RFC2756 - Hyper Text Caching Protocol (HTCP/0.0)

  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.

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.

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)
+ + + + + + + + + + + + + + + + +

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.

2.7. An HTCP/0.* DATA has the following structure:

+0 (MSB) +1 (LSB)
+ + + + + + + + + + + + + + + + +
/ /

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).

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)
+ + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + +
/ /
/ /

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
[RFC2104]), 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:

3.1. COUNTSTR is a counted string whose format is:

+0 (MSB) +1 (LSB)
/ 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 (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.)

3.3. DETAIL is used with the TST response message, defined below.
Its format is:


3.4. IDENTITY is used with the MON request and SET response message,
defined below. Its format is:


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

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.

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)
/ /

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)
/ /

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.

TST responses have the following OP-DATA, if RESPONSE is one (1):

+0 (MSB) +1 (LSB)
/ /

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)

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)
/ /

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:

/ /

RESPONSE codes are as follows:

0 identity accepted, thank you
1 identity ignored, no reason given, thank you

SET responses have no OP-DATA.

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:

/ /

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.

9. References

[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC2396,
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", RFC2616, June 1999.

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC2104, February,

[RFC2186] Wessels, D. and K. Claffy, "Internet Cache Protocol (ICP),
version 2", RFC2186, September 1997.

10. Authors' Addresses

Paul Vixie
Internet Software Consortium
950 Charter Street
Redwood City, CA 94063

Phone: +1 650 779 7001
EMail: [email protected]

Duane Wessels
National Lab for Applied Network Research
USCD, 9500 Gilman Drive
La Jolla, CA 92093

Phone: +1 303 497 1822
EMail: [email protected]

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