diff --git a/mdp_client.c b/mdp_client.c
index c7231903..f3694b9a 100644
--- a/mdp_client.c
+++ b/mdp_client.c
@@ -133,11 +133,15 @@ int overlay_mdp_send(int mdp_sockfd, overlay_mdp_frame *mdp, int flags, int time
   set_nonblock(mdp_sockfd);
   ssize_t result = sendto(mdp_sockfd, mdp, (size_t)len, 0, (struct sockaddr *)&addr, addrlen);
   set_block(mdp_sockfd);
-  if (result == -1) {
+  if ((size_t)result != (size_t)len) {
+    if (result == -1)
+      WHYF_perror("sendto(fd=%d,len=%zu,addr=%s)", mdp_sockfd, (size_t)len, alloca_sockaddr(&addr, addrlen));
+    else
+      WHYF("sendto() sent %zu bytes of MDP reply (%zu) to socket=%d", (size_t)result, (size_t)len, mdp_sockfd); 
     mdp->packetTypeAndFlags=MDP_ERROR;
     mdp->error.error=1;
     snprintf(mdp->error.message,128,"Error sending frame to MDP server.");
-    return WHYF_perror("sendto(fd=%d,len=%zu,addr=%s)", mdp_sockfd, (size_t)len, alloca_sockaddr(&addr, addrlen));
+    return -1;
   } else {
     if (!(flags&MDP_AWAITREPLY)) {       
       return 0;
diff --git a/overlay_interface.c b/overlay_interface.c
index f71f4208..8b74b60c 100644
--- a/overlay_interface.c
+++ b/overlay_interface.c
@@ -982,11 +982,16 @@ int overlay_broadcast_ensemble(struct network_destination *destination, struct o
 		(struct sockaddr *)&destination->address, sizeof(destination->address));
       ob_free(buffer);
       if (sent == -1 || (size_t)sent != (size_t)len) {
-	WHYF_perror("sendto(fd=%d,len=%zu,addr=%s)",
-	    interface->alarm.poll.fd,
-	    (size_t)len,
-	    alloca_sockaddr((struct sockaddr *)&destination->address, sizeof destination->address)
-	  );
+	if (sent == -1)
+	  WHYF_perror("sendto(fd=%d,len=%zu,addr=%s) on interface %s",
+	      interface->alarm.poll.fd,
+	      (size_t)len,
+	      alloca_sockaddr((struct sockaddr *)&destination->address, sizeof destination->address),
+	      interface->name
+	    );
+	else
+	  WHYF("sendto() sent %zu bytes of overlay frame (%zu) to interface %s (socket=%d)",
+	      (size_t)sent, (size_t)len, interface->name, interface->alarm.poll.fd);
 	// close the interface if we had any error while sending broadcast packets,
 	// unicast packets should not bring the interface down
 	if (destination == interface->destination)
diff --git a/overlay_mdp.c b/overlay_mdp.c
index 9ad5dfdd..80eb3fb8 100644
--- a/overlay_mdp.c
+++ b/overlay_mdp.c
@@ -160,10 +160,12 @@ int overlay_mdp_reply(int sock,struct sockaddr_un *recvaddr, socklen_t recvaddrl
   if (replylen == -1)
     return WHY("Invalid MDP frame (could not compute length)");
   ssize_t r = sendto(sock, (char *)mdpreply, (size_t)replylen, 0, (struct sockaddr *)recvaddr, recvaddrlen);
-  if (r == -1 || (size_t)r < (size_t)replylen) { 
+  if (r == -1) {
     WHYF_perror("sendto(fd=%d,len=%zu,addr=%s)", sock, (size_t)replylen, alloca_sockaddr((struct sockaddr *)recvaddr, recvaddrlen));
-    return WHYF("sendto() failed when sending MDP reply, sock=%d, r=%d", sock, r); 
+    return WHYF("sendto() failed when sending MDP reply"); 
   }
+  if ((size_t)r != (size_t)replylen)
+    return WHYF("sendto() sent %zu bytes of MDP reply (%zu) to socket=%d", (size_t)r, (size_t)replylen, sock); 
   return 0;  
 }
 
@@ -444,19 +446,22 @@ static int overlay_saw_mdp_frame(struct overlay_frame *frame, overlay_mdp_frame
       addr.sun_family = AF_UNIX;
       bcopy(mdp_bindings[match].socket_name, addr.sun_path, mdp_bindings[match].name_len);
       ssize_t len = overlay_mdp_relevant_bytes(mdp);
-      if (len < 0)
+      if (len == -1)
 	RETURN(WHY("unsupported MDP packet type"));
       socklen_t addrlen = sizeof addr.sun_family + mdp_bindings[match].name_len;
       ssize_t r = sendto(mdp_sock.poll.fd, mdp, (size_t)len, 0, (struct sockaddr*)&addr, addrlen);
-      if ((size_t)r == (size_t)len)
-	RETURN(0);
-      if (r == -1 && errno == ENOENT) {
-	/* far-end of socket has died, so drop binding */
-	INFOF("Closing dead MDP client '%s'",mdp_bindings[match].socket_name);
-	overlay_mdp_releasebindings(&addr,mdp_bindings[match].name_len);
+      if ((size_t)r != (size_t)len) {
+	if (r == -1) {
+	  WHYF_perror("sendto(fd=%d,len=%zu,addr=%s)", mdp_sock.poll.fd, (size_t)len, alloca_sockaddr(&addr, addrlen));
+	  if (errno == ENOENT) {
+	    /* far-end of socket has died, so drop binding */
+	    INFOF("Closing dead MDP client '%s'",mdp_bindings[match].socket_name);
+	    overlay_mdp_releasebindings(&addr,mdp_bindings[match].name_len);
+	  }
+	} else
+	  WHYF("sendto() sent %zu bytes of MDP frame (%zu) to client, socket=%d", (size_t)r, (size_t)len, mdp_sock.poll.fd); 
+	RETURN(WHY("Failed to pass received MDP frame to client"));
       }
-      WHYF_perror("sendto(fd=%d,len=%zu,addr=%s)", mdp_sock.poll.fd, (size_t)len, alloca_sockaddr(&addr, addrlen));
-      RETURN(WHY("Failed to pass received MDP frame to client"));
     } else {
       /* No socket is bound, ignore the packet ... except for magic sockets */
       RETURN(overlay_mdp_try_interal_services(frame, mdp));