Web pages are made up of many resources that must be downloaded before they can be presented to the user. The role of HTTP prioritization is to load the right bytes at the right time in order to achieve the best performance. This is a collaborative process between client and server, a client sends priority signals that the server can use to schedule the delivery of response data. In HTTP/1.1 the signal is basic, clients order requests smartly across a pool of about 6 connections. In HTTP/2 a single connection is used and clients send a signal per request, as a frame, which describes the relative dependency and weighting of the response. HTTP/3 tried to use the same approach but dependencies don’t work well when signals can be delivered out of order.
HTTP/3 is being standardised as part of the QUIC effort. As a Working Group (WG) we’ve been trying to fix the problems that non-deterministic ordering poses for HTTP priorities. However, in parallel some of us have been working on an alternative solution, the Extensible Prioritization Scheme, which fixes problems by dropping dependencies and using an absolute weighting. This is signalled in an HTTP header field meaning it can be backported to work with HTTP/2 or carried over HTTP/1.1 hops. The alternative proposal is documented in the Individual-Draft draft-kazuho-httpbis-priority-04, co-authored by Kazuho Oku (Fastly) and myself. This has now been adopted by the IETF HTTP WG as the basis of further work; It’s adopted name will be draft-ietf-httpbis-priority-00.
To some extent document adoption is the end of one journey and the start of the next; sometimes the authors of the original work are not the best people to oversee the next phase. However, I’m pleased to say that Kazuho and I have been selected as co-editors of this new document. In this role we will reflect the consensus of the WG and help steward the next chapter of HTTP prioritization standardisation. Before the next journey begins in earnest, I wanted to take the opportunity to share my thoughts on the story of developing the alternative prioritization scheme through 2019.
I’d love to explain all the details of this new approach to HTTP prioritization but the truth is I expect the standardization process to refine the design and for things to go stale quickly. However, it doesn’t hurt to give a taste of what’s in store, just be aware that it is all subject to change.
A recap on priorities
The essence of HTTP prioritization comes down to trying to download many things over constrained connectivity. To borrow some text from Pat Meenan: Web pages are made up of dozens (sometimes hundreds) of separate resources that are loaded and assembled by a browser into the final displayed content. Since it is not possible to download everything immediately, we prefer to fetch more important things before less important ones. The challenge comes in signalling the importance from client to server.
In HTTP/2, every connection has a priority tree that expresses the relative importance between requests. Servers use this to determine how to schedule sending response data. The tree starts with a single root node and as requests are made they either depend on the root or each other. Servers may use the tree to decide how to schedule sending resources but clients cannot force a server to behave in any particular way.
Once all requests are received, the server determines all requests have equal priority and that it should send response data using round-robin scheduling: send some fraction of response 1, then a fraction of response 2, then a fraction of response 3, and repeat until all responses are complete.
A single HTTP/2 request-response exchange is made up of frames that are sent on a stream.
Each region of a frame is a named field, a ‘?’ indicates the field is optional and the value in parenthesis is the length in bytes with ‘*’ meaning variable length. The Header Block Fragment field holds compressed HTTP header fields (using HPACK), Pad Length and Padding relate to optional padding, and E, Stream Dependency and Weight combined are the priority signal that controls the priority tree.
The HEADERS frame E field is the interesting bit (pun intended). A request with the field set to 1 (true) means that the dependency is exclusive and nothing else can depend on the indicated node. To illustrate, imagine a client that sends three requests which set the E field to 1. As the server receives each request, it interprets this as an exclusive dependency on the root node. Because all requests have the same dependency on root, the tree has to be shuffled around to satisfy the exclusivity rules.
The final version of the tree looks very different from our previous example. The server would schedule all of response 3, then all of response 2, then all of response 1. This could help load all of an HTML file before an image and thus improve the visual load behaviour.
In reality, clients load a lot more than three resources and use a mix of priority signals. To understand the priority of any single request, we need to understand all requests. That presents some technological challenges, especially for servers that act like proxies such as the Cloudflare edge network. Some servers have problems applying prioritization effectively.
Because not all clients send the most optimal priority signals we were motivated to develop Cloudflare’s Enhanced HTTP/2 Prioritization, announced last May during Speed Week. This was a joint project between the Speed team (Andrew Galloni, Pat Meenan, Kornel Lesiński) and Protocols team (Nick Jones, Shih-Chiang Chien) and others. It replaces the complicated priority tree with a simpler scheme that is well suited to web resources. Because the feature is implemented on the server side, we avoid requiring any modification of clients or the HTTP/2 protocol itself. Be sure to check out my colleague Nick’s blog post that details some of the technical challenges and changes needed to let our servers deliver smarter priorities.
The Extensible Prioritization Scheme proposal
The scheme specified in draft-kazuho-httpbis-priority-04, defines a way for priorities to be expressed in absolute terms. It replaces HTTP/2’s dependency-based relative prioritization, the priority of a request is independent of others, which makes it easier to reason about and easier to schedule.
Rather than send the priority signal in a frame, the scheme defines an HTTP header – tentatively named “Priority” – that can carry an urgency on a scale of 0 (highest) to 7 (lowest).