Internet DRAFT - draft-bishop-httpbis-extended-settings
draft-bishop-httpbis-extended-settings
HTTPbis M. Bishop
Internet-Draft Microsoft
Intended status: Informational November 15, 2016
Expires: May 19, 2017
HTTP/2 Extended SETTINGS Extension
draft-bishop-httpbis-extended-settings-01
Abstract
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.
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 http://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 May 19, 2017.
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Bishop Expires May 19, 2017 [Page 1]
Internet-Draft EXTENDED_SETTINGS November 2016
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. Signature Methods . . . . . . . . . . . . . . . . . . . . 6
6.2. HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting . . . . . . . . . 6
6.3. New HTTP/2 Frames . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
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.
Alternately, a number of recent and in-progress HTTP/2 extensions
describe properties of the connection that are informative to the
peer ([RFC7838], [I-D.ietf-httpbis-origin-frame]). These are
essentially settings that did not fit into a 32-bit value.
Bishop Expires May 19, 2017 [Page 2]
Internet-Draft EXTENDED_SETTINGS November 2016
Each extension could define its own SETTINGS-equivalent frame to
carry its own data, as these 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).
2. Detection of Support
An HTTP/2 peer that supports the EXTENDED_SETTINGS frame indicate
this using the HTTP/2 "SETTINGS_EXTENDED_SETTINGS" (0xSETTING-TBD)
setting.
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
3.1. EXTENDED_SETTINGS Frame
The EXTENDED_SETTINGS frame (type=0xTBD1) 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. The EXTENDED_SETTINGS frame is also used to
acknowledge the receipt of those parameters. 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
chosen. (This choice could be announced in a field of an extension
frame, or in a value in SETTINGS.)
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.
Bishop Expires May 19, 2017 [Page 3]
Internet-Draft EXTENDED_SETTINGS November 2016
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
(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 (Section 5.4.1) of type PROTOCOL_ERROR.
3.1.1. EXTENDED_SETTINGS Format
The payload of a SETTINGS frame consists of zero or more parameters,
each consisting of an unsigned 16-bit setting identifier and a
length-prefixed binary value.
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) |B| Length (15) |
+---------------------------------|-------------+---------------+
| Contents (?) ...
+---------------------------------------------------------------+
Figure 1: EXTENDED_SETTINGS frame payload
A zero-length content indicates that the setting value is a Boolean
given by the B bit. If Length is not zero, the B bit MUST be zero,
Bishop Expires May 19, 2017 [Page 4]
Internet-Draft EXTENDED_SETTINGS November 2016
and MUST be ignored by receivers. The initial value of each setting
is "false."
An implementation MUST ignore the contents for any EXTENDED_SETTINGS
identifier it does not understand.
3.2. EXTENDED_SETTINGS_ACK Frame
The EXTENDED_SETTINGS_ACK frame 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
rely on the setting 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
EXTENDED_SETTINGS.
Bishop Expires May 19, 2017 [Page 5]
Internet-Draft EXTENDED_SETTINGS November 2016
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.
The HTTP/2 "SETTINGS_EXTENDED_SETTINGS" setting is registered in
Section 6.2. Two frame types are registered in Section 6.3.
6.1. Signature Methods
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
optional.
Code: The 16-bit code assigned to the setting.
Specification: An optional reference to a specification that
describes the use of the setting.
No entries are registered by this document.
6.2. HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting
The "SETTINGS_EXTENDED_SETTINGS" setting is registered in the "HTTP/2
Settings" registry established in [RFC7540].
Name: SETTINGS_EXTENDED_SETTINGS
Code: 0xSETTING-TBD
Bishop Expires May 19, 2017 [Page 6]
Internet-Draft EXTENDED_SETTINGS November 2016
Initial Value: 0
Specification: This document.
6.3. 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
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,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
7.2. Informative References
[I-D.bishop-httpbis-http2-additional-certs]
Bishop, M. and M. Thomson, "Secondary Certificate
Authentication in HTTP/2", draft-bishop-httpbis-http2-
additional-certs-02 (work in progress), October 2016.
[I-D.ietf-httpbis-origin-frame]
Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
draft-ietf-httpbis-origin-frame-01 (work in progress),
September 2016.
Bishop Expires May 19, 2017 [Page 7]
Internet-Draft EXTENDED_SETTINGS November 2016
[MS-HTTP2E]
"Hypertext Transfer Protocol Version 2 (HTTP/2)
Extension", October 2015,
<http://download.microsoft.com/download/9/5/
E/95EF66AF-9026-4BB0-A41D-A4F81802D92C/[MS-HTTP2E].pdf>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <http://www.rfc-editor.org/info/rfc7838>.
Author's Address
Mike Bishop
Microsoft
Email: michael.bishop@microsoft.com
Bishop Expires May 19, 2017 [Page 8]
