protocol revision, and to ensure a reasonably   close approximation of accurate counts, the negotiation of metering   and usage-limiting is done hop-by-hop, not end-to-end.  If one   considers the "tree" of proxies that receive, store, and forward a   specific response, the intent of this design is that within some   (possibly null) "metering subtree", rooted at the origin server, all   proxies are using the hit-metering and/or usage-limiting requested by   the origin server.   Proxies at the leaves of this subtree will insert a "Cache-control:   s-maxage=0" directive, which forces all other proxies (below this   subtree) to check with a leaf of the metering subtree on every   request.  However, it does not prevent them from storing and using   the response, if the revalidation succeeds.   No proxy is required to implement hit-metering or usage-limiting.   However, any proxy that transmits the Meter header in a request MUST   implement every unconditional requirement of this specification,   without exception or amendment.   This is a conservative design, which may sometimes fail to take   advantage of hit-metering support in proxies outside the metering   subtree.  However, it is likely that without the reliability offered   by a conservative design, managers of origin servers with   requirements for accurate approximations will not take advantage of   any hit-metering proposal.   The hit-metering/usage-limiting mechanism is designed to avoid any   extra network round-trips in the critical path of any client request,   and (as much as possible) to avoid excessively lengthening HTTP   messages.   The Meter header is used to transmit both negotiation information and   numeric information.   A formal specification for the Meter header appears insection 5; the   following discussion uses an informal approach to improve clarity.3.1 Implementation of the "metering subtree"   The "metering subtree" approach is implemented in a simple,   straightforward way by defining the new "Meter" header as one that   MUST always be protected by a Connection header in every request or   response.  I.e., if the Meter header is present in an HTTP message,   that message:Mogul & Leach               Standards Track                     [Page 8]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      1. MUST contain "Connection: meter", and MUST be handled         according to the HTTP/1.1 specification of the Connection         header.      2. MUST NOT be sent in response to a request from a client         whose version number is less than HTTP/1.1.      3. MUST NOT be accepted from a client whose version number is         less than HTTP/1.1.   The reason for the latter two restrictions is to protect against   proxies that might not properly implement the Connection header.   Otherwise, a subtree that includes an HTTP/1.0 proxy might   erroneously appear to be a metering subtree.      Note: It appears that for the Connection header mechanism to      function correctly, a system receiving an HTTP/1.0 (or lower-      version) message that includes a Connection header must act as if      this header, and all of the headers it protects, ought to have      been removed from the message by an intermediate proxy.      AlthoughRFC2068 does not specifically require this behavior, it      appears to be implied.  Otherwise, one could not depend on the      stated property (section 14.10) that the protected options "MUST      NOT be communicated by proxies over further connections."  This      should probably be clarified in a subsequent draft of the HTTP/1.1      specification.      This specification does not, in any way, propose a modification of      the specification of the Connection header.   From the point of view of an origin server, the proxies in a metering   subtree work together to obey usage limits and to maintain accurate   usage counts.  When an origin server specifies a usage limit, a proxy   in the metering subtree may subdivide this limit among its children   in the subtree as it sees fit.  Similarly, when a proxy in the   subtree receives a usage report, it ensures that the hits represented   by this report are summed properly and reported to the origin server.   When a proxy forwards a hit-metered or usage-limited response to a   client (proxy or end-client) not in the metering subtree, it MUST   omit the Meter header, and it MUST add "Cache-control: s-maxage=0" to   the response.Mogul & Leach               Standards Track                     [Page 9]

RFC 2227            Hit-Metering and Usage-Limiting         October 19973.2 Format of the Meter header   The Meter header is used to carry zero or more directives.  Multiple   Meter headers may occur in an HTTP message, but according to the   rules insection 4.2 of the HTTP/1.1 specification [4], they may be   combined into a single header (and should be so combined, to reduce   overhead).   For example, the following sequence of Meter headers       Meter: max-uses=3       Meter: max-reuses=10       Meter: do-report   may be expressed as       Meter: max-uses=3, max-reuses=10, do-report3.3 Negotiation of hit-metering and usage-limiting   An origin server that wants to collect hit counts for a resource, by   simply forcing all requests to bypass any proxy caches, would respond   to requests on the resource with "Cache-control: s-maxage=0".  (An   origin server wishing to prevent HTTP/1.0 proxies from improperly   caching the response could also send both "Expires: <now>", to   prevent such caching, and "Cache-control: max-age=NNNN", to allow   newer proxies to cache the response).   The purpose of the Meter header is to obviate the need for "Cache-   control: s-maxage=0" within a metering subtree.  Thus, any proxy may   negotiate the use of hit-metering and/or usage-limiting with the   next-hop server.  If this server is the origin server, or is already   part of a metering subtree (rooted at the origin server), then it may   complete the negotiation, thereby extending the metering subtree to   include the new proxy.   To start the negotiation, a proxy sends its request with one of the   following Meter directives:   will-report-and-limit                   indicates that the proxy is willing and able to                   return usage reports and will obey any usage-limits.   wont-report     indicates that the proxy will obey usage-limits but                   will not send usage reports.   wont-limit      indicates that the proxy will not obey usage-limits                   but will send usage reports.Mogul & Leach               Standards Track                    [Page 10]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   A proxy willing to neither obey usage-limits nor send usage reports   MUST NOT transmit a Meter header in the request.   By definition, an empty Meter header:       Meter:   is equivalent to "Meter: will-report-and-limit", and so, by the   definition of the Connection header (seesection 14.10 of the   HTTP/1.1 specification [4]), a request that contains       Connection: Meter   and no explicit Meter header is equivalent to a request that contains       Connection: Meter       Meter: will-report-and-limit   This makes the default case more efficient.   An origin server that is not interested in metering or usage-limiting   the requested resource simply ignores the Meter header.   If the server wants the proxy to do hit-metering and/or usage-   limiting, its response should include one or more of the following   Meter directives:   For hit-metering:   do-report       specifies that the proxy MUST send usage reports to                   the server.   dont-report     specifies that the proxy SHOULD NOT send usage                   reports to the server.   timeout=NNN     sets a metering timeout of NNN minutes, from the time                   that this response was originated, for the reporting                   of a hit-count.  If the proxy has a non-zero hit                   count for this response when the timeout expires, it                   MUST send a report to the server at or before that                   time.  Implies "do-report".   By definition, an empty Meter header in a response, or any Meter   header that does not contain "dont-report", means "Meter: do-report";   this makes a common case more efficient.Mogul & Leach               Standards Track                    [Page 11]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      Note: an origin server using the metering timeout mechanism to      bound the collection period over which hit-counts are obtained      should adjust the timeout values in the responses it sends so that      all responses generated within that period reach their metering      timeouts at or before the end of that period.      If the origin server simply sends a constant metering timeout T      with each response for a resource, the reports that it receives      will reflect activity over a period whose duration is between T      and N*T (in the worst case), where N is the maximum depth of the      metering subtree.   For usage-limiting   max-uses=NNN    sets an upper limit of NNN "uses" of the response,                   not counting its immediate forwarding to the                   requesting end-client, for all proxies in the                   following subtree taken together.   max-reuses=NNN  sets an upper limit of NNN "reuses" of the response                   for all proxies in the following subtree taken                   together.   When a proxy has exhausted its allocation of "uses" or "reuses" for a   cache entry, it MUST revalidate the cache entry (using a conditional   request) before returning it in a response.  (The proxy SHOULD use   this revalidation message to send a usage report, if one was   requested and it is time to send it.  See sections3.4 and3.5.)   These Meter response-directives apply only to the specific response   that they are attached to.      Note that the limit on "uses" set by the max-uses directive does      not include the use of the response to satisfy the end-client      request that caused the proxy's request to the server.  This      counting rule supports the notion of a cache-initiated prefetch: a      cache may issue a prefetch request, receive a max-uses=0 response,      store that response, and then return that response (without      revalidation) when a client makes an actual request for the      resource.  However, each such response may be used at most once in      this way, so the origin server maintains precise control over the      number of actual uses.   A server MUST NOT send a Meter header that would require a proxy to   do something that it has not yet offered to do.  A proxy receiving a   Meter response-directive asking the proxy to do something it did not   volunteer to do SHOULD ignore that directive.Mogul & Leach               Standards Track                    [Page 12]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   A proxy receiving a Meter header in a response MUST either obey it,   or it MUST revalidate the corresponding cache entry on every access.   (I.e., if it chooses not to obey the Meter header in a response, it   MUST act as if the response included "Cache-control:  s-maxage=0".)      Note: a proxy that has not sent the Meter header in a request for      the given resource, and which has therefore not volunteered to      honor Meter directives in a response, is not required to honor      them.  If, in this situation, the server does send a Meter header      in a response, this is a protocol error.  However, based on the      robustness principle, the proxy may choose to interpret the Meter      header as an implicit request to include "Cache-control: s-      maxage=0" when it forwards the response, since this preserves the      apparent intention of the server.   A proxy that receives the Meter header in a request may ignore it   only to the extent that this is consistent with its own duty to the   next-hop server.  If the received Meter request header is   inconsistent with that duty, or if no Meter request header is   received and the response from the next-hop server requests any form   of metering or limiting, then the proxy MUST add "Cache-control: s-   maxage=0" to any response it forwards for that request.  (A proxy   SHOULD NOT add or change the Expires header or max-age Cache-control   directive.)      For example, if proxy A receives a GET request from proxy B for      URL X with "Connection: Meter", but proxy A's cached response for      URL does not include any Meter directives, then proxy A may ignore      the metering offer from proxy B.      However, if proxy A has previously told the origin server "Meter:      wont-limit" (implying will-report), and the cached response      contains "Meter: do-report", and proxy B's request includes      "Meter:  wont-report", then proxy B's offer is inconsistent with      proxy A's duty to the origin server.  Therefore, in this case      proxy A must add "Cache-control: s-maxage=0" when it returns the      cached response to proxy B, and must not include a Meter header in      this response.   If a server does not want to use the Meter mechanism, and will not   want to use it any time soon, it may send this directive:   wont-ask        recommends that the proxy SHOULD NOT send any Meter                   directives to this server.Mogul & Leach               Standards Track                    [Page 13]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   The proxy SHOULD remember this fact for up to 24 hours.  This avoids   virtually all unnecessary overheads for servers that do not wish to   use or support the Meter header.  (This directive also implies   "dont-report".)3.4 Transmission of usage reports   To transmit a usage report, a proxy sends the following Meter header   in a request on the appropriate resource:       Meter: count=NNN/MMM   The first integer indicates the count of uses of the cache entry   since the last report; the second integer indicates the count of   reuses of the entry (seesection 5.3 for rules on counting uses and   reuses).  The transmission of a "count" directive in a request with   no other Meter directive is also defined as an implicit transmission   of a "will-report-and-limit" directive, to optimize the common case.   (A proxy not willing to honor usage-limits would send "Meter:   count=NNN/MMM, wont-limit" for its reports.)   Note that when a proxy forwards a client's request and receives a   response, the response that the proxy sends immediately to the   requesting client is not counted as a "use".  I.e., the reported   count is the number of times the cache entry was used, and not the   number of times that the response was used.   A proxy SHOULD NOT transmit "Meter: count=0/0", since this conveys no   useful information.   Usage reports MUST always be transmitted as part of a conditional   request (such as a GET or HEAD), since the information in the   conditional header (e.g., If-Modified-Since or If-None-Match) is   required for the origin server to know which instance of a resource   is being counted.  Proxys forwarding usage reports up the metering   subtree MUST NOT change the contents of the conditional header, since   otherwise this would result in incorrect counting.   A usage report MUST NOT be transmitted as part of a forwarded request   that includes multiple entity tags in an If-None-Match or If-Match   header.      Note: a proxy that offers its willingness to do hit-metering      (report usage) must count both uses and reuses.  It is not      possible to negotiate the reporting of one but not the other.Mogul & Leach               Standards Track                    [Page 14]

RFC 2227            Hit-Metering and Usage-Limiting         October 19973.5 When to send usage reports   A proxy that has offered to send usage reports to its parent in the   metering subtree MUST send a usage report in each of these   situations:      1. When it forwards a conditional GET on the resource         instance on behalf of one of its clients (if the GET is         conditional on at most one entity-tag).      2. When it forwards a conditional HEAD on the resource         instance on behalf of one of its clients.      3. When it must generate a conditional GET to satisfy a         client request because the max-uses limit has been         exceeded.      4. Upon expiration of a metering timeout associated with a         cache entry that has a non-zero hit-count.      5. When it removes the corresponding non-zero hit-count entry         from its storage for any reason including:            - the proxy needs the storage space for another              hit-count entry.            - the proxy is not able to store more than one response              per resource, and a request forwarded on behalf of a              client has resulted in the receipt of a new response              (one with a different entity-tag or last-modified              time).         Note that a cache might continue to store hit-count information         even after having deleted the body of the response, so it is         not necessary to report the hit-count when deleting the body;         it is only necessary to report it if the proxy is about to         "forget" a non-zero value.   (Section 5.3 explains how hit-counts become zero or non-zero.)   If the usage report is being sent because the proxy is about to   remove the hit-count entry from its storage, or because of an expired   metering timeout:      - The proxy MUST send the report as part of a conditional        HEAD request on the resource instance.Mogul & Leach               Standards Track                    [Page 15]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      - The proxy is not required to retry the HEAD request if it        fails (this is a best-efforts design).  To improve        accuracy, however, the proxy SHOULD retry failed HEAD        requests, subject to resource constraints.      - The proxy is not required to serialize any other operation        on the completion of this request.      Note: proxy implementors are strongly encouraged to batch several      HEAD-based reports to the same server, when possible, over a      single persistent connection, to reduce network overhead as much      as possible.  This may involve a non-naive algorithm for      scheduling the deletion of hit-count entries.   If the usage count is sent because of an arriving request that also   carries a "count" directive, the proxy MUST combine its own (possibly   zero) use and reuse counts with the arriving counts, and then attempt   to forward the request.   However, the proxy is not required to forward an arriving request   with a "count" directive, provided that:      - it can reply to the request using a cached response, in        compliance with other requirements of the HTTP        specification.      - such a response does not exceed a max-uses limit.      - it is not required to forward the request because of an        expired metering timeout.   If an arriving request carries a "count" directive, and the proxy no   longer has a cache entry for the resource, the proxy MUST forward the   "count" directive.  (This is, in any case, what a proxy without a   suitable cache entry would normally do for any valid request it   receives.)3.6 Subdivision of usage-limits   When an origin server specifies a usage limit, a proxy in the   metering subtree may subdivide this limit among its children in the   subtree as it sees fit.   For example, consider the situation with two proxies P1 and P2, each   of which uses proxy P3 as a way to reach origin server S. Imagine   that S sends P3 a response withMogul & Leach               Standards Track                    [Page 16]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997       Meter: max-uses=10   The proxies use that response to satisfy the current requesting end-   client.  The max-uses directive in this example allows the   combination of P1, P2, and P3 together to satisfy 10 additional end-   client uses (unconditional GETs) for the resource.   This specification does not constrain how P3 divides up that   allocation among itself and the other proxies.  For example, P3 could   retain all of max-use allocation for itself.  In that case, it would   forward the response to P1 and/or P2 with       Meter: max-uses=0   P3 might also divide the allocation equally among P1 and P2,   retaining none for itself (which may be the right choice if P3 has   few or no other clients).  In this case, it could send       Meter: max-uses=5   to the proxy (P1 or P2) that made the initial request, and then   record in some internal data structure that it "owes" the other proxy   the rest of the allocation.   Note that this freedom to choose the max-uses value applies to the   origin server, as well.  There is no requirement that an origin   server send the same max-uses value to all caches.  For example, it   might make sense to send "max-uses=2" the first time one hears from a   cache, and then double the value (up to some maximum limit) each time   one gets a "use-count" from that cache.  The idea is that the faster   a cache is using up its max-use quota, the more likely it will be to   report a use-count value before removing the cache entry.  Also, high   and frequent use-counts imply a corresponding high efficiency benefit   from allowing caching.   Again, the details of such heuristics would be outside the scope of   this specification.4 Analysis   This section includes informal analyses of several aspects of hit-   metering:      1. the accuracy of results when applied to counting users         (section 4.1).      2. the problem of counting users whose browsers do not         include caches, such as Network Computers (section 4.2).Mogul & Leach               Standards Track                    [Page 17]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      3. delays imposed on "critical paths" for HTTP operations         (section 4.3).4.1 Approximation accuracy for counting users   For many (but not all) service operators, the single most important   aspect of the request stream is the number of distinct users who have   retrieved a particular entity within a given period (e.g., during a   given day).  The hit-metering mechanism is designed to provide an   origin server with an approximation of the number of users that   reference a given resource.  The intent of the design is that the   precision of this approximation is consistent with the goals of   simplicity and optional implementation.   Almost all Web users use client software that maintains local caches,   and the state of the art of local-caching technology is quite   effective.  (Section 4.2 discusses the case where end-client caches   are small or non-existent.)  Therefore, assuming an effective and   persistent end-client cache, each individual user who retrieves an   entity does exactly one GET request that results in a 200 or 203   response, or a 206 response that includes the first byte of the   entity. If a proxy cache maintains and reports an accurate use-count   of such retrievals, then its reported use-count will closely   approximate the number of distinct users who have retrieved the   entity.   There are some circumstances under which this approximation can break   down.  For example, if an entity stays in a proxy cache for much   longer than it persists in the typical client cache, and users often   re-reference the entity, then this scheme will tend to over-count the   number of users. Or, if the cache-management policy implemented in   typical client caches is biased against retaining certain kinds of   frequently re-referenced entities (such as very large images), the   use-counts reported will tend to overestimate the user-counts for   such entities.   Browser log analysis has shown that when a user revisits a resource,   this is almost always done very soon after the previous visit, almost   always with fewer than eight intervening references [11].  Although   this result might not apply universally, it implies that almost all   reuses will hit in the end-client cache, and will not be seen as   unconditional GETs by a proxy cache.   The existing (HTTP/1.0) "cache-busting" mechanisms for counting   distinct users will certainly overestimate the number of users behind   a proxy, since it provides no reliable way to distinguish between a   user's initial request and subsequent repeat requests that might have   been conditional GETs, had not cache-busting been employed.  TheMogul & Leach               Standards Track                    [Page 18]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   "Cache-control: s-maxage=0" feature of HTTP/1.1 does allow the   separation of use-counts and reuse-counts, provided that no HTTP/1.0   proxy caches intervene.   Note that if there is doubt about the validity of the results of   hit-metering a given set of resources, the server can employ cache-   busting techniques for short periods, to establish a baseline for   validating the hit-metering results.  Various approaches to this   problem are discussed in a paper by James Pitkow [9].4.2 What about "Network Computers"?   The analysis insection 4.1 assumed that "almost all Web users" have   client caches.  If the Network Computers (NC) model becomes popular,   however, then this assumption may be faulty: most proposed NCs have   no disk storage, and relatively little RAM.  Many Personal Digital   Assistants (PDAs), which sometimes have network access, have similar   constraints.  Such client systems may do little or no caching of HTTP   responses.  This means that a single user might well generate many   unconditional GETs that yield the same response from a proxy cache.   First note that the hit-metering design in this document, even with   such clients, provides an approximation no worse than available with   unmodified HTTP/1.1: the counts that a proxy would return to an   origin server would represent exactly the number of requests that the   proxy would forward to the server, if the server simply specifies   "Cache-control:  s-maxage=0".   However, it may be possible to improve the accuracy of these hit-   counts by use of some heuristics at the proxy.  For example, the   proxy might note the IP address of the client, and count only one GET   per client address per response.  This is not perfect: for example,   it fails to distinguish between NCs and certain other kinds of hosts.   The proxy might also use the heuristic that only those clients that   never send a conditional GET should be treated this way, although we   are not at all certain that NCs will never send conditional GETs.   Since the solution to this problem appears to require heuristics   based on the actual behavior of NCs (or perhaps a new HTTP protocol   feature that allows unambiguous detection of cacheless clients), it   appears to be premature to specify a solution.4.3 Critical-path delay analysis   In systems (such as the Web) where latency is at issue, there is   usually a tree of steps which depend on one another, in such a way   that the final result cannot be accomplished until all of its   predecessors have been.  Since the tree structure admits someMogul & Leach               Standards Track                    [Page 19]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   parallelism, it is not necessary to add up the timings for each step   to discover the latency for the entire process.  But any single path   through this dependency tree cannot be parallelized, and the longest   such path is the one whose length (in units of seconds) determines   the overall latency.  This is the "critical path", because no matter   how much shorter one makes any other path, that cannot change the   overall latency for the final result.   If one views the final result, for a Web request, as rendering a page   at a browser, or otherwise acting on the result of a request, clearly   some network round trips (e.g., exchanging TCP SYN packets if the   connection doesn't already exist) are on the critical path.  This   hit-metering design does add some round-trips for reporting non-zero   counts when a cache entry is removed, but, by design, these are off   any critical path:  they may be done in parallel with any other   operation, and require only "best efforts", so a proxy does not have   to serialize other operations with their success or failure.   Clearly, anything that changes network utilization (either increasing   or decreasing it) can indirectly affect user-perceived latency.  Our   expectation is that hit-metering, on average, will reduce loading and   so even its indirect effects should not add network round-trips in   any critical path.  But there might be a few specific instances where   the added non-critical-path operations (specifically, usage reports   upon cache-entry removal) delay an operation on a critical path.   This is an unavoidable problem in datagram networks.5 Specification5.1 Specification of Meter header and directives   The Meter general-header field is used to:      - Negotiate the use of hit-metering and usage-limiting among        origin servers and proxy caches.      - Report use counts and reuse counts.   Implementation of the Meter header is optional for both proxies and   origin servers.  However, any proxy that transmits the Meter header   in a request MUST implement every requirement of this specification,   without exception or amendment.   The Meter header MUST always be protected by a Connection header.  A   proxy that does not implement the Meter header MUST NOT pass it   through to another system (seesection 5.5 for how a non-caching   proxy may comply with this specification).  If a Meter header isMogul & Leach               Standards Track                    [Page 20]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   received in a message whose version is less than HTTP/1.1, it MUST be   ignored (because it has clearly flowed through a proxy that does not   implement Meter).   A proxy that has received a response with a version less than   HTTP/1.1, and therefore from a server (or another proxy) that does   not implement the Meter header, SHOULD NOT send Meter request   directives to that server, because these would simply waste   bandwidth.  This recommendation does not apply if the proxy is   currently hit-metering or usage-limiting any responses from that   server.  If the proxy receives a HTTP/1.1 or higher response from   such a server, it should cease its suppression of the Meter   directives.   All proxies sending the Meter header MUST adhere to the "metering   subtree" design described insection 3.       Meter = "Meter" ":" 0#meter-directive       meter-directive = meter-request-directive                       | meter-response-directive                       | meter-report-directive       meter-request-directive =                         "will-report-and-limit"                       | "wont-report"                       | "wont-limit"       meter-report-directive =                       | "count" "=" 1*DIGIT "/" 1*DIGIT       meter-response-directive =                         "max-uses" "=" 1*DIGIT                       | "max-reuses" "=" 1*DIGIT                       | "do-report"                       | "dont-report"                       | "timeout" "=" 1*DIGIT                       | "wont-ask"   A meter-request-directive or meter-report-directive may only appear   in an HTTP request message.  A meter-response-directive may only   appear in an HTTP response directive.   An empty Meter header in a request means "Meter: will-report-and-   limit".  An empty Meter header in a response, or any other response   including one or more Meter headers without the "dont-report" or   "wont-ask" directive, implies "Meter:  do-report".Mogul & Leach               Standards Track                    [Page 21]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   The meaning of the meter-request-directives are as follows:   will-report-and-limit                   indicates that the proxy is willing and able to                   return usage reports and will obey any usage-limits.   wont-report     indicates that the proxy will obey usage-limits but                   will not send usage reports.   wont-limit      indicates that the proxy will not obey usage-limits                   but will send usage reports.   A proxy willing neither to obey usage-limits nor to send usage   reports MUST NOT transmit a Meter header in the request.   The meaning of the meter-report-directives are as follows:   count "=" 1*DIGIT "/" 1*DIGIT                   Both digit strings encode decimal integers.  The                   first integer indicates the count of uses of the                   cache entry since the last report; the second integer                   indicates the count of reuses of the entry.Section 5.3 specifies the counting rules.   The meaning of the meter-response-directives are as follows:   max-uses "=" 1*DIGIT                   sets an upper limit on the number of "uses" of the                   response, not counting its immediate forwarding to                   the requesting end-client, for all proxies in the                   following subtree taken together.   max-reuses "=" 1*DIGIT                   sets an upper limit on the number of "reuses" of the                   response for all proxies in the following subtree                   taken together.   do-report       specifies that the proxy MUST send usage reports to                   the server.   dont-report     specifies that the proxy SHOULD NOT send usage                   reports to the server.   timeout "=" 1*DIGIT                   sets a metering timeout of the specified number of                   minutes (not seconds) after the origination of this                   response (as indicated by its "Date" header).  If theMogul & Leach               Standards Track                    [Page 22]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997                   proxy has a non-zero hit count for this response when                   the timeout expires, it MUST send a report to the                   server at or before that time.  Timeouts should be                   implemented with an accuracy of plus or minus one                   minute.  Implies "do-report".   wont-ask        specifies that the proxy SHOULD NOT send any Meter                   headers to the server.  The proxy should forget this                   advice after a period of no more than 24 hours.Section 5.3 specifies the counting rules, and in particular specifies   a somewhat non-obvious interpretation of the max-uses value.5.2 Abbreviations for Meter directives   To allow for the most efficient possible encoding of Meter headers,   we define abbreviated forms of all Meter directives.  These are   exactly semantically equivalent to their non-abbreviated   counterparts.  All systems implementing the Meter header MUST   implement both the abbreviated and non-abbreviated forms.   Implementations SHOULD use the abbreviated forms in normal use.   The abbreviated forms of Meter directive are shown below, with the   corresponding non-abbreviated literals in the comments:       Abb-Meter = "Meter" ":" 0#abb-meter-directive       abb-meter-directive = abb-meter-request-directive                       | abb-meter-response-directive                       | abb-meter-report-directive       abb-meter-request-directive =                         "w"           ; "will-report-and-limit"                       | "x"           ; "wont-report"                       | "y"           ; "wont-limit"       abb-meter-report-directive =                       | "c" "=" 1*DIGIT "/" 1*DIGIT   ; "count"       abb-meter-response-directive =                         "u" "=" 1*DIGIT       ; "max-uses"                       | "r" "=" 1*DIGIT       ; "max-reuses"                       | "d"                   ; "do-report"                       | "e"                   ; "dont-report"                       | "t" "=" 1*DIGIT       ; "timeout"                       | "n"                   ; "wont-ask"Mogul & Leach               Standards Track                    [Page 23]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      Note: although the Abb-Meter BNF rule is defined separately from      the Meter rule, one may freely mix abbreviated and non-abbreviated      Meter directives in the same header.5.3 Counting rules      Note: please remember that hit-counts and usage-counts are      associated with individual responses, not with resources.  A cache      entry that, over its lifetime, holds more than one response is      also not a "response", in this particular sense.   Let R be a cached response, and V be the value of the Request-URI and   selecting request-headers (if any, seesection 14.43 of the HTTP/1.1   specification [4]) that would select R if contained in a request.  We   define a "use" of R as occurring when the proxy returns its stored   copy of R in a response with any of the following status codes: a 200   (OK) status; a 203 (Non-Authoritative Information) status; or a 206   (Partial Content) status when the response contains byte #0 of the   entity (seesection 5.4 for a discussion of Range requests).      Note: when a proxy forwards a client's request and receives a      response, the response that the proxy sends immediately to the      requesting client is not counted as a "use".  I.e., the reported      count is the number of times the cache entry was used, and not the      number of times that the response was used.   We define a "reuse" of R as as occurring when the proxy responds to a   request selecting R with a 304 (Not Modified) status, unless that   request is a Range request that does not specify byte #0 of the   entity.5.3.1 Counting rules for hit-metering   A proxy participating in hit-metering for a cache response R   maintains two counters, CU and CR, associated with R. When a proxy   first stores R in its cache, it sets both CU and CR to 0 (zero).   When a subsequent client request results in a "use" of R, the proxy   increments CU.  When a subsequent client request results in a "reuse"   of R, the proxy increments CR.  When a subsequent client request   selecting R (i.e., including V) includes a "count" Meter directive,   the proxy increments CU and CR using the corresponding values in the   directive.   When the proxy sends a request selecting R (i.e., including V) to the   inbound server, it includes a "count" Meter directive with the   current CU and CR as the parameter values.  If this request was   caused by the proxy's receipt of a request from a client, upon   receipt of the server's response, the proxy sets CU and CR to theMogul & Leach               Standards Track                    [Page 24]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   number of uses and reuses, respectively, that may have occurred while   the request was in progress.  (These numbers are likely, but not   certain, to be zero.)  If the proxy's request was a final HEAD-based   report, it need no longer maintain the CU and CR values, but it may   also set them to the number of intervening uses and reuses and retain   them.5.3.2 Counting rules for usage-limiting   A proxy participating in usage-limiting for a response R maintains   either or both of two counters TU and TR, as appropriate, for that   resource.  TU and TR are incremented in just the same way as CU and   CR, respectively.  However, TU is zeroed only upon receipt of a   "max-uses" Meter directive for that response (including the initial   receipt).  Similarly, TR is zeroed only upon receipt of a "max-   reuses" Meter directive for that response.   A proxy participating in usage-limiting for a response R also stores   values MU and/or MR associated with R. When it receives a response   including only a max-uses value, it sets MU to that value and MR to   infinity.  When it receives a response including only a max-reuses   value, it sets MR to that value and MU to infinity.  When it receives   a response including both max-reuses and max-reuses values, it sets   MU and MR to those values, respectively.  When it receives a   subsequent response including neither max-reuses nor max-reuses   values, it sets both MU and MR to infinity.   If a proxy participating in usage-limiting for a response R receives   a request that would cause a "use" of R, and TU >= MU, it MUST   forward the request to the server.  If it receives a request that   would cause a "reuse" of R, and TR >= MR, it MUST forward the request   to the server.  If (in either case) the proxy has already forwarded a   previous request to the server and is waiting for the response, it   should delay further handling of the new request until the response   arrives (or times out); it SHOULD NOT have two revalidation requests   pending at once that select the same response, unless these are Range   requests selecting different subranges.   There is a special case of this rule for the "max-uses" directive: if   the proxy receives a response with "max-uses=0" and does not forward   it to a requesting client, the proxy should set a flag PF associated   with R. If R is true, then when a request arrives while if TU >= MU,   if the PF flag is set, then the request need not be forwarded to the   server (provided that this is not required by other caching rules).   However, the PF flag MUST be cleared on any use of the response.Mogul & Leach               Standards Track                    [Page 25]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997      Note: the "PF" flag is so named because this feature is useful      only for caches that could issue a "prefetch" request before an      actual client request for the response.  A proxy not implementing      prefetching need not implement the PF flag.5.3.3 Equivalent algorithms are allowed   Any other algorithm that exhibits the same external behavior (i.e.,   generates exactly the same requests from the proxy to the server) as   the one in this section is explicitly allowed.      Note: in most cases, TU will be equal to CU, and TR will be      equal to CR.  The only two cases where they could differ are:         1. The proxy issues a non-conditional request for the            resource using V, while TU and/or TR are non-zero, and            the server's response includes a new "max-uses" and/or            "max-reuses" directive (thus zeroing TU and/or TR, but            not CU and CR).         2. The proxy issues a conditional request reporting the            hit-counts (and thus zeroing CU and CR, but not TU or            TR), but the server's response does not include a new            "max-uses" and/or "max-reuses" directive.      To solve the first case, the proxy has several implementation      options         - Always store TU and TR separately from CU and CR.         - Create "shadow" copies of TU and TR when this situation           arises (analogous to "copy on write").         - Generate a HEAD-based usage report when the           non-conditional request is sent (or when the           "max-uses=0" is received), causing CU and CR to be           zeroed (analogous in some ways to a "memory barrier"           instruction).      In the second case, the server implicitly has removed the      usage-limit(s) on the response (by setting MU and/or MR to      infinity), and so the fact that, say, TU is different from CU      is not significant.      Note: It may also be possible to eliminate the PF flag by      sending extra HEAD-based usage-report requests, but we      recommend against this; it is better to allocate an extra bit      per entry than to transmit extra requests.Mogul & Leach               Standards Track                    [Page 26]

RFC 2227            Hit-Metering and Usage-Limiting         October 19975.4 Counting rules: interaction with Range requests   HTTP/1.1 allows a client to request sub-ranges of a resource.  A   client might end up issuing several requests with the net effect of   receiving one copy of the resource.  For uniformity of the results   seen by origin servers, proxies need to observe a rule for counting   these references, although it is not clear that one rule generates   accurate results in every case.   The rule established in this specification is that proxies count as a   "use" or "reuse" only those Range requests that result in the return   of byte #0 of the resource.  The rationale for this rule is that in   almost every case, an end-client will retrieve the beginning of any   resource that it references at all, and that it will seldom retrieve   any portion more than once.  Therefore, this rule appears to meet the   goal of a "best-efforts" approximation.5.5 Implementation by non-caching proxies   A non-caching proxy may participate in the metering subtree; this is   strongly recommended.   A non-caching proxy (HTTP/1.1 or higher) that participates in the   metering subtree SHOULD forward Meter headers on both requests and   responses, with the appropriate Connection headers.   If a non-caching proxy forwards Meter headers, it MUST comply with   these restrictions:      1. If the proxy forwards Meter headers in responses, such a         response MUST NOT be returned to any request except the         one that elicited it.      2. Once a non-caching proxy starts forwarding Meter headers,         it should not arbitrarily stop forwarding them (or else         reports may be lost).   A proxy that caches some responses and not others, for whatever   reason, may choose to implement the Meter header as a caching proxy   for the responses that it caches, and as a non-caching proxy for the   responses that it does not cache, as long as its external behavior   with respect to any particularly response is fully consistent with   this specification.Mogul & Leach               Standards Track                    [Page 27]

RFC 2227            Hit-Metering and Usage-Limiting         October 19975.6 Implementation by cooperating caches   Several HTTP cache implementations, most notably the Harvest/Squid   cache [2], create cooperative arrangements between several caches.   If such caches use a protocol other than HTTP to communicate between   themselves, such as the Internet Cache Protocol (ICP) [12], and if   they implement the Meter header, then they MUST act to ensure that   their cooperation does not violate the intention of this   specification.   In particular, if one member of a group of cooperating caches agrees   with a server to hit-meter a particular response, and then passes   this response via a non-HTTP protocol to a second cache in the group,   the caches MUST ensure that the server which requested the metering   receives reports that appropriately account for any uses or resues   made by the second cache.  Similarly, if the first cache agreed to   usage-limit the response, the total number of uses by the group of   caches MUST be limited to the agreed-upon number.6 Examples6.1 Example of a complete set of exchanges   This example shows how the protocol is intended to be used most of   the time: for hit-metering without usage-limiting.  Entity bodies are   omitted.   A client sends request to a proxy:       GEThttp://foo.com/bar.html HTTP/1.1   The proxy forwards request to the origin server:       GET /bar.html HTTP/1.1       Host: foo.com       Connection: Meter   thus offering (implicitly) "will-report-and-limit".   The server responds to the proxy:       HTTP/1.1 200 OK       Date: Fri, 06 Dec 1996 18:44:29 GMT       Cache-control: max-age=3600       Connection: meter       Etag: "abcde"Mogul & Leach               Standards Track                    [Page 28]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   thus (implicitly) requiring "do-report" (but not requiring   usage-limiting).   The proxy responds to the client:       HTTP/1.1 200 OK       Date: Fri, 06 Dec 1996 18:44:29 GMT       Etag: "abcde"       Cache-control: max-age=3600, proxy-mustcheck       Age: 1   Since the proxy does not know if its client is an end-system, or a   proxy that doesn't do metering, it adds the "proxy-mustcheck"   directive.   Another client soon asks for the resource:       GEThttp://foo.com/bar.html HTTP/1.1   and the proxy sends the same response as it sent to the other client,   except (perhaps) for the Age value.   After an hour has passed, a third client asks for the response:       GEThttp://foo.com/bar.html HTTP/1.1   But now the response's max-age has been exceeded, so the proxy   revalidates the response with the origin server:       GET /bar.html HTTP/1.1       If-None-Match: "abcde"       Host: foo.com       Connection: Meter       Meter: count=1/0   thus simultaneously fulfilling its duties to validate the response   and to report the one "use" that wasn't forwarded.   The origin server responds:       HTTP/1.1 304 Not Modified       Date: Fri, 06 Dec 1996 19:44:29 GMT       Cache-control: max-age=3600       Etag: "abcde"   so the proxy can use the original response to reply to the new   client; the proxy also zeros the use-count it associates with that   response.Mogul & Leach               Standards Track                    [Page 29]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   Another client soon asks for the resource:       GEThttp://foo.com/bar.html HTTP/1.1   and the proxy sends the appropriate response.   After another few hours, the proxy decides to remove the cache entry.   When it does so, it sends to the origin server:       HEAD /bar.html HTTP/1.1       If-None-Match: "abcde"       Host: foo.com       Connection: Meter       Meter: count=1/0   reporting that one more use of the response was satisfied from the   cache.6.2 Protecting against HTTP/1.0 proxies   An origin server that does not want HTTP/1.0 caches to store the   response at all, and is willing to have HTTP/1.0 end-system clients   generate excess GETs (which will be forwarded by HTTP/1.0 proxies)   could send this for its reply:       HTTP/1.1 200 OK       Cache-control: max-age=3600       Connection: meter       Etag: "abcde"       Expires: Sun, 06 Nov 1994 08:49:37 GMT   HTTP/1.0 caches will see the ancient Expires header, but HTTP/1.1   caches will see the max-age directive and will ignore Expires.      Note: although most major HTTP/1.0 proxy implementations observe      the Expires header, it is possible that some are in use that do      not.  Use of the Expires header to prevent caching by HTTP/1.0      proxies might not be entirely reliable.6.3 More elaborate examples   Here is a request from a proxy that is willing to hit-meter but is   not willing to usage-limit:       GET /bar.html HTTP/1.1       Host: foo.com       Connection: Meter       Meter: wont-limitMogul & Leach               Standards Track                    [Page 30]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   Here is a response from an origin server that does not want hit   counting, but does want "uses" limited to 3, and "reuses" limited to   6:       HTTP/1.1 200 OK       Cache-control: max-age=3600       Connection: meter       Etag: "abcde"       Expires: Sun, 06 Nov 1994 08:49:37 GMT       Meter: max-uses=3, max-reuses=6, dont-report   Here is the same example with abbreviated Meter directive names:       HTTP/1.1 200 OK       Cache-control: max-age=3600       Connection: meter       Etag: "abcde"       Expires: Sun, 06 Nov 1994 08:49:37 GMT       Meter:u=3,r=6,e7 Interactions with content negotiation   This section describes two aspects of the interaction between hit-   metering and "content-negotiated" resources:      1. treatment of responses carrying a Vary header (section7.1).      2. treatment of responses that use the proposed Transparent         Content Negotiation mechanism (section 7.2).7.1 Treatment of responses carrying a Vary header   Separate counts should be kept for each combination of the headers   named in the Vary header for the Request-URI (what [4] calls "the   selecting request-headers"), even if they map to the same entity-tag.   This rule has the effect of counting hits on each variant, if there   are multiple variants of a page available.      Note: This interaction between Vary and the hit-counting      directives allows the origin server a lot of flexibility in      specifying how hits should be counted.  In essence, the origin      server uses the Vary mechanism to divide the requests for a      resource into arbitrary categories, based on the request- headers.      (We will call these categories "request-patterns".) Since a proxy      keeps its hit-counts for each request-pattern, rather than for      each resource, the origin server can obtain separate statistics      for many aspects of an HTTP request.Mogul & Leach               Standards Track                    [Page 31]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   For example, if a page varied based on the value of the User-Agent   header in the requests, then hit counts would be kept for each   different flavor of browser. But it is in fact more general than   that; because multiple header combinations can map to the same   variant, it also enables the origin server to count the number of   times (e.g.) the Swahili version of a page was requested, even though   it is only available in English.   If a proxy does not support the Vary mechanism, then [4] says that it   MUST NOT cache any response that carries a Vary header, and hence   need not implement any aspect of this hit-counting or usage-limiting   design for varying resources.       Note: this also implies that if a proxy supports the Vary       mechanism but is not willing to maintain independent hit-counts       for each variant response in its cache, then it must follow at       least one of these rules:          1. It must not use the Meter header in a request to offer             to hit-meter or usage-limit responses.          2. If it does offer to hit-meter or usage-limit responses,             and then receives a response that includes both a Vary             header and a Meter header with a directive that it             cannot satisfy, then the proxy must not cache the             response.       In other words, a proxy is allowed to partially implement the       Vary mechanism with respect to hit-metering, as long as this has       no externally visible effect on its ability to comply with the       Meter specification.   This approach works for counting almost any aspect of the request   stream, without embedding any specific list of countable aspects in   the specification or proxy implementation.7.2 Interaction with Transparent Content Negotiation   [A description of the interaction between this design and the   proposed Transparent Content Negotiation (TCN) design [6] will be   made available in a later document.]8 A Note on Capturing Referrals   It is alleged that some advertisers want to pay content providers,   not by the "hit", but by the "nibble" -- the number of people who   actually click on the ad to get more information.Mogul & Leach               Standards Track                    [Page 32]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   Now, HTTP already has a mechanism for doing this: the "Referer"   header. However, perhaps it ought to be disabled for privacy reasons   -- according the HTTP/1.1 spec:       "Because the source of the link may be private information or may       reveal an otherwise private information source, it is strongly       recommended that the user be able to select whether or not the       Referer field is sent."   However, in the case of ads, the source of the link actually wants to   let the referred-to page know where the reference came from.   This does not require the addition of any extra mechanism, but rather   can use schemes that embed the referrer in the URI in a manner   similar to this:http://www.blah.com/ad-reference?from=site1   Such a URI should point to a resource (perhaps a CGI script) which   returns a 302 redirect to the real pagehttp://www.blah.com/ad-reference.html   Proxies which do not cache 302s will cause one hit on the redirection   page per use, but the real page will get cached. Proxies which do   cache 302s and report hits on the cached 302s will behave optimally.   This approach has the advantage that it works whether or not the   end-client has disabled the use of Referer.  Combined with the rest   of the hit-metering proposal in this design, this approach allows,   for example, an advertiser to know how often a reference to an   advertisement was made from a particular page.9 Alternative proposals   There might be a number of other ways of gathering demographic and   usage information; other mechanisms might respond to a different set   of needs than this proposal does.  This proposal certainly does not   preclude the proposal or deployment of other such mechanisms, and   many of them may be complementary to and compatible with the   mechanism proposed here.   There has been some speculation that statistical sampling methods   might be used to gather reasonably accurate data.  One such proposal   is to manipulate cache expiration times so that selected resources   are uncachable for carefully chosen periods, allowing servers to   accurately count accesses during those periods.  The hit-metering   mechanism proposed here is entirely complementary to that approach,Mogul & Leach               Standards Track                    [Page 33]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   since it could be used to reduce the cost of gathering those counts.   James Pitkow has written a paper comparing an earlier draft of this   hit-metering proposal with sampling approaches [9].   Phillip Hallam-Baker has proposed using a log-exchange protocol [5],   by which a server could request a proxy's logs by making an HTTP   request to the proxy.  This proposal asserts that it is "believed to   operate correctly in configurations involving multiple proxies", but   it is not clear that this is true if an outer proxy is used as a   (one-way) firewall.  The proposal also leaves a number of open   issues, such as how an origin server can be sure that all of the   proxies in the request subtree actually support log-exchange.  It is   also not clear how this proposal couples a proxy's support of log-   exchange to a server's permission to cache a response.   For general background on the topic of Web measurement standards, see   the discussion by Thomas P. Novak and Donna L. Hoffman [8].  Also see   the "Privacy and Demographics Overview" page maintained by by the   World Wide Web Consortium [10], which includes a pointer to some   tentative proposals for gathering consumer demographics (not just   counting references) [3].10 Security Considerations   Which outbound clients should a server (proxy or origin) trust to   report hit counts?  A malicious proxy could easily report a large   number of hits on some page, and thus perhaps cause a large payment   to a content provider from an advertiser.  To help avoid this   possibility, a proxy may choose to only relay usage counts received   from its outbound proxies to its inbound servers when the proxies   have authenticated themselves using Proxy-Authorization and/or they   are on a list of approved proxies.   It is not possible to enforce usage limits if a proxy is willing to   cheat (i.e., it offers to limit usage but then ignores a server's   Meter directive).   Regarding privacy:  it appears that the design in this document does   not reveal any more information about individual users than would   already be revealed by implementation of the existing HTTP/1.1   support for "Cache-control: max-age=0, proxy-revalidate" or "Cache-   control: s-maxage=0".  It may, in fact, help to conceal certain   aspects of the organizational structure on the outbound side of a   proxy.  In any case, the conflict between user requirements for   anonymity and origin server requirements for demographic information   cannot be resolved by purely technical means.Mogul & Leach               Standards Track                    [Page 34]

RFC 2227            Hit-Metering and Usage-Limiting         October 199711 Acknowledgments   We gratefully acknowledge the constructive comments received from   Anselm Baird-Smith, Ted Hardie, Koen Holtman (who suggested the   technique described insection 8), Dave Kristol, Ari Luotonen,   Patrick R. McManus, Ingrid Melve, and James Pitkow.12 References   1.  Bradner, S.,  "Key words for use in RFCs to Indicate Requirement       Levels",BCP 14,RFC 2119, March 1997.   2.  Anwat Chankhunthod, Peter B. Danzig, Chuck Neerdaels, Michael       F. Schwartz, and Kurt J. Worrell.  A Hierarchical Internet Object       Cache.  Proc. 1996 USENIX Technical Conf., San Diego, January,       1996, pp. 153-163.   3.  Daniel W. Connolly.  Proposals for Gathering Consumer       Demographics.http://www.w3.org/pub/WWW/Demographics/Proposals.html.   4.  Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T.       Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1,"RFC 2068,       January, 1997.   5.  Phillip M. Hallam-Baker.  Notification for Proxy Caches.  W3C       Working Draft WD-proxy-960221, World Wide Web Consortium,       February, 1996.http://www.w3.org/pub/WWW/TR/WD-proxy.html.   6.  Holtman, K., and A. Mutz, "Transparent Content Negotiation in       HTTP", Work in Progress.   7.  Mogul, J., "Forcing HTTP/1.1 proxies to revalidate responses",       Work in Progress.   8.  Thomas P. Novak and Donna L. Hoffman.  New Metrics for New Media:       Toward the Development of Web Measurement Standards.  This is a       draft paper, currently available athttp://www2000.ogsm.vanderbilt.edu/novak/web.standards/webstand.html.       Cited by permission of the author; do not quote or cite without       permission.   9.  James Pitkow.  In search of reliable usage data on the WWW.       Proc. Sixth International World Wide Web Conference, Santa Clara,       CA, April, 1997.Mogul & Leach               Standards Track                    [Page 35]

RFC 2227            Hit-Metering and Usage-Limiting         October 1997   10. Joseph Reagle, Rohit Khare, Dan Connolly, and Tim Berners-Lee.       Privacy and Demographics Overview.http://www.w3.org/pub/WWW/Demographics/.   11. Linda Tauscher and Saul Greenberg.  Revisitation Patterns in       World Wide Web Navigation.  Research Report 96/587/07, Department       of Computer Science, University of Calgary, March, 1996.http://www.cpsc.ucalgary.ca/projects/grouplab/papers/96WebReuse/TechReport96.html.   12. Wessels, D., and K. Claffy "Internet Cache Protocol (ICP),       version 2",RFC 2186, September 1997.13 Authors' Addresses   Jeffrey C. Mogul   Western Research Laboratory   Digital Equipment Corporation   250 University Avenue   Palo Alto, California, 94305, U.S.A.   EMail: mogul@wrl.dec.com   Phone: 1 415 617 3304 (email preferred)   Paul J. Leach   Microsoft   1 Microsoft Way   Redmond, Washington, 98052, U.S.A.   EMail: paulle@microsoft.comMogul & Leach               Standards Track                    [Page 36]

RFC 2227            Hit-Metering and Usage-Limiting         October 199714 Full Copyright Statement   Copyright (C) The Internet Society (1997).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implmentation may be prepared, copied, published   andand distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Mogul & Leach               Standards Track                    [Page 37]