lf or the data equipment at either end. 3.4.3 Failure Recovery To permit recovery from complete failures of the link or data equip- ment, the procedure must allow for resynchronization of various communications parameters. This means that the data equipment operating the link control procedure must continuously perform a test to see that the link is operating normally. When the link is de- termined to be inoperative, both ends must take appropriate mea- sures so they both understand that the link is inoperative, even though it may still be functioning in one direction. Once both ends know the link is down, they can begin to monitor the circuit and, after it is working correctly again, establish a new connection. PROTOCOLS AND CODES 39 3.4.4 Error Control There are many possible methods for error control over a transmis- sion link, but the most practical approach is to provide an error de- tection mechanism for each transmitted message and to allow for retransmission by the source. This retransmission can be triggered either by a negative acknowledgment from the destination when it does not receive the intended message, or by a scheme of positive acknowledgments from the destination when it does receive mes- sages. Another alternative is a timer mechanism in the source SO that it retransmits messages when it fails to receive positive acknowl- edgments. Error detection can be performed by a variety of means, such as simple parity checking, or the more complicated system of cyclic re- dundancy. When an error is detected, the destination can choose to ignore the block which can then be retransmitted by the source. More details on this subject are discussed in Section 3-12. 3.4.5 Sequencing While sequencing of traffic is not an essential property of a link con- trol procedure, it is often employed to ensure that traffic leaving a particular link is virtually the same as traffic entering. In a large packet-switching network with many links, the practice is normally not to sequence traffic on individual links since it c