Compression

Encapsulation header

length	description
1	packet type
4	retransmit mask
2	encapsulation seq
2	encapsulation ack
2	command stream seq
2	command stream ack
2	command stream len

command stream length field is completely redundant with length field in the udp header.

encapsulation seq/ack and command stream seq/ack can be 8 bit fields since it takes 30 seconds for them to wrap around even then. In case of 10 seconds of no communication between 2 peers, a special value of 0xff in them can signal that a 16bit seq/ack field follows.

retransmit mask can be replaced by 1 bit in most cases, since it is zero basically all the time. bit=0 signals mask==0, bit=1 signals mask follows.

packet type can be replaced by 2 bits, since after the initial game setup stage, only command 4 and 5 are seen on the wire. 0==command4, 1==command5, 2==8bit command follows, 3==reserved

encapsulation header savings: 6 + 31 + 4*8 + 16 bits == 85 bits of 120 bits (70%) assuming 8 * (20 + 8) bits for IP4+UDP, total savings: 85 bits of 344 (25%)

Command stream

the command stream (for when encapsulation.type == 4) consist of a sequence of command packets with this structure:

length	description
1	command code
2	length
N	contents

Known command codes

code	length	argument	description
0	4	simtick increment	sender target simtick += argument
0x32	5	playerid,simtick	sender acknowledges simtick for playerid
0x33	4	simtick	? (simtick that was executed?)
0x34	4	simtick	? (simtick that is is closed and ready to be executed?)
0xc	var	?	? (sim command?)
0x16	var	?	? (lua command? e.g.: give resources, chat)
0x3	20	hash,simtick	desync detection hash

many more command codes that are still unknown.

The deflate compression appears to achieve a ca. 50% compression ratio, albeit for a case where not many commands were given to the simulation.

example packet dump (3 player game): https://github.com/faf5678/stuff/blob/master/packet_dump