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（   ）

（ ）A message    B. Packet

C.header    D. package