组播与QOS.ppt

* * * * * * * * * * * * * * * * * * Classification and Marking in the LAN with IEEE 802.1Q IEEE 802.1p 使用的优先级字段叫做 CoS. IEEE 802.1p 支持8个 CoS位. IEEE 802.1p 用于链路直连LAN上，不支持端到端. Classification and Marking in the Enterprise DiffServ Model Describes services associated with traffic classes. Complex traffic classification and conditioning is performed at the network edge, resulting in a per-packet DSCP. No per-flow state in the core. The core only performs simple PHBs on traffic aggregates. The goal of the DiffServ model is scalability. Wide variety of services and provisioning policies. Decouple service and application in use. No application modification. No hop-by-hop signaling. Interoperability with non-DiffServ-compliant nodes. Incremental deployment. IP Precedence and DSCP Compatibility Compatibility with current IP precedence usage (RFC 1812) Differentiates probability of timely forwarding:(xyz000) = (abc000) if xyz abc(that is, if a packet has DSCP value of 011000, it has a greater probability of timely forwarding than a packet with DSCP value of 001000) Per-Hop Behaviors DSCP selects PHB throughout the network: Default PHB (FIFO, tail drop) EF PHB AF PHB Class-selector PHB (IP precedence) EF PHB EF PHB: Ensures a minimum departure rate Guarantees bandwidth—class guaranteed an amount of bandwidth with prioritized forwarding Polices bandwidth—class not allowed to exceed the guaranteed amount (excess traffic is dropped) DSCP value of 101110: Looks like IP precedence 5 to non-DiffServ-compliant devices: Bits 5 to 7: 101 = 5 (same 3 bits are used for IP precedence) Bits 3 and 4: 11 = No drop probability Bit 2: Just 0 AF PHB AF PHB: Guarantees bandwidth Allows access to extra bandwidth, if available Four standard classes: AF1, AF2, AF3, and AF4 DSCP value range of aaadd0: aaa is a binary value of the class dd is drop probability AF PHB (Cont.) Each AF class uses three DSCP values. Each AF class is independently forwarded with its guaranteed bandwidth. Congestion avoida