[tliltocatl]

Cap'n'proto doesn't have tags, but it wastes even more bytes in favor of speed. Than again, omitting tags only saves space if you are sending all the fields every time. PER uses a bitmap, which is still a bit wasteful on large sparse structs.

[cryptonector]

PER sends a bitmap only of OPTIONAL members' (fields') presence/absence. Required members are just where you expect them: right after their preceding members.