doc: document connectionless frame type

doc/hamnet70.adoc

@@ -10,7 +10,7 @@ Thomas Kolb DL5TKL
 
 == Introduction
 
-Hamnet70 intends to provide a network system in the 70 cm amateur radio band that can transfer Internet Protocol (IP) packets at a high speed.
+https://git.tkolb.de/amateurfunk/hamnet70[Hamnet70] intends to provide a network system in the 70 cm amateur radio band that can transfer Internet Protocol (IP) packets at a high speed.
 The protocols defined here are inspired by the Packet Radio network and Bluetooth Low Energy.
 
 The protocols are designed primarily for centralized infrastructure as they were common in the Packet Radio network.
@@ -238,6 +238,49 @@ Due to the long possible bursts, this system can achieve high throughput if ther
 
 === Frame Definitions
 
+==== Data Frame
+
+Data Frames carry all higher-layer data in a connection.
+Each Data Frame transfers a single layer-3 packet.
+
+The layer 2 header of Data Frames is filled as follows:
+
+- Message Type: `000` (Data Frame)
+- TX Request: `1` if this is the last packet in the burst, `0` otherwise
+- Source Address: the transmitter’s HAM-64 address
+- Destination Address: the target station’s HAM-64 address
+- TX sequence number: as required by Go-Back-N
+- RX sequence number: as required by Go-Back-N
+
+To identify how the encoded packet should be handled, the layer 3 protocol is encoded in the first byte of the layer 2 payload.
+The full layer 2 payload therefore is composed as follows:
+
+- Layer 3 protocol ID (1 Byte)
+- Layer 3 packet data (variable length)
+
+So far, the following protocols are defined and supported:
+
+[cols="2,1,1", options="header"]
+.Layer 3 protocol identifiers. EtherType is given as reference.
+|===
+| Protocol | Hamnet70 ID | EtherType
+
+| IPv6
+| `0x00`
+| `0x86DD`
+
+| IPv4
+| `0x10`
+| `0x0800`
+
+| _undefined/auto_
+| `0xFF`
+| -
+
+3+|_All other values are reserved._
+
+|===
+
 ==== Empty Frame
 
 The Empty Frame does not contain any data and therefore only consists of the header and the CRC.
@@ -436,8 +479,8 @@ The layer 2 header is filled as follows:
 - TX Request: `1` if this is the last packet in the burst, `0` otherwise
 - Source Address: the digipeater’s HAM-64 address
 - Destination Address: the client’s HAM-64 address
-- TX sequence number: as counted in the connection
-- RX sequence number: as counted in the connection
+- TX sequence number: as required by Go-Back-N
+- RX sequence number: as required by Go-Back-N
 
 The message contains exactly 1 data byte: `0x04` to indicate that this is a Disconnect Request packet.
 
@@ -460,8 +503,8 @@ The layer 2 header is filled as follows:
 - TX Request: `0`
 - Source Address: the client’s HAM-64 address
 - Destination Address: the digipeater’s HAM-64 address
-- TX sequence number: as counted in the connection
-- RX sequence number: as counted in the connection
+- TX sequence number: as required by Go-Back-N
+- RX sequence number: as required by Go-Back-N
 
 The message contains exactly 1 data byte: `0x05` to indicate that this is a Disconnect packet.
 
@@ -471,6 +514,39 @@ The sequence numbers are sent and handled the same way as in regular connection
 When the digipeater receives a disconnect packet in the regular packet flow (i.e. no previous packets are lost), it will immediately drop the connection state and not call this client again.
 Therefore, if the client wants to ensure that all previous packets are transmitted, it must wait until the digipeater confirms that by sending the corresponding RX sequence number before sending the Disconnect packet.
 
+==== Connectionless Frame
+
+Connectionless Frames are used to transfer packets between unconnected notes.
+They can be used to implement custom protocols (similar to APRS, which is implemented on top of AX.25).
+
+The layer 2 header of Connectionless Frames is filled as follows:
+
+- Message Type: `100` (Connectionless Frame)
+- TX Request: `1` if this is the last packet in the burst, `0` otherwise
+- Source Address: the transmitter’s HAM-64 address
+- Destination Address: the target station’s HAM-64 address
+- TX sequence number: user-defined
+- RX sequence number: user-defined
+
+The sequence numbers can be used in any way that is useful for the custom protocol.
+
+It is required that the first byte of each Connectionless Frame identify the protocol being used.
+Protocol numbers are centrally assigned and are listed below.
+Some protocol numbers are reserved for experimentation and development and can be self-assigned temporarily.
+
+[cols="1,3", options="header"]
+.Connectionless Frame protocol IDs
+|===
+|Protocol IDs
+|Description
+
+|`0x00 .. 0xF7`
+|_reserved_
+
+|`0xF8 .. 0xFF`
+|Available for experimentation and developmentfootnote:[If you are developing a new protocol, you can freely pick a number from this range. Please check which temporary IDs are already used around your location and pick a free one. When your protocol reaches a sufficiently stable state, please request an official ID assignment].
+|===
+
 === Message Sequence Charts
 
 ==== Connection Establishment