7.3 Sequence numbers
Sequence numbers are an alternative way of indicating freshness. If Alice is sending a stream of messages to Bob she can bind each one to a sequential serial number, and encryption will prevent an eavesdropper from altering any sequence number. If Bob is suspicious he can check that the numbers in Alice's messages are incremented sequentially. It would be a straightforward matter for him to spot a replayed message since the sequence order would be incorrect and its number would duplicate that of an earlier message.
In a connectionless packet-switched network, would sequence numbers provide effective freshness indicators?
In a packet-switched network, messages between two points could take different routes and might arrive out of sequence. It would be impossible for Bob to determine whether this was a result of network delays or some malicious intent. However, the sequence numbers would still provide a means of identifying duplicated messages.