Hyper Text Caching Protocol (HTCP/0.0)
RFC 2756

Document Type RFC - Experimental (January 2000; No errata)
Was draft-vixie-htcp-proto (individual)
Authors Duane Wessels  , Paul Vixie 
Last updated 2013-03-02
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Network Working Group                                            P. Vixie
Request for Comments: 2756                                            ISC
Category: Experimental                                         D. Wessels
                                                             January 2000

                 Hyper Text Caching Protocol (HTCP/0.0)

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

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


   This document describes HTCP, a protocol for discovering HTTP caches
   and cached data, managing sets of HTTP caches, and monitoring cache
   activity.  This is an experimental protocol, one among several
   proposals to perform these functions.

1.  Definitions, Rationale and Scope

   1.1.  HTTP/1.1 (see [RFC2616]) permits the transfer of web objects
   from "origin servers," possibly via "proxies" (which are allowed
   under some circumstances to "cache" such objects for subsequent
   reuse) to "clients" which consume the object in some way, usually by
   displaying it as part of a "web page."  HTTP/1.0 and later permit
   "headers" to be included in a request and/or a response, thus
   expanding upon the HTTP/0.9 (and earlier) behaviour of specifying
   only a URI in the request and offering only a body in the response.

   1.2.  ICP (see [RFC2186]) permits caches to be queried as to their
   content, usually by other caches who are hoping to avoid an expensive
   fetch from a distant origin server.  ICP was designed with HTTP/0.9
   in mind, such that only the URI (without any headers) is used when
   describing cached content, and the possibility of multiple compatible
   bodies for the same URI had not yet been imagined.

Vixie & Wessels               Experimental                      [Page 1]
RFC 2756         Hyper Text Caching Protocol (HTCP/0.0)     January 2000

   1.3.  This document specifies a Hyper Text Caching Protocol (HTCP)
   which permits full request and response headers to be used in cache
   management, and expands the domain of cache management to include
   monitoring a remote cache's additions and deletions, requesting
   immediate deletions, and sending hints about web objects such as the
   third party locations of cacheable objects or the measured
   uncacheability or unavailability of web objects.

2.  HTCP Protocol

   2.1.  All multi-octet HTCP protocol elements are transmitted in
   network byte order.  All RESERVED fields should be set to binary zero
   by senders and left unexamined by receivers.  Headers must be
   presented with the CRLF line termination, as in HTTP.

   2.2.  Any hostnames specified should be compatible between sender and
   receiver, such that if a private naming scheme (such as HOSTS.TXT or
   NIS) is in use, names depending on such schemes will only be sent to
   HTCP neighbors who are known to participate in said schemes.  Raw
   addresses (dotted quad IPv4, or colon-format IPv6) are universal, as
   are public DNS names.  Use of private names or addresses will require
   special operational care.

   2.3.  HTCP messages may be sent as UDP datagrams, or over TCP
   connections.  UDP must be supported.  HTCP agents must not be
   isolated from NETWORK failures and delays.  An HTCP agent should be
   prepared to act in useful ways when no response is forthcoming, or
   when responses are delayed or reordered or damaged.  TCP is optional
   and is expected to be used only for protocol debugging.  The IANA has
   assigned port 4827 as the standard TCP and UDP port number for HTCP.

   2.4.  A set of configuration variables concerning transport
   characteristics should be maintained for each agent which is capable
   of initiating HTCP transactions, perhaps with a set of per-agent
   global defaults.  These variables are:

   Maximum number of unacknowledged transactions before a "failure" is

   Maximum interval without a response to some transaction before a
   "failure" is imputed.

   Minimum interval before trying a new transaction after a failure.

Vixie & Wessels               Experimental                      [Page 2]
RFC 2756         Hyper Text Caching Protocol (HTCP/0.0)     January 2000

   2.5. An HTCP Message has the following general format:

   |        HEADER       | tells message length and protocol versions
   |         DATA        | HTCP message (varies per major version number)
   |         AUTH        | optional authentication for transaction

   2.6. An HTCP/*.* HEADER has the following format:

                 +0 (MSB)                            +1 (LSB)
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