Network Working Group M. Nottingham
Internet-Draft October 8, 2020
Intended status: Best Current Practice
Expires: April 11, 2021
Greasing HTTP
draft-nottingham-http-grease-01
Abstract
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The most recent (often, unpublished) draft is at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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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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.
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
points.
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
code).
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
practice.
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.,
'dd722785c01b')
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
possible.
4. References
4.1. Normative References
[I-D.ietf-httpbis-semantics]
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,
<https://www.rfc-editor.org/info/rfc2119>.
[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
[I-D.bishop-httpbis-grease]
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,
<https://www.rfc-editor.org/info/rfc8701>.
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
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
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