Internet DRAFT - draft-pardue-masque-dgram-priority
draft-pardue-masque-dgram-priority
MASQUE L. Pardue
Internet-Draft Cloudflare
Intended status: Experimental 25 July 2022
Expires: 26 January 2023
HTTP Datagrams, UDP Proxying, and Extensible Prioritization
draft-pardue-masque-dgram-priority-02
Abstract
Application protocols using the QUIC transport protocol rely on
streams, and optionally the unreliable datagram extension, to carry
application data. Streams and datagrams can be multiplexed in single
connections but QUIC does not define an interoperable prioritization
scheme or signaling mechanism. The HTTP Extensible Prioritization
scheme describes an application-level scheme for the prioritization
of streams in HTTP/2 and HTTP/3. This document defines how
Extensible Priorities can be augmented to apply to the multiplexing
of HTTP datagram flows with other flows or streams.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Source code and issues list for this draft can be found at
https://github.com/LPardue/draft-pardue-masque-dgram-priority
(https://github.com/LPardue/draft-pardue-masque-dgram-priority).
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 26 January 2023.
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Copyright Notice
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Signalling Datagram Priority . . . . . . . . . . . . . . . . 4
2.1. Datagram Urgency . . . . . . . . . . . . . . . . . . . . 4
3. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 5
4. Prioritization when Proxying UDP in HTTP . . . . . . . . . . 5
4.1. The CONTEXT_PRIORITY Capsule . . . . . . . . . . . . . . 5
5. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 6
6. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 6
7. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 9
A.1. Since draft-pardue-masque-dgram-priority-01 . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Application protocols using the QUIC transport protocol [QUIC] rely
on streams, and optionally the unreliable datagram extension
[QUIC-DATAGRAM], to carry application data. Streams and datagrams
can be multiplexed in single connections but QUIC does not define an
interoperable prioritization scheme or signaling mechanism. The HTTP
Extensible Prioritization scheme [PRIORITY] describes an application-
level scheme for the prioritization of streams in HTTP/2 and HTTP/3.
This document defines how Extensible Priorities can be applied to the
multiplexing of HTTP datagram [HTTP-DATAGRAM] flows with other flows
or streams.
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The Extensible Priorities scheme for HTTP describes how clients can
send priority signals related to requests in order to suggest how a
server allocates resources to serving responses. When the protocol
is HTTP/2, responses are carried on streams. When the protocol is
HTTP/3, responses are carries on QUIC streams.
While QUIC streams support multiplexing natively via use of a stream
identifier, the unreliable datagram extension does not provide any
such multiplexing identifier.
HTTP datagrams ([HTTP-DATAGRAM]) defines how multiplexed, potentially
unreliable datagrams can be sent inside an HTTP connection. All
datagrams are always associated with a request stream. In HTTP/3,
HTTP datagrams can map directly to QUIC datagrams, in which case they
carry a Quarter Stream ID - an encoding of the request stream ID -
that is used to demultiplex at the receiver; see Section 3.1 of
[HTTP-DATAGRAM]. [HTTP-DATAGRAM] also defines the DATAGRAM capsule,
which can be used for reliable delivery over all versions of HTTP;
see Section 3.5 of [HTTP-DATAGRAM]. In all cases, the prioritization
of datagrams is noted as unspecified and delegated to future
extensions.
This document describes how the Extensible Priorities scheme can be
augmented to also apply to HTTP datagrams that are multiplexed with
other flows or streams. It enhances the Priority signals sent by
clients, with a new datagram-urgency (du) parameter (Section 2.1) and
explains how this input is to be considered in server scheduling
decisions for HTTP datagrams mapped to QUIC datagrams; see Section 6.
When HTTP datagrams are used for proxying UDP, additional use cases
extending beyond UDP data transfer are supported by the use of
context IDs; see Section 4 of [HTTP-UDP-PROXY]. The CONTEXT_PRIORITY
capsule type can be used to signal the datagram-priority of
individual contexts; see Section 4.1.
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.
The term Integer is imported from [STRUCTURED-FIELDS].
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2. Signalling Datagram Priority
The Extensible Prioritization scheme [PRIORITY] describes how clients
can send priority signals related to requests. Signals are a set of
parameters, encoded using [STRUCTURED-FIELDS].
[PRIORITY] defines the urgency and incremental parameters and
provides guidance about how implementers can act on these parameters,
in combination with other inputs, to make resource allocation and
scheduling choices. Urgency communicates the client-view of request
importance, and incremental communicates how the client intends to
process response data as it arrives. Parameters are communicated in
HTTP headers or version-specific frames. A client omitting the
urgency or incremental parameters can be interpreted by the server as
a signal to apply default priorities. The core scheme is extensible,
new parameters can be defined to augment the base ones.
This specification defines the datagram-urgency (du) extension
parameter that operates in addition to the base urgency. There is no
extension to the base incremental behavior; individual datagrams,
even if belonging to the same identifier, are messages that are
expected to be processed individually as they arrive.
2.1. Datagram Urgency
The datagram-urgency (du) parameter is Integer (see Section 3.3.1 of
[STRUCTURED-FIELDS]), between 0 and 7, in descending order of
priority. This range matches the base urgency (u) parameter range;
see Section 4.1 of [PRIORITY]. However, there is no default value.
This parameter indicates the sender's recommendation, based on the
expectation that the server would transmit HTTP datagrams in the
order of their datagram-urgency values if possible. The smaller the
value, the higher the precedence. Omitting the datagram-urgency
parameter is a signal to apply the value of the urgency parameter.
The following example shows a request for a CSS file with the urgency
set to 0, any associated datagrams have the lower datagram-urgency of
2:
:method = GET
:scheme = https
:authority = example.net
:path = /style.css
priority = u=0, du=2
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Note that when the urgency parameter is omitted, it's default value
of 3 is applied. In the following example, the priority field is
omitted entirely, invoking the default behaviour of urgency and
datagram-urgency, causing them to both have the implicit value 3:
:method = GET
:scheme = https
:authority = example.net
:path = /style.css
Endpoints MUST NOT treat reception of the datagram-urgency parameter
as an error, even if HTTP datagram support is not enabled.
The datagram-urgency parameter applies only to HTTP datagrams mapped
to QUIC datagrams. Datagram capsules are sent on streams, so the
base urgency parameter applies to them.
3. Reprioritization
Reprioritization behaves similarly to existing mechanisms defined in
Section 6 of [PRIORITY]. CONTEXT_PRIORITY frames can be sent by
clients to provide updated priority signals after the initial request
has been sent.
4. Prioritization when Proxying UDP in HTTP
[HTTP-UDP-PROXY] describes how to proxy UDP using HTTP datagrams.
Client make UDP proxying requests using Extended CONNECT, which
initiates a UDP tunnel. HTTP datagrams related to this stream
correspond to the UDP tunnel by default. In order support extension
use cases, Section 4 of [HTTP-UDP-PROXY] defines context IDs, that
are sent within datagrams, in addition to the Quarter Stream ID. UDP
payloads use context ID 0, forms of data use other IDs.
Datagram priority applies to UDP proxying requests, as described in
Section 2.1. By default the same datagram-urgency applies to all
HTTP datagram contexts related to the request stream.
4.1. The CONTEXT_PRIORITY Capsule
There might be cases where it is beneficial to prioritize individual
contexts differently from one another. This document defines the
CONTEXT_PRIORITY (TBD) capsule type to carry a priority signal
related to individual contexts.
Once a UDP proxy request converts to the capsule protocol (see
Section 3 of [HTTP-UDP-PROXY], clients can send CONTEXT_PRIORITY
capsules to signal the priority of the identified context.
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A CONTEXT_PRIORITY capsule communicates a complete set of all
priority parameters in the Priority Field Value field. Omitting a
priority parameter is a signal to derive a value from defaults; see
Section 2.1. Failure to parse the Priority Field Value MAY be
treated as a connection error. In HTTP/2, the error is of type
PROTOCOL_ERROR; in HTTP/3, the error is of type
H3_GENERAL_PROTOCOL_ERROR.
TODO: describe what happens if capsules arrive before contexts
exists. Buffer? Drop?
TODO: consider if servers could send this capsule type
Context Priority Capsule {
Type (i) = CONTEXT_PRIORITY,
Length (i),
Context ID (i),
Priority Field Value (..),
}
Figure 1: CONTEXT_PRIORITY Capsule Format
The CONTEXT_PRIORITY capsule has the following fields:
Context ID: The context ID that is the target of the priority
update.
Priority Field Value: The priority update value in ASCII text,
encoded using Structured Fields; see [PRIORITY].
5. Client Scheduling
A client MAY use datagram-urgency to make local processing or
scheduling choices about HTTP datagrams related to the requests it
initiates.
6. Server Scheduling
Priority signals are input to a prioritization process. Expressing
priority is only a suggestion. The datagram-urgency parameter
introduces new scheduling considerations on top of those presented in
Section 10 of [PRIORITY].
It is RECOMMENDED that, when possible, servers respect the datagram-
urgency parameter, sending higher-urgency HTTP datagrams before
lower-urgency datagrams.
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Where streams and datagrams have equal urgency and datagram-urgency
respectively, a server needs to decide how to divide the available
sending capacity between stream and datagram data. Strict or static
preference for one type of data over another (e.g., datagrams first,
then streams) could lead to suboptimal results at the client,
depending on the nature of the data. This is a form of starvation,
as defined in Section 10 of [PRIORITY]. It applies whether the
streams are incremental or not.
Similarly, if datagrams are used for HTTP proxying and there are
multiple context IDs in use for different purposes, those purposes
might interfere or starve each other if they have the equal datagram-
urgency.
It is RECOMMENDED that servers avoid such starvation where possible.
The method for doing so is an implementation decision. One approach
is to divide the available bandwidth between stream and datagram data
in some fixed or dynamic ratio. For instance, a server could choose
to generate two classes of application data QUIC packets: STREAM-
frame-only packets and DATAGRAM-only-frame packets. The server can
control the capacity ratio split by managing the frequency of the
packet classes. A simple alternating strategy would result in a
roughly 50/50 split, while other frequencies would produce different
ratios.
When HTTP datagrams are carried in DATAGRAM capsules. It is
RECOMMENDED that servers schedule the capsules in the manner expected
for response data; see Section 10 of [PRIORITY].
7. Retransmission Scheduling
Section 12 of [PRIORITY] provides guidance about scheduling of
retransmission data vs. new data. Since QUIC datagrams are not
retransmitted, endpoints that prioritize QUIC stream retransmission
data could delay datagrams. Furthermore, since DATAGRAM capsules are
sent as stream data, they *are* subject to retransmission and could
also delay native QUIC datagrams.
8. Security Considerations
There are believed to be no additional considerations to those
presented in [PRIORITY].
9. IANA Considerations
This specification registers the following entry in the HTTP Priority
Parameters Registry
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Name: du
Description: The urgency of HTTP datagrams associated with a
response.
Reference: This document
10. References
10.1. Normative References
[HTTP-DATAGRAM]
Schinazi, D. and L. Pardue, "HTTP Datagrams and the
Capsule Protocol", Work in Progress, Internet-Draft,
draft-ietf-masque-h3-datagram-11, 17 June 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-masque-
h3-datagram-11>.
[HTTP-UDP-PROXY]
Schinazi, D., "Proxying UDP in HTTP", Work in Progress,
Internet-Draft, draft-ietf-masque-connect-udp-15, 17 June
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
masque-connect-udp-15>.
[PRIORITY] Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,
<https://www.rfc-editor.org/rfc/rfc9218>.
[QUIC-DATAGRAM]
Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", RFC 9221,
DOI 10.17487/RFC9221, March 2022,
<https://www.rfc-editor.org/rfc/rfc9221>.
[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/rfc/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/rfc/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
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10.2. Informative References
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
Appendix A. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
A.1. Since draft-pardue-masque-dgram-priority-01
* Realign with Extensible Priorities RFC
* Realign with latest HTTP datagram and capsule protocol draft
* Add CONTEXT_PRIORITY capsule for prioritizing individual contexts
* Better explain that competing stream and datagram flows should
share bandwidth but refrain from requiring any specific ratios.
Acknowledgements
This document is inspired by discussion by many people across HTTP,
QUIC and MASQUE WGs.
Author's Address
Lucas Pardue
Cloudflare
Email: lucaspardue.24.7@gmail.com
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