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IT Tech/Network

[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