A comparison with SVR4 Streams.

The paper A Stream Input-Output System (.ps) by Dennis Ritchie describes very clearly how streams work in SVR4. His plan has been successfully implemented within a real system and has thus addressed the real obstacles.

Here is a list of differences:
SVR4Tymnet Scheme
Buffering per moduleYes, better thruput for character batchesNo, better latency, better thruput for single characters
Duplex/SimplexDuplex: Addresses real problems directly.Simplex: Duplex logic can be done outside the framework.
Order of ProcessingUsually Coroutines: (a form of continuation processing)General continuation discipline
Flow ControlExplicit: conventional buffer disciplineImplicit: by management of continuations
Branching & Merging StreamsNot designed, not needed in target environmentSupported
Relation to rest of systemArchitectedIgnored
Inter-module MessagesData Blocks & Control MessagesEither “single character” (inband signaling) & invocation of continuation.
The Tymnet style single character may well be interpreted as an action message by some modules.

An interesting contrast is that in the Tymnet scheme a character could traverse several modules remaining unmolested in the A register. In the SVR4 scheme a batch of characters may remain unmolested in a buffer that is passed thru several modules, but some of those modules may need to look at the character to see if it is significant to that module.