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Versions: 01 02 03 04 05 RFC 3040

Network Working Group                                          I. Cooper
Internet-Draft                                              Mirror Image
Expires: January 2, 2001                                        I. Melve
                                                                 UNINETT
                                                            G. Tomlinson
                                                                  Novell
                                                            July 4, 2000


             Internet Web Replication and Caching Taxonomy
                    draft-ietf-wrec-taxonomy-05.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   This Internet-Draft will expire on January 2, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

   This memo specifies standard terminology and the current taxonomy of
   web replication and caching infrastructure deployed today. It
   introduces standard concepts and protocols used today within this
   application domain. Currently deployed solutions employing these
   technologies are presented to establish a standard taxonomy. Known
   problems with caching proxies are covered in an accompanying
   document[23], and are not part of this document. This document
   presents open protocols and points to published material for each
   protocol.


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Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  4
   2.      Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1     Base Terms . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.2     First order derivative terms . . . . . . . . . . . . . . .  7
   2.3     Second order derivatives . . . . . . . . . . . . . . . . .  8
   2.4     Topological terms  . . . . . . . . . . . . . . . . . . . .  8
   2.5     Automatic use of proxies . . . . . . . . . . . . . . . . .  9
   3.      Distributed System Relationships . . . . . . . . . . . . . 11
   3.1     Replication Relationships  . . . . . . . . . . . . . . . . 11
   3.1.1   Client to Replica  . . . . . . . . . . . . . . . . . . . . 11
   3.1.2   Inter-Replica  . . . . . . . . . . . . . . . . . . . . . . 11
   3.2     Proxy Relationships  . . . . . . . . . . . . . . . . . . . 12
   3.2.1   Client to Non-Interception Proxy . . . . . . . . . . . . . 12
   3.2.2   Client to Surrogate to Origin Server . . . . . . . . . . . 12
   3.2.3   Inter-Proxy  . . . . . . . . . . . . . . . . . . . . . . . 13
   3.2.3.1 (Caching) Proxy Meshes . . . . . . . . . . . . . . . . . . 13
   3.2.3.2 (Caching) Proxy Arrays . . . . . . . . . . . . . . . . . . 14
   3.2.4   Network Element to Caching Proxy . . . . . . . . . . . . . 14
   4.      Replica Selection  . . . . . . . . . . . . . . . . . . . . 16
   4.1     Navigation Hyperlinks  . . . . . . . . . . . . . . . . . . 16
   4.2     HTTP Redirection . . . . . . . . . . . . . . . . . . . . . 16
   4.3     DNS Redirection  . . . . . . . . . . . . . . . . . . . . . 17
   5.      Inter-Replica Communication  . . . . . . . . . . . . . . . 18
   5.1     Batch Driven Replication . . . . . . . . . . . . . . . . . 18
   5.2     Demand Driven Replication  . . . . . . . . . . . . . . . . 18
   5.3     Synchronized Replication . . . . . . . . . . . . . . . . . 19
   6.      User Agent to Proxy Configuration  . . . . . . . . . . . . 20
   6.1     Manual Proxy Configuration . . . . . . . . . . . . . . . . 20
   6.2     Proxy Auto Configuration (PAC) . . . . . . . . . . . . . . 20
   6.3     Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . . 21
   6.4     Web Proxy Auto-Discovery Protocol (WPAD) . . . . . . . . . 21
   7.      Inter-Proxy Communication  . . . . . . . . . . . . . . . . 23
   7.1     Loosely coupled Inter-Proxy Communication  . . . . . . . . 23
   7.1.1   Internet Cache Protocol (ICP)  . . . . . . . . . . . . . . 23
   7.1.2   Hyper Text Caching Protocol  . . . . . . . . . . . . . . . 23
   7.1.3   Cache Digest . . . . . . . . . . . . . . . . . . . . . . . 24
   7.1.4   Cache Pre-filling  . . . . . . . . . . . . . . . . . . . . 25
   7.2     Tightly Coupled Inter-Cache Communication  . . . . . . . . 26
   7.2.1   Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . . 26
   8.      Network Element Communication  . . . . . . . . . . . . . . 27
   8.1     Web Cache Control Protocol (WCCP)  . . . . . . . . . . . . 27
   8.2     Network Element Control Protocol (NECP)  . . . . . . . . . 27
   8.3     SOCKS  . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   9.      Security Considerations  . . . . . . . . . . . . . . . . . 29
   9.1     Authentication . . . . . . . . . . . . . . . . . . . . . . 29
   9.1.1   Man in the middle attacks  . . . . . . . . . . . . . . . . 29
   9.1.2   Trusted third party  . . . . . . . . . . . . . . . . . . . 29


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   9.1.3   Authentication based on IP number  . . . . . . . . . . . . 29
   9.2     Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . 30
   9.2.1   Trusted third party  . . . . . . . . . . . . . . . . . . . 30
   9.2.2   Logs and legal implications  . . . . . . . . . . . . . . . 30
   9.3     Service security . . . . . . . . . . . . . . . . . . . . . 30
   9.3.1   Denial of service  . . . . . . . . . . . . . . . . . . . . 31
   9.3.2   Replay attack  . . . . . . . . . . . . . . . . . . . . . . 31
   9.3.3   Stupid configuration of proxies  . . . . . . . . . . . . . 31
   9.3.4   Copyrighted transient copies . . . . . . . . . . . . . . . 31
   9.3.5   Application level access . . . . . . . . . . . . . . . . . 31
   10.     Acknowledgements . . . . . . . . . . . . . . . . . . . . . 32
           References . . . . . . . . . . . . . . . . . . . . . . . . 33
           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 35
           Full Copyright Statement . . . . . . . . . . . . . . . . . 37





































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1. Introduction

   Since its introduction in 1990, the World-Wide Web has evolved from
   a simple client server model into a sophisticated distributed
   architecture. This evolution has been driven largely due to the
   scaling problems associated with exponential growth. Distinct
   paradigms and solutions have emerged to satisfy specific
   requirements.  Two core infrastructure components being employed to
   meet the demands of this growth are replication and caching. In many
   cases, there is a need for web caches and replicated services to be
   able to coexist.

   There are many protocols, both open and proprietary, employed in web
   replication and caching today.  A majority of the open protocols
   include DNS[15], Cache Digests[17][19], CARP[4], HTTP[1], ICP[5],
   PAC[2], SOCKS[14], WPAD[3], and WCCP[13]. Additional protocols are
   being planned to address emerging solution requirements.

   This memo specifies standard terminology and the taxonomy of web
   replication and caching infrastructure deployed in the Internet
   today. The principal goal of this document is to establish a common
   understanding and reference point of this application domain.

   We also expect that this document will be used in the creation of a
   standard architectural framework for efficient, reliable, and
   predictable service in a web which includes both replicas and caches.

























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2. Terminology

   The following terminology provides definitions of common terms used
   within the web replication and caching community. Base terms are
   taken, where possible, from the HTTP/1.1 specification[1] and are
   included here for reference. First- and second-order derivatives are
   constructed from these base terms to help define the relationships
   that exist within this area.

   Terms that are in common usage and which are contrary to definitions
   in RFC2616 and this document are highlighted.

2.1 Base Terms

   The majority of these terms are taken as-is from RFC2616[1], and are
   included here for reference.

   client (taken from [1])
      A program that establishes connections for the purpose of sending
      requests.

   server (taken from [1])
      An application program that accepts connections in order to
      service requests by sending back responses. Any given program may
      be capable of being both a client and a server; our use of these
      terms refers only to the role being performed by the program for
      a particular connection, rather than to the program's
      capabilities in general. Likewise, any server may act as an
      origin server, proxy, gateway, or tunnel, switching behavior
      based on the nature of each request.

   proxy (taken from [1])
      An intermediary program which acts as both a server and a client
      for the purpose of making requests on behalf of other clients.
      Requests are serviced internally or by passing them on, with
      possible translation, to other servers. A proxy MUST implement
      both the client and server requirements of this specification. A
      "transparent proxy" is a proxy that does not modify the request
      or response beyond what is required for proxy authentication and
      identification. A "non-transparent proxy" is a proxy that
      modifies the request or response in order to provide some added
      service to the user agent, such as group annotation services,
      media type transformation, protocol reduction, or anonymity
      filtering. Except where either transparent or non-transparent
      behavior is explicitly stated, the HTTP proxy requirements apply
      to both types of proxies.

   Note: The term "transparent proxy" refers to a semantically
   transparent proxy as described in [1], not what is commonly


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   understood within the caching community. We recommend that the term
   "transparent proxy" is always prefixed to avoid confusion (e.g.
   "network transparent proxy").  However, see definition of
   "interception proxy" below.

   The above condition requiring implementation of both the server and
   client requirements of HTTP/1.1 is only appropriate for a
   non-network transparent proxy.

   cache (taken from [1])
      A program's local store of response messages and the subsystem
      that controls its message storage, retrieval, and deletion. A
      cache stores cacheable responses in order to reduce the response
      time and network bandwidth consumption on future, equivalent
      requests. Any client or server may include a cache, though a
      cache cannot be used by a server that is acting as a tunnel.

   Note: The term "cache" used alone often is meant as "caching proxy".

   Note: There are additional motivations for caching, for example
   reducing server load (as a further means to reduce response time).

   cacheable (taken from [1])
      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests.
      The rules for determining the cacheability of HTTP responses are
      defined in section 13. Even if a resource is cacheable, there may
      be additional constraints on whether a cache can use the cached
      copy for a particular request.

   gateway (taken from [1])
      A server which acts as an intermediary for some other server.
      Unlike a proxy, a gateway receives requests as if it were the
      origin server for the requested resource; the requesting client
      may not be aware that it is communicating with a gateway.

   tunnel (taken from [1])
      An intermediary program which is acting as a blind relay between
      two connections. Once active, a tunnel is not considered a party
      to the HTTP communication, though the tunnel may have been
      initiated by an HTTP request. The tunnel ceases to exist when
      both ends of the relayed connections are closed.

   replication (definition from FOLDOC[22])
      Creating and maintaining a duplicate copy of a database or file
      system on a different computer, typically a server.

   inbound/outbound (taken from [1])
      Inbound and outbound refer to the request and response paths for


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      messages: "inbound" means "traveling toward the origin server",
      and "outbound" means "traveling toward the user agent".

   network element
      A network device that introduces multiple paths between source
      and destination, transparent to HTTP.

2.2 First order derivative terms

   The following terms are constructed taking the above base terms as
   foundation.

   origin server (taken from [1])
      The server on which a given resource resides or is to be created.

   user agent (taken from [1])
      The client which initiates a request. These are often browsers,
      editors, spiders (web-traversing robots), or other end user tools.

   caching proxy
      A proxy with a cache, acting as a server to clients, and a client
      to servers.

      Caching proxies are often referred to as "proxy caches" or simply
      "caches". The term "proxy" is also frequently misused when
      referring to caching proxies.

   surrogate
      A gateway co-located with an origin server, or at a different
      point in the network, delegated the authority to operate on
      behalf of, and typically working in close co-operation with, one
      or more origin servers. Responses are typically delivered from an
      internal cache.

      Surrogates may derive cache entries from the origin server or
      from another of the origin server's delegates.  In some cases a
      surrogate may tunnel such requests.

      Where close co-operation between origin servers and surrogates
      exists, this enables modifications of some protocol requirements,
      including the Cache-Control directives in [1]. Such modifications
      have yet to be fully specified.

      Devices commonly known as "reverse proxies" and "(origin) server
      accelerators" are both more properly defined as surrogates.

   reverse proxy
      See "surrogate".



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   server accelerator
      See "surrogate".

2.3 Second order derivatives

   The following terms further build on first order derivatives:

   master origin server
      An origin server on which the definitive version of a resource
      resides.

   replica origin server
      An origin server holding a replica of a resource, but which may
      act as an authoritative reference for client requests.

   content consumer
      The user or system that initiates inbound requests, through use
      of a user agent.

   browser
      A special instance of a user agent that acts as a content
      presentation device for content consumers.

2.4 Topological terms

   The following definitions are added to describe caching device
   topology:

   user agent cache
      The cache within the user agent program.

   local caching proxy
      The caching proxy to which a user agent connects.

   intermediate caching proxy
      Seen from the content consumer's view, all caches participating
      in the caching mesh that are not the user agent's local caching
      proxy.

   cache server
      A server to requests made by local and intermediate caching
      proxies, but which does not act as a proxy.

   cache array
      A cluster of caching proxies, acting logically as one service and
      partitioning the resource name space across the array. Also known
      as "diffused array" or "cache cluster".

   caching mesh


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      a loosely coupled set of co-operating proxy- and (optionally)
      caching-servers, or clusters, acting independently but sharing
      cacheable content between themselves using inter-cache
      communication protocols.

2.5 Automatic use of proxies

   Network administrators may wish to force or facilitate the use of
   proxies by clients, enabling such configuration within the network
   itself or within automatic systems in user agents, such that the
   content consumer need not be aware of any such configuration issues.

   The terms that describe such configurations are given below.

   automatic user-agent proxy configuration
      The technique of discovering the availability of one or more
      proxies and the automated configuration of the user agent to use
      them. The use of a proxy is transparent to the content consumer
      but not to the user agent. The term "automatic proxy
      configuration" is also used in this sense.

   traffic interception
      The process of using a network element to examine network traffic
      to determine whether it should be redirected.

   traffic redirection
      Redirection of client requests from a network element performing
      traffic interception to a proxy. Used to deploy (caching) proxies
      without the need to manually reconfigure individual user agents,
      or to force the use of a proxy where such use would not otherwise
      occur.

   interception proxy (a.k.a. "transparent proxy", "transparent cache")
      The term "transparent proxy" has been used within the caching
      community to describe proxies used with zero configuration within
      the user agent. Such use is somewhat transparent to user agents.
      Due to discrepancies with [1] (see definition of "proxy" above),
      and objections to the use of the word "transparent", we introduce
      the term "interception proxy" to describe proxies that receive
      redirected traffic flows from network elements performing traffic
      interception.

      Interception proxies receive inbound traffic flows through the
      process of traffic redirection. (Such proxies are deployed by
      network administrators to facilitate or require the use of
      appropriate services offered by the proxy). Problems associated
      with the deployment of interception proxies are described in the
      companion document "Known HTTP Proxy/Caching Problems"[23]. The
      use of interception proxies requires zero configuration of the


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      user agent which act as though communicating directly with an
      origin server.

















































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3. Distributed System Relationships

   This section identifies the relationships that exist in a
   distributed replication and caching environment.  Having defined
   these relationships, later sections describe the communication
   protocols used in each relationship.

3.1 Replication Relationships

   The following sections describe relationships between clients and
   replicas and between replicas themselves.

3.1.1 Client to Replica

   A client may communicate with one or more replica origin servers, as
   well as with master origin servers. (In the absence of replica
   servers the client interacts directly with the origin server as is
   the normal case.)

       ------------------     -----------------     ------------------
       | Replica Origin |     | Master Origin |     | Replica Origin |
       |     Server     |     |    Server     |     |     Server     |
       ------------------     -----------------     ------------------
                \                    |                      /
                 \                   |                     /
                  -----------------------------------------
                                     |                 Client to
                              -----------------        Replica Server
                              |     Client    |
                              -----------------

   Protocols used to enable the client to use one of the replicas can
   be found in Section 4.

3.1.2 Inter-Replica

   This is the relationship between master origin server(s) and replica
   origin servers, to replicate data sets that are accessed by clients
   in the relationship shown in Section 3.1.1.

       ------------------     -----------------     ------------------
       | Replica Origin |-----| Master Origin |-----| Replica Origin |
       |     Server     |     |    Server     |     |     Server     |
       ------------------     -----------------     ------------------

   Protocols used in this relationship can be found in Section 5.





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3.2 Proxy Relationships

   There are a variety of ways in which (caching) proxies and cache
   servers communicate with each other, and with user agents.

3.2.1 Client to Non-Interception Proxy

   A client may communicate with zero or more proxies for some or all
   requests. Where the result of communication results in no proxy
   being used, the relationship is between client and (replica) origin
   server (see Section 3.1.1).

       -----------------     -----------------     -----------------
       |     Local     |     |     Local     |     |     Local     |
       |     Proxy     |     |     Proxy     |     |     Proxy     |
       -----------------     -----------------     -----------------
                \                    |                      /
                 \                   |                     /
                  -----------------------------------------
                                     |
                              -----------------
                              |     Client    |
                              -----------------

   In addition, a user agent may interact with an additional server -
   operated on behalf of a proxy for the purpose of automatic user
   agent proxy configuration.

   Schemes and protocols used in these relationships can be found in
   Section 6.

3.2.2 Client to Surrogate to Origin Server

   A client may communicate with zero or more surrogates for requests
   intended for one or more origin servers. Where a surrogate is not
   used, the client communicates directly with an origin server.  Where
   a surrogate is used the client communicates as if with an origin
   server. The surrogate fulfills the request from its internal cache,
   or acts as a gateway or tunnel to the origin server.












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              --------------  --------------   --------------
              |   Origin   |  |   Origin   |   |   Origin   |
              |   Server   |  |   Server   |   |   Server   |
              --------------  --------------   --------------
                            \        |        /
                             \       |       /
                             -----------------
                             |   Surrogate   |
                             |               |
                             -----------------
                                     |
                                     |
                               ------------
                               |  Client  |
                               ------------

3.2.3 Inter-Proxy

   Inter-Proxy relationships exist as meshes (loosely coupled) and
   clusters (tightly coupled).

3.2.3.1 (Caching) Proxy Meshes

   Within a loosely coupled mesh of (caching) proxies, communication
   can happen at the same level between peers, and with one or more
   parents.

                        ---------------------  ---------------------
             -----------|    Intermediate   |  |    Intermediate   |
             |          | Caching Proxy (D) |  | Caching Proxy (E) |
             |(peer)    ---------------------  ---------------------
       --------------             | (parent)       / (parent)
       |   Cache    |             |         ------/
       | Server (C) |             |        /
       --------------             |       /
      (peer) |            -----------------       ---------------------
             -------------| Local Caching |-------|    Intermediate   |
                          |   Proxy (A)   | (peer)| Caching Proxy (B) |
                          -----------------       ---------------------
                                  |
                                  |
                              ----------
                              | Client |
                              ----------

   Client included for illustration purposes only

   An inbound request may be routed to one of a number of intermediate
   (caching) proxies based on a determination of whether that parent is


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   better suited to resolving the request.

   For example, in the above figure, Cache Server C and Intermediate
   Caching Proxy B are peers of the Local Caching Proxy A, and may only
   be used when the resource requested by A already exists on either B
   or C. Intermediate Caching Proxies D & E are parents of A, and it is
   A's choice of which to use to resolve a particular request.

   The relationship between A & B only makes sense in a caching
   environment, while the relationships between A & D and A & E are
   also appropriate where D or E are non-caching proxies.

   Protocols used in these relationships can be found in Section 7.1.

3.2.3.2 (Caching) Proxy Arrays

   Where a user agent may have a relationship with a proxy, it is
   possible that it may instead have a relationship with an array of
   proxies arranged in a tightly coupled mesh.

                              ----------------------
                         ----------------------    |
                     ---------------------    |    |
                     |  (Caching) Proxy  |    |-----
                     |      Array        |----- ^ ^
                     --------------------- ^ ^  | |
                         ^            ^    | |--- |
                         |            |-----      |
                         --------------------------

   Protocols used in this relationship can be found in Section 7.2.

3.2.4 Network Element to Caching Proxy

   A network element performing traffic interception may choose to
   redirect requests from a client to a specific proxy within an array.
   (It may also choose not to redirect the traffic, in which case the
   relationship is between client and (replica) origin server, see
   Section 3.1.1.)












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       -----------------     -----------------     -----------------
       | Caching Proxy |     | Caching Proxy |     | Caching Proxy |
       |     Array     |     |     Array     |     |     Array     |
       -----------------     -----------------     -----------------
                 \                   |                     /
                  -----------------------------------------
                                     |
                               --------------
                               |  Network   |
                               |  Element   |
                               --------------
                                     |
                                    ///
                                     |
                                ------------
                                |  Client  |
                                ------------

   The interception proxy may be directly in-line of the flow of
   traffic - in which case the intercepting network element and
   interception proxy form parts of the same hardware system - or may
   be out-of-path, requiring the intercepting network element to
   redirect traffic to another network segment. In this latter case,
   communication protocols enable the intercepting network element to
   stop and start redirecting traffic when the interception proxy
   becomes (un)available. Details of these protocols can be found in
   Section 8.
























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4. Replica Selection

   This section describes the schemes and protocols used in the
   cooperation and communication between client and replica origin web
   servers. The ideal situation is to discover an optimal replica
   origin server for clients to communicate with. Optimality is a
   policy based decision, often based upon proximity, but may be based
   on other criteria such as load.

4.1 Navigation Hyperlinks

   Authoritative reference:
      This memo.

   Description:
      The simplest of client to replica communication mechanisms.  This
      utilizes hyperlink URIs embedded in web pages that point to the
      individual replica origin servers. The content consumer manually
      selects the link of the replica origin server they wish to use.

   Security:
      Relies on the protocol security associated with the appropriate
      URI scheme.

   Deployment:
      Probably the most commonly deployed client to replica
      communication mechanism.  Ubiquitous interoperability with humans.

   Submitter:
      Document editors.

4.2 HTTP Redirection

   Authoritative reference:
      This memo.

   Description:
      A simple and commonly used mechanism to connect clients with
      replica origin servers is to use HTTP redirection. Clients are
      redirected to an optimal replica origin server via the use of the
      HTTP[1] protocol response codes, e.g. 302 "Found", or 307
      "Temporary Redirect". A client establishes HTTP communication
      with one of the replica origin servers. The initially contacted
      replica origin server can then either choose to accept the
      service or redirect the client again. Refer to section 10.3 in
      HTTP/1.1[1] for information on HTTP response codes.

   Security:
      Relies entirely upon HTTP security.


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   Deployment:
      Observed at a number of large web sites.  Extent of usage in the
      Internet is unknown.

   Submitter:
      Document editors.

4.3 DNS Redirection

   Authoritative reference:

      *  RFC1794 DNS Support for Load Balancing Proximity[15]

      *  This memo

   Description:
      The Domain Name Service (DNS) provides a more sophisticated
      client to replica communication mechanism. This is accomplished
      by DNS servers that sort resolved IP addresses based upon quality
      of service policies. When a client resolves the name of an origin
      server, the enhanced DNS server sorts the available IP addresses
      of the replica origin servers starting with the most optimal
      replica and ending with the least optimal replica.

   Security:
      Relies entirely upon DNS security, and other protocols that may
      be used in determining the sort order.

   Deployment:
      Observed at a number of large web sites and large ISP web hosted
      services.  Extent of usage in the Internet is unknown, but is
      believed to be increasing.

   Submitter:
      Document editors.
















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5. Inter-Replica Communication

   This section describes the cooperation and communication between
   master- and replica- origin servers. Used in replicating data sets
   between origin servers.

5.1 Batch Driven Replication

   Authoritative reference:
      This memo.

   Description:
      The replica origin server to be updated initiates communication
      with a master origin server. The communication is established at
      intervals based upon queued transactions which are scheduled for
      deferred processing. The scheduling mechanism policies vary, but
      generally are re-occurring at a specified time. Once
      communication is established, data sets are copied to the
      initiating replica origin server.

   Security:
      Relies upon the protocol being used to transfer the data set.
      FTP[10] and RDIST are the most common protocols observed.

   Deployment:
      Very common for synchronization of mirror sites in the Internet.

   Submitter:
      Document editors.

5.2 Demand Driven Replication

   Authoritative reference:
      This memo.

   Description:
      Replica origin servers acquire content as needed due to client
      demand.  When a client requests a resource that is not in the
      data set of the replica origin server/surrogate, an attempt is
      made to resolve the request by acquiring the resource from the
      master origin server, returning it to the requesting client.

   Security:
      Relies upon the protocol being used to transfer the resources.
      FTP[10], Gopher[11], HTTP[1] and ICP[5] are the most common
      protocols observed.

   Deployment:
      Observed at several large web sites. Extent of usage in the


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      Internet is unknown.

   Submitter:
      Document editors.

5.3 Synchronized Replication

   Authoritative reference:
      This memo.

      Ed note: there is no IETF protocol specified at this time. The
         editors are aware of at least two open source protocols, AFS
         and CODA, along with one expired IETF draft
         <draft-leach-cifs-v1-spec-01.txt> and one proprietary protocol
         Novell NRS; none of which can be considered an authoritative
         reference.

   Description:
      Replicated origin servers cooperate using synchronized strategies
      and specialized replica protocols to keep the replica data sets
      coherent. Synchronization strategies range from tightly coherent
      (a few minutes) to loosely coherent (a few or more hours).
      Updates occur between replicas based upon the synchronization
      time constraints of the coherency model employed and are
      generally in the form of deltas only.

   Security:
      All of the known protocols utilize strong cryptographic key
      exchange methods, which are either based upon the Kerberos shared
      secret model or the public/private key RSA model.

   Deployment:
      Observed at a few sites, primarily at university campuses.

   Submitter:
      Document editors.















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6. User Agent to Proxy Configuration

   This section describes the configuration, cooperation and
   communication between user agents and proxies.

6.1 Manual Proxy Configuration

   Authoritative reference:
      This memo.

   Description:
      Each user must configure her user agent by supplying information
      pertaining to proxied protocols and local policies.

   Security:
      The potential for doing wrong is high; each user individually
      sets preferences.

   Deployment:
      Widely deployed, used in all current browsers. Most browsers also
      support additional options.

   Submitter:
      Document editors.

6.2 Proxy Auto Configuration (PAC)

   Authoritative reference:
      No RFC, no Internet-Draft; Navigator Proxy Auto-Config File
      Format[2].

   Description:
      A JavaScript script retrieved from a web server is executed for
      each URL accessed to determine the appropriate proxy (if any) to
      be used to access the resource. User agents must be configured to
      request this script upon startup. There is no bootstrap
      mechanism, manual configuration is necessary.

      Despite manual configuration, the process of proxy configuration
      is simplified by centralizing it within a script at a single
      location.

   Security:
      Common policy per organization possible but still requires
      initial manual configuration. PAC is better than "manual proxy
      configuration" since PAC administrators may update the proxy
      configuration without further user intervention.

      Interoperability of PAC files is not high, since different


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      browsers have slightly different interpretations of the same
      script, possibly leading to undesired effects.

   Deployment:
      Implemented in Netscape Navigator and Microsoft Internet Explorer.

   Submitter:
      Document editors.

6.3 Cache Array Routing Protocol (CARP) v1.0

   Authoritative reference:
      Expired Internet-Draft: draft-vinod-carp-v1-03.txt[4]

      Note: Reference kept since there is known implementation.

   Description:
      User agents may use CARP directly as a hash function based proxy
      selection mechanism. They need to be configured with the location
      of the cluster information.

   Security:
      Security considerations are not covered in the specification
      drafts.

   Deployment:
      Implemented in Microsoft Proxy Server, Squid. Implemented in user
      agents via PAC scripts.

   Submitter:
      Document editors.

6.4 Web Proxy Auto-Discovery Protocol (WPAD)

   Authoritative reference:
      Expired Internet-Draft: draft-ietf-wrec-wpad-01.txt[3]

   Description:
      WPAD uses a collection of pre-existing Internet resource
      discovery mechanisms to perform web proxy auto-discovery.

      The only goal of WPAD is to locate the PAC URL[2]. WPAD does not
      specify which proxies will be used. WPAD supplies the PAC URL,
      and the PAC script then operates as defined above to choose
      proxies per resource request.

      The WPAD protocol specifies the following:

      *  how to use each mechanism for the specific purpose of web


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         proxy auto-discovery

      *  the order in which the mechanisms should be performed

      *  the minimal set of mechanisms which must be attempted by a
         WPAD compliant user agent

      The resource discovery mechanisms utilized by WPAD are as
      follows:

      *  Dynamic Host Configuration Protocol DHCP

      *  Service Location Protocol SLP

      *  "Well Known Aliases" using DNS A records

      *  DNS SRV records

      *  "service: URLs" in DNS TXT records

   Security:
      Relies upon DNS and HTTP security.

   Deployment:
      Implemented in user agents and caching proxy servers. More than
      two independent implementations.

   Submitter:
      Josh Cohen, Microsoft, joshco@microsoft.com






















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7. Inter-Proxy Communication

7.1 Loosely coupled Inter-Proxy Communication

   This section describes the cooperation and communication between
   caching proxies.

7.1.1 Internet Cache Protocol (ICP)

   Authoritative reference:
      RFC2186 Internet Cache Protocol (ICP), version 2[5]

   Description:
      ICP is used by proxies to query other (caching) proxies about web
      resources, to see if the requested resource is present on the
      other system.

      ICP uses UDP. Since UDP is an uncorrected network transport
      protocol, an estimate of network congestion and availability may
      be calculated by ICP loss. This rudimentary loss measurement
      provides, together with round trip times, a load balancing method
      for caches.

   Security:
      See RFC2187[6]

      ICP does not convey information about HTTP headers associated
      with resources. HTTP headers may include access control and cache
      directives. Since proxies ask for the availability of resources,
      and subsequently retrieve them using HTTP, false cache hits may
      occur (object present in cache, but not accessible to a sibling
      is one example).

      ICP suffers from all the security problems of UDP.

   Deployment:
      Widely deployed. Most current caching proxy implementations
      support ICP in some form.

   Submitter:
      Document editors.

   See also Internet-Draft draft-lovric-icp-ext-02.txt[7], ICP
   development Web page[8], ICP1.4 specification[9].

7.1.2 Hyper Text Caching Protocol

   Authoritative reference:
      RFC2756 Hyper Text Caching Protocol (HTCP/0.0)[18]


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   Description:
      HTCP is a protocol for discovering HTTP caching proxies and
      cached data, managing sets of HTTP caching proxies, and
      monitoring cache activity.

      HTCP requests include HTTP header material, while ICPv2 does not,
      enabling HTCP replies to more accurately describe the behaviour
      that would occur as a result of a subsequent HTTP request for the
      same resource.

   Security:
      Optionally uses HMAC-MD5[20] shared secret authentication.
      Protocol is subject to attack if authentication is not used.

   Deployment:
      HTCP is implemented in Squid[24] and the Web Gateway
      Interceptor[25].

   Submitter:
      Document editors.

7.1.3 Cache Digest

   Authoritative reference:

      *  No RFC, no Internet-Draft; Cache Digest specification -
         version 5[17]

      *  Summary Cache[19](see note)

   Description:
      Cache Digests are a response to the problems of latency and
      congestion associated with previous inter-cache communication
      mechanisms such as the Internet Cache Protocol (ICP)[5] and the
      Hyper Text Cache Protocol[18]. Unlike these protocols, Cache
      Digests support peering between caching proxies and cache servers
      without a request-response exchange taking place for each inbound
      request. Instead, a summary of the contents in cache (the Digest)
      is fetched by other systems that peer with it. Using Cache
      Digests it is possible to determine with a relatively high degree
      of accuracy whether a given resource is cached by a particular
      system.

      Cache Digests are both an exchange protocol and a data format [17]

   Security:
      If the contents of a Digest are sensitive, they should be
      protected. Any methods which would normally be applied to secure
      an HTTP connection can be applied to Cache Digests.


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      A 'Trojan horse' attack is currently possible in a mesh: System A
      A can build a fake peer Digest for system B and serve it to B's
      peers if requested. This way A can direct traffic toward/from B.
      The impact of this problem is minimized by the 'pull' model of
      transferring Cache Digests from one system to another.

      Cache Digests provide knowledge about peer cache content on a URL
      level. Hence, they do not dictate a particular level of policy
      management and can be used to implement various policies on any
      level (user, organization, etc.).

   Deployment:
      Cache Digests are supported in Squid.

      Cache Meshes:

      *  NLANR Mesh[26]

      *  TF-CACHE mesh[27] (European Academic networks)

   Submitter:
      Alex Rousskov, NLANR, rousskov@nlanr.net for [17]
      Pei Cao for [19]

   Note: The technology of Summary Cache[19]is patent pending by the
   University of Wisconsin-Madison.

7.1.4 Cache Pre-filling

   Authoritative reference:
      Internet-Draft: draft-lovric-francetelecom-satellites-01.txt[16]

   Description:
      Cache pre-filling is a push-caching implementation. It is
      particularly well adapted to IP-multicast networks because it
      allows preselected resources to be simultaneously inserted into
      caches within the targeted multicast group.  Different
      implementations of cache pre-filling already exist, especially in
      satellite contexts.  However, there is still no standard for this
      kind of push-caching and vendors propose solutions either based
      on dedicated equipment or public domain caches extended with a
      pre-filling module.

   Security:
      Relies on the inter-cache protocols being employed.

   Deployment:
      Observed in two commercial content distribution service providers.

   Submitter:
      Ivan Lovric, France Telecom, ivan.lovric@cnet.francetelecom.fr


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7.2 Tightly Coupled Inter-Cache Communication

7.2.1 Cache Array Routing Protocol (CARP) v1.0

   Also see Section 6.3

   Authoritative reference:
      Expired Internet-Draft: draft-vinod-carp-v1-03.txt[4]

      Note: Reference kept since there is known deployment.

   Description:
      CARP is a hashing function for dividing URL-space among a cluster
      of proxies. Included in CARP is the definition of a Proxy Array
      Membership Table, and ways to download this information.

      A user agent which implements CARP v1.0 can allocate and
      intelligently route requests for the URLs to any member of the
      Proxy Array. Due to the resulting sorting of requests through
      these proxies, duplication of cache contents is eliminated and
      global cache hit rates may be improved.

   Security:
      Security considerations are not covered in the specification
      drafts.

   Deployment:
      Implemented in caching proxy servers. More than two independent
      implementations.

   Submitter:
      Document editors.



















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8. Network Element Communication

   This section describes the cooperation and communication between
   proxies and network elements. Examples of such network elements
   include routers and switches. Generally used for deploying
   interception proxies and/or diffused arrays.

8.1 Web Cache Control Protocol (WCCP)

   Authoritative reference:
      Expired Internet-Draft: draft-ietf-wrec-web-pro-00.txt[13]

   Description:
      WCCP V1 runs between a router functioning as a redirecting
      network element and out-of-path interception proxies. The
      protocol allows one or more proxies to register with a single
      router to receive redirected traffic. It also allows one of the
      proxies, the designated proxy, to dictate to the router how
      redirected traffic is distributed across the array.

   Security:
      WCCP V1 has no security features.

   Deployment:
      Network elements: WCCP V1 is deployed on a wide range of Cisco
      routers.
      Caching proxies: WCCP V1 is deployed on a number of vendors'
      caching proxies.

   Submitter:
      David Forster, CISCO, dforster@cisco.com

8.2 Network Element Control Protocol (NECP)

   Authoritative reference:
      draft-cerpa-necp-02.txt[21]

   Description:
      NECP provides methods for network elements to learn about server
      capabilities, availability, and hints as to which flows can and
      cannot be serviced. This allows network elements to perform load
      balancing across a farm of servers, redirection to interception
      proxies, and cut-through of flows that cannot be served by the
      farm.

   Security:
      Optionally uses HMAC-SHA-1[20] shared secret authentication along
      with complex sequence numbers to provide moderately strong
      security. Protocol is subject to attack if authentication is not


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      used.

   Deployment:
      NECP is a new protocol being implemented by more than two network
      element vendors and by more than two caching proxy vendors. It is
      anticipated to be broadly deployed in the Internet during the
      year 2000.

   Submitter:
      Gary Tomlinson, Novell, garyt@novell.com

8.3 SOCKS

   Authoritative reference:
      RFC1928 SOCKS Protocol Version 5[14]

   Description:
      SOCKS is primarily used as a caching proxy to firewall protocol.
      Although firewalls don't conform to the narrowly defined network
      element definition above, they are a integral part of the network
      infrastructure.  When used in conjunction with a firewall, SOCKS
      provides a authenticated tunnel between the caching proxy and the
      firewall.

   Security:
      An extensive framework provides for multiple authentication
      methods.  Currently, SSL, CHAP, DES, 3DES are known to be
      available.

   Deployment:
      SOCKS is been widely deployed in the Internet.

   Submitter:
      Document editors.

















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9. Security Considerations

   This document provides a taxonomy for web caching and replication.
   Recommended practice, architecture and protocols are not described
   in detail.

   By definition, replication and caching involve the copying of
   resources.  There are legal implications of making and keeping
   transient or permanent copies; these are not covered here.

   Information on security of each protocol referred to by this memo is
   provided in the preceding sections, and in their accompanying
   documentation. HTTP security is discussed in section 15 of
   RFC2616[1], the HTTP/1.1 specification, and to a lesser extent in
   RFC1945[12], the HTTP/1.0 specification. RFC2616 contains security
   considerations for HTTP proxies.

   Caching proxies have the same security issues as other application
   level proxies. Application level proxies are not covered in these
   security considerations. IP number based authentication is
   problematic when a proxy is involved in the communications. Details
   are not discussed here.

9.1 Authentication

   Requests for web resources, and responses to such requests, may be
   directed to replicas and/or may flow through intermediate proxies.
   The integrity of communication needs to be preserved to ensure
   protection from both loss of access and from unintended change.

9.1.1 Man in the middle attacks

   HTTP proxies are men-in-the-middle, the perfect place for a
   man-in-the-middle-attack.  A discussion of this is found in section
   15 of RFC2616[1].

9.1.2 Trusted third party

   A proxy must either be trusted to act on behalf of the origin server
   and/or client, or it must act as a tunnel. When presenting cached
   objects to clients, the clients need to trust the caching proxy to
   act on behalf on the origin server.

   A replica may get accreditation from the origin server.

9.1.3 Authentication based on IP number

   Authentication based on the client's IP number is problematic when
   connecting through a proxy, since the authenticating device only has


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   access to the proxy's IP number. One (problematic) solution to this
   is for the proxy to spoof the client's IP number for inbound
   requests.

   Authentication based on IP number assumes that the end-to-end
   properties of the Internet are preserved. This is typically not the
   case for environments containing interception proxies.

9.2 Privacy

9.2.1 Trusted third party

   When using a replication service, one must trust both the replica
   origin server and the replica selection system.

   Redirection of traffic - either by automated replica selection
   methods, or within proxies - may introduce third parties the end
   user and/or origin server must to trust. In the case of interception
   proxies, such third parties are often unknown to both end points of
   the communication. Unknown third parties may have security
   implications.

   Both proxies and replica selection services may have access to
   aggregated access information. A proxy typically knows about
   accesses by each client using it, information that is more sensitive
   than the information held by a single origin server.

9.2.2 Logs and legal implications

   Logs from proxies should be kept secure, since they provide
   information about users and their patterns of behaviour.  A proxy's
   log is even more sensitive than a web server log, as every request
   from the user population goes through the proxy. Logs from replica
   origin servers may need to be amalgamated to get aggregated
   statistics from a service, and transporting logs across borders may
   have legal implications.  Log handling is restricted by law in some
   countries.

   Requirements for object security and privacy are the same in a web
   replication and caching system as it is in the Internet at large.
   The only reliable solution is strong cryptography.  End-to-end
   encryption frequently makes resources uncacheable, as in the case of
   SSL encrypted web sessions.

9.3 Service security






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9.3.1 Denial of service

   Any redirection of traffic is susceptible to denial of service
   attacks at the redirect point, and both proxies and replica
   selection services may redirect traffic.

   By attacking a proxy, access to all servers may be denied for a
   large set of clients.

   It has been argued that introduction of an interception proxy is a
   denial of service attack, since the end-to-end nature of the
   Internet is destroyed without the content consumer's knowledge.

9.3.2 Replay attack

   A caching proxy is by definition a replay attack.

9.3.3 Stupid configuration of proxies

   It is quite easy to have a stupid configuration which will harm
   service for content consumers. This is the most common security
   problem with proxies.

9.3.4 Copyrighted transient copies

   The legislative forces of the world are considering the question of
   transient copies, like those kept in replication and caching system,
   being legal. The legal implications of replication and caching are
   subject to local law.

   Caching proxies need to preserve the protocol output, including
   headers. Replication services need to preserve the source of the
   objects.

9.3.5 Application level access

   Caching proxies are application level components in the traffic flow
   path, and may give intruders access to information that was
   previously only available at the network level in a proxy-free
   world. Some network level equipment may have required physical
   access to get sensitive information.  Introduction of application
   level components may require additional system security.









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10. Acknowledgements

   The editors would like to thank the following for their assistance:
   David Forster, Alex Rousskov, Josh Cohen, John Martin, John Dilley,
   Ivan Lovric, Joe Touch, Henrik Nordstrom, Patrick McManus, Duane
   Wessels, Wojtek Sylwestrzak, Ted Hardie, Misha Rabinovich, Larry
   Masinter, Keith Moore, Roy Fielding, Patrick Faltstrom, Hilarie
   Orman, Mark Nottingham and Oskar Batuner.











































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References

   [1]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
         L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol
         -- HTTP/1.1", RFC 2616, June 1999,
         <URL:http://www.ietf.org/rfc/rfc2616.txt>.

   [2]   Netscape, Inc., "Navigator Proxy Auto-Config File Format",
         March 1996,
         <URL:http://www.netscape.com/eng/mozilla/2.0/relnotes/demo/prox
         y-live.html>.

   [3]   Gauthier, P., Cohen, J., Dunsmuir, M. and C. Perkins, "The Web
         Proxy Auto-Discovery Protocol", draft-ietf-wrec-wpad-01.txt
         (work in progress), July 1999,
         <URL:http://www.wrec.org/Drafts/draft-ietf-wrec-wpad-01.txt>.

   [4]   Valloppillil, V. and K.W. Ross, "Cache Array Routing
         Protocol", draft-vinod-carp-v1-03.txt (work in progress),
         February 1998,
         <URL:http://www.wrec.org/Drafts/draft-vinod-carp-v1-03.txt>.

   [5]   Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP),
         Version 2", RFC 2186, September 1997,
         <URL:http://www.ietf.org/rfc/rfc2186.txt>.

   [6]   Wessels, D. and K. Claffy, "Application of Internet Cache
         Protocol (ICP), Version 2", RFC 2187, September 1997,
         <URL:http://www.ietf.org/rfc/rfc2187.txt>.

   [7]   Lovric, I., "Internet Cache Protocol Extension",
         draft-lovric-icp-ext-02.txt (work in progress), October 1999,
         <URL:http://www.wrec.org/Drafts/draft-lovric-icp-ext-02.txt>.

   [8]   Wessels, D., "ICP Home Page", July 1999,
         <URL:http://ircache.nlanr.net/Cache/ICP/>.

   [9]   University of Southern California and University of
         Colorado-Boulder, "Internet Cache Protocol Specification 1.4",
         September 1994,
         <URL:http://excalibur.usc.edu/icpdoc/icp.html>.

   [10]  Postel, J. and J.K. Reynolds, "File Transfer Protocol (FTP)",
         RFC 959, Oct 1985,
         <URL:http://www.ietf.org/rfc/rfc0959.txt>.

   [11]  Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
         Torrey, D. and B. Alberti, "The Internet Gopher Protocol", RFC
         1436, Mar 1993,


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         <URL:http://www.ietf.org/rfc/rfc1436.txt>.

   [12]  Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext
         Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996,
         <URL:http://www.ietf.org/rfc/rfc1945.txt>.

   [13]  Cieslak, M. and D. Forster, "Cisco Web Cache Control Protocol
         V1.0", draft-ietf-wrec-web-pro-00.txt (work in progress), June
         1999,
         <URL:http://www.wrec.org/Drafts/draft-ietf-wrec-web-pro-00.txt>.

   [14]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L.
         Jones, "SOCKS Protocol Version 5", RFC 1928, March 1996,
         <URL:http://www.ietf.org/rfc/rfc1928.txt>.

   [15]  Brisco, T., "DNS Support for Load Balancing", RFC 1794, April
         1995,
         <URL:http://www.ietf.org/rfc/rfc1794.txt>.

   [16]  Goutard, C., Lovric, I. and E. Maschio-Esposito, "Pre-filling
         a cache - A satellite overview",
         draft-lovric-francetelecom-satellites-01.txt (work in
         progress), February 2000,
         <URL:http://www.ietf.org/internet-drafts/draft-lovric-francetel
         ecom-satellites-01.txt>.

   [17]  Hamilton, M., Rousskov, A. and D. Wessels, "Cache Digest
         specification - version 5", December 1998,
         <URL:http://www.squid-cache.org/CacheDigest/cache-digest-v5.txt
         >.

   [18]  Vixie, P. and D. Wessels, "Hyper Text Caching Protocol
         (HTCP/0.0)", RFC 2756, January 2000,
         <URL:http://www.ietf.org/rfc/rfc2756.txt>.

   [19]  Fan, L., Cao, P., Almeida, J. and A. Broder, "Summary Cache: A
         Scalable Wide-Area Web Cache Sharing Protocol", Proceedings of
         ACM SIGCOMM'98 pp. 254-265, September 1998,
         <URL:http://www.cs.wisc.edu/~cao/papers/summarycache.html>.

   [20]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997,
         <URL:http://www.ietf.org/rfc/rfc2104.txt>.

   [21]  Cerpa, A., Elson, J., Beheshti, H., Chankhunthod, A., Danzig,
         P., Jalan, R., Neerdaels, C., Shroeder, T. and G. Tomlinson,
         "NECP: The Network Element Control Protocol",
         draft-cerpa-necp-02.txt (work in progress), February 2000,
         <URL:http://www.ietf.org/internet-drafts/draft-cerpa-necp-02.tx


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         t>.

   [22]  FOLDOC, "Free Online Dictionary of Computing: Replication",
         December 1997,
         <URL:http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?replication>.

   [23]  Cooper, I. and J. Dilley, "Known HTTP Proxy/Caching Problems",
         draft-ietf-wrec-known-prob-02.txt (work in progress), July
         2000,
         <URL:http://www.ietf.org/internet-drafts/draft-ietf-wrec-known-
         prob-02.txt>.

   [24]  <URL:http://www.squid-cache.org/>

   [25]  <URL:http://www.vix.com/vix/wgi.html>

   [26]  <URL:http://ircache.nlanr.net/Cache/>

   [27]  <URL:http://www.terena.nl/task-force/tf-cache/>


Authors' Addresses

   Ian Cooper
   Mirror Image Internet, Inc.
   49 Dragon Court
   Woburn, MA  01801
   USA

   Phone: +1 781 376 1109
   EMail: ian.cooper@mirror-image.com


   Ingrid Melve
   UNINETT
   Tempeveien 22
   Trondheim  N-7465
   Norway

   Phone: +47 73 55 79 07
   EMail: Ingrid.Melve@uninett.no










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   Gary Tomlinson
   Novell Inc.
   122 East 1700 South
   Provo, Utah  84606
   USA

   Phone: +1 801 861 7021
   EMail: garyt@novell.com











































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Full Copyright Statement

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