Warum werden 5 PRIORITY-Frames vom HTTP2-Client gesendet, bevor ein HEADERS-Frame gesendet wird? Sind sie für eine erfolgreiche HTTP2-Verbindung notwendig?

Question

Ich habe festgestellt, dass einige HTTP2-Clients (Firefox und nghttp) 5 PRIORITY-Frames für Stream 3, 5, 7, 9, 11 senden, nachdem http2-Protokoll vereinbart wurde und bevor sie ein HEADERS-Frame senden. Ich bin neugierig warum? Ich kenne die Bedeutung von PRIORITY-Frames, aber ich verstehe es nicht, es zu senden, aber Firefox und nghttp tun es, also muss es einige Gründe geben.

Und am wichtigsten, diese Ströme (Strom 3, 5, 7, 9, 11) werden nicht nach PRIORITY Rahmen gemäß dem Protokoll verwendet.

Ausgabeinformationen vom Server für Firefox werden wie folgt angezeigt.

$ nghttpd -d ~/Proxy 8080 key.pem cert.pem -v 
Enter PEM pass phrase: 
IPv4: listen 0.0.0.0:8080 
IPv6: listen :::8080 
[ALPN] client offers: 
* h2 
* spdy/3.1 
* http/1.1 
SSL/TLS handshake completed 
The negotiated protocol: h2 
[id=1] [ 18.917] send SETTINGS frame <length=6, flags=0x00, stream_id=0> 
     (niv=1) 
     [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100] 
[id=1] [ 18.920] recv SETTINGS frame <length=12, flags=0x00, stream_id=0> 
     (niv=2) 
     [SETTINGS_INITIAL_WINDOW_SIZE(0x04):131072] 
     [SETTINGS_MAX_FRAME_SIZE(0x05):16384] 
[id=1] [ 18.920] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0> 
     (window_size_increment=12517377) 
[id=1] [ 18.920] recv PRIORITY frame <length=5, flags=0x00, stream_id=3> 
     (dep_stream_id=0, weight=201, exclusive=0) 
[id=1] [ 18.920] recv PRIORITY frame <length=5, flags=0x00, stream_id=5> 
     (dep_stream_id=0, weight=101, exclusive=0) 
[id=1] [ 18.920] recv PRIORITY frame <length=5, flags=0x00, stream_id=7> 
     (dep_stream_id=0, weight=1, exclusive=0) 
[id=1] [ 18.920] recv PRIORITY frame <length=5, flags=0x00, stream_id=9> 
     (dep_stream_id=7, weight=1, exclusive=0) 
[id=1] [ 18.920] recv PRIORITY frame <length=5, flags=0x00, stream_id=11> 
     (dep_stream_id=3, weight=1, exclusive=0) 
[id=1] [ 18.920] send SETTINGS frame <length=0, flags=0x01, stream_id=0> 
     ; ACK 
     (niv=0) 
[id=1] [ 18.926] recv SETTINGS frame <length=0, flags=0x01, stream_id=0> 
     ; ACK 
     (niv=0) 
[id=1] [ 25.773] recv (stream_id=13) :method: GET 
[id=1] [ 25.773] recv (stream_id=13) :path: /plaintext.txt 
[id=1] [ 25.773] recv (stream_id=13) :authority: 127.0.0.1:8080 
[id=1] [ 25.773] recv (stream_id=13) :scheme: https 
[id=1] [ 25.773] recv (stream_id=13) user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:45.0) Gecko/20100101 Firefox/45.0 
[id=1] [ 25.773] recv (stream_id=13) accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 
[id=1] [ 25.773] recv (stream_id=13) accept-language: en-US,en;q=0.5 
[id=1] [ 25.773] recv (stream_id=13) accept-encoding: gzip, deflate, br 
[id=1] [ 25.773] recv (stream_id=13) if-modified-since: Wed, 27 Apr 2016 13:10:07 GMT 
[id=1] [ 25.773] recv HEADERS frame <length=196, flags=0x25, stream_id=13> 
     ; END_STREAM | END_HEADERS | PRIORITY 
     (padlen=0, dep_stream_id=11, weight=32, exclusive=0) 
     ; Open new stream 
[id=1] [ 25.774] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=13> 
     (window_size_increment=12451840) 
[id=1] [ 25.774] send HEADERS frame <length=42, flags=0x05, stream_id=13> 
     ; END_STREAM | END_HEADERS 
     (padlen=0) 
     ; First response header 
     :status: 304 
     server: nghttpd nghttp2/1.8.0 
     date: Fri, 29 Apr 2016 08:07:25 GMT 
[id=1] [ 25.774] stream_id=13 closed 

Aber Chrome sendet sie nicht, wie unten gezeigt. Sie sind also nicht notwendig für eine erfolgreiche Verbindung?

$ nghttpd -d ~/Proxy 8080 key.pem cert.pem -v 
Enter PEM pass phrase: 
IPv4: listen 0.0.0.0:8080 
IPv6: listen :::8080 
[ALPN] client offers: 
* h2 
* spdy/3.1 
* http/1.1 
SSL/TLS handshake completed 
The negotiated protocol: h2 
[id=1] [ 16.069] send SETTINGS frame <length=6, flags=0x00, stream_id=0> 
     (niv=1) 
     [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100] 
[id=1] [ 16.083] closed 
[ALPN] client offers: 
* h2 
* spdy/3.1 
* http/1.1 
SSL/TLS handshake completed 
The negotiated protocol: h2 
[id=2] [ 16.298] send SETTINGS frame <length=6, flags=0x00, stream_id=0> 
     (niv=1) 
     [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100] 
[id=2] [ 16.299] closed 
[ALPN] client offers: 
* h2 
* spdy/3.1 
* http/1.1 
SSL/TLS handshake completed 
The negotiated protocol: h2 
[id=3] [ 16.303] send SETTINGS frame <length=6, flags=0x00, stream_id=0> 
     (niv=1) 
     [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):100] 
[id=3] [ 16.303] recv SETTINGS frame <length=12, flags=0x00, stream_id=0> 
     (niv=2) 
     [SETTINGS_MAX_CONCURRENT_STREAMS(0x03):1000] 
     [SETTINGS_INITIAL_WINDOW_SIZE(0x04):6291456] 
[id=3] [ 16.303] recv WINDOW_UPDATE frame <length=4, flags=0x00, stream_id=0> 
     (window_size_increment=15663105) 
[id=3] [ 16.303] send SETTINGS frame <length=0, flags=0x01, stream_id=0> 
     ; ACK 
     (niv=0) 
[id=3] [ 16.303] recv SETTINGS frame <length=0, flags=0x01, stream_id=0> 
     ; ACK 
     (niv=0) 
[id=3] [ 16.311] recv (stream_id=1) :method: GET 
[id=3] [ 16.311] recv (stream_id=1) :authority: 127.0.0.1:8080 
[id=3] [ 16.311] recv (stream_id=1) :scheme: https 
[id=3] [ 16.311] recv (stream_id=1) :path: /plaintext.txt 
[id=3] [ 16.311] recv (stream_id=1) accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 
[id=3] [ 16.311] recv (stream_id=1) upgrade-insecure-requests: 1 
[id=3] [ 16.311] recv (stream_id=1) user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.86 Safari/537.36 
[id=3] [ 16.311] recv (stream_id=1) accept-encoding: gzip, deflate, sdch 
[id=3] [ 16.311] recv (stream_id=1) accept-language: en-US,en;q=0.8 
[id=3] [ 16.311] recv HEADERS frame <length=238, flags=0x25, stream_id=1 
.... 

Answer 1

Sie sind nicht für eine erfolgreiche Verbindung erforderlich.

Für den Rest rate ich nur hier. Firefox verwendet diese Streams als "virtuelle Streams". Dann werden weitere reale Streams von diesen virtuellen Streams abhängig gemacht. Der Effekt sollte die Erstellung unterschiedlicher Prioritätsgruppen sein, ohne Prioritäten für (reale) Streams angeben zu müssen.