首先看IPv4包头如下
其中，Qos用到的是Tos定义有下面两种：
老的IPv4 TOS Byte定义和值
新的DSCP定义和值
DSCP值
DSCP Value | Meaning | Drop Probability | Equivalent IP Precedence Value |
101 110 (46) | High Priority Expedited Forwarding (EF) | N/A | 101 – Critical |
000 000 (0) | Best Effort | N/A | 000 – Routine |
001 010 (10) | AF11 | Low | 001 – Priority |
001 100 (12) | AF12 | Medium | 001 – Priority |
001 110 (14) | AF13 | High | 001 – Priority |
010 010 (18) | AF21 | Low | 001 – Immediate |
010 100 (20) | AF22 | Medium | 001 – Immediate |
010 110 (22) | AF23 | High | 001 – Immediate |
011 010 (26) | AF31 | Low | 011 – Flash |
011 100 (28) | AF32 | Medium | 011 – Flash |
011 110 (30) | AF33 | High | 011 – Flash |
100 010 (34) | AF41 | Low | 100 – Flash Override |
100 100 (36) | AF42 | Medium | 100 – Flash Override |
100 110 (38) | AF43 | High | 100 – Flash Override |
