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A common carrier is a private company that provides communications services for a free.
Time Services:
With the launch of the so-called Global Positioning System (GPS) satellites, microsecond accuracy became possible in workstations equipped with inexpensive radio receivers. Unfortunately, however, accurate clocks remain a major problem in the most widely used computer workstations and network technologies.
At the time of this writing, the usual clock for a PC or workstation consists of a quartz-based chip much like the one in a common wristwatch, accurate to within a few seconds per year. The initial value of such a clock is either set by the vendor or by the user, when the computer is booted. As a result, in any network of workstations, clocks can give widely divergent readings and can drift with respect to one another at significant rates. For these reasons, there has been considerable study of algorithms for clock synchronization, whereby the clocks on individual machines can be adjusted to give behavior approximating that of a shared global clock.
However, much of this work has a limited lifetime. GPS receivers can give extremely accurate time, and GPS signals are transmitted frequently enough so that even inexpensive hardware can potentially maintain time accurate to microseconds. By broadcasting GPS time values, this information can be propagated within a network of computers, and although some accuracy is necessarily lost when doing so, the resulting clocks are still accurate and comparable to within tens of microseconds. This development can be expected to have a major impact on the way that distributed software is designed-from a world of asynchronous communication and clocks that can be inaccurate by many times the average message latency in the network, GPS-based time could catapult us into a domain in which clock resolutions considerably exceed the average latency between sending a message and when it is received. Such developments make it very reasonable to talk about synchronous (time-based) styles of software design and the use of time in algorithms of all sorts.
Even coarsely synchronized clocks can be of value in distributed software for example, when comparing versions of files, microsecond accuracy is not needed to decide if one version is more current than another: Accuracy of seconds or even tens of seconds may be adequate. Security systems often have a concept of expiration associated with keys, but for these to be at risk of attacks an intruder would need a way to set a clock back by days, not fractions of a second. And although we will see that RFC protocols use time to detect and ignore very old, stale messages, as in the case of a security mechanism a clock would need to be extremely inaccurate for such a system to malfunction.
Security Services:
In the context of an RFC environment, security is usually concerned with the authentication problem. Briefly stated, this is the problem of providing applications with accurate information about the user-ID on behalf of which a request is being performed. Obviously, one would hope that the user-ID is related in some way to the user, although this is frequently the weak link in security architecture. Given an accurate source of user identifications, however, the basic idea is to avoid intrusions that can compromise user-ID security through break-ins on individual computers and even replacements of system components on some machines with versions that have been compromised and hence could malfunction.
To accomplish authentication, a typical -security mechanism will request some form of password or one-time key from the user at log-in time, and periodically thereafter, as keys expire on the basis of elapsed time. This information is used to compute a form of secure user identification, which can be employed during connection establishment. When a client binds to a server, the security mechanism authenticates both ends and (at the option of the programmer) arranges for data to be encrypted on the wire, so that intruders who witness messages being exchanged between the client and server have no way to decode the data contained within them. (Unfortunately, however, this step is so costly that many applications disable encryption and simply rely upon the security available from the initial connection setup.) Notice that for such a system to work correctly there must be a way to trust the authentication server itself: The user needs a way to confirm that it is actually talking to the authentication server and to legitimate representatives of the services it wishes to use. Given the anonymity of network communication, these are potentially difficult problems.
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