At least with IPv4 you can easily compute the start of layer 4, it's just ipv4[0]&0xf * 4 - trivially implemented with little real-estate in hardware. IPv6 re

At least with IPv4 you can easily compute the start of layer 4, it's just ipv4[0]&0xf * 4 - trivially implemented with little real-estate in hardware. IPv6 requires you to loop through all extension headers to reach layer 4, and you need to know about a handful of those extension headers as not all follow the same format.