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diff --git a/doc/rfc/rfc3507.txt b/doc/rfc/rfc3507.txt new file mode 100644 index 0000000..ceff707 --- /dev/null +++ b/doc/rfc/rfc3507.txt @@ -0,0 +1,2747 @@ + + + + + + +Network Working Group J. Elson +Request for Comments: 3507 A. Cerpa +Category: Informational UCLA + April 2003 + + + Internet Content Adaptation Protocol (ICAP) + +Status of this Memo + + This memo provides information for the Internet community. It does + not specify an Internet standard of any kind. Distribution of this + memo is unlimited. + +Copyright Notice + + Copyright (C) The Internet Society (2003). All Rights Reserved. + +IESG Note + + The Open Pluggable Services (OPES) working group has been chartered + to produce a standards track protocol specification for a protocol + intended to perform the same of functions as ICAP. However, since + ICAP is already in widespread use the IESG believes it is appropriate + to document existing usage by publishing the ICAP specification as an + informational document. The IESG also notes that ICAP was developed + before the publication of RFC 3238 and therefore does not address the + architectural and policy issues described in that document. + +Abstract + + ICAP, the Internet Content Adaption Protocol, is a protocol aimed at + providing simple object-based content vectoring for HTTP services. + ICAP is, in essence, a lightweight protocol for executing a "remote + procedure call" on HTTP messages. It allows ICAP clients to pass + HTTP messages to ICAP servers for some sort of transformation or + other processing ("adaptation"). The server executes its + transformation service on messages and sends back responses to the + client, usually with modified messages. Typically, the adapted + messages are either HTTP requests or HTTP responses. + + + + + + + + + + + +Elson & Cerpa Informational [Page 1] + +RFC 3507 ICAP April 2003 + + +Table of Contents + + 1. Introduction............................................3 + 2. Terminology.............................................5 + 3. ICAP Overall Operation..................................8 + 3.1 Request Modification..............................8 + 3.2 Response Modification............................10 + 4. Protocol Semantics.....................................11 + 4.1 General Operation................................11 + 4.2 ICAP URIs........................................11 + 4.3 ICAP Headers.....................................12 + 4.3.1 Headers Common to Requests and + Responses................................12 + 4.3.2 Request Headers..........................13 + 4.3.3 Response Headers.........................14 + 4.3.4 ICAP-Related Headers in HTTP + Messages.................................15 + 4.4 ICAP Bodies: Encapsulation of HTTP + Messages.........................................16 + 4.4.1 Expected Encapsulated Sections...........16 + 4.4.2 Encapsulated HTTP Headers................18 + 4.5 Message Preview..................................18 + 4.6 "204 No Content" Responses outside of + Previews.........................................22 + 4.7 ISTag Response Header............................22 + 4.8 Request Modification Mode........................23 + 4.8.1 Request..................................23 + 4.8.2 Response.................................24 + 4.8.3 Examples.................................24 + 4.9 Response Modification Mode.......................27 + 4.9.1 Request..................................27 + 4.9.2 Response.................................27 + 4.9.3 Examples.................................28 + 4.10 OPTIONS Method...................................29 + 4.10.1 OPTIONS request..........................29 + 4.10.2 OPTIONS response.........................30 + 4.10.3 OPTIONS examples.........................33 + 5. Caching................................................33 + 6. Implementation Notes...................................34 + 6.1 Vectoring Points.................................34 + 6.2 Application Level Errors.........................35 + 6.3 Use of Chunked Transfer-Encoding.................37 + 6.4 Distinct URIs for Distinct Services..............37 + 7. Security Considerations................................37 + 7.1 Authentication...................................37 + 7.2 Encryption.......................................38 + 7.3 Service Validation...............................38 + 8. Motivations and Design Alternatives....................39 + + + +Elson & Cerpa Informational [Page 2] + +RFC 3507 ICAP April 2003 + + + 8.1 To Be HTTP, or Not to Be.........................39 + 8.2 Mandatory Use of Chunking........................39 + 8.3 Use of the null-body directive in the + Encapsulated header..............................40 + 9. References.............................................40 + 10. Contributors...........................................41 + Appendix A BNF Grammar for ICAP Messages..................45 + Authors' Addresses..........................................48 + Full Copyright Statement....................................49 + +1. Introduction + + As the Internet grows, so does the need for scalable Internet + services. Popular web servers are asked to deliver content to + hundreds of millions of users connected at ever-increasing + bandwidths. The model of centralized, monolithic servers that are + responsible for all aspects of every client's request seems to be + reaching the end of its useful life. + + To keep up with the growth in the number of clients, there has been a + move towards architectures that scale better through the use of + replication, distribution, and caching. On the content provider + side, replication and load-balancing techniques allow the burden of + client requests to be spread out over a myriad of servers. Content + providers have also begun to deploy geographically diverse content + distribution networks that bring origin-servers closer to the "edge" + of the network where clients are attached. These networks of + distributed origin-servers or "surrogates" allow the content provider + to distribute their content whilst retaining control over the + integrity of that content. The distributed nature of this type of + deployment and the proximity of a given surrogate to the end-user + enables the content provider to offer additional services to a user + which might be based, for example, on geography where this would have + been difficult with a single, centralized service. + + ICAP, the Internet Content Adaption Protocol, is a protocol aimed at + providing simple object-based content vectoring for HTTP services. + ICAP is, in essence, a lightweight protocol for executing a "remote + procedure call" on HTTP messages. It allows ICAP clients to pass + HTTP messages to ICAP servers for some sort of transformation or + other processing ("adaptation"). The server executes its + transformation service on messages and sends back responses to the + client, usually with modified messages. The adapted messages may be + either HTTP requests or HTTP responses. Though transformations may + be possible on other non-HTTP content, they are beyond the scope of + this document. + + + + + +Elson & Cerpa Informational [Page 3] + +RFC 3507 ICAP April 2003 + + + This type of Remote Procedure Call (RPC) is useful in a number of + ways. For example: + + o Simple transformations of content can be performed near the edge + of the network instead of requiring an updated copy of an object + from an origin server. For example, a content provider might want + to provide a popular web page with a different advertisement every + time the page is viewed. Currently, content providers implement + this policy by marking such pages as non-cachable and tracking + user cookies. This imposes additional load on the origin server + and the network. In our architecture, the page could be cached + once near the edges of the network. These edge caches can then + use an ICAP call to a nearby ad-insertion server every time the + page is served to a client. + + Other such transformations by edge servers are possible, either + with cooperation from the content provider (as in a content + distribution network), or as a value-added service provided by a + client's network provider (as in a surrogate). Examples of these + kinds of transformations are translation of web pages to different + human languages or to different formats that are appropriate for + special physical devices (e.g., PDA-based or cell-phone-based + browsers). + + o Surrogates or origin servers can avoid performing expensive + operations by shipping the work off to other servers instead. + This helps distribute load across multiple machines. For example, + consider a user attempting to download an executable program via a + surrogate (e.g., a caching proxy). The surrogate, acting as an + ICAP client, can ask an external server to check the executable + for viruses before accepting it into its cache. + + o Firewalls or surrogates can act as ICAP clients and send outgoing + requests to a service that checks to make sure the URI in the + request is allowed (for example, in a system that allows parental + control of web content viewed by children). In this case, it is a + *request* that is being adapted, not an object returned by a + response. + + In all of these examples, ICAP is helping to reduce or distribute the + load on origin servers, surrogates, or the network itself. In some + cases, ICAP facilitates transformations near the edge of the network, + allowing greater cachability of the underlying content. In other + examples, devices such as origin servers or surrogates are able to + reduce their load by distributing expensive operations onto other + machines. In all cases, ICAP has also created a standard interface + for content adaptation to allow greater flexibility in content + distribution or the addition of value added services in surrogates. + + + +Elson & Cerpa Informational [Page 4] + +RFC 3507 ICAP April 2003 + + + There are two major components in our architecture: + + 1. Transaction semantics -- "How do I ask for adaptation?" + + 2. Control of policy -- "When am I supposed to ask for adaptation, + what kind of adaptation do I ask for, and from where?" + + Currently, ICAP defines only the transaction semantics. For example, + this document specifies how to send an HTTP message from an ICAP + client to an ICAP server, specify the URI of the ICAP resource + requested along with other resource-specific parameters, and receive + the adapted message. + + Although a necessary building-block, this wire-protocol defined by + ICAP is of limited use without the second part: an accompanying + application framework in which it operates. The more difficult + policy issue is beyond the scope of the current ICAP protocol, but is + planned in future work. + + In initial implementations, we expect that implementation-specific + manual configuration will be used to define policy. This includes + the rules for recognizing messages that require adaptation, the URIs + of available adaptation resources, and so on. For ICAP clients and + servers to interoperate, the exact method used to define policy need + not be consistent across implementations, as long as the policy + itself is consistent. + + IMPORTANT: + Note that at this time, in the absence of a policy-framework, it + is strongly RECOMMENDED that transformations SHOULD only be + performed on messages with the explicit consent of either the + content-provider or the user (or both). Deployment of + transformation services without the consent of either leads to, at + best, unpredictable results. For more discussion of these issues, + see Section 7. + + Once the full extent of the typical policy decisions are more fully + understood through experience with these initial implementations, + later follow-ons to this architecture may define an additional policy + control protocol. This future protocol may allow a standard policy + definition interface complementary to the ICAP transaction interface + defined here. + +2. Terminology + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this + document are to be interpreted as described in BCP 14, RFC 2119 [2]. + + + +Elson & Cerpa Informational [Page 5] + +RFC 3507 ICAP April 2003 + + + The special terminology used in this document is defined below. The + majority of these terms are taken as-is from HTTP/1.1 [4] and are + reproduced here for reference. A thorough understanding of HTTP/1.1 + is assumed on the part of the reader. + + connection: + A transport layer virtual circuit established between two programs + for the purpose of communication. + + message: + The basic unit of HTTP communication, consisting of a structured + sequence of octets matching the syntax defined in Section 4 of + HTTP/1.1 [4] and transmitted via the connection. + + request: + An HTTP request message, as defined in Section 5 of HTTP/1.1 [4]. + + response: + An HTTP response message, as defined in Section 6 of HTTP/1.1 [4]. + + resource: + A network data object or service that can be identified by a URI, + as defined in Section 3.2 of HTTP/1.1 [4]. Resources may be + available in multiple representations (e.g., multiple languages, + data formats, size, resolutions) or vary in other ways. + + client: + A program that establishes connections for the purpose of sending + requests. + + server: + An application program that accepts connections in order to + service requests by sending back responses. Any given program may + be capable of being both a client and a server; our use of these + terms refers only to the role being performed by the program for a + particular connection, rather than to the program's capabilities + in general. Likewise, any server may act as an origin server, + surrogate, gateway, or tunnel, switching behavior based on the + nature of each request. + + origin server: + The server on which a given resource resides or is to be created. + + + + + + + + + +Elson & Cerpa Informational [Page 6] + +RFC 3507 ICAP April 2003 + + + proxy: + An intermediary program which acts as both a server and a client + for the purpose of making requests on behalf of other clients. + Requests are serviced internally or by passing them on, with + possible translation, to other servers. A proxy MUST implement + both the client and server requirements of this specification. + + cache: + A program's local store of response messages and the subsystem + that controls its message storage, retrieval, and deletion. A + cache stores cachable responses in order to reduce the response + time and network bandwidth consumption on future, equivalent + requests. Any client or server may include a cache, though a + cache cannot be used by a server that is acting as a tunnel. + + cachable: + A response is cachable if a cache is allowed to store a copy of + the response message for use in answering subsequent requests. + The rules for determining the cachability of HTTP responses are + defined in Section 13 of [4]. Even if a resource is cachable, + there may be additional constraints on whether a cache can use the + cached copy for a particular request. + + surrogate: + A gateway co-located with an origin server, or at a different + point in the network, delegated the authority to operate on behalf + of, and typically working in close co-operation with, one or more + origin servers. Responses are typically delivered from an + internal cache. Surrogates may derive cache entries from the + origin server or from another of the origin server's delegates. + In some cases a surrogate may tunnel such requests. + + Where close co-operation between origin servers and surrogates + exists, this enables modifications of some protocol requirements, + including the Cache-Control directives in [4]. Such modifications + have yet to be fully specified. + + Devices commonly known as "reverse proxies" and "(origin) server + accelerators" are both more properly defined as surrogates. + + New definitions: + + ICAP resource: + Similar to an HTTP resource as described above, but the URI refers + to an ICAP service that performs adaptations of HTTP messages. + + + + + + +Elson & Cerpa Informational [Page 7] + +RFC 3507 ICAP April 2003 + + + ICAP server: + Similar to an HTTP server as described above, except that the + application services ICAP requests. + + ICAP client: + A program that establishes connections to ICAP servers for the + purpose of sending requests. An ICAP client is often, but not + always, a surrogate acting on behalf of a user. + +3. ICAP Overall Operation + + Before describing ICAP's semantics in detail, we will first give a + general overview of the protocol's major functions and expected uses. + As described earlier, ICAP focuses on modification of HTTP requests + (Section 3.1), and modification of HTTP responses (Section 3.2). + +3.1 Request Modification + + In "request modification" (reqmod) mode, an ICAP client sends an HTTP + request to an ICAP server. The ICAP server may then: + + 1) Send back a modified version of the request. The ICAP client may + then perform the modified request by contacting an origin server; + or, pipeline the modified request to another ICAP server for + further modification. + + 2) Send back an HTTP response to the request. This is used to + provide information useful to the user in case of an error (e.g., + "you sent a request to view a page you are not allowed to see"). + + 3) Return an error. + + ICAP clients MUST be able to handle all three types of responses. + However, in line with the guidance provided for HTTP surrogates in + Section 13.8 of [4], ICAP client implementors do have flexibility in + handling errors. If the ICAP server returns an error, the ICAP + client may (for example) return the error to the user, execute the + unadapted request as it arrived from the client, or re-try the + adaptation again. + + We will illustrate this method with an example application: content + filtering. Consider a surrogate that receives a request from a + client for a web page on an origin server. The surrogate, acting as + an ICAP client, sends the client's request to an ICAP server that + performs URI-based content filtering. If access to the requested URI + is allowed, the request is returned to the ICAP client unmodified. + However, if the ICAP server chooses to disallow access to the + requested resources, it may either: + + + +Elson & Cerpa Informational [Page 8] + +RFC 3507 ICAP April 2003 + + + 1) Modify the request so that it points to a page containing an error + message instead of the original URI. + + 2) Return an encapsulated HTTP response that indicates an HTTP error. + + This method can be used for a variety of other applications; for + example, anonymization, modification of the Accept: headers to handle + special device requirements, and so forth. + + Typical data flow: + + origin-server + | /|\ + | | + 5 | | 4 + | | + \|/ | 2 + ICAP-client --------------> ICAP-resource + (surrogate) <-------------- on ICAP-server + | /|\ 3 + | | + 6 | | 1 + | | + \|/ | + client + + 1. A client makes a request to a ICAP-capable surrogate (ICAP client) + for an object on an origin server. + + 2. The surrogate sends the request to the ICAP server. + + 3. The ICAP server executes the ICAP resource's service on the + request and sends the possibly modified request, or a response to + the request back to the ICAP client. + + If Step 3 returned a request: + + 4. The surrogate sends the request, possibly different from original + client request, to the origin server. + + 5. The origin server responds to request. + + 6. The surrogate sends the reply (from either the ICAP server or the + origin server) to the client. + + + + + + + +Elson & Cerpa Informational [Page 9] + +RFC 3507 ICAP April 2003 + + +3.2 Response Modification + + In the "response modification" (respmod) mode, an ICAP client sends + an HTTP response to an ICAP server. (The response sent by the ICAP + client typically has been generated by an origin server.) The ICAP + server may then: + + 1) Send back a modified version of the response. + + 2) Return an error. + + The response modification method is intended for post-processing + performed on an HTTP response before it is delivered to a client. + Examples include formatting HTML for display on special devices, + human language translation, virus checking, and so forth. + + Typical data flow: + + origin-server + | /|\ + | | + 3 | | 2 + | | + \|/ | 4 + ICAP-client --------------> ICAP-resource + (surrogate) <-------------- on ICAP-server + | /|\ 5 + | | + 6 | | 1 + | | + \|/ | + client + + 1. A client makes a request to a ICAP-capable surrogate (ICAP client) + for an object on an origin server. + + 2. The surrogate sends the request to the origin server. + + 3. The origin server responds to request. + + 4. The ICAP-capable surrogate sends the origin server's reply to the + ICAP server. + + 5. The ICAP server executes the ICAP resource's service on the origin + server's reply and sends the possibly modified reply back to the + ICAP client. + + + + + +Elson & Cerpa Informational [Page 10] + +RFC 3507 ICAP April 2003 + + + 6. The surrogate sends the reply, possibly modified from the original + origin server's reply, to the client. + +4. Protocol Semantics + +4.1 General Operation + + ICAP is a request/response protocol similar in semantics and usage to + HTTP/1.1 [4]. Despite the similarity, ICAP is not HTTP, nor is it an + application protocol that runs over HTTP. This means, for example, + that ICAP messages can not be forwarded by HTTP surrogates. Our + reasons for not building directly on top of HTTP are discussed in + Section 8.1. + + ICAP uses TCP/IP as a transport protocol. The default port is 1344, + but other ports may be used. The TCP flow is initiated by the ICAP + client to a passively listening ICAP server. + + ICAP messages consist of requests from client to server and responses + from server to client. Requests and responses use the generic + message format of RFC 2822 [3] -- that is, a start-line (either a + request line or a status line), a number of header fields (also known + as "headers"), an empty line (i.e., a line with nothing preceding the + CRLF) indicating the end of the header fields, and a message-body. + + The header lines of an ICAP message specify the ICAP resource being + requested as well as other meta-data such as cache control + information. The message body of an ICAP request contains the + (encapsulated) HTTP messages that are being modified. + + As in HTTP/1.1, a single transport connection MAY (perhaps even + SHOULD) be re-used for multiple request/response pairs. The rules + for doing so in ICAP are the same as described in Section 8.1.2.2 of + [4]. Specifically, requests are matched up with responses by + allowing only one outstanding request on a transport connection at a + time. Multiple parallel connections MAY be used as in HTTP. + +4.2 ICAP URIs + + All ICAP requests specify the ICAP resource being requested from the + server using an ICAP URI. This MUST be an absolute URI that + specifies both the complete hostname and the path of the resource + being requested. For definitive information on URL syntax and + semantics, see "Uniform Resource Identifiers (URI): Generic Syntax + and Semantics," RFC 2396 [1], Section 3. The URI structure defined + by ICAP is roughly: + + + + + +Elson & Cerpa Informational [Page 11] + +RFC 3507 ICAP April 2003 + + + ICAP_URI = Scheme ":" Net_Path [ "?" Query ] + + Scheme = "icap" + + Net_Path = "//" Authority [ Abs_Path ] + + Authority = [ userinfo "@" ] host [ ":" port ] + + ICAP adds the new scheme "icap" to the ones defined in RFC 2396. If + the port is empty or not given, port 1344 is assumed. An example + ICAP URI line might look like this: + + icap://icap.example.net:2000/services/icap-service-1 + + An ICAP server MUST be able to recognize all of its hosts names, + including any aliases, local variations, and numeric IP addresses of + its interfaces. + + Any arguments that an ICAP client wishes to pass to an ICAP service + to modify the nature of the service MAY be passed as part of the + ICAP-URI, using the standard "?"-encoding of attribute-value pairs + used in HTTP. For example: + + icap://icap.net/service?mode=translate&lang=french + +4.3 ICAP Headers + + The following sections define the valid headers for ICAP messages. + Section 4.3.1 describes headers common to both requests and + responses. Request-specific and response-specific headers are + described in Sections 4.3.2 and 4.3.3, respectively. + + User-defined header extensions are allowed. In compliance with the + precedent established by the Internet mail format [3] and later + adopted by HTTP [4], all user-defined headers MUST follow the "X-" + naming convention ("X-Extension-Header: Foo"). ICAP implementations + MAY ignore any "X-" headers without loss of compliance with the + protocol as defined in this document. + + Each header field consists of a name followed by a colon (":") and + the field value. Field names are case-insensitive. ICAP follows the + rules describe in section 4.2 of [4]. + +4.3.1 Headers Common to Requests and Responses + + The headers of all ICAP messages MAY include the following + directives, defined in ICAP the same as they are in HTTP: + + + + +Elson & Cerpa Informational [Page 12] + +RFC 3507 ICAP April 2003 + + + Cache-Control + Connection + Date + Expires + Pragma + Trailer + Upgrade + + Note in particular that the "Transfer-Encoding" option is not + allowed. The special transfer-encoding requirements of ICAP bodies + are described in Section 4.4. + + The Upgrade header MAY be used to negotiate Transport-Layer Security + on an ICAP connection, exactly as described for HTTP/1.1 in [4]. + + The ICAP-specific headers defined are: + + Encapsulated (See Section 4.4) + +4.3.2 Request Headers + + Similar to HTTP, ICAP requests MUST start with a request line that + contains a method, the complete URI of the ICAP resource being + requested, and an ICAP version string. The current version number of + ICAP is "1.0". + + This version of ICAP defines three methods: + + REQMOD - for Request Modification (Section 4.8) + RESPMOD - for Response Modification (Section 4.9) + OPTIONS - to learn about configuration (Section 4.10) + + The OPTIONS method MUST be implemented by all ICAP servers. All + other methods are optional and MAY be implemented. + + User-defined extension methods are allowed. Before attempting to use + an extension method, an ICAP client SHOULD use the OPTIONS method to + query the ICAP server's list of supported methods; see Section 4.10. + (If an ICAP server receives a request for an unknown method, it MUST + give a 501 error response as described in the next section.) + + Given the URI rules described in Section 4.2, a well-formed ICAP + request line looks like the following example: + + RESPMOD icap://icap.example.net/translate?mode=french ICAP/1.0 + + + + + + +Elson & Cerpa Informational [Page 13] + +RFC 3507 ICAP April 2003 + + + A number of request-specific headers are allowed in ICAP requests, + following the same semantics as the corresponding HTTP request + headers (Section 5.3 of [4]). These are: + + Authorization + Allow (see Section 4.6) + From (see Section 14.22 of [4]) + Host (REQUIRED in ICAP as it is in HTTP/1.1) + Referer (see Section 14.36 of [4]) + User-Agent + + In addition to HTTP-like headers, there are also request headers + unique to ICAP defined: + + Preview (see Section 4.5) + +4.3.3 Response Headers + + ICAP responses MUST start with an ICAP status line, similar in form + to that used by HTTP, including the ICAP version and a status code. + For example: + + ICAP/1.0 200 OK + + Semantics of ICAP status codes in ICAP match the status codes defined + by HTTP (Section 6.1.1 and 10 of [4]), except where otherwise + indicated in this document; n.b. 100 (Section 4.5) and 204 (Section + 4.6). + + ICAP error codes that differ from their HTTP counterparts are: + + 100 - Continue after ICAP Preview (Section 4.5). + + 204 - No modifications needed (Section 4.6). + + 400 - Bad request. + + 404 - ICAP Service not found. + + 405 - Method not allowed for service (e.g., RESPMOD requested for + service that supports only REQMOD). + + 408 - Request timeout. ICAP server gave up waiting for a request + from an ICAP client. + + 500 - Server error. Error on the ICAP server, such as "out of disk + space". + + + + +Elson & Cerpa Informational [Page 14] + +RFC 3507 ICAP April 2003 + + + 501 - Method not implemented. This response is illegal for an + OPTIONS request since implementation of OPTIONS is mandatory. + + 502 - Bad Gateway. This is an ICAP proxy and proxying produced an + error. + + 503 - Service overloaded. The ICAP server has exceeded a maximum + connection limit associated with this service; the ICAP client + should not exceed this limit in the future. + + 505 - ICAP version not supported by server. + + As in HTTP, the 4xx class of error codes indicate client errors, and + the 5xx class indicate server errors. + + ICAP's response-header fields allow the server to pass additional + information in the response that cannot be placed in the ICAP's + status line. + + A response-specific header is allowed in ICAP requests, following the + same semantics as the corresponding HTTP response headers (Section + 6.2 of [4]). This is: + + Server (see Section 14.38 of [4]) + + In addition to HTTP-like headers, there is also a response header + unique to ICAP defined: + + ISTag (see Section 4.7) + +4.3.4 ICAP-Related Headers in HTTP Messages + + When an ICAP-enabled HTTP surrogate makes an HTTP request to an + origin server, it is often useful to advise the origin server of the + surrogate's ICAP capabilities. Origin servers can use this + information to modify its response accordingly. For example, an + origin server may choose not to insert an advertisement into a page + if it knows that a downstream ICAP server can insert the ad instead. + + Although this ICAP specification can not mandate how HTTP is used in + communication between HTTP clients and servers, we do suggest a + convention: such headers (if used) SHOULD start with "X-ICAP". HTTP + clients with ICAP services SHOULD minimally include an "X-ICAP- + Version: 1.0" header along with their application-specific headers. + + + + + + + +Elson & Cerpa Informational [Page 15] + +RFC 3507 ICAP April 2003 + + +4.4 ICAP Bodies: Encapsulation of HTTP Messages + + The ICAP encapsulation model is a lightweight means of packaging any + number of HTTP message sections into an encapsulating ICAP message- + body, in order to allow the vectoring of requests, responses, and + request/response pairs to an ICAP server. + + This is accomplished by concatenating interesting message parts + (encapsulatED sections) into a single ICAP message-body (the + encapsulatING message). The encapsulated sections may be the headers + or bodies of HTTP messages. + + Encapsulated bodies MUST be transferred using the "chunked" + transfer-coding described in Section 3.6.1 of [4]. However, + encapsulated headers MUST NOT be chunked. In other words, an ICAP + message-body switches from being non-chunked to chunked as the body + passes from the encapsulated header to encapsulated body section. + (See Examples in Sections 4.8.3 and 4.9.3.). The motivation behind + this decision is described in Section 8.2. + +4.4.1 The "Encapsulated" Header + + The offset of each encapsulated section's start relative to the start + of the encapsulating message's body is noted using the "Encapsulated" + header. This header MUST be included in every ICAP message. For + example, the header + + Encapsulated: req-hdr=0, res-hdr=45, res-body=100 + + indicates a message that encapsulates a group of request headers, a + group of response headers, and then a response body. Each of these + is included at the byte-offsets listed. The byte-offsets are in + decimal notation for consistency with HTTP's Content-Length header. + + The special entity "null-body" indicates there is no encapsulated + body in the ICAP message. + + The syntax of an Encapsulated header is: + + encapsulated_header: "Encapsulated: " encapsulated_list + encapsulated_list: encapsulated_entity | + encapsulated_entity ", " encapsulated_list + encapsulated_entity: reqhdr | reshdr | reqbody | resbody | optbody + reqhdr = "req-hdr" "=" (decimal integer) + reshdr = "res-hdr" "=" (decimal integer) + reqbody = { "req-body" | "null-body" } "=" (decimal integer) + resbody = { "res-body" | "null-body" } "=" (decimal integer) + optbody = { "opt-body" | "null-body" } "=" (decimal integer) + + + +Elson & Cerpa Informational [Page 16] + +RFC 3507 ICAP April 2003 + + + There are semantic restrictions on Encapsulated headers beyond the + syntactic restrictions. The order in which the encapsulated parts + appear in the encapsulating message-body MUST be the same as the + order in which the parts are named in the Encapsulated header. In + other words, the offsets listed in the Encapsulated line MUST be + monotonically increasing. In addition, the legal forms of the + Encapsulated header depend on the method being used (REQMOD, RESPMOD, + or OPTIONS). Specifically: + + REQMOD request encapsulated_list: [reqhdr] reqbody + REQMOD response encapsulated_list: {[reqhdr] reqbody} | + {[reshdr] resbody} + RESPMOD request encapsulated_list: [reqhdr] [reshdr] resbody + RESPMOD response encapsulated_list: [reshdr] resbody + OPTIONS response encapsulated_list: optbody + + In the above grammar, note that encapsulated headers are always + optional. At most one body per encapsulated message is allowed. If + no encapsulated body is presented, the "null-body" header is used + instead; this is useful because it indicates the length of the header + section. + + Examples of legal Encapsulated headers: + + /* REQMOD request: This encapsulated HTTP request's headers start + * at offset 0; the HTTP request body (e.g., in a POST) starts + * at 412. */ + Encapsulated: req-hdr=0, req-body=412 + + /* REQMOD request: Similar to the above, but no request body is + * present (e.g., a GET). We use the null-body directive instead. + * In both this case and the previous one, we can tell from the + * Encapsulated header that the request headers were 412 bytes + * long. */ + Encapsulated: req-hdr=0, null-body=412 + + /* REQMOD response: ICAP server returned a modified request, + * with body */ + Encapsulated: req-hdr=0, req-body=512 + + /* RESPMOD request: Request headers at 0, response headers at 822, + * response body at 1655. Note that no request body is allowed in + * RESPMOD requests. */ + Encapsulated: req-hdr=0, res-hdr=822, res-body=1655 + + /* RESPMOD or REQMOD response: header and body returned */ + Encapsulated: res-hdr=0, res-body=749 + + + + +Elson & Cerpa Informational [Page 17] + +RFC 3507 ICAP April 2003 + + + /* OPTIONS response when there IS an options body */ + Encapsulated: opt-body=0 + + /* OPTIONS response when there IS NOT an options body */ + Encapsulated: null-body=0 + +4.4.2 Encapsulated HTTP Headers + + By default, ICAP messages may encapsulate HTTP message headers and + entity bodies. HTTP headers MUST start with the request-line or + status-line for requests and responses, respectively, followed by + interesting HTTP headers. + + The encapsulated headers MUST be terminated by a blank line, in order + to make them human readable, and in order to terminate line-by-line + HTTP parsers. + + HTTP/1.1 makes a distinction between end-to-end headers and hop-by- + hop headers (see Section 13.5.1 of [4]). End-to-end headers are + meaningful to the ultimate recipient of a message, whereas hop-by-hop + headers are meaningful only for a single transport-layer connection. + Hop-by-hop headers include Connection, Keep-Alive, and so forth. All + end-to-end HTTP headers SHOULD be encapsulated, and all hop-by-hop + headers MUST NOT be encapsulated. + + Despite the above restrictions on encapsulation, the hop-by-hop + Proxy-Authenticate and Proxy-Authorization headers MUST be forwarded + to the ICAP server in the ICAP header section (not the encapsulated + message). This allows propagation of client credentials that might + have been sent to the ICAP client in cases where the ICAP client is + also an HTTP surrogate. Note that this does not contradict HTTP/1.1, + which explicitly states "A proxy MAY relay the credentials from the + client request to the next proxy if that is the mechanism by which + the proxies cooperatively authenticate a given request." (Section + 14.34). + + The Via header of an encapsulated message SHOULD be modified by an + ICAP server as if the encapsulated message were traveling through an + HTTP surrogate. The Via header added by an ICAP server MUST specify + protocol as ICAP/1.0. + +4.5 Message Preview + + ICAP REQMOD or RESPMOD requests sent by the ICAP client to the ICAP + server may include a "preview". This feature allows an ICAP server + to see the beginning of a transaction, then decide if it wants to + + + + + +Elson & Cerpa Informational [Page 18] + +RFC 3507 ICAP April 2003 + + + opt-out of the transaction early instead of receiving the remainder + of the request message. Previewing can yield significant performance + improvements in a variety of situations, such as the following: + + - 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. + + + +Elson & Cerpa Informational [Page 19] + +RFC 3507 ICAP April 2003 + + + For example, to effect a Preview consisting of only encapsulated HTTP + headers, the ICAP client would add the following header to the ICAP + request: + + Preview: 0 + + This indicates that the ICAP client will send only the encapsulated + header sections to the ICAP server, then it will send a zero-length + chunk and stop and wait for a "go ahead" to send more encapsulated + body bytes to the ICAP server. + + Similarly, the ICAP header: + + Preview: 4096 + + Indicates that the ICAP client will attempt to send 4096 bytes of + origin server data in the encapsulated body of the ICAP request to + the ICAP server. It is important to note that the actual transfer + may be less, because the ICAP client is acting like a surrogate and + is not looking ahead to find the total length of the origin server + response. The entire ICAP encapsulated header section(s) will be + sent, followed by up to 4096 bytes of encapsulated HTTP body. The + chunk body terminator "0\r\n\r\n" is always included in these + transactions. + + After sending the preview, the ICAP client will wait for a response + from the ICAP server. The response MUST be one of the following: + + - 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 + + + +Elson & Cerpa Informational [Page 20] + +RFC 3507 ICAP April 2003 + + + unexpectedly arrives during the preview process. This is a + particularly useful optimization if a header-only HTTP response + arrives at the ICAP client (i.e., zero bytes of body); only a single + round trip will be needed for the complete ICAP server response. + + We define an HTTP chunk-extension of "ieof" to indicate that an ICAP + chunk is the last chunk (see [4]). The ICAP server MUST strip this + chunk extension before passing the chunk data to an ICAP application + process. + + For example, consider an ICAP client that has just received HTTP + response headers from an origin server and initiates an ICAP RESPMOD + transaction to an ICAP server. It does not know yet how many body + bytes will be arriving from the origin server because the server is + not using the Content-Length header. The ICAP client informs the + ICAP server that it will be sending a 1024-byte preview using a + "Preview: 1024" request header. If the HTTP origin server then + closes its connection to the ICAP client before sending any data + (i.e., it provides a zero-byte body), the corresponding zero-byte + preview for that zero-byte origin response would appear as follows: + + \r\n + 0; ieof\r\n\r\n + + If an ICAP server sees this preview, it knows from the presence of + "ieof" that the client will not be sending any more chunk data. In + this case, the server MUST respond with the modified response or a + 204 No Content message right away. It MUST NOT send a 100-Continue + response in this case. (In contrast, if the origin response had been + 1 byte or larger, the "ieof" would not have appeared. In that case, + an ICAP server MAY reply with 100-Continue, a modified response, or + 204 No Content.) + + In another example, if the preview is 1024 bytes and the origin + response is 1024 bytes in two chunks, then the encapsulation would + appear as follows: + + 200\r\n + <512 bytes of data>\r\n + 200\r\n + <512 bytes of data>\r\n + 0; ieof\r\n\r\n + + <204 or modified response> (100 Continue disallowed due to ieof) + + If the preview is 1024 bytes and the origin response is 1025 bytes + (and the ICAP server responds with 100-continue), then these chunks + would appear on the wire: + + + +Elson & Cerpa Informational [Page 21] + +RFC 3507 ICAP April 2003 + + + 200\r\n + <512 bytes of data>\r\n + 200\r\n + <512 bytes of data>\r\n + 0\r\n + + <100 Continue Message> + + 1\r\n + <1 byte of data>\r\n + 0\r\n\r\n <no ieof because we are no longer in preview mode> + + Once the ICAP server receives the eof indicator, it finishes reading + the current chunk stream. + + Note that when offering a Preview, the ICAP client is committing to + temporarily buffer the previewed portion of the message so that it + can honor a "204 No Content" response. The remainder of the message + is not necessarily buffered; it might be pipelined directly from + another source to the ICAP server after a 100-Continue. + +4.6 "204 No Content" Responses outside of Previews + + An ICAP client MAY choose to honor "204 No Content" responses for an + entire message. This is the decision of the client because it + imposes a burden on the client of buffering the entire message. + + An ICAP client MAY include "Allow: 204" in its request headers, + indicating that the server MAY reply to the message with a "204 No + Content" response if the object does not need modification. + + If an ICAP server receives a request that does not have "Allow: 204", + it MUST NOT reply with a 204. In this case, an ICAP server MUST + return the entire message back to the client, even though it is + identical to the message it received. + + The ONLY EXCEPTION to this rule is in the case of a message preview, + as described in the previous section. If this is the case, an ICAP + server can respond with a 204 No Content message in response to a + message preview EVEN if the original request did not have the "Allow: + 204" header. + +4.7 ISTag Response Header + + The ISTag ("ICAP Service Tag") response-header field provides a way + for ICAP servers to send a service-specific "cookie" to ICAP clients + that represents a service's current state. It is a 32-byte-maximum + alphanumeric string of data (not including the null character) that + + + +Elson & Cerpa Informational [Page 22] + +RFC 3507 ICAP April 2003 + + + may, for example, be a representation of the software version or + configuration of a service. An ISTag validates that previous ICAP + server responses can still be considered fresh by an ICAP client that + may be caching them. If a change on the ICAP server invalidates + previous responses, the ICAP server can invalidate portions of the + ICAP client's cache by changing its ISTag. The ISTag MUST be + included in every ICAP response from an ICAP server. + + For example, consider a virus-scanning ICAP service. The ISTag might + be a combination of the virus scanner's software version and the + release number of its virus signature database. When the database is + updated, the ISTag can be changed to invalidate all previous + responses that had been certified as "clean" and cached with the old + ISTag. + + ISTag is similar, but not identical, to the HTTP ETag. While an ETag + is a validator for a particular entity (object), an ISTag validates + all entities generated by a particular service (URI). A change in + the ISTag invalidates all the other entities provided a service with + the old ISTag, not just the entity whose response contained the + updated ISTag. + + The syntax of an ISTag is simply: + ISTag = "ISTag: " quoted-string + + In this document we use the quoted-string definition defined in + section 2.2 of [4]. + + For example: + ISTag: "874900-1994-1c02798" + +4.8 Request Modification Mode + + In this method, described in Section 3.1, an ICAP client sends an + HTTP request to an ICAP server. The ICAP server returns a modified + version of the request, an HTTP response, or (if the client indicates + it supports 204 responses) an indication that no modification is + required. + +4.8.1 Request + + In REQMOD mode, the ICAP request MUST contain an encapsulated HTTP + request. The headers and body (if any) MUST both be encapsulated, + except that hop-by-hop headers are not encapsulated. + + + + + + + +Elson & Cerpa Informational [Page 23] + +RFC 3507 ICAP April 2003 + + +4.8.2 Response + + The response from the ICAP server back to the ICAP client may take + one of four forms: + + - 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. + + + + +Elson & Cerpa Informational [Page 24] + +RFC 3507 ICAP April 2003 + + + ICAP Request Modification Example 1 - ICAP Request + ---------------------------------------------------------------- + REQMOD icap://icap-server.net/server?arg=87 ICAP/1.0 + Host: icap-server.net + Encapsulated: req-hdr=0, null-body=170 + + GET / HTTP/1.1 + Host: www.origin-server.com + Accept: text/html, text/plain + Accept-Encoding: compress + Cookie: ff39fk3jur@4ii0e02i + If-None-Match: "xyzzy", "r2d2xxxx" + + ---------------------------------------------------------------- + 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 + + + +Elson & Cerpa Informational [Page 25] + +RFC 3507 ICAP April 2003 + + + POST /origin-resource/form.pl HTTP/1.1 + Host: www.origin-server.com + Accept: text/html, text/plain + Accept-Encoding: compress + Pragma: no-cache + + 1e + I am posting this information. + 0 + + ---------------------------------------------------------------- + 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 + + ---------------------------------------------------------------- + + + +Elson & Cerpa Informational [Page 26] + +RFC 3507 ICAP April 2003 + + + ICAP Request Modification Example 3 - ICAP Response + ---------------------------------------------------------------- + ICAP/1.0 200 OK + Date: Mon, 10 Jan 2000 09:55:21 GMT + Server: ICAP-Server-Software/1.0 + Connection: close + ISTag: "W3E4R7U9-L2E4-2" + Encapsulated: res-hdr=0, res-body=213 + + HTTP/1.1 403 Forbidden + Date: Wed, 08 Nov 2000 16:02:10 GMT + Server: Apache/1.3.12 (Unix) + Last-Modified: Thu, 02 Nov 2000 13:51:37 GMT + ETag: "63600-1989-3a017169" + Content-Length: 58 + Content-Type: text/html + + 3a + Sorry, you are not allowed to access that naughty content. + 0 + + ---------------------------------------------------------------- + +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, + + + +Elson & Cerpa Informational [Page 27] + +RFC 3507 ICAP April 2003 + + + - 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 + + + + + + + +Elson & Cerpa Informational [Page 28] + +RFC 3507 ICAP April 2003 + + + 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 + + + + + +Elson & Cerpa Informational [Page 29] + +RFC 3507 ICAP April 2003 + + + Other headers are also allowed as described in Section 4.3.1 and + Section 4.3.2 (for example, Host). + +4.10.2 OPTIONS Response + + The OPTIONS response consists of a status line as described in + section 4.3.3 followed by a series of header field names-value pairs + optionally followed by an opt-body. Multiple values in the value + field MUST be separated by commas. If an opt-body is present in the + OPTIONS response, the Opt-body-type header describes the format of + the opt-body. + + The OPTIONS headers supported in this version of the protocol are: + + -- 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. + + + + + +Elson & Cerpa Informational [Page 30] + +RFC 3507 ICAP April 2003 + + + 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 RFC 1123 compliant date/time + string. This header MAY be included in the OPTIONS response. + + For example: + Date: Fri, 15 Jun 2001 04:33:55 GMT + + -- Service-ID: + + A short label identifying the ICAP service. It MAY be used in + attribute header names. This header MAY be included in the + OPTIONS response. + + For example: + Service-ID: xyztech + + + + + +Elson & Cerpa Informational [Page 31] + +RFC 3507 ICAP April 2003 + + + -- 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 + + + +Elson & Cerpa Informational [Page 32] + +RFC 3507 ICAP April 2003 + + + Note: If any of Transfer-* are sent, exactly one of them MUST contain + the wildcard value "*" to specify the default. If no Transfer-* are + sent, all responses will be sent in their entirety (without Preview). + +4.10.3 OPTIONS Examples + + In example 5, an ICAP Client sends an OPTIONS Request to an ICAP + Service named icap.server.net/sample-service in order to get + configuration information for the service provided. + + ICAP OPTIONS Example 5 - ICAP OPTIONS Request + ---------------------------------------------------------------- + OPTIONS icap://icap.server.net/sample-service ICAP/1.0 + Host: icap.server.net + User-Agent: BazookaDotCom-ICAP-Client-Library/2.3 + + ---------------------------------------------------------------- + + 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 + + + +Elson & Cerpa Informational [Page 33] + +RFC 3507 ICAP April 2003 + + + Modification mode, the ICAP server MAY add or modify the HTTP caching + directives located in the encapsulated HTTP response (NOT in the ICAP + header section). Consequently, the ICAP client SHOULD look for + caching directives in the ICAP headers in case of REQMOD, and in the + encapsulated HTTP response in case of RESPMOD. + + In cases where an ICAP server returns a modified version of an object + created by an origin server, such as in Response Modification mode, + the expiration of the ICAP-modified object MUST NOT be longer than + that of the origin object. In other words, ICAP servers MUST NOT + extend the lifetime of origin server objects, but MAY shorten it. + + In cases where the ICAP server is the authoritative source of an ICAP + response, such as in Request Modification mode, the ICAP server is + not restricted in its expiration policy. + + Note that the ISTag response-header may also be used to providing + caching hints to clients; see Section 4.7. + +6. Implementation Notes + +6.1 Vectoring Points + + The definition of the ICAP protocol itself only describes two + different adaptation channels: modification (and satisfaction) of + requests, and modifications of replies. However, an ICAP client + implementation is likely to actually distinguish among four different + classes of adaptation: + + 1. Adaptation of client requests. This is adaptation done every + time a request arrives from a client. This is adaptation done + when a request is "on its way into the cache". Factors such as + the state of the objects currently cached will determine whether + or not this request actually gets forwarded to an origin server + (instead of, say, getting served off the cache's disk). An + example of this type of adaptation would be special access + control or authentication services that must be performed on a + per-client basis. + + 2. Adaptation of requests on their way to an origin server. + Although this type of adaptation is also an adaptation of + requests similar to (1), it describes requests that are "on their + way out of the cache"; i.e., if a request actually requires that + an origin server be contacted. These adaptation requests are not + necessarily specific to particular clients. An example would be + addition of "Accept:" headers for special devices; these + adaptations can potentially apply to many clients. + + + + +Elson & Cerpa Informational [Page 34] + +RFC 3507 ICAP April 2003 + + + 3. Adaptations of responses coming from an origin server. This is + the adaptation of an object "on its way into the cache". In + other words, this is adaptation that a surrogate might want to + perform on an object before caching it. The adapted object may + subsequently served to many clients. An example of this type of + adaptation is virus checking: a surrogate will want to check an + incoming origin reply for viruses once, before allowing it into + the cache -- not every time the cached object is served to a + client. + + Adaptation of responses coming from the surrogate, heading back + to the client. Although this type of adaptation, like (3), is + the adaptation of a response, it is client-specific. Client + reply adaptation is adaptation that is required every time an + object is served to a client, even if all the replies come from + the same cached object off of disk. Ad insertion is a common + form of this kind of adaptation; e.g., if a popular (cached) + object that rarely changes needs a different ad inserted into it + every time it is served off disk to a client. Note that the + relationship between adaptations of type (3) and (4) is analogous + to the relationship between types (2) and (1). + + Although the distinction among these four adaptation points is + critical for ICAP client implementations, the distinction is not + significant for the ICAP protocol itself. From the point of view of + an ICAP server, a request is a request -- the ICAP server doesn't + care what policy led the ICAP client to generate the request. We + therefore did not make these four channels explicit in ICAP for + simplicity. + +6.2 Application Level Errors + + Section 4 described "on the wire" protocol errors that MUST be + standardized across implementations to ensure interoperability. In + this section, we describe errors that are communicated between ICAP + software and the clients and servers on which they are implemented. + Although such errors are implementation dependent and do not + necessarily need to be standardized because they are "within the + box", they are presented here as advice to future implementors based + on past implementation experience. + + + + + + + + + + + +Elson & Cerpa Informational [Page 35] + +RFC 3507 ICAP April 2003 + + + Error name Value + ==================================================== + ICAP_CANT_CONNECT 1000 + ICAP_SERVER_RESPONSE_CLOSE 1001 + ICAP_SERVER_RESPONSE_RESET 1002 + ICAP_SERVER_UNKNOWN_CODE 1003 + ICAP_SERVER_UNEXPECTED_CLOSE_204 1004 + ICAP_SERVER_UNEXPECTED_CLOSE 1005 + + 1000 ICAP_CANT_CONNECT: + "Cannot connect to ICAP server". + + The ICAP server is not connected on the socket. Maybe the ICAP + server is dead or it is not connected on the socket. + + 1001 ICAP_SERVER_RESPONSE_CLOSE: + "ICAP Server closed connection while reading response". + + The ICAP server TCP-shutdowns the connection before the ICAP + client can send all the body data. + + 1002 ICAP_SERVER_RESPONSE_RESET: + "ICAP Server reset connection while reading response". + + The ICAP server TCP-reset the connection before the ICAP client + can send all the body data. + + 1003 ICAP_SERVER_UNKNOWN_CODE: + "ICAP Server sent unknown response code". + + An unknown ICAP response code (see Section 4.x) was received by + the ICAP client. + + 1004 ICAP_SERVER_UNEXPECTED_CLOSE_204: + "ICAP Server closed connection on 204 without 'Connection: close' + header". + + An ICAP server MUST send the "Connection: close" header if + intends to close after the current transaction. + + 1005 ICAP_SERVER_UNEXPECTED_CLOSE: + "ICAP Server closed connection as ICAP client wrote body + preview". + + + + + + + + +Elson & Cerpa Informational [Page 36] + +RFC 3507 ICAP April 2003 + + +6.3 Use of Chunked Transfer-Encoding + + For simplicity, ICAP messages MUST use the "chunked" transfer- + encoding within the encapsulated body section as defined in HTTP/1.1 + [4]. This requires that ICAP client implementations convert incoming + objects "on the fly" to chunked from whatever transfer-encoding on + which they arrive. However, the transformation is simple: + + - 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 RFC 2617. 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. + + + + + + +Elson & Cerpa Informational [Page 37] + +RFC 3507 ICAP April 2003 + + + - If authentication is required between an ICAP client and ICAP + server, hop-by-hop Proxy Authentication as described in RFC 2617 + MUST be used. + + There are potential applications where a user (as opposed to ICAP + client) might have rights to access an ICAP service. In this version + of the protocol, we assume that ICAP clients and ICAP servers are + under the same administrative domain, and contained in a single trust + domain. Therefore, in these cases, we assume that it is sufficient + for users to authenticate themselves to the ICAP client (which is a + surrogate from the point of view from the user). This type of + authentication will also be Proxy Authentication as described in RFC + 2617. + + This standard explicitly excludes any method for a user to + authenticate directly to an ICAP server; the ICAP client MUST be + involved as described above. + +7.2 Encryption + + Users of ICAP should note well that ICAP messages are not encrypted + for transit by default. In the absence of some other form of + encryption at the link or network layers, eavesdroppers may be able + to record the unencrypted transactions between ICAP clients and + servers. As described in Section 4.3.1, the Upgrade header MAY be + used to negotiate transport-layer security for an ICAP connection + [5]. + + Note also that end-to-end encryption between a client and origin + server is likely to preclude the use of value-added services by + intermediaries such as surrogates. An ICAP server that is unable to + decrypt a client's messages will, of course, be unable to perform any + transformations on it. + +7.3 Service Validation + + Normal HTTP surrogates, when operating correctly, should not affect + the end-to-end semantics of messages that pass through them. This + forms a well-defined criterion to validate that a surrogate is + working correctly: a message should look the same before the + surrogate as it does after the surrogate. + + In contrast, ICAP is meant to cause changes in the semantics of + messages on their way from origin servers to users. The criteria for + a correctly operating surrogate are no longer as easy to define. + This will make validation of ICAP services significantly more + difficult. Incorrect adaptations may lead to security + vulnerabilities that were not present in the unadapted content. + + + +Elson & Cerpa Informational [Page 38] + +RFC 3507 ICAP April 2003 + + +8. Motivations and Design Alternatives + + This section describes some of our design decisions in more detail, + and describes the ideas and motivations behind them. This section + does not define protocol requirements, but hopefully sheds light on + the requirements defined in previous sections. Nothing in this + section carries the "force of law" or is part of the formal protocol + specification. + + In general, our guiding principle was to make ICAP the simplest + possible protocol that would do the job, and no simpler. Some + features were rejected where alternative (non-protocol-based) + solutions could be found. In addition, we have intentionally left a + number of issues at the discretion of the implementor, where we + believe that doing so does not compromise interoperability. + +8.1 To Be HTTP, or Not To Be + + ICAP was initially designed as an application-layer protocol built to + run on top of HTTP. This was desirable for a number of reasons. + HTTP is well-understood in the community and has enjoyed significant + investments in software infrastructure (clients, servers, parsers, + etc.). Our initial designs focused on leveraging that existing work; + we hoped that it would be possible to implement ICAP services simply, + using CGI scripts run by existing web servers. + + However, the devil (as always) proved to be in the details. Certain + features that we considered important were impossible to implement + with HTTP. For example, ICAP clients can stop and wait for a "100 + Continue" message in the midst of a message-body; HTTP clients may + only wait between the header and body. In addition, certain + transformations of HTTP messages by surrogates are legal (and + harmless for HTTP), but caused problems with ICAP's "header-in- + header" encapsulation and other features. + + Ultimately, we decided that the tangle of workarounds required to fit + ICAP into HTTP was more complex and confusing than moving away from + HTTP and defining a new (but similar) protocol. + +8.2 Mandatory Use of Chunking + + Chunking is mandatory in ICAP encapsulated bodies for three reasons. + First, efficiency is important, and the chunked encoding allows both + the client and server to keep the transport-layer connection open for + later reuse. Second, ICAP servers (and their developers) should be + encouraged to produce "incremental" responses where possible, to + reduce the latency perceived by users. Chunked encoding is the only + way to support this type of implementation. Finally, by + + + +Elson & Cerpa Informational [Page 39] + +RFC 3507 ICAP April 2003 + + + standardizing on a single encapsulation mechanism, we avoid the + complexity that would be required in client and server software to + support multiple mechanisms. This simplifies ICAP, particularly in + the "body preview" feature described in Section 4.5. + + While chunking of encapsulated bodies is mandatory, encapsulated + headers are not chunked. There are two reasons for this decision. + First, in cases where a chunked HTTP message body is being + encapsulated in an ICAP message, the ICAP client (HTTP server) can + copy it directly from the HTTP client to the ICAP server without un- + chunking and then re-chunking it. Second, many header-parser + implementations have difficulty dealing with headers that come in + multiple chunks. Earlier drafts of this document mandated that a + chunk boundary not come within a header. For clarity, chunking of + encapsulated headers has simply been disallowed. + +8.3 Use of the null-body directive in the Encapsulated header + + There is a disadvantage to not using the chunked transfer-encoding + for encapsulated header part of an ICAP message. Specifically, + parsers do not know in advance how much header data is coming (e.g., + for buffer allocation). ICAP does not allow chunking in the header + part for reasons described in Section 8.2. To compensate, the + "null-body" directive allows the final header's length to be + determined, despite it not being chunked. + +9. References + + [1] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource + Identifiers (URI): Generic Syntax and Semantics", RFC 2396, + August 1998. + + [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement + Levels", BCP 14, RFC 2119, March 1997. + + [3] Resnick, P., "Internet Message Format", RFC 2822, April 2001. + + [4] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., + Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- + HTTP/1.1", RFC 2616, June 1999. + + [5] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1", + RFC 2817, May 2000. + + + + + + + + +Elson & Cerpa Informational [Page 40] + +RFC 3507 ICAP April 2003 + + +10. Contributors + + ICAP is based on an original idea by John Martin and Peter Danzig. + Many individuals and organizations have contributed to the + development of ICAP, including the following contributors (past and + present): + + Lee Duggs + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: lee.duggs@netapp.com + + Paul Eastham + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: eastham@netapp.com + + Debbie Futcher + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: deborah.futcher@netapp.com + + Don Gillies + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: gillies@netapp.com + + Steven La + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: steven.la@netapp.com + + + + + +Elson & Cerpa Informational [Page 41] + +RFC 3507 ICAP April 2003 + + + John Martin + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: jmartin@netapp.com + + Jeff Merrick + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: jeffrey.merrick@netapp.com + + John Schuster + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: john.schuster@netapp.com + + Edward Sharp + Network Appliance, Inc. + 495 East Java Dr. + Sunnyvale, CA 94089 USA + + Phone: (408) 822-6000 + EMail: edward.sharp@netapp.com + + Peter Danzig + Akamai Technologies + 1400 Fashion Island Blvd + San Mateo, CA 94404 USA + + Phone: (650) 372-5757 + EMail: danzig@akamai.com + + Mark Nottingham + Akamai Technologies + 1400 Fashion Island Blvd + San Mateo, CA 94404 USA + + Phone: (650) 372-5757 + EMail: mnot@akamai.com + + + + +Elson & Cerpa Informational [Page 42] + +RFC 3507 ICAP April 2003 + + + Nitin Sharma + Akamai Technologies + 1400 Fashion Island Blvd + San Mateo, CA 94404 USA + + Phone: (650) 372-5757 + EMail: nitin@akamai.com + + Hilarie Orman + Novell, Inc. + 122 East 1700 South + Provo, UT 84606 USA + + Phone: (801) 861-7021 + EMail: horman@novell.com + + Craig Blitz + Novell, Inc. + 122 East 1700 South + Provo, UT 84606 USA + + Phone: (801) 861-7021 + EMail: cblitz@novell.com + + Gary Tomlinson + Novell, Inc. + 122 East 1700 South + Provo, UT 84606 USA + + Phone: (801) 861-7021 + EMail: garyt@novell.com + + Andre Beck + Bell Laboratories / Lucent Technologies + 101 Crawfords Corner Road + Holmdel, New Jersey 07733-3030 + + Phone: (732) 332-5983 + EMail: abeck@bell-labs.com + + Markus Hofmann + Bell Laboratories / Lucent Technologies + 101 Crawfords Corner Road + Holmdel, New Jersey 07733-3030 + + Phone: (732) 332-5983 + EMail: hofmann@bell-labs.com + + + + +Elson & Cerpa Informational [Page 43] + +RFC 3507 ICAP April 2003 + + + David Bryant + CacheFlow, Inc. + 650 Almanor Avenue + Sunnyvale, California 94086 + + Phone: (888) 462-3568 + EMail: david.bryant@cacheflow.com + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Elson & Cerpa Informational [Page 44] + +RFC 3507 ICAP April 2003 + + +Appendix A BNF Grammar for ICAP Messages + + This grammar is specified in terms of the augmented Backus-Naur Form + (BNF) similar to that used by the HTTP/1.1 specification (See Section + 2.1 of [4]). Implementors will need to be familiar with the notation + in order to understand this specification. + + Many header values (where noted) have exactly the same grammar and + semantics as in HTTP/1.1. We do not reproduce those grammars here. + + ICAP-Version = "ICAP/1.0" + + ICAP-Message = Request | Response + + Request = Request-Line + *(Request-Header CRLF) + CRLF + [ Request-Body ] + + Request-Line = Method SP ICAP_URI SP ICAP-Version CRLF + + Method = "REQMOD" ; Section 4.8 + | "RESPMOD" ; Section 4.9 + | "OPTIONS" ; Section 4.10 + | Extension-Method ; Section 4.3.2 + + Extension-Method = token + + ICAP_URI = Scheme ":" Net_Path [ "?" Query ] ; Section 4.2 + + Scheme = "icap" + + Net_Path = "//" Authority [ Abs_Path ] + + Authority = [ userinfo "@" ] host [ ":" port ] + + + Request-Header = Request-Fields ":" [ Generic-Field-Value ] + + Request-Fields = Request-Field-Name + | Common-Field-Name + + ; Header fields specific to requests + Request-Field-Name = "Authorization" ; Section 4.3.2 + | "Allow" ; Section 4.3.2 + | "From" ; Section 4.3.2 + | "Host" ; Section 4.3.2 + | "Referer" ; Section 4.3.2 + + + +Elson & Cerpa Informational [Page 45] + +RFC 3507 ICAP April 2003 + + + | "User-Agent" ; Section 4.3.2 + | "Preview" ; Section 4.5 + + ; Header fields common to both requests and responses + Common-Field-Name = "Cache-Control" ; Section 4.3.1 + | "Connection" ; Section 4.3.1 + | "Date" ; Section 4.3.1 + | "Expires" ; Section 4.3.1 + | "Pragma" ; Section 4.3.1 + | "Trailer" ; Section 4.3.1 + | "Upgrade" ; Section 4.3.1 + | "Encapsulated" ; Section 4.4 + | Extension-Field-Name ; Section 4.3 + + Extension-Field-Name = "X-" token + + Generic-Field-Value = *( Generic-Field-Content | LWS ) + Generic-Field-Content = <the OCTETs making up the field-value + and consisting of either *TEXT or + combinations of token, separators, + and quoted-string> + + Request-Body = *OCTET ; See Sections 4.4 and 4.5 for semantics + + Response = Status-Line + *(Response-Header CRLF) + CRLF + [ Response-Body ] + + Status-Line = ICAP-Version SP Status-Code SP Reason-Phrase CRLF + + Status-Code = "100" ; Section 4.5 + | "101" ; Section 10.1.2 of [4] + | "200" ; Section 10.2.1 of [4] + | "201" ; Section 10.2.2 of [4] + | "202" ; Section 10.2.3 of [4] + | "203" ; Section 10.2.4 of [4] + | "204" ; Section 4.6 + | "205" ; Section 10.2.6 of [4] + | "206" ; Section 10.2.7 of [4] + | "300" ; Section 10.3.1 of [4] + | "301" ; Section 10.3.2 of [4] + | "302" ; Section 10.3.3 of [4] + | "303" ; Section 10.3.4 of [4] + | "304" ; Section 10.3.5 of [4] + | "305" ; Section 10.3.6 of [4] + | "306" ; Section 10.3.7 of [4] + | "307" ; Section 10.3.8 of [4] + + + +Elson & Cerpa Informational [Page 46] + +RFC 3507 ICAP April 2003 + + + | "400" ; Section 4.3.3 + | "401" ; Section 10.4.2 of [4] + | "402" ; Section 10.4.3 of [4] + | "403" ; Section 10.4.4 of [4] + | "404" ; Section 4.3.3 + | "405" ; Section 4.3.3 + | "406" ; Section 10.4.7 of [4] + | "407" ; Section 10.4.8 of [4] + | "408" ; Section 4.3.3 + | "409" ; Section 10.4.10 of [4] + | "410" ; Section 10.4.11 of [4] + | "411" ; Section 10.4.12 of [4] + | "412" ; Section 10.4.13 of [4] + | "413" ; Section 10.4.14 of [4] + | "414" ; Section 10.4.15 of [4] + | "415" ; Section 10.4.16 of [4] + | "416" ; Section 10.4.17 of [4] + | "417" ; Section 10.4.18 of [4] + | "500" ; Section 4.3.3 + | "501" ; Section 4.3.3 + | "502" ; Section 4.3.3 + | "503" ; Section 4.3.3 + | "504" ; Section 10.5.5 of [4] + | "505" ; Section 4.3.3 + | Extension-Code + + Extension-Code = 3DIGIT + + Reason-Phrase = *<TEXT, excluding CR, LF> + + Response-Header = Response-Fields ":" [ Generic-Field-Value ] + + Response-Fields = Response-Field-Name + | Common-Field-Name + + Response-Field-Name = "Server" ; Section 4.3.3 + | "ISTag" ; Section 4.7 + + Response-Body = *OCTET ; See Sections 4.4 and 4.5 for semantics + + + + + + + + + + + + +Elson & Cerpa Informational [Page 47] + +RFC 3507 ICAP April 2003 + + +Authors' Addresses + + Jeremy Elson + University of California Los Angeles + Department of Computer Science + 3440 Boelter Hall + Los Angeles CA 90095 + + Phone: (310) 206-3925 + EMail: jelson@cs.ucla.edu + + + Alberto Cerpa + University of California Los Angeles + Department of Computer Science + 3440 Boelter Hall + Los Angeles CA 90095 + + Phone: (310) 206-3925 + EMail: cerpa@cs.ucla.edu + + + ICAP discussion currently takes place at + icap-discussions@yahoogroups.com. + For more information, see + http://groups.yahoo.com/group/icap-discussions/. + + + + + + + + + + + + + + + + + + + + + + + + + +Elson & Cerpa Informational [Page 48] + +RFC 3507 ICAP April 2003 + + +Full Copyright Statement + + Copyright (C) The Internet Society (2003). All Rights Reserved. + + This document and translations of it may be copied and furnished to + others, and derivative works that comment on or otherwise explain it + or assist in its implementation may be prepared, copied, published + and distributed, in whole or in part, without restriction of any + kind, provided that the above copyright notice and this paragraph are + included on all such copies and derivative works. However, this + document itself may not be modified in any way, such as by removing + the copyright notice or references to the Internet Society or other + Internet organizations, except as needed for the purpose of + developing Internet standards in which case the procedures for + copyrights defined in the Internet Standards process must be + followed, or as required to translate it into languages other than + English. + + The limited permissions granted above are perpetual and will not be + revoked by the Internet Society or its successors or assigns. + + This document and the information contained herein is provided on an + "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING + TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING + BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION + HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF + MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Acknowledgement + + Funding for the RFC Editor function is currently provided by the + Internet Society. + + + + + + + + + + + + + + + + + + + +Elson & Cerpa Informational [Page 49] + |