This is xnu-8019. See this file in:
Directory listing for xnu-8019//libsyscall/wrappers/skywalk
cpu_copy_in_cksum.s
cpu_copy_in_cksum_gen.c
cpu_in_cksum.s
cpu_in_cksum_gen.c
os_channel.c
os_channel_event.c
os_nexus.c
os_packet.c