Difference:
TOSAnd DSCP are the fields in the three tier protocol (IP layer).
COSIt is the field in the two tier protocol.
Detailed:
TOSDefined in different protocols, RFC 791, RFC 122, RFC 1349; RFC 1349 abolished the previous two RFC definitions, and now most devices use RFC 1349.
DSCPDefined by RFC2474, the 1 byte of the TOS and Traffic Class in the IPv4 header is renamed. The new name is the DS field, which is still used by QoS tools to annotate data.
0 1 2 3 4 5 6 7
+—+—+—+—+—+—+—+—+
| DSCP | CU |
+—+—+—+—+—+—+—+—+
DSCP: differentiated services codepoin
CU: currently unused
COSThe 802.1Q standard defines a two-byte label (TPID) and two-byte control information (TCI) in a two-layer data frame (802.1q frame), and the first three bits of the TCI define 802.1p priority.
When implementing Qos strategy, mapping mechanism between Cos and Tos or DSCP is usually done.