## 10.5 Shared Memory Communications - Direct Memory Access 

This section describes the configuration of the Shared Memory Communiction - Direct Memory Access (SMC-D) connections on an IBM z14 ZR1. 

## 10.5.1 SMC-D overview 

SMC-D is a protocol that allows TCP socket applications to transparently use Internal Shared Memory (ISM). ISM is a virtual channel that is similar to IQD for HiperSockets. A virtual adapter is created in each z/OS LPAR and by using the SMC protocol, the memory is logically shared. The virtual network is provided by firmware. 

SMC-R requires a TCP/IP connection and preserves the entire network infrastructure. SMC-D is also a “hybrid” solution. It uses a TCP connection to establish the SMC-D connection. The TCP path can be through an OSA-Express port or HiperSockets connection. 

A TCP option (called SMCD) controls switching from TCP to “out of band” SMC-D. The SMC-D information is exchanged within the TCP data stream. Socket application data is exchanged through ISM (write operations). The TCP connection remains established to control the SMC-D connection. 

For more information about SMC-D, see IBM z14 Technical Guide , SG24-8451, and IBM z/OS V2R2 Communications Server TCP/IP Implementation Volume 1 , SG24-8360. 

## 10.5.2 Planning for SMC-D configuration 

For more information about planning considerations for SMC-D, see “Shared Memory Communications - RDMA” on page 22. 

## 10.5.3 Configuring SMC-D 

For more information about configuring an SMC-D connection by using HCD, see 15.2.2, “Defining an ISM PCIe function” on page 372. 

## 10.5.4 SMC-D management 

From an operational standpoint, SMC-D is similar to SMC-R. However, SMC-D uses direct memory access (DMA) instead of an RDMA. It also uses a virtual PCI adapter that is called ISM rather than an RNIC. The ISM interfaces are associated with IP interfaces (for example, HiperSockets or OSA-Express) and are dynamically created, automatically started and stopped, and auto-discovered. 

SMC-D over ISM does not use QP technology like SMC-R. Therefore, links and Link Groups that are based on QPs (or other hardware constructs) are not applicable to ISM. SMC-D protocol features a design concept of a “logical point-to-point connection” called an SMC-D link. 

Note:  The SMC-D information in the netstat  command displays is related to ISM link information (not Link Groups). 

