Internet DRAFT - draft-wessels-icp-v2


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Network Working Group                                         D. Wessels
Internet-Draft                           National Laboratory for Applied
Obsoletes <draft-wessels-icp-v2-01.txt>            Network Research/UCSD
Expires: November 22, 1997                                 22 April 1997

                Internet Cache Protocol (ICP), version 2

Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   To learn the current status of any Internet-Draft, please check the
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   This draft document describes the Internet Cache Protocol (ICP) as
   currently implemented in a couple of World-Wide Web proxy cache
   packages.  A companion document (RFCXXXX, <draft-wessels-icp-
   v2-appl-00.txt>) describes the application of ICP to Web caches.  ICP
   was initially developed by Peter Danzig, et. al. at the University of
   Southern California.  It evolved as an important part of hierarchical
   caching on the Harvest research project.

1.  Introduction

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   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
   neighbor caches.  One cache will send an ICP query to its neighbors.
   The neighbors will send back ICP replies indicating a ``HIT'' or a

   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 sec-
   ond 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 imme-
   diate 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 appropri-
   ate source from which to retrieve an object.

ICP Message Format

   The ICP message format consists of a 20-octet fixed header plus a
   variable sized payload:

     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                     |

Wessels                                                         [Page 2]
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   |                                                               |
   |                            Payload                            |
   /                                                               /
   /                                                               /
   |                                                               |

   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.

   Message Length
      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 ver-
      sion 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.

   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.

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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.

      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                     /
      /                                                               /
      |                                                               |

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      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 pay-
      load 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 mes-

      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).


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      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 run-
      ning, 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 par-
      ticular 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 mes-
      sages 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 mes-
      sages from that address until some sort of administrative inter-
      vention occurs.

      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 autho-
      rization 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.

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      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.

      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.

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   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 mea-
      sured 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, RFCXXXX (<draft-wessels-icp-v2-appl-00.txt>).

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 Wes-
   sels D., "The Harvest Information Discovery and Access System",
   Internet Research Task Force - Resource Discovery, http://har-

   [4] Wessels D., Claffy K., "ICP and the Squid Web Cache", National

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   Laboratory for Applied Network Research,

6.  Acknowledgments

   Paul A Vixie <>

7.  Author's  Address:

   Duane Wessels
   National Laboratory for Applied Network Research

   K Claffy
   National Laboratory for Applied Network Research

Wessels                                                         [Page 9]