Library /sys$common/syshlp/ncphelp.hlb LOOP, NODE, LENGTH number  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Specifies the length (in bytes) of the blocks to be sent during
 loopback testing. The length must be a decimal integer in the range
 of 1  through 4096.  If the parameter is omitted, a block length of 4096
 bytes is used.