Synchronization protocols work in one or more association modes, depending on the protocol design. In this mode, a client synchronizes to a stateless server as in the conventional RPC model. NTP also supports symmetric mode, which allows either of two peer servers to synchronize to the other, in order to provide mutual backup. DTSS and NTP support a broadcast mode which allows many clients to synchronize to one or a few servers, reducing network traffic when large numbers of clients are involved. In NTP, IP multicast can be used when the subnet spans multiple networks.
Configuration management can be a serious problem in large subnets.
Various schemes which index public databases and network directory services are used in DTSS and NTP to discover servers. A reliable network time service requires provisions to prevent accidental or malicious attacks on the servers and clients in the network. Ubiquity requires that any client can verify the authenticity of any server using only public information. This is especially important in such ubiquitous network services as directory services, cryptographic key management and time synchronization. NTP includes provisions to cryptographically authenticate individual servers using symmetric-key cryptography in which clients authenticate servers using shared secret keys.