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en:impulsion

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 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.

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.

en/impulsion.txt · Last modified: 2020/01/15 14:53 by Mike