the packet is the last fragment in a series of fragmented packets. The third or high-order bit is not
used.
Fragment Offset – This 13-bit field indicates the position of the fragment data relative to the
beginning of the data in the original datagram, which allows the destination IP process to properly
reconstruct the original datagram.
Time-to-Live – This is a counter that gradually decrements down to zero, at which point the datagram
is discarded. This keeps packets from looping endlessly.
Protocol – Indicates which upper-layer protocol receives incoming packets after IP processing is
complete.
Header Checksum – Helps ensure IP header integrity. Since some header fields change, e.g., Time
To Live, this is recomputed and verified at each point that the Internet header is processed.
Source Address – Specifies the sending node.
Destination Address – Specifies the receiving node.
Options – Allows IP to support various options, such as security.
Data – Contains upper-layer information.
6.4 IP Precedence
IP Precedence is a QoS method that combines a priority value with different on/off parameters; Delay,
Throughput, Reliability and Cost. The MBZ (Must be Zero) bit is not used.
Using the ToS bits, you can define up to eight classes of service. Other devices configured throughout
the network can then use these bits to determine how to treat the packet in regard to the type of
service to grant it. These other QoS features can assign appropriate traffic-handling policies including
congestion management and bandwidth allocation. By setting IP Precedence levels on incoming traffic
and using them in combination with QoS queuing features, you can create differentiated service.
Table 6-3 Type of Service Field (IP Precedence - i Ref. REC 1349)
3 Bits 1 Bit 1 Bit 1 Bit 1 Bit 1 Bit
IP Precedence Value Delay Throughput Reliability Cost MBZ (must be zero)
IP Precedence Value
Value Binary Value Description
0 000 Routine
1 001 Priority
2 010 Immediate
3 011 Flash
4 100 Flash Override
5 101 CRITIC/ECP
6 110 Internetwork Control
7 111 Network Control
ISSUE 1.0 SL2100
Networking Manual 6-15
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