Also see RFC 1948/6528 – timestamps have been augmented by a PRNG since to avoid sequence number attacks.

@klickverbot Is there some source of truth for which RFCs are actually authoritative for TCP? RFC 793 is hopelessly outdated and has errata, RFC 1122 fixes some of that, highlights a few common errors, many of which I did make, but also piles completely useless junk on top of it (I think every ICMP message it specifically mentions except unreachables and echo request/reply is deprecated, strongly discouraged and ignored by any modern implementation).

@whitequark: Unfortunately, I don't know of any up to date list of RFCs relevant for the various areas, but I found the review in RFC 7414 to be quite useful (from 2015).

@klickverbot Thanks

The timeouts for sending out IGMP reports could also use a PRNG.

Could one be built with just an Instant timestamp as input?

The TCP specification dictates a particular algorithm for the PRNG. While not great it's more complex than just the timestamp...