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INFORMATIONAL
Network Working Group                                          I. CooperRequest for Comments: 3040                                 Equinix, Inc.Category: Informational                                         I. Melve                                                                 UNINETT                                                            G. Tomlinson                                                          CacheFlow Inc.                                                            January 2001Internet Web Replication and Caching TaxonomyStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This memo specifies standard terminology and the taxonomy of web   replication and caching infrastructure as 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 the document titled   "Known HTTP Proxy/Caching Problems", and are not part of this   document.  This document presents open protocols and points to   published material for each protocol.Table of Contents1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .32.      Terminology  . . . . . . . . . . . . . . . . . . . . . . .32.1     Base Terms . . . . . . . . . . . . . . . . . . . . . . . .42.2     First order derivative terms . . . . . . . . . . . . . . .62.3     Second order derivatives . . . . . . . . . . . . . . . . .72.4     Topological terms  . . . . . . . . . . . . . . . . . . . .72.5     Automatic use of proxies . . . . . . . . . . . . . . . . .83.      Distributed System Relationships . . . . . . . . . . . . .93.1     Replication Relationships  . . . . . . . . . . . . . . . .93.1.1   Client to Replica  . . . . . . . . . . . . . . . . . . . .93.1.2   Inter-Replica  . . . . . . . . . . . . . . . . . . . . . .93.2     Proxy Relationships  . . . . . . . . . . . . . . . . . . .103.2.1   Client to Non-Interception Proxy . . . . . . . . . . . . .10Cooper, et al.               Informational                      [Page 1]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20013.2.2   Client to Surrogate to Origin Server . . . . . . . . . . .103.2.3   Inter-Proxy  . . . . . . . . . . . . . . . . . . . . . . .113.2.3.1 (Caching) Proxy Meshes . . . . . . . . . . . . . . . . . .113.2.3.2 (Caching) Proxy Arrays . . . . . . . . . . . . . . . . . .123.2.4   Network Element to Caching Proxy . . . . . . . . . . . . .124.      Replica Selection  . . . . . . . . . . . . . . . . . . . .134.1     Navigation Hyperlinks  . . . . . . . . . . . . . . . . . .134.2     Replica HTTP Redirection . . . . . . . . . . . . . . . . .144.3     DNS Redirection  . . . . . . . . . . . . . . . . . . . . .145.      Inter-Replica Communication  . . . . . . . . . . . . . . .155.1     Batch Driven Replication . . . . . . . . . . . . . . . . .155.2     Demand Driven Replication  . . . . . . . . . . . . . . . .165.3     Synchronized Replication . . . . . . . . . . . . . . . . .166.      User Agent to Proxy Configuration  . . . . . . . . . . . .176.1     Manual Proxy Configuration . . . . . . . . . . . . . . . .176.2     Proxy Auto Configuration (PAC) . . . . . . . . . . . . . .176.3     Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . .186.4     Web Proxy Auto-Discovery Protocol (WPAD) . . . . . . . . .187.      Inter-Proxy Communication  . . . . . . . . . . . . . . . .197.1     Loosely coupled Inter-Proxy Communication  . . . . . . . .197.1.1   Internet Cache Protocol (ICP)  . . . . . . . . . . . . . .197.1.2   Hyper Text Caching Protocol  . . . . . . . . . . . . . . .207.1.3   Cache Digest . . . . . . . . . . . . . . . . . . . . . . .217.1.4   Cache Pre-filling  . . . . . . . . . . . . . . . . . . . .227.2     Tightly Coupled Inter-Cache Communication  . . . . . . . .227.2.1   Cache Array Routing Protocol (CARP) v1.0 . . . . . . . . .228.      Network Element Communication  . . . . . . . . . . . . . .238.1     Web Cache Control Protocol (WCCP)  . . . . . . . . . . . .238.2     Network Element Control Protocol (NECP)  . . . . . . . . .248.3     SOCKS  . . . . . . . . . . . . . . . . . . . . . . . . . .259.      Security Considerations  . . . . . . . . . . . . . . . . .259.1     Authentication . . . . . . . . . . . . . . . . . . . . . .269.1.1   Man in the middle attacks  . . . . . . . . . . . . . . . .269.1.2   Trusted third party  . . . . . . . . . . . . . . . . . . .269.1.3   Authentication based on IP number  . . . . . . . . . . . .269.2     Privacy  . . . . . . . . . . . . . . . . . . . . . . . . .269.2.1   Trusted third party  . . . . . . . . . . . . . . . . . . .269.2.2   Logs and legal implications  . . . . . . . . . . . . . . .279.3     Service security . . . . . . . . . . . . . . . . . . . . .279.3.1   Denial of service  . . . . . . . . . . . . . . . . . . . .279.3.2   Replay attack  . . . . . . . . . . . . . . . . . . . . . .279.3.3   Stupid configuration of proxies  . . . . . . . . . . . . .289.3.4   Copyrighted transient copies . . . . . . . . . . . . . . .289.3.5   Application level access . . . . . . . . . . . . . . . . .2810.     Acknowledgements . . . . . . . . . . . . . . . . . . . . .28           References . . . . . . . . . . . . . . . . . . . . . . . .28           Authors' Addresses . . . . . . . . . . . . . . . . . . . .31           Full Copyright Statement . . . . . . . . . . . . . . . . .32Cooper, et al.               Informational                      [Page 2]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20011. Introduction   Since its introduction in 1990, the World-Wide Web has evolved from a   simple client server model into a complex 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.   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.   It is also expected 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.   Some of the protocols which this memo examines are specified only by   company technical white papers or work in progress documents.  Such   references are included to demonstrate the existence of such   protocols, their experimental deployment in the Internet today, or to   aid the reader in their understanding of this technology area.   There are many protocols, both open and proprietary, employed in web   replication and caching today.  A majority of the open protocols   include DNS [8], Cache Digests [21][10], CARP [14], HTTP [1], ICP   [2], PAC [12], SOCKS [7], WPAD [13], and WCCP [18][19].  These   protocols, and their use within the caching and replication   environments, are discussed below.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   inRFC 2616 and this document are highlighted.Cooper, et al.               Informational                      [Page 3]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20012.1 Base Terms   The majority of these terms are taken as-is fromRFC 2616 [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   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.Cooper, et al.               Informational                      [Page 4]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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 insection 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      "Creating and maintaining a duplicate copy of a database or file      system on a different computer, typically a server."  - Free      Online Dictionary of Computing (FOLDOC)   inbound/outbound (taken from [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.Cooper, et al.               Informational                      [Page 5]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20012.2 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".   server accelerator      See "surrogate".Cooper, et al.               Informational                      [Page 6]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20012.3 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".Cooper, et al.               Informational                      [Page 7]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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 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 theCooper, et al.               Informational                      [Page 8]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      document "Known HTTP Proxy/Caching Problems" [23].  The use of      interception proxies requires zero configuration of the user agent      which act as though communicating directly with an origin server.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 inSection 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 inSection 3.1.1.Cooper, et al.               Informational                      [Page 9]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      ------------------     -----------------     ------------------      | Replica Origin |-----| Master Origin |-----| Replica Origin |      |     Server     |     |    Server     |     |     Server     |      ------------------     -----------------     ------------------   Protocols used in this relationship can be found inSection 5.3.2 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 (seeSection 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 inSection 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.Cooper, et al.               Informational                     [Page 10]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001            --------------  --------------   --------------            |   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 onlyCooper, et al.               Informational                     [Page 11]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   An inbound request 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 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 inSection 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 inSection 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, seeSection 3.1.1.)Cooper, et al.               Informational                     [Page 12]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      -----------------     -----------------     -----------------      | 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 inSection 8.4. 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   Best known 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.Cooper, et al.               Informational                     [Page 13]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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 Replica HTTP Redirection   Best known 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 tosection 10.3 in HTTP/1.1      [1] for information on HTTP response codes.   Security:      Relies entirely upon HTTP security.   Deployment:      Observed at a number of large web sites.  Extent of usage in the      Internet is unknown.   Submitter:      Document editors.4.3 DNS Redirection   Best known references:      *RFC 1794 DNS Support for Load Balancing Proximity [8]      *  This memo   Description:      The Domain Name Service (DNS) provides a more sophisticated client      to replica communication mechanism.  This is accomplished by DNSCooper, et al.               Informational                     [Page 14]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      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.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   Best known 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      [4] and RDIST are the most common protocols observed.   Deployment:      Very common for synchronization of mirror sites in the Internet.   Submitter:      Document editors.Cooper, et al.               Informational                     [Page 15]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20015.2 Demand Driven Replication   Best known 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      [4], Gopher [5], HTTP [1] and ICP [2] are the most common      protocols observed.   Deployment:      Observed at several large web sites.  Extent of usage in the      Internet is unknown.   Submitter:      Document editors.5.3 Synchronized Replication   Best known reference:      This memo.   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.Cooper, et al.               Informational                     [Page 16]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   Note:      The editors are aware of at least two open source protocols - AFS      and CODA - as well as the proprietary NRS protocol from Novell.6. User Agent to Proxy Configuration   This section describes the configuration, cooperation and   communication between user agents and proxies.6.1 Manual Proxy Configuration   Best known 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)   Best known reference:      "Navigator Proxy Auto-Config File Format" [12]   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 proxyCooper, et al.               Informational                     [Page 17]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      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.   Deployment:      Implemented in Netscape Navigator and Microsoft Internet Explorer.   Submitter:      Document editors.6.3 Cache Array Routing Protocol (CARP) v1.0   Best known references:      *  "Cache Array Routing Protocol" [14] (work in progress)      *  "Cache Array Routing Protocol (CARP) v1.0 Specifications" [15]      *  "Cache Array Routing Protocol and Microsoft Proxy Server 2.0"         [16]   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 works      in progress.   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)   Best known reference:      "The Web Proxy Auto-Discovery Protocol" [13] (work in progress)   Description:      WPAD uses a collection of pre-existing Internet resource discovery      mechanisms to perform web proxy auto-discovery.Cooper, et al.               Informational                     [Page 18]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      The only goal of WPAD is to locate the PAC URL [12].  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 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 some user agents and caching proxy servers.  More      than two independent implementations.   Submitter:      Josh Cohen7. Inter-Proxy Communication7.1 Loosely coupled Inter-Proxy Communication   This section describes the cooperation and communication between   caching proxies.7.1.1 Internet Cache Protocol (ICP)   Best known reference:RFC 2186  Internet Cache Protocol (ICP), version 2 [2]Cooper, et al.               Informational                     [Page 19]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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:      SeeRFC 2187 [3]      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 Cache Protocol Extension" [17] (work in progress)7.1.2 Hyper Text Caching Protocol   Best known reference:RFC 2756 Hyper Text  Caching Protocol (HTCP/0.0) [9]   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.Cooper, et al.               Informational                     [Page 20]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   Security:      Optionally uses HMAC-MD5 [11] shared secret authentication.      Protocol is subject to attack if authentication is not used.   Deployment:      HTCP is implemented in Squid and the "Web Gateway Interceptor".   Submitter:      Document editors.7.1.3 Cache Digest      Best known references:      *  "Cache Digest Specification - version 5" [21]      *  "Summary Cache: A Scalable Wide-Area Web Cache Sharing         Protocol" [10] (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) [2] and the      Hyper Text Cache Protocol [9].  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.      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.      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.Cooper, et al.               Informational                     [Page 21]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001      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; TF-CACHE Mesh (European Academic      networks   Submitter:      Alex Rousskov for [21], Pei Cao for [10].   Note: The technology of Summary Cache [10] is patent pending by the   University of Wisconsin-Madison.7.1.4 Cache Pre-filling   Best known reference:      "Pre-filling a cache - A satellite overview" [20] (work in      progress)   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 Lovric7.2 Tightly Coupled Inter-Cache Communication7.2.1 Cache Array Routing Protocol (CARP) v1.0   Also seeSection 6.3Cooper, et al.               Informational                     [Page 22]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   Best known references:      *  "Cache Array Routing Protocol" [14] (work in progress)      *  "Cache Array Routing Protocol (CARP) v1.0 Specifications" [15]      *  "Cache Array Routing Protocol and Microsoft Proxy Server 2.0"         [16]   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 works      in progress.   Deployment:      Implemented in caching proxy servers.  More than two independent      implementations.   Submitter:      Document editors.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)   Best known references:      "Web Cache Control Protocol" [18][19] (work in progress)      Note: The name used for this protocol varies, sometimes referred      to as the "Web Cache Coordination Protocol", but frequently just      "WCCP" to avoid confusionCooper, et al.               Informational                     [Page 23]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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.      WCCP V2 additionally runs between multiple routers and the      proxies.   Security:      WCCP V1 has no security features.      WCCP V2 provides optional authentication of protocol packets.   Deployment:      Network elements: WCCP is deployed on a wide range of Cisco      routers.      Caching proxies: WCCP is deployed on a number of vendors' caching      proxies.   Submitter:      David Forster      Document editors.8.2 Network Element Control Protocol (NECP)   Best known reference:      "NECP: The Network Element Control Protocol" [22] (work in      progress)   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 [11] shared secret authentication along      with complex sequence numbers to provide moderately strong      security.  Protocol is subject to attack if authentication is not      used.   Deployment:      Unknown at present; several network element and caching proxy      vendors have expressed intent to implement the protocol.Cooper, et al.               Informational                     [Page 24]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   Submitter:      Gary Tomlinson8.3 SOCKS   Best known reference:RFC 1928 SOCKS Protocol Version 5 [7]   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 widely deployed in the Internet.   Submitter:      Document editors.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 insection 15 of RFC 2616   [1], the HTTP/1.1 specification, and to a lesser extent inRFC 1945   [6], the HTTP/1.0 specification.RFC 2616 contains security   considerations for HTTP proxies.Cooper, et al.               Informational                     [Page 25]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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 insection 15 of   RFC 2616 [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   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 Privacy9.2.1 Trusted third party   When using a replication service, one must trust both the replica   origin server and the replica selection system.Cooper, et al.               Informational                     [Page 26]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   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 security9.3.1 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.Cooper, et al.               Informational                     [Page 27]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 20019.3.3 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.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, Patrik Faltstrom, Hilarie Orman,   Mark Nottingham and Oskar Batuner.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]   Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP),         Version 2",RFC 2186, September 1997.   [3]   Wessels, D. and K. Claffy, "Application of Internet Cache         Protocol (ICP), Version 2",RFC 2187, September 1997.Cooper, et al.               Informational                     [Page 28]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   [4]   Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD         9,RFC 959, October 1985.   [5]   Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,         D. and B. Alberti, "The Internet Gopher Protocol",RFC 1436,         March 1993.   [6]   Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext         Transfer Protocol -- HTTP/1.0",RFC 1945, May 1996.   [7]   Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L.         Jones, "SOCKS Protocol Version 5",RFC 1928, March 1996.   [8]   Brisco, T., "DNS Support for Load Balancing",RFC 1794, April         1995.   [9]   Vixie, P. and D. Wessels, "Hyper Text Caching Protocol         (HTCP/0.0)",RFC 2756, January 2000.   [10]  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.   [11]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing         for Message Authentication",RFC 2104, February 1997.   [12]  Netscape, Inc., "Navigator Proxy Auto-Config File Format",         March 1996,         <URL:http://www.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-         live.html>.   [13]  Gauthier, P., Cohen, J., Dunsmuir, M. and C. Perkins, "The Web         Proxy Auto-Discovery Protocol", Work in Progress.   [14]  Valloppillil, V. and K. Ross,"Cache Array Routing Protocol",         Work in Progress.   [15]  Microsoft Corporation, "Cache Array Routing Protocol (CARP)         v1.0 Specifications, Technical Whitepaper", August 1999,         <URL:http://www.microsoft.com/Proxy/Guide/carpspec.asp>.   [16]  Microsoft Corporation, "Cache Array Routing Protocol and         Microsoft Proxy Server 2.0, Technical White Paper", August         1998,         <URL:http://www.microsoft.com/proxy/documents/CarpWP.exe>.   [17]  Lovric, I.,"Internet Cache Protocol Extension", Work in         Progress.Cooper, et al.               Informational                     [Page 29]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001   [18]  Cieslak, M. and D. Forster, "Cisco Web Cache Coordination         Protocol V1.0", Work in Progress.   [19]  Cieslak, M., Forster, D., Tiwana, G. and R. Wilson, "Cisco Web         Cache Coordination Protocol V2.0", Work in Progress.   [20]  Goutard, C., Lovric, I. and E. Maschio-Esposito, "Pre-filling a         cache - A satellite overview", Work in Progress.   [21]  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>.   [22]  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", Work in Progress.   [23]  Cooper, I. and J. Dilley,"Known HTTP Proxy/Caching Problems",         Work in Progress.Cooper, et al.               Informational                     [Page 30]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001Authors' Addresses   Ian Cooper   Equinix, Inc.   2450 Bayshore Parkway   Mountain View, CA  94043   USA   Phone: +1 650 316 6065   EMail: icooper@equinix.com   Ingrid Melve   UNINETT   Tempeveien 22   Trondheim  N-7465   Norway   Phone: +47 73 55 79 07   EMail: Ingrid.Melve@uninett.no   Gary Tomlinson   CacheFlow Inc.   12034 134th Ct. NE, Suite 201   Redmond, WA  98052   USA   Phone: +1 425 820 3009   EMail: gary.tomlinson@cacheflow.comCooper, et al.               Informational                     [Page 31]

RFC 3040      Internet Web Replication & Caching Taxonomy   January 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  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.Cooper, et al.               Informational                     [Page 32]

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