Main Program and Common Routines
(NTP Reference Implementation) (Computer Network Time Synchronization)

The four routines shown in Figure 14.3 stand alone and are not part of any
process. The main program main() initializes the program, allocates the
persistent associations, and then loops waiting for a received packet. When
a packet arrives, the routine captures the receive timestamp and calls the
receive() routine. The mobilize() routine allocates and initializes an
association and starts the poll timer. The clear() routine resets or
demobilizes an association and returns its resources to the system. The
md5() routine computes a 128-bit message digest from a given key and message
block. For simplicity, the routine shown here is used for both generating
and checking. For checking purposes, the routine returns the codes shown in
Table 14.4.



Main program and common routines.

FIGURE 14.3

Main program and common routines.

The md5() routine uses the MD5 algorithm described in Request for Comments
(RFC) 1321 [1] but modified to incorporate a symmetric key. The key itself
is a 128-bit value held in a special cache where each key is associated with
a 32-bit key identifier. The message digest is computed by first prepending
the key value to the block and then computing the message digest over the
combined key and block.

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        Computer Network Time Synchronization
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            Processing Time Values
            Correctness and Accuracy Expectations