🛡️ Guard Flag Calculation Logic

Per Tor Directory Specification, a relay qualifies for Guard if ALL of the following are true:

1. Fast flag: Relay has the Fast flag (bandwidth ≥100 KB/s OR in top 7/8ths)
2. Stable flag: Relay has the Stable flag (sufficient uptime/MTBF)
3. WFU ≥ guard-wfu threshold: Weighted Fractional Uptime at or above the median for "familiar" relays (typically 98%)
4. Time Known ≥ guard-tk threshold: Relay is "familiar" - authority has tracked it for sufficient time (typically 8 days)
5. Bandwidth requirement (one of):
   • Option A: Observed bandwidth ≥ AuthDirGuardBWGuarantee (default 2 MB/s)
   • Option B: Observed bandwidth in top 25% of relays (≥ guard-bw-inc-exits shown in table)
6. V2Dir flag: Relay supports directory protocol v2+

Important: The bandwidth check uses observed_bandwidth from the relay's server descriptor (actual capacity in bytes/s), NOT the scaled consensus weight shown in votes. The 2 MB/s guarantee means any relay with ≥2 MB/s observed bandwidth can be a Guard, regardless of network size.
⚙️ AuthDirGuardBWGuarantee: This is a hardcoded Tor configuration parameter (default: 2 MB/s = 2,097,152 bytes/s). It is NOT present in vote files - authorities use their compiled-in default. This value rarely changes and provides a stable minimum bandwidth guarantee for Guard eligibility independent of network size fluctuations. [View in Tor source - Line 21]
Sources: dir-spec: Assigning Flags | dir-spec: Computing BW Weights | Bandwidth Scanners Blog

⚡ Fast Flag Calculation Logic

Per Tor Directory Specification, a relay qualifies for Fast if its observed bandwidth is:

• In the top 7/8ths (87.5%) for known active routers (≥ fast-speed threshold shown above)
• OR at least 100 KB/s (absolute minimum fallback)

The fast-speed threshold in the table is dynamically calculated by each authority based on the bandwidth distribution of all relays. It typically equals or exceeds 100 KB/s. The 100 KB/s fallback ensures relays on low-bandwidth networks can still qualify.
Source: dir-spec: Assigning Flags in a Vote

🏃 Running Flag Calculation Logic

Per Tor Directory Specification:

• A directory authority considers a relay "Running" when the relay is reachable/active by connecting to the relay's OR port

When an authority votes for a relay, that means the authority considers it Running (reachable). If an authority cannot reach a relay, it will not include the relay in its vote. A relay must be Running from a majority of authorities (≥5/9) to be included in the final consensus.
Source: dir-spec: Assigning Flags in a Vote

⚓ Stable Flag Calculation Logic

Per Tor Directory Specification, a relay qualifies for Stable if:

• Weighted MTBF ≥ stable-mtbf threshold (median MTBF of all active relays)
• OR Weighted MTBF ≥ 7 days (604800 seconds, absolute minimum)

How Directory Authorities Measure MTBF:

• Tracks reachability — Each authority independently tests if it can connect to relay's OR port
• Records "up" intervals — Tracks periods when the relay was successfully reachable
• Calculates Weighted MTBF — Weighted mean of all intervals when relay was "observed to be up"
• Weights recent data more — Uses discount factor of 0.95 every 12 hours (research paper)
• Requires sufficient data — At least 4 days of MTBF data needed for valid assessment

MTBF vs Uptime — Key Distinction:

Metric	Source	Meaning
MTBF (stats mtbf=X)	Authority-measured	Mean Time Between Failures — authority's observation of relay's reachability history
Uptime (last_restarted)	Relay self-reported	Time since last restart — from relay's own descriptor

Key Point: Authority does NOT use relay's self-reported uptime for MTBF. It independently measures reachability. The Stable flag is based on uptime history, not current uptime — a relay with good history that recently restarted may keep Stable.

From dir-spec § Assigning Flags:

"To calculate weighted MTBF, compute the weighted mean of the lengths of all intervals when the router was observed to be up, weighting intervals by α^n, where n is the amount of time that has passed since the interval ended, and α is chosen so that measurements over approximately one month old no longer influence the weighted MTBF much."

⚠️ Study Finding: Our analysis of 10k+ relays shows authorities assigning Stable to relays with MTBF as low as ~5 days, below both the published threshold and 7-day minimum. The stable-mtbf in flag-thresholds is the median MTBF value, not the minimum threshold. See Stable Flag Threshold Study.
Sources: dir-spec | Relay Stability Research (2011)

✅ Valid Flag Calculation Logic

Per Tor Directory Specification, a relay qualifies for Valid if:

• The relay is Running (reachable by the authority)
• The relay's identity key is not blacklisted
• The relay has a valid descriptor

Unlike Running (which just means reachable), Valid confirms the relay is verified and allowed to participate. Almost all Running relays also have Valid unless they've been manually blacklisted.
Source: dir-spec: Assigning Flags in a Vote