Information Technology Reference
In-Depth Information
Ta b l e 8 - 4
IP Precedence Bit Values
Decimal
Binary
Description
0
000
Routine
1
001
Priority
2
010
Immediate
3
011
Flash
4
100
Flash override
5
101
Critical
6
110
Internetwork control
7
111
Network control
All default traffic is set with 000 in the precedence bits. Voice traffic is usually set to 101
(critical) to give it priority over normal traffic. Applications such as FTP are assigned a
normal priority because it tolerates network latency and packet loss. Packet retransmis-
sions are typically acceptable for normal traffic.
Note:
It is common to see voice traffic classified as IP precedence 5, video traffic classi-
fied as IP precedence 4, and voice and video signaling classified as IP precedence 3. Default
traffic remains as IP precedence 0.
RFC 1349 redefined bits 3 and 6 (expanding for ToS bits) to reflect a desired type of serv-
ice optimization. Table 8-5 shows the ToS field values that indicate service parameters to
use for IP packets.
Ta b l e 8 - 5
To S F i e l d Va l u e s
To S B i t s 3 t o 6
Description
0000
Normal service
1000
Minimize delay
0100
Maximize throughput
0010
Maximize reliability
0001
Minimize monetary cost
In 1998, RFC 2474 redefined the ToS octet as the Differentiated Services (DS) field and
further specified bits 0 through 5 as the Differentiated Services Codepoint (DSCP) bits to
tion of an Explicit Congestion Notification (ECN) field.
