Depending on circumstances, the protocol stack might have to cope with other failures beyond damage, loss and duplication of a data packet. One important class of failures, termed amnesia failures, involves loss of information on the sender or the recipient. An amnesia failure can cause the sender to forget the unique identifiers of the packets that were sent, or the recipient to forget the unique identifiers of the packets that were delivered. Generally, this makes it impossible to guarantee the exactly once delivery semantics.

As an alternative to the exactly once delivery semantics, the at most once and at least once variants are considered in practice. The at most once delivery semantics guarantees reliable delivery except for failures that make reliable delivery impossible. When such a failure occurs, it is guaranteed that no packets were duplicated, but some packets could be lost. The at least once delivery semantics also guarantees reliable delivery except for failures that make reliable delivery impossible. When such a failure occurs, it is guaranteed that no packets were lost, but some packets could be duplicated.