Circuits as a Local Option
With this protocol the nodes at the two ends of a link agree to compress SS information so as to save link load but more importantly to avoid SS processing.
The SS protocol so far described would often lead to a flow of many packets with identical SSes over a particular link, with constant senders and receivers.
The notion is to agree, for periods of time to compress these using a fully associative software map from SS value to channel number for transmission over the link.
This sounds like more work rather than less until you consider passage of a packet thru several links all using this compression.
As a packet arrives at a node on an interface, the channel number of the packet is used to index into a table, which is associated with the incoming link.
In that table is found: (outgoing link number, outgoing channel number).
All logic of examining and manipulating turn ops is avoided.
Indeed the aforementioned tables matching channel numbers to SS values are unneeded in intermediate nodes.
Tymnet had something a bit like SS fields but they were used only on circuit building and they were generated only by network supervisors.