Updated text:

   The main decision is whether the message must be sent in C-mode or
   D-mode.  Reasons for using C-mode are:

[snip]

   o  message size: a message whose size (including the GIST header,
      GIST objects and any NSLP payload, and an allowance for the IP and
      transport layer encapsulation required by D-mode) exceeds a
      fragmentation-related threshold MUST be sent over C-mode, using a
      messaging association that supports fragmentation and reassembly
      internally.  The allowance for IP and transport layer
      encapsulation is 64 bytes.  If the Path MTU to the next peer is
      known, the message size MUST NOT exceed that Path MTU; if the Path
      MTU is not known, the message size MUST NOT exceed 576 bytes.  The
      same limit applies to IPv4 and IPv6.