IP Type of Service

IP Precedence, TOS & DSCP[转]

The IP Type of Service Header Field

 

The Type of Service field in the IP header was originally defined in RFC 791. It defined a mechanism for assigning a priority to each IP packet as well as a mechanism to request specific treatment such as high throughput, high reliability or low latency.

In practice, only the IP Precedence part of the field was ever used. At its simplest, the higher the value of the IP Precedence field, the higher the priority of the IP packet. Simple...

In RFC 2474 the definition of this entire field was changed. It is now called the "DS" (Differentiated Services) field and the upper 6 bits contain a value called the "DSCP" (Differentiated Services Code Point). Since RFC 3168, the remaining two bits (the two least siginficant bits) are used for Explicit Congestion Notification.

The diagram below illustrates the relationship between the bits in the Type of Services/Differeniated Services field in the IP header:-

 

 

Type of Service (TOS)	X	X	X	X	X	X	X	0
	7	6	5	4	3	2	1	0
Differentiated Services (DS)	X	X	X	X	X	X	X	X

Interpretation of the TOS/DS Field

RFC791/RFC1349 Interpretation

Bits	Meaning
7-5	IP Precedence: 111 Network Control 110 Internetwork Control 101 Critic/ECP 100 Flash Override 011 Flash 010 Immediate 001 Priority 000 Routine
4	1 = Low Delay; 0 = Normal Delay
3	1 = High Throughput; 0 = Normal Throughput
2	1 = High Reliability; 0 = Normal Reliability
1	1 = Minimise monetary cost (RFC 1349)
0	Must be 0

RFC 2474 (Differentiated Services) Interpretation

Bits	Meaning
7-2	DSCP
1-0	ECN (Explicit Congestion Notification)

 

The DSCP

The most signficant three bits of the DSCP (corresponding directly to the IP Precedence) are referred to as the Class Selector.

 

DSCP <=> IP Precedence Conversion Table

DSCP Name	DS Field Value		IP Precedence
	Binary	Decimal
CS0	000 000	0	0
CS1	001 000	8	1
AF11	001 010	10	1
AF12	001 100	12	1
AF13	001 110	14	1
CS2	010 000	16	2
AF21	010 010	18	2
AF22	010 100	20	2
AF23	010 110	22	2
CS3	011 000	24	3
AF31	011 010	26	3
AF32	011 100	28	3
AF33	011 110	30	3
CS4	100 000	32	4
AF41	100 010	34	4
AF42	100 100	36	4
AF43	100 110	38	4
CS5	101 000	40	5
EF	101 110	46	5
CS6	110 000	48	6
CS7	111 000	56	7

CS	Class Selector (RFC 2474)
AFxy	Assured Forwarding (x=class, y=drop precedence) (RFC2597)
EF	Expedited Forwarding (RFC 3246)

 

 

转自 ： http://bogpeople.com/networking/dscp.shtml