Rdma Technology Overview; D.2.2 Shared Memory Communications Over Rdma - IBM z13s Technical Manual

A reliable "lossless" Ethernet network fabric (LAN for layer 2 data center network distance)
An RDMA network interface card (RNIC)
Memory
Rkey A
Figure D-1 RDMA technology overview
RDMA technology is now available on Ethernet. RoCE uses an existing Ethernet fabric
(switches with Global Pause enabled) and requires advanced Ethernet hardware
(RDMA-capable NICs on the host).

D.2.2 Shared Memory Communications over RDMA

SMC-R is a protocol that allows TCP socket applications to transparently use RDMA.
SMC-R is a "hybrid" solution as shown in Figure D-2 on page 478:
It uses an existing TCP connection to establish the SMC-R connection.
A TCP option (SMCR) controls switching from TCP to "out of band" SMC-R.
The SMC-R information is exchanged within the TCP data stream.
Socket application data is exchanged through RDMA (write operations).
The TCP connection remains to control the SMC-R connection.
This model preserves many critical operational and network management features of
TCP/IP.
Host A
A
CPU
RNIC
Ethernet Fabric
Host B
Memory B
CPU
RNIC
Rkey B
Appendix D. Shared Memory Communications
477