| HTTP | K. Oku |
| Internet-Draft | Fastly |
| Intended status: Standards Track | L. Pardue |
| Expires: May 24, 2020 | Cloudflare |
| November 21, 2019 |
Extensible Prioritization Scheme for HTTP
draft-kazuho-httpbis-priority-latest
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.
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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 representation 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 ([RFC7540]) 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.
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 sh-token and sh-boolean are imported from [STRUCTURED-HEADERS].
Example HTTP requests and responses use the HTTP/2-style formatting from [RFC7540].
This document uses the variable-length integer encoding from [I-D.ietf-quic-transport].
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 signaling scheme that used a combination of dependencies and weights, formed into an unbalanced tree. This scheme has suffered from poor deployment and interoperability.
The rich flexibility of client-driven HTTP/2 prioritization tree building is rarely exercised; experience shows that clients either choose a single model optimized for a web use case (and don’t vary it) or do nothing at all. But every client builds their prioritization tree in a different 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 favoring instead bespoke server-driven schemes based on heuristics and other hints, like the content type of resources and the order in which requests arrive. 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 [I-D.ietf-quic-http] 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.
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 [RFC7540]; Section 5.4.1) of type PROTOCOL_ERROR.
Endpoints MUST send this SETTINGS parameter as part of the first SETTINGS frame. When the peer receives the first SETTINGS frame, it learns the sender has deprecated the HTTP/2 priority scheme if it receives the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with the value of 1.
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 both the priority signal defined in the HTTP/2 priority scheme and also that of this prioritization scheme. Once the client learns that the HTTP/2 priority scheme is deprecated, it SHOULD stop sending the HTTP/2 priority signals. If the client learns that the HTTP/2 priority scheme is not deprecated, it SHOULD stop sending PRIORITY_UPDATE frames, but MAY continue sending the Priority header field, 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.
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 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 are used by clients to transmit the same information on a single hop. If intermediaries want to specify prioritizaton on a multiplexed HTTP connection, it 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 Headers Dictionary ([STRUCTURED-HEADERS]).
This document defines the urgency(u) and incremental(i) parameters. When used, these parameters MUST be accompanied by values. When any of the defined parameters are omitted, or if the Priority header field is not used, their default values SHOULD be applied.
Unknown parameters, parameters with out-of-range values or values of unexpected types MUST be ignored.
The urgency(u) parameter takes an integer between 0 and 7, in descending order of priority, as shown below:
| Urgency | Definition |
|---|---|
| 0 | prerequisite (Section 3.1.1) |
| 1 | default (Section 3.1.2) |
| between 2 and 6 | supplementary (Section 3.1.3) |
| 7 | background (Section 3.1.4) |
The value is encoded as an sh-integer. The default value is 1.
A server SHOULD transmit HTTP responses in the order of their urgency values. The lower 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 = example.net :path = /style.css priority = u=0
The definition of the urgencies and their expected use-case are described below. Endpoints SHOULD respect the definition of the values when assigning urgencies.
The prerequisite urgency (value 0) indicates that the response prevents other responses with an urgency of prerequisite or default from being used until it is fully transmitted.
For example, use of an external stylesheet can block a web browser from rendering the HTML. In such case, the stylesheet is given the prerequisite urgency.
The default urgency (value 1) indicates a response that is to be used as it is delivered to the client, but one that does not block other responses from being used.
For example, when a user using a web browser navigates to a new HTML document, the request for that HTML is given the default urgency. When that HTML document uses a custom font, the request for that custom font SHOULD also be given the default urgency. This is because the availability of the custom font is likely a precondition for the user to use that portion of the HTML document, which is to be rendered by that font.
The supplementary urgencies (values 2 to 6) indicate a response that is helpful to the client using a composition of responses, even though the response itself is not mandatory for using those responses.
For example, inline images (i.e., images being fetched and displayed as part of the document) are visually important elements of an HTML document. As such, users will typically not be prevented from using the document, at least to some degree, before any or all of these images are loaded. Display of those images are thus considered to be an improvement for visual clients rather than a prerequisite for all user agents. Therefore, such images will be given the supplementary urgency.
Values between 2 and 6 are used to represent this urgency, to provide flexibility to the endpoints for giving some responses more or less precedence than others that belong to the supplementary group. Section 6 explains how these values might be used.
Clients SHOULD NOT use values 2 and 6. Servers MAY use these values to prioritize a response above or below other supplementary responses.
Clients MAY use values 3 to indicate that a request is given relatively high priority, or 5 to indicate relatively low priority, within the supplementary urgency group.
For example, an image certain to be visible at the top of the page, might be assigned a value of 3 instead of 4, as it will have a high visual impact for the user. Conversely, an asynchronously loaded JavaScript file might be assigned an urgency value of 5, as it is less likely to have a visual impact.
When none of the considerations above is applicable, the value of 3 SHOULD be used.
The background urgency (value 7) is used for responses of which the delivery can be postponed without having an impact on using other responses.
As an example, the download of a large file in a web browser would be assigned the background urgency so it would not impact further page loads on the same connection.
The incremental(i) parameter takes an sh-boolean as the value that indicates if a 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 0.
A server SHOULD distribute the bandwidth of a connection between incremental responses that share the same urgency.
A server SHOULD transmit non-incremental responses one by one, preferably in the order the requests were generated. Doing so maximizes the chance of the client making progress in using the composition of the HTTP responses at the earliest moment.
The following example shows a request for a JPEG file with the urgency parameter set to 4 and the incremental parameter set to 1.
:method = GET :scheme = https :authority = example.net :path = /image.jpg priority = u=4, i=?1
When attempting to extend priorities, care must be taken to ensure any use of existing parameters are either unchanged or modified in a way that is backwards compatible for peers that are unaware of the extended meaning.
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 6).
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).
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 frame with the priority field value set to u=0 (prerequisite).
In HTTP/2 and HTTP/3, after a request message is sent on a stream, the stream transitions to a state that prevents the client from sending additional frames on the stream. Therefore, a client cannot reprioritize a response by using the Priority header field. Modifying this behavior would require a semantic change to the protocol, but this is avoided by restricting the stream on which a PRIORITY_UPDATE frame can be sent. In HTTP/2 the frame is on stream zero and in HTTP/3 it is sent on the control stream ([I-D.ietf-quic-http], Section 6.2.1).
This document specifies a new PRIORITY_UPDATE frame type for HTTP/2 ([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]) which enables reprioritization. It carries updated priority parameters and references the target of the reprioritization based on a version-specific identifier; in HTTP/2 this is the Stream ID, in HTTP/3 this is either the Stream ID or Push ID.
Unlike the header field, the reprioritization frame is a hop-by-hop signal.
The HTTP/2 PRIORITY_UPDATE frame (type=0xF) carries the stream ID of the response that is being reprioritized, and the updated priority in ASCII text, using the same representation as that of the Priority header field value.
The Stream Identifier field ([RFC7540], Section 4.1) in the PRIORITY_UPDATE frame header MUST be zero (0x0).
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| Stream ID (31) | +---------------------------------------------------------------+ | Priority Field Value (*) ... +---------------------------------------------------------------+
Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload
The PRIORITY_UPDATE frame payload has the following fields:
The HTTP/2 PRIORITY_UPDATE frame MUST NOT be sent prior to opening the stream. If a PRIORITY_UPDATE is received prior to the stream being opened, it MAY be treated as a connection error of type PROTOCOL_ERROR.
TODO: add more description of how to handle things like receiving PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid ID, etc.
The HTTP/3 PRIORITY_UPDATE frame (type=0xF) carries the identifier of the element that is being reprioritized, and the updated priority in ASCII text, using the same representation as that of the Priority header field value.
The PRIORITY_UPDATE frame MUST be sent on the control stream ([I-D.ietf-quic-http], Section 6.2.1).
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |T| Empty | Prioritized Element ID (i) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Priority Field Value (*) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: HTTP/3 PRIORITY_UPDATE Frame Payload
The PRIORITY_UPDATE frame payload has the following fields:
The HTTP/3 PRIORITY_UPDATE frame MUST NOT be sent with an invalid identifier, including before the request stream has been opened or before a promised request has been received. If a server receives a PRIORITY_UPDATE specifying a push ID that has not been promised, it SHOULD be treated as a connection error of type H3_ID_ERROR.
Because the HTTP/3 PRIORITY_UPDATE frame is sent on the control stream and there are no ordering guarantees between streams, a client that reprioritizes a request before receiving the response data might cause the server to receive a PRIORITY_UPDATE for an unknown request. If the request stream ID is within bidirectional stream limits, the PRIORITY_UPDATE frame SHOULD be buffered until the stream is opened and applied immediately after the request message has been processed. Holding PRIORITY_UPDATES consumes extra state on the peer, although the size of the state is bounded by bidirectional stream limits. There is no bound on the number of PRIORITY_UPDATES that can be sent, so an endpoint SHOULD store only the most recently received frame.
TODO: add more description of how to handle things like receiving PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid ID, etc.
It is not always the case that the client has the best understanding of how the HTTP responses deserve to be prioritized. 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.
By using the “Priority” response header, a server can override the prioritization hints provided by the client. When used, the parameters found in the response header field overrides those specified by the client.
For example, when the client sends an HTTP request with
:method = GET :scheme = https :authority = example.net :path = /menu.png priority = u=4, i=?1
and the origin responds with
:status = 200 content-type = image/png priority = u=2
the intermediary’s understanding of the urgency is promoted from 4 to 2, because the server-provided value overrides the value provided by the client. The incremental value continues to be 1, the value specified by the client, as the server did not specify the incremental(i) parameter.
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.
TODO: Discuss if we should add a signal that mitigates this issue. For example, we might add a SETTINGS parameter that indicates the next hop that the connection is NOT coalesced (see https://github.com/kazuho/draft-kazuho-httpbis-priority/issues/99).
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 end client to be delayed totally after those going to another.
In order to mitigate this fairness problem, when a server responds to a request that is known to have come through an intermediary, the server SHOULD prioritize the response as if it was assigned the priority of u=1, i=?1 (i.e. round-robin) regardless of the value of the Priority header field being transmitted, unless the server knows the intermediary is not coalescing requests from multiple clients.
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:
Responding to requests coming through an intermediary in a round-robin manner works well when the network bottleneck exists between the intermediary and the end client, as the intermediary would be buffering the responses and then be forwarding the chunks of those buffered responses based on the prioritization scheme it implements. A sophisticated server MAY use a weighted round-robin reflecting the urgencies expressed in the requests, so that less urgent responses would receive less bandwidth in case the bottleneck exists between the server and the intermediary.
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.
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.
Also, a client MAY use the priority values for making local scheduling choices for the requests it initiates.
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.
One of the aims of this specification is to define a mechanism for merging client- and server-provided hints for prioritizing the responses. For that to work, each urgency level needs to have a well-defined meaning. As an example, a server can assign the highest precedence among the supplementary responses to an HTTP response carrying an icon, because the meaning of u=2 is shared among the endpoints.
This specification restricts itself to defining a minimum set of urgency levels in order to provide sufficient granularity for prioritizing responses for ordinary web browsing, at minimal complexity.
However, that does not mean that the prioritization scheme would forever be stuck to the eight levels. The design provides extensibility. If deemed necessary, it would be possible to subdivide any of the eight urgency levels that are currently defined. Or, a graphical user-agent could send a visible parameter to indicate if the resource being requested is within the viewport.
A server can combine the hints provided in the Priority header field with other information in order to improve the prioritization of responses. For example, 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.
This specification registers the following entry in the Permanent Message Header Field Names registry established by [RFC3864]:
This specification registers the following entry in the HTTP/2 Settings registry established by [RFC7540]:
This specification registers the following entry in the HTTP/2 Frame Type registry established by [RFC7540]:
This specification registers the following entries in the HTTP/3 Frame Type registry established by [I-D.ietf-quic-http]:
| [I-D.ietf-quic-http] | Bishop, M., "Hypertext Transfer Protocol Version 3 (HTTP/3)", Internet-Draft draft-ietf-quic-http-23, September 2019. |
| [I-D.ietf-quic-transport] | Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-23, September 2019. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC7230] | Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014. |
| [RFC7540] | Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015. |
| [STRUCTURED-HEADERS] | Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Internet-Draft draft-ietf-httpbis-header-structure-14, October 2019. |
| [I-D.lassey-priority-setting] | Lassey, B. and L. Pardue, "Declaring Support for HTTP/2 Priorities", Internet-Draft draft-lassey-priority-setting-00, July 2019. |
| [RFC3864] | Klyne, G., Nottingham, M. and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004. |
| [RFC7234] | Fielding, R., Nottingham, M. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014. |
| [RFC7239] | Petersson, A. and M. Nilsson, "Forwarded HTTP Extension", RFC 7239, DOI 10.17487/RFC7239, June 2014. |
| [RFC8081] | Lilley, C., "The "font" Top-Level Media Type", RFC 8081, DOI 10.17487/RFC8081, February 2017. |
| [RFC8586] | Ludin, S., Nottingham, M. and N. Sullivan, "Loop Detection in Content Delivery Networks (CDNs)", RFC 8586, DOI 10.17487/RFC8586, April 2019. |
Roy Fielding presented the idea of using a header field for representing priorities in http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf. In https://github.com/pmeenan/http3-prioritization-proposal, Patrick Meenan advocates for representing the priorities using a tuple of urgency and concurrency. The ability to deprecate HTTP/2 priortization 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.