The diffserv approach  to providing QoS in networks employs a small,well-defined set   of building blocks from which you can build a variety of(      ).Its aim is to define the differentiated services(DS)byte,the Type of Service(ToS)byte from the Internet Protocol Version  4(       )and the Traffic  Class byte  from  IP Version 6,and mark the standardized  DS byte of the packet  such that it receives a particular forwarding treatment,or  per-   hop  behavior(PHB),at  each  network  node.The  diffserv  architecture  provides a(    )within which service providers can offer customers a range of network   services,each  differentiated  based  on  performance.A  customer  can  choose the(________ )level needed on a packet-by-packet basis by simply marking the packet's Differentiated  Services Code Point(DSCP)field to a specific value.This (      ) specifies the PHB given to the packet within the service provider network。横线处应选（   ）

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