Электронная библиотека
Программисту веб-дизайнеру
Другие материалы
HTTP/1.1, протокол передачи гипертекста
part of Hypertext Transfer Protocol -- HTTP/1.119 Appendices
19.1 Internet Media Type message/http and application/http
In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type "message/http" and "application/http". The message/http type can be used to enclose a single HTTP request or response message, provided that it obeys the MIME restrictions for all "message" types regarding line length and encodings. The application/http type can be used to enclose a pipeline of one or more HTTP request or response messages (not intermixed). The following is to be registered with IANA [17].
Media Type name: message Media subtype name: http Required parameters: none Optional parameters: version, msgtype version: The HTTP-Version number of the enclosed message (e.g., "1.1"). If not present, the version can be determined from the first line of the body. msgtype: The message type -- "request" or "response". If not present, the type can be determined from the first line of the body. Encoding considerations: only "7bit", "8bit", or "binary" are permitted Security considerations: none
Media Type name: application Media subtype name: http Required parameters: none Optional parameters: version, msgtype version: The HTTP-Version number of the enclosed messages (e.g., "1.1"). If not present, the version can be determined from the first line of the body. msgtype: The message type -- "request" or "response". If not present, the type can be determined from the first line of the body. Encoding considerations: HTTP messages enclosed by this type are in "binary" format; use of an appropriate Content-Transfer-Encoding is required when transmitted via E-mail. Security considerations: none
19.2 Internet Media Type multipart/byteranges
When an HTTP 206 (Partial Content) response message includes the content of multiple ranges (a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart message-body. The media type for this purpose is called "multipart/byteranges".
The multipart/byteranges media type includes two or more parts, each with its own Content-Type and Content-Range fields. The required boundary parameter specifies the boundary string used to separate each body-part.
Media Type name: multipart Media subtype name: byteranges Required parameters: boundary Optional parameters: none Encoding considerations: only "7bit", "8bit", or "binary" are permitted Security considerations: none
For example:
HTTP/1.1 206 Partial Content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT Content-type: multipart/byteranges; boundary=THIS_StrING_SEPARATES
--THIS_StrING_SEPARATES Content-type: application/pdf Content-range: bytes 500-999/8000
...the first range... --THIS_StrING_SEPARATES Content-type: application/pdf Content-range: bytes 7000-7999/8000
...the second range --THIS_StrING_SEPARATES--
Notes:
1) Additional CRLFs may precede the first boundary string in the entity.
2) Although RFC 2046 [40] permits the boundary string to be quoted, some existing implementations handle a quoted boundary string incorrectly.
3) A number of browsers and servers were coded to an early draft of the byteranges specification to use a media type of multipart/x-byteranges, which is almost, but not quite compatible with the version documented in HTTP/1.1.
19.3 Tolerant Applications
Although this document specifies the requirements for the generation of HTTP/1.1 messages, not all applications will be correct in their implementation. We therefore recommend that operational applications be tolerant of deviations whenever those deviations can be interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line and servers tolerant when parsing the Request-Line. In particular, they SHOULD accept any amount of SP or HT characters between fields, even though only a single SP is required.
The line terminator for message-header fields is the sequence CRLF. However, we recommend that applications, when parsing such headers, recognize a single LF as a line terminator and ignore the leading CR.
The character set of an entity-body SHOULD be labeled as the lowest common denominator of the character codes used within that body, with the exception that not labeling the entity is preferred over labeling the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1 and 3.4.1.
Additional rules for requirements on parsing and encoding of dates and other potential problems with date encodings include:
- HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve the "year 2000" problem).
- An HTTP/1.1 implementation MAY internally represent a parsed Expires date as earlier than the proper value, but MUST NOT internally represent a parsed Expires date as later than the proper value.
- All expiration-related calculations MUST be done in GMT. The local time zone MUST NOT influence the calculation or comparison of an age or expiration time.
- If an HTTP header incorrectly carries a date value with a time zone other than GMT, it MUST be converted into GMT using the most conservative possible conversion.
19.4 Differences Between HTTP Entities and RFC 2045 Entities
HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to allow entities to be transmitted in an open variety of representations and with extensible mechanisms. However, RFC 2045 discusses mail, and HTTP has a few features that are different from those described in RFC 2045. These differences were carefully chosen to optimize performance over binary connections, to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients.
This appendix describes specific areas where HTTP differs from RFC 2045. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions might be required.
19.4.1 MIME-Version
HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY include a single MIME-Version general-header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field indicates that the message is in full compliance with the MIME protocol (as defined in RFC 2045[7]). Proxies/gateways are responsible for ensuring full compliance (where possible) when exporting HTTP messages to strict MIME environments.
MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
MIME version "1.0" is the default for use in HTTP/1.1. However, HTTP/1.1 message parsing and semantics are defined by this document and not the MIME specification.
19.4.2 Conversion to Canonical Form
RFC 2045 [7] requires that an Internet mail entity be converted to canonical form prior to being transferred, as described in section 4 of RFC 2049 [48]. Section 3.7.1 of this document describes the forms allowed for subtypes of the "text" media type when transmitted over HTTP. RFC 2046 requires that content with a type of "text" represent line breaks as CRLF and forbids the use of CR or LF outside of line
break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content when a message is transmitted over HTTP.
Where it is possible, a proxy or gateway from HTTP to a strict MIME environment SHOULD translate all line breaks within the text media types described in section 3.7.1 of this document to the RFC 2049 canonical form of CRLF. Note, however, that this might be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some character sets which do not use octets 13 and 10 to represent CR and LF, as is the case for some multi-byte character sets.
Implementors should note that conversion will break any cryptographic checksums applied to the original content unless the original content is already in canonical form. Therefore, the canonical form is recommended for any content that uses such checksums in HTTP.
19.4.3 Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to simplify the process of date comparison. Proxies and gateways from other protocols SHOULD ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary.
19.4.4 Introduction of Content-Encoding
RFC 2045 does not include any concept equivalent to HTTP/1.1's Content-Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols MUST either change the value of the Content-Type header field or decode the entity-body before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=<content-coding>" to perform a function equivalent to Content-Encoding. However, this parameter is not part of RFC 2045.)
19.4.5 No Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any non-identity CTE ("quoted-printable" or "base64") encoding prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.
19.4.6 Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field (section 14.41). Proxies/gateways MUST remove any transfer-coding prior to forwarding a message via a MIME-compliant protocol.
A process for decoding the "chunked" transfer-coding (section 3.6) can be represented in pseudo-code as:
length := 0
read chunk-size, chunk-extension (if any) and CRLF
while (chunk-size > 0) {
read chunk-data and CRLF
append chunk-data to entity-body
length := length + chunk-size
read chunk-size and CRLF
}
read entity-header
while (entity-header not empty) {
append entity-header to existing header fields
read entity-header
}
Content-Length := length
Remove "chunked" from Transfer-Encoding
19.4.7 MHTML and Line Length Limitations
HTTP implementations which share code with MHTML [45] implementations need to be aware of MIME line length limitations. Since HTTP does not have this limitation, HTTP does not fold long lines. MHTML messages being transported by HTTP follow all conventions of MHTML, including line length limitations and folding, canonicalization, etc., since HTTP transports all message-bodies as payload (see section 3.7.2) and does not interpret the content or any MIME header lines that might be contained therein.
19.5 Additional Features
RFC 1945 and RFC 2068 document protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementors are advised to be aware of these features, but cannot rely upon their presence in, or interoperability with, other HTTP/1.1 applications. Some of these
describe proposed experimental features, and some describe features that experimental deployment found lacking that are now addressed in the base HTTP/1.1 specification.
A number of other headers, such as Content-Disposition and Title, from SMTP and MIME are also often implemented (see RFC 2076 [37]).
19.5.1 Content-Disposition
The Content-Disposition response-header field has been proposed as a means for the origin server to suggest a default filename if the user requests that the content is saved to a file. This usage is derived from the definition of Content-Disposition in RFC 1806 [35].
content-disposition = "Content-Disposition" ":" disposition-type *( ";" disposition-parm ) disposition-type = "attachment" | disp-extension-token disposition-parm = filename-parm | disp-extension-parm filename-parm = "filename" "=" quoted-string disp-extension-token = token disp-extension-parm = token "=" ( token | quoted-string )
An example is
Content-Disposition: attachment; filename="fname.ext"
The receiving user agent SHOULD NOT respect any directory path information present in the filename-parm parameter, which is the only parameter believed to apply to HTTP implementations at this time. The filename SHOULD be treated as a terminal component only.
If this header is used in a response with the application/octet- stream Content-Type, the implied suggestion is that the user agent should not display the response, but directly enter a `save response as...' dialog.
See section 15.5 for Content-Disposition security issues.
19.6 Compatibility with Previous Versions
It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP/1.1 was deliberately designed, however, to make supporting previous versions easy. It is worth noting that, at the time of composing this specification (1996), we would expect commercial HTTP/1.1 servers to:
- recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 requests;
- understand any valid request in the format of HTTP/0.9, 1.0, or 1.1;
- respond appropriately with a message in the same major version used by the client.
And we would expect HTTP/1.1 clients to:
- recognize the format of the Status-Line for HTTP/1.0 and 1.1 responses;
- understand any valid response in the format of HTTP/0.9, 1.0, or 1.1.
For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the server after sending the response. Some implementations implement the Keep-Alive version of persistent connections described in section 19.7.1 of RFC 2068 [33].
19.6.1 Changes from HTTP/1.0
This section summarizes major differences between versions HTTP/1.0 and HTTP/1.1.
19.6.1.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
Addresses
The requirements that clients and servers support the Host request- header, report an error if the Host request-header (section 14.23) is missing from an HTTP/1.1 request, and accept absolute URIs (section 5.1.2) are among the most important changes defined by this specification.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP addresses and servers; there was no other established mechanism for distinguishing the intended server of a request than the IP address to which that request was directed. The changes outlined above will allow the Internet, once older HTTP clients are no longer common, to support multiple Web sites from a single IP address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created serious problems. The Internet will also be able to recover the IP addresses that have been allocated for the sole purpose of allowing special-purpose domain names to be used in root-level HTTP URLs. Given the rate of growth of the Web, and the number of servers already deployed, it is extremely
important that all implementations of HTTP (including updates to existing HTTP/1.0 applications) correctly implement these requirements:
- Both clients and servers MUST support the Host request-header.
- A client that sends an HTTP/1.1 request MUST send a Host header.
- Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 request does not include a Host request-header.
- Servers MUST accept absolute URIs.
19.6.2 Compatibility with HTTP/1.0 Persistent Connections
Some clients and servers might wish to be compatible with some previous implementations of persistent connections in HTTP/1.0 clients and servers. Persistent connections in HTTP/1.0 are explicitly negotiated as they are not the default behavior. HTTP/1.0 experimental implementations of persistent connections are faulty, and the new facilities in HTTP/1.1 are designed to rectify these problems. The problem was that some existing 1.0 clients may be sending Keep-Alive to a proxy server that doesn't understand Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection and result in a hung HTTP/1.0 proxy waiting for the close on the response. The result is that HTTP/1.0 clients must be prevented from using Keep-Alive when talking to proxies.
However, talking to proxies is the most important use of persistent connections, so that prohibition is clearly unacceptable. Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when talking to an old proxy that ignores Connection. Persistent connections are the default for HTTP/1.1 messages; we introduce a new keyword (Connection: close) for declaring non-persistence. See section 14.10.
The original HTTP/1.0 form of persistent connections (the Connection: Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]
19.6.3 Changes from RFC 2068
This specification has been carefully audited to correct and disambiguate key word usage; RFC 2068 had many problems in respect to the conventions laid out in RFC 2119 [34].
Clarified which error code should be used for inbound server failures (e.g. DNS failures). (Section 10.5.5).
CREATE had a race that required an Etag be sent when a resource is first created. (Section 10.2.2).
Content-Base was deleted from the specification: it was not implemented widely, and there is no simple, safe way to introduce it without a robust extension mechanism. In addition, it is used in a similar, but not identical fashion in MHTML [45].
Transfer-coding and message lengths all interact in ways that required fixing exactly when chunked encoding is used (to allow for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are computed. (Sections 3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16)
A content-coding of "identity" was introduced, to solve problems discovered in caching. (section 3.5)
Quality Values of zero should indicate that "I don't want something" to allow clients to refuse a representation. (Section 3.9)
The use and interpretation of HTTP version numbers has been clarified by RFC 2145. Require proxies to upgrade requests to highest protocol version they support to deal with problems discovered in HTTP/1.0 implementations (Section 3.1)
Charset wildcarding is introduced to avoid explosion of character set names in accept headers. (Section 14.2)
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections 13.4, 14.8, 14.9, 14.9.3)
The Cache-Control: max-age directive was not properly defined for responses. (Section 14.9.3)
There are situations where a server (especially a proxy) does not know the full length of a response but is capable of serving a byterange request. We therefore need a mechanism to allow byteranges with a content-range not indicating the full length of the message. (Section 14.16)
Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send needed headers in a 206 response, this problem can be avoided. (Section 10.2.7, 13.5.3, and 14.27)
Fix problem with unsatisfiable range requests; there are two cases: syntactic problems, and range doesn't exist in the document. The 416 status code was needed to resolve this ambiguity needed to indicate an error for a byte range request that falls outside of the actual contents of a document. (Section 10.4.17, 14.16)
Rewrite of message transmission requirements to make it much harder for implementors to get it wrong, as the consequences of errors here can have significant impact on the Internet, and to deal with the following problems:
1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where this was incorrectly placing a requirement on the behavior of an implementation of a future version of HTTP/1.x
2. Made it clear that user-agents should retry requests, not "clients" in general.
3. Converted requirements for clients to ignore unexpected 100 (Continue) responses, and for proxies to forward 100 responses, into a general requirement for 1xx responses.
4. Modified some TCP-specific language, to make it clearer that non-TCP transports are possible for HTTP.
5. Require that the origin server MUST NOT wait for the request body before it sends a required 100 (Continue) response.
6. Allow, rather than require, a server to omit 100 (Continue) if it has already seen some of the request body.
7. Allow servers to defend against denial-of-service attacks and broken clients.
This change adds the Expect header and 417 status code. The message transmission requirements fixes are in sections 8.2, 10.4.18, 8.1.2.2, 13.11, and 14.20.
Proxies should be able to add Content-Length when appropriate. (Section 13.5.2)
Clean up confusion between 403 and 404 responses. (Section 10.4.4, 10.4.5, and 10.4.11)
Warnings could be cached incorrectly, or not updated appropriately. (Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
Transfer-coding had significant problems, particularly with interactions with chunked encoding. The solution is that transfer- codings become as full fledged as content-codings. This involves adding an IANA registry for transfer-codings (separate from content codings), a new header field (TE) and enabling trailer headers in the future. Transfer encoding is a major performance benefit, so it was worth fixing [39]. TE also solves another, obscure, downward interoperability problem that could have occurred due to interactions between authentication trailers, chunked encoding and HTTP/1.0 clients.(Section 3.6, 3.6.1, and 14.39)
The PATCH, LINK, UNLINK methods were defined but not commonly implemented in previous versions of this specification. See RFC 2068 [33].
The Alternates, Content-Version, Derived-From, Link, URI, Public and Content-Base header fields were defined in previous versions of this specification, but not commonly implemented. See RFC 2068 [33].