reassembly of TCP segments required to achieve transport and delivery through the multiple networks and intercon- necting gateways (routers). The IP also carries information on the precedence, security classification, and compartmentation of the TCP segments, SO this information can be communicated end-to-end across multiple networks. 12.3.4.2 TCP Mechanisms. TCP builds its services on top of the network layer's potentially unreliable services with mechanisms such as error detection, positive acknowl- edgments, sequence numbers, and flow control. These mechanisms require certain address- ing and control information to be initialized and maintained during data transfer. This collection of information is called a TCP connection. The following paragraphs describe the purpose and operation of the major TCP mechanisms. Par Mechanism. TCP uses a positive acknowledgment with retransmission (PAR) mechanism to recover from the loss of a segment by the lower layers. The strategy with PAR is for sending a TCP to retransmit a segment at timed intervals until a posi- tive acknowledgment is returned. The choice of retransmission interval affects efficiency. An interval that is too long reduces data throughput while one that is too short floods ENTERPRISE NETWORKS II: WIDE AREA NETWORKS the transmission media with superfluous segments. In TCP, the timeout is expected to be dynamically adjusted to approximate the segment round-trip time plus a factor for internal processing; otherwise performance degradation may occur. TCP uses a simple checksum to detect segments damaged in transit. Such segments are discarded without being acknowledged. Hence, damaged segments are treated identically to lost segments and are compensated for by the PAR mechanism. TCP assigns sequence numbers to iden- tify each octet of the data stream. These enable a receiving TCP to detect duplicate and out-of-order segments. Sequence numbers are also used to extend the PAR mechanism by allowing a single acknowledgment to cover many segments worth of data. Thu