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rfc:rfc3507

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] RFC 3507 ICAP April 2003

      8.1   To Be HTTP, or Not to Be.........................39
      8.2   Mandatory Use of Chunking........................39
      8.3   Use of the null-body directive in the
            Encapsulated header..............................40
 9.   References.............................................40
 10.  Contributors...........................................41
 Appendix A   BNF Grammar for ICAP Messages..................45
 Authors' Addresses..........................................48
 Full Copyright Statement....................................49

1. Introduction

 As the Internet grows, so does the need for scalable Internet
 services.  Popular web servers are asked to deliver content to
 hundreds of millions of users connected at ever-increasing
 bandwidths.  The model of centralized, monolithic servers that are
 responsible for all aspects of every client's request seems to be
 reaching the end of its useful life.
 To keep up with the growth in the number of clients, there has been a
 move towards architectures that scale better through the use of
 replication, distribution, and caching.  On the content provider
 side, replication and load-balancing techniques allow the burden of
 client requests to be spread out over a myriad of servers.  Content
 providers have also begun to deploy geographically diverse content
 distribution networks that bring origin-servers closer to the "edge"
 of the network where clients are attached.  These networks of
 distributed origin-servers or "surrogates" allow the content provider
 to distribute their content whilst retaining control over the
 integrity of that content.  The distributed nature of this type of
 deployment and the proximity of a given surrogate to the end-user
 enables the content provider to offer additional services to a user
 which might be based, for example, on geography where this would have
 been difficult with a single, centralized service.
 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.  The adapted messages may be
 either HTTP requests or HTTP responses.  Though transformations may
 be possible on other non-HTTP content, they are beyond the scope of
 this document.

Elson & Cerpa Informational [Page 3] RFC 3507 ICAP April 2003

 This type of Remote Procedure Call (RPC) is useful in a number of
 ways.  For example:
 o  Simple transformations of content can be performed near the edge
    of the network instead of requiring an updated copy of an object
    from an origin server.  For example, a content provider might want
    to provide a popular web page with a different advertisement every
    time the page is viewed.  Currently, content providers implement
    this policy by marking such pages as non-cachable and tracking
    user cookies.  This imposes additional load on the origin server
    and the network.  In our architecture, the page could be cached
    once near the edges of the network.  These edge caches can then
    use an ICAP call to a nearby ad-insertion server every time the
    page is served to a client.
    Other such transformations by edge servers are possible, either
    with cooperation from the content provider (as in a content
    distribution network), or as a value-added service provided by a
    client's network provider (as in a surrogate).  Examples of these
    kinds of transformations are translation of web pages to different
    human languages or to different formats that are appropriate for
    special physical devices (e.g., PDA-based or cell-phone-based
    browsers).
 o  Surrogates or origin servers can avoid performing expensive
    operations by shipping the work off to other servers instead.
    This helps distribute load across multiple machines.  For example,
    consider a user attempting to download an executable program via a
    surrogate (e.g., a caching proxy).  The surrogate, acting as an
    ICAP client, can ask an external server to check the executable
    for viruses before accepting it into its cache.
 o  Firewalls or surrogates can act as ICAP clients and send outgoing
    requests to a service that checks to make sure the URI in the
    request is allowed (for example, in a system that allows parental
    control of web content viewed by children).  In this case, it is a
    *request* that is being adapted, not an object returned by a
    response.
 In all of these examples, ICAP is helping to reduce or distribute the
 load on origin servers, surrogates, or the network itself.  In some
 cases, ICAP facilitates transformations near the edge of the network,
 allowing greater cachability of the underlying content.  In other
 examples, devices such as origin servers or surrogates are able to
 reduce their load by distributing expensive operations onto other
 machines.  In all cases, ICAP has also created a standard interface
 for content adaptation to allow greater flexibility in content
 distribution or the addition of value added services in surrogates.

Elson & Cerpa Informational [Page 4] RFC 3507 ICAP April 2003

 There are two major components in our architecture:
 1. Transaction semantics -- "How do I ask for adaptation?"
 2. Control of policy -- "When am I supposed to ask for adaptation,
    what kind of adaptation do I ask for, and from where?"
 Currently, ICAP defines only the transaction semantics.  For example,
 this document specifies how to send an HTTP message from an ICAP
 client to an ICAP server, specify the URI of the ICAP resource
 requested along with other resource-specific parameters, and receive
 the adapted message.
 Although a necessary building-block, this wire-protocol defined by
 ICAP is of limited use without the second part: an accompanying
 application framework in which it operates.  The more difficult
 policy issue is beyond the scope of the current ICAP protocol, but is
 planned in future work.
 In initial implementations, we expect that implementation-specific
 manual configuration will be used to define policy.  This includes
 the rules for recognizing messages that require adaptation, the URIs
 of available adaptation resources, and so on.  For ICAP clients and
 servers to interoperate, the exact method used to define policy need
 not be consistent across implementations, as long as the policy
 itself is consistent.
 IMPORTANT:
    Note that at this time, in the absence of a policy-framework, it
    is strongly RECOMMENDED that transformations SHOULD only be
    performed on messages with the explicit consent of either the
    content-provider or the user (or both).  Deployment of
    transformation services without the consent of either leads to, at
    best, unpredictable results.  For more discussion of these issues,
    see Section 7.
 Once the full extent of the typical policy decisions are more fully
 understood through experience with these initial implementations,
 later follow-ons to this architecture may define an additional policy
 control protocol.  This future protocol may allow a standard policy
 definition interface complementary to the ICAP transaction interface
 defined here.

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119 [2].

Elson & Cerpa Informational [Page 5] RFC 3507 ICAP April 2003

 The special terminology used in this document is defined below.  The
 majority of these terms are taken as-is from HTTP/1.1 [4] and are
 reproduced here for reference.  A thorough understanding of HTTP/1.1
 is assumed on the part of the reader.
 connection:
    A transport layer virtual circuit established between two programs
    for the purpose of communication.
 message:
    The basic unit of HTTP communication, consisting of a structured
    sequence of octets matching the syntax defined in Section 4 of
    HTTP/1.1 [4] and transmitted via the connection.
 request:
    An HTTP request message, as defined in Section 5 of HTTP/1.1 [4].
 response:
    An HTTP response message, as defined in Section 6 of HTTP/1.1 [4].
 resource:
    A network data object or service that can be identified by a URI,
    as defined in Section 3.2 of HTTP/1.1 [4].  Resources may be
    available in multiple representations (e.g., multiple languages,
    data formats, size, resolutions) or vary in other ways.
 client:
    A program that establishes connections for the purpose of sending
    requests.
 server:
    An application program that accepts connections in order to
    service requests by sending back responses.  Any given program may
    be capable of being both a client and a server; our use of these
    terms refers only to the role being performed by the program for a
    particular connection, rather than to the program's capabilities
    in general. Likewise, any server may act as an origin server,
    surrogate, gateway, or tunnel, switching behavior based on the
    nature of each request.
 origin server:
    The server on which a given resource resides or is to be created.

Elson & Cerpa Informational [Page 6] RFC 3507 ICAP April 2003

 proxy:
    An intermediary program which acts as both a server and a client
    for the purpose of making requests on behalf of other clients.
    Requests are serviced internally or by passing them on, with
    possible translation, to other servers.  A proxy MUST implement
    both the client and server requirements of this specification.
 cache:
    A program's local store of response messages and the subsystem
    that controls its message storage, retrieval, and deletion.  A
    cache stores cachable responses in order to reduce the response
    time and network bandwidth consumption on future, equivalent
    requests.  Any client or server may include a cache, though a
    cache cannot be used by a server that is acting as a tunnel.
 cachable:
    A response is cachable if a cache is allowed to store a copy of
    the response message for use in answering subsequent requests.
    The rules for determining the cachability of HTTP responses are
    defined in Section 13 of [4].  Even if a resource is cachable,
    there may be additional constraints on whether a cache can use the
    cached copy for a particular request.
 surrogate:
    A gateway co-located with an origin server, or at a different
    point in the network, delegated the authority to operate on behalf
    of, and typically working in close co-operation with, one or more
    origin servers.  Responses are typically delivered from an
    internal cache.  Surrogates may derive cache entries from the
    origin server or from another of the origin server's delegates.
    In some cases a surrogate may tunnel such requests.
    Where close co-operation between origin servers and surrogates
    exists, this enables modifications of some protocol requirements,
    including the Cache-Control directives in [4].  Such modifications
    have yet to be fully specified.
    Devices commonly known as "reverse proxies" and "(origin) server
    accelerators" are both more properly defined as surrogates.
 New definitions:
 ICAP resource:
    Similar to an HTTP resource as described above, but the URI refers
    to an ICAP service that performs adaptations of HTTP messages.

Elson & Cerpa Informational [Page 7] RFC 3507 ICAP April 2003

 ICAP server:
    Similar to an HTTP server as described above, except that the
    application services ICAP requests.
 ICAP client:
    A program that establishes connections to ICAP servers for the
    purpose of sending requests.  An ICAP client is often, but not
    always, a surrogate acting on behalf of a user.

3. ICAP Overall Operation

 Before describing ICAP's semantics in detail, we will first give a
 general overview of the protocol's major functions and expected uses.
 As described earlier, ICAP focuses on modification of HTTP requests
 (Section 3.1), and modification of HTTP responses (Section 3.2).

3.1 Request Modification

 In "request modification" (reqmod) mode, an ICAP client sends an HTTP
 request to an ICAP server.  The ICAP server may then:
 1) Send back a modified version of the request.  The ICAP client may
    then perform the modified request by contacting an origin server;
    or, pipeline the modified request to another ICAP server for
    further modification.
 2) Send back an HTTP response to the request.  This is used to
    provide information useful to the user in case of an error (e.g.,
    "you sent a request to view a page you are not allowed to see").
 3) Return an error.
 ICAP clients MUST be able to handle all three types of responses.
 However, in line with the guidance provided for HTTP surrogates in
 Section 13.8 of [4], ICAP client implementors do have flexibility in
 handling errors.  If the ICAP server returns an error, the ICAP
 client may (for example) return the error to the user, execute the
 unadapted request as it arrived from the client, or re-try the
 adaptation again.
 We will illustrate this method with an example application: content
 filtering.  Consider a surrogate that receives a request from a
 client for a web page on an origin server.  The surrogate, acting as
 an ICAP client, sends the client's request to an ICAP server that
 performs URI-based content filtering.  If access to the requested URI
 is allowed, the request is returned to the ICAP client unmodified.
 However, if the ICAP server chooses to disallow access to the
 requested resources, it may either:

Elson & Cerpa Informational [Page 8] RFC 3507 ICAP April 2003

 1) Modify the request so that it points to a page containing an error
    message instead of the original URI.
 2) Return an encapsulated HTTP response that indicates an HTTP error.
 This method can be used for a variety of other applications; for
 example, anonymization, modification of the Accept: headers to handle
 special device requirements, and so forth.
 Typical data flow:
    origin-server
        | /|\
        |  |
     5  |  |  4
        |  |
       \|/ |              2
    ICAP-client    -------------->   ICAP-resource
    (surrogate)    <--------------   on ICAP-server
        | /|\             3
        |  |
     6  |  |  1
        |  |
       \|/ |
       client
 1. A client makes a request to a ICAP-capable surrogate (ICAP client)
    for an object on an origin server.
 2. The surrogate sends the request to the ICAP server.
 3. The ICAP server executes the ICAP resource's service on the
    request and sends the possibly modified request, or a response to
    the request back to the ICAP client.
 If Step 3 returned a request:
 4. The surrogate sends the request, possibly different from original
    client request, to the origin server.
 5. The origin server responds to request.
 6. The surrogate sends the reply (from either the ICAP server or the
    origin server) to the client.

Elson & Cerpa Informational [Page 9] RFC 3507 ICAP April 2003

3.2 Response Modification

 In the "response modification" (respmod) mode, an ICAP client sends
 an HTTP response to an ICAP server.  (The response sent by the ICAP
 client typically has been generated by an origin server.)  The ICAP
 server may then:
 1) Send back a modified version of the response.
 2) Return an error.
 The response modification method is intended for post-processing
 performed on an HTTP response before it is delivered to a client.
 Examples include formatting HTML for display on special devices,
 human language translation, virus checking, and so forth.
 Typical data flow:
    origin-server
        | /|\
        |  |
     3  |  |  2
        |  |
       \|/ |            4
    ICAP-client    -------------->   ICAP-resource
    (surrogate)    <--------------   on ICAP-server
        | /|\            5
        |  |
     6  |  |  1
        |  |
       \|/ |
       client
 1. A client makes a request to a ICAP-capable surrogate (ICAP client)
    for an object on an origin server.
 2. The surrogate sends the request to the origin server.
 3. The origin server responds to request.
 4. The ICAP-capable surrogate sends the origin server's reply to the
    ICAP server.
 5. The ICAP server executes the ICAP resource's service on the origin
    server's reply and sends the possibly modified reply back to the
    ICAP client.

Elson & Cerpa Informational [Page 10] RFC 3507 ICAP April 2003

 6. The surrogate sends the reply, possibly modified from the original
    origin server's reply, to the client.

4. Protocol Semantics

4.1 General Operation

 ICAP is a request/response protocol similar in semantics and usage to
 HTTP/1.1 [4].  Despite the similarity, ICAP is not HTTP, nor is it an
 application protocol that runs over HTTP.  This means, for example,
 that ICAP messages can not be forwarded by HTTP surrogates.  Our
 reasons for not building directly on top of HTTP are discussed in
 Section 8.1.
 ICAP uses TCP/IP as a transport protocol.  The default port is 1344,
 but other ports may be used.  The TCP flow is initiated by the ICAP
 client to a passively listening ICAP server.
 ICAP messages consist of requests from client to server and responses
 from server to client.  Requests and responses use the generic
 message format of RFC 2822 [3] -- that is, a start-line (either a
 request line or a status line), a number of header fields (also known
 as "headers"), an empty line (i.e., a line with nothing preceding the
 CRLF) indicating the end of the header fields, and a message-body.
 The header lines of an ICAP message specify the ICAP resource being
 requested as well as other meta-data such as cache control
 information. The message body of an ICAP request contains the
 (encapsulated) HTTP messages that are being modified.
 As in HTTP/1.1, a single transport connection MAY (perhaps even
 SHOULD) be re-used for multiple request/response pairs.  The rules
 for doing so in ICAP are the same as described in Section 8.1.2.2 of
 [4].  Specifically, requests are matched up with responses by
 allowing only one outstanding request on a transport connection at a
 time.  Multiple parallel connections MAY be used as in HTTP.

4.2 ICAP URIs

 All ICAP requests specify the ICAP resource being requested from the
 server using an ICAP URI.  This MUST be an absolute URI that
 specifies both the complete hostname and the path of the resource
 being requested.  For definitive information on URL syntax and
 semantics, see "Uniform Resource Identifiers (URI): Generic Syntax
 and Semantics," RFC 2396 [1], Section 3.  The URI structure defined
 by ICAP is roughly:

Elson & Cerpa Informational [Page 11] RFC 3507 ICAP April 2003

    ICAP_URI = Scheme ":" Net_Path [ "?" Query ]
    Scheme = "icap"
    Net_Path = "//" Authority [ Abs_Path ]
    Authority = [ userinfo "@" ] host [ ":" port ]
 ICAP adds the new scheme "icap" to the ones defined in RFC 2396.  If
 the port is empty or not given, port 1344 is assumed.  An example
 ICAP URI line might look like this:
    icap://icap.example.net:2000/services/icap-service-1
 An ICAP server MUST be able to recognize all of its hosts names,
 including any aliases, local variations, and numeric IP addresses of
 its interfaces.
 Any arguments that an ICAP client wishes to pass to an ICAP service
 to modify the nature of the service MAY be passed as part of the
 ICAP-URI, using the standard "?"-encoding of attribute-value pairs
 used in HTTP. For example:
    icap://icap.net/service?mode=translate&lang=french

4.3 ICAP Headers

 The following sections define the valid headers for ICAP messages.
 Section 4.3.1 describes headers common to both requests and
 responses.  Request-specific and response-specific headers are
 described in Sections 4.3.2 and 4.3.3, respectively.
 User-defined header extensions are allowed.  In compliance with the
 precedent established by the Internet mail format [3] and later
 adopted by HTTP [4], all user-defined headers MUST follow the "X-"
 naming convention ("X-Extension-Header: Foo").  ICAP implementations
 MAY ignore any "X-" headers without loss of compliance with the
 protocol as defined in this document.
 Each header field consists of a name followed by a colon (":") and
 the field value.  Field names are case-insensitive.  ICAP follows the
 rules describe in section 4.2 of [4].

4.3.1 Headers Common to Requests and Responses

 The headers of all ICAP messages MAY include the following
 directives, defined in ICAP the same as they are in HTTP:

Elson & Cerpa Informational [Page 12] RFC 3507 ICAP April 2003

    Cache-Control
    Connection
    Date
    Expires
    Pragma
    Trailer
    Upgrade
 Note in particular that the "Transfer-Encoding" option is not
 allowed.  The special transfer-encoding requirements of ICAP bodies
 are described in Section 4.4.
 The Upgrade header MAY be used to negotiate Transport-Layer Security
 on an ICAP connection, exactly as described for HTTP/1.1 in [4].
 The ICAP-specific headers defined are:
    Encapsulated  (See Section 4.4)

4.3.2 Request Headers

 Similar to HTTP, ICAP requests MUST start with a request line that
 contains a method, the complete URI of the ICAP resource being
 requested, and an ICAP version string.  The current version number of
 ICAP is "1.0".
 This version of ICAP defines three methods:
    REQMOD  - for Request Modification (Section 4.8)
    RESPMOD - for Response Modification (Section 4.9)
    OPTIONS - to learn about configuration (Section 4.10)
 The OPTIONS method MUST be implemented by all ICAP servers.  All
 other methods are optional and MAY be implemented.
 User-defined extension methods are allowed.  Before attempting to use
 an extension method, an ICAP client SHOULD use the OPTIONS method to
 query the ICAP server's list of supported methods; see Section 4.10.
 (If an ICAP server receives a request for an unknown method, it MUST
 give a 501 error response as described in the next section.)
 Given the URI rules described in Section 4.2, a well-formed ICAP
 request line looks like the following example:
    RESPMOD icap://icap.example.net/translate?mode=french ICAP/1.0

Elson & Cerpa Informational [Page 13] RFC 3507 ICAP April 2003

 A number of request-specific headers are allowed in ICAP requests,
 following the same semantics as the corresponding HTTP request
 headers (Section 5.3 of [4]).  These are:
    Authorization
    Allow (see Section 4.6)
    From  (see Section 14.22 of [4])
    Host (REQUIRED in ICAP as it is in HTTP/1.1)
    Referer (see Section 14.36 of [4])
    User-Agent
 In addition to HTTP-like headers, there are also request headers
 unique to ICAP defined:
    Preview (see Section 4.5)

4.3.3 Response Headers

 ICAP responses MUST start with an ICAP status line, similar in form
 to that used by HTTP, including the ICAP version and a status code.
 For example:
    ICAP/1.0 200 OK
 Semantics of ICAP status codes in ICAP match the status codes defined
 by HTTP (Section 6.1.1 and 10 of [4]), except where otherwise
 indicated in this document; n.b. 100 (Section 4.5) and 204 (Section
 4.6).
 ICAP error codes that differ from their HTTP counterparts are:
 100 - Continue after ICAP Preview (Section 4.5).
 204 - No modifications needed (Section 4.6).
 400 - Bad request.
 404 - ICAP Service not found.
 405 - Method not allowed for service (e.g., RESPMOD requested for
       service that supports only REQMOD).
 408 - Request timeout.  ICAP server gave up waiting for a request
       from an ICAP client.
 500 - Server error.  Error on the ICAP server, such as "out of disk
       space".

Elson & Cerpa Informational [Page 14] RFC 3507 ICAP April 2003

 501 - Method not implemented.  This response is illegal for an
       OPTIONS request since implementation of OPTIONS is mandatory.
 502 - Bad Gateway.  This is an ICAP proxy and proxying produced an
       error.
 503 - Service overloaded.  The ICAP server has exceeded a maximum
       connection limit associated with this service; the ICAP client
       should not exceed this limit in the future.
 505 - ICAP version not supported by server.
 As in HTTP, the 4xx class of error codes indicate client errors, and
 the 5xx class indicate server errors.
 ICAP's response-header fields allow the server to pass additional
 information in the response that cannot be placed in the ICAP's
 status line.
 A response-specific header is allowed in ICAP requests, following the
 same semantics as the corresponding HTTP response headers (Section
 6.2 of [4]).  This is:
    Server (see Section 14.38 of [4])
 In addition to HTTP-like headers, there is also a response header
 unique to ICAP defined:
    ISTag (see Section 4.7)

4.3.4 ICAP-Related Headers in HTTP Messages

 When an ICAP-enabled HTTP surrogate makes an HTTP request to an
 origin server, it is often useful to advise the origin server of the
 surrogate's ICAP capabilities.  Origin servers can use this
 information to modify its response accordingly.  For example, an
 origin server may choose not to insert an advertisement into a page
 if it knows that a downstream ICAP server can insert the ad instead.
 Although this ICAP specification can not mandate how HTTP is used in
 communication between HTTP clients and servers, we do suggest a
 convention: such headers (if used) SHOULD start with "X-ICAP".  HTTP
 clients with ICAP services SHOULD minimally include an "X-ICAP-
 Version: 1.0" header along with their application-specific headers.

Elson & Cerpa Informational [Page 15] RFC 3507 ICAP April 2003

4.4 ICAP Bodies: Encapsulation of HTTP Messages

 The ICAP encapsulation model is a lightweight means of packaging any
 number of HTTP message sections into an encapsulating ICAP message-
 body, in order to allow the vectoring of requests, responses, and
 request/response pairs to an ICAP server.
 This is accomplished by concatenating interesting message parts
 (encapsulatED sections) into a single ICAP message-body (the
 encapsulatING message).  The encapsulated sections may be the headers
 or bodies of HTTP messages.
 Encapsulated bodies MUST be transferred using the "chunked"
 transfer-coding described in Section 3.6.1 of [4].  However,
 encapsulated headers MUST NOT be chunked.  In other words, an ICAP
 message-body switches from being non-chunked to chunked as the body
 passes from the encapsulated header to encapsulated body section.
 (See Examples in Sections 4.8.3 and 4.9.3.).  The motivation behind
 this decision is described in Section 8.2.

4.4.1 The "Encapsulated" Header

 The offset of each encapsulated section's start relative to the start
 of the encapsulating message's body is noted using the "Encapsulated"
 header.  This header MUST be included in every ICAP message.  For
 example, the header
    Encapsulated: req-hdr=0, res-hdr=45, res-body=100
 indicates a message that encapsulates a group of request headers, a
 group of response headers, and then a response body.  Each of these
 is included at the byte-offsets listed.  The byte-offsets are in
 decimal notation for consistency with HTTP's Content-Length header.
 The special entity "null-body" indicates there is no encapsulated
 body in the ICAP message.
 The syntax of an Encapsulated header is:
 encapsulated_header: "Encapsulated: " encapsulated_list
 encapsulated_list: encapsulated_entity |
                    encapsulated_entity ", " encapsulated_list
 encapsulated_entity: reqhdr | reshdr | reqbody | resbody | optbody
 reqhdr  = "req-hdr" "=" (decimal integer)
 reshdr  = "res-hdr" "=" (decimal integer)
 reqbody = { "req-body" | "null-body" } "=" (decimal integer)
 resbody = { "res-body" | "null-body" } "=" (decimal integer)
 optbody = { "opt-body" | "null-body" } "=" (decimal integer)

Elson & Cerpa Informational [Page 16] RFC 3507 ICAP April 2003

 There are semantic restrictions on Encapsulated headers beyond the
 syntactic restrictions.  The order in which the encapsulated parts
 appear in the encapsulating message-body MUST be the same as the
 order in which the parts are named in the Encapsulated header.  In
 other words, the offsets listed in the Encapsulated line MUST be
 monotonically increasing.  In addition, the legal forms of the
 Encapsulated header depend on the method being used (REQMOD, RESPMOD,
 or OPTIONS).  Specifically:
 REQMOD  request  encapsulated_list: [reqhdr] reqbody
 REQMOD  response encapsulated_list: {[reqhdr] reqbody} |
                                     {[reshdr] resbody}
 RESPMOD request  encapsulated_list: [reqhdr] [reshdr] resbody
 RESPMOD response encapsulated_list: [reshdr] resbody
 OPTIONS response encapsulated_list: optbody
 In the above grammar, note that encapsulated headers are always
 optional.  At most one body per encapsulated message is allowed.  If
 no encapsulated body is presented, the "null-body" header is used
 instead; this is useful because it indicates the length of the header
 section.
 Examples of legal Encapsulated headers:
 /* REQMOD request: This encapsulated HTTP request's headers start
  * at offset 0; the HTTP request body (e.g., in a POST) starts
  * at 412. */
 Encapsulated: req-hdr=0, req-body=412
 /* REQMOD request: Similar to the above, but no request body is
  * present (e.g., a GET).  We use the null-body directive instead.
  * In both this case and the previous one, we can tell from the
  * Encapsulated header that the request headers were 412 bytes
  * long. */
 Encapsulated: req-hdr=0, null-body=412
 /* REQMOD response: ICAP server returned a modified request,
  * with body */
 Encapsulated: req-hdr=0, req-body=512
 /* RESPMOD request: Request headers at 0, response headers at 822,
  * response body at 1655.  Note that no request body is allowed in
  * RESPMOD requests. */
 Encapsulated: req-hdr=0, res-hdr=822, res-body=1655
 /* RESPMOD or REQMOD response: header and body returned */
 Encapsulated: res-hdr=0, res-body=749

Elson & Cerpa Informational [Page 17] RFC 3507 ICAP April 2003

 /* OPTIONS response when there IS an options body */
 Encapsulated: opt-body=0
 /* OPTIONS response when there IS NOT an options body */
 Encapsulated: null-body=0

4.4.2 Encapsulated HTTP Headers

 By default, ICAP messages may encapsulate HTTP message headers and
 entity bodies.  HTTP headers MUST start with the request-line or
 status-line for requests and responses, respectively, followed by
 interesting HTTP headers.
 The encapsulated headers MUST be terminated by a blank line, in order
 to make them human readable, and in order to terminate line-by-line
 HTTP parsers.
 HTTP/1.1 makes a distinction between end-to-end headers and hop-by-
 hop headers (see Section 13.5.1 of [4]).  End-to-end headers are
 meaningful to the ultimate recipient of a message, whereas hop-by-hop
 headers are meaningful only for a single transport-layer connection.
 Hop-by-hop headers include Connection, Keep-Alive, and so forth.  All
 end-to-end HTTP headers SHOULD be encapsulated, and all hop-by-hop
 headers MUST NOT be encapsulated.
 Despite the above restrictions on encapsulation, the hop-by-hop
 Proxy-Authenticate and Proxy-Authorization headers MUST be forwarded
 to the ICAP server in the ICAP header section (not the encapsulated
 message).  This allows propagation of client credentials that might
 have been sent to the ICAP client in cases where the ICAP client is
 also an HTTP surrogate.  Note that this does not contradict HTTP/1.1,
 which explicitly states "A proxy MAY relay the credentials from the
 client request to the next proxy if that is the mechanism by which
 the proxies cooperatively authenticate a given request."  (Section
 14.34).
 The Via header of an encapsulated message SHOULD be modified by an
 ICAP server as if the encapsulated message were traveling through an
 HTTP surrogate.  The Via header added by an ICAP server MUST specify
 protocol as ICAP/1.0.

4.5 Message Preview

 ICAP REQMOD or RESPMOD requests sent by the ICAP client to the ICAP
 server may include a "preview".  This feature allows an ICAP server
 to see the beginning of a transaction, then decide if it wants to

Elson & Cerpa Informational [Page 18] RFC 3507 ICAP April 2003

 opt-out of the transaction early instead of receiving the remainder
 of the request message.  Previewing can yield significant performance
 improvements in a variety of situations, such as the following:
  1. Virus-checkers can certify a large fraction of files as "clean"

just by looking at the file type, file name extension, and the

    first few bytes of the file.  Only the remaining files need to be
    transmitted to the virus-checking ICAP server in their entirety.
  1. Content filters can use Preview to decide if an HTTP entity needs

to be inspected (the HTTP file type alone is not enough in cases

    where "text" actually turns out to be graphics data).  The magic
    numbers at the front of the file can identify a file as a JPEG or
    GIF.
  1. If an ICAP server wants to transcode all GIF87 files into GIF89

files, then the GIF87 files could quickly be detected by looking

    at the first few body bytes of the file.
  1. If an ICAP server wants to force all cacheable files to expire in

24 hours or less, then this could be implemented by selecting HTTP

    messages with expiries more than 24 hours in the future.
 ICAP servers SHOULD use the OPTIONS method (see Section 4.10) to
 specify how many bytes of preview are needed for a particular ICAP
 application on a per-resource basis.  Clients SHOULD be able to
 provide Previews of at least 4096 bytes.  Clients furthermore SHOULD
 provide a Preview when using any ICAP resource that has indicated a
 Preview is useful.  (This indication might be provided via the
 OPTIONS method, or some other "out-of-band" configuration.)  Clients
 SHOULD NOT provide a larger Preview than a server has indicated it is
 willing to accept.
 To effect a Preview, an ICAP client MUST add a "Preview:" header to
 its request headers indicating the length of the preview.  The ICAP
 client then sends:
  1. all of the encapsulated header sections, and
  1. the beginning of the encapsulated body section, if any, up to the

number of bytes advertised in the Preview (possibly 0).

 After the Preview is sent, the client stops and waits for an
 intermediate response from the ICAP server before continuing.  This
 mechanism is similar to the "100-Continue" feature found in HTTP,
 except that the stop-and-wait point can be within the message body.
 In contrast, HTTP requires that the point must be the boundary
 between the headers and body.

Elson & Cerpa Informational [Page 19] RFC 3507 ICAP April 2003

 For example, to effect a Preview consisting of only encapsulated HTTP
 headers, the ICAP client would add the following header to the ICAP
 request:
    Preview: 0
 This indicates that the ICAP client will send only the encapsulated
 header sections to the ICAP server, then it will send a zero-length
 chunk and stop and wait for a "go ahead" to send more encapsulated
 body bytes to the ICAP server.
 Similarly, the ICAP header:
    Preview: 4096
 Indicates that the ICAP client will attempt to send 4096 bytes of
 origin server data in the encapsulated body of the ICAP request to
 the ICAP server.  It is important to note that the actual transfer
 may be less, because the ICAP client is acting like a surrogate and
 is not looking ahead to find the total length of the origin server
 response.  The entire ICAP encapsulated header section(s) will be
 sent, followed by up to 4096 bytes of encapsulated HTTP body.  The
 chunk body terminator "0\r\n\r\n" is always included in these
 transactions.
 After sending the preview, the ICAP client will wait for a response
 from the ICAP server.  The response MUST be one of the following:
  1. 204 No Content. The ICAP server does not want to (or can not)

modify the ICAP client's request. The ICAP client MUST treat this

    the same as if it had sent the entire message to the ICAP server
    and an identical message was returned.
  1. ICAP reqmod or respmod response, depending what method was the

original request. See Section 4.8.2 and 4.9.2 for the format of

    reqmod and respmod responses.
  1. 100 Continue. If the entire encapsulated HTTP body did not fit

in the preview, the ICAP client MUST send the remainder of its

    ICAP message, starting from the first chunk after the preview.  If
    the entire message fit in the preview (detected by the "EOF"
    symbol explained below), then the ICAP server MUST NOT respond
    with 100 Continue.
 When an ICAP client is performing a preview, it may not yet know how
 many bytes will ultimately be available in the arriving HTTP message
 that it is relaying to the HTTP server.  Therefore, ICAP defines a
 way for ICAP clients to indicate "EOF" to ICAP servers if one

Elson & Cerpa Informational [Page 20] RFC 3507 ICAP April 2003

 unexpectedly arrives during the preview process.  This is a
 particularly useful optimization if a header-only HTTP response
 arrives at the ICAP client (i.e., zero bytes of body); only a single
 round trip will be needed for the complete ICAP server response.
 We define an HTTP chunk-extension of "ieof" to indicate that an ICAP
 chunk is the last chunk (see [4]).  The ICAP server MUST strip this
 chunk extension before passing the chunk data to an ICAP application
 process.
 For example, consider an ICAP client that has just received HTTP
 response headers from an origin server and initiates an ICAP RESPMOD
 transaction to an ICAP server.  It does not know yet how many body
 bytes will be arriving from the origin server because the server is
 not using the Content-Length header.  The ICAP client informs the
 ICAP server that it will be sending a 1024-byte preview using a
 "Preview:  1024" request header.  If the HTTP origin server then
 closes its connection to the ICAP client before sending any data
 (i.e., it provides a zero-byte body), the corresponding zero-byte
 preview for that zero-byte origin response would appear as follows:
    \r\n
    0; ieof\r\n\r\n
 If an ICAP server sees this preview, it knows from the presence of
 "ieof" that the client will not be sending any more chunk data.  In
 this case, the server MUST respond with the modified response or a
 204 No Content message right away.  It MUST NOT send a 100-Continue
 response in this case.  (In contrast, if the origin response had been
 1 byte or larger, the "ieof" would not have appeared.  In that case,
 an ICAP server MAY reply with 100-Continue, a modified response, or
 204 No Content.)
 In another example, if the preview is 1024 bytes and the origin
 response is 1024 bytes in two chunks, then the encapsulation would
 appear as follows:
    200\r\n
    <512 bytes of data>\r\n
    200\r\n
    <512 bytes of data>\r\n
    0; ieof\r\n\r\n
    <204 or modified response> (100 Continue disallowed due to ieof)
 If the preview is 1024 bytes and the origin response is 1025 bytes
 (and the ICAP server responds with 100-continue), then these chunks
 would appear on the wire:

Elson & Cerpa Informational [Page 21] RFC 3507 ICAP April 2003

    200\r\n
    <512 bytes of data>\r\n
    200\r\n
    <512 bytes of data>\r\n
    0\r\n
    <100 Continue Message>
    1\r\n
    <1 byte of data>\r\n
    0\r\n\r\n  <no ieof because we are no longer in preview mode>
 Once the ICAP server receives the eof indicator, it finishes reading
 the current chunk stream.
 Note that when offering a Preview, the ICAP client is committing to
 temporarily buffer the previewed portion of the message so that it
 can honor a "204 No Content" response.  The remainder of the message
 is not necessarily buffered; it might be pipelined directly from
 another source to the ICAP server after a 100-Continue.

4.6 "204 No Content" Responses outside of Previews

 An ICAP client MAY choose to honor "204 No Content" responses for an
 entire message.  This is the decision of the client because it
 imposes a burden on the client of buffering the entire message.
 An ICAP client MAY include "Allow: 204" in its request headers,
 indicating that the server MAY reply to the message with a "204 No
 Content" response if the object does not need modification.
 If an ICAP server receives a request that does not have "Allow: 204",
 it MUST NOT reply with a 204.  In this case, an ICAP server MUST
 return the entire message back to the client, even though it is
 identical to the message it received.
 The ONLY EXCEPTION to this rule is in the case of a message preview,
 as described in the previous section.  If this is the case, an ICAP
 server can respond with a 204 No Content message in response to a
 message preview EVEN if the original request did not have the "Allow:
 204" header.

4.7 ISTag Response Header

 The ISTag ("ICAP Service Tag") response-header field provides a way
 for ICAP servers to send a service-specific "cookie" to ICAP clients
 that represents a service's current state.  It is a 32-byte-maximum
 alphanumeric string of data (not including the null character) that

Elson & Cerpa Informational [Page 22] RFC 3507 ICAP April 2003

 may, for example, be a representation of the software version or
 configuration of a service.  An ISTag validates that previous ICAP
 server responses can still be considered fresh by an ICAP client that
 may be caching them.  If a change on the ICAP server invalidates
 previous responses, the ICAP server can invalidate portions of the
 ICAP client's cache by changing its ISTag.  The ISTag MUST be
 included in every ICAP response from an ICAP server.
 For example, consider a virus-scanning ICAP service.  The ISTag might
 be a combination of the virus scanner's software version and the
 release number of its virus signature database.  When the database is
 updated, the ISTag can be changed to invalidate all previous
 responses that had been certified as "clean" and cached with the old
 ISTag.
 ISTag is similar, but not identical, to the HTTP ETag.  While an ETag
 is a validator for a particular entity (object), an ISTag validates
 all entities generated by a particular service (URI).  A change in
 the ISTag invalidates all the other entities provided a service with
 the old ISTag, not just the entity whose response contained the
 updated ISTag.
 The syntax of an ISTag is simply:
    ISTag = "ISTag: " quoted-string
 In this document we use the quoted-string definition defined in
 section 2.2 of [4].
 For example:
    ISTag: "874900-1994-1c02798"

4.8 Request Modification Mode

 In this method, described in Section 3.1, an ICAP client sends an
 HTTP request to an ICAP server.  The ICAP server returns a modified
 version of the request, an HTTP response, or (if the client indicates
 it supports 204 responses) an indication that no modification is
 required.

4.8.1 Request

 In REQMOD mode, the ICAP request MUST contain an encapsulated HTTP
 request.  The headers and body (if any) MUST both be encapsulated,
 except that hop-by-hop headers are not encapsulated.

Elson & Cerpa Informational [Page 23] RFC 3507 ICAP April 2003

4.8.2 Response

 The response from the ICAP server back to the ICAP client may take
 one of four forms:
  1. An error indication,
  1. A 204 indicating that the ICAP client's request requires no

adaptation (see Section 4.6 for limitations of this response),

  1. An encapsulated, adapted version of the ICAP client's request, or
  1. An encapsulated HTTP error response. Note that Request

Modification requests may only be satisfied with HTTP responses in

    cases when the HTTP response is an error (e.g., 403 Forbidden).
 The first line of the response message MUST be a status line as
 described in Section 4.3.3.  If the return code is a 2XX, the ICAP
 client SHOULD continue its normal execution of the request.  If the
 ICAP client is a surrogate, this may include serving an object from
 its cache or forwarding the modified request to an origin server.
 Note it is valid for a 2XX ICAP response to contain an encapsulated
 HTTP error response, which in turn should be returned to the
 downstream client by the ICAP client.
 For other return codes that indicate an error, the ICAP client MAY
 (for example) return the error to the downstream client or user,
 execute the unadapted request as it arrived from the client, or re-
 try the adaptation again.
 The modified request headers, if any, MUST be returned to the ICAP
 client using appropriate encapsulation as described in Section 4.4.

4.8.3 Examples

 Consider the following example, in which a surrogate receives a
 simple GET request from a client.  The surrogate, acting as an ICAP
 client, then forwards this request to an ICAP server for
 modification.  The ICAP server modifies the request headers and sends
 them back to the ICAP client.  Our hypothetical ICAP server will
 modify several headers and strip the cookie from the original
 request.
 In all of our examples, we include the extra meta-data added to the
 message due to chunking the encapsulated message body (if any).  We
 assume that end-of-line terminations, and blank lines, are two-byte
 "CRLF" sequences.

Elson & Cerpa Informational [Page 24] RFC 3507 ICAP April 2003

 ICAP Request Modification Example 1 - ICAP Request
 ----------------------------------------------------------------
 REQMOD icap://icap-server.net/server?arg=87 ICAP/1.0
 Host: icap-server.net
 Encapsulated: req-hdr=0, null-body=170
 GET / HTTP/1.1
 Host: www.origin-server.com
 Accept: text/html, text/plain
 Accept-Encoding: compress
 Cookie: ff39fk3jur@4ii0e02i
 If-None-Match: "xyzzy", "r2d2xxxx"
  1. —————————————————————

ICAP Request Modification Example 1 - ICAP Response

  1. —————————————————————

ICAP/1.0 200 OK

 Date: Mon, 10 Jan 2000  09:55:21 GMT
 Server: ICAP-Server-Software/1.0
 Connection: close
 ISTag: "W3E4R7U9-L2E4-2"
 Encapsulated: req-hdr=0, null-body=231
 GET /modified-path HTTP/1.1
 Host: www.origin-server.com
 Via: 1.0 icap-server.net (ICAP Example ReqMod Service 1.1)
 Accept: text/html, text/plain, image/gif
 Accept-Encoding: gzip, compress
 If-None-Match: "xyzzy", "r2d2xxxx"
  1. —————————————————————
 The second example is similar to the first, except that the request
 being modified in this case is a POST instead of a GET.  Note that
 the encapsulated Content-Length argument has been modified to reflect
 the modified body of the POST message.  The outer ICAP message does
 not need a Content-Length header because it uses chunking (not
 shown).
 In this second example, the Encapsulated header shows the division
 between the forwarded header and forwarded body, for both the request
 and the response.
 ICAP Request Modification Example 2 - ICAP Request
 ----------------------------------------------------------------
 REQMOD icap://icap-server.net/server?arg=87 ICAP/1.0
 Host: icap-server.net
 Encapsulated: req-hdr=0, req-body=147

Elson & Cerpa Informational [Page 25] RFC 3507 ICAP April 2003

 POST /origin-resource/form.pl HTTP/1.1
 Host: www.origin-server.com
 Accept: text/html, text/plain
 Accept-Encoding: compress
 Pragma: no-cache
 1e
 I am posting this information.
 0
  1. —————————————————————

ICAP Request Modification Example 2 - ICAP Response

  1. —————————————————————

ICAP/1.0 200 OK

 Date: Mon, 10 Jan 2000  09:55:21 GMT
 Server: ICAP-Server-Software/1.0
 Connection: close
 ISTag: "W3E4R7U9-L2E4-2"
 Encapsulated: req-hdr=0, req-body=244
 POST /origin-resource/form.pl HTTP/1.1
 Host: www.origin-server.com
 Via: 1.0 icap-server.net (ICAP Example ReqMod Service 1.1)
 Accept: text/html, text/plain, image/gif
 Accept-Encoding: gzip, compress
 Pragma: no-cache
 Content-Length: 45
 2d
 I am posting this information.  ICAP powered!
 0
  1. —————————————————————

Finally, this third example shows an ICAP server returning an error

 response when it receives a Request Modification request.
 ICAP Request Modification Example 3 - ICAP Request
 ----------------------------------------------------------------
 REQMOD icap://icap-server.net/content-filter ICAP/1.0
 Host: icap-server.net
 Encapsulated: req-hdr=0, null-body=119
 GET /naughty-content HTTP/1.1
 Host: www.naughty-site.com
 Accept: text/html, text/plain
 Accept-Encoding: compress
  1. —————————————————————

Elson & Cerpa Informational [Page 26] RFC 3507 ICAP April 2003

 ICAP Request Modification Example 3 - ICAP Response
 ----------------------------------------------------------------
 ICAP/1.0 200 OK
 Date: Mon, 10 Jan 2000  09:55:21 GMT
 Server: ICAP-Server-Software/1.0
 Connection: close
 ISTag: "W3E4R7U9-L2E4-2"
 Encapsulated: res-hdr=0, res-body=213
 HTTP/1.1 403 Forbidden
 Date: Wed, 08 Nov 2000 16:02:10 GMT
 Server: Apache/1.3.12 (Unix)
 Last-Modified: Thu, 02 Nov 2000 13:51:37 GMT
 ETag: "63600-1989-3a017169"
 Content-Length: 58
 Content-Type: text/html
 3a
 Sorry, you are not allowed to access that naughty content.
 0
  1. —————————————————————

4.9 Response Modification Mode

 In this method, described in Section 3.2, an ICAP client sends an
 origin server's HTTP response to an ICAP server, and (if available)
 the original client request that caused that response.  Similar to
 Request Modification method, the response from the ICAP server can be
 an adapted HTTP response, an error, or a 204 response code indicating
 that no adaptation is required.

4.9.1 Request

 Using encapsulation described in Section 4.4, the header and body of
 the HTTP response to be modified MUST be included in the ICAP body.
 If available, the header of the original client request SHOULD also
 be included.  As with the other method, the hop-by-hop headers of the
 encapsulated messages MUST NOT be forwarded.  The Encapsulated header
 MUST indicate the byte-offsets of the beginning of each of these four
 parts.

4.9.2 Response

 The response from the ICAP server looks just like a reply in the
 Request Modification method (Section 4.8); that is,
  1. An error indication,

Elson & Cerpa Informational [Page 27] RFC 3507 ICAP April 2003

  1. An encapsulated and potentially modified HTTP response header and

response body, or

  1. An HTTP response 204 indicating that the ICAP client's request

requires no adaptation.

 The first line of the response message MUST be a status line as
 described in Section 4.3.3.  If the return code is a 2XX, the ICAP
 client SHOULD continue its normal execution of the response.  The
 ICAP client MAY re-examine the headers in the response's message
 headers in order to make further decisions about the response (e.g.,
 its cachability).
 For other return codes that indicate an error, the ICAP client SHOULD
 NOT return these directly to downstream client, since these errors
 only make sense in the ICAP client/server transaction.
 The modified response headers, if any, MUST be returned to the ICAP
 client using appropriate encapsulation as described in Section 4.4.

4.9.3 Examples

 In Example 4, an ICAP client is requesting modification of an entity
 that was returned as a result of a client GET.  The original client
 GET was to an origin server at "www.origin-server.com"; the ICAP
 server is at "icap.example.org".
 ICAP Response Modification Example 4 - ICAP Request
 ----------------------------------------------------------------
 RESPMOD icap://icap.example.org/satisf ICAP/1.0
 Host: icap.example.org
 Encapsulated: req-hdr=0, res-hdr=137, res-body=296
 GET /origin-resource HTTP/1.1
 Host: www.origin-server.com
 Accept: text/html, text/plain, image/gif
 Accept-Encoding: gzip, compress
 HTTP/1.1 200 OK
 Date: Mon, 10 Jan 2000 09:52:22 GMT
 Server: Apache/1.3.6 (Unix)
 ETag: "63840-1ab7-378d415b"
 Content-Type: text/html
 Content-Length: 51

Elson & Cerpa Informational [Page 28] RFC 3507 ICAP April 2003

 33
 This is data that was returned by an origin server.
 0
  1. —————————————————————
 ICAP Response Modification Example 4 - ICAP Response
 ----------------------------------------------------------------
 ICAP/1.0 200 OK
 Date: Mon, 10 Jan 2000  09:55:21 GMT
 Server: ICAP-Server-Software/1.0
 Connection: close
 ISTag: "W3E4R7U9-L2E4-2"
 Encapsulated: res-hdr=0, res-body=222
 HTTP/1.1 200 OK
 Date: Mon, 10 Jan 2000  09:55:21 GMT
 Via: 1.0 icap.example.org (ICAP Example RespMod Service 1.1)
 Server: Apache/1.3.6 (Unix)
 ETag: "63840-1ab7-378d415b"
 Content-Type: text/html
 Content-Length: 92
 5c
 This is data that was returned by an origin server, but with
 value added by an ICAP server.
 0
  1. —————————————————————

4.10 OPTIONS Method

 The ICAP "OPTIONS" method is used by the ICAP client to retrieve
 configuration information from the ICAP server.  In this method, the
 ICAP client sends a request addressed to a specific ICAP resource and
 receives back a response with options that are specific to the
 service named by the URI.  All OPTIONS requests MAY also return
 options that are global to the server (i.e., apply to all services).

4.10.1 OPTIONS Request

 The OPTIONS method consists of a request-line, as described in
 Section 4.3.2, such as the following example:
 OPTIONS icap://icap.server.net/sample-service ICAP/1.0 User-Agent:
 ICAP-client-XYZ/1.001

Elson & Cerpa Informational [Page 29] RFC 3507 ICAP April 2003

 Other headers are also allowed as described in Section 4.3.1 and
 Section 4.3.2 (for example, Host).

4.10.2 OPTIONS Response

 The OPTIONS response consists of a status line as described in
 section 4.3.3 followed by a series of header field names-value pairs
 optionally followed by an opt-body.  Multiple values in the value
 field MUST be separated by commas.  If an opt-body is present in the
 OPTIONS response, the Opt-body-type header describes the format of
 the opt-body.
 The OPTIONS headers supported in this version of the protocol are:
  1. - Methods:
    The method that is supported by this service.  This header MUST be
    included in the OPTIONS response.  The OPTIONS method MUST NOT be
    in the Methods' list since it MUST be supported by all the ICAP
    server implementations.  Each service should have a distinct URI
    and support only one method in addition to OPTIONS (see Section
    6.4).
    For example:
    Methods: RESPMOD
  1. - Service:
    A text description of the vendor and product name.  This header
    MAY be included in the OPTIONS response.
    For example:
    Service: XYZ Technology Server 1.0
  1. - ISTag:
    See section 4.7 for details.  This header MUST be included in the
    OPTIONS response.
    For example:
    ISTag: "5BDEEEA9-12E4-2"
  1. - Encapsulated:
    This header MUST be included in the OPTIONS response; see Section
    4.4.

Elson & Cerpa Informational [Page 30] RFC 3507 ICAP April 2003

    For example:
    Encapsulated: opt-body=0
  1. - Opt-body-type:
    A token identifying the format of the opt-body.  (Valid opt-body
    types are not defined by ICAP.)  This header MUST be included in
    the OPTIONS response ONLY if an opt-body type is present.
    For example:
    Opt-body-type: XML-Policy-Table-1.0
  1. - Max-Connections:
    The maximum number of ICAP connections the server is able to
    support.  This header MAY be included in the OPTIONS response.
    For example:
    Max-Connections: 1500
  1. - Options-TTL:
    The time (in seconds) for which this OPTIONS response is valid.
    If none is specified, the OPTIONS response does not expire.  This
    header MAY be included in the OPTIONS response.  The ICAP client
    MAY reissue an OPTIONS request once the Options-TTL expires.
    For example:
    Options-TTL: 3600
  1. - Date:
    The server's clock, specified as an RFC 1123 compliant date/time
    string.  This header MAY be included in the OPTIONS response.
    For example:
    Date: Fri, 15 Jun 2001 04:33:55 GMT
  1. - Service-ID:
    A short label identifying the ICAP service.  It MAY be used in
    attribute header names.  This header MAY be included in the
    OPTIONS response.
    For example:
    Service-ID: xyztech

Elson & Cerpa Informational [Page 31] RFC 3507 ICAP April 2003

  1. - Allow:
    A directive declaring a list of optional ICAP features that this
    server has implemented.  This header MAY be included in the
    OPTIONS response.  In this document we define the value "204" to
    indicate that the ICAP server supports a 204 response.
    For example:
    Allow: 204
  1. - Preview:
    The number of bytes to be sent by the ICAP client during a
    preview.  This header MAY be included in the OPTIONS response.
    For example:
    Preview: 1024
  1. - Transfer-Preview:
    A list of file extensions that should be previewed to the ICAP
    server before sending them in their entirety.  This header MAY be
    included in the OPTIONS response.  Multiple file extensions values
    should be separated by commas.  The wildcard value "*" specifies
    the default behavior for all the file extensions not specified in
    any other Transfer-* header (see below).
    For example:
    Transfer-Preview: *
  1. - Transfer-Ignore:
    A list of file extensions that should NOT be sent to the ICAP
    server.  This header MAY be included in the OPTIONS response.
    Multiple file extensions should be separated by commas.
    For example:
    Transfer-Ignore: html
  1. - Transfer-Complete:
    A list of file extensions that should be sent in their entirety
    (without preview) to the ICAP server.  This header MAY be included
    in the OPTIONS response.  Multiple file extensions values should
    be separated by commas.
    For example:
    Transfer-Complete: asp, bat, exe, com, ole

Elson & Cerpa Informational [Page 32] RFC 3507 ICAP April 2003

 Note: If any of Transfer-* are sent, exactly one of them MUST contain
 the wildcard value "*" to specify the default.  If no Transfer-* are
 sent, all responses will be sent in their entirety (without Preview).

4.10.3 OPTIONS Examples

 In example 5, an ICAP Client sends an OPTIONS Request to an ICAP
 Service named icap.server.net/sample-service in order to get
 configuration information for the service provided.
 ICAP OPTIONS Example 5 - ICAP OPTIONS Request
 ----------------------------------------------------------------
 OPTIONS icap://icap.server.net/sample-service ICAP/1.0
 Host: icap.server.net
 User-Agent: BazookaDotCom-ICAP-Client-Library/2.3
  1. —————————————————————
 ICAP OPTIONS Example 5 - ICAP OPTIONS Response
 ----------------------------------------------------------------
 ICAP/1.0 200 OK
 Date: Mon, 10 Jan 2000  09:55:21 GMT
 Methods: RESPMOD
 Service: FOO Tech Server 1.0
 ISTag: "W3E4R7U9-L2E4-2"
 Encapsulated: null-body=0
 Max-Connections: 1000
 Options-TTL: 7200
 Allow: 204
 Preview: 2048
 Transfer-Complete: asp, bat, exe, com
 Transfer-Ignore: html
 Transfer-Preview: *
  1. —————————————————————

5. Caching

 ICAP servers' responses MAY be cached by ICAP clients, just as any
 other surrogate might cache HTTP responses.  Similar to HTTP, ICAP
 clients MAY always store a successful response (see sections 4.8.2
 and 4.9.2) as a cache entry, and MAY return it without validation if
 it is fresh. ICAP servers use the caching directives described in
 HTTP/1.1 [4].
 In Request Modification mode, the ICAP server MAY include caching
 directives in the ICAP header section of the ICAP response (NOT in
 the encapsulated HTTP request of the ICAP message body).  In Response

Elson & Cerpa Informational [Page 33] RFC 3507 ICAP April 2003

 Modification mode, the ICAP server MAY add or modify the HTTP caching
 directives located in the encapsulated HTTP response (NOT in the ICAP
 header section).  Consequently, the ICAP client SHOULD look for
 caching directives in the ICAP headers in case of REQMOD, and in the
 encapsulated HTTP response in case of RESPMOD.
 In cases where an ICAP server returns a modified version of an object
 created by an origin server, such as in Response Modification mode,
 the expiration of the ICAP-modified object MUST NOT be longer than
 that of the origin object.  In other words, ICAP servers MUST NOT
 extend the lifetime of origin server objects, but MAY shorten it.
 In cases where the ICAP server is the authoritative source of an ICAP
 response, such as in Request Modification mode, the ICAP server is
 not restricted in its expiration policy.
 Note that the ISTag response-header may also be used to providing
 caching hints to clients; see Section 4.7.

6. Implementation Notes

6.1 Vectoring Points

 The definition of the ICAP protocol itself only describes two
 different adaptation channels: modification (and satisfaction) of
 requests, and modifications of replies.  However, an ICAP client
 implementation is likely to actually distinguish among four different
 classes of adaptation:
 1.  Adaptation of client requests.  This is adaptation done every
     time a request arrives from a client.  This is adaptation done
     when a request is "on its way into the cache".  Factors such as
     the state of the objects currently cached will determine whether
     or not this request actually gets forwarded to an origin server
     (instead of, say, getting served off the cache's disk).  An
     example of this type of adaptation would be special access
     control or authentication services that must be performed on a
     per-client basis.
 2.  Adaptation of requests on their way to an origin server.
     Although this type of adaptation is also an adaptation of
     requests similar to (1), it describes requests that are "on their
     way out of the cache"; i.e., if a request actually requires that
     an origin server be contacted.  These adaptation requests are not
     necessarily specific to particular clients.  An example would be
     addition of "Accept:"  headers for special devices; these
     adaptations can potentially apply to many clients.

Elson & Cerpa Informational [Page 34] RFC 3507 ICAP April 2003

 3.  Adaptations of responses coming from an origin server.  This is
     the adaptation of an object "on its way into the cache".  In
     other words, this is adaptation that a surrogate might want to
     perform on an object before caching it.  The adapted object may
     subsequently served to many clients.  An example of this type of
     adaptation is virus checking: a surrogate will want to check an
     incoming origin reply for viruses once, before allowing it into
     the cache -- not every time the cached object is served to a
     client.
     Adaptation of responses coming from the surrogate, heading back
     to the client.  Although this type of adaptation, like (3), is
     the adaptation of a response, it is client-specific.  Client
     reply adaptation is adaptation that is required every time an
     object is served to a client, even if all the replies come from
     the same cached object off of disk.  Ad insertion is a common
     form of this kind of adaptation; e.g., if a popular (cached)
     object that rarely changes needs a different ad inserted into it
     every time it is served off disk to a client.  Note that the
     relationship between adaptations of type (3) and (4) is analogous
     to the relationship between types (2) and (1).
 Although the distinction among these four adaptation points is
 critical for ICAP client implementations, the distinction is not
 significant for the ICAP protocol itself.  From the point of view of
 an ICAP server, a request is a request -- the ICAP server doesn't
 care what policy led the ICAP client to generate the request.  We
 therefore did not make these four channels explicit in ICAP for
 simplicity.

6.2 Application Level Errors

 Section 4 described "on the wire" protocol errors that MUST be
 standardized across implementations to ensure interoperability.  In
 this section, we describe errors that are communicated between ICAP
 software and the clients and servers on which they are implemented.
 Although such errors are implementation dependent and do not
 necessarily need to be standardized because they are "within the
 box", they are presented here as advice to future implementors based
 on past implementation experience.

Elson & Cerpa Informational [Page 35] RFC 3507 ICAP April 2003

 Error name                                     Value
 ====================================================
 ICAP_CANT_CONNECT                               1000
 ICAP_SERVER_RESPONSE_CLOSE                      1001
 ICAP_SERVER_RESPONSE_RESET                      1002
 ICAP_SERVER_UNKNOWN_CODE                        1003
 ICAP_SERVER_UNEXPECTED_CLOSE_204                1004
 ICAP_SERVER_UNEXPECTED_CLOSE                    1005
 1000 ICAP_CANT_CONNECT:
     "Cannot connect to ICAP server".
     The ICAP server is not connected on the socket.  Maybe the ICAP
     server is dead or it is not connected on the socket.
 1001 ICAP_SERVER_RESPONSE_CLOSE:
     "ICAP Server closed connection while reading response".
     The ICAP server TCP-shutdowns the connection before the ICAP
     client can send all the body data.
 1002 ICAP_SERVER_RESPONSE_RESET:
     "ICAP Server reset connection while reading response".
     The ICAP server TCP-reset the connection before the ICAP client
     can send all the body data.
 1003 ICAP_SERVER_UNKNOWN_CODE:
     "ICAP Server sent unknown response code".
     An unknown ICAP response code (see Section 4.x) was received by
     the ICAP client.
 1004 ICAP_SERVER_UNEXPECTED_CLOSE_204:
     "ICAP Server closed connection on 204 without 'Connection: close'
     header".
     An ICAP server MUST send the "Connection: close" header if
     intends to close after the current transaction.
 1005 ICAP_SERVER_UNEXPECTED_CLOSE:
     "ICAP Server closed connection as ICAP client wrote body
     preview".

Elson & Cerpa Informational [Page 36] RFC 3507 ICAP April 2003

6.3 Use of Chunked Transfer-Encoding

 For simplicity, ICAP messages MUST use the "chunked" transfer-
 encoding within the encapsulated body section as defined in HTTP/1.1
 [4].  This requires that ICAP client implementations convert incoming
 objects "on the fly" to chunked from whatever transfer-encoding on
 which they arrive.  However, the transformation is simple:
  1. For objects arriving using "Content-Length" headers, one big chunk

can be created of the same size as indicated in the Content-Length

    header.
  1. For objects arriving using a TCP close to signal the end of the

object, each incoming group of bytes read from the OS can be

    converted into a chunk (by writing the length of the bytes read,
    followed by the bytes themselves)
  1. For objects arriving using chunked encoding, they can be

retransmitted as is (without re-chunking).

6.4 Distinct URIs for Distinct Services

 ICAP servers SHOULD assign unique URIs to each service they provide,
 even if such services might theoretically be differentiated based on
 their method.  In other words, a REQMOD and RESPMOD service should
 never have the same URI, even if they do something that is
 conceptually the same.
 This situation in ICAP is similar to that found in HTTP where it
 might, in theory, be possible to perform a GET or a POST to the same
 URI and expect two different results.  This kind of overloading of
 URIs only causes confusion and should be avoided.

7. Security Considerations

7.1 Authentication

 Authentication in ICAP is very similar to proxy authentication in
 HTTP as specified in RFC 2617.  Specifically, the following rules
 apply:
  1. WWW-Authenticate challenges and responses are for end-to-end

authentication between a client (user) and an origin server. As

    any proxy, ICAP clients and ICAP servers MUST forward these
    headers without modification.

Elson & Cerpa Informational [Page 37] RFC 3507 ICAP April 2003

  1. If authentication is required between an ICAP client and ICAP

server, hop-by-hop Proxy Authentication as described in RFC 2617

    MUST be used.
 There are potential applications where a user (as opposed to ICAP
 client) might have rights to access an ICAP service.  In this version
 of the protocol, we assume that ICAP clients and ICAP servers are
 under the same administrative domain, and contained in a single trust
 domain. Therefore, in these cases, we assume that it is sufficient
 for users to authenticate themselves to the ICAP client (which is a
 surrogate from the point of view from the user).  This type of
 authentication will also be Proxy Authentication as described in RFC
 2617.
 This standard explicitly excludes any method for a user to
 authenticate directly to an ICAP server; the ICAP client MUST be
 involved as described above.

7.2 Encryption

 Users of ICAP should note well that ICAP messages are not encrypted
 for transit by default.  In the absence of some other form of
 encryption at the link or network layers, eavesdroppers may be able
 to record the unencrypted transactions between ICAP clients and
 servers.  As described in Section 4.3.1, the Upgrade header MAY be
 used to negotiate transport-layer security for an ICAP connection
 [5].
 Note also that end-to-end encryption between a client and origin
 server is likely to preclude the use of value-added services by
 intermediaries such as surrogates.  An ICAP server that is unable to
 decrypt a client's messages will, of course, be unable to perform any
 transformations on it.

7.3 Service Validation

 Normal HTTP surrogates, when operating correctly, should not affect
 the end-to-end semantics of messages that pass through them.  This
 forms a well-defined criterion to validate that a surrogate is
 working correctly: a message should look the same before the
 surrogate as it does after the surrogate.
 In contrast, ICAP is meant to cause changes in the semantics of
 messages on their way from origin servers to users.  The criteria for
 a correctly operating surrogate are no longer as easy to define.
 This will make validation of ICAP services significantly more
 difficult.  Incorrect adaptations may lead to security
 vulnerabilities that were not present in the unadapted content.

Elson & Cerpa Informational [Page 38] RFC 3507 ICAP April 2003

8. Motivations and Design Alternatives

 This section describes some of our design decisions in more detail,
 and describes the ideas and motivations behind them.  This section
 does not define protocol requirements, but hopefully sheds light on
 the requirements defined in previous sections.  Nothing in this
 section carries the "force of law" or is part of the formal protocol
 specification.
 In general, our guiding principle was to make ICAP the simplest
 possible protocol that would do the job, and no simpler.  Some
 features were rejected where alternative (non-protocol-based)
 solutions could be found.  In addition, we have intentionally left a
 number of issues at the discretion of the implementor, where we
 believe that doing so does not compromise interoperability.

8.1 To Be HTTP, or Not To Be

 ICAP was initially designed as an application-layer protocol built to
 run on top of HTTP.  This was desirable for a number of reasons.
 HTTP is well-understood in the community and has enjoyed significant
 investments in software infrastructure (clients, servers, parsers,
 etc.).  Our initial designs focused on leveraging that existing work;
 we hoped that it would be possible to implement ICAP services simply,
 using CGI scripts run by existing web servers.
 However, the devil (as always) proved to be in the details.  Certain
 features that we considered important were impossible to implement
 with HTTP.  For example, ICAP clients can stop and wait for a "100
 Continue" message in the midst of a message-body; HTTP clients may
 only wait between the header and body.  In addition, certain
 transformations of HTTP messages by surrogates are legal (and
 harmless for HTTP), but caused problems with ICAP's "header-in-
 header" encapsulation and other features.
 Ultimately, we decided that the tangle of workarounds required to fit
 ICAP into HTTP was more complex and confusing than moving away from
 HTTP and defining a new (but similar) protocol.

8.2 Mandatory Use of Chunking

 Chunking is mandatory in ICAP encapsulated bodies for three reasons.
 First, efficiency is important, and the chunked encoding allows both
 the client and server to keep the transport-layer connection open for
 later reuse.  Second, ICAP servers (and their developers) should be
 encouraged to produce "incremental" responses where possible, to
 reduce the latency perceived by users.  Chunked encoding is the only
 way to support this type of implementation.  Finally, by

Elson & Cerpa Informational [Page 39] RFC 3507 ICAP April 2003

 standardizing on a single encapsulation mechanism, we avoid the
 complexity that would be required in client and server software to
 support multiple mechanisms.  This simplifies ICAP, particularly in
 the "body preview" feature described in Section 4.5.
 While chunking of encapsulated bodies is mandatory, encapsulated
 headers are not chunked.  There are two reasons for this decision.
 First, in cases where a chunked HTTP message body is being
 encapsulated in an ICAP message, the ICAP client (HTTP server) can
 copy it directly from the HTTP client to the ICAP server without un-
 chunking and then re-chunking it.  Second, many header-parser
 implementations have difficulty dealing with headers that come in
 multiple chunks.  Earlier drafts of this document mandated that a
 chunk boundary not come within a header.  For clarity, chunking of
 encapsulated headers has simply been disallowed.

8.3 Use of the null-body directive in the Encapsulated header

 There is a disadvantage to not using the chunked transfer-encoding
 for encapsulated header part of an ICAP message.  Specifically,
 parsers do not know in advance how much header data is coming (e.g.,
 for buffer allocation).  ICAP does not allow chunking in the header
 part for reasons described in Section 8.2.  To compensate, the
 "null-body" directive allows the final header's length to be
 determined, despite it not being chunked.

9. References

 [1]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
      Identifiers (URI): Generic Syntax and Semantics", RFC 2396,
      August 1998.
 [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [3]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.
 [4]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
      Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
      HTTP/1.1", RFC 2616, June 1999.
 [5]  Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1",
      RFC 2817, May 2000.

Elson & Cerpa Informational [Page 40] RFC 3507 ICAP April 2003

10. Contributors

 ICAP is based on an original idea by John Martin and Peter Danzig.
 Many individuals and organizations have contributed to the
 development of ICAP, including the following contributors (past and
 present):
 Lee Duggs
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: lee.duggs@netapp.com
 Paul Eastham
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: eastham@netapp.com
 Debbie Futcher
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: deborah.futcher@netapp.com
 Don Gillies
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: gillies@netapp.com
 Steven La
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: steven.la@netapp.com

Elson & Cerpa Informational [Page 41] RFC 3507 ICAP April 2003

 John Martin
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: jmartin@netapp.com
 Jeff Merrick
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: jeffrey.merrick@netapp.com
 John Schuster
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: john.schuster@netapp.com
 Edward Sharp
 Network Appliance, Inc.
 495 East Java Dr.
 Sunnyvale, CA 94089 USA
 Phone: (408) 822-6000
 EMail: edward.sharp@netapp.com
 Peter Danzig
 Akamai Technologies
 1400 Fashion Island Blvd
 San Mateo, CA 94404 USA
 Phone: (650) 372-5757
 EMail: danzig@akamai.com
 Mark Nottingham
 Akamai Technologies
 1400 Fashion Island Blvd
 San Mateo, CA 94404 USA
 Phone: (650) 372-5757
 EMail: mnot@akamai.com

Elson & Cerpa Informational [Page 42] RFC 3507 ICAP April 2003

 Nitin Sharma
 Akamai Technologies
 1400 Fashion Island Blvd
 San Mateo, CA 94404 USA
 Phone: (650) 372-5757
 EMail: nitin@akamai.com
 Hilarie Orman
 Novell, Inc.
 122 East 1700 South
 Provo, UT 84606 USA
 Phone: (801) 861-7021
 EMail: horman@novell.com
 Craig Blitz
 Novell, Inc.
 122 East 1700 South
 Provo, UT 84606 USA
 Phone: (801) 861-7021
 EMail: cblitz@novell.com
 Gary Tomlinson
 Novell, Inc.
 122 East 1700 South
 Provo, UT 84606 USA
 Phone: (801) 861-7021
 EMail: garyt@novell.com
 Andre Beck
 Bell Laboratories / Lucent Technologies
 101 Crawfords Corner Road
 Holmdel, New Jersey 07733-3030
 Phone: (732) 332-5983
 EMail: abeck@bell-labs.com
 Markus Hofmann
 Bell Laboratories / Lucent Technologies
 101 Crawfords Corner Road
 Holmdel, New Jersey 07733-3030
 Phone: (732) 332-5983
 EMail: hofmann@bell-labs.com

Elson & Cerpa Informational [Page 43] RFC 3507 ICAP April 2003

 David Bryant
 CacheFlow, Inc.
 650 Almanor Avenue
 Sunnyvale, California 94086
 Phone: (888) 462-3568
 EMail: david.bryant@cacheflow.com

Elson & Cerpa Informational [Page 44] RFC 3507 ICAP April 2003

Appendix A BNF Grammar for ICAP Messages

 This grammar is specified in terms of the augmented Backus-Naur Form
 (BNF) similar to that used by the HTTP/1.1 specification (See Section
 2.1 of [4]).  Implementors will need to be familiar with the notation
 in order to understand this specification.
 Many header values (where noted) have exactly the same grammar and
 semantics as in HTTP/1.1.  We do not reproduce those grammars here.
 ICAP-Version = "ICAP/1.0"
 ICAP-Message = Request | Response
 Request      = Request-Line
                *(Request-Header CRLF)
                CRLF
                [ Request-Body ]
 Request-Line = Method SP ICAP_URI SP ICAP-Version CRLF
 Method       = "REQMOD"         ; Section 4.8
              | "RESPMOD"        ; Section 4.9
              | "OPTIONS"        ; Section 4.10
              | Extension-Method ; Section 4.3.2
 Extension-Method = token
 ICAP_URI = Scheme ":" Net_Path [ "?" Query ]  ; Section 4.2
 Scheme      = "icap"
 Net_Path    = "//" Authority [ Abs_Path ]
 Authority   = [ userinfo "@" ] host [ ":" port ]
 Request-Header     = Request-Fields ":" [ Generic-Field-Value ]
 Request-Fields     = Request-Field-Name
                    | Common-Field-Name
 ; Header fields specific to requests
 Request-Field-Name = "Authorization"   ; Section 4.3.2
                    | "Allow"           ; Section 4.3.2
                    | "From"            ; Section 4.3.2
                    | "Host"            ; Section 4.3.2
                    | "Referer"         ; Section 4.3.2

Elson & Cerpa Informational [Page 45] RFC 3507 ICAP April 2003

                    | "User-Agent"      ; Section 4.3.2
                    | "Preview"         ; Section 4.5
 ; Header fields common to both requests and responses
 Common-Field-Name  = "Cache-Control"   ; Section 4.3.1
                    | "Connection"      ; Section 4.3.1
                    | "Date"            ; Section 4.3.1
                    | "Expires"         ; Section 4.3.1
                    | "Pragma"          ; Section 4.3.1
                    | "Trailer"         ; Section 4.3.1
                    | "Upgrade"         ; Section 4.3.1
                    | "Encapsulated"    ; Section 4.4
                    | Extension-Field-Name   ; Section 4.3
 Extension-Field-Name  = "X-" token
 Generic-Field-Value   = *( Generic-Field-Content | LWS )
 Generic-Field-Content = <the OCTETs making up the field-value
                          and consisting of either *TEXT or
                          combinations of token, separators,
                          and quoted-string>
 Request-Body = *OCTET   ; See Sections 4.4 and 4.5 for semantics
 Response    = Status-Line
               *(Response-Header CRLF)
               CRLF
               [ Response-Body ]
 Status-Line = ICAP-Version SP Status-Code SP Reason-Phrase CRLF
 Status-Code = "100"  ; Section 4.5
             | "101"  ; Section 10.1.2 of [4]
             | "200"  ; Section 10.2.1 of [4]
             | "201"  ; Section 10.2.2 of [4]
             | "202"  ; Section 10.2.3 of [4]
             | "203"  ; Section 10.2.4 of [4]
             | "204"  ; Section 4.6
             | "205"  ; Section 10.2.6 of [4]
             | "206"  ; Section 10.2.7 of [4]
             | "300"  ; Section 10.3.1 of [4]
             | "301"  ; Section 10.3.2 of [4]
             | "302"  ; Section 10.3.3 of [4]
             | "303"  ; Section 10.3.4 of [4]
             | "304"  ; Section 10.3.5 of [4]
             | "305"  ; Section 10.3.6 of [4]
             | "306"  ; Section 10.3.7 of [4]
             | "307"  ; Section 10.3.8 of [4]

Elson & Cerpa Informational [Page 46] RFC 3507 ICAP April 2003

             | "400"  ; Section 4.3.3
             | "401"  ; Section 10.4.2 of [4]
             | "402"  ; Section 10.4.3 of [4]
             | "403"  ; Section 10.4.4 of [4]
             | "404"  ; Section 4.3.3
             | "405"  ; Section 4.3.3
             | "406"  ; Section 10.4.7 of [4]
             | "407"  ; Section 10.4.8 of [4]
             | "408"  ; Section 4.3.3
             | "409"  ; Section 10.4.10 of [4]
             | "410"  ; Section 10.4.11 of [4]
             | "411"  ; Section 10.4.12 of [4]
             | "412"  ; Section 10.4.13 of [4]
             | "413"  ; Section 10.4.14 of [4]
             | "414"  ; Section 10.4.15 of [4]
             | "415"  ; Section 10.4.16 of [4]
             | "416"  ; Section 10.4.17 of [4]
             | "417"  ; Section 10.4.18 of [4]
             | "500"  ; Section 4.3.3
             | "501"  ; Section 4.3.3
             | "502"  ; Section 4.3.3
             | "503"  ; Section 4.3.3
             | "504"  ; Section 10.5.5 of [4]
             | "505"  ; Section 4.3.3
             | Extension-Code
 Extension-Code = 3DIGIT
 Reason-Phrase = *<TEXT, excluding CR, LF>
 Response-Header     = Response-Fields ":" [ Generic-Field-Value ]
 Response-Fields     = Response-Field-Name
                     | Common-Field-Name
 Response-Field-Name = "Server"         ; Section 4.3.3
                     | "ISTag"          ; Section 4.7
 Response-Body = *OCTET  ; See Sections 4.4 and 4.5 for semantics

Elson & Cerpa Informational [Page 47] RFC 3507 ICAP April 2003

Authors' Addresses

 Jeremy Elson
 University of California Los Angeles
 Department of Computer Science
 3440 Boelter Hall
 Los Angeles CA 90095
 Phone: (310) 206-3925
 EMail: jelson@cs.ucla.edu
 Alberto Cerpa
 University of California Los Angeles
 Department of Computer Science
 3440 Boelter Hall
 Los Angeles CA 90095
 Phone: (310) 206-3925
 EMail: cerpa@cs.ucla.edu
 ICAP discussion currently takes place at
         icap-discussions@yahoogroups.com.
 For more information, see
         http://groups.yahoo.com/group/icap-discussions/.

Elson & Cerpa Informational [Page 48] RFC 3507 ICAP April 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
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Elson & Cerpa Informational [Page 49]

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