Riccardo Gusella, Stefano Zatti, James M. Bloom
Computer Systems Research Group
Computer Science Division
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, CA 94720
Engineering Computer Network
Department of Electrical Engineering
West Lafayette, IN 47906
The clock synchronization service for the UNIX 4.3BSD operating system is composed of a collection of time daemons (timed) running on the machines in a local area network. The algorithms implemented by the service is based on a master-slave scheme. The time daemons communicate with each other using the Time Synchronization Protocol (TSP) which is built on the DARPA UDP protocol and described in detail in .
A time daemon has a twofold function. First, it supports the synchronization of the clocks of the various hosts in a local area network. Second, it starts (or takes part in) the election that occurs among slave time daemons when, for any reason, the master disappears. The synchronization mechanism and the election procedure employed by the program timed are described in other documents [1,2,3]. The next paragraphs are a brief overview of how the time daemon works. This document is mainly concerned with the administrative and technical issues of running timed at a particular site.
A master time daemon measures the time differences between the clock of the machine on which it is running and those of all other machines. The master computes the network time as the average of the times provided by nonfaulty clocks.[note 1] It then sends to each slave time daemon the correction that should be performed on the clock of its machine. This process is repeated periodically. Since the correction is expressed as a time difference rather than an absolute time, transmission delays do not interfere with the accuracy of the synchronization. When a machine comes up and joins the network, it starts a slave time daemon which will ask the master for the correct time and will reset the machine's clock before any user activity can begin. The time daemons are able to maintain a single network time in spite of the drift of clocks away from each other. The present implementation keeps processor clocks synchronized within 20 milliseconds.
To ensure that the service provided is continuous and reliable, it is necessary to implement an election algorithm to elect a new master should the machine running the current master crash, the master terminate (for example, because of a run-time error), or the network be partitioned. Under our algorithm, slaves are able to realize when the master has stopped functioning and to elect a new master from among themselves. It is important to note that, since the failure of the master results only in a gradual divergence of clock values, the election need not occur immediately.
The machines that are gateways between distinct local area networks require particular care. A time daemon on such machines may act as a submaster. This artifact depends on the current inability of transmission protocols to broadcast a message on a network other than the one to which the broadcasting machine is connected. The submaster appears as a slave on one network, and as a master on one or more of the other networks to which it is connected.
A submaster classifies each network as one of three types. A slave network is a network on which the submaster acts as a slave. There can only be one slave network. A master network is a network on which the submaster acts as a master. An ignored network is any other network which already has a valid master. The submaster tries periodically to become master on an ignored network, but gives up immediately if a master already exists.
While the synchronization algorithm is quite general, the election one, requiring a broadcast mechanism, puts constraints on the kind of network on which time daemons can run. The time daemon will only work on networks with broadcast capability augmented with point-to-point links. Machines that are only connected to point-to-point, non-broadcast networks may not use the time daemon.
If we exclude submasters, there will normally be, at most, one master time daemon in a local area internetwork. During an election, only one of the slave time daemons will become the new master. However, because of the characteristics of its machine, a slave can be prevented from becoming the master. Therefore, a subset of machines must be designated as potential master time daemons. A master time daemon will require CPU resources proportional to the number of slaves, in general, more than a slave time daemon, so it may be advisable to limit master time daemons to machines with more powerful processors or lighter loads. Also, machines with inaccurate clocks should not be used as masters. This is a purely administrative decision: an organization may well allow all of its machines to run master time daemons.
At the administrative level, a time daemon on a machine with multiple network interfaces, may be told to ignore all but one network or to ignore one network. This is done with the -n network and -i network options respectively at start-up time. Typically, the time daemon would be instructed to ignore all but the networks belonging to the local administrative control.
There are some limitations to the current implementation of the time daemon. It is expected that these limitations will be removed in future releases. The constant NHOSTS in /usr/src/etc/timed/globals.h limits the maximum number of machines that may be directly controlled by one master time daemon. The current maximum is 29 (NHOSTS - 1). The constant must be changed and the program recompiled if a site wishes to run timed on a larger (inter)network.
In addition, there is a pathological situation to be avoided at all costs, that might occur when time daemons run on multiply-connected local area networks. In this case, as we have seen, time daemons running on gateway machines will be submasters and they will act on some of those networks as master time daemons. Consider machines A and B that are both gateways between networks X and Y. If time daemons were started on both A and B without constraints, it would be possible for submaster time daemon A to be a slave on network X and the master on network Y, while submaster time daemon B is a slave on network Y and the master on network X. This loop of master time daemons will not function properly or guarantee a unique time on both networks, and will cause the submasters to use large amounts of system resources in the form of network bandwidth and CPU time. In fact, this kind of loop can also be generated with more than two master time daemons, when several local area networks are interconnected.
In order to start the time daemon on a given machine, the following lines should be added to the local daemons section in the file /etc/rc.local:
if [ -f /etc/timed ]; then /etc/timed flags & echo -n ' timed' >/dev/console fi
In any case, they must appear after the network is configured via ifconfig(8).
Also, the file /etc/services should contain the following line:
timed 525/udp timeserver
The flags are:
Timedc(8) is used to control the operation of the time daemon. It may be used to:
See the manual page on timed(8) and timedc(8) for more detailed information.
The date(1) command can be used to set the network date. In order to set the time on a single machine, the -n flag can be given to date(1).