[네트워크] DSCP 비트의 진화, IP 헤더 첫 4Byte의 진화

Originally, the IP header had 3 precedence bits and 3 ToS bits, as well as 2 unused bits. The precedence bits were (and still are) used to make various decisions about packet treatment. Precedence values 0 through 5 are designated for user data; precedence values 6 and 7 are reserved to make network control traffic. RFC 1349 reassigned one of the unused bits to the ToS bits, giving the IP header a total of 3 precedence bits, 4 ToS bits, and one unused bit.

Use of ToS bits was never well-defined or well-deployed. The original intent was to be able to mark packets that preferred low delay, high throughput, or high-reliability paths, but service architectures were never designed or built around these bits.

The DiffServ set of RFCs (RFCs 2474 and 2475) redefined the entire ToS byte. The ToS byte now contains 6 bits of information that declare desired packet treatment — DSCP bits. The remaining two bits in the ToS byte are used for a TCP mechanism called Explicit Congestion Notification (ECN), which isn't addressed here but is defined in RFC 3168.

DSCP Bits to IP Precedence Bits

IP Precedence (Decimal)	IP Precedence (Bits)	DSCP (Decimal)	DSCP (Bits)
0	000	0	000000
1	001	8	001000
2	010	16	010000
3	011	24	011000
4	100	32	100000
5	101	40	101000
6	110	48	110000
7	111	56	111000

Additional DSCP Values
(RFC 2597, RFC2598)

Name	DSCP (Decimal)	DSCP (Bits)
Default	0	000000
AF11	10	001010
AF12	12	001100
AF13	14	001110
AF21	18	010010
AF22	20	010100
AF23	22	010110
AF31	26	011010
AF32	28	011100
AF33	30	011110
AF41	34	100010
AF42	36	100100
AF43	38	100110
EF	46	101110

--> 21개의 클래스를 정의하고 있지만, 실제 frame-mode network에서는 0~7까지의 8개 클래스가 사용됨.








[Source]
http://cisco-press-traffic-engineering.org.ua/1587050315/ch06lev1sec3.html