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Versions: 00 01                                                         
Network Working Group                                      M. Nottingham
Internet-Draft                                           October 8, 2020
Intended status: Best Current Practice
Expires: April 11, 2021

                             Greasing HTTP


   Like many network protocols, HTTP is vulnerable to ossification of
   its extensibility points.  This draft explains why HTTP ossification
   is a problem and establishes guidelines for exercising those
   extensions by 'greasing' the protocol to combat it.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   The issues list for this draft can be found at
   https://github.com/mnot/I-D/labels/http-grease [1].

   The most recent (often, unpublished) draft is at
   https://mnot.github.io/I-D/http-grease/ [2].

   Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
   pages/http-grease [3].

   See also the draft's current status in the IETF datatracker, at
   https://datatracker.ietf.org/doc/draft-nottingham-http-grease/ [4].

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 https://datatracker.ietf.org/drafts/current/.

   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 April 11, 2021.

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Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Ossification and HTTP . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Greasing HTTP Request Header Fields . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     4.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Like many network protocols, HTTP is vulnerable to ossification of
   its extensibility points.  Ossification happens when a significant
   number of the systems that generate, transmit, handle, or consume the
   protocol don't accept a new extension, thereby making it more
   difficult to deploy extensions.

   For example, TCP has effectively been ossified by middleboxes that
   assume that new TCP options will not be deployed; likewise, the
   Protocol field in IP has been effectively ossified as well, since so
   many networks will only accept TCP or UDP traffic.

   Addressing this issue is important; protocol extensibility allows
   adaptation to new circumstances as well as application to new use
   cases.  Inability to deploy new extensions creates pressure to misuse
   the protocol - often leading to undesirable side effects - or to use
   other protocols, reducing the value that the community gets from a
   shared, standard protocol.

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   While there are a few ways that protocol designers can mitigate
   ossification, this document focuses on a technique that's well suited
   to many of the ossification risks in HTTP: 'greasing' extensibility
   points by exercising them, so that they don't become 'rusted shut.'

   [RFC8701]) pioneered greasing techniques in IETF protocols; this
   document explains how they apply to HTTP.  It focuses on generic HTTP
   features; other documents cover versioned extensibility points (e.g.,
   see [I-D.bishop-httpbis-grease]).

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.  Ossification and HTTP

   As an application protocol, HTTP has several extensibility points.
   For example, methods, status codes, header and trailer fields, cache
   directives, range units and content codings are all HTTP extension

   Each extension point defines how unrecognised values should be
   handled; in most cases, they should be ignored (e.g. header fields,
   cache directives and range units), while in a few cases they have
   other handling (e.g., unrecognised methods result in a 405 status
   code; unrecognised status codes devolve to a more generic x00 status

   Implementations and other components that diverge from these defined
   behaviours risk ossifying that extensibility point.

   For example, it is increasingly common for Web Application Firewalls
   (WAFs), bot detection services and similar components to reject HTTP
   requests that contain header fields with certain characters or
   strings, even though syntactically valid, and even though the header
   fields are not necessarily recognised by the recipient.

   This behaviour has become prevalent enough to make it difficult for
   Web browsers and other clients to introduce new request header
   fields.  That difficulty is aggravated by two factors:

   1.  A relatively large number of vendors create these components, but
       have little coordination between them, leading to wide variances
       in behaviour, and

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   2.  Many of these components' deployments are not updated regularly
       and reliably, leading to difficulty in addressing ossification
       issues even when they are identified.

   To avoid ossification of request header fields, it is Best Current
   Practice to grease them, as explained below.  Other HTTP
   extensibility points might be added in the future, and it is not to
   be inferred that greasing other HTTP extensibility points is not good

2.1.  Greasing HTTP Request Header Fields

   HTTP clients SHOULD grease request header fields.  There are two aims
   in doing so:

   1.  Preserving the ability to add new request header fields over time

   2.  Preserving the ability to add new request header fields with
       values containing common syntax

   Clients can grease a given request at their discretion.  For example,
   a client implementation might add one or more grease request header
   fields to every request it makes, or it might add one to every third
   or tenth request.

   Depending on the deployment model of the client, it might do this in
   production releases automatically (especially if there are ways that
   it can modify how grease values are sent with a high degree of
   control, in case too many errors are encountered), or it might do so
   only in pre-releases.

   Grease field names SHOULD be hard to predict; e.g., they SHOULD NOT
   have any identifying prefix, suffix, or pattern.  However, they MUST
   NOT be likely to conflict with unregistered or future field names,
   and the grease coordinator MUST avoid potentially offensive or
   confusing terms.  They also MUST conform to the syntactic
   requirements for field names in HTTP ([I-D.ietf-httpbis-semantics],
   Section 4.3).

   This can be achieved in different ways (which SHOULD vary from time
   to time), for example:

   o  Combine two or three dictionary words or proper nouns with a
      hyphen (e.g., 'Skateboard-Clancy', 'Murray-Fortnight-Scout')

   o  Append digits to a dictionary word (e.g., 'Turnstile23')

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   o  Generate a string using a hash or similar function (e.g.,

   Grease field names are not required to be registered in the IANA HTTP
   Field Name Registry, unless they are intended to be used over an
   extended period of time (e.g., more than one year).  However, they
   MAY be registered as Provisional with a reference to this RFC or
   another explanatory resource, to help interested parties to find out
   what they are used for.  Such registered values SHOULD be removed
   after the client stops using that field.

   Greasing clients SHOULD not reuse other clients' grease fields names,
   unless they coordinate.

   Grease field values can be fixed strings, or dynamically generated at
   runtime.  It is RECOMMENDED that greasing clients exercise the
   various types in [I-D.ietf-httpbis-header-structure].

   If an error is encountered by a greasing client, it SHOULD NOT re-
   issue the request without the grease value, since hiding the
   consequences of the failure doesn't serve the purpose of greasing.

   Greasing clients SHOULD announce new field names they intend to
   grease on the http-grease@ietf.org mailing list.

3.  Security Considerations

   Some HTTP extensibility points are becoming (or have become) ossified
   because of security considerations; receiving implementations believe
   that it is more secure to reject unknown values, or that they can
   identify undesirable peers through their use of extensions.

   This document does not directly address these concerns, nor does it
   directly disallow such behaviour.  Instead, it aims to encourage the
   ability to accommodate new extensions more quickly than is now

4.  References

4.1.  Normative References

              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", draft-ietf-httpbis-semantics-12 (work in
              progress), October 2020.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

4.2.  Informative References

              Bishop, M., "GREASE for HTTP/2", draft-bishop-httpbis-
              grease-01 (work in progress), June 2020.

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", draft-ietf-httpbis-header-structure-19 (work in
              progress), June 2020.

   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,

4.3.  URIs

   [1] https://github.com/mnot/I-D/labels/http-grease

   [2] https://mnot.github.io/I-D/http-grease/

   [3] https://github.com/mnot/I-D/commits/gh-pages/http-grease

   [4] https://datatracker.ietf.org/doc/draft-nottingham-http-grease/

Author's Address

   Mark Nottingham
   made in
   Prahran, VIC

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

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