HTTP Working GroupK. Oku
Intended status: Standards TrackL. Pardue
Expires: January 13, 2022Cloudflare
July 12, 2021

Extensible Prioritization Scheme for HTTP


This document describes a scheme for prioritizing HTTP responses. This scheme expresses the priority of each HTTP response using absolute values, rather than as a relative relationship between a group of HTTP responses.

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 the responses. These share a common format structure that is designed to provide future extensibility.

Note to Readers

RFC EDITOR: please remove this section before publication

Discussion of this draft takes place on the HTTP working group mailing list (, which is archived at

Working Group information can be found at; source code and issues list for this draft can be found at

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1. Introduction

It is common for an HTTP ([RFC7230]) resource representation to have relationships to one or more other resources. Clients will often discover these relationships while processing a retrieved representation, leading to further retrieval requests. Meanwhile, the nature of the relationship determines whether the client is blocked from continuing to process locally available resources. For example, visual rendering of an HTML document could be blocked by the retrieval of a CSS file that the document refers to. In contrast, inline images do not block rendering and get drawn incrementally as the chunks of the images arrive.

To provide meaningful presentation of a document at the earliest moment, it is important for an HTTP server to prioritize the HTTP responses, or the chunks of those HTTP responses, that it sends.

HTTP/2 ([HTTP2]) provides such a prioritization scheme. A client sends a series of PRIORITY frames to communicate to the server a "priority tree"; this represents the client's preferred ordering and weighted distribution of the bandwidth among the HTTP responses. However, the design and implementation of this scheme has been observed to have shortcomings, explained in Section 2.

This document defines the Priority HTTP header field that can be used by both client and server to specify the precedence of HTTP responses in a standardized, extensible, protocol-version-independent, end-to-end format. Along with the protocol-version-specific frame for reprioritization, this prioritization scheme acts as a substitute for the original prioritization scheme of HTTP/2.

1.1. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The terms sf-token and sf-boolean 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 [HTTP3], Section 6.2.1.

2. Motivation for Replacing HTTP/2 Priorities

An important feature of any implementation of a protocol that provides multiplexing is the ability to prioritize the sending of information. This was an important realization in the design of HTTP/2. Prioritization is a difficult problem, so it will always be suboptimal, particularly if one endpoint operates in ignorance of the needs of its peer.

HTTP/2 introduced a complex prioritization scheme that uses a combination of stream dependencies and weights to describe an unbalanced tree. This scheme has suffered from poor deployment and interoperability.

Clients build an HTTP/2 prioritization tree through a series of individual stream relationships, which are transferred to the server using HTTP/2 priority signals in either of three forms. First, a HEADERS frame with the PRIORITY flag set is an explicit signal that includes an Exclusive flag, Stream Dependency field, and Weight field. Second, a HEADERS frame with no PRIORITY flag is an implicit signal to use the default priority. Third, the PRIORITY frame, which is always explicit since it always contains an Exclusive flag, Stream Dependency field, and Weight field.

The rich flexibility of tree building is rarely exercised. Experience has shown that clients tend to choose a single model optimized for a web use case and experiment within the model constraints, or do nothing at all. Furthermore, many clients build their prioritization tree in a unique way, which makes it difficult for servers to understand their intent and act or intervene accordingly.

Many HTTP/2 server implementations do not include support for the priority scheme. Some instead favor custom server-driven schemes based on heuristics or other hints, such as resource content type or request generation order. For example, a server, with knowledge of the document structure, might want to prioritize the delivery of images that are critical to user experience above other images, but below the CSS files. Since client trees vary, it is impossible for the server to determine how such images should be prioritized against other responses.

The HTTP/2 scheme allows intermediaries to coalesce multiple client trees into a single tree that is used for a single upstream HTTP/2 connection. However, most intermediaries do not support this. The scheme does not define a method that can be used by a server to express the priority of a response. Without such a method, intermediaries cannot coordinate client-driven and server-driven priorities.

HTTP/2 describes denial-of-service considerations for implementations. On 2019-08-13 Netflix issued an advisory notice about the discovery of several resource exhaustion vectors affecting multiple HTTP/2 implementations. One attack, [CVE-2019-9513] aka "Resource Loop", is based on manipulation of the priority tree.

The HTTP/2 scheme depends on in-order delivery of signals, leading to challenges in porting the scheme to protocols that do not provide global ordering. For example, the scheme cannot be used in HTTP/3 [HTTP3] without changing the signal and its processing.

Considering the problems with deployment and adaptability to HTTP/3, retaining the HTTP/2 priority scheme increases the complexity of the entire system without any evidence that the value it provides offsets that complexity. In fact, multiple experiments from independent research have shown that simpler schemes can reach at least equivalent performance characteristics compared to the more complex HTTP/2 setups seen in practice, at least for the web use case.

2.1. Disabling HTTP/2 Priorities

The problems and insights set out above are motivation for allowing endpoints to opt out of using the HTTP/2 priority scheme, in favor of using an alternative such as the scheme defined in this specification. The SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting described below enables endpoints to understand their peer's intention. The value of the parameter MUST be 0 or 1. Any value other than 0 or 1 MUST be treated as a connection error (see [HTTP2], Section 5.4.1) of type PROTOCOL_ERROR.

Endpoints MUST send this SETTINGS parameter as part of the first SETTINGS frame. A sender MUST NOT change the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter value after the first SETTINGS frame. Detection of a change by a receiver MUST be treated as a connection error of type PROTOCOL_ERROR.

Until the client receives the SETTINGS frame from the server, the client SHOULD send the signals of the HTTP/2 priority scheme (see Section 2) and the signals of this prioritization scheme (see Section 5 and Section 7.1). When the client receives the first SETTINGS frame that contains the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with value of 1, it SHOULD stop sending the HTTP/2 priority signals. If the value was 0 or if the settings parameter was absent, it SHOULD stop sending PRIORITY_UPDATE frames (Section 7.1), but 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.

The SETTINGS frame precedes any priority signal sent from a client in HTTP/2, so a server can determine if it should respect the HTTP/2 scheme before building state. A server that receives SETTINGS_DEPRECATE_HTTP2_PRIORITIES with value of 1 MUST ignore HTTP/2 priority signals.

Where both endpoints disable HTTP/2 priorities, the client is expected to send this scheme's priority signal. Handling of omitted signals is described in Section 4.

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.

Where HTTP extensions change stream behavior or define new data carriage mechanisms, they MAY also define how this priority scheme can 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 frames (Section 7.1 and Section 7.2) are used by clients to transmit the same information on a single hop. If intermediaries want to specify prioritization on a multiplexed HTTP connection, they SHOULD use a PRIORITY_UPDATE frame and SHOULD NOT change the Priority header field.

In both cases, the set of priority parameters is encoded as a Structured Fields Dictionary ([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.

Unknown parameters, 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. This range provides sufficient granularity for prioritizing responses for ordinary web browsing, at minimal complexity.

The value is encoded as an sf-integer. The default value is 3.

This parameter indicates the sender's recommendation, based on the expectation that the server would transmit HTTP responses in the order of their urgency values if possible. The smaller the value, the higher the precedence.

The following example shows a request for a CSS file with the urgency set to 0:

:method = GET
:scheme = https
:authority =
:path = /style.css
priority = u=0

A client that fetches a document that likely consists of multiple HTTP resources (e.g., HTML) SHOULD assign the default urgency level to the main resource. This convention allows servers to refine the urgency using knowledge specific to the web-site (see Section 8).

The lowest urgency level (7) is reserved for background tasks such as delivery of software updates. This urgency level SHOULD NOT be used for fetching responses that have impact on user interaction.

4.2. Incremental

The incremental parameter (i) takes an sf-boolean as the value that indicates if an HTTP response can be processed incrementally, i.e. provide some meaningful output as chunks of the response arrive.

The default value of the incremental parameter is false (0).

A server might distribute the bandwidth of a connection between incremental responses that share the same urgency, hoping that providing those responses in parallel would be more helpful to the client than delivering the responses one by one.

If a client makes concurrent requests with the incremental parameter set to false, there is no benefit serving responses in parallel because the client is not going to process those responses incrementally. Serving non-incremental responses one by one, in the order in which those requests were generated is considered to be the best strategy.

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 =
:path = /image.jpg
priority = u=5, i

4.3. Defining New 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 predefined 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.

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.

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).

4.3.1. Registration

New Priority parameters can be defined by registering them in the HTTP Priority Parameters Registry.

Registration requests are reviewed and approved by a Designated Expert, as per [RFC8126], Section 4.5. A specification document is appreciated, but not required.

The Expert(s) should consider the following factors when evaluating requests:

  • Community feedback
  • If the parameters are sufficiently well-defined and adhere to the guidance provided in Section 4.3.

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]
  • Reference: [to a specification defining this parameter]

See the registry at for details on where to send registration requests.

5. The Priority HTTP Header Field

The Priority HTTP header field can appear in requests and responses. A client uses it to specify the priority of the response. A server uses it to inform the client that the priority was overwritten. An intermediary can use the Priority information from client requests and server responses to correct or amend the precedence to suit it (see Section 8).

The Priority header field is an end-to-end signal of the request priority from the client or the response priority from the server.

As is the ordinary case for HTTP caching ([RFC7234]), 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 endpoint that fails to parse the Priority header field SHOULD use default parameter values.

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. The PRIORITY_UPDATE frame (Section 7) can be used for such reprioritization.


This document specifies a new PRIORITY_UPDATE frame for HTTP/2 ([HTTP2]) and HTTP/3 ([HTTP3]). It carries priority parameters and references the target of the prioritization based on a version-specific identifier. In HTTP/2, this identifier is the Stream ID; in 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 MUST 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_FRAME_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 there is no limit to the number of PRIORITY_UPDATES that can be sent, storing only the most recently received frame limits resource commitment.


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 Priority header field value.

The Stream Identifier field ([HTTP2], Section 4.1) in the PRIORITY_UPDATE frame header MUST be zero (0x0). Receiving a PRIORITY_UPDATE frame with a field of any other value MUST be treated as a connection error of type PROTOCOL_ERROR.

  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
 |R|                Prioritized Stream ID (31)                   |
 |                   Priority Field Value (*)                  ...

Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload

The PRIORITY_UPDATE frame payload has the following fields:

A reserved 1-bit field. The semantics of this bit are undefined, and the bit MUST remain unset (0x0) when sending and MUST be ignored when receiving.
Prioritized Stream ID:
A 31-bit stream identifier for the stream that is the target of the priority update.
Priority Field Value:
The priority update value in ASCII text, encoded using Structured Fields.

When the PRIORITY_UPDATE frame applies to a request stream, clients SHOULD provide a Prioritized Stream ID that refers to a stream in the "open", "half-closed (local)", or "idle" state. Servers can discard frames where the Prioritized Stream ID refers to a stream in the "half-closed (local)" or "closed" state. The number of streams which have been prioritized but remain in the "idle" state plus the number of active streams (those in the "open" or either "half-closed" state; see section 5.1.2 of [HTTP2]) MUST NOT exceed the value of the SETTINGS_MAX_CONCURRENT_STREAMS parameter. Servers that receive such a PRIORITY_UPDATE MUST respond with a connection error of type PROTOCOL_ERROR.

When the PRIORITY_UPDATE frame applies to a push stream, clients SHOULD provide a Prioritized Stream ID that refers to a stream in the "reserved (remote)" or "half-closed (local)" state. Servers can discard frames where the Prioritized Stream ID refers to a stream in the "closed" state. Clients MUST NOT provide a Prioritized Stream ID that refers to a push stream in the "idle" state. Servers that receive a PRIORITY_UPDATE for a push stream in the "idle" state MUST respond with a connection error of type PROTOCOL_ERROR.

If a PRIORITY_UPDATE frame is received with a Prioritized Stream ID of 0x0, the recipient MUST respond with a connection error of type PROTOCOL_ERROR.

If a client receives a PRIORITY_UPDATE frame, it MUST respond with a connection error of type PROTOCOL_ERROR.


The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by clients to signal the initial priority of a response, or to reprioritize a response or push stream. It carries the identifier of the element that is being prioritized, and the updated priority in ASCII text, using the same representation as that of the Priority header field value. PRIORITY_UPDATE with a frame type of 0xF0700 is used for request streams, while PRIORITY_UPDATE with a frame type of 0xF0701 is used for push streams.

The PRIORITY_UPDATE frame MUST be sent on the client control stream ([HTTP3], Section 6.2.1). Receiving a PRIORITY_UPDATE frame on a stream other than the client control stream MUST be treated as a connection error of type H3_FRAME_UNEXPECTED.

  Type (i) = 0xF0700..0xF0701,
  Length (i),
  Prioritized Element ID (i),
  Priority Field Value (..),


The PRIORITY_UPDATE frame payload has the following fields:

Prioritized Element ID:
The stream ID or push ID that is the target of the priority update.
Priority Field Value:
The priority update value in ASCII text, encoded using Structured Fields.

The request-stream variant of PRIORITY_UPDATE (type=0xF0700) MUST reference a request stream. If a server receives a PRIORITY_UPDATE (type=0xF0700) for a Stream ID that is not a request stream, this MUST be treated as a connection error of type H3_ID_ERROR. The Stream ID MUST be within the client-initiated bidirectional stream limit. If a server receives a PRIORITY_UPDATE (type=0xF0700) with a Stream ID that is beyond the stream limits, this SHOULD be treated as a connection error of type H3_ID_ERROR.

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

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 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).

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 =
:path = /menu.png
priority = u=5, i

and the origin responds with

:status = 200
content-type = image/png
priority = u=1

the intermediary might alter its understanding of the urgency from 5 to 1, because it prefers the server-provided value over the client's. The incremental value continues to be true, the value specified by 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.

Clients cannot depend on particular treatment based on priority signals. Servers can use other information to prioritize responses.

It is RECOMMENDED that, when possible, servers respect the urgency parameter (Section 4.1), sending higher urgency responses before lower urgency responses.

It is RECOMMENDED that, when possible, servers respect the incremental parameter (Section 4.2). Non-incremental responses of the same urgency SHOULD be served one-by-one based on 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 in round-robin manner.

The incremental parameter indicates how a client processes response bytes as they arrive. Non-incremental resources are only used when all of the response payload has been received. Incremental resources are used as parts, or chunks, of the response payload are received. Therefore, the timing of response data reception at the client, such as the time to early bytes or the time to receive the entire payload, plays an important role in perceived performance. Timings depend on resource size but this scheme provides no explicit guidance about how a server should use size as an input to prioritization. Instead, the following examples demonstrate how a server that strictly abides the scheduling guidance based on urgency and request generation order could find that early requests prevent serving of later requests.

  1. At the same urgency level, a non-incremental request for a large resource followed by an incremental request for a small resource.
  2. At the same urgency level, an incremental request of indeterminate length followed by a non-incremental large resource.

It is RECOMMENDED that servers avoid such starvation where possible. The method to do so is an implementation decision. For example, a server might pre-emptively send responses of a particular incremental type based on other information such as content size.

Optimal scheduling of server push is difficult, especially when pushed resources contend with active concurrent requests. Servers can consider many factors when scheduling, such as the type or size of resource being pushed, the priority of the request that triggered the push, the count of active concurrent responses, the priority of other active concurrent responses, etc. There is no general guidance on the best way to apply these. A server that is too simple could easily push at too high a priority and block client requests, or push at too low a priority and delay the response, negating intended goals of server push.

Priority signals are a factor for server push scheduling. The concept of parameter value defaults applies slightly differently because there is no explicit client-signalled initial priority. A server can apply priority signals provided in an origin response; see the merging guidance given in Section 8. In the absence of origin signals, applying default parameter values could be suboptimal. How ever a server decides to schedule a pushed response, it can signal the intended priority to the client by including the Priority field in a PUSH_PROMISE or HEADERS frame.

10.1. Intermediaries with Multiple Backend Connections

An intermediary serving an HTTP connection might split requests over multiple backend connections. When it applies prioritization rules strictly, low priority requests cannot make progress while requests with higher priorities are inflight. This blocking can propagate to backend connections, which the peer might interpret as a connection stall. Endpoints often implement protections against stalls, such as abruptly closing connections after a certain time period. To reduce the possibility of this occurring, intermediaries can avoid strictly following prioritization and instead allocate small amounts of bandwidth for all the requests that they are forwarding, so that 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.

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 this section discusses considerations when using QUIC.

[QUIC], Section 13.3 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.

[QUIC-RECOVERY], Section 6.2.4 also highlights consideration of application priorities when sending probe packets after PTO timer expiration. An 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 schedule 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.

The remainder of this section discusses scenarios where unfairness is problematic and presents possible mitigations, or where unfairness is desirable.

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.

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.

In order to mitigate this fairness problem, a server could use knowledge about the intermediary as another signal in its prioritization decisions. For instance, if a server knows the intermediary is coalescing requests, then it could serve the responses in 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 following header fields:

13.2. HTTP/1.x Back Ends

It is common for CDN infrastructure to support different HTTP versions on the front end and back end. For instance, the client-facing edge might support HTTP/2 and HTTP/3 while communication to back end servers is done using HTTP/1.1. Unlike with connection coalescing, the CDN will "de-mux" requests into discrete connections to the back end. As HTTP/1.1 and older do not provide a way to concurrently transmit multiple responses, there is no immediate fairness issue in protocol. However, back end servers MAY still use client headers for request scheduling. Back end servers SHOULD only schedule based on client priority information where that information can be scoped to individual end clients. Authentication and other session information might provide this linkability.

13.3. Intentional Introduction of Unfairness

It is sometimes beneficial to deprioritize the transmission of one connection over others, knowing that doing so introduces a certain 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.

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.

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

[CVE-2019-9513] aka "Resource Loop", is a DoS attack based on manipulation of the HTTP/2 priority tree. Extensible priorities does not use stream dependencies, which mitigates this vulnerability.

TBD: depending on the outcome of reprioritization discussions, following paragraphs may change or be removed.

[HTTP2], Section 5.3.4 describes a scenario where closure of streams in the priority tree could cause suboptimal prioritization. To avoid this, [HTTP2] states that "an endpoint SHOULD retain stream prioritization state for a period after streams become closed". Retaining state for streams no longer counted towards stream concurrency consumes server resources. Furthermore, [HTTP2] identifies that reprioritization of a closed stream could affect dependents; it recommends updating the priority tree if sufficient state is stored, which will also consume server resources. To limit this commitment, it is stated that "The amount of prioritization state that is retained MAY be limited" and "If a limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.". Extensible priorities does not use stream dependencies, which minimizes most of the resource concerns related to this scenario.

[HTTP2], Section 5.3.4 also presents considerations about the state required to store priority information about streams in an "idle" state. This state can be limited by adopting the guidance about concurrency limits described above. Extensible priorities is subject to a similar consideration because PRIORITY_UPDATE frames may arrive before the request that they reference. A server is required to store the information in order to apply the most up-to-date signal to the request. However, HTTP/3 implementations might have practical barriers to determining reasonable stream concurrency limits depending on the information that is available to them from the QUIC transport layer. TODO: so what can we suggest?

16. IANA Considerations

This specification registers the following entry in the Permanent Message Header Field Names registry established by [RFC3864]:

Header field name:
Applicable protocol:
Author/change controller:
Specification document(s):
This document
Related information:

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

Initial value:
This document

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

Frame Type:
This document

This specification registers the following entries in the HTTP/3 Frame Type registry established by [HTTP3]:

Frame Type:
0xF0700 and 0xF0701
This document

Upon publication, please create the HTTP Priority Parameters registry at and populate it with the types defined in Section 4; see Section 4.3.1 for its associated procedures.

17. References

17.2. Informative References

Common Vulnerabilities and Exposures, “CVE-2019-9513”, March 2019, <>.
Lassey, B. and L. , “Declaring Support for HTTP/2 Priorities”, Internet-Draft draft-lassey-priority-setting-00 (work in progress), July 2019.
Iyengar, J., Ed. and I. Swett, Ed., “QUIC Loss Detection and Congestion Control”, RFC 9002, DOI 10.17487/RFC9002, May 2021, <>.
Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004, <>.
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, RFC 7234, DOI 10.17487/RFC7234, June 2014, <>.
Petersson, A. and M. Nilsson, “Forwarded HTTP Extension”, RFC 7239, DOI 10.17487/RFC7239, June 2014, <>.
Lilley, C., “The "font" Top-Level Media Type”, RFC 8081, DOI 10.17487/RFC8081, February 2017, <>.

Appendix A. Acknowledgements

Roy Fielding presented the idea of using a header field for representing priorities in In, Patrick Meenan advocates for representing the priorities using a tuple of urgency and concurrency. The ability to deprecate HTTP/2 prioritization is based on [I-D.lassey-priority-setting], authored by Brad Lassey and Lucas Pardue, with modifications based on feedback that was not incorporated into an update to that document.

The motivation for defining an alternative to HTTP/2 priorities is drawn from discussion within the broad HTTP community. Special thanks to Roberto Peon, Martin Thomson and Netflix for text that was incorporated explicitly in this document.

In addition to the people above, this document owes a lot to the extensive discussion in the HTTP priority design team, consisting of Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku, Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy Fielding.

Appendix B. Change Log

B.1. 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.2. 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)

B.3. 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.4. 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.5. 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.6. 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.7. 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.8. Since draft-kazuho-httpbis-priority-01 📄 🔍

  • Explain how reprioritization might be supported.

B.9. Since draft-kazuho-httpbis-priority-00 📄 🔍

  • Expand urgency levels from 3 to 8.

Authors' Addresses

Kazuho Oku
Lucas Pardue