HTTP method registrations MUST include the following fields:
o Method Name (see Section 4)
o Safe ("yes" or "no", see Section 4.2.1)
o Idempotent ("yes" or "no", see Section 4.2.2)
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
8.1.2. Considerations for New Methods
Standardized methods are generic; that is, they are potentially
applicable to any resource, not just one particular media type, kind
of resource, or application. As such, it is preferred that new
methods be registered in a document that isn't specific to a single
application or data format, since orthogonal technologies deserve
Since message parsing (Section 3.3 of [RFC7230]) needs to be
independent of method semantics (aside from responses to HEAD),
definitions of new methods cannot change the parsing algorithm or
prohibit the presence of a message body on either the request or the
response message. Definitions of new methods can specify that only a
zero-length message body is allowed by requiring a Content-Length
header field with a value of "0".
A new method definition needs to indicate whether it is safe
(Section 4.2.1), idempotent (Section 4.2.2), cacheable
(Section 4.2.3), what semantics are to be associated with the payload
body if any is present in the request and what refinements the method
makes to header field or status code semantics. If the new method is
cacheable, its definition ought to describe how, and under what
conditions, a cache can store a response and use it to satisfy a
subsequent request. The new method ought to describe whether it can
be made conditional (Section 5.2) and, if so, how a server responds
when the condition is false. Likewise, if the new method might have
some use for partial response semantics ([RFC7233]), it ought to
document this, too.
Note: Avoid defining a method name that starts with "M-", since
that prefix might be misinterpreted as having the semantics
assigned to it by [RFC2774].
8.2.2. Considerations for New Status Codes
When it is necessary to express semantics for a response that are not
defined by current status codes, a new status code can be registered.
Status codes are generic; they are potentially applicable to any
resource, not just one particular media type, kind of resource, or
application of HTTP. As such, it is preferred that new status codes
be registered in a document that isn't specific to a single
New status codes are required to fall under one of the categories
defined in Section 6. To allow existing parsers to process the
response message, new status codes cannot disallow a payload,
although they can mandate a zero-length payload body.
Proposals for new status codes that are not yet widely deployed ought
to avoid allocating a specific number for the code until there is
clear consensus that it will be registered; instead, early drafts can
use a notation such as "4NN", or "3N0" .. "3N9", to indicate the
class of the proposed status code(s) without consuming a number
The definition of a new status code ought to explain the request
conditions that would cause a response containing that status code
(e.g., combinations of request header fields and/or method(s)) along
with any dependencies on response header fields (e.g., what fields
are required, what fields can modify the semantics, and what header
field semantics are further refined when used with the new status
The definition of a new status code ought to specify whether or not
it is cacheable. Note that all status codes can be cached if the
response they occur in has explicit freshness information; however,
status codes that are defined as being cacheable are allowed to be
cached without explicit freshness information. Likewise, the
definition of a status code can place constraints upon cache
behavior. See [RFC7234] for more information.
Finally, the definition of a new status code ought to indicate
whether the payload has any implied association with an identified
resource (Section 220.127.116.11).
The status code registry has been updated with the registrations
8.3.1. Considerations for New Header Fields
Header fields are key:value pairs that can be used to communicate
data about the message, its payload, the target resource, or the
connection (i.e., control data). See Section 3.2 of [RFC7230] for a
general definition of header field syntax in HTTP messages.
The requirements for header field names are defined in [BCP90].
Authors of specifications defining new fields are advised to keep the
name as short as practical and not to prefix the name with "X-"
unless the header field will never be used on the Internet. (The
"X-" prefix idiom has been extensively misused in practice; it was
intended to only be used as a mechanism for avoiding name collisions
inside proprietary software or intranet processing, since the prefix
would ensure that private names never collide with a newly registered
Internet name; see [BCP178] for further information).
New header field values typically have their syntax defined using
ABNF ([RFC5234]), using the extension defined in Section 7 of
[RFC7230] as necessary, and are usually constrained to the range of
US-ASCII characters. Header fields needing a greater range of
characters can use an encoding such as the one defined in [RFC5987].
Leading and trailing whitespace in raw field values is removed upon
field parsing (Section 3.2.4 of [RFC7230]). Field definitions where
leading or trailing whitespace in values is significant will have to
use a container syntax such as quoted-string (Section 3.2.6 of
Because commas (",") are used as a generic delimiter between
field-values, they need to be treated with care if they are allowed
in the field-value. Typically, components that might contain a comma
are protected with double-quotes using the quoted-string ABNF
For example, a textual date and a URI (either of which might contain
a comma) could be safely carried in field-values like these:
Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double-quote delimiters almost always are used with the
quoted-string production; using a different syntax inside
double-quotes will likely cause unnecessary confusion.
Many header fields use a format including (case-insensitively) named
parameters (for instance, Content-Type, defined in Section 18.104.22.168).
Allowing both unquoted (token) and quoted (quoted-string) syntax for
the parameter value enables recipients to use existing parser
components. When allowing both forms, the meaning of a parameter
value ought to be independent of the syntax used for it (for an
example, see the notes on parameter handling for media types in
Authors of specifications defining new header fields are advised to
o Whether the field is a single value or whether it can be a list
(delimited by commas; see Section 3.2 of [RFC7230]).
If it does not use the list syntax, document how to treat messages
where the field occurs multiple times (a sensible default would be
to ignore the field, but this might not always be the right
Note that intermediaries and software libraries might combine
multiple header field instances into a single one, despite the
field's definition not allowing the list syntax. A robust format
enables recipients to discover these situations (good example:
"Content-Type", as the comma can only appear inside quoted
strings; bad example: "Location", as a comma can occur inside a
o Under what conditions the header field can be used; e.g., only in
responses or requests, in all messages, only on responses to a
particular request method, etc.
o Whether the field should be stored by origin servers that
understand it upon a PUT request.
o Whether the field semantics are further refined by the context,
such as by existing request methods or status codes.
o Whether it is appropriate to list the field-name in the Connection
header field (i.e., if the header field is to be hop-by-hop; see
Section 6.1 of [RFC7230]).
o Under what conditions intermediaries are allowed to insert,
delete, or modify the field's value.
o Whether it is appropriate to list the field-name in a Vary
response header field (e.g., when the request header field is used
by an origin server's content selection algorithm; see
o Whether the header field is useful or allowable in trailers (see
Section 4.1 of [RFC7230]).
o Whether the header field ought to be preserved across redirects.
o Whether it introduces any additional security considerations, such
as disclosure of privacy-related data.
The "Message Headers" registry has been updated with the following
| Header Field Name | Protocol | Status | Reference |
| Accept | http | standard | Section 5.3.2 |
| Accept-Charset | http | standard | Section 5.3.3 |
| Accept-Encoding | http | standard | Section 5.3.4 |
| Accept-Language | http | standard | Section 5.3.5 |
| Allow | http | standard | Section 7.4.1 |
| Content-Encoding | http | standard | Section 22.214.171.124 |
| Content-Language | http | standard | Section 126.96.36.199 |
| Content-Location | http | standard | Section 188.8.131.52 |
| Content-Type | http | standard | Section 184.108.40.206 |
| Date | http | standard | Section 220.127.116.11 |
| Expect | http | standard | Section 5.1.1 |
| From | http | standard | Section 5.5.1 |
| Location | http | standard | Section 7.1.2 |
| Max-Forwards | http | standard | Section 5.1.2 |
| MIME-Version | http | standard | Appendix A.1 |
| Referer | http | standard | Section 5.5.2 |
| Retry-After | http | standard | Section 7.1.3 |
| Server | http | standard | Section 7.4.2 |
| User-Agent | http | standard | Section 5.5.3 |
| Vary | http | standard | Section 7.1.4 |
The change controller for the above registrations is: "IETF
(firstname.lastname@example.org) - Internet Engineering Task Force".
8.4. Content Coding Registry
The "HTTP Content Coding Registry" defines the namespace for content
coding names (Section 4.2 of [RFC7230]). The content coding registry
is maintained at <http://www.iana.org/assignments/http-parameters>.
Content coding registrations MUST include the following fields:
o Pointer to specification text
Names of content codings MUST NOT overlap with names of transfer
codings (Section 4 of [RFC7230]), unless the encoding transformation
is identical (as is the case for the compression codings defined in
Section 4.2 of [RFC7230]).
Values to be added to this namespace require IETF Review (see Section
4.1 of [RFC5226]) and MUST conform to the purpose of content coding
defined in this section.
The "HTTP Content Coding Registry" has been updated with the
| Name | Description | Reference |
| identity | Reserved (synonym for "no encoding" in | Section 5.3.4 |
| | Accept-Encoding) | |
9. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns relevant to HTTP semantics and
its use for transferring information over the Internet.
Considerations related to message syntax, parsing, and routing are
discussed in Section 9 of [RFC7230].
The list of considerations below is not exhaustive. Most security
concerns related to HTTP semantics are about securing server-side
applications (code behind the HTTP interface), securing user agent
processing of payloads received via HTTP, or secure use of the
Internet in general, rather than security of the protocol. Various
organizations maintain topical information and links to current
research on Web application security (e.g., [OWASP]).
9.1. Attacks Based on File and Path Names
Origin servers frequently make use of their local file system to
manage the mapping from effective request URI to resource
representations. Most file systems are not designed to protect
against malicious file or path names. Therefore, an origin server
needs to avoid accessing names that have a special significance to
the system when mapping the request target to files, folders, or
For example, UNIX, Microsoft Windows, and other operating systems use
".." as a path component to indicate a directory level above the
current one, and they use specially named paths or file names to send
data to system devices. Similar naming conventions might exist
within other types of storage systems. Likewise, local storage
systems have an annoying tendency to prefer user-friendliness over
security when handling invalid or unexpected characters,
recomposition of decomposed characters, and case-normalization of
Attacks based on such special names tend to focus on either denial-
of-service (e.g., telling the server to read from a COM port) or
disclosure of configuration and source files that are not meant to be
9.2. Attacks Based on Command, Code, or Query Injection
Origin servers often use parameters within the URI as a means of
identifying system services, selecting database entries, or choosing
a data source. However, data received in a request cannot be
trusted. An attacker could construct any of the request data
elements (method, request-target, header fields, or body) to contain
data that might be misinterpreted as a command, code, or query when
passed through a command invocation, language interpreter, or
For example, SQL injection is a common attack wherein additional
query language is inserted within some part of the request-target or
header fields (e.g., Host, Referer, etc.). If the received data is
used directly within a SELECT statement, the query language might be
interpreted as a database command instead of a simple string value.
This type of implementation vulnerability is extremely common, in
spite of being easy to prevent.
In general, resource implementations ought to avoid use of request
data in contexts that are processed or interpreted as instructions.
Parameters ought to be compared to fixed strings and acted upon as a
result of that comparison, rather than passed through an interface
that is not prepared for untrusted data. Received data that isn't
based on fixed parameters ought to be carefully filtered or encoded
to avoid being misinterpreted.
Similar considerations apply to request data when it is stored and
later processed, such as within log files, monitoring tools, or when
included within a data format that allows embedded scripts.
9.3. Disclosure of Personal Information
Clients are often privy to large amounts of personal information,
including both information provided by the user to interact with
resources (e.g., the user's name, location, mail address, passwords,
encryption keys, etc.) and information about the user's browsing
activity over time (e.g., history, bookmarks, etc.). Implementations
need to prevent unintentional disclosure of personal information.
9.4. Disclosure of Sensitive Information in URIs
URIs are intended to be shared, not secured, even when they identify
secure resources. URIs are often shown on displays, added to
templates when a page is printed, and stored in a variety of
unprotected bookmark lists. It is therefore unwise to include
information within a URI that is sensitive, personally identifiable,
or a risk to disclose.
Authors of services ought to avoid GET-based forms for the submission
of sensitive data because that data will be placed in the
request-target. Many existing servers, proxies, and user agents log
or display the request-target in places where it might be visible to
third parties. Such services ought to use POST-based form submission
Since the Referer header field tells a target site about the context
that resulted in a request, it has the potential to reveal
information about the user's immediate browsing history and any
personal information that might be found in the referring resource's
URI. Limitations on the Referer header field are described in
Section 5.5.2 to address some of its security considerations.
9.5. Disclosure of Fragment after Redirects
Although fragment identifiers used within URI references are not sent
in requests, implementers ought to be aware that they will be visible
to the user agent and any extensions or scripts running as a result
of the response. In particular, when a redirect occurs and the
original request's fragment identifier is inherited by the new
reference in Location (Section 7.1.2), this might have the effect of
disclosing one site's fragment to another site. If the first site
uses personal information in fragments, it ought to ensure that
redirects to other sites include a (possibly empty) fragment
component in order to block that inheritance.
9.6. Disclosure of Product Information
The User-Agent (Section 5.5.3), Via (Section 5.7.1 of [RFC7230]), and
Server (Section 7.4.2) header fields often reveal information about
the respective sender's software systems. In theory, this can make
it easier for an attacker to exploit known security holes; in
practice, attackers tend to try all potential holes regardless of the
apparent software versions being used.
Proxies that serve as a portal through a network firewall ought to
take special precautions regarding the transfer of header information
that might identify hosts behind the firewall. The Via header field
allows intermediaries to replace sensitive machine names with
9.7. Browser Fingerprinting
Browser fingerprinting is a set of techniques for identifying a
specific user agent over time through its unique set of
characteristics. These characteristics might include information
related to its TCP behavior, feature capabilities, and scripting
environment, though of particular interest here is the set of unique
characteristics that might be communicated via HTTP. Fingerprinting
is considered a privacy concern because it enables tracking of a user
agent's behavior over time without the corresponding controls that
the user might have over other forms of data collection (e.g.,
cookies). Many general-purpose user agents (i.e., Web browsers) have
taken steps to reduce their fingerprints.
There are a number of request header fields that might reveal
information to servers that is sufficiently unique to enable
fingerprinting. The From header field is the most obvious, though it
is expected that From will only be sent when self-identification is
desired by the user. Likewise, Cookie header fields are deliberately
designed to enable re-identification, so fingerprinting concerns only
apply to situations where cookies are disabled or restricted by the
user agent's configuration.
The User-Agent header field might contain enough information to
uniquely identify a specific device, usually when combined with other
characteristics, particularly if the user agent sends excessive
details about the user's system or extensions. However, the source
of unique information that is least expected by users is proactive
negotiation (Section 5.3), including the Accept, Accept-Charset,
Accept-Encoding, and Accept-Language header fields.
In addition to the fingerprinting concern, detailed use of the
Accept-Language header field can reveal information the user might
consider to be of a private nature. For example, understanding a
given language set might be strongly correlated to membership in a
particular ethnic group. An approach that limits such loss of
privacy would be for a user agent to omit the sending of
Accept-Language except for sites that have been whitelisted, perhaps
via interaction after detecting a Vary header field that indicates
language negotiation might be useful.
In environments where proxies are used to enhance privacy, user
agents ought to be conservative in sending proactive negotiation
header fields. General-purpose user agents that provide a high
degree of header field configurability ought to inform users about
the loss of privacy that might result if too much detail is provided.
As an extreme privacy measure, proxies could filter the proactive
negotiation header fields in relayed requests.
See Section 10 of [RFC7230].
11.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4647] Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
Tags", BCP 47, RFC 4647, September 2006.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, June 2014.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, June 2014.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
11.2. Informative References
[BCP13] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013.
[BCP178] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012.
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
[OWASP] van der Stock, A., Ed., "A Guide to Building Secure Web
Applications and Web Services", The Open Web Application
Security Project (OWASP) 2.0.1, July 2005,
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures",
Doctoral Dissertation, University of California, Irvine,
[RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997.
[RFC2295] Holtman, K. and A. Mutz, "Transparent Content Negotiation
in HTTP", RFC 2295, March 1998.
[RFC2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, August 1998.
[RFC2557] Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
"MIME Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, March 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2774] Frystyk, H., Leach, P., and S. Lawrence, "An HTTP
Extension Framework", RFC 2774, February 2000.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322,
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, March 2010.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC5987] Reschke, J., "Character Set and Language Encoding for
Hypertext Transfer Protocol (HTTP) Header Field
Parameters", RFC 5987, August 2010.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
[RFC7238] Reschke, J., "The Hypertext Transfer Protocol (HTTP)
Status Code 308 (Permanent Redirect)", RFC 7238,
Appendix A. Differences between HTTP and MIME
HTTP/1.1 uses many of the constructs defined for the Internet Message
Format [RFC5322] and the Multipurpose Internet Mail Extensions (MIME)
[RFC2045] to allow a message body to be transmitted in an open
variety of representations and with extensible header fields.
However, RFC 2045 is focused only on email; applications of HTTP have
many characteristics that differ from email; hence, HTTP has features
that differ from MIME. 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
This appendix describes specific areas where HTTP differs from MIME.
Proxies and gateways to and from strict MIME environments need to be
aware of these differences and provide the appropriate conversions
HTTP is not a MIME-compliant protocol. However, messages can include
a single MIME-Version 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
conformance with the MIME protocol (as defined in [RFC2045]).
Senders are responsible for ensuring full conformance (where
possible) when exporting HTTP messages to strict MIME environments.
A.2. Conversion to Canonical Form
MIME requires that an Internet mail body part be converted to
canonical form prior to being transferred, as described in Section 4
of [RFC2049]. Section 18.104.22.168 of this document describes the forms
allowed for subtypes of the "text" media type when transmitted over
HTTP. [RFC2046] 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.
A proxy or gateway from HTTP to a strict MIME environment ought to
translate all line breaks within the text media types described in
Section 22.214.171.124 of this document to the RFC 2049 canonical form of
CRLF. Note, however, this might be complicated by the presence of a
Content-Encoding and by the fact that HTTP allows the use of some
charsets that do not use octets 13 and 10 to represent CR and LF,
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.
A.3. Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (Section 126.96.36.199) to
simplify the process of date comparison. Proxies and gateways from
other protocols ought to 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.
A.4. Conversion of Content-Encoding
MIME 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 ought to either change the value of the Content-Type header
field or decode the representation 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 the MIME standards).
A.5. Conversion of Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding field of MIME.
Proxies and gateways from MIME-compliant protocols to HTTP need to
remove any Content-Transfer-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 ought to transform and label the data with an
appropriate Content-Transfer-Encoding if doing so will improve the
likelihood of safe transport over the destination protocol.
A.6. MHTML and Line Length Limitations
HTTP implementations that share code with MHTML [RFC2557]
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 transfers message-bodies as
payload and, aside from the "multipart/byteranges" type (Appendix A
of [RFC7233]), does not interpret the content or any MIME header
lines that might be contained therein.
Appendix B. Changes from RFC 2616
The primary changes in this revision have been editorial in nature:
extracting the messaging syntax and partitioning HTTP semantics into
separate documents for the core features, conditional requests,
partial requests, caching, and authentication. The conformance
language has been revised to clearly target requirements and the
terminology has been improved to distinguish payload from
representations and representations from resources.
A new requirement has been added that semantics embedded in a URI be
disabled when those semantics are inconsistent with the request
method, since this is a common cause of interoperability failure.
An algorithm has been added for determining if a payload is
associated with a specific identifier. (Section 188.8.131.52)
The default charset of ISO-8859-1 for text media types has been
removed; the default is now whatever the media type definition says.
Likewise, special treatment of ISO-8859-1 has been removed from the
Accept-Charset header field. (Section 184.108.40.206 and Section 5.3.3)
The definition of Content-Location has been changed to no longer
affect the base URI for resolving relative URI references, due to
poor implementation support and the undesirable effect of potentially
breaking relative links in content-negotiated resources.
To be consistent with the method-neutral parsing algorithm of
[RFC7230], the definition of GET has been relaxed so that requests
can have a body, even though a body has no meaning for GET.
Servers are no longer required to handle all Content-* header fields
and use of Content-Range has been explicitly banned in PUT requests.
Definition of the CONNECT method has been moved from [RFC2817] to
this specification. (Section 4.3.6)
The OPTIONS and TRACE request methods have been defined as being
safe. (Section 4.3.7 and Section 4.3.8)
The Expect header field's extension mechanism has been removed due to
widely-deployed broken implementations. (Section 5.1.1)
The Max-Forwards header field has been restricted to the OPTIONS and
TRACE methods; previously, extension methods could have used it as
well. (Section 5.1.2)
The "about:blank" URI has been suggested as a value for the Referer
header field when no referring URI is applicable, which distinguishes
that case from others where the Referer field is not sent or has been
removed. (Section 5.5.2)
The following status codes are now cacheable (that is, they can be
stored and reused by a cache without explicit freshness information
present): 204, 404, 405, 414, 501. (Section 6)
The 201 (Created) status description has been changed to allow for
the possibility that more than one resource has been created.
The definition of 203 (Non-Authoritative Information) has been
broadened to include cases of payload transformations as well.
The set of request methods that are safe to automatically redirect is
no longer closed; user agents are able to make that determination
based upon the request method semantics. The redirect status codes
301, 302, and 307 no longer have normative requirements on response
payloads and user interaction. (Section 6.4)
The status codes 301 and 302 have been changed to allow user agents
to rewrite the method from POST to GET. (Sections 6.4.2 and 6.4.3)
The description of the 303 (See Other) status code has been changed
to allow it to be cached if explicit freshness information is given,
and a specific definition has been added for a 303 response to GET.
The 305 (Use Proxy) status code has been deprecated due to security
concerns regarding in-band configuration of a proxy. (Section 6.4.5)
The 400 (Bad Request) status code has been relaxed so that it isn't
limited to syntax errors. (Section 6.5.1)
The 426 (Upgrade Required) status code has been incorporated from
[RFC2817]. (Section 6.5.15)
The target of requirements on HTTP-date and the Date header field
have been reduced to those systems generating the date, rather than
all systems sending a date. (Section 7.1.1)
The syntax of the Location header field has been changed to allow all
URI references, including relative references and fragments, along
with some clarifications as to when use of fragments would not be
appropriate. (Section 7.1.2)
Allow has been reclassified as a response header field, removing the
option to specify it in a PUT request. Requirements relating to the
content of Allow have been relaxed; correspondingly, clients are not
required to always trust its value. (Section 7.4.1)
A Method Registry has been defined. (Section 8.1)
The Status Code Registry has been redefined by this specification;
previously, it was defined in Section 7.1 of [RFC2817].
Registration of content codings has been changed to require IETF
Review. (Section 8.4)
The Content-Disposition header field has been removed since it is now
defined by [RFC6266].
The Content-MD5 header field has been removed because it was
inconsistently implemented with respect to partial responses.
Appendix C. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
(line feed), OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character).
The rules below are defined in [RFC7230]:
BWS = <BWS, see [RFC7230], Section 3.2.3>
OWS = <OWS, see [RFC7230], Section 3.2.3>
RWS = <RWS, see [RFC7230], Section 3.2.3>
URI-reference = <URI-reference, see [RFC7230], Section 2.7>
absolute-URI = <absolute-URI, see [RFC7230], Section 2.7>
comment = <comment, see [RFC7230], Section 3.2.6>
field-name = <comment, see [RFC7230], Section 3.2>
partial-URI = <partial-URI, see [RFC7230], Section 2.7>