write a little about the network protocol

README.md

@@ -7,7 +7,95 @@ The lower layer takes care of p2p coordination while the upper chat layer sits o
 
 ### P2P
 
-TODO
+This protocol layer takes care of establishing a coherent Peer-to-Peer network over which arbitrary data can be transported from one node to another without regard to the underlying network topology.
+
+It requires datagram transport from the IP layer between nodes full messh reachability is not required.
+To upper layers (e.g. the chat protocol) it offers unreliable transport between arbitrary nodes of the network and addressing by node IDs instead of IP addresses.
+
+#### Concepts
+
+- **netid**: A unique 32bit identifier of each node
+- **local neighbor**: A local neighbor of node A is another node B that can be directly contacted by node A without NAT or firewalling.
+- **remote neighbor**: A remote neighbor of node A is another node B that is part of the same P2P network but which cannot be reached directly via UDP datagrams and only through traversing the P2P network.
+
+#### General Protocol Provisions
+
+The protocol is encoded in binary and uses the following common header:
+
+```
+0          7 8        15 16       23 24        31 
++-----------+-----------+-----------+------------+
+|  magic    |  version  | message   |   flags    |
+|           |           |  type     |            |
++-----------+-----------+-----------+------------+
+|                   nonce                        |
+|                                                |
++-----------+-----------+-----------+------------+
+|                 src netid                      |
+|                                                |
++-----------+-----------+-----------+------------+
+|                 dst netid                      |
+|                                                |
++-----------+-----------+-----------+------------+
+```
+
+- *magic*: The 8 bit constant value `0xAC`. It identifies this message as a P2P message.
+- *version*: 8 bit unsigned integer denoting the protocol version. In this spec always `0x01`.
+- *message type*: 8 bit identifying the type of payload this message carries.
+- *nonce*: A 32 bit, randomly generated, unique number associated with each packet.
+- *src netid*: The unique identifier of the node who originated this message.
+- *dst netidc*: The unique identifier of the node who this message is intended for.
+
+| Flag Bit | Name | Description |
+| -------- | :--: | ----------- |
+| `10000000` | NO_FORWARD | This message should not be forwarded even if the recipient is not the target node |
+| `01000000` | *reserved* |   |
+| `00100000` | *reserved* |   |
+| `00010000` | *reserved* |   |
+| `00001000` | *reserved* |   |
+| `00000100` | *reserved* |   |
+| `00000010` | *reserved* |   |
+| `00000001` | *reserved* |   |
+
+If a node receives a message without the NO_FORWARD flag set, and its own netid is not the *dst netid*, then it SHOULD forward the message unaltered to all its connected neighbors.
+To prevent packet loops, all participating nodes MUST store a hash of each received packet and not forward packets again that it has already processed. The local hash table SHOULD NOT be cleared more often than every 5 minutes.
+
+#### Special netids
+
+The following special netids are defined:
+
+| netid | name | description |
+| :---: | ---- | ----------- |
+| `0`   | any node | For message forwarding, all nodes consider this *dst netid* their own and do not forward the message |
+| `0xFFFFFFFF` | all nodes | For message forwarding, all nodes consider *dst netid* their own and forward the message to all their neighbors |
+
+#### Message Type *Hello*
+
+- Message Type: `0x01`
+
+This message is the introductory message that a new node sends to any node of the network it can reach and which identifies the new one.
+It has no payload.
+
+#### Message Type *Goodbye*
+
+- Message Type: `0x02`
+
+This message type MAY be sent by a node who is going offline or which is intending to leave the network.
+It has no payload.
+
+#### Message Type *Ping*
+
+- Message Type `0x03`
+
+A message which MAY be originated by a node to check for reachability of other nodes in the network.
+Nodes which receive this message SHOULD respond with a *Pong* message.
+
+#### Message Type *Pong*
+
+- Message Type `0x04`
+
+A message which MUST ONLY be originated by a node in response to a previous *Ping* message.
+
 
 ### Chat