| The Network Time Protocol (NTP) keeps Internet-connected clocks synchronized, which core applications and protocols depend on for proper functioning, including DNS caches, RPKI, and TLS. Next to free time servers (NIST, NTP Pool), large cloud providers and OS vendors such as Microsoft, Apple, and Google have begun operating their own time services and setting their billions of systems/devices to use their NTP servers by default. This creates a level of service centralization around these providers, and yet there has been little scrutiny of these services and their characteristics. In this paper, we characterize seven large time service providers in terms of replication architecture, client mapping, accuracy and service provided. We found a large variation in service replication (DNS and anycast), how they map clients to servers, and in their accuracy and (security) feature support. This includes violations of best practices, servers clocks being incorrect by up to 50ms on one occasion, the use of outdated NTP versions, and a lack of support for NTS, RPKI, and DNSSEC. |