Type of service

 

Tos字段被主机用来标识一个数据包的优先级

字段格式：

 

0	1	2	3	4	5	6	7
Precedence			D	T	R	Unused

 

0 = Most Significant Bit; 7 = Least Significant Bit.

 Precedence bits have a from 0 to 7 and are used to indicate the importance of a datagram (default is 000, higher is better).

 

Precedence	Hexadecimal	Decimal	Description
111	0xE0	224	Network Control
110	0xC0	192	Internetwork Control
101	0xA0	160	CRITIC/ECP
100	0x80	128	Flash Override
011	0x60	96	Flash
010	0x40	64	Immediate
001	0x20	32	Priority
000	0x00	0	Routine

The decimal and hexadecimal values above, assumes bits 3 to 7 are set to 0.

Bits 3 4 5 represent the following:

• D: requests low delay
• T: requests high throughput
• R: requests high reliability

Bit 6 was later defined by RFC 1349 as "Minimize Monetary Cost
 

例:

20:56:04.852978 In IP (tos 0x10, ttl 63, id 62037, len 60) 130.130.0.1.3465 >
10.0.3.4.5060: S 1914879600:1914879600(0) win 16384 <mss 1460,nop,wscale 0,
nop,nop,timestamp 558487 0> (DF)

tos0x10,When broken down into binary, this yields a 000 100 00 pattern (the added spacing helps to call out that the first three bits in ToS field code a precedence setting of 0). The next three bits code the Delay, Throughput, and Reliability indicators, which in this case are coded to indicate that delay is the optimal metric for this traffic (telnet, being interactive and character based,is a delay-sensitive application) The last two bits of the precedence/ToS file are reserved and set to zero。