PTP vs NTP: Comparación del Protocolo de Tiempo de Precisión

The Accuracy Gap

While NTP can synchronize clocks across the internet to within a few milliseconds, Precision Time Protocol (PTP, IEEE 1588) can synchronize clocks on a local network to within nanoseconds — three to six orders of magnitude more accurate. The key difference: PTP can use hardware timestamping in network cards and switches, eliminating software stack latency.

How PTP Works

PTP uses a master-slave hierarchy. The grandmaster clock (typically connected to GPS or an atomic clock) distributes time to boundary clocks and ordinary clocks:

# PTP message sequence (simplified)
1. Grandmaster sends Sync message with T1 timestamp
2. Slave receives Sync at T2
3. Grandmaster sends Follow_Up with precise T1
4. Slave sends Delay_Req at T3
5. Master receives Delay_Req at T4, sends Delay_Resp
6. Slave calculates:
   offset = ((T2-T1) - (T4-T3)) / 2
   delay  = ((T2-T1) + (T4-T3)) / 2

Hardware Timestamping: The Key Advantage

NTP timestamps are generated in software — the OS scheduler and network stack add variable latency (microseconds to milliseconds). PTP can timestamp packets at the hardware level in the network interface card as they leave or arrive, eliminating nearly all software jitter.

# Check if your NIC supports PTP hardware timestamping
ethtool -T eth0 | grep -i "hardware"
# hardware-transmit     (hardware TX timestamp)
# hardware-receive      (hardware RX timestamp)
# hardware-raw-clock    (hardware clock)

# linuxptp — configure PTP on Linux
ptp4l -i eth0 -m    # Start PTP daemon
phc2sys -s eth0 -c CLOCK_REALTIME -w  # Sync system clock to PTP

NTP vs PTP Comparison

• NTP accuracy: 1–10ms over internet, <1ms on LAN
• PTP accuracy: <100ns on LAN with hardware timestamping
• NTP complexity: Simple — one daemon, standard NTP servers available
• PTP complexity: Requires PTP-aware switches, hardware NIC support, tuning
• NTP cost: Free, works on any hardware
• PTP cost: Requires PTP-capable NICs and switches (more expensive)

When to Use PTP

• Financial trading: MiFID II requires microsecond timestamping for trade reporting
• Telecommunications: 5G networks require sub-microsecond timing
• Industrial automation: Synchronized motion control, power grid synchronization
• Audio/video production: Professional AV over IP requires sample-accurate sync
• Scientific instruments: CERN LHC, radio telescope arrays

Cloud PTP

AWS, GCP, and Azure all offer PTP endpoints for their EC2/Compute instances, providing sub-microsecond accuracy for cloud workloads that need it:

# AWS EC2 PTP endpoint
# 169.254.169.123 supports NTP and PTP
ptp4l -i eth0 --ptp_dst_mac 01:1B:19:00:00:00 -m