rfc3040.txt

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      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.





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   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 to section 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 DNS



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      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.




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5.2 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.



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   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 proxy



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      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.



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      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 Cohen

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)

   Best known reference:
      RFC 2186  Internet Cache Protocol (ICP), version 2 [2]



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   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.