Sequence numbers have been used by a variety of network protocols as freshness identifiers to achieve reliable transmission and provide protection against replay attacks. The number of bits allocated for a sequence number shall not be too small in order to avoid frequent wraparounds and replay attacks. However, in a sensor network consisting of resource-constrained sensor nodes, transmitting a full sequence number along with a message represents considerable overhead that is desirable to be avoided. In this paper, we propose LOFT, a protocol that lowers the overhead of transmitting freshness identifiers along with messages. In LOFT, the sender and receiver still maintain a sequence number of the same number of bits, but when the sender sends a new message, it only transmits a less significant portion of the sequence number bits in order to save energy. All the bits of the sequence number are involved in the computation of a message authentication code, so that the receiver can verify the freshness of the message. Moreover, LOFT makes use of Bloom filter to mitigate the overhead of freshness check caused by DoS attacks. We use simulations to analyze how much a portion of the sequence number should be transmitted to achieve best performance in terms of efficiency and effectiveness.