Two ways to skip a body

Three requests for the same page. The first sends If-None-Match with the tag the client already holds, "v7", so the server answers 304 Not Modified (RFC 9110 §13.1.2). The second is a HEAD, answered 200 with the headers a GET would get (RFC 9110 §9.3.2). Each of those answers is one HEADERS frame with END_STREAM set, and no DATA follows (RFC 9113 §8.1). The third is a plain GET, and only it carries the 24 KB body.

Connection · one pipe, many streams

DATA: a block as wide as its time on the wire, in its stream's colour
HEADERS: hatched
SETTINGS, ACK, PING, WINDOW_UPDATE, GOAWAY: a tick with a glyph
RST_STREAM: a cut; NO_ERROR is not a failure
END_STREAM: a thick trailing edge
a window: the filled part is the credit left, in bytes, not in time
a gauge for a stream with no window yet: dashed, not the same as empty
bytes ready with no credit
current frame
in flight: from its last byte to its landing at the other end
why: the frame whose landing set the current one off
faded: not sent yet
close ticks step to a second line so two close glyphs don't overlap

The pipe

client windows

server windows

Step 0 of 23

Streams at this step
s0	connection SET	1 frames
s1	cached	not open yet
s3	head	not open yet
s5	full	not open yet

Frame

Reset

Step 0 · SETTINGS · stream 0 · leaves client → server

The client sends its SETTINGS

Before any frame, the client sends the 24-octet connection preface, PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n. RFC 9113 §3.4 chose it so that a large proportion of HTTP/1.x servers and intermediaries do not process further frames. It is not a frame, so it is not drawn. Then SETTINGS states what the client will accept: frames of up to 16 384 bytes (MAX_FRAME_SIZE), 65 535 bytes per stream before the server must wait for more window (INITIAL_WINDOW_SIZE). It also turns server push off (ENABLE_PUSH 0); the hub does not model it anyway.

moment: leaves the client
sent at: 0 ms
received at: 21 ms
length: 18 bytes
flags: none

Settings

SETTINGS_ENABLE_PUSH: 0
SETTINGS_INITIAL_WINDOW_SIZE: 65 535
SETTINGS_MAX_FRAME_SIZE: 16 384

RFC 9113 §3.4 · RFC 9113 §6.5.2

Lifecycle

RFC 9113 §5.1

ES is END_STREAM. Select a state to read about it. RFC 9113 §5.1 also has two reserved states for server push; they are left out because push is not modelled.

The client's actions, in order. Each runs when its trigger is met. Change any of them and the trace is worked out again.

cached GET /index.html not sent yet

at start
head HEAD /index.html not sent yet

at start
full GET /index.html not sent yet

at start