IBM z13s, Technical Manual

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Appendix D. Shared Memory Communications
477
򐂰 A reliable “lossless” Ethernet network fabric (LAN for layer 2 data center network distance)
򐂰 An RDMA network interface card (RNIC)
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
Memory
Host B
Memory
CPU
CPU
Rkey A
Rkey B
A B
RNIC
RNIC
Ethernet Fabric