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HTTP                                                           L. Pardue
Internet-Draft                                                Cloudflare
Intended status: Standards Track                            13 July 2020
Expires: 14 January 2021


    HTTP Alternative Services That Do Not Support Desired Extensions
                draft-pardue-alt-svc-ext-unsupported-00

Abstract

   This document describes an error case when an HTTP Alternative
   Service does not support the extension points of the original
   connection.  It defines an error code that can be used by endpoints
   to signal the encounter.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the ALT Working Group
   mailing list (alt@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/alt/
   (https://mailarchive.ietf.org/arch/browse/alt/).

   Source for this draft and an issue tracker can be found at
   https://github.com/LPardue/draft-pardue-alt-svc-ext-unsupported
   (https://github.com/LPardue/draft-pardue-alt-svc-ext-unsupported).

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 14 January 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
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Detecting and Signalling  . . . . . . . . . . . . . . . . . .   3
   4.  HTTP/2 Error Code . . . . . . . . . . . . . . . . . . . . . .   4
   5.  HTTP/3 Error Code . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   HTTP/2 ([HTTP2]) and HTTP/3 ([HTTP3]) provide several extension
   points, only some of which require negotiation before use.  For any
   HTTP connection that is actively using extensions, it is possible
   that the server might advertise an Alternative Service ([ALT-SVC])
   that contains no indication of whether the current extension points
   are supported by indicated server.  A client has to proactively
   engage with the Alternative in order to determine what extension
   points it supports, which will often require the client to establish
   a new transport-layer connection and HTTP handshake.  The client
   might determine that the Alternative does not support the same
   extensions as the current connection and decide to close the
   connection.  For an extension that is negotiated via SETTINGS frames,
   the client might be able to detect the lack of extensions early in
   the lifecycle.  For other forms of extension, the problem might not
   be detectable until later on, for instance when extension frame
   exchanges fail.




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   The guidance in [ALT-SVC] has lead to clients that are robust to
   different types of failures.  Following a "make-before-break"
   approach avoids active transfers being interupted by an opportunistic
   change in connection.  As such, testing an Alternative and finding
   that its extension points are incompatible is unlikely to be a hard
   failure cases.  However, Alt-Svc is the primary method of switching
   between HTTP/2 and HTTP/3 and there is a possibility that extensions
   offered in one version of a connection are not provided in another.
   It is also feasible that support for extension points are not
   unilaterally deployed across a fleet of servers, whether in the same
   organization domain or not.  A client might encounter problems with
   the Alternative due to permanent error or transient erro due to
   synchronization or coordination issues between the origin and the
   Alternative.

   Advertising Alternative Services has quite a low barrier and does
   require up front coordination, meaning it is quite easy for an origin
   administrator to configure things that are logically invalid.
   Alternatives can detect problems and take action; [ALT-SVC] defines
   status 421 Misdirected Request that allows the Alternative to signal
   such a problem.  However, there is are cases where clients will be
   misdirected to Alternatives that cannot satisfy the extension
   expectations of the client.  A server that detects this issue can
   either close the connection or degrade into a mode that does not use
   the extension.

   This document defines an error code for HTTP/2 and HTTP/3 that can be
   sent by an endpoint when closing streams or connections due to a
   failure with extenstion expectations.  This clearly disambiguates the
   failure case from other types, i.e. general protocol error, which can
   allow the peer to take more specific meaures, such as identifying
   configuration errors.

2.  Conventions and Definitions

   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.

3.  Detecting and Signalling

   HTTP/2 and HTTP/3 endpoints might use various means to detect that a
   desired extension is not supported on the connection, no specific
   measures are prescribed by this document.





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   When an endpoint detects that a desired extension is unsupported it
   MAY terminate a stream or a connection with the error code
   EXTENSION_UNSUPPORTED or H3_EXTENSION_UNSUPPORTED.

4.  HTTP/2 Error Code

   EXTENSION_UNSUPPORTED (0xf10): The endpoint detected that its peer
   wants to use an extension that is not locally supported, or the peer
   does not support an extension that is desired.

5.  HTTP/3 Error Code

   H3_EXTENSION_UNSUPPORTED (0x1f10): The endpoint detected that its
   peer wants to use an extension that is not locally supported, or the
   peer does not support an extension that is desired.

6.  Security Considerations

   None, probably.

7.  IANA Considerations

   This specification registers the following entry in the HTTP/2 Error
   Code registry established by [HTTP2]:

   Name:  EXTENSION_UNSUPPORTED

   Code:  0xf10

   Description:  Extension unsupported

   Specification:  This document

   This specification registers the following entry in the HTTP/3 Error
   Code registry established by [HTTP3]:

   Name:  EXTENSION_UNSUPPORTED

   Code:  0x1f10

   Description:  Extension unsupported

   Specification:  This document

8.  References

8.1.  Normative References




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   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-29, 9 June 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-quic-http-29.txt>.

   [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>.

8.2.  Informative References

   [ALT-SVC]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.

Acknowledgments

   Patrick McManus, Mike Bishop and Ryan Hamilton shared some opinions
   on the error cases of Alt-Svc, in various channels, that have been
   incorporated into this document.

Author's Address

   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com














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