Today I tried to understand the first client packet with CHLO (Client Hello) tag. There was one undocumented 16 byte field that I could not understand. This field was just after the public packet and before the CHLO tag. The value of this field was different for every request.
I dug into proto-quic sources to find the answer. It took me almost a day. And unfortunately I did not find the answer. At the end of the day I posted a question on Google Groups https://groups.google.com/a/chromium.org/forum/#!topic/proto-quic/2T8QB6p3kIg.
Turns out that this field is a packet hash. And the hash function is FNV1a.