====== Impulsion ======

Impulsion is a bit-optimised format for communication between NeL server and client.

===== Impulsion Format =====

The basic header format for impulsions is simple:

  packetNumber: 32-bit signed integer
  systemMode: 1-bit bool, if this is a system impulsion or not

For system-mode impulsions (which are used for connection and deconnection "meta" messages) there are 10 types, identified by the next number:

  systemMessageType: 8-bit unsigned integer, meaning:

| 0 | SYSTEM_LOGIN_CODE          | client -> server |
| 1 | SYSTEM_SYNC_CODE           | server -> client |
| 2 | SYSTEM_ACK_SYNC_CODE       | client -> server |
| 3 | SYSTEM_PROBE_CODE          | server -> client |
| 4 | SYSTEM_ACK_PROBE_CODE      | client -> server |
| 5 | SYSTEM_DISCONNECTION_CODE  | client -> server |
| 6 | SYSTEM_STALLED_CODE        | server -> client |
| 7 | SYSTEM_SERVER_DOWN_CODE    | server -> client |
| 8 | SYSTEM_QUIT_CODE           | client -> server |
| 9 | SYSTEM_ACK_QUIT_CODE       | server -> client |

For normal impulsions:

  ackNumber: 32-bit signed integer (packetNumber which is acked by this packet?)

Then there are three levels, each of which looks like follows:

  channelPresent: 1-bit bool, if there is anything sent in this channel or not
  shortCode: 1-bit bool, if the action code is only 2 bits or 8 bits
  actionCode: either 2-bit or 8-bit unsigned integer, 2-bit if shortCode is true, meaning:

| 0  | ACTION_POSITION_CODE            | for sending position of a creature (not checked)         |
| 1  | ACTION_GENERIC_CODE             | for sending generic data in just one impulsion           |
| 2  | ACTION_GENERIC_MULTI_PART_CODE  | for sending generic data split over impulsions           |
| 3  | ACTION_SINT64                   | for sending any properties, how to decode correct size?  |
|                                                                                               |||
| 10 | ACTION_SYNC_CODE                | ?                                                        |
| 11 | ACTION_DISCONNECTION_CODE       | ?                                                        |
| 12 | ACTION_ASSOCIATION_CODE         | ?                                                        |
| 13 | ACTION_LOGIN_CODE               | ?                                                        |
|                                                                                               |||
| 40 | ACTION_TARGET_SLOT_CODE         | ?                                                        |
|                                                                                               |||
| 99 | ACTION_DUMMY_CODE               | ?                                                        |


===== Wireshark =====

The [[https://git.khaganat.net/causec/nel-layer5-wireshark-dissector|git repository]] with the layer5 (inter-service communication on server) lua dissector for wireshark, now also includes a start for impulsion dissection. For now it just dissects specified UDP port (47851) so you might have to change that for it to work (in nelimpulsion.lua:198). Also for now it just dissects some system messages (types 0-4) and a start for normal impulsion dissection.

<WRAP center round important 60%>
Since it's somewhat complicated, and bit-wise dissection in wireshark is quite annoying to do, this is very much work in progress still. A lot has to be understood and correctly dissected still.
</WRAP>


{{tag>NeL Network Communication}}