[gizmo686]

You could implement it as a generic 'application metadata' field in the IP header. From the perspective of IP, it one more length prefixed field in the IP header. Routers may interpret it in conjunction with the value of the protocol field; otherwise they are just required to leave it in unchanged in the header (including in all fragments).

For packets that don't want to use it, this is just 1 byte of overhead to set the size to 0.