Wednesday, January 7, 2015

HTTP/2 Dependency Priorities in Firefox 37

Next week Firefox 35 will be in general release, and Firefox 37 will be promoted to the Developer Edition channel (aka Firefox Aurora).

HTTP/2 support will be enabled for the first time by default on a release channel in Firefox 35. Use it in good health on sites like Its awesome.

This post is about a feature that landed in Firefox 37 - the use of HTTP/2 priority dependencies and groupings. In earlier releases prioritization was done strictly through relative weightings - similar to SPDY or UNIX nice values. H2 lets us take that a step further and say that some resources are dependent on other resources in addition to using relative weights between transactions that are dependent on the same thing.

There are some simple cases where you really want a strict dependency relationship - for example two frames of the same video should be serialized on the wire rather than sharing bandwidth. More complicated relationships can be expressed through the use of grouping streams. Grouping streams are H2 streams that are never actually opened with a HEADERS frame - they exist simply to be nodes in the dependency tree that other streams depend on.

The canonical use case involves better prioritization for loading html pages that include js, css, and lots of images. When doing so over H1 the browser will actually defer sending the request for the images while the js and css load - the reason is that the transfer of the js/css blocks any rendering of the page and the high byte count of the images slows down the higher priority js/css if done in parallel. The workaround, not requesting the images at all while the js/css is loading, has some downsides - it incurs at least one extra round trip delay and it doesn't utilize the available bandwidth effectively in some cases.

The weighting mechanisms of H2 (and SPDY) can help here - and they are what are used prior to Firefox 37. Full parallelism is restored, but some unwanted bandwidth sharing still goes on.

I've implemented a scheme for H2 using 5 fixed dependency groups (known informally as leader, follower, unblocked, background, and speculative). They are created with the PRIORITY frame upon session establishment and every new stream depends on one of them.

Streams for things like js and css are dependent on the leader group and images are dependent on the follower group. The use of group nodes, rather than being dependent on the js/css directly, greatly simplifies dependency management when some streams complete and new streams of the same class are created - no reprioritization needs to be done when the group nodes are used.

This is experimental - the tree organization and its weights will evolve over time. Various types of resource loads will still have to be better classified into the right groups within Firefox and that too will evolve over time. There is an obvious implication for prioritization of tabs as well, and that will also follow over time. Nonetheless its a start - and I'm excited about it.

One last note to H2 server implementors, if you should be reading this, there is a very strong implication here that you need to pay attention to the dependency information and not simply implement the individual resource weightings. Given the tree above, think about what would happen if there was a stream dependent on leaders with a weight of 1 and a stream dependent on speculative with a weight of 255. Because the entire leader group exists at the same level of the tree as background (and by inclusion speculative) the leader descendents should dominate the bandwidth allocation due to the relative weights of those group nodes - but looking only at the weights of the individual streams gives the incorrect conclusion.