Difference between revisions of "Latency"

Latest revision as of 16:55, 29 July 2005

Latency (or lag) occurs when a client's average ping is very different from its current ping. This happens because clients that receive a laggy client's position packets can incorrectly predict the lagger's position, as per the following equation: PredictedXCoord = SentXCoord + (RateInXDirection * LagTime). Now this will work fine as long as the laggy ship is flying at a constant velocity, and it usually does, since some lag always exists, and ships are always at predicted positions and not actual ones. But if the ship changes velocity (either speed or direction), the equation will predict incorrect positions and the ship will "jump around" on other clients' screens as the true positions are received.

Since this latency can easily disrupt gameplay, both Subgame and ASSS have configurable Latency Settings.

Timing

There's no way to know one-way latency on a network; one can only estimate it. The Subspace protocol does this as follows:

• The client sends a packet with its current time (in milliseconds) to the server every 5 seconds.
• When the server receives this packet, it immediately replies, sending back the client's time and the current server time, which is different from the client time.
• When (if) the client receives this reply, it now knows: how long it took (round trip) for the sync exchange, and the server time at some point in the "middle".

Example:

Client send time: 100
Client receive time: 300
Server time: 1000

Round trip: (300 - 100) = 200
Server time estimate (at client time 300): 1000 + 200 / 2 = 1100

The actual clients do not use 50%, but 60%. Don't ask why.

All packets that require time use the server time. This includes movement, weapons, and ball packets. If the sync packet exchange suffers an extra delay of 1000ms at a router, you would get something like this:

Client send time: 100
Client receive time: 1300
Server time: 1000

Round trip: (1300 - 100) = 1200
Server time estimate (at client time 1300): 1000 + 1200 / 2 = 1600
Actual server time (at client time 1300): 2100

This creates "fake" C2S latency. So for the next 5 seconds, the client will send estimated times that are early by about 500ms, making the server think they were sent much further in the past than they were. If the client were allowed to continue to play under these circumstances, their ship would be displayed incorrectly by about 160 pixels at full speed in PB, and even more for weapons. Even though this latency is "fake", the sync issue is still bad enough that the client cannot be allowed to stay in game.

Source

This document was compiled by catid's addendum and Mr Ekted's latency post.