PacketHeader

Every packet starts with a small header (see packet). It contains:

• the packet type (data vs fragment, essentially)
• the packet id (a wrapping u16)
• the last ack-ed packet id received by the sender
• an ack bitfield covering the 32 packets before that id (so 33 acks in total)
• the current tick

This schema is adopted from the GafferOnGames blogpost.