The Data Defense Network (DDN) is a large packet switching network that services elements of the U.S. Department of Defense (DOD). The emphasis of this report is to perform probabilistic analysis of certain features of the DDN system with a view of enhancing or 'optimizing' measures of service such as data base throughput and the waiting times experienced by data-base-transfer customers. The particular questions addressed, and the models constructed, are in response to tasking statements supplied by personnel from the Defense Communications Engineering Center, Reston, Virginia. In Section 2 models are presented to study the optimal length of a packet subject to transmission errors. When a data transfer is to occur the total collection of bits that comprise the data base is divided into packets, i.e., subcollections of contiguous bits from the data base plus a header carrying address information. In Section 3 models are introduced to study the effect on D's buffer of all sources retransmitting at a retransmission interval of length 8. We also model the behavior of one form of congestion control, exponential backoff, a procedure that increases successive time-out intervals possibly from 8 to 28, 28 to 48, etc. In summary, probabilistic models have been constructed to study the effect of packet size and message size on system throughput and response time of a data-transfer operation. The effect of packet retransmission on response times has also been evaluated. (KR)