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.


   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 (RFC2187) 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.

      One of the opcodes defined below.

      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.

      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.

      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
      22    ICP_OP_DENIED
      23    ICP_OP_HIT_OBJ

      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.

      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:

      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.

      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

Wessels & Claffy             Informational                      [Page 5]

RFC 2186                          ICP                     September 1997

      An ICP_OP_HIT response indicates that the requested URL exists in
      this cache and that the requester is allowed to retrieve it.

      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.

      An ICP_OP_ERR response indicates some kind of error in parsing or
      handling the query message (e.g. invalid URL).

      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.

      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

      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

      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

      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

      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, RFC2187.

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,

   [4] Wessels D., Claffy K., "ICP and the Squid Web Cache", National
   Laboratory for Applied Network Research,

   [5] Wessels D., "The Squid Internet Object Cache", National
   Laboratory for Applied Network Research,

6.  Acknowledgments

   The authors wish to thank Paul A Vixie <> 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


   K. Claffy
   National Laboratory for Applied Network Research
   10100 Hopkins Drive
   La Jolla, CA 92093


Wessels & Claffy             Informational                      [Page 9]