Internet Content Adaptation Protocol (ICAP)
RFC 3507

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Network Working Group                                           J. Elson
Request for Comments: 3507                                      A. Cerpa
Category: Informational                                             UCLA
                                                              April 2003

              Internet Content Adaptation Protocol (ICAP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

IESG Note

   The Open Pluggable Services (OPES) working group has been chartered
   to produce a standards track protocol specification for a protocol
   intended to perform the same of functions as ICAP.  However, since
   ICAP is already in widespread use the IESG believes it is appropriate
   to document existing usage by publishing the ICAP specification as an
   informational document.  The IESG also notes that ICAP was developed
   before the publication of RFC 3238 and therefore does not address the
   architectural and policy issues described in that document.

Abstract

   ICAP, the Internet Content Adaption Protocol, is a protocol aimed at
   providing simple object-based content vectoring for HTTP services.
   ICAP is, in essence, a lightweight protocol for executing a "remote
   procedure call" on HTTP messages.  It allows ICAP clients to pass
   HTTP messages to ICAP servers for some sort of transformation or
   other processing ("adaptation").  The server executes its
   transformation service on messages and sends back responses to the
   client, usually with modified messages.  Typically, the adapted
   messages are either HTTP requests or HTTP responses.

Elson & Cerpa                Informational                      [Page 1]
RFC 3507                          ICAP                        April 2003

Table of Contents

   1.   Introduction............................................3
   2.   Terminology.............................................5
   3.   ICAP Overall Operation..................................8
        3.1   Request Modification..............................8
        3.2   Response Modification............................10
   4.   Protocol Semantics.....................................11
        4.1   General Operation................................11
        4.2   ICAP URIs........................................11
        4.3   ICAP Headers.....................................12
              4.3.1   Headers Common to Requests and
                      Responses................................12
              4.3.2   Request Headers..........................13
              4.3.3   Response Headers.........................14
              4.3.4   ICAP-Related Headers in HTTP
                      Messages.................................15
        4.4   ICAP Bodies: Encapsulation of HTTP
              Messages.........................................16
              4.4.1   Expected Encapsulated Sections...........16
              4.4.2   Encapsulated HTTP Headers................18
        4.5   Message Preview..................................18
        4.6   "204 No Content" Responses outside of
              Previews.........................................22
        4.7   ISTag Response Header............................22
        4.8   Request Modification Mode........................23
              4.8.1   Request..................................23
              4.8.2   Response.................................24
              4.8.3   Examples.................................24
        4.9   Response Modification Mode.......................27
              4.9.1   Request..................................27
              4.9.2   Response.................................27
              4.9.3   Examples.................................28
        4.10  OPTIONS Method...................................29
              4.10.1  OPTIONS request..........................29
              4.10.2  OPTIONS response.........................30
              4.10.3  OPTIONS examples.........................33
   5.   Caching................................................33
   6.   Implementation Notes...................................34
        6.1   Vectoring Points.................................34
        6.2   Application Level Errors.........................35
        6.3   Use of Chunked Transfer-Encoding.................37
        6.4   Distinct URIs for Distinct Services..............37
   7.   Security Considerations................................37
        7.1   Authentication...................................37
        7.2   Encryption.......................................38
        7.3   Service Validation...............................38
   8.   Motivations and Design Alternatives....................39

Elson & Cerpa                Informational                      [Page 2]