If two communicating entities are in different hosts connected by a network，there is a risk that PDUs will not arrive in the order in which they were sent，because they may traverse(71)paths through the network. If each PDUs is given a unique number，and numbers are assigned(72)，then it is a logically simple task for the receiving entity to reorder(73)PDUs on the basis of sequence number. A problem with this scheme is that，with a(74)sequence number field，sequence number will repeat. Evidently，the maximum sequence number must be (75)than the maximum number of PDUs that could be outstanding at any time.
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