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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)
 0: |                             LENGTH                            |
    +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +
 2: |                             LENGTH                            |
 2: |             MAJOR             |             MINOR             |
 LENGTH  is the message length, inclusive of all header and data
         octets, including the LENGTH field itself.  This field will
         be equal to the datagram payload size ("record length") if a
         datagram protocol is in use, and can include padding, i.e.,
         not all octets of the message need be used in the DATA and
         AUTH sections.
 MAJOR   is the major version number (0 for this specification).  The
         DATA section of an HTCP message need not be upward or
         downward compatible between different major version numbers.
 MINOR   is the minor version number (0 for this specification).
         Feature levels and interpretation rules can vary depending on
         this field, in particular RESERVED fields can take on new
         (though optional) meaning in successive minor version numbers
         within the same major version number.
 2.6.1.  It is expected that an HTCP initiator will know the version
 number of a prospective HTCP responder, or that the initiator will
 probe using declining values for MINOR and MAJOR (beginning with the
 highest locally supported value) and locally cache the probed version
 number of the responder.
 2.6.2.  Higher MAJOR numbers are to be preferred, as are higher MINOR
 numbers within a particular MAJOR number.

Vixie & Wessels Experimental [Page 3] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 2.7. An HTCP/0.* DATA has the following structure:
               +0 (MSB)                            +1 (LSB)
 0: |                             LENGTH                            |
 2: |    OPCODE     |   RESPONSE    |        RESERVED       |F1 |RR |
 4: |                           TRANS-ID                            |
    +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +
 6: |                           TRANS-ID                            |
 8: |                                                               |
    /                            OP-DATA                            /
    /                                                               /
 LENGTH    is the number of octets of the message which are reserved
           for the DATA section, including the LENGTH field itself.
           This number can include padding, i.e., not all octets
           reserved by LENGTH need be used in OP-DATA.
 OPCODE    is the operation code of an HTCP transaction.  An HTCP
           transaction can consist of multiple HTCP messages, e.g., a
           request (sent by the initiator), or a response (sent by the
 RESPONSE  is a numeric code indicating the success or failure of a
           transaction.  It should be set to zero (0) by requestors
           and ignored by responders.  Each operation has its own set
           of response codes, which are described later.  The overall
           message has a set of response codes which are as follows:
           0   authentication wasn't used but is required
           1   authentication was used but unsatisfactorily
           2   opcode not implemented
           3   major version not supported
           4   minor version not supported (major version is ok)
           5   inappropriate, disallowed, or undesirable opcode
           The above response codes all indicate errors and all depend
           for their visibility on MO=1 (as specified below).
 RR        is a flag indicating whether this message is a request (0)
           or response (1).

Vixie & Wessels Experimental [Page 4] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 F1        is overloaded such that it is used differently by
           requestors than by responders.  If RR=0, then F1 is defined
           as RD.  If RR=1, then F1 is defined as MO.
 RD        is a flag which if set to 1 means that a response is
           desired.  Some OPCODEs require RD to be set to 1 to be
 MO        (em-oh) is a flag which indicates whether the RESPONSE code
           is to be interpreted as a response to the overall message
           (fixed fields in DATA or any field of AUTH) [MO=1] or as a
           response to fields in the OP-DATA [MO=0].
 TRANS-ID  is a 32-bit value which when combined with the initiator's
           network address, uniquely identifies this HTCP transaction.
           Care should be taken not to reuse TRANS-ID's within the
           life-time of a UDP datagram.
 OP-DATA   is opcode-dependent and is defined below, per opcode.
 2.8. An HTCP/0.0 AUTH has the following structure:
               +0 (MSB)                            +1 (LSB)
  0: |                             LENGTH                            |
  2: |                            SIG-TIME                           |
     +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +
  4: |                            SIG-TIME                           |
  6: |                           SIG-EXPIRE                          |
     +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +   +
  8: |                           SIG-EXPIRE                          |
 10: |                                                               |
     /                            KEY-NAME                           /
     /                                                               /
  n: |                                                               |
     /                            SIGNATURE                          /
     /                                                               /

Vixie & Wessels Experimental [Page 5] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 LENGTH      is the number of octets used by the AUTH, including the
             LENGTH field itself.  If the optional AUTH is not being
             transmitted, this field should be set to 2 (two).  LENGTH
             can include padding, which means that not all octets
             reserved by LENGTH will necessarily be consumed by
 SIG-TIME    is an unsigned binary count of the number of seconds
             since 00:00:00 1-Jan-70 UTC at the time the SIGNATURE is
 SIG-EXPIRE  is an unsigned binary count of the number of seconds
             since 00:00:00 1-Jan-70 UTC at the time the SIGNATURE is
             considered to have expired.
 KEY-NAME    is a COUNTSTR [3.1] which specifies the name of a shared
             secret.  (Each HTCP implementation is expected to allow
             configuration of several shared secrets, each of which
             will have a name.)
 SIGNATURE   is a COUNTSTR [3.1] which holds the HMAC-MD5 digest (see
             [RFC 2104]), with a B value of 64, of the following
             elements, each of which is digested in its "on the wire"
             format, including transmitted padding if any is covered
             by a field's associated LENGTH:
             IP SRC ADDR                           [4 octets]
             IP SRC PORT                           [2 octets]
             IP DST ADDR                           [4 octets]
             IP DST PORT                           [2 octets]
             HTCP MAJOR version number             [1 octet]
             HTCP MINOR version number             [1 octet]
             SIG-TIME                              [4 octets]
             SIG-EXPIRE                            [4 octets]
             HTCP DATA                             [variable]
             KEY-NAME (the whole COUNTSTR [3.1])   [variable]
 2.8.1.  Shared secrets should be cryptorandomly generated and should
 be at least a few hundred octets in size.

3. Data Types

 HTCP/0.* data types are defined as follows:

Vixie & Wessels Experimental [Page 6] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 3.1. COUNTSTR is a counted string whose format is:
               +0 (MSB)                            +1 (LSB)
 0: |                             LENGTH                            |
 2: |                                                               |
    /                              TEXT                             /
    /                                                               /
 LENGTH  is the number of octets which will follow in TEXT.  This
         field is *not* self-inclusive as is the case with other HTCP
         LENGTH fields.
 TEXT    is a stream of uninterpreted octets, usually ISO8859-1
 3.2.  SPECIFIER is used with the TST and CLR request messages,
 defined below.  Its format is:
    |        METHOD       | : COUNTSTR
    |         URI         | : COUNTSTR
    |       VERSION       | : COUNTSTR
    |       REQ-HDRS      | : COUNTSTR
 METHOD    (Since HTCP only returns headers, methods GET and HEAD are
 URI       (If the URI is a URL, it should always include a ":"<port>
           specifier, but in its absense, port 80 should be imputed by
           a receiver.)
 VERSION   is an entire HTTP version string such as" HTTP/1.1".
           VERSION strings with prefixes other than "HTTP/" or with
           version numbers less than "1.1" are outside the domain of
           this specification.
 REQ-HDRS  are those presented by an HTTP initiator.  These headers
           should include end-to-end but NOT hop-by-hop headers, and
           they can be canonicalized (aggregation of "Accept:" is
           permitted, for example.)

Vixie & Wessels Experimental [Page 7] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 3.3.  DETAIL is used with the TST response message, defined below.
 Its format is:
    |      RESP-HDRS      | : COUNTSTR
    |     ENTITY-HDRS     | : COUNTSTR
    |     CACHE-HDRS      | : COUNTSTR
 3.4.  IDENTITY is used with the MON request and SET response message,
 defined below.  Its format is:
    |      SPECIFIER      |
    |        DETAIL       |

4. Cache Headers

 HTCP/0.0 CACHE-HDRS consist of zero or more of the following headers:
 Cache-Vary: <header-name> ...
    The sender of this header has learned that content varies on a set
    of headers different from the set given in the object's Vary:
    header.  Cache-Vary:, if present, overrides the object's Vary:
 Cache-Location: <cache-hostname>:<port> ...
    The sender of this header has learned of one or more proxy caches
    who are holding a copy of this object.  Probing these caches with
    HTCP may result in discovery of new, close-by (preferrable to
    current) HTCP neighbors.
 Cache-Policy: [no-cache] [no-share] [no-cache-cookie]
    The sender of this header has learned that the object's caching
    policy has more detail than is given in its response headers.
    no-cache          means that it is uncacheable (no reason given),
                      but may be shareable between simultaneous
    no-share          means that it is unshareable (no reason given),
                      and per-requestor tunnelling is always

Vixie & Wessels Experimental [Page 8] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

    no-cache-cookie   means that the content could change as a result
                      of different, missing, or even random cookies
                      being included in the request headers, and that
                      caching is inadvisable.
 Cache-Flags: [incomplete]
    The sender of this header has modified the object's caching policy
    locally, such that requesters may need to treat this response
    specially, i.e., not necessarily in accordance with the object's
    actual policy.
    incomplete   means that the response headers and/or entity headers
                 given in this response are not known to be complete,
                 and may not be suitable for use as a cache key.
 Cache-Expiry: <date>
    The sender of this header has learned that this object should be
    considered to have expired at a time different than that indicated
    by its response headers.  The format is the same as HTTP/1.1
 Cache-MD5: <discovered content MD5>
    The sender of this header has computed an MD5 checksum for this
    object which is either different from that given in the object's
    Content-MD5:  header, or is being supplied since the object has no
    Content-MD5 header.  The format is the same as HTTP/1.1 Content-
 Cache-to-Origin: <origin> <rtt> <samples> <hops>
    The sender of this header has measured the round trip time to an
    origin server (given as a hostname or literal address).  The <rtt>
    is the average number of seconds, expressed as decimal ASCII with
    arbitrary precision and no exponent.  <Samples> is the number of
    RTT samples which have had input to this average.  <Hops> is the
    number of routers between the cache and the origin, expressed as
    decimal ASCII with arbitrary precision and no exponent, or 0 if
    the cache doesn't know.

Vixie & Wessels Experimental [Page 9] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

6. HTCP Operations

 HTCP/0.* opcodes and their respective OP-DATA are defined below:
 6.1. NOP (OPCODE 0):
 This is an HTCP-level "ping."  Responders are encouraged to process
 NOP's with minimum delay since the requestor may be using the NOP RTT
 (round trip time) for configuration or mapping purposes.  The
 RESPONSE code for a NOP is always zero (0).  There is no OP-DATA for
 a NOP.  NOP requests with RD=0 cause no processing to occur at all.
 6.2. TST (OPCODE 1):
 Test for the presence of a specified content entity in a proxy cache.
 TST requests with RD=0 cause no processing to occur at all.
 TST requests have the following OP-DATA:
               +0 (MSB)                            +1 (LSB)
 0: |                                                               |
    /                          SPECIFIER                            /
    /                                                               /
 RESPONSE codes for TST are as follows:
 0   entity is present in responder's cache
 1   entity is not present in responder's cache
 TST responses have the following OP-DATA, if RESPONSE is zero (0):
               +0 (MSB)                            +1 (LSB)
 0: |                                                               |
    /                             DETAIL                            /
    /                                                               /
 Note:  The response headers returned by a positive TST can be of a
        stale object.  Requestors should be prepared to cope with this
        condition, either by using the responder as a source for this
        object (which could cause the responder to simply refresh it)
        or by choosing a different responder.

Vixie & Wessels Experimental [Page 10] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 TST responses have the following OP-DATA, if RESPONSE is one (1):
               +0 (MSB)                            +1 (LSB)
 0: |                                                               |
    /                           CACHE-HDRS                          /
    /                                                               /
 6.3. MON (OPCODE 2):
 Monitor activity in a proxy cache's local object store (adds, deletes,
 replacements, etc).  Since interleaving of HTCP transactions over a
 single pair of UDP endpoints is not supported, it is recommended that a
 unique UDP endpoint be allocated by the requestor for each concurrent
 MON request.  MON requests with RD=0 are equivalent to those with RD=1
 and TIME=0; that is, they will cancel any outstanding MON transaction.
 MON requests have the following OP-DATA structure:
                +0 (MSB)
 0: |             TIME              |
 TIME  is the number of seconds of monitoring output desired by the
       initiator.  Subsequent MON requests from the same initiator
       with the same TRANS-ID should update the time on a ongoing MON
       transaction.  This is called "overlapping renew."
 RESPONSE codes for MON are as follows:
 0   accepted, OP-DATA is present and valid
 1   refused (quota error -- too many MON's are active)
 MON responses have the following OP-DATA structure, if RESPONSE is
 zero (0):
               +0 (MSB)                            +1 (LSB)
 0: |             TIME              |     ACTION    |     REASON    |
 2: |                                                               |
    /                            IDENTITY                           /
    /                                                               /

Vixie & Wessels Experimental [Page 11] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 TIME      is the number of seconds remaining for this MON
 ACTION    is a numeric code indicating a cache population action.
           Codes are:
           0   an entity has been added to the cache
           1   an entity in the cache has been refreshed
           2   an entity in the cache has been replaced
           3   an entity in the cache has been deleted
 REASON    is a numeric code indicating the reason for an ACTION.
           Codes are:
           0   some reason not covered by the other REASON codes
           1   a proxy client fetched this entity
           2   a proxy client fetched with caching disallowed
           3   the proxy server prefetched this entity
           4   the entity expired, per its headers
           5   the entity was purged due to caching storage limits
 6.4. SET (OPCODE 3):
 Inform a cache of the identity of an object.  This is a "push"
 transaction, whereby cooperating caches can share information such as
 updated Age/Date/Expires headers (which might result from an origin
 "304 Not modified" HTTP response) or updated cache headers (which
 might result from the discovery of non-authoritative "vary"
 conditions or from learning of second or third party cache locations
 for this entity.  RD is honoured.
 SET requests have the following OP-DATA structure:
 0: |                                                               |
    /                            IDENTITY                           /
    /                                                               /
 RESPONSE  codes are as follows:
           0   identity accepted, thank you
           1   identity ignored, no reason given, thank you
 SET responses have no OP-DATA.

Vixie & Wessels Experimental [Page 12] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

 6.5. CLR (OPCODE 4):
 Tell a cache to completely forget about an entity.  RD is honoured.
 CLR requests have the following OP-DATA structure:
 0: |                   RESERVED                    |     REASON    |
 2: |                                                               |
    /                           SPECIFIER                           /
    /                                                               /
 REASON    is a numeric code indicating the reason why the requestor
           is asking that this entity be removed.  The codes are as
           0   some reason not better specified by another code
           1   the origin server told me that this entity does not
 RESPONSE  codes are as follows:
           0   i had it, it's gone now
           1   i had it, i'm keeping it, no reason given.
           2   i didn't have it
 CLR responses have no OP-DATA.
 Clearing a URI without specifying response, entity, or cache headers
 means to clear all entities using that URI.

7. Security Considerations

 If the optional AUTH element is not used, it is possible for
 unauthorized third parties to both view and modify a cache using the
 HTCP protocol.

8. Acknowledgements

 Mattias Wingstedt of Idonex brought key insights to the development
 of this protocol.  David Hankins helped clarify this document.

Vixie & Wessels Experimental [Page 13] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

9. References

 [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
           Resource Identifiers (URI): Generic Syntax", RFC 2396,
           August 1998.
 [RFC2616] 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.
 [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
           Hashing for Message Authentication", RFC 2104, February,
 [RFC2186] Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP),
           version 2", RFC 2186, September 1997.

10. Authors' Addresses

 Paul Vixie
 Internet Software Consortium
 950 Charter Street
 Redwood City, CA 94063
 Phone: +1 650 779 7001
 Duane Wessels
 National Lab for Applied Network Research
 USCD, 9500 Gilman Drive
 La Jolla, CA 92093
 Phone: +1 303 497 1822

Vixie & Wessels Experimental [Page 14] RFC 2756 Hyper Text Caching Protocol (HTCP/0.0) January 2000

11. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  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
 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


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

Vixie & Wessels Experimental [Page 15]

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