The second byte in an IPv4 header is called the Differentiated Services Field.
Quoting RFC 2474:
2. Terminology Used in This Document
...
Differentiated Services Field: the IPv4 header TOS octet or the IPv6
Traffic Class octet when interpreted in conformance with the
definition given in this document. Also DS field.
It's a historical accident that the ECN bits were shoehorned into this octet.
--
Regards
Brian Carpenter
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