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(   )paths through the network. If each PDUs is given a unique number，and numbers are assigned(   )，then it is a logically simple task for the receiving entity to reorder(   )PDUs on the basis of sequence number. A problem with this scheme is that，with a(  )sequence number field，sequence  number  will  repeat. Evidently，the  maximum  sequence  number  must  be (   )than  the  maximum  number  of  PDUs  that  could  be  outstanding  at any  time.