[1] Prior to the redefinition, the ToS field could specify a datagram's priority and request a route for low-latency, high-throughput, or highly-reliable service.
Based on these ToS values, a packet would be placed in a prioritized outgoing queue,[2] or take a route with appropriate latency, throughput, or reliability.
However, a great deal of experimental, research, and deployment work has focused on how to make use of these eight bits, resulting in the current DS field definition.
The definition was largely derived from a US DoD Specification JANAP-128, which defines message multilevel precedence and preemption.
IPv6 has never had an IPv4-like "traditional" ToS field, partially because the authors were aware of DiffServ efforts at its drafting (RFC 2460 Section 7).
Although not frequently used, IP ToS definitions are widely found in netinet/ip.h of Unix-like or Unix operating systems as IPTOS_FIELDNAME macros.
[5] The "lowcost" field is commented out in OpenBSD due to its newer use for indicating ECN support.
An old Apache module "mod_iptos", once packaged in Ubuntu, notes that a way to use multiple RFC 1349 option bits together emerged after some point.