RoCE maximum transmission unit

RDMA protocols, including SMC-R, define a unique set of supported maximum transmission unit (MTU) sizes. SMC-R protocols support MTU sizes of 256, 512, 1024, 2048, and 4096 bytes, but z/OS® Communications Server requests an MTU size of at least 1024 bytes. When an SMC-R link is initially established between two peer hosts, the MTU size is exchanged and negotiated to the lowest value for both hosts. The negotiated MTU size must account for transport headers and cyclic redundancy check (CRC) information that is used by the underlying RoCE protocols.

Guidelines:

The default MTU value for Communications Server is 1024, which can be used for most z/OS application workloads.
In some cases, such as streaming or bulk data transfer, you can improve throughput by using an MTU setting of 2048. You can configure the MTU value on the PFID subparameter of the GLOBALCONFIG statement.
If you set the MTU size to 2048, you must also enable jumbo frames on all switches in the path for all peer hosts.
Define the same MTU size for all PFIDs that are associated with the same physical network (PNet ID).