There isn't one! TCP running on top of UDP would have been a reasonable design. TCP/IP is full of warts like this. The URG pointer. The conflicting length fields.
The fact of ICMP not being itself a UDP protocol caused major problems for systems programmers, because it meant that OS kernels all "owned" ICMP, provided only a baroque sockopt programming interface to like 5% if it, and required userland programs to hold suser privileges to do any real ICMP work. Awful design. And ICMP is slow-pathed by routers, because it isn't UDP.
UDP literally doesn't do anything but multiplex raw IP. Unless you're actually worried about the 8 bytes the header takes up, there's no reason, none at all, to slide a new IP protocol anywhere but on top of UDP. Again: that's why UDP was designed in the first place. You can go look this up! David Reed still talks about it!
