Tor Relay Capacity Planning: Right-Sizing Your Network Contribution

For Tor relay performance, the limiting factor changes with scale. At low-medium bandwidth (under 100 Mbit/s): network bandwidth is typically the bottleneck, not CPU or RAM. A 1 vCPU, 1 GB RAM VPS with 100 Mbit/s unmetered port is well-matched. At high bandwidth (100 Mbit/s - 1 Gbit/s): cryptographic operations (AES for Tor circuit encryption) become CPU-bound. Tor uses OpenSSL's AES-NI hardware acceleration when available - VPS providers offering CPUs with AES-NI significantly increase per-core throughput. At very high bandwidth (above 1 Gbit/s): RAM for connection state and file descriptor limits may constrain performance alongside CPU. Configure ulimit -n 65535 for file descriptors. Budget guidance: minimum viable relay ($10/month): 1 vCPU, 1 GB RAM, 1 Gbit/s unmetered port. High-performance relay ($20-30/month): 2 vCPU, 2 GB RAM, 1 Gbit/s unmetered port.

Key VPS selection criteria for relay operators: (1) Unmetered bandwidth at port speed (no data transfer cap): many providers offer 'unlimited' bandwidth plans that throttle at sustained use - verify with provider that the plan supports continuous sustained relay traffic. (2) Tor relay-friendly TOS: check good-bad-isps.md in the Tor Project wiki. (3) Geographic diversity: Tor needs relay diversity. Eastern Europe, Iceland, Nordic, and non-US/UK jurisdictions contribute to geographic diversity. (4) Uptime SLA: target 99.9%+ SLA for Stable flag maintenance. (5) Price per Mbit/s of sustainable bandwidth: calculate as monthly price divided by sustainable relay bandwidth. A $10/month server with 100 Mbit/s costs $0.10/Mbit/s - compare across providers for value optimization.

Operators running multiple relays contribute more to the Tor network but must configure the relays as a family to avoid clients selecting two relays from the same operator in the same circuit (which would reduce circuit diversity). Configure MyFamily in torrc with the fingerprints of all sibling relays. To get a relay's fingerprint: cat /var/lib/tor/keys/ed25519_signing_cert | head or check Tor Metrics after the relay appears in the consensus. Update MyFamily on ALL family members when adding a new relay. Tor path selection excludes relays from the same family from being used in the same circuit. For maximum Tor network contribution, spread multiple relays across different geographic locations, different ASNs (autonomous systems), and different physical data centers.

Calculating cost effectiveness of different relay configurations. Scenario A: budget relay $10/month, 50 Mbit/s sustainable bandwidth (derated from 100 Mbit/s port due to relay overhead) = $0.20/Mbit/s. Scenario B: standard relay $20/month, 200 Mbit/s sustainable bandwidth = $0.10/Mbit/s. Scenario C: high-performance relay $40/month, 500 Mbit/s sustainable bandwidth = $0.08/Mbit/s. The high-performance relay is most cost-efficient per Mbit contributed. However, geographic and ASN diversity has value beyond raw bandwidth - two relays in different countries at $0.20/Mbit/s each contribute more to network resilience than one relay at $0.08/Mbit/s. For operators with limited budgets, one medium-performance relay in a well-connected, privacy-friendly jurisdiction (Iceland, Netherlands) provides good value per dollar while contributing meaningfully to network diversity.

Some VPS providers offer data-transfer-cap plans (e.g., 10 TB/month included) rather than unmetered bandwidth. For these plans, calculate sustainable relay bandwidth: 10,000 GB / 30 days / 86,400 seconds = 3.86 MB/s = 30 Mbit/s. Configure AccountingMax in torrc to prevent exceeding the cap: AccountingMax 9 TB (90% of cap as safety margin). AccountingStart month 1 00:00 resets the counter on the 1st of each month. When the relay reaches AccountingMax, Tor automatically pauses new circuits until the next accounting period. This prevents overage charges while automatically resuming at the start of each period. Monthly data transfer usage can be tracked with vnstat: vnstat -m shows monthly totals per interface.