Resource Reservation Protocol (RSVP) functionality, which is the case in 99 percent of applications that use IP. Figure summarizes these points.
Content 4.1 Introducing Classification and Marking 4.1.5 IP Precedence and DSCP Compatibility Figure shows the structure of the header. The introduction of DSCP replaces IP precedence, a 3-bit field in the ToS byte of the IP header originally used to classify and prioritize types of traffic as shown in . However, DiffServ maintains interoperability with non-DiffServ-compliant devices (those that still use IP precedence). Because of this backward compatibility, DiffServ can be deployed gradually in large networks. The meaning of the 8 bits in the DiffServ field of the IP packet has changed over time to meet the expanding requirements of IP networks. Originally, the field was referred to as the ToS field, and the first three bits of the field (bits 7 to 5) defined a packet IP precedence value. A packet could be assigned one of six priorities based on the value of the IP precedence value (eight total values minus two reserved ones). IP precedence 5 (101) was the highest priority that could be assigned (RFC 791). In December 1998, RFC 2474 replaced the ToS field with the DiffServ field, in which a range of eight values (class selector) is used for backward compatibility with IP precedence. There is no compatibility with other bits used by the ToS field. The class selector PHB was defined to provide backward compatibility for DSCP with ToS-based IP precedence. RFC 1812 simply prioritizes packets according to the precedence value. In this sense, the PHB is defined as the probability of timely forwarding. For example, consider a service provider offering so-called “Olympic” service classes (Gold, Silver, and Bonze) so that packets in the gold class experience lighter load, and thus have greater probability for timely forwarding, than packets assigned to the silver class. Packets with higher IP precedence should be (on average) forwarded in less time than packets with lower IP precedence. The last 3 bits of the DSCP (bits 2 to 4) set to 0 identify a class-selector PHB.
Content 4.1 Introducing Classification and Marking 4.1.6 Per-Hop Behaviors A PHB is a description of the externally observable forwarding behavior of a DiffServ node applied to a particular DiffServ behavior aggregate (BA). Forwarding behavior is a general concept in this context. For example, in the event that only one BA occupies a link, the observable forwarding behavior (that is, loss, delay, jitter) will often depend only on the relative loading of the link (that is, in the event that the behavior assumes a work-conserving scheduling discipline). Useful behavioral distinctions are mainly observed when multiple BAs compete for buffer and bandwidth resources on a node. The PHB is the means by which a node allocates resources to behavior aggregates, and it is on top of this basic hop-by-hop resource allocation mechanism that useful differentiated services may be constructed. A simple example of a PHB is one that guarantees a minimal bandwidth allocation of x% of a link (over some reasonable time interval) to a behavior aggregate. This PHB is easily measured under a variety of competing traffic conditions. A slightly more complex PHB would guarantee a minimal bandwidth allocation of x% of a link, with proportional fair sharing of any excess link capacity. In general, the observable behavior of a PHB may depend on certain constraints on the traffic characteristics of the associated BA, or the characteristics of other BAs. PHBs may be specified in terms of their resource (for example, buffer, bandwidth) priority relative to other PHBs, or in terms of their relative observable traffic characteristics (for example, delay, loss). These PHBs may be used as building blocks to allocate resources and should be specified as a group (PHB group) for consistency. PHB groups will usually share a common constraint applying to each PHB within the group, such as a packet scheduling or buffer management policy. The relationship between PHBs in a group may be in terms of absolute or relative priority (for example, discard priority by means of deterministic or stochastic thresholds), but this is not required (for example, N equal link shares). A single PHB defined in isolation is a special case of a PHB group. PHBs are implemented in nodes by means of some buffer management and packet scheduling mechanisms. PHBs are defined in terms of behavior characteristics relevant to service provisioning policies, and not in terms of particular implementation mechanisms. In general, a variety of implementation mechanisms may be suitable for implementing a particular PHB group. Furthermore, it is likely that more than one PHB group may be implemented on a node and utilized within a domain. PHB groups should be defined in such a way that the proper resource allocation between groups can be inferred and integrated mechanisms can be implemented that can simultaneously support two or more groups. A PHB group definition should indicate possible conflicts with previously documented PHB groups that might prevent simultaneous operation. The DiffServ architecture defines the DiffServ (DS) field, which supersedes the ToS field in IPv4 to make per-hop behavior (PHB) decisions about packet classification and traffic conditioning functions, such as metering, marking, shaping, and policing. To review: The IETF defines the following PHBs: EF PHB
Figure is a detailed view of the EF PHB used in DSCP. The EF PHB is identified based on the following: Packets requiring EF should be marked with DSCP binary value 101110 (46 or 0x2E). Non-DiffServ-compliant devices regard EF DSCP value 101110 as IP precedence 5 (101). This precedence is the highest user-definable IP precedence and is typically used for delay-sensitive traffic (such as VoIP). Bits 5 to 7 of the EF DSCP value are 101, which matches IP precedence 5 and allows backward compatibility. AF PHB
Figure is a detailed view of the AF PHBs used in DSCP. The AF PHB is identified based on the following: Packets requiring AF PHB should be marked with DSCP value aaadd0, where aaa is the number of the class and dd is the drop probability. There are four standard AF classes defined: AF1, AF2, AF3, and AF4. Each