diff --git a/src/core/ddsc/src/dds_sertopic_builtintopic.c b/src/core/ddsc/src/dds_sertopic_builtintopic.c
index 18cd330..3479fa4 100644
--- a/src/core/ddsc/src/dds_sertopic_builtintopic.c
+++ b/src/core/ddsc/src/dds_sertopic_builtintopic.c
@@ -29,16 +29,7 @@
 struct ddsi_sertopic *new_sertopic_builtintopic (enum ddsi_sertopic_builtintopic_type type, const char *name, const char *typename, struct q_globals *gv)
 {
   struct ddsi_sertopic_builtintopic *tp = ddsrt_malloc (sizeof (*tp));
-  tp->c.iid = ddsi_iid_gen();
-  tp->c.name = dds_string_dup (name);
-  tp->c.type_name = dds_string_dup (typename);
-  const size_t name_typename_size = strlen (tp->c.name) + 1 + strlen (tp->c.type_name) + 1;
-  tp->c.name_type_name = dds_alloc (name_typename_size);
-  snprintf (tp->c.name_type_name, name_typename_size, "%s/%s", tp->c.name, tp->c.type_name);
-  tp->c.ops = &ddsi_sertopic_ops_builtintopic;
-  tp->c.serdata_ops = &ddsi_serdata_ops_builtintopic;
-  tp->c.serdata_basehash = ddsi_sertopic_compute_serdata_basehash (tp->c.serdata_ops);
-  ddsrt_atomic_st32 (&tp->c.refc, 1);
+  ddsi_sertopic_init (&tp->c, name, typename, &ddsi_sertopic_ops_builtintopic, &ddsi_serdata_ops_builtintopic, false);
   tp->type = type;
   tp->gv = gv;
   return &tp->c;
@@ -46,9 +37,7 @@ struct ddsi_sertopic *new_sertopic_builtintopic (enum ddsi_sertopic_builtintopic
 
 static void sertopic_builtin_free (struct ddsi_sertopic *tp)
 {
-  ddsrt_free (tp->name_type_name);
-  ddsrt_free (tp->name);
-  ddsrt_free (tp->type_name);
+  ddsi_sertopic_fini (tp);
   ddsrt_free (tp);
 }
 
diff --git a/src/core/ddsc/src/dds_topic.c b/src/core/ddsc/src/dds_topic.c
index 52fa5d8..e9f0fc5 100644
--- a/src/core/ddsc/src/dds_topic.c
+++ b/src/core/ddsc/src/dds_topic.c
@@ -454,12 +454,9 @@ err_invalid_qos:
 
 dds_entity_t dds_create_topic (dds_entity_t participant, const dds_topic_descriptor_t *desc, const char *name, const dds_qos_t *qos, const dds_listener_t *listener)
 {
-  char *key = NULL;
   struct ddsi_sertopic_default *st;
-  const char *typename;
   nn_plist_t plist;
   dds_entity_t hdl;
-  size_t keysz;
   struct dds_entity *ppent;
   dds_return_t ret;
 
@@ -469,21 +466,9 @@ dds_entity_t dds_create_topic (dds_entity_t participant, const dds_topic_descrip
   if ((ret = dds_entity_pin (participant, &ppent)) < 0)
     return ret;
 
-  typename = desc->m_typename;
-  keysz = strlen (name) + strlen (typename) + 2;
-  key = dds_alloc (keysz);
-  (void) snprintf (key, keysz, "%s/%s", name, typename);
-
   st = dds_alloc (sizeof (*st));
 
-  ddsrt_atomic_st32 (&st->c.refc, 1);
-  st->c.iid = ddsi_iid_gen ();
-  st->c.name_type_name = key;
-  st->c.name = ddsrt_strdup (name);
-  st->c.type_name = ddsrt_strdup (typename);
-  st->c.ops = &ddsi_sertopic_ops_default;
-  st->c.serdata_ops = desc->m_nkeys ? &ddsi_serdata_ops_cdr : &ddsi_serdata_ops_cdr_nokey;
-  st->c.serdata_basehash = ddsi_sertopic_compute_serdata_basehash (st->c.serdata_ops);
+  ddsi_sertopic_init (&st->c, name, desc->m_typename, &ddsi_sertopic_ops_default, desc->m_nkeys ? &ddsi_serdata_ops_cdr : &ddsi_serdata_ops_cdr_nokey, (desc->m_nkeys == 0));
   st->native_encoding_identifier = (DDSRT_ENDIAN == DDSRT_LITTLE_ENDIAN ? CDR_LE : CDR_BE);
   st->serpool = ppent->m_domain->gv.serpool;
   st->type = (void*) desc;
diff --git a/src/core/ddsi/include/dds/ddsi/ddsi_sertopic.h b/src/core/ddsi/include/dds/ddsi/ddsi_sertopic.h
index b423101..e23333c 100644
--- a/src/core/ddsi/include/dds/ddsi/ddsi_sertopic.h
+++ b/src/core/ddsi/include/dds/ddsi/ddsi_sertopic.h
@@ -22,16 +22,13 @@ extern "C" {
 
 struct ddsi_serdata;
 struct ddsi_serdata_ops;
-
-struct dds_topic;
-typedef void (*topic_cb_t) (struct dds_topic * topic);
-
 struct ddsi_sertopic_ops;
 
 struct ddsi_sertopic {
   const struct ddsi_sertopic_ops *ops;
   const struct ddsi_serdata_ops *serdata_ops;
   uint32_t serdata_basehash;
+  bool topickind_no_key;
   char *name_type_name;
   char *name;
   char *type_name;
@@ -39,6 +36,18 @@ struct ddsi_sertopic {
   ddsrt_atomic_uint32_t refc; /* counts refs from entities, not from data */
 };
 
+/* The old and the new happen to have the same memory layout on a 64-bit machine
+   and so any user that memset's the ddsi_sertopic to 0 before filling out the
+   required fields gets unchanged behaviour.  32-bit machines have a different
+   layout and no such luck.
+
+   There are presumably very few users of this type outside Cyclone DDS itself,
+   but the ROS2 RMW implementation does use it -- indeed, it prompted the change.
+   This define makes it possible to have a single version of the source that is
+   compatible with the old and the new definition, even if it is only partially
+   binary compatible. */
+#define DDSI_SERTOPIC_HAS_TOPICKIND_NO_KEY 1
+
 /* Called when the refcount dropped to zero */
 typedef void (*ddsi_sertopic_free_t) (struct ddsi_sertopic *tp);
 
@@ -59,6 +68,9 @@ struct ddsi_sertopic_ops {
   ddsi_sertopic_free_samples_t free_samples;
 };
 
+DDS_EXPORT void ddsi_sertopic_init (struct ddsi_sertopic *tp, const char *name, const char *type_name, const struct ddsi_sertopic_ops *sertopic_ops, const struct ddsi_serdata_ops *serdata_ops, bool topickind_no_key);
+DDS_EXPORT void ddsi_sertopic_init_anon (struct ddsi_sertopic *tp, const struct ddsi_sertopic_ops *sertopic_ops, const struct ddsi_serdata_ops *serdata_ops, bool topickind_no_key);
+DDS_EXPORT void ddsi_sertopic_fini (struct ddsi_sertopic *tp);
 DDS_EXPORT struct ddsi_sertopic *ddsi_sertopic_ref (const struct ddsi_sertopic *tp);
 DDS_EXPORT void ddsi_sertopic_unref (struct ddsi_sertopic *tp);
 DDS_EXPORT uint32_t ddsi_sertopic_compute_serdata_basehash (const struct ddsi_serdata_ops *ops);
diff --git a/src/core/ddsi/include/dds/ddsi/q_entity.h b/src/core/ddsi/include/dds/ddsi/q_entity.h
index 3aba1f7..fb5b825 100644
--- a/src/core/ddsi/include/dds/ddsi/q_entity.h
+++ b/src/core/ddsi/include/dds/ddsi/q_entity.h
@@ -410,6 +410,7 @@ int is_builtin_endpoint (nn_entityid_t id, nn_vendorid_t vendorid);
 bool is_local_orphan_endpoint (const struct entity_common *e);
 int is_writer_entityid (nn_entityid_t id);
 int is_reader_entityid (nn_entityid_t id);
+int is_keyed_endpoint_entityid (nn_entityid_t id);
 nn_vendorid_t get_entity_vendorid (const struct entity_common *e);
 
 /* Interface for glue code between the OpenSplice kernel and the DDSI
diff --git a/src/core/ddsi/src/ddsi_sertopic.c b/src/core/ddsi/src/ddsi_sertopic.c
index 3ce5ef1..968a1de 100644
--- a/src/core/ddsi/src/ddsi_sertopic.c
+++ b/src/core/ddsi/src/ddsi_sertopic.c
@@ -16,9 +16,11 @@
 
 #include "dds/ddsrt/heap.h"
 #include "dds/ddsrt/md5.h"
+#include "dds/ddsrt/string.h"
 #include "dds/ddsi/q_bswap.h"
 #include "dds/ddsi/q_config.h"
 #include "dds/ddsi/q_freelist.h"
+#include "dds/ddsi/ddsi_iid.h"
 #include "dds/ddsi/ddsi_sertopic.h"
 #include "dds/ddsi/ddsi_serdata.h"
 
@@ -41,6 +43,41 @@ void ddsi_sertopic_unref (struct ddsi_sertopic *sertopic)
   }
 }
 
+void ddsi_sertopic_init (struct ddsi_sertopic *tp, const char *name, const char *type_name, const struct ddsi_sertopic_ops *sertopic_ops, const struct ddsi_serdata_ops *serdata_ops, bool topickind_no_key)
+{
+  ddsrt_atomic_st32 (&tp->refc, 1);
+  tp->iid = ddsi_iid_gen ();
+  tp->name = ddsrt_strdup (name);
+  tp->type_name = ddsrt_strdup (type_name);
+  size_t ntn_sz = strlen (tp->name) + 1 + strlen (tp->type_name) + 1;
+  tp->name_type_name = ddsrt_malloc (ntn_sz);
+  (void) snprintf (tp->name_type_name, ntn_sz, "%s/%s", tp->name, tp->type_name);
+  tp->ops = sertopic_ops;
+  tp->serdata_ops = serdata_ops;
+  tp->serdata_basehash = ddsi_sertopic_compute_serdata_basehash (tp->serdata_ops);
+  tp->topickind_no_key = topickind_no_key;
+}
+
+void ddsi_sertopic_init_anon (struct ddsi_sertopic *tp, const struct ddsi_sertopic_ops *sertopic_ops, const struct ddsi_serdata_ops *serdata_ops, bool topickind_no_key)
+{
+  ddsrt_atomic_st32 (&tp->refc, 1);
+  tp->iid = ddsi_iid_gen ();
+  tp->name = NULL;
+  tp->type_name = NULL;
+  tp->name_type_name = NULL;
+  tp->ops = sertopic_ops;
+  tp->serdata_ops = serdata_ops;
+  tp->serdata_basehash = ddsi_sertopic_compute_serdata_basehash (tp->serdata_ops);
+  tp->topickind_no_key = topickind_no_key;
+}
+
+void ddsi_sertopic_fini (struct ddsi_sertopic *tp)
+{
+  ddsrt_free (tp->name);
+  ddsrt_free (tp->type_name);
+  ddsrt_free (tp->name_type_name);
+}
+
 uint32_t ddsi_sertopic_compute_serdata_basehash (const struct ddsi_serdata_ops *ops)
 {
   ddsrt_md5_state_t md5st;
diff --git a/src/core/ddsi/src/ddsi_sertopic_default.c b/src/core/ddsi/src/ddsi_sertopic_default.c
index 3400a0d..0fbfec6 100644
--- a/src/core/ddsi/src/ddsi_sertopic_default.c
+++ b/src/core/ddsi/src/ddsi_sertopic_default.c
@@ -22,13 +22,9 @@
 #include "dds/ddsi/ddsi_sertopic.h"
 #include "dds/ddsi/ddsi_serdata_default.h"
 
-/* FIXME: sertopic /= ddstopic so a lot of stuff needs to be moved here from dds_topic.c and the free function needs to be implemented properly */
-
 static void sertopic_default_free (struct ddsi_sertopic *tp)
 {
-  ddsrt_free (tp->name_type_name);
-  ddsrt_free (tp->name);
-  ddsrt_free (tp->type_name);
+  ddsi_sertopic_fini (tp);
   ddsrt_free (tp);
 }
 
diff --git a/src/core/ddsi/src/q_entity.c b/src/core/ddsi/src/q_entity.c
index 1974ffc..06afbbf 100644
--- a/src/core/ddsi/src/q_entity.c
+++ b/src/core/ddsi/src/q_entity.c
@@ -145,6 +145,21 @@ int is_reader_entityid (nn_entityid_t id)
   }
 }
 
+int is_keyed_endpoint_entityid (nn_entityid_t id)
+{
+  switch (id.u & NN_ENTITYID_KIND_MASK)
+  {
+    case NN_ENTITYID_KIND_READER_WITH_KEY:
+    case NN_ENTITYID_KIND_WRITER_WITH_KEY:
+      return 1;
+    case NN_ENTITYID_KIND_READER_NO_KEY:
+    case NN_ENTITYID_KIND_WRITER_NO_KEY:
+      return 0;
+    default:
+      return 0;
+  }
+}
+
 int is_builtin_entityid (nn_entityid_t id, nn_vendorid_t vendorid)
 {
   if ((id.u & NN_ENTITYID_SOURCE_MASK) == NN_ENTITYID_SOURCE_BUILTIN)
@@ -402,7 +417,7 @@ static bool update_qos_locked (struct entity_common *e, dds_qos_t *ent_qos, cons
   return true;
 }
 
-static dds_return_t pp_allocate_entityid(nn_entityid_t *id, unsigned kind, struct participant *pp)
+static dds_return_t pp_allocate_entityid(nn_entityid_t *id, uint32_t kind, struct participant *pp)
 {
   uint32_t id1;
   int ret = 0;
@@ -2092,14 +2107,20 @@ static void reader_qos_mismatch (struct reader * rd, dds_qos_policy_id_t reason)
   }
 }
 
-static bool qos_match_p_lock (struct entity_common *ea, const dds_qos_t *a, struct entity_common *eb, const dds_qos_t *b, dds_qos_policy_id_t *reason)
+static bool topickind_qos_match_p_lock (struct entity_common *rd, const dds_qos_t *rdqos, struct entity_common *wr, const dds_qos_t *wrqos, dds_qos_policy_id_t *reason)
 {
-  assert (ea != eb);
-  ddsrt_mutex_t * const locks[] = { &ea->qos_lock, &eb->qos_lock, &ea->qos_lock };
-  const int shift = (uintptr_t) ea > (uintptr_t) eb;
+  assert (is_reader_entityid (rd->guid.entityid));
+  assert (is_writer_entityid (wr->guid.entityid));
+  if (is_keyed_endpoint_entityid (rd->guid.entityid) != is_keyed_endpoint_entityid (wr->guid.entityid))
+  {
+    *reason = DDS_INVALID_QOS_POLICY_ID;
+    return false;
+  }
+  ddsrt_mutex_t * const locks[] = { &rd->qos_lock, &wr->qos_lock, &rd->qos_lock };
+  const int shift = (uintptr_t) rd > (uintptr_t) wr;
   for (int i = 0; i < 2; i++)
     ddsrt_mutex_lock (locks[i + shift]);
-  bool ret = qos_match_p (a, b, reason);
+  bool ret = qos_match_p (rdqos, wrqos, reason);
   for (int i = 0; i < 2; i++)
     ddsrt_mutex_unlock (locks[i + shift]);
   return ret;
@@ -2115,7 +2136,7 @@ static void connect_writer_with_proxy_reader (struct writer *wr, struct proxy_re
     return;
   if (wr->e.onlylocal)
     return;
-  if (!isb0 && !qos_match_p_lock (&prd->e, prd->c.xqos, &wr->e, wr->xqos, &reason))
+  if (!isb0 && !topickind_qos_match_p_lock (&prd->e, prd->c.xqos, &wr->e, wr->xqos, &reason))
   {
     writer_qos_mismatch (wr, reason);
     return;
@@ -2134,7 +2155,7 @@ static void connect_proxy_writer_with_reader (struct proxy_writer *pwr, struct r
     return;
   if (rd->e.onlylocal)
     return;
-  if (!isb0 && !qos_match_p_lock (&rd->e, rd->xqos, &pwr->e, pwr->c.xqos, &reason))
+  if (!isb0 && !topickind_qos_match_p_lock (&rd->e, rd->xqos, &pwr->e, pwr->c.xqos, &reason))
   {
     reader_qos_mismatch (rd, reason);
     return;
@@ -2176,7 +2197,7 @@ static void connect_writer_with_reader (struct writer *wr, struct reader *rd, nn
     return;
   if (ignore_local_p (&wr->e.guid, &rd->e.guid, wr->xqos, rd->xqos))
     return;
-  if (!qos_match_p_lock (&rd->e, rd->xqos, &wr->e, wr->xqos, &reason))
+  if (!topickind_qos_match_p_lock (&rd->e, rd->xqos, &wr->e, wr->xqos, &reason))
   {
     writer_qos_mismatch (wr, reason);
     reader_qos_mismatch (rd, reason);
@@ -2912,17 +2933,20 @@ dds_return_t new_writer (struct writer **wr_out, struct q_globals *gv, struct nn
 {
   struct participant *pp;
   dds_return_t rc;
+  uint32_t kind;
 
   if ((pp = ephash_lookup_participant_guid (gv->guid_hash, ppguid)) == NULL)
   {
     GVLOGDISC ("new_writer - participant "PGUIDFMT" not found\n", PGUID (*ppguid));
     return DDS_RETCODE_BAD_PARAMETER;
   }
+
   /* participant can't be freed while we're mucking around cos we are
      awake and do not touch the thread's vtime (ephash_lookup already
      verifies we're awake) */
   wrguid->prefix = pp->e.guid.prefix;
-  if ((rc = pp_allocate_entityid (&wrguid->entityid, NN_ENTITYID_KIND_WRITER_WITH_KEY, pp)) < 0)
+  kind = topic->topickind_no_key ? NN_ENTITYID_KIND_WRITER_NO_KEY : NN_ENTITYID_KIND_WRITER_WITH_KEY;
+  if ((rc = pp_allocate_entityid (&wrguid->entityid, kind, pp)) < 0)
     return rc;
   return new_writer_guid (wr_out, wrguid, group_guid, pp, topic, xqos, whc, status_cb, status_cb_arg);
 }
@@ -3391,6 +3415,7 @@ dds_return_t new_reader
 {
   struct participant * pp;
   dds_return_t rc;
+  uint32_t kind;
 
   if ((pp = ephash_lookup_participant_guid (gv->guid_hash, ppguid)) == NULL)
   {
@@ -3398,7 +3423,8 @@ dds_return_t new_reader
     return DDS_RETCODE_BAD_PARAMETER;
   }
   rdguid->prefix = pp->e.guid.prefix;
-  if ((rc = pp_allocate_entityid (&rdguid->entityid, NN_ENTITYID_KIND_READER_WITH_KEY, pp)) < 0)
+  kind = topic->topickind_no_key ? NN_ENTITYID_KIND_READER_NO_KEY : NN_ENTITYID_KIND_READER_WITH_KEY;
+  if ((rc = pp_allocate_entityid (&rdguid->entityid, kind, pp)) < 0)
     return rc;
   return new_reader_guid (rd_out, rdguid, group_guid, pp, topic, xqos, rhc, status_cb, status_cbarg);
 }
diff --git a/src/core/ddsi/src/q_init.c b/src/core/ddsi/src/q_init.c
index ab46018..54601d9 100644
--- a/src/core/ddsi/src/q_init.c
+++ b/src/core/ddsi/src/q_init.c
@@ -744,15 +744,11 @@ static struct ddsi_sertopic *make_special_topic (struct serdatapool *serpool, ui
        (kinda natural if they stop being "default" ones) */
   struct ddsi_sertopic_default *st = ddsrt_malloc (sizeof (*st));
   memset (st, 0, sizeof (*st));
-  ddsrt_atomic_st32 (&st->c.refc, 1);
-  st->c.ops = &ddsi_sertopic_ops_default;
-  st->c.serdata_ops = ops;
-  st->c.serdata_basehash = ddsi_sertopic_compute_serdata_basehash (st->c.serdata_ops);
-  st->c.iid = ddsi_iid_gen ();
+  ddsi_sertopic_init_anon (&st->c, &ddsi_sertopic_ops_default, ops, false);
   st->native_encoding_identifier = enc_id;
   st->serpool = serpool;
   st->nkeys = 1;
-  return (struct ddsi_sertopic *)st;
+  return (struct ddsi_sertopic *) st;
 }
 
 static void make_special_topics (struct q_globals *gv)