Split last PC into unicast and multicast values.

It has been reported that some WiFi networks delay multicast
packets by up to 200ms, which breaks the PC check.  Split
the receiver's state into two last PC values, one for multicast
and one for unicast.  This is safe as the destination address
is covered by the MAC.

CHANGES

+babel 1.12.1 (unreleased)
+
+  * Implement separate PC values for unicast and multicast, which avoids
+    dropping packets protected by MAC when WiFi powersave is active.
+    Thanks to Daniel Gröber.
+
 5 May 2022: babeld-1.12
 
   * Implement v4-via-v6 routing (RFC 9229), which allows a router with

message.c

@@ -551,7 +551,8 @@ preparse_packet(const unsigned char *from, struct interface *ifp,
     } else if(challenge_success) {
         neigh->index_len = index_len;
         memcpy(neigh->index, index, index_len);
-        memcpy(neigh->pc, pc, 4);
+        memcpy(neigh->pc_m, pc, 4);
+        memcpy(neigh->pc_u, pc, 4);
         accept_packet = 1;
     } else {
         neigh = neigh != NULL ? neigh : find_neighbour(from, ifp);
@@ -563,12 +564,19 @@ preparse_packet(const unsigned char *from, struct interface *ifp,
             rc = send_challenge_request(neigh);
             if(rc < -1)
                 fputs("Could not send challenge request.\n", stderr);
-        } else if(memcmp(pc, neigh->pc, 4) <= 0) {
-            debugf("Out of order PC.\n");
-            nonce = NULL;
         } else {
-            memcpy(neigh->pc, pc, 4);
-            accept_packet = 1;
+            unsigned char *last_pc;
+            if(IN6_IS_ADDR_MULTICAST(to))
+                last_pc = neigh->pc_m;
+            else
+                last_pc = neigh->pc_u;
+            if(memcmp(pc, last_pc, 4) <= 0) {
+                debugf("Out of order PC.\n");
+                nonce = NULL;
+            } else {
+                memcpy(last_pc, pc, 4);
+                accept_packet = 1;
+            }
         }
     }
 
@@ -645,7 +653,7 @@ parse_packet(const unsigned char *from, struct interface *ifp,
         } else {
             neigh = preparse_packet(from, ifp, packet, bodylen, to);
             if(neigh == NULL) {
-                debugf("Received packet with wrong PC.\n");
+                debugf("PC check failed.\n");
                 return;
             }
         }

neighbour.h

@@ -47,7 +47,7 @@ struct neighbour {
     struct timeval echo_receive_time;
     unsigned int rtt;
     struct timeval rtt_time;
-    unsigned char pc[4];
+    unsigned char pc_u[4], pc_m[4];
     int index_len; /* This is -1 when index is undefined */
     unsigned char index[32];
     unsigned char nonce[NONCE_LEN];