Learning HTTP/2

Flow Control
A new feature in h2 is stream flow control. Unlike h1 where the server will send data
just about as fast as the client will consume it, h2 provides the ability for the client to
pace the delivery. (And, as just about everything in h2 is symmetrical, the server can
do the same thing.) Flow control information is indicated in WINDOW_UPDATE
frames. Each frame tells the peer endpoint how many bytes the sender is willing to
receive. As the end point received and consumes sent data it will send out a WIN‐
DOWS_UPDATE frame to indicate its updated ability to consume bytes. (Many an
early implementor spent a good deal of time debugging window updates to answer
the “Why am I not getting data?” question). It is the responsibility of the sender to
honor these limits.
A client may want to use flow control for a variety of reasons. One very practical reason
may be to make certain one stream does not choke out others. Or a client may
have limited bandwidth or memory available and forcing the data to come down in
manageable chunks will lead to efficiency gains. Though flow control cannot be
turned off, setting the maximum value of 2^31-1 effectively disables it, at least for
files under 2 GB in size. Another case to keep in mind is intermediaries. Very often
content is delivered through a proxy or content delivery network which terminated
the HTTP connections. Because the different sides of the proxy could have different
throughput capabilities, flow control allows a proxy to keep the two side closely in
sync to minimize the need for overly taxing proxy resournces.
Flow Control Example
At the start of every stream, the window defaults to 65,535 ( 2^16-1 ) bytes. Assume a
client end point A sticks with that default and their peer, B, sends 10,000 bytes. B
keeps track of the window ( now 55,535 bytes ). Now, say A takes its time and consumes
5,000 bytes and sends out a WINDOW_UPDATE frame indicating that its
window is now 60,535 bytes. B gets this and starts to send a large file ( 4 GB ). Each
send can only send up to the window size, 60,535 in this case. If at any point A wants
to adjust how much data it receives it can raise or lower its window at will via WIN‐
DOW_UPDATE frame.
Priority
The last important characteristic of streams is dependencies. Modern browser are
very careful to ask for the most important elements on a web page first. In this way it
improves performance by fetching objects in an optimal order. Once it has the HTML
in hand, the browser generally needs things like cascading style sheets (CSS) and critical
javascript before it can start painting the screen. Without multiplexing it needs to
wait for a response to complete before it can ask for a new object. With h2, the client
34 | Chapter 3: The Protocol