HTTP adaptation with OPES

This section documents the HTTP profile for the OPES Callout Protocol (OCP) Core . Familiarity with OCP Core is required to understand the HTTP profile. This section uses OCP Core conventions, terminology, and mechanisms. OPES processor communicates its desire to adapt HTTP messages via a &NO_t; message with HTTP-specific feature identifiers documented in . HTTP-specific OCP optimization mechanisms can be negotiated at the same time. A callout server that supports adaptation of HTTP messages has a chance to negotiate what HTTP message parts will participate in adaptation, including negotiation of HTTP request parts as metadata for HTTP response adaptation. Negotiable HTTP message parts are documented in . HTTP profile introduces a new parameter for the &AMS_t; message to communicate known HTTP message length (HTTP headers often do not convey length information reliably or at all). This parameter is documented in . documents a mechanism to report HTTP message part with &DUM_t; messages. The remaining OCP sections document various OCP marshaling corner cases such as handling of HTTP transfer encodings and 100 Continue responses.

An HTTP message may have several well-known parts: headers, body, and trailers. HTTP OPES processors are likely to have information about HTTP message parts because they have to isolate and interpret HTTP headers and find HTTP message boundaries. Callout servers may either not care about certain parts or may benefit from reusing HTTP OPES processor work on isolating and categorizing interesting parts. The following is the declaration of am-part (application message part) type using OCP Core Protocol Element Type Declaration Mnemonic (PETDM):

am-part: extends atom; am-parts: extends list of am-part; The following six "am-part" atoms are valid values: The start-line of an HTTP request message, all request message headers, and the CRLF separator at the end of HTTP headers (compare with section 4.1 of ). The message body of an HTTP request message as defined in section 4.3 of but not including the trailer. The entity headers of the trailer of an HTTP request message in chunked transfer encoding. This part follows the same syntax as the trailer defined in section 3.6.1 of . The start-line of an HTTP response message, all response message headers, and the CRLF separator at the end of HTTP headers (compare with section 4.1 of ). The message body of an HTTP response message as defined in section 4.3 of but not including the trailer. The entity headers of the trailer of an HTTP response message in chunked transfer encoding. This part follows the same syntax as the trailer defined in section 3.6.1 of .

This document defines two HTTP profiles for OCP: request and response profiles. These two profiles are described below. Each profile has a unique feature identifier, a list of original application message parts, and a list of adapted application message parts: http://iana.org/opes/ocp/HTTP/request request-header, request-body, request-trailer request-header, request-body, request-trailer response-header, response-body, response-trailer http://iana.org/opes/ocp/HTTP/response request-header (aux), request-body (aux), request-trailer (aux), response-header, response-body, response-trailer response-header, response-body, response-trailer The request profile contains two variants of adapted part lists: HTTP request parts and HTTP response parts. Parts marked with an "(aux)" suffix are auxiliary parts that can only be used if explicitly negotiated for a profile. See for specific rules governing negotiation and use of am-parts. The scope of a negotiated profile is the OCP connection (default) or the service group specified via the SG parameter.

An OCP agent MUST send application message parts in the order implied by the profile parts lists above. An OCP agent receiving an out-of-order part MAY terminate the transaction with an error. An OPES processor MUST NOT send parts that are not listed as "original" in the negotiated profile. An callout server MUST NOT send parts that are not listed as "adapted" in the negotiated profile. An OCP agent receiving an not-listed part MUST terminate the transaction with an error. The informal rationale for the last requirement is to reduce the number of subtle interoperability problems where an agent thinks that the parts it is sending are understood/used by the other agent when, in fact, they are being ignored or skipped because they are not expected. Some HTTP messages lack certain parts. For example, many HTTP requests do not have bodies, and most HTTP messages do not have trailers. An OCP agent MUST NOT send (i.e., must skip) absent application message parts. An OCP agent MUST send present non-auxiliary parts and it MUST send those present auxiliary parts that were negotiated via the Aux-Parts parameter. OCP agents MUST NOT send auxiliary parts that were not negotiated via the Aux-Parts parameter. An OCP agent receiving a message part in violation of the above requirements MAY terminate the corresponding transaction with an error. By design, original parts not included in the adapted parts list cannot be adapted. In other words, a callout service can only adapt parts in the adapted parts list even though it may have access to other parts. In the request profile, the callout server MUST send either adapted request parts or adapted response parts. An OPES processor receiving adapted flow with application message parts from both lists (in violation of the previous rule) MUST terminate the OCP transaction with an error. Informally, the callout server sends adapted response parts to "short-circuit" the HTTP transaction, forcing the OPES processor to return an HTTP response without forwarding an adapted HTTP request. This short-circuiting is useful for responding, for example, to an HTTP request that the callout service defines as forbidden. Unless explicitly configured to do otherwise, an OPES processor MUST offer all non-auxiliary original parts in &NO_t; messages. See for this rule rationale and examples of harmful side-effects from selective adaptation.

An HTTP application profile feature extends semantics of the feature type of OCP Core while adding the following named parameters to that type: Aux-Parts Pause-At-Body Stop-Receiving-Body Preservation-Interest-Body Content-Encodings The definition of the HTTP profile feature structure using PETDM follows:

HTTP-Profile: extends Feature with { [Aux-Parts: am-parts]; [Pause-At-Body: size]; [Stop-Receiving-Body: size]; [Preservation-Interest-Body: size]; [Content-Encodings: codings]; }; An HTTP profile structure can be used in feature lists of &NO_t; messages and as anonymous parameter of an &NR_t; message. All profile parameters apply to any OCP transaction within profile scope.

The Aux-Parts parameter of an HTTP response profile can be used to negotiate the inclusion of auxiliary application message parts into the original data flow. The parameter is a possibly empty list of am-part tokens. An OPES processor MAY send an Aux-Parts parameter to advertise availability of auxiliary application message parts. A callout server MAY respond with a possibly empty subset of the parts it needs. The callout server response defines the subset of successfully negotiated auxiliary message parts. When receiving a &NO_t; message, the callout server MUST ignore any non-auxiliary part listed in the Aux-Parts parameter. When sending a &NR_t; message, the callout server MUST NOT select any application message part that was not explicitly listed in the negotiation offer. In case of a violation of the last rule, the OPES processor MUST terminate the transaction. An OPES processor MUST send each negotiated auxiliary part to the callout server, unless the part is absent.

Example: Aux-Parts: (request-header,request-body)

A callout server MAY use the Pause-At-Body parameter to request a pause in original application message body transmission before original dataflow starts. The parameter's value is of type "offset". The parameter specifies the start of the non-auxiliary application message body suffix that the sender is temporary not interested in seeing.

[headers][ body prefix | body suffix ][trailer] <-- offset --><-- ? ---------------->]]> ]]> When an OPES processor receives a Pause-At-Body parameter, it MUST behave as if it has received a Want Data Paused (DWP) message with the corresponding org-offset. Note that the latter offset is different from the Pause-At-Body offset and is unknown until the size of the HTTP message headers is known. For example, if the Pause-At-Body value is zero, the OPES processor should send a &DPM_t; message just before it sends the first &DUM_t; message with the response-body part in the HTTP response profile. If the Pause-At-Body value is 300, the OPES processor should send a &DPM; message after transmitting 300 OCTETs for that application message part.

Example: Pause-At-Body: 0

A callout server MAY use the Stop-Receiving-Body parameter to imply a &DWSR; message behavior before the original dataflow starts. The parameter's value is of type "offset". The parameter specifies an offset into the original, non-auxiliary message body part (request-body in request profile and response-body in response profile). A callout service MAY send a Stop-Receiving-Body parameter with its negotiation response if there is a fixed offset into the message body for all transactions of a profile for which a &DWSR_t; message would be sent. An OPES processor MUST behave as if it has received a DWSR message with the corresponding offset. Note that the latter offset is different from the Stop-Receiving-Body offset and is unknown until the size of the HTTP message headers is known. For example, if the Stop-Receiving-Body value is zero in an HTTP response profile, the OPES processor should send an AME message with result code 206 immediately after sending the response-header message part and before starting with the response-body message part.

Example: Stop-Receiving-Body: 0

The Preservation-Interest-Body parameter can be used to optimize data preservation at the OPES processor. The parameter's value is of type "size" and denominates a prefix size of the original, non-auxiliary message body part (request-body in HTTP request profile and response-body in response profile). A callout service MAY send a Preservation-Interest-Body parameter with its negotiation response if there is a fixed-size prefix of the application message body for which a &DPI_t; message would be sent. An OPES processor MUST behave as if it receives a &DPI; message with org-offset zero and org-size equal to the value of the Preservation-Interest-Body parameter. For example, if the Preservation-Interest-Body value is zero in an HTTP response profile, the callout server must not send any &DUY; message for the response-body part; the OPES processor may use this information to optimize its data preservation behavior even before it makes the decision to preserve data.

Example: Preservation-Interest-Body: 0

A callout server MAY send a Content-Encodings list to indicate its preferences in content encodings. Encodings listed first are preferred to other encodings. An OPES processor MAY use any content encoding when sending application messages to a callout server. The list of preferred content encodings does not imply lack of support for other encodings. The OPES processor MUST NOT bypass a service just because the actual content encoding does not match the service's preferences. If an OCP agent receives an application message that it cannot handle due to specific content encoding, the usual transaction termination rules apply.

content-coding: extends atom; content-codings: extends list of content-coding; Example: Content-Encodings: (gzip) The semantics of content-coding is defined in section 3.5 of .

Example: P: NO ({"38:http://iana.org/opes/ocp/HTTP/response" Aux-Parts: (request-header,request-body) }) SG: 5 ; S: NR {"38:http://iana.org/opes/ocp/HTTP/response" Aux-Parts: (request-header) Pause-At-Body: 30 Preservation-Interest-Body: 0 Content-Encodings: (gzip) } SG: 5 ; This example shows a negotiation offer made by an OPES processor for a service group (id 5) that has already been created; the callout server sends an adequate negotiation response. The OPES processor offers one profile feature for HTTP response messages. Besides the standard message parts, the OPES processor is able to add the header and body of the original HTTP request as auxiliary message parts. The callout server requests the auxiliary request-header part, but is not interested in receiving the request-body part. The OPES processor sends at most the following message parts, in the specified order, for all transactions in service group 5: request-header, response-header, response-body, response-trailer. Note that the request-body part is not included (because it is an auxiliary part that was not explicitly requested). Some of the response parts may not be sent if the original message lacks them. The callout server indicates through the Preservation-Interest-Body parameter with size zero that it will not send any DUY messages. The OPES processor may therefore preserve no preservation for any transaction of this profile. By sending a Pause-At-Body value of 30, the callout server requests a data pause. The OPES processor sends a &DPM_t; message immediately after sending at least 30 OCTETs of the response-body part. Thereafter, the OPES processor waits for a &DWM_t; message from the callout service.

A new named parameter for Application Message Start (AMS) messages is introduced.

AM-EL: size AM-EL value is the size of the request-body part in the HTTP request profile, and is the size of the response-body part in the HTTP response profile, before any transfer codings have been applied (or after all transfer codings have been removed). This definition is consistent with the HTTP entity length definition. An OCP agent that knows the exact length of the HTTP message entity (see Section 7.2.2 "Entity Length" in ) at the time it sends the AMS message, SHOULD announce this length using the AM-EL named parameter of an AMS message. If the exact entity length is not known, an OCP agent MUST NOT send an AM-EL parameter. Relaying correct entity length can have significant performance advantages for the recipient, and implementations are strongly encouraged to relay known entity lengths. Similarly, relaying incorrect entity length can have drastic correctness consequences for the recipient, and implementations are urged to exercise great care when relaying entity length. An OPES processor receiving an AM-EL parameter SHOULD use the parameter's value in a Content-Length HTTP entity header when constructing an HTTP message, provided a Content-Length HTTP entity header is allowed for the given application message by HTTP (see ).

A new named parameter for &DUM_t; messages is introduced.

AM-Part: am-part An OCP agent MUST send an AM-Part parameter with every DUM message that is a part of an OCP transaction with an HTTP profile. The AM-Part parameter value is a single am-part token. As implied by the syntax, a DUM message can only contain data of a single application message part. One message part can be fragmented into any number of DUM messages with the same AM-Part parameter. The following example shows three DUM messages containing an abridged HTTP response message. The response-body part is fragmented and sent within two DUM messages.

Example: P: DUM 88 1 0 Kept: 0 AM-Part: response-header 64:HTTP/1.1 200 OK Content-Type: text/html Content-Length: 51 ; P: DUM 88 1 64 Kept: 64 AM-Part: response-body 19:<html><body>This is ; P: DUM 88 1 83 Kept: 83 AM-Part: response-body 32: a simple message.</body></html> ;

The HTTP profile for OCP applies to all HTTP messages. That scope includes HTTP messages such as 1xx (Informational) responses, POST, CONNECT, and OPTIONS requests as well as responses with extension status codes and requests with extension methods. Unless specifically configured to do otherwise, an OPES processor MUST forward all HTTP messages for adaptation at callout servers. OPES bypass instructions, configured HTTP message handling rules, and OCP-negotiation with a callout server are all examples of an acceptable "specific configuration" that provides an exception to this rule. While it may seem useless to attempt to adapt "control" messages such as a 100 (Continue) response, skipping such messages by default may lead to serious security flaws and interoperability problems. For example, sensitive company information might be relayed via a carefully crafted 100 Continue response or a malicious CONNECT request may not get logged if OPES processor does not forward these messages to a callout service that is supposed to handle them. By design, OPES processor implementation cannot unilaterally decide that an HTTP message is not worth adapting. It needs a callout server opinion, a configuration setting, or another external information to make the decision.

HTTP defines several ho-by-hop headers (e.g., Connection) and allows for extension headers to be specified as hop-by-hop ones (via the Connection header mechanism). Depending on the environment and configuration, an OPES processor MAY forward hop-by-hop headers to callout servers and MAY use hop-by-hop headers returned by callout servers to build an HTTP message for the next application hop. However, see for requirements specific to the Transfer-Encoding header. For example, a logging or statistics collection service may want to see hop-by-hop headers sent by the previous application hop to the OPES processor and/or hop-by-hop headers sent by the OPES processor to the next application hop. Another service may actually handle HTTP logic of removing and adding hop-by-hop headers. Many services will ignore hop-by-hop headers. This specification does not define a mechanism for distinguishing these use cases.

HTTP messages may use transfer encodings, a hop-by-hop encoding feature of HTTP. Adaptations that use HTTP transfer encodings have to be explicitly negotiated. This specification does not document such negotiations. In the absence of explicit transfer-encoding negotiations, an OCP agent MUST NOT send transfer-encoded application message bodies. Informally, the above rule means that the agent or its environment have to make sure that all transfer encodings are stripped from an HTTP message body before it enters OCP scope. An agent MUST terminate the OCP transaction if it has to send an application message body but cannot remove all transfer encodings. Violations of these rules lead to interoperability problems. If an OCP agent receives transfer-encoded application data in violation of the above requirement, the agent MAY terminate the corresponding OCP transaction. An OPES processor removing transfer encodings MUST remove the Transfer-Encoding header before sending the header part to the callout service. A callout server receiving a Transfer-Encoding header MAY assume that original application data is still transfer-encoded (and terminate the transaction). The OPES processor MUST send a correct Transfer-Encoding header to the next HTTP recipient independent of what header (if any) the callout server returned. Logging and wiretapping are the examples where negotiating acceptable transfer encodings may be worthwhile. While a callout server may not be able to strip an encoding, it may still want to log the entire message "as is". In most cases, however, the callout server would not be able to meaningfully handle unknown transfer encodings.

When communicating with HTTP applications, OPES processors MUST ensure correctness of all computable HTTP headers documented in specifications that the processors intend to be compliant with. A computable header is defined as a header which value can be computed based on the message body alone. For example, the correctness of Content-Length and Content-MD5 headers has to be ensured by processors claiming compliance with HTTP/1.1 (). Informally and by default, the OPES processor has to validate and eventually recalculate, add, or remove computable HTTP headers in order to build a compliant HTTP message from an adapted application message returned by the callout server. If a particular OPES processor trusts certain HTTP headers that a callout service sends, it can use those headers "as is". An OPES processor MAY forward a partially adapted HTTP message from a callout server to the next callout server, without verifying HTTP header correctness. Consequently, a callout service cannot assume that the HTTP headers it receives are correct or final from an HTTP point of view. The following subsections present guidelines for the recalculation of some HTTP headers.

By default, an OCP agent MUST NOT trust the Content-Length header that is sent within an HTTP header message part. The message length could be modified by a callout service without adaptation of the HTTP message headers. Before sending the HTTP message to the HTTP peer, the OPES processor has to ensure correctness of the message length indication according to section 4.4 of . Besides ensuring HTTP message correctness, good OPES processors set up the message to optimize performance, including minimizing delivery latency. Specifically, indicating the end of a message by closing the HTTP connection ought to be the last resort: If the callout server sends an AM-EL parameter with its AMS message, the OPES processor SHOULD use this value to create a Content-Length header to be able to keep a persistent HTTP connection. Note that HTTP rules prohibit a Content-Length header to be used in transfer encoded messages. If AM-EL parameter or equivalent entity length information is not available, and HTTP rules allow for chunked transfer encoding, the OPES processor SHOULD use chunked transfer encoding. Note that any Content-Length header has to be removed in this case. If the message size is not known a priori and chunked transfer coding cannot be used, but the OPES processor can wait for the OCP transaction to finish before forwarding the adapted HTTP message on a persistent HTTP connection, then the processor SHOULD compute and add a Content-Length header. Finally, if all optimizations are not applicable, the OPES processor SHOULD delete any Content-Length header and forward adapted data immediately, while indicating the message end by closing the HTTP connection.

By default, the OPES processor MUST assume that the callout service modifies the content in a way that the MD5 checksum of the message body becomes invalid. According to section 14.15 of , HTTP intermediaries must not generate Content-MD5 headers. A recalculation is therefore possible only if the OPES processor is considered authoritative for the entity being adapted. An un-authoritative OPES processor MUST remove the Content-MD5 header unless it detects that the HTTP message was not modified; in this case it MAY leave the Content-MD5 header in the message. When such detection significantly increases message latency, deleting the Content-MD5 header may be a better option.

This is a possible OCP message flow using an HTTP request profile. An end-user wants to access the home page of www.restricted-content.org, through the proxy, but access is denied by a URL blocking service running on the callout server used by the proxy. OCP messages from the OPES processor are marked with "P:" and OCP messages from the callout server are marked with "S:". The OCP connection is not closed at the end but kept open for the next OCP transaction.

Example: P: CS; S: CS; P: SGC 11 ({"23:ocp-test.com/url-filter"}); P: NO ({"37:http://iana.org/opes/ocp/HTTP/request"}) SG: 11 ; S: NR {"37:http://iana.org/opes/ocp/HTTP/request"} SG: 11 ; P: TS 55 11; P: AMS 55 AM-EL: 0 ; P: DUM 55 0 Kept: 0 AM-Part: request-header 235:GET http://www.restricted-content.org/ HTTP/1.1 Accept: */* Accept-Language: de Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; Windows NT 5.0) Host: www.restricted-content.org Proxy-Connection: Keep-Alive ; P: AME 55; S: AMS 55; S: DUM 55 0 AM-Part: response-header 76:HTTP/1.1 403 Forbidden Content-Type: text/html Proxy-Connection: close ; S: DUM 55 0 AM-Part: response-body 67:<html><body>You are not allowed to access this page.</body></html> ; S: AME 55; P: TE 55; S: TE 55; The next example is a language translation of a small plain text file that gets transferred in an HTTP response. In this example, OCP agents negotiate a profile for the whole OCP connection. The OCP connection remains open in the end of the OCP transaction.

Example: P: CS; S: CS; P: NO ({"38:http://iana.org/opes/ocp/HTTP/response"}); S: NR {"38:http://iana.org/opes/ocp/HTTP/response"}; P: SGC 12 ({"45:ocp-test.com/translate?from=EN&to=DE"}); P: TS 89 12; P: AMS 89 AM-EL: 86 ; P: DUM 89 0 AM-Part: response-header 65:HTTP/1.1 200 OK Content-Type: text/plain Content-Length: 86 ; P: DUM 89 65 AM-Part: response-body 86:Whether 'tis nobler in the mind to suffer The slings and arrows of outrageous fortune ; P: AME 89; S: AMS 89 AM-EL: 78 ; P: TE 89; S: DUM 89 0 AM-Part: response-header 65:HTTP/1.1 200 OK Content-Type: text/plain Content-Length: 78 ; S: DUM 89 63 AM-Part: response-body 80:Ob�s edler im Gemuet, die Pfeil und Schleudern des wuetenden Geschicks erdulden ; S: AME 89; S: TE 89; The following example shows modification of an HTML resource and demonstrates data preservation optimization. The callout server uses a DUY message to send back an unchanged response header part but because it does not know the size of the altered HTML resource at the time it sends the AMS message, the callout server omits the AM-EL parameter; the OPES processor is responsible for adjusting the Content-Length header.

Example: P: CS; S: CS; P: SGC 10 ({"22:ocp-test.com/ad-filter"}); P: NO ({"38:http://iana.org/opes/ocp/HTTP/response" Aux-Parts: (request-header,request-body) },{"29:http://iana.org/opes/ocp/MIME"}) SG: 10 ; S: NR {"38:http://iana.org/opes/ocp/HTTP/response" Aux-Parts: (request-header) Content-Encodings: (gzip) } SG: 10 ; P: TS 88 10; P: AMS 88 AM-EL: 95 ; P: DUM 88 0 AM-Part: request-header 65:GET /opes/adsample.html HTTP/1.1 Host: www.martin-stecher.de ; P: DUM 88 65 Kept: 65 64 AM-Part: response-header 64:HTTP/1.1 200 OK Content-Type: text/html Content-Length: 95 ; P: DUM 88 129 Kept: 65 90 AM-Part: response-body 26:<html> <body> This is my ; S: AMS 88; P: DUM 88 155 Kept: 65 158 AM-Part: response-body 68: new ad: <img src="my_ad.gif" width=88 height=31> </body> </html> ; S: DUY 88 65 64 S: DPI 88 129 2147483647; P: AME 88; S: DUM 88 0 AM-Part: response-body 52:<html> <body> This is my new ad: </body> </html> ; S: DPI 88 129 0; P: TE 88; S: AME 88; S: TE 88;