High Level design

UCX code consists of 3 parts:

• Protocol Layer - UCP
• Transport Layer - UCT
• Services - UCS

Protocol Layer

Supports all functionality described in the API, and does not require knowledge of particular hardware. It would try to provide best "out-of-box" performance, combining different hardware mechanisms and transports. It may emulate features which are not directly supported in hardware, such as one-sided operations. In addition, it would support common software protocols which are not implemented in hardware, such as tag matching and generic active messages. More details here

Transport Layer

Provides direct access to hardware capabilities, without decision logic which would prefer one hardware mechanism over another. Some functionality may not be supported, due to hardware limitations. The capabilities are exposed in the interface.
More details here.

Services

Collection of generic services, data structures, debug aids, etc.

Responsibilities of each layer

What	Where	Why
Tag matching	High level	Software protocol
RMA/AMO emulation	High level	Software protocol
Fragmentation	High level	Software protocol
Multi-transport/channel/rail	High level	OOB optimization
Select inline/bcopy/zcopy	High level	optimization logic
Reliability (e.g UD)	Low level	Transport specific
DMA buffer ownership	Low level	Transport specific
Memory registration cache	Low level	Transport dependent

See also:

• Send side progress completion
• Design discussion