This ports handle various link-related low level services. It most notably host the null packet used by the radio link to pull downlink packets.
Packets sent to the Echo channel are send back unaltered.
When receiving any packet on the source channel, the Crazyflie sends back a 32 bytes packet on the source channel.
Since protocol version 1, this packet contains the string “Bitcraze Crazyflie” followed by zeros. Before version 1 the content of the packet was not defined. This allows to detect firmware protocol version bellow 1 (the getProtocolVersion packet was only implemented after version 1 of the protocol).
Packet sent to the sink channel are dropped and ignored.
Null packets must be dropped. The data part of NULL packet is used for some out-of-band communication at the link level or by the bootloader. The Crazyflie firmware and lib should ignore them.