QoS加速转发(EF)模型

确保转发(AF)模型用于为不同的数据应用程序提供优先级值。加速转发(EF)模型用于向延迟(延迟)敏感的实时交互流量提供资源。EF模型使用一个标记——dscp46。DSCP 46向后兼容IP优先值5，如下面的二进制模式所示:101110 = DSCP 46 46的EF标记不遵循有保证转发模型的drop preference规则。请不要认为11意味着高滴偏好。EF模式在大多数供应商的电话中被默认用于IP上的语音媒体流量(RTP)。Cisco IP电话将信令包(SCCP或SIP)标记为CS3(24)，而media (RTP)默认标记为EF (DSCP 46)。所有EF流量通常被映射到Cisco交换机和路由器上的优先队列(PQ)。优先级队列保证三个关键的服务:•包丢失•延迟•抖动(延迟变化)101的三个最重要的位只有在使用IP优先级时才会被考虑。当只考虑三个数字时，使用4 2 1的二进制数字分解101的二进制模式。 The DSCP binary pattern of 101110 (46) uses six digits or binary values-32 16 8 4 2 1. It is good to know how to convert a DSCP decimal value to an entire ToS octet (byte) values as well. The ToS byte uses all eight bits, while the DSCP is only using the leading six digits. The EF pattern discussed above, will becomes 10111000 when considering the entire octet. Notice the two least significant zeros that were added to the 101110 binary pattern. Many network management utilities will only allow administrators to configure or display the entire ToS byte. A ping –V from a Microsoft operating system requires setting the entire ToS byte. An extended ping from a Cisco router will also allow administrators to see the entire ToS byte. Sniffer Pro LAN and Wire Shark sniffers show the entire ToS field as well. IP accounting shows the entire ToS byte, while Netflow shows the ToS byte in hexadecimal format. The ToS byte value for EF is as follows: 128 64 32 16 8 4 2 1 1 0 1 1 1 0 0 0 A DSCP value of 46 results in a ToS byte value of 184. Although you can mark a ping with a ToS value of 184, the ICMP (ping) traffic will probably not be mapped to the proper application class. In the next blog, we will learn QoS models for using markings for different application classes.