HTTP/2 Extended SETTINGS Extension

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HTTP                                                           M. Bishop
Internet-Draft                                                 Microsoft
Intended status: Standards Track                           June 13, 2016
Expires: December 15, 2016

                   HTTP/2 Extended SETTINGS Extension


   HTTP/2 defines the SETTINGS frame to contain a single 32-bit value
   per setting.  While this is sufficient to convey everything used in
   the core HTTP/2 specification, some protocols will require more
   complex values, such as arrays of code-points or strings.

   For such protocols, this extension defines a parallel to the SETTINGS
   frame, EXTENDED_SETTINGS, where the value of a setting is not a
   32-bit value, but a variable-length opaque data blob whose
   interpretation is subject entirely to the definition of the protocol
   using it.

Status of This Memo

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

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   This Internet-Draft will expire on December 15, 2016.

Copyright Notice

   Copyright (c) 2016 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

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   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.  Detection of Support  . . . . . . . . . . . . . . . . . . . .   3
   3.  Extension Frame Types . . . . . . . . . . . . . . . . . . . .   3
     3.1.  EXTENDED_SETTINGS Frame . . . . . . . . . . . . . . . . .   3
       3.1.1.  EXTENDED_SETTINGS Format  . . . . . . . . . . . . . .   4
     3.2.  EXTENDED_SETTINGS_ACK Frame . . . . . . . . . . . . . . .   5
   4.  Settings Synchronization  . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Extended Settings Identifiers . . . . . . . . . . . . . .   6
     6.2.  New HTTP/2 Frames . . . . . . . . . . . . . . . . . . . .   7
     6.3.  HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting  . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In [I-D.bishop-httpbis-http2-additional-certs], values for which IANA
   registries already exist must be communicated between two HTTP/2
   implementations.  Since the SETTINGS frame constrains setting values
   to a 32-bit value, the existing version of that draft divides the
   32-bit value into halves and dedicates bits to each currently-known
   value.  This requires the creation of two duplicative IANA
   registries, and enormously constrains future extensibility since each
   future supported value will consume one of only sixteen bits.  It
   also causes divergence from other places in the protocol where a
   bitmask is not required and a more sensible value can be used.

   [MS-HTTP2E], likewise, defines a very limited bitmap in the 32-bit
   value - two bits are defined, all others are reserved (and not
   useful).  The setting fits easily in a single byte, and need not
   consume a four-byte value every time it is transferred.  With minor
   changes to semantics, a simple flag would have been sufficient - the
   primary requirement is communication to the remote endpoint that an
   extension is supported.

   Alternately, a number of recent and in-progress HTTP/2 extensions
   describe properties of the connection that are informative to the

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   peer ([RFC7838], [I-D.ietf-httpbis-origin-frame]).  These can be
   viewed as connection properties that do not fit into a 32-bit value
   and therefore have new frame types defined to carry them instead.

   Each extension could define its own SETTINGS-equivalent frame to
   carry this data, as some extensions already have, but to do so every
   time a new extension might require such a capability seems similarly
   wasteful, given the limited frame type space (also an IANA registry).

   Instead, this draft proposes a unified EXTENDED_SETTINGS frame (and
   accompanying settings synchronization logic) which permits values of
   lengths other than 32 bits.  These values are interpreted as defined
   by the specification defining them, operate under similar semantics
   to the original SETTINGS frame, and should make development of future
   extensions much simpler.

2.  Detection of Support

   An HTTP/2 peer that supports the EXTENDED_SETTINGS frame indicates

   The initial value for the "SETTINGS_EXTENDED_SETTINGS" setting is 0
   (0x00), indicating that the peer does not support the
   EXTENDED_SETTINGS frame.  A peer that is able to parse the
   EXTENDED_SETTINGS frame MUST set this value to 1 (0x01).

   This setting MUST be sent before any of the frame types in Section 3
   are sent, but those frames MAY be sent before the setting is
   acknowledged and MAY be sent regardless of whether the peer has sent
   this setting.

3.  Extension Frame Types


   The EXTENDED_SETTINGS frame (type=0xFRAME-TBD1) conveys configuration
   parameters that affect how endpoints communicate, such as preferences
   and constraints on peer behavior which occur in a form other than a
   32-bit value.  Individually, an EXTENDED_SETTINGS parameter can also
   be referred to as a "setting".

   EXTENDED_SETTINGS parameters are not negotiated; they describe
   characteristics of the sending peer, which are used by the receiving
   peer.  However, a negotiation can be implied by the use of
   EXTENDED_SETTINGS - a peer uses EXTENDED_SETTINGS to advertise a set
   of supported values.  The recipient can then choose which entries
   from this list are also acceptable and proceed with the value it has

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   chosen.  (This choice could be announced in a field of an extension
   frame, or in a different setting.)

   Different values for the same parameter can be advertised by each
   peer.  For example, a server might support many different signing
   algorithms, while a resource constrained client has only one or two
   that it can validate.

   An EXTENDED_SETTINGS frame MAY be sent at any time by either endpoint
   over the lifetime of the connection.

   Each parameter in an EXTENDED_SETTINGS frame replaces any existing
   value for that parameter.  Parameters are processed in the order in
   which they appear, and a receiver of an EXTENDED_SETTINGS frame does
   not need to maintain any state other than the current value of its
   parameters.  Therefore, the value of a EXTENDED_SETTINGS parameter is
   the last value that is seen by a receiver.

   EXTENDED_SETTINGS parameters can request acknowledgement by the
   receiving peer.  To enable this, the EXTENDED_SETTINGS frame defines
   the following flag:

   REQUEST_ACK (0x1):  When set, bit 0 indicates that this frame
      contains values which the sender wants to know were understood and
      applied.  For more information, see Section 4.

   Like SETTINGS frames, EXTENDED_SETTINGS frames always apply to a
   connection, never a single stream.  The stream identifier for an
   EXTENDED_SETTINGS frame MUST be zero (0x0).  If an endpoint receives
   an EXTENDED_SETTINGS frame whose stream identifier field is anything
   other than 0x0, the endpoint MUST respond with a connection error
   ([RFC7540], section 5.4.1) of type PROTOCOL_ERROR.

   The EXTENDED_SETTINGS frame affects connection state.  A badly formed
   or incomplete EXTENDED_SETTINGS frame MUST be treated as a connection
   error ([RFC7540], section 5.4.1) of type PROTOCOL_ERROR.


   The payload of an EXTENDED_SETTINGS frame consists of zero or more
   parameters, each consisting of an unsigned 16-bit setting identifier
   and a length-prefixed binary value.

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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |        Identifier (16)        |         Length (16)           |
   |                          Contents (?)                       ...

                 Figure 1: EXTENDED_SETTINGS frame payload

   In some cases (e.g. indications of feature support), the presence and
   acknowledgement of a setting may be sufficient, and a value
   superfluous.  In order to accomodate such cases, implementations MUST
   track identifiers with zero-length values differently from never-seen
   identifiers.  The initial value of each setting is "never-seen."

   An implementation MUST ignore the contents for any EXTENDED_SETTINGS
   identifier it does not understand.


   The EXTENDED_SETTINGS_ACK frame (type=0xFRAME-TBD2) acknowledges
   receipt and application of specific values in the peer's SETTINGS
   frame.  It contains a list of EXTENDED_SETTINGS identifiers which the
   sender has understood and applied.  This list MAY be empty.

   Any EXTENDED_SETTINGS_ACK frame whose length is not a multiple of two
   bytes MUST be treated as a connection error ([RFC7540] section 5.4.1)
   of type "FRAME_SIZE_ERROR".

4.  Settings Synchronization

   Some values in EXTENDED_SETTINGS benefit from or require an
   understanding of when the peer has received and applied the changed
   parameter values.  In order to provide such synchronization
   timepoints, the recipient of a EXTENDED_SETTINGS frame MUST apply the
   updated parameters as soon as possible upon receipt.  The values in
   the EXTENDED_SETTINGS frame MUST be processed in the order they
   appear, with no other frame processing between values.  Unsupported
   parameters MUST be ignored.

   Once all values have been processed, if the REQUEST_ACK flag was set,
   the recipient MUST immediately emit a EXTENDED_SETTINGS_ACK frame
   listing the identifiers whose values were understood and applied.
   (If none of the values were understood, the EXTENDED_SETTINGS_ACK
   frame will be empty, but MUST still be sent.)  Upon receiving an
   EXTENDED_SETTINGS_ACK frame, the sender of the altered parameters can

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   know which settings were understood and can rely on that subset
   having been applied.

   If the sender of an EXTENDED_SETTINGS frame with the "REQUEST_ACK"
   flag set does not receive an acknowledgement from a peer that has
   sent the "SETTINGS_EXTENDED_SETTINGS" setting within a reasonable
   amount of time, it MAY issue a connection error ([RFC7540]
   Section 5.4.1) of type SETTINGS_TIMEOUT.  This error MUST NOT be sent
   if the peer has not previously advertised support for

5.  Security Considerations

   Because these frames can be used to request that peers retain
   potentially-large state, implementations need to use caution in their
   retention policies.  Values which are not understood MUST be
   discarded in order to protect against increased memory usage.
   Specifications which make use of EXTENDED_SETTINGS MUST include
   details about how the contents can be parsed and stored, and SHOULD
   include details about how the information can be compressed and when
   it can safely be discarded.

6.  IANA Considerations

   This draft establishes one new registry and add three entries across
   two existing registries.

   A registry for Extended Settings identifiers is created in
   Section 6.1.  Two frame types are registered in Section 6.2.  The
   HTTP/2 "SETTINGS_EXTENDED_SETTINGS" setting is registered in
   Section 6.3.

6.1.  Extended Settings Identifiers

   This document establishes a registry for HTTP/2 extended settings.
   The "HTTP/2 Extended Settings" registry manages a 16-bit space.  The
   "HTTP/2 Extended Settings" registry operates under the "Expert
   Review" policy [RFC5226] for values in the range from 0x0000 to
   0xefff, with values between and 0xf000 and 0xffff being reserved for
   Experimental Use.

   New registrations are advised to provide the following information:

   Name:  A symbolic name for the setting.  Specifying a setting name is

   Code:  The 16-bit code assigned to the setting.

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   Specification: An optional reference to a specification that
   describes the use of the setting.

   No entries are registered by this document.

6.2.  New HTTP/2 Frames

   Two new frame types are registered in the "HTTP/2 Frame Types"
   registry established in [RFC7540].  The entries in the following
   table are registered by this document.

   | Frame Type            | Code         | Specification           |
   | EXTENDED_SETTINGS     | 0xFRAME-TBD1 | {{settings-frame}}      |
   | EXTENDED_SETTINGS_ACK | 0xFRAME-TBD2 | {{ack}}                 |

                                 Figure 2


   The "SETTINGS_EXTENDED_SETTINGS" setting is registered in the "HTTP/2
   Settings" registry established in [RFC7540].


   Code:  0xSETTING-TBD

   Initial Value:  0

   Specification:  This document.

7.  References

7.1.  Normative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

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

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7.2.  Informative References

              Bishop, M. and M. Thomson, "Secondary Certificate
              Authentication in HTTP/2", draft-bishop-httpbis-http2-
              additional-certs-01 (work in progress), May 2016.

              Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              draft-ietf-httpbis-origin-frame-00 (work in progress), May

              Microsoft Corporation, "Hypertext Transfer Protocol
              Version 2 (HTTP/2) Extension", October 2015,

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

Author's Address

   Mike Bishop


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