HTTP                                                       M. Nottingham
Internet-Draft                                             April 2, 2018
Obsoletes: 3205 (if approved)
Intended status: Best Current Practice
Expires: October 4, 2018

                   On the use of HTTP as a Substrate


   HTTP is often used as a substrate for other application protocols.
   This document specifies best practices for these protocols' use of

   This document obsoletes RFC 3205.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (, which is archived at [1].

   Working Group information can be found at
   [2]; source code and issues list for this draft can be found at [3].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 4, 2018.

Nottingham               Expires October 4, 2018                [Page 1]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Is HTTP Being Used? . . . . . . . . . . . . . . . . . . . . .   4
   3.  What's Important About HTTP . . . . . . . . . . . . . . . . .   5
     3.1.  Generic Semantics . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Links . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Rich Functionality  . . . . . . . . . . . . . . . . . . .   7
   4.  Best Practices for Using HTTP . . . . . . . . . . . . . . . .   7
     4.1.  Specifying the Use of HTTP  . . . . . . . . . . . . . . .   8
     4.2.  Defining HTTP Resources . . . . . . . . . . . . . . . . .   8
     4.3.  Specifying Client Behaviours  . . . . . . . . . . . . . .   9
     4.4.  HTTP URLs . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.4.1.  Initial URL Discovery . . . . . . . . . . . . . . . .  11
       4.4.2.  URL Schemes . . . . . . . . . . . . . . . . . . . . .  11
       4.4.3.  Transport Ports . . . . . . . . . . . . . . . . . . .  12
     4.5.  HTTP Methods  . . . . . . . . . . . . . . . . . . . . . .  12
       4.5.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.6.  HTTP Status Codes . . . . . . . . . . . . . . . . . . . .  14
     4.7.  HTTP Header Fields  . . . . . . . . . . . . . . . . . . .  15
     4.8.  Defining Message Payloads . . . . . . . . . . . . . . . .  16
     4.9.  HTTP Caching  . . . . . . . . . . . . . . . . . . . . . .  16
     4.10. Application State . . . . . . . . . . . . . . . . . . . .  17
     4.11. Client Authentication . . . . . . . . . . . . . . . . . .  17
     4.12. Co-Existing with Web Browsing . . . . . . . . . . . . . .  18
     4.13. Application Boundaries  . . . . . . . . . . . . . . . . .  19
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  22
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  25

Nottingham               Expires October 4, 2018                [Page 2]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   Appendix A.  Changes from RFC 3205  . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   HTTP [RFC7230] is often used as a substrate for other application
   protocols.  This is done for a variety of reasons, including:

   o  familiarity by implementers, specifiers, administrators,
      developers and users,

   o  availability of a variety of client, server and proxy

   o  ease of use,

   o  ubiquity of Web browsers,

   o  reuse of existing mechanisms like authentication and encryption,

   o  presence of HTTP servers and clients in target deployments, and

   o  its ability to traverse firewalls.

   In many cases, these protocols are ad hoc; they are intended for only
   deployment on the server side, and consumption by a limited set of
   clients.  A body of practices and tools has arisen around defining
   such "HTTP APIs" that favours these conditions.

   However, when such a protocol is standarised, it is typically
   deployed on multiple servers, implemented a number of times, and
   might be consumed by a broader variety of clients.  Such diversity
   brings a different set of concerns, and tools and practices intended
   for a single-server deployment might not be suitable.

   In particular, standards-defined HTTP APIs need to more carefully
   consider how extensibility and evolution will be handled, how
   different deployment requirements will be accommodated, and how
   clients will evolve with the API.

   At the same time, the Internet community has a tradition of protocol
   reuse (e.g., Telnet [RFC0854] as a substrate for FTP [RFC0959] and
   SMTP [RFC2821]), but less experience using HTTP as a substrate.
   Because HTTP is extensible in many ways, a number of questions arise,
   such as:

   o  Should an application using HTTP define a new URL scheme?  Use new

Nottingham               Expires October 4, 2018                [Page 3]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   o  Should it use standard HTTP methods and status codes, or define
      new ones?

   o  How can the maximum value be extracted from the use of HTTP?

   o  How does it coexist with other uses of HTTP - especially Web

   o  How can interoperability problems and "protocol dead ends" be

   This document contains best current practices regarding the use of
   HTTP by applications other than Web browsing.  Section 2 defines what
   applications it applies to; Section 3 surveys the properties of HTTP
   that are important to preserve, and Section 4 conveys best practices
   for those applications that do use HTTP.

   It is written primarily to guide IETF efforts to define application
   protocols using HTTP for deployment on the Internet, but might be
   applicable in other situations.  Note that the requirements herein do
   not necessarily apply to the development of generic HTTP extensions.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Is HTTP Being Used?

   Different applications have different goals when using HTTP.  In this
   document, we say an application is _using HTTP_ when any of the
   following conditions are true:

   o  The transport port in use is 80 or 443,

   o  The URL scheme "http" or "https" is used,

   o  The ALPN protocol ID [RFC7301] generically identifies HTTP (e.g.,
      "http/1.1", "h2", "h2c"), or

   o  The IANA registries defined for HTTP are updated or modified.

   When an application is using HTTP, all of the requirements of the
   HTTP protocol suite (including but not limited to [RFC7230],

Nottingham               Expires October 4, 2018                [Page 4]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   [RFC7231], [RFC7232], [RFC7233], [RFC7234], [RFC7235] and [RFC7540])
   are in force.

   An application might not be _using HTTP_ according to this
   definition, but still relying upon the HTTP specifications in some
   manner.  For example, an application might wish to avoid re-
   specifying parts of the message format, but change others; or, it
   might want to use a different set of methods.

   Such applications are referred to as _protocols based upon HTTP_ in
   this document.  These have more freedom to modify protocol operation,
   but are also likely to lose at least a portion of the benefits
   outlined above, as most HTTP implementations won't be easily
   adaptable to these changes, and as the protocol diverges from HTTP,
   the benefit of mindshare will be lost.

   Protocols that are based upon HTTP MUST NOT reuse HTTP's URL schemes,
   transport ports, ALPN protocol IDs or IANA registries; rather, they
   are encouraged to establish their own.

3.  What's Important About HTTP

   There are many ways that applications using HTTP are defined and
   deployed, and sometimes they are brought to the IETF for
   standardisation.  In that process, what might be workable for
   deployment in a limited fashion isn't appropriate for standardisation
   and the corresponding broader deployment.

   This section examines the facets of the protocol that are important
   to preserve in these situations.

3.1.  Generic Semantics

   When writing an application's specification, it's often tempting to
   specify exactly how HTTP is to be implemented, supported and used.

   However, this can easily lead to an unintended profile of HTTP's
   behaviour.  For example, it's common to see specifications with
   language like this:

   A `200 OK` response means that the widget has successfully been

   This sort of specification is bad practice, because it is adding new
   semantics to HTTP's status codes and methods, respectively; a
   recipient - whether it's an origin server, client library,
   intermediary or cache - now has to know these extra semantics to
   understand the message.

Nottingham               Expires October 4, 2018                [Page 5]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   Some applications even require specific behaviours, such as:

   A `POST` request MUST result in a `201 Created` response.

   This forms an expectation in the client that the response will always
   be "201 Created", when in fact there are a number of reasons why the
   status code might differ in a real deployment.  If the client does
   not anticipate this, the application's deployment is brittle.

   Much of the value of HTTP is in its _generic semantics_ - that is,
   the protocol elements defined by HTTP are potentially applicable to
   every resource, not specific to a particular context.  Application-
   specific semantics are expressed in the payload; mostly, in the body,
   but also in header fields.

   This allows a HTTP message to be examined by generic HTTP software
   (e.g., HTTP servers, intermediaries, client implementations), and its
   handling to be correctly determined.  It also allows people to
   leverage their knowledge of HTTP semantics without special-casing
   them for a particular application.

   Therefore, applications that use HTTP MUST NOT re-define, refine or
   overlay the semantics of defined protocol elements.  Instead, they
   should focus their specifications on protocol elements that are
   specific to that application; namely their HTTP resources.

   See Section 4.2 for details.

3.2.  Links

   Another common practice is assuming that the HTTP server's name space
   (or a portion thereof) is exclusively for the use of a single
   application.  This effectively overlays special, application-specific
   semantics onto that space, precludes other applications from using

   As explained in [RFC7320], such "squatting" on a part of the URL
   space by a standard usurps the server's authority over its own
   resources, can cause deployment issues, and is therefore bad practice
   in standards.

   Instead of statically defining URL components like paths, it is
   RECOMMENDED that applications using HTTP define links in payloads, to
   allow flexibility in deployment.

   Using runtime links in this fashion has a number of other benefits.
   For example, navigating with a link allows a request to be routed to
   a different server without the overhead of a redirection, thereby

Nottingham               Expires October 4, 2018                [Page 6]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   supporting deployment across machines well.  It becomes possible to
   "mix" different applications on the same server, and offers a natural
   path for extensibility, versioning and capability management.

3.3.  Rich Functionality

   The simplest possible use of HTTP is to POST data to a single URL,
   thereby effectively tunnelling through the protocol.

   This "RPC" style of communication does get some benefit from using
   HTTP - namely, message framing and the availability of
   implementations - but fails to realise many others when used

   o  Caching for server scalability, latency and bandwidth reduction,
      and reliability;

   o  Granularity of access control (through use of a rich space of

   o  Partial content to selectively request part of a response;

   o  Definition of an information space using URLs; and

   o  The ability to interact with the application easily using a Web

   Using such a high-level protocol to tunnel simple semantics has
   downsides too; because of its more advanced capabilities, breadth of
   deployment and age, HTTP's complexity can cause interoperability
   problems that could be avoided by using a simpler substrate (e.g.,
   WebSockets [RFC6455], if browser support is necessary, or TCP
   [RFC0793] if not), or making the application be _based upon HTTP_,
   instead of using it (as defined in Section 2).

   Applications that use HTTP are encouraged to accommodate the various
   features that the protocol offers, so that their users receive the
   maximum benefit from it.  This document does not require specific
   features to be used, since the appropriate design tradeoffs are
   highly specific to a given situation.  However, following the
   practices in Section 4 will help make them available.

4.  Best Practices for Using HTTP

   This section contains best practices regarding the use of HTTP by
   applications, including practices for specific HTTP protocol

Nottingham               Expires October 4, 2018                [Page 7]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

4.1.  Specifying the Use of HTTP

   When specifying the use of HTTP, an application SHOULD use [RFC7230]
   as the primary reference; it is not necessary to reference all of the
   specifications in the HTTP suite unless there are specific reasons to
   do so (e.g., a particular feature is called out).

   Applications using HTTP MAY specify a minimum version to be supported
   (HTTP/1.1 is suggested), and MUST NOT specify a maximum version, to
   preserve the protocol's ability to evolve.

   Likewise, applications need not specify what HTTP mechanisms - such
   as redirection, caching, authentication, proxy authentication, and so
   on - are to be supported.  For example, an application can specify
   that it uses HTTP like this:

   Foo Application uses HTTP [RFC7230]. Implementations MUST support
   HTTP/1.1, and MAY support later versions.

   When specifying examples of protocol interactions, applications
   SHOULD document both the request and response messages, with full
   headers, preferably in HTTP/1.1 format.  For example:

   GET /thing HTTP/1.1
   Accept: application/things+json
   User-Agent: Foo/1.0

   HTTP/1.1 200 OK
   Content-Type: application/things+json
   Content-Length: 500
   Server: Bar/2.2

   [payload here]

4.2.  Defining HTTP Resources

   Applications that use HTTP should focus on defining the following
   application-specific protocol elements:

   o  Media types [RFC6838], often based upon a format convention such
      as JSON [RFC8259],

   o  HTTP header fields, as per Section 4.7, and

   o  The behaviour of resources, as identified by link relations

Nottingham               Expires October 4, 2018                [Page 8]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   By composing these protocol elements, an application can define a set
   of resources, identified by link relations, that implement specified
   behaviours, including:

   o  Retrieval of their state using GET, in one or more formats
      identified by media type;

   o  Resource creation or update using POST or PUT, with an
      appropriately identified request body format;

   o  Data processing using POST and identified request and response
      body format(s); and

   o  Resource deletion using DELETE.

   For example, an application might specify:

   Resources linked to with the "example-widget" link relation type are
   Widgets. The state of a Widget can be fetched in the
   "application/example-widget+json" format, and can be updated by PUT
   to the same link. Widget resources can be deleted.

   The "Example-Count" response header field on Widget representations
   indicates how many Widgets are held by the sender.

   The "application/example-widget+json" format is a JSON [RFC8259]
   format representing the state of a Widget. It contains links to
   related information in the link indicated by the Link header field
   value with the "example-other-info" link relation type.

4.3.  Specifying Client Behaviours

   HTTP does not mandate some behaviours that have nevertheless become
   very common; if these are not explicitly specified by applications
   using HTTP, there may be confusion and interoperability problems.
   This section recommends default handling for these mechanisms.

   o  Redirect handling - applications using HTTP SHOULD specify that
      3xx redirect status codes be followed automatically.  See
      [RFC7231], Section 6.4.

   o  Redirect methods - applications using HTTP SHOULD specify that 301
      and 302 redirect status codes rewrite the POST method to GET.  See
      [RFC7231], Section 6.4.

   o  Cookies - Applications using HTTP MUST explicitly reference the
      Cookie specification [RFC6265] if they are required.

Nottingham               Expires October 4, 2018                [Page 9]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   o  Certificates - Applications using HTTP MUST specify that TLS
      certificates are to be checked according to [RFC2818] when HTTPS
      is used.

   In general, applications using HTTP ought to align their usage as
   closely as possible with Web browsers, to avoid interoperability
   issues when they are used.  See Section 4.12.

   If an application using HTTP has browser compatibility as a goal,
   client interaction ought to be defined in terms of [FETCH], since
   that is the abstraction that browsers use for HTTP; it enforces many
   of these best practices.

   Applications using HTTP MUST NOT require HTTP features that are
   usually negotiated to be supported.  For example, requiring that
   clients support responses with a certain content-encoding ([RFC7231],
   Section instead of negotiating for it ([RFC7231],
   Section 5.3.4) means that otherwise conformant clients cannot
   interoperate with the application.  Applications MAY encourage the
   implementation of such features, though.

4.4.  HTTP URLs

   In HTTP, URLs are opaque identifiers under the control of the server.
   As outlined in [RFC7320], standards cannot usurp this space, since it
   might conflict with existing resources, and constrain implementation
   and deployment.

   In other words, applications that use HTTP shouldn't associate
   application semantics with specific URL paths on arbitrary servers.
   Doing so inappropriately conflates the identity of the resource (its
   URL) with the capabilities that resource supports, bringing about
   many of the same interoperability problems that [RFC4367] warns of.

   For example, specifying that a "GET to the URL /foo retrieves a bar
   document" is bad practice.  Likewise, specifying "The widget API is
   at the path /bar" violates [RFC7320].

   Instead, applications that use HTTP are encouraged to ensure that
   URLs are discovered at runtime, allowing HTTP-based services to
   describe their own capabilities.  One way to do this is to use typed
   links [RFC8288] to convey the URIs that are in use, as well as the
   semantics of the resources that they identify.  See Section 4.2 for

Nottingham               Expires October 4, 2018               [Page 10]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

4.4.1.  Initial URL Discovery

   Generally, a client will begin interacting with a given application
   server by requesting an initial document that contains information
   about that particular deployment, potentially including links to
   other relevant resources.

   Applications that use HTTP are encouraged to allow an arbitrary URL
   to be used as that entry point.  For example, rather than specifying
   "the initial document is at "/foo/v1", they should allow a deployment
   to use any URL as the entry point for the application.

   In cases where doing so is impractical (e.g., it is not possible to
   convey a whole URL, but only a hostname) standard applications that
   use HTTP can request a well-known URL [RFC5785] as an entry point.

4.4.2.  URL Schemes

   Applications that use HTTP will typically employ the "http" and/or
   "https" URL schemes. "https" is preferred to provide authentication,
   integrity and confidentiality, as well as mitigate pervasive
   monitoring attacks [RFC7258].

   However, application-specific schemes can be defined as well.

   When defining an URL scheme for an application using HTTP, there are
   a number of tradeoffs and caveats to keep in mind:

   o  Unmodified Web browsers will not support the new scheme.  While it
      is possible to register new URL schemes with Web browsers (e.g.
      registerProtocolHandler() in [HTML5], as well as several
      proprietary approaches), support for these mechanisms is not
      shared by all browsers, and their capabilities vary.

   o  Existing non-browser clients, intermediaries, servers and
      associated software will not recognise the new scheme.  For
      example, a client library might fail to dispatch the request; a
      cache might refuse to store the response, and a proxy might fail
      to forward the request.

   o  Because URLs occur in and are generated in HTTP artefacts
      commonly, often without human intervention (e.g., in the
      "Location" response header), it can be difficult to assure that
      the new scheme is used consistently.

   o  The resources identified by the new scheme will still be available
      using "http" and/or "https" URLs.  Those URLs can "leak" into use,
      which can present security and operability issues.  For example,

Nottingham               Expires October 4, 2018               [Page 11]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

      using a new scheme to assure that requests don't get sent to a
      "normal" Web site is likely to fail.

   o  Features that rely upon the URL's origin [RFC6454], such as the
      Web's same-origin policy, will be impacted by a change of scheme.

   o  HTTP-specific features such as cookies [RFC6265], authentication
      [RFC7235], caching [RFC7234], and CORS [FETCH] might or might not
      work correctly, depending on how they are defined and implemented.
      Generally, they are designed and implemented with an assumption
      that the URL will always be "http" or "https".

   o  Web features that require a secure context [SECCTXT] will likely
      treat a new scheme as insecure.

   See [RFC7595] for more information about minting new URL schemes.

4.4.3.  Transport Ports

   Applications that use HTTP can use the applicable default port (80
   for HTTP, 443 for HTTPS), or they can be deployed upon other ports.
   This decision can be made at deployment time, or might be encouraged
   by the application's specification (e.g., by registering a port for
   that application).

   In either case, non-default ports will need to be reflected in the
   authority of all URLs for that resource; the only mechanism for
   changing a default port is changing the scheme (see Section 4.4.2).

   Using a port other than the default has privacy implications (i.e.,
   the protocol can now be distinguished from other traffic), as well as
   operability concerns (as some networks might block or otherwise
   interfere with it).  Privacy implications should be documented in
   Security Considerations.

   See [RFC7605] for further guidance.

4.5.  HTTP Methods

   Applications that use HTTP MUST confine themselves to using
   registered HTTP methods such as GET, POST, PUT, DELETE, and PATCH.

   New HTTP methods are rare; they are required to be registered with
   IETF Review (see [RFC7232]), and are also required to be _generic_.
   That means that they need to be potentially applicable to all
   resources, not just those of one application.

Nottingham               Expires October 4, 2018               [Page 12]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   While historically some applications (e.g., [RFC4791]) have defined
   non-generic methods, [RFC7231] now forbids this.

   When authors believe that a new method is required, they are
   encouraged to engage with the HTTP community early, and document
   their proposal as a separate HTTP extension, rather than as part of
   an application's specification.

4.5.1.  GET

   GET is one of the most common and useful HTTP methods; its retrieval
   semantics allow caching, side-effect free linking and forms the basis
   of many of the benefits of using HTTP.

   A common use of GET is to perform queries, often using the query
   component of the URL; this is this a familiar pattern from Web
   browsing, and the results can be cached, improving efficiency of an
   often expensive process.

   In some cases, however, GET might be unwieldy for expressing queries,
   because of the limited syntax of the URL; in particular, if binary
   data forms part of the query terms, it needs to be encoded to conform
   to URL syntax.

   While this is not an issue for short queries, it can become one for
   larger query terms, or ones which need to sustain a high rate of
   requests.  Additionally, some HTTP implementations limit the size of
   URLs they support - although modern HTTP software has much more
   generous limits than previously (typically, considerably more than
   8000 octets, as required by [RFC7230] Section 3.1.1).

   In these cases, an application using HTTP might consider using POST
   to express queries in the request body; doing so avoids encoding
   overhead and URL length limits in implementations.  However, in doing
   so it should be noted that the benefits of GET such as caching and
   linking to query results are lost.  Therefore, applications using
   HTTP that feel a need to allow POST queries ought consider allowing
   both methods.

   Applications that use HTTP SHOULD NOT define GET requests to have
   side effects, since implementations can and do retry HTTP GET
   requests that fail.

   Finally, note that while HTTP allows GET requests to have a body
   syntactically, this is done only to allow parsers to be generic; as
   per [RFC7231], Section 4.3.1, a body on a GET has no meaning, and
   will be either ignored or rejected by generic HTTP software.  As a

Nottingham               Expires October 4, 2018               [Page 13]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   result, applications that use HTTP SHOULD NOT define GET to have any
   sde effects upon their resources.

4.6.  HTTP Status Codes

   Applications that use HTTP MUST only use registered HTTP status

   As with methods, new HTTP status codes are rare, and required (by
   [RFC7231]) to be registered with IETF review.  Similarly, HTTP status
   codes are generic; they are required (by [RFC7231]) to be potentially
   applicable to all resources, not just to those of one application.

   When authors believe that a new status code is required, they are
   encouraged to engage with the HTTP community early, and document
   their proposal as a separate HTTP extension, rather than as part of
   an application's specification.

   The primary function of status codes is to convey HTTP semantics for
   the benefit of generic HTTP software, not application-specific
   semantics.  Therefore, applications MUST NOT specify additional
   semantics or refine existing semantics for status codes.

   In particular, specifying that a particular status code has a
   specific meaning in the context of an application is harmful, as
   these are not generic semantics, since the consumer needs to be in
   the context of the application to understand them.

   Furthermore, applications using HTTP MUST NOT re-specify the
   semantics of HTTP status codes, even if it is only by copying their
   definition.  They MUST NOT require specific reason phrases to be
   used; the reason phrase has no function in HTTP, and is not
   guaranteed to be preserved by implementations.  The reason phrase is
   not carried in the [RFC7540] message format.

   Typically, applications using HTTP will convey application-specific
   information in the message body and/or HTTP header fields, not the
   status code.  [RFC7807] provides one way for applications using HTTP
   to do this.

   Specifications sometimes also create a "laundry list" of potential
   status codes, in an effort to be helpful.  The problem with doing so
   is that such a list is never complete; for example, if a network
   proxy is interposed, the client might encounter a "407 Proxy
   Authentication Required" response; or, if the server is rate limiting
   the client, it might receive a "429 Too Many Requests" response.

Nottingham               Expires October 4, 2018               [Page 14]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   Since the list of HTTP status codes can be added to, it's safer to
   specify behaviours in terms of general response classes (e.g.,
   "successful response" for 2xx; "client error" for 4xx and "server
   error" for 5xx), pointing out that clients SHOULD be able to handle
   all applicable protocol elements gracefully (i.e., falling back to
   the generic "n00" semantics of a given status code; e.g., "499" can
   be safely handled as "400" by clients that don't recognise it).

4.7.  HTTP Header Fields

   Applications that use HTTP MAY define new HTTP header fields,
   following the advice in [RFC7231], Section 8.3.1.

   Typically, using HTTP header fields is appropriate in a few different

   o  Their content is useful to intermediaries (who often wish to avoid
      parsing the body), and/or

   o  Their content is useful to generic HTTP software (e.g., clients,
      servers), and/or

   o  It is not possible to include their content in the message body
      (usually because a format does not allow it).

   New header fields MUST be registered, as per [RFC7231] and [RFC3864].

   It is RECOMMENDED that header field names be short (even when HTTP/2
   header compression is in effect, there is an overhead) but
   appropriately specific.  In particular, if a header field is specific
   to an application, an identifier for that application SHOULD form a
   prefix to the header field name, separated by a "-".

   For example, if the "example" application needs to create three
   headers, they might be called "example-foo", "example-bar" and
   "example-baz".  Note that the primary motivation here is to avoid
   consuming more generic header names, not to reserve a portion of the
   namespace for the application; see [RFC6648] for related

   The semantics of existing HTTP header fields MUST NOT be re-defined
   without updating their registration or defining an extension to them
   (if allowed).  For example, an application using HTTP cannot specify
   that the "Location" header has a special meaning in a certain

Nottingham               Expires October 4, 2018               [Page 15]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   See Section 4.9 for the interaction between headers and HTTP caching;
   in particular, request headers that are used to "select" a response
   have impact there, and need to be carefully considered.

   See Section 4.10 for considerations regarding header fields that
   carry application state (e.g,.  Cookie).

4.8.  Defining Message Payloads

   There are many potential formats for payloads; for example, JSON
   [RFC8259], XML [XML], and CBOR [RFC7049].  Best practices for their
   use are out of scope for this document.

   Applications SHOULD register distinct media types for each format
   they define; this makes it possible to identify them unambiguously
   and negotiate for their use.  See [RFC6838] for more information.

4.9.  HTTP Caching

   HTTP caching [RFC7234] is one of the primary benefits of using HTTP
   for applications; it provides scalability, reduces latency and
   improves reliability.  Furthermore, HTTP caches are readily available
   in browsers and other clients, networks as forward and reverse
   proxies, Content Delivery Networks and as part of server software.

   Assigning even a short freshness lifetime ([RFC7234], Section 4.2) -
   e.g., 5 seconds - allows a response to be reused to satisfy multiple
   clients, and/or a single client making the same request repeatedly.
   In general, if it is safe to reuse something, consider assigning a
   freshness lifetime; cache implementations take active measures to
   expire content intelligently when they are out of space, so "it will
   fill up the cache" is not a valid concern.

   Understand that stale responses (e.g., one with "Cache-Control: max-
   age=0") can be reused when the cache is disconnected from the origin
   server; this can be useful for handling network issues.  See
   [RFC7234], Section 4.2.4, and also [RFC5861] for additional controls
   over stale content.

   Stale responses can be refreshed by assigning a validator, saving
   both transfer bandwidth and latency for large responses; see

   In some situations, responses without explicit cache directives
   (e.g., Cache-Control or Expires) will be stored and served using a
   heuristic freshness lifetime; see [RFC7234], Section 4.2.2.  As the
   heuristic is not under control of the application, it is generally
   preferable to set an explicit freshness lifetime.

Nottingham               Expires October 4, 2018               [Page 16]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   If caching of a response is not desired, the appropriate response
   directive is "Cache-Control: no-store".  This only need be sent in
   situations where the response might be cached; see [RFC7234],
   Section 3.

   If an application defines a request header field that might be used
   by a server to change the response's headers or body, authors should
   point out that this has implications for caching; in general, such
   resources need to either make their responses uncacheable (e.g., with
   the "no-store" cache-control directive defined in [RFC7234],
   Section or consistently send the Vary response header
   ([RFC7231], Section 7.1.4).

   When an application has a need to express a lifetime that's separate
   from the freshness lifetime, this should be expressed separately,
   either in the response's body or in a separate header field.  When
   this happens, the relationship between HTTP caching and that lifetime
   need to be carefully considered, since the response will be used as
   long as it is considered fresh.

   Like other functions, HTTP caching is generic; it does not have
   knowledge of the application in use.  Therefore, caching extensions
   need to be backwards-compatible, as per [RFC7234], Section 5.2.3.

4.10.  Application State

   Applications that use HTTP MAY use stateful cookies [RFC6265] to
   identify a client and/or store client-specific data to contextualise

   When used, it is important to carefully specify the scoping and use
   of cookies; if the application exposes sensitive data or capabilities
   (e.g., by acting as an ambient authority), exploits are possible.
   Mitigations include using a request-specific token to assure the
   intent of the client.

   Applications MUST NOT make assumptions about the relationship between
   separate requests on a single transport connection; doing so breaks
   many of the assumptions of HTTP as a stateless protocol, and will
   cause problems in interoperability, security, operability and

4.11.  Client Authentication

   Applications that use HTTP MAY use HTTP authentication [RFC7235] to
   identify clients.  The Basic authentication scheme [RFC7617] MUST NOT
   be used unless the underlying transport is authenticated, integrity-
   protected and confidential (e.g., as provided the "HTTPS" URL scheme,

Nottingham               Expires October 4, 2018               [Page 17]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   or another using TLS).  The Digest scheme [RFC7616] MUST NOT be used
   unless the underlying transport is similarly secure, or the chosen
   hash algorithm is not "MD5".

   When used, it is important to carefully specify the scoping and use
   of authentication; if the application exposes sensitive data or
   capabilities (e.g., by acting as an ambient authority), exploits are
   possible.  Mitigations include using a request-specific token to
   assure the intent of the client.

4.12.  Co-Existing with Web Browsing

   Even if there is not an intent for an application that uses HTTP to
   be used with a Web browser, its resources will remain available to
   browsers and other HTTP clients.

   This means that all such applications need to consider how browsers
   will interact with them, particularly regarding security.

   For example, if an application's state can be changed using a POST
   request, a Web browser can easily be coaxed into making that request
   by a HTML form on an arbitrary Web site.

   Or, if a resource reflects data from the request into a response,
   that can be used to perform a Cross-Site Scripting attack on Web
   browsers directed to it.

   This is only a small sample of the kinds of issues that applications
   using HTTP must consider.  Generally, the best approach is to
   consider the application _as_ a Web application, and to follow best
   practices for their secure development.

   A complete enumeration of such practices is out of scope for this
   document, but some considerations include:

   o  Using Strict Transport Security [RFC6797] to assure that HTTPS is

   o  Using Content-Security-Policy [W3C.WD-CSP3-20160913] to constrain
      the capabilities of content, thereby mitigating Cross-Site
      Scripting attacks (which are possible if client-provided data is
      exposed in any part of a response in the application)

   o  Using X-Frame-Options [RFC7034] to prevent content from being
      included in a HTML frame from another origin, thereby enabling

Nottingham               Expires October 4, 2018               [Page 18]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   o  Using Referrer-Policy [W3C.CR-referrer-policy-20170126] to prevent
      sensitive data in URLs from being leaked in the Referer request

   o  Using the 'HttpOnly' flag on Cookies to assure that cookies are
      not exposed to browser scripting languages [RFC6265]

   Depending on how they are intended to be deployed, specifications for
   applications using HTTP might require the use of these mechanisms in
   specific ways, or might merely point them out in Security

   If an application using HTTP has browser compatibility as a goal,
   client interaction ought to be defined in terms of [FETCH], since
   that is the abstraction that browsers use for HTTP; it enforces many
   of these best practices.

4.13.  Application Boundaries

   Because the origin [RFC6454] is how many HTTP capabilities are
   scoped, applications also need to consider how deployments might
   interact with other applications (including Web browsing) on the same

   For example, if Cookies [RFC6265] are used to carry application
   state, they will be sent with all requests to the origin by default,
   unless scoped by path, and the application might receive cookies from
   other applications on the origin.  This can lead to security issues,
   as well as collision in cookie names.

   One solution to these issues is to require a dedicated hostname for
   the application, so that it has a unique origin.  However, it is
   often desirable to allow multiple applications to be deployed on a
   single hostname; doing so provides the most deployment flexibility
   and enables them to be "mixed" together (See [RFC7320] for details).
   Therefore, applications using HTTP should strive to allow multiple
   applications on an origin.

   To enable this, when specifying the use of Cookies, HTTP
   authentication realms [RFC7235], or other origin-wide HTTP
   mechanisms, applications using HTTP SHOULD NOT mandate the use of a
   particular identifier, but instead let deployments configure them.
   Consideration SHOULD be given to scoping them to part of the origin,
   using their specified mechanisms for doing so.

   Modern Web browsers constrain the ability of content from one origin
   to access resources from another, to avoid leaking private
   information.  As a result, applications that wish to expose cross-

Nottingham               Expires October 4, 2018               [Page 19]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   origin data to browsers will need to implement the CORS protocol; see

5.  IANA Considerations

   This document has no requirements for IANA.

6.  Security Considerations

   Section 4.10 discusses the impact of using stateful mechanisms in the
   protocol as ambient authority, and suggests a mitigation.

   Section 4.4.2 requires support for 'https' URLs, and discourages the
   use of 'http' URLs, to provide authentication, integrity and
   confidentiality, as well as mitigate pervasive monitoring attacks.

   Section 4.12 highlights the implications of Web browsers'
   capabilities on applications that use HTTP.

   Section 4.13 discusses the issues that arise when applications are
   deployed on the same origin as Web sites (and other applications).

   Applications that use HTTP in a manner that involves modification of
   implementations - for example, requiring support for a new URL
   scheme, or a non-standard method - risk having those implementations
   "fork" from their parent HTTP implementations, with the possible
   result that they do not benefit from patches and other security
   improvements incorporated upstream.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

Nottingham               Expires October 4, 2018               [Page 20]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648,
              DOI 10.17487/RFC6648, June 2012,

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,

Nottingham               Expires October 4, 2018               [Page 21]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

7.2.  Informative References

   [FETCH]    WHATWG, "Fetch - Living Standard", n.d.,

   [HTML5]    WHATWG, "HTML - Living Standard", n.d.,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
              1983, <>.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985,

   [RFC2821]  Klensin, J., Ed., "Simple Mail Transfer Protocol",
              RFC 2821, DOI 10.17487/RFC2821, April 2001,

Nottingham               Expires October 4, 2018               [Page 22]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   [RFC3205]  Moore, K., "On the use of HTTP as a Substrate", BCP 56,
              RFC 3205, DOI 10.17487/RFC3205, February 2002,

   [RFC4367]  Rosenberg, J., Ed. and IAB, "What's in a Name: False
              Assumptions about DNS Names", RFC 4367,
              DOI 10.17487/RFC4367, February 2006,

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              DOI 10.17487/RFC4791, March 2007,

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

   [RFC7034]  Ross, D. and T. Gondrom, "HTTP Header Field X-Frame-
              Options", RFC 7034, DOI 10.17487/RFC7034, October 2013,

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

Nottingham               Expires October 4, 2018               [Page 23]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,

   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

   [SECCTXT]  West, M., "Secure Contexts", World Wide Web Consortium CR
              CR-secure-contexts-20160915, September 2016,

              Eisinger, J. and E. Stark, "Referrer Policy", World Wide
              Web Consortium CR CR-referrer-policy-20170126, January

              West, M., "Content Security Policy Level 3", World Wide
              Web Consortium WD WD-CSP3-20160913, September 2016,

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, November 2008,

Nottingham               Expires October 4, 2018               [Page 24]

Internet-Draft      On the use of HTTP as a Substrate         April 2018

7.3.  URIs




Appendix A.  Changes from RFC 3205

   [RFC3205] captured the Best Current Practice in the early 2000's,
   based on the concerns facing protocol designers at the time.  Use of
   HTTP has changed considerably since then, and as a result this
   document is substantially different.  As a result, the changes are
   too numerous to list individually.

Author's Address

   Mark Nottingham


Nottingham               Expires October 4, 2018               [Page 25]