Tor Entry Guard Bandwidth Requirements

This article provides a concise overview of the bandwidth, stability, and configuration criteria needed to operate a Tor entry guard. To become a guard, a relay must not only provide a minimum sustained data throughput but also maintain high availability and rank within the top tier of network relays as determined dynamically by Tor Directory Authorities.

Minimum and Recommended Bandwidth Thresholds

To qualify for the Guard flag, a relay must meet both absolute baseline thresholds and relative dynamic standards set by the Tor network consensus:

• Minimum Absolute Bandwidth: The Tor directory specifications generally require a sustained consensus bandwidth of at least 2 MByte/s (2,000 KB/s or ~16 Mbps). Relays falling below this floor will not be assigned the Guard flag regardless of uptime.
• Dynamic Network Median: The Directory Authorities constantly calculate the median bandwidth of all active relays. To earn the Guard flag, a relay must meet or exceed either the network-wide median bandwidth or the fixed minimum threshold, whichever is higher.
• Recommended Capacity: Network operators recommend provisioning at least 10 to 25 Mbps (unmetered) of sustained bandwidth to comfortably achieve and maintain guard status without losing the flag during peak network load.

Consensus Weight and Measurement

The bandwidth requirement is evaluated through active measurements rather than self-reported figures:

1. Bandwidth Authorities (bwauths): Tor utilizes specialized scanner relays to regularly measure the real-world throughput of your node.
2. Consensus Bandwidth: Your relay’s measured performance is combined into a consensus weight. The Guard flag is automatically granted when this weight crosses the current network threshold.
3. Warm-Up Phase: New relays go through an initial testing period (often lasting several days to a few weeks) where traffic gradually scales up as authorities verify sustained capacity.

Crucial Uptime and Stability Requirements

Bandwidth alone is insufficient; an entry guard must demonstrate high reliability to prevent circuit disruptions:

• Weighted Fractional Uptime (WFU): The relay must maintain high availability, generally requiring an uptime score exceeding the network median (typically at least 98% availability over recent days).
• Mean Time Between Failures (MTBF): The relay must show a long historical track record of staying online, often requiring at least 8 to 30 days of consistent operation.

Example Configuration

In the torrc configuration file, operators should ensure bandwidth parameters accommodate the guard threshold:

# Ensure sufficient rate limits (example values for a 20 Mbps allocation)
RelayBandwidthRate 2500 KBytes   # 2.5 MB/s (~20 Mbps)
RelayBandwidthBurst 3500 KBytes  # 3.5 MB/s peak allowance

If the relay maintains this throughput consistently alongside high uptime, the Directory Authorities will automatically append the Guard flag to the node’s consensus status.