Game Lag White Paper › Netcode design

Player-hosted server (host) Listen server / host advantage

Cause ID sy-host · Primary owner Game team (Server development) · Also Game team (Client development), Infra team (Server infrastructure)

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When one player’s PC acts as the server, that player’s connection and PC performance decide how the game feels for everyone.

Why The host’s PC acts as the server (P2P, listen server) → Effect If the host’s connection or PC is slow, everyone feels it, while the host has zero ping → On screen Only the host has the advantage, and when the host leaves everyone freezes or disconnects

Symptoms

Stutter, Freeze, Disconnect

Factors

Latency, Stall

Who’s affected

Specific zone/channel

When

Randomly

Owner

Primary owner Game team (Server development) · Also Game team (Client development), Infra team (Server infrastructure)

Game team action items

Server: move to dedicated servers that make the calls, until then pick a player with a good connection and PC as host during matchmaking. Client: support host migration, measure and report ping to other participants, upload speed, and PC performance during matchmaking.

Infra team action items

Secure server machines or instances for dedicated servers, place them close to regions with many players.

On the graph

Outliers only · Lag reports and disconnects per host

Where to look

Match logs with the host’s upload speed, each participant’s RTT to the host, the host PC’s frame time, and when the host left, with lag and disconnect reports grouped by host. Players can also confirm it by playing again with the same group and only the host changed

Confirmed if

Lag and disconnects concentrate in one host’s matches, all participants get worse together when that host’s upload speed is low or frame time is long, and without host migration everyone disconnects the moment the host leaves

Ruled out if

Only participants in the same region are affected, regardless of host: connection or route problem. With dedicated servers, this isn’t the cause

Check with

Game server or client logs and metrics

Sources

1. Networking Overview for Unreal Engine Epic Games
   A listen server’s host has an advantage over other clients and carries a heavy load, running both the server and rendering
2. Distributed authority topologies (Netcode for GameObjects 2.5) Unity
   When the session owner leaves, a new owner is elected automatically from the remaining clients
3. A Survey and Taxonomy of Latency Compensation Techniques for Network Computer Games (ACM Computing Surveys, 2022) ACM
   P2P games can also be seen as client-server, with the host doubling as the server

See also

Same layer: Netcode design

• Feedback only after the server responds (request-response)
• Chatty protocol (many sequential round trips)
• No skill input buffering
• Short timing windows eaten up by ping
• Hit registration without lag compensation
• Too much lag compensation
• Client authority
• Lockstep waiting on the slowest player
• Rollback netcode misprediction
• Events played on arrival without timestamps
• Double tick wait
• Overly strict server validation
• Server rejects after client-side feedback
• Pathfinding mismatch in command sync
• Low snapshot send rate

Same symptom (Stutter), other layers

• Frame time spike L1 Client game process
• Client garbage collection L1 Client game process
• Synchronous loading and shader compilation on the main thread L1 Client game process
• Slow storage delays asset streaming L1 Client game process
• Rendering load from large crowds L1 Client game process
• Missing or too-short interpolation buffer L1 Client game process
• Excessive extrapolation (dead reckoning) L1 Client game process
• Fixed-timestep catch-up spiral L1 Client game process

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