NTP is the Network Time Protocol defined in RFC 1305, Network Time Protocol (Version 3) Specification, Implementation and Analysis and RFC 5905, Network Time Protocol Version 4: Protocol and Algorithms Specification. It allows for the participating network nodes to keep time more accurately and maintain time in a more synchronized fashion among all participating network nodes.
NTP uses stratum levels to define the number of hops from a reference clock. The reference clock is considered to be a Stratum-0 device that is assumed to be accurate with little or no delay. Stratum-0 servers cannot be used in a network. However, they can be directly connected to devices that operate as Stratum-1 servers. A Stratum-1 server is an NTP server with a directly connected device that provides Coordinated Universal Time (UTC), such as a GNSS or atomic clock.
The higher stratum levels are separated from the Stratum-1 server over a network path; therefore a Stratum-2 server receives its time over a network link from a Stratum-1 server. A Stratum-3 server receives its time over a network link from a Stratum-2 server.
The 7705 SAR runs a single NTP clock that operates NTP message exchanges with external NTP clocks. Exchanges can be made with external NTP clients, servers, and peers. These exchanges can be through the base, management, or VPRN routing instances.
When NTP is enabled, the NTP clock in the 7705 SAR operates as an NTP client by default. The 7705 SAR typically operates as a Stratum-2 device, relying on an external Stratum-1 server to source accurate time into the network.
Alternatively, the NTP clock in the 7705 SAR can recover time from a local PTP or GNSS source. This is achieved by configuring the PTP clock or GNSS receiver as the internal system time. The internal system time can then be identified as the preferred source of NTP timing into the network with the command config>system>time>ntp>server>system-time>prefer. This configuration makes the local PTP or GNSS source appear as a Stratum-0 server. When the internal PTP clock or GNSS is identified as the server for NTP, NTP promotes the internal NTP server (the 7705 SAR) to Stratum-1 level, which may affect the NTP network topology.
The 7705 SAR can also operate as an NTP server and provide timing to downstream clients with the ntp-server command. When the NTP server is enabled with authentication, any NTP clients must authenticate using the correct key.
In server mode, the 7705 SAR advertises the ability to act as a clock source for other network elements. By default, the router transmits NTP packets in NTP version 4 mode. Server mode is supported on the CSM Management port, in the base routing context, and in the VPRN routing context.
As an NTP server, the 7705 SAR can peer with an external NTP server in another router that is considered more trustworthy or accurate than other routers carrying NTP in the system. This allows the peers to act as mutual backups where they can obtain time from or supply time to the other server as required. If both servers are peering each other, the router is in symmetric active mode. This mode requires that the peer association is set on both routers so that the local and remote router designate each other as a peer. If only one server is peering the other (that is, the other peer has not specifically configured the peer association), the router is in symmetric passive mode.
The 7705 SAR can be configured to transmit broadcast NTP packets on a specified interface with the broadcast command. The interface can be the management interface, interfaces in the base routing context, or an interface in the VPRN context. The messages are transmitted using a destination address that is the NTP broadcast address. Only IPv4 addressing is supported.
The 7705 SAR can also be configured to receive broadcast NTP packets on interfaces in the base routing context or on the management interface with the broadcastclient command.
The router can be configured to transmit or receive multicast NTP packets on the CSM Management port. The multicast command configures the transmission of NTP multicast messages. The multicastclient command configures the receipt of multicast NTP packets. When receiving or sending multicast NTP messages, the default address 224.0.1.1 is used. Only IPv4 addressing is supported.
The following NTP elements are supported:
authentication keys — both DES and MD5 authentication are supported as well as multiple keys, to provide increased security support in carrier and other networks
server and peer addressing — external servers and external peers may be defined using IPv4 or IPv6 addresses
alert when NTP server is not available — when none of the configured servers are reachable on the node, the system reverts to manual timekeeping and issues a critical alarm. When a server becomes available, a trap is issued indicating that standard operation has resumed.
NTP and SNTP — if both NTP and SNTP are enabled on the router, SNTP transitions to an operationally down state. If NTP is removed from the configuration or shut down, SNTP resumes an operationally up state.
NTP priority — if a higher-priority time source such as GNSS or PTP is selected on the router, NTP transitions to an operationally down state. If the higher-priority time source is disqualified or disabled, NTP resumes an operationally up state.
gradual clock adjustment — because several applications (such as Service Assurance Agent (SAA)) can use the clock, if a major adjustment (128 ms or more) must be performed, the adjustment is performed by programmatically setting the clock. If a minor adjustment (less than 128 ms) must be performed, the adjustment is performed by either speeding up or slowing down the clock.
to facilitate correct operation when the standby CSM takes over from the active CSM, the time on the secondary CSM must be synchronized with the clock of the active CSM
to prevent the generation of too many events and traps, the NTP module rate-limits the generation of events and traps to three per second. At that point, a single trap is generated that indicates that event/trap blocking is taking place.
NTP accuracy depends on the accuracy of NTP packet timestamping. By default, NTP packets are timestamped by the CSM where the NTP protocol is executed. However, an enhanced NTP mode is available where the timestamping is performed on the adapter card by the network processor. This reduces variations introduced by packet delay within the router as well as by a busy CPU in the CSM. This enhanced mode is only available for in-band NTP over a network interface. When enhanced NTP mode is used, NTP authentication is not supported.