Network Working Group                                          D. Wessels
Request for Comments: 2187                                      K. Claffy
Category: Informational                   National Laboratory for Applied
                                                    Network Research/UCSD
                                                           September 1997

        Application of 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 the application of ICPv2 (Internet Cache
   Protocol version 2, RFC2186) to Web caching.  ICPv2 is a lightweight
   message format used for communication among Web caches.  Several
   independent caching implementations now use ICP[3,5], making it
   important to codify the existing practical uses of ICP for those
   trying to implement, deploy, and extend its use.

   ICP queries and replies refer to the existence of URLs (or objects)
   in neighbor caches.  Caches exchange ICP messages and use the
   gathered information to select the most appropriate location from
   which to retrieve an object.  A companion document (RFC2186)
   describes the format and syntax of the protocol itself.  In this
   document we focus on issues of ICP deployment, efficiency, security,
   and interaction with other aspects of Web traffic behavior.

Table of Contents

   1.   Introduction.................................................  2
   2.   Web Cache Hierarchies........................................  3
   3.   What is the Added Value of ICP?..............................  5
   4.   Example Configuration of ICP Hierarchy.......................  5
     4.1. Configuring the `proxy.customer.org' cache.................  6
     4.2. Configuring the `cache.isp.com' cache......................  6
   5.   Applying the Protocol........................................  7
     5.1. Sending ICP Queries........................................  8
     5.2. Receiving ICP Queries and Sending Replies.................. 10
     5.3. Receiving ICP Replies...................................... 11
     5.4. ICP Options................................................ 13
   6.   Firewalls.................................................... 14
   7.   Multicast.................................................... 14
   8.   Lessons Learned.............................................. 16
     8.1. Differences Between ICP and HTTP........................... 16




 
RFC 2187                          ICP                     September 1997


     8.2. Parents, Siblings, Hits and Misses......................... 16
     8.3. Different Roles of ICP..................................... 17
     8.4. Protocol Design Flaws of ICPv2............................. 17
   9.   Security Considerations...................................... 18
     9.1. Inserting Bogus ICP Queries................................ 19
     9.2. Inserting Bogus ICP Replies................................ 19
     9.3. Eavesdropping.............................................. 20
     9.4. Blocking ICP Messages...................................... 20
     9.5. Delaying ICP Messages...................................... 20
     9.6. Denial of Service.......................................... 20
     9.7. Altering ICP Fields........................................ 21
     9.8. Summary.................................................... 22
   10.  References................................................... 23
   11.  Acknowledgments.............................................. 24
   12.  Authors' Addresses........................................... 24

1.  Introduction

   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 for use in selecting the most appropriate location from
   which to retrieve an object.

   This document describes the implementation of ICP in software.  For a
   description of the protocol and message format, please refer to the
   companion document (RFC2186).  We avoid making judgments about
   whether or how ICP should be used in particular Web caching
   configurations.  ICP may be a "net win" in some situations, and a
   "net loss" in others.  We recognize that certain practices described
   in this document are suboptimal. Some of these exist for historical
   reasons.  Some aspects have been improved in later versions.  Since
   this document only serves to describe current practices, we focus on
   documenting rather than evaluating.  However, we do address known
   security problems and other shortcomings.

   The remainder of this document is written as follows.  We first
   describe Web cache hierarchies, explain motivation for using ICP, and