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 RFC3238 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.

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

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.

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.

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, RFC2119 [2].

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.

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.

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:

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.

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.

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 RFC2822 [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," RFC2396 [1], Section 3. The URI structure defined
by ICAP is roughly:

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 RFC2396. 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:

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

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".

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.

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)

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

/* 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

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:

- 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.

- 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.

- 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.

- 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:

- all of the encapsulated header sections, and

- 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.

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 "0rnrn" 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:

- 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.

- 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.

- 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

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:

rn
0; ieofrnrn

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:

200rn
<512 bytes of data>rn
200rn
<512 bytes of data>rn
0; ieofrnrn

<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:

200rn
<512 bytes of data>rn
200rn
<512 bytes of data>rn
0rn

<100 Continue Message>

1rn
<1 byte of data>rn
0rnrn <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

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.

4.8.2 Response

The response from the ICAP server back to the ICAP client may take
one of four forms:

- An error indication,

- A 204 indicating that the ICAP client's request requires no
adaptation (see Section 4.6 for limitations of this response),

- An encapsulated, adapted version of the ICAP client's request, or

- 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.

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"

----------------------------------------------------------------
ICAP Request Modification Example 1 - 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: 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"

----------------------------------------------------------------

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

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

----------------------------------------------------------------
ICAP Request Modification Example 2 - 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: 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

----------------------------------------------------------------
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

----------------------------------------------------------------

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

----------------------------------------------------------------

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,

- An error indication,

- An encapsulated and potentially modified HTTP response header and
response body, or

- 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

33
This is data that was returned by an origin server.
0

----------------------------------------------------------------

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

----------------------------------------------------------------

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

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:

-- 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

-- 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

-- ISTag:

See section 4.7 for details. This header MUST be included in the
OPTIONS response.

For example:
ISTag: "5BDEEEA9-12E4-2"

-- Encapsulated:

This header MUST be included in the OPTIONS response; see Section
4.4.

For example:
Encapsulated: opt-body=0

-- 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

-- 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

-- 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

-- Date:

The server's clock, specified as an RFC1123 compliant date/time
string. This header MAY be included in the OPTIONS response.

For example:
Date: Fri, 15 Jun 2001 04:33:55 GMT

-- 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

-- 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

-- 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

-- 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: *

-- 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

-- 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

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

----------------------------------------------------------------

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: *

----------------------------------------------------------------

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

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.

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.

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".

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:

- For objects arriving using "Content-Length" headers, one big chunk
can be created of the same size as indicated in the Content-Length
header.

- 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)

- 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 RFC2617. Specifically, the following rules
apply:

- 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.

- If authentication is required between an ICAP client and ICAP
server, hop-by-hop Proxy Authentication as described in RFC2617
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.

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

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", RFC2396,
August 1998.

[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[3] Resnick, P., "Internet Message Format", RFC2822, 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", RFC2616, June 1999.

[5] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1",
RFC2817, May 2000.

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

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

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

David Bryant
CacheFlow, Inc.
650 Almanor Avenue
Sunnyvale, California 94086

Phone: (888) 462-3568
EMail: david.bryant@cacheflow.com

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

"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]

"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

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/.

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