qos部分乾颐堂qos03-差分服务和phb

1、乾颐堂网络实验室QoS 服务质量差分服务的类基础Understanding the Differentiated Services Model-PHB每一跳行为Differentiated Services ModelThe Differentiated Services model describes services associated with traffic classes.差分服务模型描述了不同的流量分类Complex traffic classification and conditioning is performed at the network edge, resultin

2、g in a per-packet复杂的流量分类和条件可以在网络边界实施No per-flow state in the core.-核心没有基于流的状态The core only performs simple “per-hop behaviors” on traffic aggregates.核心仅仅基于简单的PHBThe goal is scalability.分类和标记分类工具通过检查数据帧、信元和数据包头将数据包分类，同时QoS工具的分类功能还可以修改包头的字段，便于分类很多QoS工具都依靠分类功能用以决定该为哪种流量应用该工具标记功能将一个值放入数据帧、数据包或者信元头部中的一个字

3、段，该字段是转为QoS标记功能设计的，通过标记数据包，其他的QoS功能可以根据头部内的标记字段来执行数据包分类。标记功能简化了网络的QoS设计，简化了其他QoS工具的配置分类和标记不直接影响网络流量的带宽、演示、抖动和丢包，但他们却是应用其他QoS工具的前提可以被扩展ACL匹配的字段可以被CB直接匹配的字段分类标准CB的一些例子按照（所谓的）1层特性来给数据分类，比如来自某个接口的数据R2(config)#class-map A-定义一个class-map名为A，默认为匹配所有（match-all）R2(config-cmap)#match input-interface e0/0-匹配来e0

4、/0接口的流量根据二层特征来给数据分类R3(config)#class-map BR3(config-cmap)#match source-address mac cc00.41fc.0000匹配三层特性来分类R3(config)#class-map TestR3(config-cmap)#match ip precedence 1 2 3 4-这里需要特别指出的是，在这里，和route-map的逻辑相同，在同一行中是任意的意思，即或的关系，匹配1-4的任意一个ip优先级分类标准CB依赖四层或者高层信息R4(config)#class-map match-any Ender-这里我们匹配的是任

5、意一个条件，即匹配TCP或者UDP的流量R4(config-cmap)#match access-group name TCPR4(config-cmap)#match access-group name UDP!R4(config)#ip access-list extended TCPR4(config-ext-nacl)#permit tcp any host 44.1.1.1 eq 23IP包头中用于QoS的字段接下来的部分列出了您可以标记的头部字段，并且说明了该字段最适合用于何种场景QoS中最常用的两个标识字段是IP优先级和DSCPDSCP EncodingDiffServ fiel

6、d: The IP version 4 header ToS octet or the IPv6 traffic class octet, when interpreted in conformance with the definition given in RFC 2474 . IPv4中取出三位作为IP优先级位IP优先级举例：101(000XX)=160（5*32）DSCP: The first six bits of the DiffServ field, used to select a PHB (forwarding and queuing method)-六位的DSCP，用于选择

7、PHB（即在每一个设备所受的服务，转发和队列方法）优先级字段优先级字段的值应该随着重要性而增长，对QoS行为来说值越大越重要DSCP与此不同，在定义逐跳行为的RFC中，建议使用的DSCP值并不是越大越好IP precedencenetwork control precedence (7)网络控制internetwork control precedence (6)网间控制critical precedence (5，101)，voice关键flash override precedence (4)，video疾速flash precedence (3)，voice control闪速immed

8、iate precedence (2)，data快速priority precedence (1)，data优先routine precedence (0)普通IP Precedence bit settings 6 and 7 are reserved for network control information such as routing updates.优先级6和7保留给网络控制信息Per-Hop BehaviorsDSCP selects PHB throughout the network.四类PHBDefault PHB (FIFO, tail drop)EF Expedit

9、ed Forwarding（101110）-快速转发 AF Assured Forwarding 保证转发Class-Selector (兼容IP Precedence) PHB后三位为0，称之为CS。兼容ip优先级，CS1=P1，类推至cs7默认0没有QoS机制101110的EFAF保证转发类选择器PHB基于差分服务模型的网络，QoS特性无法支持差分服务是不可想象的DSCP中定义了一个特用于兼容IP优先级的CS的PHB（每一跳行为），即CS越高优先级越高Backward Compatibility Using the Class Selector用CS向后兼容IP优先级Class-Selec

10、tor “xxx000” DSCP Compatibility with current IP Precedence usage (RFC 1812) = maps IP Precedence to DSCP-和ip 优先级兼容（后三位等于0）Differentiates probability of timely forwarding 不同的转发可能(xyz000) = (abc000) if xyz abcIf a packet has DSCP = “011000,” it has a greater probability of timely forwarding than a pac

11、ket with DSCP = “001000.”DSCP example af11 Match packets with AF11 dscp (001010) af12 Match packets with AF12 dscp (001100) af13 Match packets with AF13 dscp (001110) af21 Match packets with AF21 dscp (010010) af22 Match packets with AF22 dscp (010100) af23 Match packets with AF23 dscp (010110) af31

12、 Match packets with AF31 dscp (011010) af32 Match packets with AF32 dscp (011100) af33 Match packets with AF33 dscp (011110) af41 Match packets with AF41 dscp (100010) af42 Match packets with AF42 dscp (100100) af43 Match packets with AF43 dscp (100110) cs1 Match packets with CS1(precedence 1) dscp

13、(001000) cs2 Match packets with CS2(precedence 2) dscp (010000) cs3 Match packets with CS3(precedence 3) dscp (011000) cs4 Match packets with CS4(precedence 4) dscp (100000) cs5 Match packets with CS5(precedence 5) dscp (101000) cs6 Match packets with CS6(precedence 6) dscp (110000) cs7 Match packet

14、s with CS7(precedence 7) dscp (111000) default Match packets with default dscp (000000) ef Match packets with EF dscp (101110)Per-Hop Behaviors (Cont.)EF PHB:Ensures a minimum departure rate-保证最小转发能力Guarantees bandwidth (The class is guaranteed an amount of bandwidth with prioritized forwarding.)-保证

15、带宽，优先转发Polices bandwidth (The class is not allowed to exceed the guaranteed amountexcess traffic is dropped.)-管制流量，超过的会被丢弃DSCP value “101110”: looks like IP Precedence 5 to pliant devicesBits 5 to 7: “101” = 5 (Same three bits used for IP Precedence) Bits 3 to 4: “11” = drop probability high 丢弃可能性最大

16、，可以理解为管制后都丢弃Bit 2: Just “0”EF=快速转发：101110-固定值如果仅仅识别IP优先级，则优先级5，等同EFPer-Hop Behaviors (Cont.)AF PHB-确保转发:Guarantees bandwidth-带宽保证Allows access to extra bandwidth, if available-可以占用更多的出接口Four standard classes (af1, af2, af3, and af4；001，010，011，100)DSCP value range: “aaadd0”Where “aaa” is a binary va

17、lue of the classWhere “dd” is drop probability-代表丢弃可能性，值越高，丢弃可能性越大Per-Hop Behaviors (Cont.)Each AF class uses three DSCP values.-每个AF类用三个DSCP值（如AF11，AF12，AF13）Each AF class is independently forwarded with its guaranteed bandwidth.每个AF类依据保证带宽独立的转发它们数据Congestion avoidance is used within each class to

18、prevent congestion within the class.拥塞避免针对不同的AF有不同的待遇AF13丢弃可能性大于AF11;AF43的包对比AF21，更易被丢弃dd位的取值：00，01，10，11;00被CS占用Per-Hop Behaviors (Cont.)A DiffServ node must allocate a configurable, minimum amount of forwarding resources (buffer space and bandwidth) per AF class.Excess resources may be allocated between nonidle classes. The manner must be specified.Reordering of IP packets of the same flow is not allow