Add documentation for CRTP link services

docs/functional-areas/crtp/crtp_link.md

@@ -0,0 +1,37 @@
+---
+title: Link layer services
+page_id: crtp_link
+---
+
+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 |
+|------|---------|----------|
+| 13   | 0       | [Echo](#platform-commands) |
+| 13   | 1       | [Source](#platform-commands)  |
+| 13   | 2       | [Sink](#app-channel) |
+| 13   | 3       | [*Null packets*](#null-packet) |
+
+## Echo
+
+Packets sent to the Echo channel are send back unaltered.
+
+## Source
+
+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).
+
+## Sink
+
+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.

docs/functional-areas/crtp/crtp_platform.md

@@ -64,7 +64,7 @@ Answer:
 | 1    | Version |
 
 Returns the CRTP protocol version. See the
-[protocl versioning and stability guarantee](../#protocol-version-and-stability-guarantee) documentation for more
+[protocl versioning and stability guarantee](index.md#protocol-version-and-stability-guarantee) documentation for more
 information.
 
 ### Get firmware version
@@ -114,4 +114,4 @@ Returns a String representation of the device type the firmware is running on. T
 
 The app channel is intended to be used by user apps on the Crazyflie and on the ground to exchange data. Every packet
 sent and received from the app channel (port:channel) (13:2) will be available through the
-[app channel API](/docs/userguides/app_layer/#app-channel-packet-based-communication-between-the-crazyflie-and-the-python-lib).
+[app channel API](/docs/userguides/app_layer.md#app-channel-packet-based-communication-between-the-crazyflie-and-the-python-lib).

docs/functional-areas/crtp/index.md

@@ -117,7 +117,7 @@ its handling on the ground.
 |  7       | [Generic Setpoint](crtp_generic_setpoint.md) | Generic instantaneous setpoints (ie. position control and more) |
 |  13      | [Platform](crtp_platform.md)                 | Used for misc platform control, like debugging and power off |
 |  14      | Client-side debugging                        | Debugging the UI and exists only in the Crazyflie Python API and not in the Crazyflie itself.|
-|  15      | Link layer                                   | Low level link-related service. For example *echo* to ping the Crazyflie |
+|  15      | [Link layer](crtp_link.md)                   | Low level link-related service. For example *echo* to ping the Crazyflie |
 
 Connection procedure
 --------------------