The Cisco CMTS can
function as a ToD server that provides the current date and time to the cable
modems and other customer premises equipment (CPE) devices connected to its
cable interfaces. This allows the cable modems and CPE devices to accurately
timestamp their Simple Network Management Protocol (SNMP) messages and error
log entries, as well as ensure that all of the system clocks on the cable
network are synchronized to the same system time.
The DOCSIS 1.0 and
1.1 specifications require that all DOCSIS cable modems request the following
time-related fields in the DHCP request they send during their initial power-on
- Time Offset (option
2)—Specifies the time zone for the cable modem or CPE device, in the form of
the number of seconds that the device’s timestamp is offset from Greenwich Mean
- Time Server Option (option
4)—Specifies one or more IP addresses for a ToD server.
After a cable modem
successfully acquires a DHCP lease time, it then attempts to contact one of the
ToD servers provided in the list provided by the DHCP server. If successful,
the cable modem updates its system clock with the time offset and timestamp
received from the ToD server.
If a ToD server
cannot be reached or if it does not respond, the cable modem eventually times
out, logs the failure with the CMTS, and continues on with the initialization
process. The cable modem can come online without receiving a reply from a ToD
server, but it must periodically continue to reach the ToD server at least once
in every five-minute period until it successfully receives a ToD reply. Until
it reaches a ToD server, the cable modem must initialize its system clock to
midnight on January 1, 1970 GMT.
of the DOCSIS 1.0 specification specified that the cable device must obtain a
valid response from a ToD server before continuing with the initialization
process. This requirement was removed in the released DOCSIS 1.0 specification
and in the DOCSIS 1.1 specifications. Cable devices running older firmware that
is compliant with the initial DOCSIS 1.0 specification, however, might require
receiving a reply from a ToD server before being able to come online.
modems will repeatedly retry connecting with a ToD server until they receive a
successful reply, you should consider activating the ToD server on the Cisco
CMTS, even if you have one or more other ToD servers at the headend. This
ensures that an online cable modem will always be able to connect with the ToD
server on the Cisco CMTS, even if the other servers go down or are unreachable
because of network congestion, and therefore will not send repeated ToD
To be able to use
the Cisco CMTS as the ToD server, you must configure the DHCP server to provide
the IP address Cisco CMTS as one of the valid ToD servers (DHCP option 4) for
although the DOCSIS specifications do not require that a cable modem
successfully obtain a response from a ToD server before coming online, not
obtaining a timestamp could prevent the cable modem from coming online in the
- If DOCSIS configuration
files are being timestamped, to prevent cable modems from caching the files and
replaying them, the clocks on the cable modem and CMTS must be synchronized.
Otherwise, the cable modem cannot determine whether a DOCSIS configuration file
has the proper timestamp.
- If cable modems register
using Baseline Privacy Interface Plus (BPI+) authentication and encryption, the
clocks on the cable modem and CMTS must be synchronized. This is because BPI+
authorization requires that the CMTS and cable modem verify the timestamps on
the digital certificates being used for authentication. If the timestamps on
the CMTS and cable modem are not synchronized, the cable modem cannot come
online using BPI+ encryption.
modems must use
RFC 868 -compliant ToD
server to obtain the current system time. They cannot use the Network Time
Protocol (NTP) or Simple Network Time Protocol (SNTP) service for this purpose.
However, the Cisco CMTS can use an NTP or SNTP server to set its own system
clock, which can then be used by the ToD server. Otherwise, you must manually
set the clock on the CMTS using the
set command each time that the CMTS boots up.
servers can be provided by workstations or PCs installed at the cable headend.
UNIX and Solaris systems typically include a ToD server as part of the
operating system, which can be enabled by putting the appropriate line in the
inetd.conf file. Windows systems can use shareware servers such as Greyware and
Tardis. The DOCSIS specifications require that the ToD servers use the User
Datagram Protocol (UDP) protocol instead of the TCP protocol for its packets.