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IETF RFC 2186
Internet Cache Protocol (ICP), version 2
Last modified on Thursday, September 4th, 1997
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Network Working Group D. Wessels
Request for Comments: 2186 K. Claffy
Category: Informational National Laboratory for Applied
Network Research/UCSD
September 1997
Internet Cache Protocol (ICP), version 2
 Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document describes version 2 of the Internet Cache Protocol
(ICPv2) as currently implemented in two World-Wide Web proxy cache
packages[3,5]. ICP is a lightweight message format used for
communicating among Web caches. ICP is used to exchange hints about
the existence of URLs in neighbor caches. Caches exchange ICP
queries and replies to gather information to use in selecting the
most appropriate location from which to retrieve an object.
This document describes only the format and fields of ICP messages.
A companion document (RFC 2187) describes the application of ICP to
Web caches. Several independent caching implementations now use ICP,
and we consider it important to codify the existing practical uses of
ICP for those trying to implement, deploy, and extend its use for
their own purposes.
1. Introduction
ICP is a message format used for communicating between Web caches.
Although Web caches use HTTP[1] for the transfer of object data,
caches benefit from a simpler, lighter communication protocol. ICP
is primarily used in a cache mesh to locate specific Web objects in
neighboring caches. One cache sends an ICP query to its neighbors.
The neighbors send back ICP replies indicating a "HIT" or a "MISS."
Wessels & Claffy Informational PAGE 1 
RFC 2186 ICP September 1997
In current practice, ICP is implemented on top of UDP, but there is
no requirement that it be limited to UDP. We feel that ICP over UDP
offers features important to Web caching applications. An ICP
query/reply exchange needs to occur quickly, typically within a
second or two. A cache cannot wait longer than that before beginning
to retrieve an object. Failure to receive a reply message most
likely means the network path is either congested or broken. In
either case we would not want to select that neighbor. As an
indication of immediate network conditions between neighbor caches,
ICP over a lightweight protocol such as UDP is better than one with
the overhead of TCP.
In addition to its use as an object location protocol, ICP messages
can be used for cache selection. Failure to receive a reply from a
cache may indicate a network or system failure. The ICP reply may
include information that could assist selection of the most
appropriate source from which to retrieve an object.
ICP was initially developed by Peter Danzig, et. al. at the
University of Southern California as a central part of hierarchical
caching in the Harvest research project[3].
ICP Message Format
The ICP message format consists of a 20-octet fixed header plus a
variable sized payload (see Figure 1).
NOTE: All fields must be represented in network byte order.
Opcode
One of the opcodes defined below.
Version
The ICP protocol version number. At the time of this writing,
both versions two and three are in use. This document describes
only version two. The version number field allows for future
development of this protocol.
Wessels & Claffy Informational PAGE 2
RFC 2186 ICP September 1997
Message Length
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode | Version | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Host Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Payload |
/ /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FIGURE 1: ICP message format.
The total length (octets) of the ICP message. ICP messages MUST
not exceed 16,384 octets in length.
Request Number
An opaque identifier. When responding to a query, this value must
be copied into the reply message.
Options
A 32-bit field of option flags that allows extension of this
version of the protocol in certain, limited ways. See "ICP Option
Flags" below.
Option Data
A four-octet field to support optional features. The following
ICP features make use of this field:
The ICP_FLAG_SRC_RTT option uses the low 16-bits of Option Data to
return RTT measurements. The ICP_FLAG_SRC_RTT option is further
described below.
Wessels & Claffy Informational PAGE 3
RFC 2186 ICP September 1997
Sender Host Address
The IPv4 address of the host sending the ICP message. This field
should probably not be trusted over what is provided by getpeer-
name(), accept(), and recvfrom(). There is some ambiguity over
the original purpose of this field. In practice it is not used.
Payload
The contents of the Payload field vary depending on the Opcode,
but most often it contains a null-terminated URL string.
2. ICP Opcodes
The following table shows currently defined ICP opcodes:
Value Name
----- -----------------
0 ICP_OP_INVALID
1 ICP_OP_QUERY
2 ICP_OP_HIT
3 ICP_OP_MISS
4 ICP_OP_ERR
5-9 UNUSED
10 ICP_OP_SECHO
11 ICP_OP_DECHO
12-20 UNUSED
21 ICP_OP_MISS_NOFETCH
22 ICP_OP_DENIED
23 ICP_OP_HIT_OBJ
ICP_OP_INVALID
A place holder to detect zero-filled or malformed messages. A
cache must never intentionally send an ICP_OP_INVALID message.
ICP_OP_ERR should be used instead.
ICP_OP_QUERY
A query message. NOTE this opcode has a different payload format
than most of the others. First is the requester's IPv4 address,
followed by a URL. The Requester Host Address is not that of the
cache generating the ICP message, but rather the address of the
caches's client that originated the request. The Requester Host
Address is often zero filled. An ICP message with an all-zero
Requester Host Address address should be taken as one where the
requester address is not specified; it does not indicate a valid
IPv4 address.
Wessels & Claffy Informational PAGE 4
RFC 2186 ICP September 1997
ICP_OP_QUERY payload format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requester Host Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Null-Terminated URL /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In response to an ICP_OP_QUERY, the recipient must return one of:
ICP_OP_HIT, ICP_OP_MISS, ICP_OP_ERR, ICP_OP_MISS_NOFETCH,
ICP_OP_DENIED, or ICP_OP_HIT_OBJ.
ICP_OP_SECHO
Similar to ICP_OP_QUERY, but for use in simulating a query to an
origin server. When ICP is used to select the closest neighbor,
the origin server can be included in the algorithm by bouncing an
ICP_OP_SECHO message off it's echo port. The payload is simply
the null-terminated URL.
NOTE: the echo server will not interpret the data (i.e. we could
send it anything). This opcode is used to tell the difference
between a legitimate query or response, random garbage, and an
echo response.
ICP_OP_DECHO
Similar to ICP_OP_QUERY, but for use in simulating a query to a
cache which does not use ICP. When ICP is used to choose the
closest neighbor, a non-ICP cache can be included in the algorithm
by bouncing an ICP_OP_DECHO message off it's echo port. The
payload is simply the null-terminated URL.
NOTE: one problem with this approach is that while a system's echo
port may be functioning perfectly, the cache software may not be
running at all.
One of the following six ICP opcodes are sent in response to an
ICP_OP_QUERY message. Unless otherwise noted, the payload must be
the null-terminated URL string. Both the URL string and the Request
Number field must be exactly the same as from the ICP_OP_QUERY
message.
Wessels & Claffy Informational PAGE 5
RFC 2186 ICP September 1997
ICP_OP_HIT
An ICP_OP_HIT response indicates that the requested URL exists in
this cache and that the requester is allowed to retrieve it.
ICP_OP_MISS
An ICP_OP_MISS response indicates that the requested URL does not
exist in this cache. The querying cache may still choose to fetch
the URL from the replying cache.
ICP_OP_ERR
An ICP_OP_ERR response indicates some kind of error in parsing or
handling the query message (e.g. invalid URL).
ICP_OP_MISS_NOFETCH
An ICP_OP_MISS_NOFETCH response indicates that this cache is up,
but is in a state where it does not want to handle cache misses.
An example of such a state is during a startup phase where a cache
might be rebuilding its object store. A cache in such a mode may
wish to return ICP_OP_HIT for cache hits, but not ICP_OP_MISS for
misses. ICP_OP_MISS_NOFETCH essentially means "I am up and
running, but please don't fetch this URL from me now."
Note, ICP_OP_MISS_NOFETCH has a different meaning than
ICP_OP_MISS. The ICP_OP_MISS reply is an invitation to fetch the
URL from the replying cache (if their relationship allows it), but
ICP_OP_MISS_NOFETCH is a request to NOT fetch the URL from the
replying cache.
ICP_OP_DENIED
An ICP_OP_DENIED response indicates that the querying site is not
allowed to retrieve the named object from this cache. Caches and
proxies may implement complex access controls. This reply must be
be interpreted to mean "you are not allowed to request this
particular URL from me at this particular time."
Caches receiving a high percentage of ICP_OP_DENIED replies are
probably misconfigured. Caches should track percentage of all
replies which are ICP_OP_DENIED and disable a neighbor which
exceeds a certain threshold (e.g. 95% of 100 or more queries).
Similarly, a cache should track the percent of ICP_OP_DENIED
messages that are sent to a given address. If the percent of
denied messages exceeds a certain threshold (e.g. 95% of 100 or
more), the cache may choose to ignore all subsequent ICP_OP_QUERY
messages from that address until some sort of administrative
intervention occurs.
Wessels & Claffy Informational PAGE 6
RFC 2186 ICP September 1997
ICP_OP_HIT_OBJ
Just like an ICP_OP_HIT response, but the actual object data has
been included in this reply message. Many requested objects are
small enough that it is possible to include them in the query
response and avoid the need to make a subsequent HTTP request for
the object.
CAVEAT: ICP_OP_HIT_OBJ has some negative side effects which make
its use undesirable. It transfers object data without HTTP and
therefore bypasses the standard HTTP processing, including
authorization and age validation. Another negative side effect is
that ICP_OP_HIT_OBJ messages will often be much larger than the
path MTU, thereby causing fragmentation to occur on the UDP
packet. For these reasons, use of ICP_OP_HIT_OBJ is NOT
recommended.
A cache must not send an ICP_OP_HIT_OBJ unless the
ICP_FLAG_HIT_OBJ flag is set in the query message Options field.
ICP_OP_HIT_OBJ payload format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Null-Terminated URL /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Object Size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
/ Object Data /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The receiving application must check to make sure it actually
receives Object Size octets of data. If it does not, then it
should treat the ICP_OP_HIT_OBJ reply as though it were a normal
ICP_OP_HIT.
NOTE: the Object Size field does not necessarily begin on a 32-bit
boundary as shown in the diagram above. It begins immediately
following the NULL byte of the URL string.
Wessels & Claffy Informational PAGE 7
RFC 2186 ICP September 1997
UNRECOGNIZED OPCODES
ICP messages with unrecognized or unused opcodes should be
ignored, i.e. no reply generated. The application may choose to
note the anomalous behaviour in a log file.
3. ICP Option Flags
0x80000000 ICP_FLAG_HIT_OBJ
This flag is set in an ICP_OP_QUERY message indicating that it is
okay to respond with an ICP_OP_HIT_OBJ message if the object data
will fit in the reply.
0x40000000 ICP_FLAG_SRC_RTT
This flag is set in an ICP_OP_QUERY message indicating that the
requester would like the ICP reply to include the responder's
measured RTT to the origin server.
Upon receipt of an ICP_OP_QUERY with ICP_FLAG_SRC_RTT bit set, a
cache should check an internal database of RTT measurements. If
available, the RTT value MUST be expressed as a 16-bit integer, in
units of milliseconds. If unavailable, the responder may either
set the RTT value to zero, or clear the ICP_FLAG_SRC_RTT bit in
the ICP reply. The ICP reply MUST not be delayed while waiting
for the RTT measurement to occur.
This flag is set in an ICP reply message (ICP_OP_HIT, ICP_OP_MISS,
ICP_OP_MISS_NOFETCH, or ICP_OP_HIT_OBJ) to indicate that the low
16-bits of the Option Data field contain the measured RTT to the
host given in the requested URL. If ICP_FLAG_SRC_RTT is clear in
the query then it MUST also be clear in the reply. If
ICP_FLAG_SRC_RTT is set in the query, then it may or may not be
set in the reply.
4. Security Considerations
The security issues relating to ICP are discussed in the companion
document, RFC 2187.
Wessels & Claffy Informational PAGE 8
RFC 2186 ICP September 1997
5. References
[1] Fielding, R., et. al, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, UC Irvine, January 1997.
[2] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota,
December 1994.
[3] Bowman M., Danzig P., Hardy D., Manber U., Schwartz M., and
Wessels D., "The Harvest Information Discovery and Access System",
Internet Research Task Force - Resource Discovery,
http://harvest.transarc.com/.
[4] Wessels D., Claffy K., "ICP and the Squid Web Cache", National
Laboratory for Applied Network Research,
http://www.nlanr.net/~wessels/Papers/icp-squid.ps.gz
[5] Wessels D., "The Squid Internet Object Cache", National
Laboratory for Applied Network Research,
http://squid.nlanr.net/Squid/
6. Acknowledgments
The authors wish to thank Paul A Vixie <paul@vix.com> for providing
excellent feedback on this document.
7. Authors' Addresses
Duane Wessels
National Laboratory for Applied Network Research
10100 Hopkins Drive
La Jolla, CA 92093
EMail: wessels@nlanr.net
K. Claffy
National Laboratory for Applied Network Research
10100 Hopkins Drive
La Jolla, CA 92093
EMail: kc@nlanr.net
Wessels & Claffy Informational PAGE 9
Internet Cache Protocol (ICP), version 2
RFC TOTAL SIZE: 18808 bytes
PUBLICATION DATE: Thursday, September 4th, 1997
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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