The first frame after connection preface

[oimou]

RFC7540 says

That is, the connection preface starts with the string "PRI *
HTTP/2.0\r\n\r\nSM\r\n\r\n"). This sequence MUST be followed by a
SETTINGS frame (Section 6.5), which MAY be empty.

in 3.5 HTTP/2 Connection Preface.

So, the first frame after connection preface should be SETTINGS. However, according to this output of h2i, h2o server seems to accept HEADERS frame as the first frame after connection preface.

 % ./h2o -c h2o.conf &
...

 % h2i -insecure localhost:8081
Connecting to localhost:8081 ...
Connected to [::1]:8081
Negotiated protocol "h2"
[FrameHeader SETTINGS len=12]
  [MAX_CONCURRENT_STREAMS = 100]
  [INITIAL_WINDOW_SIZE = 16777216]
h2i> HEADERS
(as HTTP/1.1)> GET / HTTP/1.1
(as HTTP/1.1)>
Opening Stream-ID 1:
 :authority =
 :method = GET
 :path = /
 :scheme = https
[FrameHeader HEADERS flags=END_HEADERS stream=1 len=63]
  :status = "404"
  server = "h2o/1.8.0-alpha1"
  date = "Sat, 20 Feb 2016 19:04:32 GMT"
  content-type = "text/plain; charset=utf-8"
  content-length = "9"
[FrameHeader DATA flags=END_STREAM stream=1 len=9]
  "not found"
h2i>

Is it expected behaviour?

[kazuho]

according to this output of h2i, h2o server seems to accept HEADERS frame as the first frame after connection preface.

Thank you for pointing out. I think your observation is correct.

However, according to this output of h2i, h2o server seems to accept HEADERS frame as the first frame after connection preface.

I thought that h2i always sent SETTINGS frame as part of the preface without logging it. Is my understanding wrong, or did you use a modified version of h2i?

[oimou]

Thank you for your response. Anyway, I don't use a modified version. I looked into an implementation of h2i a bit and it seems not send implicit SETTINGS frame after client preface.
https://github.com/bradfitz/http2/blob/master/h2i%2Fh2i.go#L165