doc: document data frames

doc/hamnet70.adoc

@@ -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.