Re: Weird MAX_CONCURRENT_STREAMS value with empty HTTP/2 SETTINGS frame

Hi,

Thank you both for your replies.

> Seems like it might be a nghttp2 issue though, as it tells curl there's a
> settings frame and the curl code simply reads what nghttp2 told it...
>
> Is this reproducible against a public site?

It seems that <https://twitter.com> answers with an empty SETTINGS frame and curl/nghttp2 says it allows 4294967295 MAX_CONCURRENT_STREAMS (which is maybe true, but seems a lot compared to the value of ~100 concurrent streams seen on most other HTTP/2 servers).

% curl -v --http2 --output /dev/null https://twitter.com
...
* Using Stream ID: 1 (easy handle 0x55f48bccf040)
> GET / HTTP/2
> Host: twitter.com
> user-agent: curl/7.68.0
> accept: */*
>
* Connection state changed (MAX_CONCURRENT_STREAMS == 4294967295)!
< HTTP/2 200
...

% nghttp -nv https://twitter.com
[ 0.139] Connected
The negotiated protocol: h2
[ 0.371] recv SETTINGS frame <length=6, flags=0x00, stream_id=0>
          (niv=1)
          [SETTINGS_INITIAL_WINDOW_SIZE(0x04):65536]
...
[ 0.372] send HEADERS frame <length=36, flags=0x25, stream_id=13>
          ; END_STREAM | END_HEADERS | PRIORITY
          (padlen=0, dep_stream_id=11, weight=16, exclusive=0)
          ; Open new stream
          :method: GET
          :path: /
          :scheme: https
          :authority: twitter.com
          accept: */*
          accept-encoding: gzip, deflate
          user-agent: nghttp2/1.36.0
[ 0.486] recv SETTINGS frame <length=0, flags=0x01, stream_id=0>
          ; ACK
          (niv=0)
[ 0.529] recv (stream_id=13) :status: 200
...

> The default max concurrent streams in nghttp2 is 4294967295.
> https://github.com/nghttp2/nghttp2/blob/v1.41.0/lib/nghttp2_session.c#L4425-L4429
> https://github.com/nghttp2/nghttp2/blob/v1.41.0/lib/nghttp2_session.h#L102-L103

Thanks for the links. I understand nghttp2 defines this value to UINT32_MAX, due to the fact that this is the maximum allowed by the RFC <https://github.com/nghttp2/nghttp2/commit/16c46114dc724278beaa6d59462f8396f35fa4a9>.

As I see it, the HTTP/2 server didn't explicitly send the number of MAX_CONCURRENT_STREAMS supported in a SETTINGS frame, so nghttp2 uses its default value (4294967295) and curl reads this value. I am right or did I miss something ?

Regards,

Alex
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Received on 2020-06-08