Information Technology Reference
In-Depth Information
Ta b l e 8 - 3
IPv4 Header Fields
Field
Length
Description
Source Address
32 bits
IP address of the sending host.
Destination Ad-
dress
32 bits
IP address of the destination host.
IP Options
Va r i a ble
Options for security, loose source routing, record route, and
timestamp.
Padding
Va r i a ble
Added to ensure that the header ends in a 32-bit boundary.
To S
The ToS field of the IP header is used to specify QoS parameters. Routers and Layer 3
switches look at the ToS field to apply policies, such as priority, to IP packets based on the
markings. An example is a router prioritizing time-sensitive IP packets over regular data
traffic like web or email, which are not time-sensitive.
Key
To p i c
The ToS field has undergone several definitions since RFC 791. Figure 8-2 shows the sev-
eral formats of the ToS service field based on the evolution of RFCs 791 (1981), 1349
(1992), 2474 (1998), and 3168 (2001). The follow ing parag raphs de s c r ibe this evolut ion.
0 1 2 3 4 5 6 7
IP precedence
ToS
0
0
RFC 791 (1981)
IP precedence
ToS
0
RFC 1349 (1992)
RFC 2474 (1998)
DSCP
0
0
RFC 3168 (2001)
DSCP
ECN
Figure 8-2
Evolution of the IPv4 ToS Field
The first 3 (leftmost) bits are the IP precedence bits. These bits define values that are used
by QoS methods. The precedence bits especially help in marking packets to give them dif-
ferent iated t reat ment w ith different pr ior it ie s. For example, Voice over IP (VoIP) packets
can get preferential treatment over regular data packets. RFC 791 describes the prece-
dence bits as shown in Table 8-4.
 
 
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