[RFCs/IDs] [Plain Text] [From draft-elson-icap]
INFORMATIONAL
Errata
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.
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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
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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.
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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.
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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].
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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.
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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.
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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:
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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.
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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.
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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:
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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:
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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
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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".
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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.
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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)
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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
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/* 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
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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.
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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:
- 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
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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:
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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
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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.
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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.
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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
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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
----------------------------------------------------------------
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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,
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- 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
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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
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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.
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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: