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<head><title>HTTP/1.1: Appendices</title></head>
<body><address>part of <a rev='Section' href='rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a><br />
RFC 2616 Fielding, et al.</address>
<h2><a id='sec19'>19</a> Appendices</h2>
<h3><a id='sec19.1'>19.1</a> Internet Media Type message/http and application/http</h3>
<p>
   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 <a rel='bibref' href='rfc2616-sec17.html#bib17'>[17]</a>.
</p>
<pre>       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
</pre>
<pre>       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
</pre>
<h3><a id='sec19.2'>19.2</a> Internet Media Type multipart/byteranges</h3>
<p>
   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".
</p>
<p>
   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.
</p>
<pre>       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
</pre>
<p>
   For example:
</p>
<p>
   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
</p>
<p>
   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 500-999/8000
</p>
<p>
   ...the first range...
   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 7000-7999/8000
</p>
<p>
   ...the second range
   --THIS_STRING_SEPARATES--
</p>
<pre>      Notes:
</pre>
<pre>      1) Additional CRLFs may precede the first boundary string in the
         entity.
</pre>
<pre>      2) Although RFC 2046 [40] permits the boundary string to be
         quoted, some existing implementations handle a quoted boundary
         string incorrectly.
</pre>
<pre>      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.
</pre>
<h3><a id='sec19.3'>19.3</a> Tolerant Applications</h3>
<p>
   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.
</p>
<p>
   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.
</p>
<p>
   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.
</p>
<p>
   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 <a rel='xref' href='rfc2616-sec3.html#sec3.7.1'>3.7.1</a>
   and <a rel='xref' href='rfc2616-sec3.html#sec3.4.1'>3.4.1</a>.
</p>
<p>
   Additional rules for requirements on parsing and encoding of dates
   and other potential problems with date encodings include:
</p>
<pre>      - 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).
</pre>
<pre>      - 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.
</pre>
<pre>      - 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.
</pre>
<pre>      - 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.
</pre>
<h3><a id='sec19.4'>19.4</a> Differences Between HTTP Entities and RFC 2045 Entities</h3>
<p>
   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.
</p>
<p>
   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.
</p>
<h3><a id='sec19.4.1'>19.4.1</a> MIME-Version</h3>
<p>
   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<a rel='bibref' href='rfc2616-sec17.html#bib7'>[7]</a>).
   Proxies/gateways are responsible for ensuring full compliance (where
   possible) when exporting HTTP messages to strict MIME environments.
</p>
<pre>       MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
</pre>
<p>
   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.
</p>
<h3><a id='sec19.4.2'>19.4.2</a> Conversion to Canonical Form</h3>
<p>
   RFC 2045 <a rel='bibref' href='rfc2616-sec17.html#bib7'>[7]</a> requires that an Internet mail entity be converted to
   canonical form prior to being transferred, as described in section 4
   of RFC 2049 <a rel='bibref' href='rfc2616-sec17.html#bib48'>[48]</a>. Section <a rel='xref' href='rfc2616-sec3.html#sec3.7.1'>3.7.1</a> 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
</p>
<p>
   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.
</p>
<p>
   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 <a rel='xref' href='rfc2616-sec3.html#sec3.7.1'>3.7.1</a> 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.
</p>
<p>
   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.
</p>
<h3><a id='sec19.4.3'>19.4.3</a> Conversion of Date Formats</h3>
<p>
   HTTP/1.1 uses a restricted set of date formats (section <a rel='xref' href='rfc2616-sec3.html#sec3.3.1'>3.3.1</a>) 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.
</p>
<h3><a id='sec19.4.4'>19.4.4</a> Introduction of Content-Encoding</h3>
<p>
   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=&lt;content-coding>" to perform
   a function equivalent to Content-Encoding. However, this parameter is
   not part of RFC 2045.)
</p>
<h3><a id='sec19.4.5'>19.4.5</a> No Content-Transfer-Encoding</h3>
<p>
   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.
</p>
<p>
   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
</p>
<p>
   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.
</p>
<h3><a id='sec19.4.6'>19.4.6</a> Introduction of Transfer-Encoding</h3>
<p>
   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.
</p>
<p>
   A process for decoding the "chunked" transfer-coding (section <a rel='xref' href='rfc2616-sec3.html#sec3.6'>3.6</a>)
   can be represented in pseudo-code as:
</p>
<pre>       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
</pre>
<h3><a id='sec19.4.7'>19.4.7</a> MHTML and Line Length Limitations</h3>
<p>
   HTTP implementations which share code with MHTML <a rel='bibref' href='rfc2616-sec17.html#bib45'>[45]</a> 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 <a rel='xref' href='rfc2616-sec3.html#sec3.7.2'>3.7.2</a>) and
   does not interpret the content or any MIME header lines that might be
   contained therein.
</p>
<h3><a id='sec19.5'>19.5</a> Additional Features</h3>
<p>
   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
</p>
<p>
   describe proposed experimental features, and some describe features
   that experimental deployment found lacking that are now addressed in
   the base HTTP/1.1 specification.
</p>
<p>
   A number of other headers, such as Content-Disposition and Title,
   from SMTP and MIME are also often implemented (see RFC 2076 [37]).
</p>
<h3><a id='sec19.5.1'>19.5.1</a> Content-Disposition</h3>
<p>
   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 <a rel='bibref' href='rfc2616-sec17.html#bib35'>[35]</a>.
</p>
<pre>        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 )
</pre>
<p>
   An example is
</p>
<pre>        Content-Disposition: attachment; filename="fname.ext"
</pre>
<p>
   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.
</p>
<p>
   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.
</p>
<p>
   See section <a rel='xref' href='rfc2616-sec15.html#sec15.5'>15.5</a> for Content-Disposition security issues.
</p>
<h3><a id='sec19.6'>19.6</a> Compatibility with Previous Versions</h3>
<p>
   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:
</p>
<pre>      - recognize the format of the Request-Line for HTTP/0.9, 1.0, and
        <a rel='xref' href='rfc2616-sec1.html#sec1.1'>1.1</a> requests;
</pre>
<pre>      - understand any valid request in the format of HTTP/0.9, 1.0, or
        <a rel='xref' href='rfc2616-sec1.html#sec1.1'>1.1</a>;
</pre>
<pre>      - respond appropriately with a message in the same major version
        used by the client.
</pre>
<p>
   And we would expect HTTP/1.1 clients to:
</p>
<pre>      - recognize the format of the Status-Line for HTTP/1.0 and 1.1
        responses;
</pre>
<pre>      - understand any valid response in the format of HTTP/0.9, 1.0, or
        <a rel='xref' href='rfc2616-sec1.html#sec1.1'>1.1</a>.
</pre>
<p>
   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 <a rel='xref' href='rfc2616-sec19.html#sec19.7.1'>19.7.1</a> of RFC
   2068 <a rel='bibref' href='rfc2616-sec17.html#bib33'>[33]</a>.
</p>
<h3><a id='sec19.6.1'>19.6.1</a> Changes from HTTP/1.0</h3>
<p>
   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.
</p>
<h3><a id='sec19.6.1.1'>19.6.1.1</a> Changes to Simplify Multi-homed Web Servers and Conserve IP</h3>
<pre>         Addresses
</pre>
<p>
   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
   <a rel='xref' href='rfc2616-sec5.html#sec5.1.2'>5.1.2</a>) are among the most important changes defined by this
   specification.
</p>
<p>
   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
</p>
<p>
   important that all implementations of HTTP (including updates to
   existing HTTP/1.0 applications) correctly implement these
   requirements:
</p>
<pre>      - Both clients and servers MUST support the Host request-header.
</pre>
<pre>      - A client that sends an HTTP/1.1 request MUST send a Host header.
</pre>
<pre>      - Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
        request does not include a Host request-header.
</pre>
<pre>      - Servers MUST accept absolute URIs.
</pre>
<h3><a id='sec19.6.2'>19.6.2</a> Compatibility with HTTP/1.0 Persistent Connections</h3>
<p>
   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 <a rel='xref' href='rfc2616-sec1.html#sec1.0'>1.0</a> 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.
</p>
<p>
   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 <a rel='xref' href='rfc2616-sec14.html#sec14.10'>14.10</a>.
</p>
<p>
   The original HTTP/1.0 form of persistent connections (the Connection:
   Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]
</p>
<h3><a id='sec19.6.3'>19.6.3</a> Changes from RFC 2068</h3>
<p>
   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 <a rel='bibref' href='rfc2616-sec17.html#bib34'>[34]</a>.
</p>
<p>
   Clarified which error code should be used for inbound server failures
   (e.g. DNS failures). (Section 10.5.5).
</p>
<p>
   CREATE had a race that required an Etag be sent when a resource is
   first created. (Section 10.2.2).
</p>
<p>
   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 <a rel='bibref' href='rfc2616-sec17.html#bib45'>[45]</a>.
</p>
<p>
   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
   <a rel='xref' href='rfc2616-sec3.html#sec3.6'>3.6</a>, <a rel='xref' href='rfc2616-sec4.html#sec4.4'>4.4</a>, <a rel='xref' href='rfc2616-sec7.html#sec7.2.2'>7.2.2</a>, <a rel='xref' href='rfc2616-sec13.html#sec13.5.2'>13.5.2</a>, <a rel='xref' href='rfc2616-sec14.html#sec14.13'>14.13</a>, <a rel='xref' href='rfc2616-sec14.html#sec14.16'>14.16</a>)
</p>
<p>
   A content-coding of "identity" was introduced, to solve problems
   discovered in caching. (section 3.5)
</p>
<p>
   Quality Values of zero should indicate that "I don't want something"
   to allow clients to refuse a representation. (Section 3.9)
</p>
<p>
   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 <a rel='xref' href='rfc2616-sec3.html#sec3.1'>3.1</a>)
</p>
<p>
   Charset wildcarding is introduced to avoid explosion of character set
   names in accept headers. (Section 14.2)
</p>
<p>
   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,
   <a rel='xref' href='rfc2616-sec14.html#sec14.9.3'>14.9.3</a>)
</p>
<p>
   The Cache-Control: max-age directive was not properly defined for
   responses. (Section 14.9.3)
</p>
<p>
   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 <a rel='xref' href='rfc2616-sec14.html#sec14.16'>14.16</a>)
</p>
<p>
   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
   <a rel='xref' href='rfc2616-sec10.html#sec10.2.7'>10.2.7</a>, <a rel='xref' href='rfc2616-sec13.html#sec13.5.3'>13.5.3</a>, and <a rel='xref' href='rfc2616-sec14.html#sec14.27'>14.27</a>)
</p>
<p>
   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 <a rel='xref' href='rfc2616-sec10.html#sec10.4.17'>10.4.17</a>, <a rel='xref' href='rfc2616-sec14.html#sec14.16'>14.16</a>)
</p>
<p>
   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:
</p>
<pre>      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
</pre>
<pre>      2. Made it clear that user-agents should retry requests, not
         "clients" in general.
</pre>
<pre>      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.
</pre>
<pre>      4. Modified some TCP-specific language, to make it clearer that
         non-TCP transports are possible for HTTP.
</pre>
<pre>      5. Require that the origin server MUST NOT wait for the request
         body before it sends a required 100 (Continue) response.
</pre>
<pre>      6. Allow, rather than require, a server to omit 100 (Continue) if
         it has already seen some of the request body.
</pre>
<pre>      7. Allow servers to defend against denial-of-service attacks and
         broken clients.
</pre>
<p>
   This change adds the Expect header and 417 status code. The message
   transmission requirements fixes are in sections 8.2, 10.4.18,
   <a rel='xref' href='rfc2616-sec8.html#sec8.1.2.2'>8.1.2.2</a>, <a rel='xref' href='rfc2616-sec13.html#sec13.11'>13.11</a>, and <a rel='xref' href='rfc2616-sec14.html#sec14.20'>14.20</a>.
</p>
<p>
   Proxies should be able to add Content-Length when appropriate.
   (Section 13.5.2)
</p>
<p>
   Clean up confusion between 403 and 404 responses. (Section <a rel='xref' href='rfc2616-sec10.html#sec10.4.4'>10.4.4</a>,
   10.4.5, and 10.4.11)
</p>
<p>
   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.
</p>
<p>
   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 <a rel='bibref' href='rfc2616-sec17.html#bib39'>[39]</a>. 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 <a rel='xref' href='rfc2616-sec3.html#sec3.6'>3.6</a>, <a rel='xref' href='rfc2616-sec3.html#sec3.6.1'>3.6.1</a>, and <a rel='xref' href='rfc2616-sec14.html#sec14.39'>14.39</a>)
</p>
<p>
   The PATCH, LINK, UNLINK methods were defined but not commonly
   implemented in previous versions of this specification. See RFC 2068
   <a rel='bibref' href='rfc2616-sec17.html#bib33'>[33]</a>.
</p>
<p>
   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 <a rel='bibref' href='rfc2616-sec17.html#bib33'>[33]</a>.
</p>
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