12.3. Representation of TOS

All OSPF link state advertisements (with the exception of network links advertisements) specify metrics. In router links advertisements, the metrics indicate the costs of the described interfaces. In summary link and AS external link advertisements, the metric indicates the cost of the described path. In all of these advertisements, a separate metric can be specified for each IP TOS. The encoding of TOS in OSPF link state advertisements is specified in Table 17. That table relates the OSPF encoding to the IP packet header's TOS field (defined in [RFC 1349]). The OSPF encoding is expressed as a decimal integer, and the IP packet header's TOS field is expressed in the binary TOS values used in [RFC 1349].

                    OSPF encoding   RFC 1349 TOS values
                    ___________________________________________
                    0               0000 normal service
                    2               0001 minimize monetary cost
                    4               0010 maximize reliability
                    6               0011
                    8               0100 maximize throughput
                    10              0101
                    12              0110
                    14              0111
                    16              1000 minimize delay
                    18              1001
                    20              1010
                    22              1011
                    24              1100
                    26              1101
                    28              1110
                    30              1111

                        Table 17: Representing TOS in OSPF.

Each OSPF link state advertisement must specify the TOS 0 metric. Other TOS metrics, if they appear, must appear in order of increasing TOS encoding. For example, the TOS 8 (maximize throughput) metric must always appear before the TOS 16 (minimize delay) metric when both are specified. If a metric for some non-zero TOS is not specified, its cost defaults to the cost for TOS 0, unless the T-bit is reset in the advertisement's Options field (see Section 12.1.2 for more details).