After some discussion on the mailing list, the text updates below are updated as
follows:

In 5.1:

   MA-Hello: This message MUST be sent only in C-mode.  It contains the
   common header, with a NSLPID of zero, and a message identifier, the
   Hello-ID.  If sent with R=0 and null Hello-ID, it indicates that a
   node wishes to keep a messaging association open.  A node MAY also
   invoke a diagnostic request/response exchange by setting R=1and
   providing a non-zero Hello-ID; if this case, the peer MUST send
   another MA-Hello back along the messaging association echoing the
   same Hello-ID and with R=0.  Use of this diagnostic is entirely at
   the discretion of the initiating node.

In 5.2.2:

   Hello-ID:  The Hello-ID is a 32-bit quantity that is used to
      correlate messages in an MA-Hello request/response exchange.  A
      non-zero value MUST be used in a request (messages sent with R=1)
      and the same value must be returned in the response (which has
      R=0).  The value zero MUST be used for all other messages; if a
      message is received with R=1 and Hello-ID=0, an "Object Value
      Error" message (Appendix A.4.4.10) with subcode 1 ("Value Not
      Supported") MUST be returned and the message dropped.  Nodes MAY
      use any algorithm to generate the Hello-ID; a suitable approach is
      a local sequence number with a random starting point.

and in 6.4:

   At any time in the Connected or Idle states, a node MAY test the
   connectivity to its peer and the liveness of the GIST instance at
   that peer by sending a MA-Hello request with R=1.  Failure to receive
   a response with a matching Hello-ID within a timeout MAY be taken as
   a reason to trigger er_MAFailure.  Initiation of such a test and the
   timeout setting are left to the discretion of the implementaion.
   Note that er_MAFailure may also be signalled by indications from the
   underlying messaging association protocols.