--- 1/draft-ietf-httpbis-priority-10.txt 2021-12-08 18:13:27.341872849 -0800 +++ 2/draft-ietf-httpbis-priority-11.txt 2021-12-08 18:13:27.401874356 -0800 @@ -1,63 +1,63 @@ HTTP K. Oku Internet-Draft Fastly Intended status: Standards Track L. Pardue -Expires: 23 May 2022 Cloudflare - 19 November 2021 +Expires: 12 June 2022 Cloudflare + 9 December 2021 Extensible Prioritization Scheme for HTTP - draft-ietf-httpbis-priority-10 + draft-ietf-httpbis-priority-11 Abstract This document describes a scheme that allows an HTTP client to communicate its preferences for how the upstream server prioritizes responses to its requests, and also allows a server to hint to a downstream intermediary how its responses should be prioritized when they are forwarded. This document defines the Priority header field for communicating the initial priority in an HTTP version-independent manner, as well as HTTP/2 and HTTP/3 frames for reprioritizing responses. These share a common format structure that is designed to provide future extensibility. -Note to Readers +About This Document - _RFC EDITOR: please remove this section before publication_ + This note is to be removed before publishing as an RFC. - Discussion of this draft takes place on the HTTP working group - mailing list (ietf-http-wg@w3.org), which is archived at - https://lists.w3.org/Archives/Public/ietf-http-wg/ - (https://lists.w3.org/Archives/Public/ietf-http-wg/). + Status information for this document may be found at + https://datatracker.ietf.org/doc/draft-ietf-httpbis-priority/. - Working Group information can be found at https://httpwg.org/ - (https://httpwg.org/); source code and issues list for this draft can - be found at https://github.com/httpwg/http-extensions/labels/ - priorities (https://github.com/httpwg/http-extensions/labels/ - priorities). + Discussion of this document takes place on the HTTP Working Group + mailing list (mailto:ietf-http-wg@w3.org), which is archived at + https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group + information can be found at https://httpwg.org/. + + Source for this draft and an issue tracker can be found at + https://github.com/httpwg/http-extensions/labels/priorities. 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 23 May 2022. + This Internet-Draft will expire on 12 June 2022. Copyright Notice Copyright (c) 2021 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 @@ -68,64 +68,65 @@ Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 5 2. Motivation for Replacing RFC 7540 Priorities . . . . . . . . 5 2.1. Disabling RFC 7540 Priorities . . . . . . . . . . . . . . 6 2.1.1. Advice when Using Extensible Priorities as the Alternative . . . . . . . . . . . . . . . . . . . . . 7 3. Applicability of the Extensible Priority Scheme . . . . . . . 7 - 4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 7 + 4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 8 4.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 9 - 4.3. Defining New Parameters . . . . . . . . . . . . . . . . . 10 + 4.3. Defining New Priority Parameters . . . . . . . . . . . . 10 4.3.1. Registration . . . . . . . . . . . . . . . . . . . . 10 5. The Priority HTTP Header Field . . . . . . . . . . . . . . . 11 6. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 12 7. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 12 7.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 13 7.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 14 - 8. Merging Client- and Server-Driven Parameters . . . . . . . . 15 - 9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 16 + 8. Merging Client- and Server-Driven Priority Parameters . . . . 16 + 9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 17 10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 17 - 10.1. Intermediaries with Multiple Backend Connections . . . . 18 + 10.1. Intermediaries with Multiple Backend Connections . . . . 19 11. Scheduling and the CONNECT Method . . . . . . . . . . . . . . 19 12. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 19 13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 20 13.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 20 13.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 21 13.3. Intentional Introduction of Unfairness . . . . . . . . . 21 14. Why use an End-to-End Header Field? . . . . . . . . . . . . . 21 15. Security Considerations . . . . . . . . . . . . . . . . . . . 22 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 17.1. Normative References . . . . . . . . . . . . . . . . . . 23 17.2. Informative References . . . . . . . . . . . . . . . . . 24 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25 - Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 25 - B.1. Since draft-ietf-httpbis-priority-09 . . . . . . . . . . 25 - B.2. Since draft-ietf-httpbis-priority-08 . . . . . . . . . . 25 - B.3. Since draft-ietf-httpbis-priority-07 . . . . . . . . . . 26 - B.4. Since draft-ietf-httpbis-priority-06 . . . . . . . . . . 26 - B.5. Since draft-ietf-httpbis-priority-05 . . . . . . . . . . 26 - B.6. Since draft-ietf-httpbis-priority-04 . . . . . . . . . . 26 - B.7. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 26 - B.8. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 27 - B.9. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 27 - B.10. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 27 - B.11. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 27 - B.12. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 28 - B.13. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 28 - B.14. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 28 - B.15. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 28 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 + Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 26 + B.1. Since draft-ietf-httpbis-priority-10 . . . . . . . . . . 26 + B.2. Since draft-ietf-httpbis-priority-09 . . . . . . . . . . 26 + B.3. Since draft-ietf-httpbis-priority-08 . . . . . . . . . . 26 + B.4. Since draft-ietf-httpbis-priority-07 . . . . . . . . . . 26 + B.5. Since draft-ietf-httpbis-priority-06 . . . . . . . . . . 26 + B.6. Since draft-ietf-httpbis-priority-05 . . . . . . . . . . 27 + B.7. Since draft-ietf-httpbis-priority-04 . . . . . . . . . . 27 + B.8. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 27 + B.9. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 27 + B.10. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 27 + B.11. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 28 + B.12. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 28 + B.13. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 28 + B.14. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 28 + B.15. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 29 + B.16. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 29 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 1. Introduction It is common for representations of an HTTP [HTTP] resource to have relationships to one or more other resources. Clients will often discover these relationships while processing a retrieved representation, which may lead to further retrieval requests. Meanwhile, the nature of the relationship determines whether the client is blocked from continuing to process locally available resources. An example of this is visual rendering of an HTML @@ -154,77 +155,76 @@ RFC 7540 [RFC7540] stream priority allowed a client to send a series of priority signals that communicate to the server a "priority tree"; the structure of this tree represents the client's preferred relative ordering and weighted distribution of the bandwidth among HTTP responses. Servers could use these priority signals as input into prioritization decision making. The design and implementation of RFC 7540 stream priority was observed to have shortcomings, explained in Section 2. HTTP/2 [HTTP2] has consequently deprecated the use of these stream priority - signals. + signals. The prioritization scheme and priority signals defined + herein can act as a substitute for RFC 7540 stream priority. This document describes an extensible scheme for prioritizing HTTP responses that uses absolute values. Section 4 defines priority parameters, which are a standardized and extensible format of priority information. Section 5 defines the Priority HTTP header field, a protocol-version-independent and end-to-end priority signal. - Clients can use this header to signal priority to servers in order to - specify the precedence of HTTP responses. Similarly, servers behind - an intermediary can use it to signal priority to the intermediary. - Section 7.1 and Section 7.2 define version-specific frames that carry - parameters, which clients can use for reprioritization. + Clients can send this header field to signal their view of how + responses should be prioritized. Similarly, servers behind an + intermediary can use it to signal priority to the intermediary. + After sending a request, a client can change the priority of the + response (see Section 6) using HTTP-version-specific frames defined + in Section 7.1 and Section 7.2. Header field and frame priority signals are input to a server's response prioritization process. They are only a suggestion and do not guarantee any particular processing or transmission order for one response relative to any other response. Section 10 and Section 12 provide consideration and guidance about how servers might act upon signals. - The prioritization scheme and priority signals defined herein can act - as a substitute for RFC 7540 stream priority. - 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 terms Dictionary, sf-boolean, sf-dictionary, and sf-integer are imported from [STRUCTURED-FIELDS]. Example HTTP requests and responses use the HTTP/2-style formatting from [HTTP2]. This document uses the variable-length integer encoding from [QUIC]. - The term control stream is used to describe the HTTP/2 stream with - identifier 0x0, and HTTP/3 control stream; see Section 6.2.1 of - [HTTP3]. + The term control stream is used to describe both the HTTP/2 stream + with identifier 0x0, and the HTTP/3 control stream; see Section 6.2.1 + of [HTTP3]. The term HTTP/2 priority signal is used to describe the priority information sent from clients to servers in HTTP/2 frames; see Section 5.3.2 of [HTTP2]. 2. Motivation for Replacing RFC 7540 Priorities RFC 7540 stream priority (see Section 5.3 of [RFC7540]) is a complex system where clients signal stream dependencies and weights to describe an unbalanced tree. It suffered from limited deployment and interoperability and was deprecated in a revision of HTTP/2 [HTTP2]. HTTP/2 retains these protocol elements in order to maintain wire compatibility (see Section 5.3.2 of [HTTP2]), which means that they - might still be used in the absence of alternative signaling, such as - the scheme this document describes. + might still be used even in the presence of alternative signaling, + such as the scheme this document describes. Many RFC 7540 server implementations do not act on HTTP/2 priority signals. Prioritization can use information that servers have about resources or the order in which requests are generated. For example, a server, with knowledge of an HTML document structure, might want to prioritize the delivery of images that are critical to user experience above other images. With RFC 7540 it is difficult for servers to interpret signals from clients for prioritization as the @@ -235,24 +235,24 @@ RFC 7540 does not define a method that can be used by a server to provide a priority signal for intermediaries. RFC 7540 priority is expressed relative to other requests on the same connection. Many requests are generated without knowledge of how other requests might share a connection, which makes this difficult to use reliably, especially in protocols that do not have strong ordering guarantees, like HTTP/3 [HTTP3]. - Multiple experiments from independent research have shown that - simpler schemes can reach at least equivalent performance - characteristics compared to the more complex RFC 7540 setups seen in - practice, at least for the web use case. + Multiple experiments from independent research ([MARX], [MEENAN]) + have shown that simpler schemes can reach at least equivalent + performance characteristics compared to the more complex RFC 7540 + setups seen in practice, at least for the web use case. 2.1. Disabling RFC 7540 Priorities The problems and insights set out above provided the motivation for an alternative to RFC 7540 stream priority (see Section 5.3 of [HTTP2]). The SETTINGS_NO_RFC7540_PRIORITIES HTTP/2 setting is defined by this document in order to allow endpoints to omit or ignore HTTP/2 priority signals (see Section 5.3.2 of [HTTP2]), as described below. @@ -300,63 +300,79 @@ Similarly, if the client receives SETTINGS_NO_RFC7540_PRIORITIES with value of 0 or if the settings parameter was absent, it SHOULD stop sending PRIORITY_UPDATE frames (Section 7.1), since those frames are likely to be ignored. However, the client MAY continue sending the Priority header field (Section 5), as it is an end-to-end signal that might be useful to nodes behind the server that the client is directly connected to. 3. Applicability of the Extensible Priority Scheme - The priority scheme defined by this document considers only the - prioritization of HTTP messages and tunnels, see Section 9, - Section 10, and Section 11. + The priority scheme defined by this document is primarily focused on + the prioritization of HTTP response messages (see Section 3.4 of + [HTTP]). It defines new priority parameters (Section 4) and their + conveyors (Section 5 and Section 7) intended to communicate the + priority of responses to a server that is responsible for + prioritizing them. Section 10 provides considerations for servers + about acting on those signals in combination with other inputs and + factors. - Where HTTP extensions change stream behavior or define new data - carriage mechanisms, they can also define how this priority scheme - can be applied. + The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to + establish tunnels. Signaling applies similarly to tunnels; + additional considerations for server prioritization are given in + Section 11. + + Section 9 describes how clients can optionally apply elements of this + scheme locally to the request messages that they generate. + + Some forms of HTTP extensions might change HTTP/2 or HTTP/3 stream + behavior or define new data carriage mechanisms. Such extensions can + define themselves how this priority scheme is to be applied. 4. Priority Parameters The priority information is a sequence of key-value pairs, providing room for future extensions. Each key-value pair represents a priority parameter. The Priority HTTP header field (Section 5) is an end-to-end way to - transmit this set of parameters when a request or a response is - issued. In order to reprioritize a request, HTTP-version-specific - PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) are used by - clients to transmit the same information on a single hop. + transmit this set of priority parameters when a request or a response + is issued. In order to reprioritize a request (Section 6), HTTP- + version-specific PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) + are used by clients to transmit the same information on a single hop. Intermediaries can consume and produce priority signals in a PRIORITY_UPDATE frame or Priority header field. Sending a PRIORITY_UPDATE frame preserves the signal from the client, but provides a signal that overrides that for the next hop; see Section 14. Replacing or adding a Priority header field overrides any signal from a client and can affect prioritization for all subsequent recipients. For both the Priority header field and the PRIORITY_UPDATE frame, the set of priority parameters is encoded as a Structured Fields Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]). - This document defines the urgency(u) and incremental(i) parameters. - When receiving an HTTP request that does not carry these priority - parameters, a server SHOULD act as if their default values were - specified. Note that handling of omitted parameters is different - when processing an HTTP response; see Section 8. + This document defines the urgency(u) and incremental(i) priority + parameters. When receiving an HTTP request that does not carry these + priority parameters, a server SHOULD act as if their default values + were specified. + + An intermediary can combine signals from requests and responses that + it forwards. Note that omission of priority parameters in responses + is handled differently from omission in requests; see Section 8. Receivers parse the Dictionary as defined in Section 4.2 of [STRUCTURED-FIELDS]. Where the Dictionary is successfully parsed, this document places the additional requirement that unknown priority - parameters, parameters with out-of-range values, or values of - unexpected types MUST be ignored. + parameters, priority parameters with out-of-range values, or values + of unexpected types MUST be ignored. 4.1. Urgency The urgency parameter (u) takes an integer between 0 and 7, in descending order of priority. The value is encoded as an sf-integer. The default value is 3. Endpoints use this parameter to communicate their view of the precedence of HTTP responses. The chosen value of urgency can be @@ -407,107 +423,107 @@ The following example shows a request for a JPEG file with the urgency parameter set to 5 and the incremental parameter set to true. :method = GET :scheme = https :authority = example.net :path = /image.jpg priority = u=5, i -4.3. Defining New Parameters +4.3. Defining New Priority Parameters - When attempting to define new parameters, care must be taken so that - they do not adversely interfere with prioritization performed by - existing endpoints or intermediaries that do not understand the newly - defined parameter. Since unknown parameters are ignored, new - parameters should not change the interpretation of, or modify, the - urgency (see Section 4.1) or incremental (see Section 4.2) parameters - in a way that is not backwards compatible or fallback safe. + When attempting to define new priority parameters, care must be taken + so that they do not adversely interfere with prioritization performed + by existing endpoints or intermediaries that do not understand the + newly defined priority parameter. Since unknown priority parameters + are ignored, new priority parameters should not change the + interpretation of, or modify, the urgency (see Section 4.1) or + incremental (see Section 4.2) priority parameters in a way that is + not backwards compatible or fallback safe. For example, if there is a need to provide more granularity than eight urgency levels, it would be possible to subdivide the range - using an additional parameter. Implementations that do not recognize - the parameter can safely continue to use the less granular eight - levels. + using an additional priority parameter. Implementations that do not + recognize the parameter can safely continue to use the less granular + eight levels. Alternatively, the urgency can be augmented. For example, a - graphical user agent could send a visible parameter to indicate if - the resource being requested is within the viewport. + graphical user agent could send a visible priority parameter to + indicate if the resource being requested is within the viewport. - Generic parameters are preferred over vendor-specific, application- - specific or deployment-specific values. If a generic value cannot be - agreed upon in the community, the parameter's name should be - correspondingly specific (e.g., with a prefix that identifies the - vendor, application or deployment). + Generic priority parameters are preferred over vendor-specific, + application-specific or deployment-specific values. If a generic + value cannot be agreed upon in the community, the parameter's name + should be correspondingly specific (e.g., with a prefix that + identifies the vendor, application or deployment). 4.3.1. Registration - New Priority parameters can be defined by registering them in the + New priority parameters can be defined by registering them in the HTTP Priority Parameters Registry. The registry governs the keys - (short textual strings) used in Structured Fields Dictionary (see + (short textual strings) used in the Structured Fields Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]). Since each HTTP request can have associated priority signals, there is value in having short key lengths, especially single-character strings. In order to encourage extension while avoiding unintended conflict among attractive key values, the HTTP Priority Parameters Registry operates two registration policies depending on key length. - * Registration requests for parameters with a key length of one use - the Specification Required policy, as per Section 4.6 of + * Registration requests for priority parameters with a key length of + one use the Specification Required policy, as per Section 4.6 of [RFC8126]. - * Registration requests for parameters with a key length greater - than one use the Expert Review policy, as per Section 4.5 of - [RFC8126]. A specification document is appreciated, but not + * Registration requests for priority parameters with a key length + greater than one use the Expert Review policy, as per Section 4.5 + of [RFC8126]. A specification document is appreciated, but not required. When reviewing registration requests, the designated expert(s) can consider the additional guidance provided in Section 4.3 but cannot use it as a basis for rejection. Registration requests should use the following template: Name: [a name for the Priority Parameter that matches key] - Description: [a description of the parameter semantics and value] + Description: [a description of the priority parameter semantics and + value] - Reference: [to a specification defining this parameter] + Reference: [to a specification defining this priority parameter] See the registry at https://iana.org/assignments/http-priority (https://iana.org/assignments/http-priority) for details on where to send registration requests. 5. The Priority HTTP Header Field - The Priority HTTP header field carries priority parameters Section 4. - It can appear in requests and responses. It is an end-to-end signal - of the request priority from the client or the response priority from - the server. Section 8 describes how intermediaries can combine the - priority information from client requests and server responses to - correct or amend the precedence. Clients cannot interpret the - appearance or omission of a Priority response header as - acknowledgement that any prioritization has occurred. Guidance for - how endpoints can act on Priority header values is given in - Section 10 and Section 9. + The Priority HTTP header field carries priority parameters (see + Section 4). It can appear in requests and responses. It is an end- + to-end signal of the request priority from the client or the response + priority from the server. Section 8 describes how intermediaries can + combine the priority information sent from clients and servers. + Clients cannot interpret the appearance or omission of a Priority + response header field as acknowledgement that any prioritization has + occurred. Guidance for how endpoints can act on Priority header + values is given in Section 10 and Section 9. Priority is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]): Priority = sf-dictionary - - As is the ordinary case for HTTP caching [CACHING], a response with a - Priority header field might be cached and re-used for subsequent - requests. When an origin server generates the Priority response - header field based on properties of an HTTP request it receives, the - server is expected to control the cacheability or the applicability - of the cached response, by using header fields that control the - caching behavior (e.g., Cache-Control, Vary). + An HTTP request with a Priority header field might be cached and re- + used for subsequent requests; see [CACHING]. When an origin server + generates the Priority response header field based on properties of + an HTTP request it receives, the server is expected to control the + cacheability or the applicability of the cached response, by using + header fields that control the caching behavior (e.g., Cache-Control, + Vary). 6. Reprioritization After a client sends a request, it may be beneficial to change the priority of the response. As an example, a web browser might issue a prefetch request for a JavaScript file with the urgency parameter of the Priority request header field set to u=7 (background). Then, when the user navigates to a page which references the new JavaScript file, while the prefetch is in progress, the browser would send a reprioritization signal with the priority field value set to u=0. @@ -523,40 +539,40 @@ HTTP/3, the identifier is either the Stream ID or Push ID. Unlike the Priority header field, the PRIORITY_UPDATE frame is a hop-by-hop signal. PRIORITY_UPDATE frames are sent by clients on the control stream, allowing them to be sent independent from the stream that carries the response. This means they can be used to reprioritize a response or a push stream; or signal the initial priority of a response instead of the Priority header field. - A PRIORITY_UPDATE frame communicates a complete set of all parameters - in the Priority Field Value field. Omitting a parameter is a signal - to use the parameter's default value. 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 + A PRIORITY_UPDATE frame communicates a complete set of all priority + parameters in the Priority Field Value field. Omitting a priority + parameter is a signal to use its default value. 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. A client MAY send a PRIORITY_UPDATE frame before the stream that it references is open (except for HTTP/2 push streams; see Section 7.1). Furthermore, HTTP/3 offers no guaranteed ordering across streams, which could cause the frame to be received earlier than intended. Either case leads to a race condition where a server receives a PRIORITY_UPDATE frame that references a request stream that is yet to be opened. To solve this condition, for the purposes of scheduling, the most recently received PRIORITY_UPDATE frame can be considered as the most up-to-date information that overrides any other signal. Servers SHOULD buffer the most recently received PRIORITY_UPDATE frame and apply it once the referenced stream is opened. Holding PRIORITY_UPDATE frames for each stream requires server resources, - which can can be bound by local implementation policy. Although + which can can be bounded by local implementation policy. Although there is no limit to the number of PRIORITY_UPDATES that can be sent, storing only the most recently received frame limits resource commitment. 7.1. HTTP/2 PRIORITY_UPDATE Frame The HTTP/2 PRIORITY_UPDATE frame (type=0x10) is used by clients to signal the initial priority of a response, or to reprioritize a response or push stream. It carries the stream ID of the response and the priority in ASCII text, using the same representation as the @@ -676,46 +692,46 @@ The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference a promised push stream. If a server receives a PRIORITY_UPDATE (type=0xF0701) with a Push ID that is greater than the maximum Push ID or which has not yet been promised, this MUST be treated as a connection error of type H3_ID_ERROR. PRIORITY_UPDATE frames of either type are only sent by clients. If a client receives a PRIORITY_UPDATE frame, this MUST be treated as a connection error of type H3_FRAME_UNEXPECTED. -8. Merging Client- and Server-Driven Parameters +8. Merging Client- and Server-Driven Priority Parameters It is not always the case that the client has the best understanding of how the HTTP responses deserve to be prioritized. The server might have additional information that can be combined with the client's indicated priority in order to improve the prioritization of the response. For example, use of an HTML document might depend heavily on one of the inline images; existence of such dependencies is typically best known to the server. Or, a server that receives requests for a font [RFC8081] and images with the same urgency might give higher precedence to the font, so that a visual client can render textual information at an early moment. An origin can use the Priority response header field to indicate its view on how an HTTP response should be prioritized. An intermediary - that forwards an HTTP response can use the parameters found in the - Priority response header field, in combination with the client + that forwards an HTTP response can use the priority parameters found + in the Priority response header field, in combination with the client Priority request header field, as input to its prioritization process. No guidance is provided for merging priorities, this is left as an implementation decision. Absence of a priority parameter in an HTTP response indicates the server's disinterest in changing the client-provided value. This is - different from the logic being defined for the request header field, - in which omission of a priority parameter implies the use of their - default values (see Section 4). + different from the the request header field, in which omission of a + priority parameter implies the use of their default values (see + Section 4). As a non-normative example, when the client sends an HTTP request with the urgency parameter set to 5 and the incremental parameter set to true :method = GET :scheme = https :authority = example.net :path = /menu.png priority = u=5, i @@ -732,47 +748,48 @@ the client, as the server did not specify the incremental(i) parameter. 9. Client Scheduling A client MAY use priority values to make local processing or scheduling choices about the requests it initiates. 10. Server Scheduling - Priority signals are input to a prioritization process. They do not - guarantee any particular processing or transmission order for one - response relative to any other response. An endpoint cannot force a - peer to process concurrent request in a particular order using - priority. Expressing priority is therefore only a suggestion. - - A server can use priority signals along with other inputs to make - scheduling decisions. No guidance is provided about how this can or - should be done. Factors such as implementation choices or deployment - environment also play a role. Any given connection is likely to have - many dynamic permutations. For these reasons, there is no unilateral - perfect scheduler and this document only provides some basic - recommendations for implementations. + It is generally beneficial for an HTTP server to send all responses + as early as possible. However, when serving multiple requests on a + single connection, there could be competition between the requests + for resources such as connection bandwidth. This section describes + considerations regarding how servers can schedule the order in which + the competing responses will be sent, when such competition exists. - Clients cannot depend on particular treatment based on priority - signals. Servers can use other information to prioritize responses. + Server scheduling is a prioritization process based on many inputs, + with priority signals being only one form of input. Factors such as + implementation choices or deployment environment also play a role. + Any given connection is likely to have many dynamic permutations. + For these reasons, there is no unilateral perfect scheduler. This + document provides some basic, non-exhaustive, recommendations for how + servers might act on priority parameters. It does not describe in + detail how servers might combine priority signals with other factors. + Endpoints cannot depend on particular treatment based on priority + signals. Expressing priority is only a suggestion. It is RECOMMENDED that, when possible, servers respect the urgency parameter (Section 4.1), sending higher urgency responses before lower urgency responses. The incremental parameter indicates how a client processes response bytes as they arrive. It is RECOMMENDED that, when possible, servers - respect the incremental parameter (Section 4.2). Non-incremental - resources can only be used when all of the response payload has been - received. Therefore, non-incremental responses of the same urgency - SHOULD be served in their entirety, one-by-one, based on the stream + respect the incremental parameter (Section 4.2). + + Non-incremental responses of the same urgency SHOULD be served by + prioritizing bandwidth allocation in ascending order of the stream ID, which corresponds to the order in which clients make requests. Doing so ensures that clients can use request ordering to influence response order. Incremental responses of the same urgency SHOULD be served by sharing bandwidth amongst them. Incremental resources are used as parts, or chunks, of the response payload are received. A client might benefit more from receiving a portion of all these resources rather than the entirety of a single resource. How large a portion of the resource is needed to be useful in improving performance varies. Some @@ -837,83 +854,86 @@ every request can make some progress over time. Similarly, servers SHOULD allocate some amount of bandwidths to streams acting as tunnels. 11. Scheduling and the CONNECT Method When a request stream carries the CONNECT method, the scheduling guidance in this document applies to the frames on the stream. A client that issues multiple CONNECT requests can set the incremental - parameter to true, servers that implement the recommendation in - Section 10 will schedule these fairly. + parameter to true. Servers that implement the recommendations for + handling of the incremental parameter in Section 10 are likely to + schedule these fairly, avoiding one CONNECT stream from blocking + others. 12. Retransmission Scheduling Transport protocols such as TCP and QUIC provide reliability by - detecting packet losses and retransmitting lost information. While - this document specifies HTTP-layer prioritization, its effectiveness - can be further enhanced if the transport layer factors priority into - scheduling both new data and retransmission data. The remainder of + detecting packet losses and retransmitting lost information. In + addition to the considerations in Section 10, scheduling of + retransmission data could compete with new data. The remainder of this section discusses considerations when using QUIC. Section 13.3 of [QUIC] states "Endpoints SHOULD prioritize retransmission of data over sending new data, unless priorities specified by the application indicate otherwise". When an HTTP/3 application uses the priority scheme defined in this document and the QUIC transport implementation supports application indicated stream priority, a transport that considers the relative priority of streams when scheduling both new data and retransmission data might better match the expectations of the application. However, there are no requirements on how a transport chooses to schedule based on this information because the decision depends on several factors and trade-offs. It could prioritize new data for a higher urgency stream over retransmission data for a lower priority stream, or it could prioritize retransmission data over new data irrespective of urgencies. - Section 6.2.4 of [QUIC-RECOVERY], also highlights consideration of + Section 6.2.4 of [QUIC-RECOVERY] also highlights consideration of application priorities when sending probe packets after Probe Timeout timer expiration. A QUIC implementation supporting application- indicated priorities might use the relative priority of streams when choosing probe data. 13. Fairness - As a general guideline, a server SHOULD NOT use priority information - for making scheduling decisions across multiple connections, unless - it knows that those connections originate from the same client. Due - to this, priority information conveyed over a non-coalesced HTTP - connection (e.g., HTTP/1.1) might go unused. + Typically, HTTP implementations depend on the underlying transport to + maintain fairness between connections competing for bandwidth. When + HTTP requests are forwarded through intermediaries, progress made by + each connection originating from end clients can become different + over time, depending on how intermediaries coalesce or split requests + into backend connections. This unfairness can expand if priority + signals are used. Section 13.1 and Section 13.2 discuss mitigations + against this expansion of unfairness. - The remainder of this section discusses scenarios where unfairness is - problematic and presents possible mitigations, or where unfairness is - desirable. + Conversely, Section 13.3 discusses how servers might intentionally + allocate unequal bandwidth to some connections depending on the + priority signals. 13.1. Coalescing Intermediaries When an intermediary coalesces HTTP requests coming from multiple clients into one HTTP/2 or HTTP/3 connection going to the backend - server, requests that originate from one client might have higher - precedence than those coming from others. + server, requests that originate from one client might carry signals + indicating higher priority than those coming from others. It is sometimes beneficial for the server running behind an - intermediary to obey to the value of the Priority header field. As - an example, a resource-constrained server might defer the - transmission of software update files that would have the background - urgency being associated. However, in the worst case, the asymmetry - between the precedence declared by multiple clients might cause - responses going to one user agent to be delayed totally after those - going to another. + intermediary to obey Priority header field values. As an example, a + resource-constrained server might defer the transmission of software + update files that would have the background urgency being associated. + However, in the worst case, the asymmetry between the priority + declared by multiple clients might cause responses going to one user + agent to be delayed totally after those going to another. In order to mitigate this fairness problem, a server could use - knowledge about the intermediary as another signal in its + knowledge about the intermediary as another input in its prioritization decisions. For instance, if a server knows the intermediary is coalescing requests, then it could avoid serving the responses in their entirety and instead distribute bandwidth (for example, in a round-robin manner). This can work if the constrained resource is network capacity between the intermediary and the user agent, as the intermediary buffers responses and forwards the chunks based on the prioritization scheme it implements. A server can determine if a request came from an intermediary through configuration, or by consulting if that request contains one of the @@ -945,52 +965,39 @@ amount of unfairness between the connections and therefore between the requests served on those connections. For example, a server might use a scavenging congestion controller on connections that only convey background priority responses such as software update images. Doing so improves responsiveness of other connections at the cost of delaying the delivery of updates. 14. Why use an End-to-End Header Field? - Contrary to the prioritization scheme of HTTP/2 that uses a hop-by- - hop frame, the Priority header field is defined as end-to-end. + In contrast to the prioritization scheme of HTTP/2 that uses a hop- + by-hop frame, the Priority header field is defined as end-to-end. - The rationale is that the Priority header field transmits how each - response affects the client's processing of those responses, rather - than how relatively urgent each response is to others. The way a - client processes a response is a property associated to that client - generating that request. Not that of an intermediary. Therefore, it - is an end-to-end property. How these end-to-end properties carried - by the Priority header field affect the prioritization between the - responses that share a connection is a hop-by-hop issue. + The way that a client processes a response is a property associated + with the client generating that request. Not that of an + intermediary. Therefore, it is an end-to-end property. How these + end-to-end properties carried by the Priority header field affect the + prioritization between the responses that share a connection is a + hop-by-hop issue. Having the Priority header field defined as end-to-end is important for caching intermediaries. Such intermediaries can cache the value of the Priority header field along with the response, and utilize the value of the cached header field when serving the cached response, only because the header field is defined as end-to-end rather than hop-by-hop. - It should also be noted that the use of a header field carrying a - textual value makes the prioritization scheme extensible; see the - discussion below. - 15. Security Considerations - [RFC7540] stream prioritization relies on dependencies. - Considerations are presented to implementations, describing how - limiting state or work commitments can avoid some types of problems. - In addition, [CVE-2019-9513] aka "Resource Loop", is an example of a - DoS attack that abuses stream dependencies. Extensible priorities - does not use dependencies, which avoids these issues. - Section 7 describes considerations for server buffering of PRIORITY_UPDATE frames. Section 10 presents examples where servers that prioritize responses in a certain way might be starved of the ability to transmit payload. The security considerations from [STRUCTURED-FIELDS] apply to processing of priority parameters defined in Section 4. 16. IANA Considerations @@ -1028,38 +1035,48 @@ Frame Type: PRIORITY_UPDATE Code: 0xF0700 and 0xF0701 Specification: This document Upon publication, please create the HTTP Priority Parameters registry at https://iana.org/assignments/http-priority (https://iana.org/assignments/http-priority) and populate it with the - types defined in Section 4; see Section 4.3.1 for its associated - procedures. + entries in Table 1; see Section 4.3.1 for its associated procedures. + + +======+==================================+===============+ + | Name | Description | Specification | + +======+==================================+===============+ + | u | The urgency of an HTTP response. | Section 4.1 | + +------+----------------------------------+---------------+ + | i | Whether an HTTP response can be | Section 4.2 | + | | processed incrementally. | | + +------+----------------------------------+---------------+ + + Table 1: Initial Priority Parameters 17. References 17.1. Normative References [HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf- httpbis-semantics-19, 12 September 2021, . [HTTP2] Thomson, M. and C. Benfield, "Hypertext Transfer Protocol Version 2 (HTTP/2)", Work in Progress, Internet-Draft, - draft-ietf-httpbis-http2bis-05, 26 September 2021, + draft-ietf-httpbis-http2bis-06, 18 November 2021, . + http2bis-06>. [HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3 (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf- quic-http-34, 2 February 2021, . [QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, @@ -1085,37 +1102,44 @@ . 17.2. Informative References [CACHING] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Caching", Work in Progress, Internet-Draft, draft-ietf- httpbis-cache-19, 12 September 2021, . - [CVE-2019-9513] - Common Vulnerabilities and Exposures, "CVE-2019-9513", 1 - March 2019, . - [FORWARDED] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension", RFC 7239, DOI 10.17487/RFC7239, June 2014, . [I-D.lassey-priority-setting] Lassey, B. and L. Pardue, "Declaring Support for HTTP/2 Priorities", Work in Progress, Internet-Draft, draft- lassey-priority-setting-00, 25 July 2019, . + [MARX] Marx, R., Decker, T.D., Quax, P., and W. Lamotte, "Of the + Utmost Importance: Resource Prioritization in HTTP/3 over + QUIC", DOI 10.5220/0008191701300143, + SCITEPRESS Proceedings of the 15th International + Conference on Web Information Systems and Technologies + (pages 130-143), September 2019, + . + + [MEENAN] Meenan, P., "Better HTTP/2 Prioritization for a Faster + Web", 14 May 2019, . + [QUIC-RECOVERY] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, May 2021, . [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, . @@ -1147,148 +1171,154 @@ Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku, Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy Fielding. Yang Chi contributed the section on retransmission scheduling. Appendix B. Change Log _RFC EDITOR: please remove this section before publication_ -B.1. Since draft-ietf-httpbis-priority-09 +B.1. Since draft-ietf-httpbis-priority-10 * Editorial changes -B.2. Since draft-ietf-httpbis-priority-08 + * Add clearer IANA instructions for Priority Parameter initial + population + +B.2. Since draft-ietf-httpbis-priority-09 + + * Editorial changes + +B.3. Since draft-ietf-httpbis-priority-08 * Changelog fixups -B.3. Since draft-ietf-httpbis-priority-07 +B.4. Since draft-ietf-httpbis-priority-07 * Relax requirements of receiving SETTINGS_NO_RFC7540_PRIORITIES that changes value (#1714, #1725) * Clarify how intermediaries might use frames vs. headers (#1715, #1735) * Relax requirement when receiving a PRIORITY_UPDATE with an invalid structured field value (#1741, #1756) -B.4. Since draft-ietf-httpbis-priority-06 +B.5. Since draft-ietf-httpbis-priority-06 * Focus on editorial changes * Clarify rules about Sf-Dictionary handling in headers - * Split policy for parameter IANA registry into two sections based on key length -B.5. Since draft-ietf-httpbis-priority-05 +B.6. Since draft-ietf-httpbis-priority-05 * Renamed SETTINGS_DEPRECATE_RFC7540_PRIORITIES to SETTINGS_NO_RFC7540_PRIORITIES * Clarify that senders of the HTTP/2 setting can use any alternative (#1679, #1705) -B.6. Since draft-ietf-httpbis-priority-04 +B.7. Since draft-ietf-httpbis-priority-04 * Renamed SETTINGS_DEPRECATE_HTTP2_PRIORITIES to SETTINGS_DEPRECATE_RFC7540_PRIORITIES (#1601) * Reoriented text towards RFC7540bis (#1561, #1601) * Clarify intermediary behavior (#1562) -B.7. Since draft-ietf-httpbis-priority-03 +B.8. Since draft-ietf-httpbis-priority-03 * Add statement about what this scheme applies to. Clarify extensions can use it but must define how themselves (#1550, #1559) * Describe scheduling considerations for the CONNECT method (#1495, #1544) * Describe scheduling considerations for retransmitted data (#1429, #1504) * Suggest intermediaries might avoid strict prioritization (#1562) -B.8. Since draft-ietf-httpbis-priority-02 +B.9. Since draft-ietf-httpbis-priority-02 * Describe considerations for server push prioritization (#1056, #1345) * Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261, #1344) - * Add a Parameters registry (#1371) + * Add a Priority Parameters registry (#1371) -B.9. Since draft-ietf-httpbis-priority-01 +B.10. Since draft-ietf-httpbis-priority-01 * PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267, #1271) * Add section to describe server scheduling considerations (#1215, #1232, #1266) * Remove specific instructions related to intermediary fairness (#1022, #1264) -B.10. Since draft-ietf-httpbis-priority-00 +B.11. Since draft-ietf-httpbis-priority-00 * Move text around (#1217, #1218) * Editorial change to the default urgency. The value is 3, which was always the intent of previous changes. -B.11. Since draft-kazuho-httpbis-priority-04 +B.12. Since draft-kazuho-httpbis-priority-04 * Minimize semantics of Urgency levels (#1023, #1026) * Reduce guidance about how intermediary implements merging priority signals (#1026) * Remove mention of CDN-Loop (#1062) * Editorial changes * Make changes due to WG adoption * Removed outdated Consideration (#118) -B.12. Since draft-kazuho-httpbis-priority-03 +B.13. Since draft-kazuho-httpbis-priority-03 * Changed numbering from [-1,6] to [0,7] (#78) * Replaced priority scheme negotiation with HTTP/2 priority deprecation (#100) * Shorten parameter names (#108) * Expand on considerations (#105, #107, #109, #110, #111, #113) -B.13. Since draft-kazuho-httpbis-priority-02 +B.14. Since draft-kazuho-httpbis-priority-02 * Consolidation of the problem statement (#61, #73) * Define SETTINGS_PRIORITIES for negotiation (#58, #69) * Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51) * Explain fairness issue and mitigations (#56) -B.14. Since draft-kazuho-httpbis-priority-01 +B.15. Since draft-kazuho-httpbis-priority-01 * Explain how reprioritization might be supported. -B.15. Since draft-kazuho-httpbis-priority-00 +B.16. Since draft-kazuho-httpbis-priority-00 * Expand urgency levels from 3 to 8. Authors' Addresses Kazuho Oku Fastly Email: kazuhooku@gmail.com