Single Roundtrip for remote transfers

I believe you are referring to the SiRT feature with SRDF/s introduced with Enginuity 5772, see eLab Distance Extension document page 56. Excerpt below. This feature eliminates the transfer-ready IO by locally acknowledging the transfer-ready IO. 

VPLEX supports both FC and IP connectivity for wan-com.  This feature is only applicable to FC connected environments.

With VPLEX-Metro utilizing FC for wan-com, we do not optimize the transfer ready as of VPLEX 5.1, there is an outstanding enhancement request to add this functionality, date and version that it will be supported on is TBD. Customers that are concerned or believe they need this functionality can utilize a number of FC distance Extension solutions to accomplish this, again see eLab Distance Extension document.

Excerpt from the above linked doc.

Single RoundTrip (SiRT) for Fibre Channel SRDF directors (RFs) was

introduced in EMC Enginuity™  5772 for SRDF/S mode only. It is

dynamically enabled for SRDF/S links > 12 Km for block sizes up to

32K in Enginuity 5773 code. SiRT is compatible with Fast Write/Write

Acceleration switches and extenders, as it will measure link latency

and disable automatically if connected to these devices. As a best

practice, it is recommended that either the EMC SiRT feature or the

third-party fast write feature should be used. Both should not be

enabled simultaneously.