How do we interpret the following server-timing header example?

server-timing: cfL4;desc="?proto=TCP&rtt=7337&sent=8&recv=8&lost=0&retrans=0&sent_bytes=3419&recv_bytes=832&delivery_rate=548023&cwnd=25&unsent_bytes=0&cid=94dae6b578f91145&ts=225

ts - it is in milliseconds, but I don't know what it means by data captured. Does this mean first byte from the downstream servers?

rtt - I cannot tell what unit this is in... Likely it is in microseconds, seems like the only logical assumption. But I hope someone can clarify.

I have tried the following references

References:

• /

There does not seems to be a standard for the fields and names merely the format of the server-timing header and how to concatenate the fields.