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

Versions: 01 02 03 04 05 rfc3040                                        
Network Working Group                                           I. Melve
Internet-Draft                                                   UNINETT
Expires: April 21, 2000                                     G. Tomlinson
                                                                  Novell
                                                               I. Cooper
                                                            Mirror Image
                                                        October 22, 1999


             Internet Web Replication and Caching Taxonomy
                    draft-ietf-wrec-taxonomy-02.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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   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
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   To view the entire list of Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 21, 2000.

Copyright Notice

   Copyright (C) The Internet Society (1999). 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 uses today within this
   application domain. Currently deployed solutions employing this
   technologies are presented to establish a standard taxonomy.
   Research issues and HTTP proxy caching known problems are covered in
   two accompanying document, and are not part of this document. This
   document presents open protocols and points to published RFCs 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 . . . . . . . . . . . . . . . . .  7
   2.4     Topological terms  . . . . . . . . . . . . . . . . . . . .  8
   2.5     Automatic use of proxies . . . . . . . . . . . . . . . . .  8
   3.      Distributed System Relationships . . . . . . . . . . . . . 10
   3.1     Replication Relationships  . . . . . . . . . . . . . . . . 10
   3.1.1   Client to Replica  . . . . . . . . . . . . . . . . . . . . 10
   3.1.2   Inter-Replica  . . . . . . . . . . . . . . . . . . . . . . 10
   3.2     Proxy Relationships  . . . . . . . . . . . . . . . . . . . 11
   3.2.1   Client to Non-Network Transparent Proxy  . . . . . . . . . 11
   3.2.2   Surrogate to Origin Server . . . . . . . . . . . . . . . . 11
   3.2.3   Inter-Proxy  . . . . . . . . . . . . . . . . . . . . . . . 12
   3.2.3.1 (Caching) Proxy Meshes . . . . . . . . . . . . . . . . . . 12
   3.2.3.2 (Caching) Proxy Clusters . . . . . . . . . . . . . . . . . 13
   3.2.4   Network Element to Caching Proxy . . . . . . . . . . . . . 13
   4.      Client to Replica Communication  . . . . . . . . . . . . . 15
   4.1     Navigation Hyperlinks  . . . . . . . . . . . . . . . . . . 15
   4.2     URL Redirection  . . . . . . . . . . . . . . . . . . . . . 15
   4.3     DNS Redirection  . . . . . . . . . . . . . . . . . . . . . 16
   5.      Inter-Replica Communication  . . . . . . . . . . . . . . . 17
   5.1     Batch Driven Replication . . . . . . . . . . . . . . . . . 17
   5.2     Demand Driven Replication  . . . . . . . . . . . . . . . . 17
   5.3     Synchronized Replication . . . . . . . . . . . . . . . . . 18
   6.      Client to Proxy Configuration  . . . . . . . . . . . . . . 19
   6.1     Manual Proxy Configuration . . . . . . . . . . . . . . . . 19
   6.2     Proxy Auto Configuration (PAC) . . . . . . . . . . . . . . 19
   6.3     Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . . 20
   6.4     Web Proxy Auto-Discovery Protocol (WPAD) . . . . . . . . . 20
   7.      Inter-Proxy Communication  . . . . . . . . . . . . . . . . 22
   7.1     Loosely coupled Inter-Proxy Communication  . . . . . . . . 22
   7.1.1   Internet Cache Protocol (ICP)  . . . . . . . . . . . . . . 22
   7.1.2   Hyper Text Caching Protocol (HTCP/0.0) . . . . . . . . . . 22
   7.1.3   Cache Digest . . . . . . . . . . . . . . . . . . . . . . . 23
   7.1.4   Cache Pre-filling  . . . . . . . . . . . . . . . . . . . . 24
   7.2     Tightly Coupled Inter-Cache Communication  . . . . . . . . 24
   7.2.1   Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . . 25
   8.      Network Element Communication  . . . . . . . . . . . . . . 26
   8.1     Web Cache Coordination Protocol (WCCP) . . . . . . . . . . 26
   8.2     SOCKS  . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   9.      Security Considerations  . . . . . . . . . . . . . . . . . 28
   9.1     Authentication . . . . . . . . . . . . . . . . . . . . . . 28
   9.1.1   Man in the middle attacks  . . . . . . . . . . . . . . . . 28
   9.1.2   Trusted third party  . . . . . . . . . . . . . . . . . . . 28
   9.1.3   Authentication based on IP number  . . . . . . . . . . . . 29


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






































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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 infrastructural 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[13], Cache Digests [15][17], CARP[4], HTTP[1], ICP[5],
   PAC[2], SOCKS[12], WPAD[3], and WCCP[11]. 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 the
   definitions in RFC2616 and this document are highlighted.

2.1 Base Terms

   The majority of these terms are taken as-is from RFC 2616[1], and
   are included here for reference.

   client (as given in [1])
      A program that establishes connections for the purpose of sending
      requests.

   server (as given in [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 (as given in [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").

   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 (as given in [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 (as given in [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.

   tunnel (as given in [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 (as given in [19])
      Creating and maintaining a duplicate copy of a database or file
      system on a different computer, typically a server.

   inbound/outbound (as given in [1])
      Inbound and outbound refer to the request and response paths for
      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.



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2.2 First order derivative terms

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

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

   user agent (as given in [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 mis-used when
      referring to caching proxies.

   surrogate (a.k.a. "reverse proxies", "server accelerators")
      An intermediary program which acts as a server or tunnel for the
      purpose of responding to requests on behalf of one or more origin
      servers. Requests are serviced internally from a cache or by
      tunnelling them on to origin servers. The implementation
      requirements for surrogates have not been standardized; depending
      on the implementation, surrogates may or may not respond to the
      cache directives defined in [1]. Surrogates are also known as
      "reverse proxies" and "(origin) server accelerators".

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.



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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
      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 (typically caching 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 client to use
      them. The use of a proxy is transparent to the user but not to
      the client. The term "automatic proxy configuration" is also used
      in this sense.

   traffic interception


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      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.

   (network) transparent proxy
      A proxy that receives traffic as a result of network traffic
      redirection. The term "transparent proxy" is typically used to
      refer to a network transparent proxy and the additional systems
      that perform traffic redirection. The use of this type of proxy
      is transparent to both user and client.  Due to a conflicting
      definition in [1], caution should be exercised when referring to
      a "transparent proxy". As stated above, it is recommended that
      the phrase "transparent proxy" is prepended with appropriate
      terminology to avoid confusion.































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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 clients.

3.2.1 Client to Non-Network Transparent 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 cache and origin server or
   replica origin server (see Section 3.1.1).


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


   In addition, a client may interact with an additional server -
   operated on behalf of a proxy - to aid the configuration of the
   client to use that proxy.

   Protocols used in this relationship can be found in Section 6.

3.2.2 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
   available, the client communicates directly with an 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



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   An outbound request from a local (caching) proxy may be routed to
   one of a number of intermediate (caching) proxies based on a
   determination of whether that parent is 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 is 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 query.

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

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

3.2.3.2 (Caching) Proxy Clusters

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
























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4. Client to Replica Communication

   This section describes the cooperation and communication between
   clients 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
      mirror sites.  The human user 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 URL Redirection

   Authoritative reference:
      This memo.

   Description:
      A simple and commonly used mechanism to connect web clients with
      origin server replicas is to use URL redirection. Clients are
      redirected to a optimal web server replica via the use of the
      HTTP[1] protocol response codes, e.g. 302 "Found", or 307
      "Temporary Redirect". A web client establishes HTTP communication
      with one of the web server replicas. The initially contacted
      replica origin web server can either choose to accept the service
      or redirect the client to the proper replica. Refer to section
      10.3 in HTTP/1.1 RFC2616 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 at this time.

   Submitter:
      Document editors.

4.3 DNS Redirection

   Authoritative reference:

      *  RFC1794 DNS Support for Load Balancing Proximity[13]

      *  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 at this
      time.

   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:
      In this model, 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 reoccuring 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
      and RDIST are the most common protocols observed.

   Deployment:
      Very common for mirror synchronization in the Internet.

   Submitter:
      Document editors.

5.2 Demand Driven Replication

   Authoritative reference:
      This memo.

   Description:
      In this model the replica origin server acquires the content as
      needed due to client demand. This is generally done by a
      surrogate. When a client requests a resource that is not in the
      data set of the replica origin server/surrogate, the surrogate
      attempts to acquire it from the master origin server and then
      forwards it to the requesting client.

   Security:
      Relies upon the protocol being used to transfer the resources.
      FTP, Gopher, HTTP and ICP 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 at this time.

   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:
      In this model, the 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. Client to Proxy Configuration

   This section describes the configuration, cooperation and
   communication between end user clients (browsers and applications) a
   proxy.

6.1 Manual Proxy Configuration

   Authoritative reference:
      This memo.

   Description:
      Each user needs to 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 to
      determine an appropriate proxy (if any) for the resource being
      requested. User agents must be configured to request this
      JavaScript resource upon startup. No bootstrap mechanism, manual
      configuration necessary.

      Manual configuration is made easier by centralizing the script to
      one URI.

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


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   Deployment:
      Implemented in most browsers.

   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:
      Clients 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
      clients via PAC scripts.

   Submitter:
      Document editors.

6.4 Web Proxy Auto-Discovery Protocol (WPAD)

   Authoritative reference:
      Internet-Draft: draft-ietf-wrec-wpad-00.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 gets you to 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
         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 web client


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      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 web clients 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:
      RFC 2186 Internet Cache Protocol (ICP), version 2[5]

   Description:
      ICP is used by caches to query other caches about web objects, to
      see if a web object is present at the other cache.

      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
      does, together with round trip times provide a load balancing
      method for caches.

   Security:
      See RFC 2187[6]

      ICP does not convey information about HTTP headers associated
      with a web object. HTTP headers may include access control and
      cache directives, Since caches ask for objects, and then download
      the objects using HTTP, false cache hits may occur (object
      present in cache, but not accessible for sibling cache 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 (HTCP/0.0)

   Authoritative reference:
      Internet-Draft: draft-vixie-htcp-proto-05.txt[16]

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


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      monitoring cache activity.

      HTCP includes HTTP headers, while ICPv2 does not. HTTP headers
      are vital information for caching proxies.

   Security:
      Optionally uses HMAC-MD5[18]shared secret authentication.
      Protocol is subject to attack if authentication is not used.

   Deployment:
      HTCP is implemented in Squid and the Web Gateway Interceptor[20].

   Submitter:
      Document editors.

7.1.3 Cache Digest

   Authoritative reference:

      *  No RFC, no Internet-Draft; Cache Digest specification -
         version 5[15]

      *  Summary Cache[17] (see note)

   Description:
      Cache Digests are a response to the problems of latency and
      congestion associated with previous inter-cache communications
      mechanisms such as the Internet Cache Protocol (ICP)[5] and the
      HyperText Cache Protocol[16]. Unlike most of these protocols,
      Cache Digests support peering between caching proxies and cache
      servers without a request-response exchange taking place.
      Instead, a summary of the contents of the server (the Digest) is
      fetched by other servers which peer with it. Using Cache Digests
      it is possible to determine with a relatively high degree of
      accuracy whether a given URL is cached by a particular server.

      Cache Digests are both an exchange protocol and a data format
      [15].

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

      A 'Trojan horse' attack is currently possible in a mesh: Cache A
      can build a fake peer Digest for cache 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


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      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

      *  TF-CACHE mesh (European Academic networks)

   Submitter:
      Alex Rousskov, NLANR, rousskov@nlanr.net for [15]
      Pei Cao for [17]

   Note: The technology of Summary Cache[17] is patent pending by the
   University of Wisconsin-Madison.

7.1.4 Cache Pre-filling

   Authoritative reference:
      Expired Internet-Draft:
      draft-lovric-francetelecom-satellites-00.txt[14]

   Description:
      Cache pre-filling is a push-caching implementation. It is
      particularly well adapted to IP-multicast networks because it
      allows preselected URLs to be inserted in one single time within
      all the caches that belong to 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 equipments 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

7.2 Tightly Coupled Inter-Cache Communication






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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 proxy caches. Included in CARP is the definition of a Proxy
      Array Membership Table, and ways to download this information.

      An HTTP client agent (either a proxy server or a client browser)
      which implements CARP v1.0 can allocate and intelligently route
      requests for the correct 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
   caching proxy and network elements.  Examples include routers and
   switches.  Generally used for transparent caching and/or diffused
   arrays.

8.1 Web Cache Coordination Protocol (WCCP)

   Authoritative reference:
      Internet-Draft: draft-ietf-wrec-web-pro-00.txt[11]

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

   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'
      caches.

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

8.2 SOCKS

   Authoritative reference:
      RFC1928 SOCKS Protocol Version 5[12]

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

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


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      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.

   Replication and caching means copying objects. There are legal
   implications of making and keeping transient or permanent copies;
   these are not covered in the security considerations.

   Information on security in each protocol is provided in the
   preceding description of the protocol, 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[10], 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. Authentication based on client IP number is
   problematic when connecting through a proxy, details are not
   discussed here.

9.1 Authentication

   Requests for web objects and responses to such requests may go to
   replicas and/or flow through proxies. The integrity of the
   communication needs to be preserved, to ensure protection of access
   to the communication and protect the communication exchange from
   unintended change. In the case of security breach, the culprit needs
   to be identified

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 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.





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9.1.3 Authentication based on IP number

   Authentication based on client IP number is problematic when
   connecting through a proxy, as the authenticating server sees the
   proxy's IP number. One (not recommended) solution to this is
   spoofing the client's IP number.

   Authentication based on IP number assumes that the end-to-end
   properties of the Internet are preserved. This is typically not the
   case for a network transparent proxy.

9.2 Privacy

9.2.1 Trusted third party

   When using a replication service, you need to trust both the replica
   and the object location service. A object location service is used
   to find the replicated object. Current examples include DNS round
   robin, manual mirror lists, URNs, HTTP redirecting.

   Redirection of traffic, either by redirecting to replicas or by
   redirection done by proxies, may introduce third parties the end
   user and/or origin server need to trust. In the case of network
   transparent proxies, such trusted third parties are often unknown to
   both end points of the communication. Unknown trusted third parties
   may have security implications.

   Both proxies and location services may have access to aggregated
   access information. A proxy typically knows about all access by all
   the clients using it, information that is more sensitive than the
   information held by one origin server.

9.2.2 Logs and legal implications

   Logs from proxies need to be kept secure, as they provide
   information about users and end user patterns.  A proxy log is even
   more sensitive than a web server log, as all requests from the user
   population goes through the proxy. Logs from replication servers may
   need to be amalgamated to get aggregated statistics from a service,
   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 does not necessarily make objects cacheable, as is the
   case of SSL encrypted web sessions.




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9.3 Service security

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 location
   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 a network transparent proxy
   is denial of service since the end to end nature of the Internet is
   destroyed without the end users 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 end users. 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. Legal implications of replication and caching is
   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 only
   available at network level equipment in a proxy-free world. Some
   network level equipment may have required physical access to get
   sensitive information, and introducing 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, and Keith Moore.












































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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.

   [2]  Netscape, Inc., "Navigator Proxy Auto-Config File Format",
        External reference http://www.netscape.com/eng/mozilla/2.0/
        relnotes/demo/proxy-live.html, March 1996.

   [3]  Gauthier, P., Cohen, J., Dunsmuir, M. and C. Perkins, "The Web
        Proxy Auto-Discovery Protocol", Internet Draft
        draft-ietf-wrec-wpad-01.txt, July 1999.

   [4]  Valloppillil, V. and K.W. Ross, "Cache Array Routing Protocol",
        Expired Internet Draft draft-vinod-carp-v1-03.txt available at
        http://ircache.nlanr.net/Cache/ICP/carp.txt, February 1998.

   [5]  Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP),
        Version 2", RFC 2186, September 1997.

   [6]  Wessels, D. and K. Claffy, "Application of Internet Cache
        Protocol (ICP), Version 2", RFC 2187, September 1997.

   [7]  Lovric, I., "Internet Cache Protocol Extension", Internet Draft
        draft-lovric-icp-ext-02.txt, October 1999.

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

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

   [10]  Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext
         Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [11]  Cisco Systems, "Cisco Web Cache Coordination Protocol V1.0",
         Internet Draft draft-ietf-wrec-web-pro-00.txt, June 1999.

   [12]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L.
         Jones, "SOCKS Protocol Version 5", RFC 1928, March 1996.

   [13]  Brisco, T., "DNS Support for Load Balancing", RFC 1794, April
         1995.

   [14]  Goutard, C., Lovric, I. and E. Maschio-Esposito, "Pre-filling
         a cache - A satellite overview", Expired Internet Draft


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         draft-lovric-francetelecom-satellites-00.txt, February 1999.

   [15]  Hamilton, M., Rousskov, A. and D. Wessels, "Cache Digest
         specification - version 5", External reference
         http://squid.nlanr.net/CacheDigest/cache-digest-v5.txt,
         December 1998.

   [16]  Vixie, P. and D. Wessels, "Hyper Text Caching Protocol
         (HTCP/0.0)", Internet Draft draft-vixie-htcp-proto-05.txt,
         August 1999.

   [17]  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.

   [18]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

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

   [20]  http://www.vix.com/vix/wgi.html

Authors' Addresses

   Ingrid Melve
   UNINETT
   Tempeveien 22
   Trondheim
   Norway

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

   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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   Ian Cooper
   Mirror Image Internet, Inc.
   49 Dragon Court
   2nd floor
   Woburn, MA  01801
   USA

   Phone: +1 781 939 0735
   EMail: ian@mirror-image.com










































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Full Copyright Statement

   Copyright (C) The Internet Society (1999). 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 implementation may be prepared, copied, published
   and 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.

Acknowledgement

   Funding for the RFC editor function is currently provided by the
   Internet Society.



















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