In the original specification, the server cookie was of variable length
and rules for its generation were left to the server implementers. This
resulted in interoperability problems if servers from multiple vendors
were placed behind the same public address. Thus, RFC 9018 defined a
standard mechanism of the content and generation of the cookie. This
type is such a standard server cookie.
This standard server cookie consists of a 1 octet version number
(currently 1), 3 reserved octets that must be zero, a 4 octet timestamp
as seconds since the Unix epoch, and 8 octets of hash value.
In version 1, the hash is calculated feeding the SipHash-2-4 that has been
initialized with a server secret the concatenation of client cookie,
version, reserved, timestamp, client IP address. Generatin and checking
the hash is available if the siphasher feature is enabled.