Sockets Direct Protocol (SDP) is a byte-stream transport protocol implementing the TCP SOCK_STREAM semantics utilizing transport offloading capabilities of the InfiniBand fabric. Under the hood, SDP supports Zero-Copy (ZCopy) operation mode, using the InfiniBand RDMA capability to transfer data directly between application buffers. Alternatively, in Buffer Copy (BCopy) mode, data is copied to and from transport buffers.

In the initial open-source SDP implementation, ZCopy mode was restricted to Asynchronous I/O operations. We added a prototype ZCopy support for send()/recv() synchronous socket calls.

This paper presents the major architectural aspects of the SDP protocol, the ZCopy implementation, and a preliminary performance evaluation. We show substantial benefits of ZCopy when multiple connections are running in parallel on the same host. For example, when 8 connections are simultaneously active, enabling ZCopy yields a bandwidth growth from 500MB/s to 700MB/s, while CPU utilization decreases 8 times.