- 1) Introduction
- 2) Notational Conventions and Generic Grammar
- 3) Protocol Parameters
- 1) HTTP Version
- 2) Uniform Resource Identifiers
- 3) Date/Time Formats
- 4) Character Sets
- 5) Content Codings
- 6) Transfer Codings
- 7) Media Types
- 8) Product Tokens
- 9) Quality Values
- 10) Language Tags
- 11) Entity Tags
- 12) Range Units
- 4) HTTP Message
- 5) Request
- 6) Response
- 7) Entity
- 8) Connections
- 1) Persistent Connections
- 2) Message Transmission Requirements
- 9) Method Definitions
- 10) Status Code Definitions
- 1) Informational 1xx
- 2) Successful 2xx
- 3) Redirection 3xx
- 4) Client Error 4xx
- 1) 400 Bad Request
- 2) 401 Unauthorized
- 3) 402 Payment Required
- 4) 403 Forbidden
- 5) 404 Not Found
- 6) 405 Method Not Allowed
- 7) 406 Not Acceptable
- 8) 407 Proxy Authentication Required
- 9) 408 Request Timeout
- 10) 409 Conflict
- 11) 410 Gone
- 12) 411 Length Required
- 13) 412 Precondition Failed
- 14) 413 Request Entity Too Large
- 15) 414 Request-URI Too Long
- 16) 415 Unsupported Media Type
- 17) 416 Requested Range Not Satisfiable
- 18) 417 Expectation Failed
- 5) Server Error 5xx
- 11) Access Authentication
- 12) Content Negotiation
- 13) Caching in HTTP
- 1) ..
- 2) Expiration Model
- 3) Validation Model
- 4) Response Cacheability
- 5) Constructing Responses From Caches
- 6) Caching Negotiated Responses
- 7) Shared and Non-Shared Caches
- 8) Errors or Incomplete Response Cache Behavior
- 9) Side Effects of GET and HEAD
- 10) Invalidation After Updates or Deletions
- 11) Write-Through Mandatory
- 12) Cache Replacement
- 13) History Lists
- 14) Header Field Definitions
- 1) Accept
- 2) Accept-Charset
- 3) Accept-Encoding
- 4) Accept-Language
- 5) Accept-Ranges
- 6) Age
- 7) Allow
- 8) Authorization
- 9) Cache-Control
- 10) Connection
- 11) Content-Encoding
- 12) Content-Language
- 13) Content-Length
- 14) Content-Location
- 15) Content-MD5
- 16) Content-Range
- 17) Content-Type
- 18) Date
- 19) ETag
- 20) Expect
- 21) Expires
- 22) From
- 23) Host
- 24) If-Match
- 25) If-Modified-Since
- 26) If-None-Match
- 27) If-Range
- 28) If-Unmodified-Since
- 29) Last-Modified
- 30) Location
- 31) Max-Forwards
- 32) Pragma
- 33) Proxy-Authenticate
- 34) Proxy-Authorization
- 35) Range
- 36) Referer
- 37) Retry-After
- 38) Server
- 39) TE
- 40) Trailer
- 41) Transfer-Encoding
- 42) Upgrade
- 43) User-Agent
- 44) Vary
- 45) Via
- 46) Warning
- 47) WWW-Authenticate
- 15) Security Considerations
- 1) Personal Information
- 2) Attacks Based On File and Path Names
- 3) DNS Spoofing
- 4) Location Headers and Spoofing
- 5) Content-Disposition Issues
- 6) Authentication Credentials and Idle Clients
- 7) Proxies and Caching
- 16) Acknowledgments
- 17) References
- 18) Authors' Addresses
- 19) Appendices
- 1) Internet Media Type message/http and application/http
- 2) Internet Media Type multipart/byteranges
- 3) Tolerant Applications
- 4) Differences Between HTTP Entities and RFC 2045 Entities
- 5) Additional Features
- 6) Compatibility with Previous Versions
- 20) Index
- 21) Full Copyright Statement
- 22) Acknowledgement
3 Protocol Parameters
3.1 HTTP Version
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication. No change is made to the version number for the addition of message components which do not affect communication behavior or which only add to extensible field values. The <minor> number is incremented when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may add to the message semantics and imply additional capabilities of the sender. The <major> number is incremented when the format of a message within the protocol is changed. See RFC 2145  for a fuller explanation.
The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message.
= "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers MUST be treated as separate integers and that each MAY be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and MUST NOT be sent.
An application that sends a request or response message that includes HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant with this specification. Applications that are at least conditionally compliant with this specification SHOULD use an HTTP-Version of "HTTP/1.1" in their messages, and MUST do so for any message that is not compatible with HTTP/1.0. For more details on when to send specific HTTP-Version values, see RFC 2145 .
The HTTP version of an application is the highest HTTP version for which the application is at least conditionally compliant.
Proxy and gateway applications need to be careful when forwarding messages in protocol versions different from that of the application. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway MUST NOT send a message with a version indicator which is greater than its actual version. If a higher version request is received, the proxy/gateway MUST either downgrade the request version, or respond with an error, or switch to tunnel behavior.
Due to interoperability problems with HTTP/1.0 proxies discovered since the publication of RFC 2068, caching proxies MUST, gateways MAY, and tunnels MUST NOT upgrade the request to the highest version they support. The proxy/gateway's response to that request MUST be in the same major version as the request.
Note: Converting between versions of HTTP may involve modification of header fields required or forbidden by the versions involved.
3.2 Uniform Resource Identifiers
URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers , and finally the combination of Uniform Resource Locators (URL)  and Names (URN) . As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location, or any other characteristic--a resource.
3.2.1 General Syntax
URIs in HTTP can be represented in absolute form or relative to some known base URI , depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with a scheme name followed by a colon. For definitive information on URL syntax and semantics, see "Uniform Resource Identifiers (URI): Generic Syntax and Semantics," RFC 2396  (which replaces RFCs 1738  and RFC 1808 ). This specification adopts the definitions of "URI-reference", "absoluteURI", "relativeURI", "port", "host","abs_path", "rel_path", and "authority" from that specification.
The HTTP protocol does not place any a priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return 414 (Request-URI Too Long) status if a URI is longer than the server can handle (see Section 10.4.15).
Note: Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations might not properly support these lengths.
3.2.2 http URL
The "http" scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax and semantics for http URLs.
http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
If the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path (Section 5.1.2). The use of IP addresses in URLs SHOULD be avoided whenever possible (see RFC 1900 ). If the abs_path is not present in the URL, it MUST be given as "/" when used as a Request-URI for a resource (Section 5.1.2). If a proxy receives a host name which is not a fully qualified domain name, it MAY add its domain to the host name it received. If a proxy receives a fully qualified domain name, the proxy MUST NOT change the host name.
3.2.3 URI Comparison
When comparing two URIs to decide if they match or not, a client SHOULD use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions:
- A port that is empty or not given is equivalent to the default port for that URI-reference; - Comparisons of host names MUST be case-insensitive;
- Comparisons of scheme names MUST be case-insensitive.
- An empty abs_path is equivalent to an abs_path of "/".
Characters other than those in the "reserved" and "unsafe" sets (seeRFC 2396 ) are equivalent to their ""%" HEX HEX" encoding.
For example, the following three URIs are equivalent:
3.3 Date/Time Formats
3.3.1 Full Date
HTTP applications have historically allowed three different formats for the representation of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123  (an update to RFC 822 ). The second format is in common use, but is based on the obsolete RFC 850  date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value MUST accept all three formats (for compatibility with HTTP/1.0), though they MUST only generate the RFC 1123 format for representing HTTP-date values in header fields. See Section 19.3 for further information.
Note: Recipients of date values are encouraged to be robust in accepting date values that may have been sent by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.
All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated Universal Time). This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and MUST be assumed when reading the asctime format. HTTP-date is case sensitive and MUST NOT include additional LWS beyond that specifically included as SP in the grammar.
= rfc1123-date | rfc850-date | asctime-date
= wkday "," SP date1 SP time SP "GMT"
= wkday "," SP date1 SP time SP "GMT"
= wkday SP date3 SP time SP 4DIGIT
= 2DIGIT SP month SP 4DIGIT ; day month year (e.g., 02 Jun 1982)
=2DIGIT "-" month "-" 2DIGIT ; day-month-year (e.g., 02-Jun-82)
=month SP ( 2DIGIT|( SP 1DIGIT )) ; month day (e.g., Jun 2)
=2DIGIT":"2DIGIT":"2DIGIT;00:00:00 - 23:59:59
="Monday"|"Tuesday"|"Wednesday"|"Thursday" |"Friday" |"Saturday" |"Sunday"
= "Jan" | "Feb" | "Mar" | "Apr" | "May" | "Jun" | "Jul" | "Aug" | "Sep" | "Oct" | "Nov" | "Dec"
Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc.
3.3.2 Delta Seconds
Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after the time that the message was received.
delta-seconds = 1*DIGIT
3.4 Character Sets
HTTP uses the same definition of the term "character set" as that described for MIME: The term "character set" is used in this document to refer to a method used with one or more tables to convert a sequence of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that not all characters may be available in a given character set and a character set may provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encoding, from simple single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO-2022's techniques. However, the definition associated with a MIME character set name MUST fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information to determine the exact mapping is not permitted.
Note: This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME share the same registry, it is important that the terminology also be shared.
HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry .
charset = token
Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry  MUST represent the character set defined by that registry. Applications SHOULD limit their use of character sets to those defined by the IANA registry.
Implementors should be aware of IETF character set requirements  .
3.4.1 Missing Charset
Some HTTP/1.0 software has interpreted a Content-Type header without charset parameter incorrectly to mean "recipient should guess." Senders wishing to defeat this behavior MAY include a charset parameter even when the charset is ISO-8859-1 and SHOULD do so when it is known that it will not confuse the recipient.
Unfortunately, some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. HTTP/1.1 recipients MUST respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset MUST use the charset from the content-type field if they support that charset, rather than the recipient's preference, when initially displaying a document. See Section 3.7.1.
3.5 Content Codings
Content coding values indicate an encoding transformation that has been or can be applied to an entity. Content codings are primarily used to allow a document to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Frequently, the entity is stored in coded form, transmitted directly, and only decoded by the recipient.
content-coding = token
All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (Section 14.3) and Content-Encoding (Section 14.11) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding.
The Internet Assigned Numbers Authority (IANA) acts as a registry for content-coding value tokens. Initially, the registry contains the following tokens:
An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 . This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).
Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings. Their use here is representative of historical practice, not good design. For compatibility with previous implementations of HTTP, applications SHOULD consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.
The "zlib" format defined in RFC 1950  in combination with the "deflate" compression mechanism described in RFC 1951 .
The default (identity) encoding; the use of no transformation whatsoever. This content-coding is used only in the Accept- Encoding header, and SHOULD NOT be used in the Content-Encoding header.
New content-coding value tokens SHOULD be registered; to allow interoperability between clients and servers, specifications of the content coding algorithms needed to implement a new value SHOULD be publicly available and adequate for independent implementation, and conform to the purpose of content coding defined in this section.
3.6 Transfer Codings
Transfer-coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to an entity-body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer-coding is a property of the message, not of the original entity.
= "chunked" | transfer-extension
= token *( ";" parameter )
Parameters are in the form of attribute/value pairs.
= attribute "=" value
= token | quoted-string
All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer-coding values in the TE header field (Section 14.39) and in the Transfer-Encoding header field (Section 14.41).
Whenever a transfer-coding is applied to a message-body, the set of transfer-codings MUST include "chunked", unless the message is terminated by closing the connection. When the "chunked" transfer- coding is used, it MUST be the last transfer-coding applied to the message-body. The "chunked" transfer-coding MUST NOT be applied more than once to a message-body. These rules allow the recipient to determine the transfer-length of the message (Section 4.4).
Transfer-codings are analogous to the Content-Transfer-Encoding values of MIME , which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty in determining the exact body length (Section 7.2.2), or the desire to encrypt data over a shared transport.
The Internet Assigned Numbers Authority (IANA) acts as a registry for transfer-coding value tokens. Initially, the registry contains the following tokens: "chunked" (Section 3.6.1), "identity" (section 3.6.2), "gzip" (Section 3.5), "compress" (Section 3.5), and "deflate" (Section 3.5).
New transfer-coding value tokens SHOULD be registered in the same way as new content-coding value tokens (Section 3.5).
A server which receives an entity-body with a transfer-coding it does not understand SHOULD return 501 (Unimplemented), and close the connection. A server MUST NOT send transfer-codings to an HTTP/1.0 client.
3.6.1 Chunked Transfer Coding
The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an OPTIONAL trailer containing entity-header fields. This allows dynamically produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.
= *chunk last-chunk trailer CRLF
=chunk-size[ chunk-extension] CRLF chunk-data CRLF
= 1*("0") [ chunk-extension ] CRLF
= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
= token | quoted-strin
= *(entity-header CRLF)
The chunk-size field is a string of hex digits indicating the size of the chunk. The chunked encoding is ended by any chunk whose size is zero, followed by the trailer, which is terminated by an empty line.
The trailer allows the sender to include additional HTTP header fields at the end of the message. The Trailer header field can be used to indicate which header fields are included in a trailer (see Section 14.40).
A server using chunked transfer-coding in a response MUST NOT use the trailer for any header fields unless at least one of the following is true:
a)the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as described in Section 14.39; or,
b)the server is the origin server for the response, the trailer fields consist entirely of optional metadata, and the recipient could use the message (in a manner acceptable to the origin server) without receiving this metadata. In other words, the origin server is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.
This requirement prevents an interoperability failure when the message is being received by an HTTP/1.1 (or later) proxy and forwarded to an HTTP/1.0 recipient. It avoids a situation where compliance with the protocol would have necessitated a possibly infinite buffer on the proxy.
An example process for decoding a Chunked-Body is presented in Appendix 19.4.6.
All HTTP/1.1 applications MUST be able to receive and decode the "chunked" transfer-coding, and MUST ignore chunk-extension extensions they do not understand.
3.7 Media Types
HTTP uses Internet Media Types  in the Content-Type (Section 14.17) and Accept (Section 14.1) header fields in order to provide open and extensible data typing and type negotiation.
= type "/" subtype *( ";" parameter )
Parameters MAY follow the type/subtype in the form of attribute/value pairs (as defined in Section 3.6).
The type, subtype, and parameter attribute names are case- insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. Linear white space (LWS) MUST NOT be used between the type and subtype, nor between an attribute and its value. The presence or absence of a parameter might be significant to the processing of a media-type, depending on its definition within the media type registry. Note that some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations SHOULD only use media type parameters when they are required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number Authority (IANA ). The media type registration process is outlined in RFC 1590 . Use of non-registered media types is discouraged.
3.7.1 Canonicalization and Text Defaults
Internet media types are registered with a canonical form. An entity-body transferred via HTTP messages MUST be represented in the appropriate canonical form prior to its transmission except for "text" types, as defined in the next paragraph.
When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire entity-body. HTTP applications MUST accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP. In addition, if the text is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the entity-body; a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).
If an entity-body is encoded with a content-coding, the underlying data MUST be in a form defined above prior to being encoded.
The "charset" parameter is used with some media types to define the character set (Section 3.4) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value. See Section 3.4.1 for compatibility problems.
3.7.2 Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of one or more entities within a single message-body. All multipart types share a common syntax, as defined in Section 5.1.1 of RFC 2046 , and MUST include a boundary parameter as part of the media type value. The message body is itself a protocol element and MUST therefore use only CRLF to represent line breaks between body-parts. Unlike in RFC 2046, the epilogue of any multipart message MUST be empty; HTTP applications MUST NOT transmit the epilogue (even if the original multipart contains an epilogue). These restrictions exist in order to preserve the self-delimiting nature of a multipart message- body, wherein the "end" of the message-body is indicated by the ending multipart boundary.
In general, HTTP treats a multipart message-body no differently than any other media type: strictly as payload. The one exception is the "multipart/byteranges" type (Appendix 19.2) when it appears in a 206 (Partial Content) response, which will be interpreted by some HTTP caching mechanisms as described in Sections 13.5.4 and 14.16. In all other cases, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within each body-part of a multipart message- body do not have any significance to HTTP beyond that defined by their MIME semantics.
In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 .
3.8 Product Tokens
Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by white space. By convention, the products are listed in order of their significance for identifying the application.
= token ["/" product-version]
Product tokens SHOULD be short and to the point. They MUST NOT be used for advertising or other non-essential information. Although any token character MAY appear in a product-version, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value).
3.9 Quality Values
HTTP content negotiation (Section 12) uses short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has a quality value of 0, then content with this parameter is `not acceptable' for the client. HTTP/1.1 applications MUST NOT generate more than three digits after the decimal point. User configuration of these values SHOULD also be limited in this fashion.
= ( "0" [ "." 0*3DIGIT ] ) | ( "1" [ "." 0*3("0") ] )
"Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.
3.10 Language Tags
A language tag identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to other human beings. Computer languages are explicitly excluded. HTTP uses language tags within the Accept-Language and Content- Language fields.
The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 . In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags:
= primary-tag *( "-" subtag )
White space is not allowed within the tag and all tags are case- insensitive. The name space of language tags is administered by the IANA. Example tags include: en, en-US, en-cockney, i-cherokee, x-pig-latin where any two-letter primary-tag is an ISO-639 language abbreviation and any two-letter initial subtag is an ISO-3166 country code. (The last three tags above are not registered tags; all but the last are examples of tags which could be registered in future.)
3.11 Entity Tags
Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the ETag (Section 14.19), If-Match (Section 14.24), If-None-Match (Section 14.26), and If-Range (Section 14.27) header fields. The definition of how they are used and compared as cache validators is in Section 13.3.3. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
= [ weak ] opaque-tag
A "strong entity tag" MAY be shared by two entities of a resource only if they are equivalent by octet equality.
A "weak entity tag," indicated by the "W/" prefix, MAY be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no significant change in semantics. A weak entity tag can only be used for weak comparison.
An entity tag MUST be unique across all versions of all entities associated with a particular resource. A given entity tag value MAY be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities obtained by requests on different URIs does not imply the equivalence of those entities.
3.12 Range Units
HTTP/1.1 allows a client to request that only part (a range of) the response entity be included within the response. HTTP/1.1 uses range units in the Range (Section 14.35) and Content-Range (Section 14.16) header fields. An entity can be broken down into subranges according to various structural units.
= bytes-unit | other-range-unit
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 implementations MAY ignore ranges specified using other units.
HTTP/1.1 has been designed to allow implementations of applications that do not depend on knowledge of ranges.