The (effective) "+42" computation was, in fact, the incorrect answer
in this case (gasp!).
We should just take the max_msg_size from the command (which came from
the libfabric endpoint max_msg_size attribute in the client) and
subtract off the max header size: 68 (which is explained in the
comment). This will result in a "large" message size which is likely
slightly smaller than the MTU, but still right up near the MTU, and
therefore good enough.
Note: the old computation (i.e., -(68-42)) worked fine when we asked
for Libfabric API v1.1 because the usnic provider would return a
max_msg_size that was already less than the MTU due to FI_PREFIX
behavior shenanigans. Once we started asking for Libfabric API v1.4,
the usnic Libfabric provider started returning (MTU + prefix_size),
and the -(68-42) computation started giving a value that was over the
MTU. This caused sendto() on the connectivity checker UDP socket
to fail.
This commit also removes an old/misleading comment.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
545d8f2e66