TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Service Type: The service type is an indication of the quality of service
requested for this IP datagram. This field contains the information illustrated
in Figure 3-18.
Figure 3-18 IP: Service type
Where:
– Precedence: This field specifies the nature and priority of the datagram:
000: Routine
001: Priority
010: Immediate
011: Flash
100: Flash override
101: Critical
110: Internetwork control
111: Network control
– TOS: Specifies the type of service value:
1000: Minimize delay
0100: Maximize throughput
0010: Maximize reliability
0001: Minimize monetary cost
0000: Normal service
100 TCP/IP Tutorial and Technical Overview
A detailed description of the type of service is in the RFC 1349 (refer to
8.1, “Why QoS?” on page 288).
– MBZ: Reserved for future use.
► Total Length: The total length of the datagram, header and data.
► Identification: A unique number assigned by the sender to aid in reassembling
a fragmented datagram. Each fragment of a datagram has the same
identification number.
► Flags: This field contains control flags illustrated in Figure 3-19.
0 1 2
0
D
F
0 1 2 3 4 5 6 7
Figure 3-19 IP: Flags
precedence TOS MBZ
M
F