Application-Layer Protocol Negotiation (ALPN). It is a byte sequence defining
the upper layer protocol in use over the connection. The most common use case
will be to pass the exact copy of the ALPN extension of the Transport Layer
Security (TLS) protocol as defined by RFC7301 [9].
Contains the host name value passed by the client, as an UTF8-encoded string.
In case of TLS being used on the client connection, this is the exact copy of
the “server_name” extension as defined by RFC3546 [10], section 3.1, often
referred to as “SNI”. There are probably other situations where an authority
can be mentioned on a connection without TLS being involved at all.
The TLV of this type should be ignored when parsed. The value is zero or more
bytes. Can be used for data padding or alignment. Note that it can be used
to align only by 3 or more bytes because a TLV can not be smaller than that.
The value of the type PP2_TYPE_UNIQUE_ID is an opaque byte sequence of up to
128 bytes generated by the upstream proxy that uniquely identifies the
connection.