diff --git a/nqptp-shm-structures.h b/nqptp-shm-structures.h
index 82837778..0791197d 100644
--- a/nqptp-shm-structures.h
+++ b/nqptp-shm-structures.h
@@ -1,6 +1,6 @@
 /*
  * This file is part of the nqptp distribution (https://github.com/mikebrady/nqptp).
- * Copyright (c) 2021 Mike Brady.
+ * Copyright (c) 2021--2022 Mike Brady.
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -23,11 +23,24 @@
 #define NQPTP_SHM_STRUCTURES_VERSION 7
 #define NQPTP_CONTROL_PORT 9000
 
-// The control port will accept a UDP packet with the first letter being:
-// "T", followed by the name of the shared memory interface, followed by
-// a space and then a space-delimited list of ip numbers, either IPv4 or IPv6
-// the whole not to exceed 4096 characters in total
-// The IPs will become the new list of timing peers, replacing any previous
+// The control port expects a UDP packet with the first space-delimited string
+// being the name of the shared memory interface (SMI) to be used.
+// This allows client applications to have a dedicated named SMI interface with
+// a timing peer list independent of other clients.
+// The name given must be a valid SMI name and must contain no spaces.
+// If the named SMI interface doesn't exist it will be created by NQPTP.
+// The SMI name should be delimited by a space and followed by a command letter.
+// At present, the only command is "T", which must followed by nothing or by
+// a space and a space-delimited list of IPv4 or IPv6 numbers,
+// the whole not to exceed 4096 characters in total.
+// The IPs, if provided, will become the new list of timing peers, replacing any
+// previous list. If the master clock of the new list is the same as that of the old list,
+// the master clock is retained without resynchronisation; this means that non-master
+// devices can be added and removed without disturbing the SMI's existing master clock.
+// If no timing list is provided, the existing timing list is deleted.
+// (In future version of NQPTP the SMI interface may also be deleted at this point.)
+// SMI interfaces are not currently deleted or garbage collected.
+
 
 #include <inttypes.h>
 #include <pthread.h>