HTTP requirements for cache behavior and cacheable responses are
   defined in section 13.

   In fact, there are a wide variety of architectures and configurations
   of caches and proxies currently being experimented with or deployed
   across the World Wide Web. These systems include national hierarchies
   of proxy caches to save transoceanic bandwidth, systems that
   broadcast or multicast cache entries, organizations that distribute
   subsets of cached data via CD-ROM, and so on. HTTP systems are used
   in corporate intranets over high-bandwidth links, and for access via
   PDAs with low-power radio links and intermittent connectivity. The
   goal of HTTP/1.1 is to support the wide diversity of configurations
   already deployed while introducing protocol constructs that meet the
   needs of those who build web applications that require high
   reliability and, failing that, at least reliable indications of
   failure.

   HTTP communication usually takes place over TCP/IP connections. The
   default port is TCP 80 [19], but other ports can be used. This does
   not preclude HTTP from being implemented on top of any other protocol
   on the Internet, or on other networks. HTTP only presumes a reliable
   transport; any protocol that provides such guarantees can be used;
   the mapping of the HTTP/1.1 request and response structures onto the
   transport data units of the protocol in question is outside the scope
   of this specification.





 
RFC 2616                        HTTP/1.1                       June 1999


   In HTTP/1.0, most implementations used a new connection for each
   request/response exchange. In HTTP/1.1, a connection may be used for
   one or more request/response exchanges, although connections may be
   closed for a variety of reasons (see section 8.1).

2 Notational Conventions and Generic Grammar

2.1 Augmented BNF

   All of the mechanisms specified in this document are described in
   both prose and an augmented Backus-Naur Form (BNF) similar to that
   used by RFC 822 [9]. Implementors will need to be familiar with the
   notation in order to understand this specification. The augmented BNF
   includes the following constructs:

   name = definition
      The name of a rule is simply the name itself (without any
      enclosing "<" and ">") and is separated from its definition by the
      equal "=" character. White space is only significant in that
      indentation of continuation lines is used to indicate a rule
      definition that spans more than one line. Certain basic rules are
      in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
      brackets are used within definitions whenever their presence will
      facilitate discerning the use of rule names.

   "literal"
      Quotation marks surround literal text. Unless stated otherwise,
      the text is case-insensitive.

   rule1 | rule2
      Elements separated by a bar ("|") are alternatives, e.g., "yes |
      no" will accept yes or no.

   (rule1 rule2)
      Elements enclosed in parentheses are treated as a single element.
      Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
      foo elem" and "elem bar elem".

   *rule
      The character "*" preceding an element indicates repetition. The
      full form is "*element" indicating at least  and at most
       occurrences of element. Default values are 0 and infinity so
      that "*(element)" allows any number, including zero; "1*element"
      requires at least one; and "1*2element" allows one or two.

   [rule]
      Square brackets enclose optional elements; "[foo bar]" is
      equivalent to "*1(foo bar)".




 
RFC 2616                        HTTP/1.1                       June 1999


   N rule
      Specific repetition: "(element)" is equivalent to
      "*(element)"; that is, exactly  occurrences of (element).
      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
      alphabetic characters.

   #rule
      A construct "#" is defined, similar to "*", for defining lists of
      elements. The full form is "#element" indicating at least
       and at most  elements, each separated by one or more commas