Link layer services

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.

CRTP channels

port channel Function
15 0 Echo
15 1 Source
15 2 Sink
15 3 Null 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 packet

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.