Redefining the ToS byte as the Differentiated Services (DiffServ) field, with the 6 most significant bits called the DSCP, has provided much more flexibility and capability to the new IP QoS efforts. The 2 least significant bits of the DiffServ field are used for flow control and are called explicit congestion notification (ECN) bits. DSCP is backward compatible with IP Precedence (IPP), providing the opportunity for gradual deployment of DSCP-based QoS in IP networks. The current DSCP value definitions include four PHBs:
■ Class selector PHB—With the least significant 3 bits of the DSCP set to 000, the class selector PHB provides backward compatibility with ToS-based IP Precedence. When DSCP-compliant network devices receive IP packets from non-DSCP compliant network devices, they can be configured only to process and interpret the IP precedence bits. When IP packets are sent from DSCP-compliant devices to the non-DSCP-compliant devices, only the 3 most significant bits of the DiffServ field (equivalent to IP precedence bits) are set; the rest of the bits are set to 0.
■ Default PHB—With the 3 most significant bits of the DiffServ/DSCP field set to 000, the Default PHB is used for best effort (BE) service. If the DSCP value of a packet is not mapped to a PHB, it is consequently assigned to the default PHB.
■ Assured forwarding (AF) PHB—With the most significant 3 bits of the DSCP field set to 001, 010, 011, or 100 (these are also called AF1, AF2, AF3, and AF4), the AF PHB is used for guaranteed bandwidth service.
■ Expedited forwarding (EF) PHB—With the most significant 3 bits of the DSCP field set to 101 (the whole DSCP field is set to 101110, decimal value of 46), the EF PHB provides low delay service.