Babel: Implement IPv6 prefix compression on outgoing updates

Previously, the Babel protocol would never use prefix compression on outgoing
updates (but would parse it on incoming ones). This adds compression of IPv6
addresses of outgoing updates.

The compression only works to the extent that the FIB is walked in lexicographic
order; i.e. a prefix is only compressed if it shares bytes with the previous
prefix in the same packet.

Thanks to Toke Høiland-Jørgensen <toke@toke.dk> for the patch.

proto/babel/packets.c

@@ -133,6 +133,8 @@ struct babel_write_state {
   u8 router_id_seen;
   ip_addr next_hop_ip4;
   ip_addr next_hop_ip6;
+  u8 def_ip6_prefix[16];	/* Implicit IPv6 prefix in network order */
+  u8 def_ip6_pxlen;
 };
 
 
@@ -150,6 +152,14 @@ struct babel_write_state {
 
 #define NET_SIZE(n) BYTES(net_pxlen(n))
 
+static inline uint
+bytes_equal(u8 *b1, u8 *b2, uint maxlen)
+{
+  uint i;
+  for (i = 0; (i < maxlen) && (*b1 == *b2); i++, b1++, b2++)
+    ;
+  return i;
+}
 
 static inline u16
 get_time16(const void *p)
@@ -696,7 +706,29 @@ babel_write_update(struct babel_tlv *hdr, union babel_msg *m,
   {
     tlv->ae = BABEL_AE_IP6;
     tlv->plen = net6_pxlen(&msg->net);
-    put_ip6_px(tlv->addr, &msg->net);
+
+    /* Address compression - omit initial matching bytes */
+    u8 buf[16], omit;
+    put_ip6(buf, net6_prefix(&msg->net));
+    omit = bytes_equal(buf, state->def_ip6_prefix,
+		       MIN(tlv->plen, state->def_ip6_pxlen) / 8);
+
+    if (omit > 0)
+    {
+      memcpy(tlv->addr, buf + omit, NET_SIZE(&msg->net) - omit);
+
+      tlv->omitted = omit;
+      tlv->length -= omit;
+      len -= omit;
+    }
+    else
+    {
+      put_ip6_px(tlv->addr, &msg->net);
+      tlv->flags |= BABEL_FLAG_DEF_PREFIX;
+
+      put_ip6(state->def_ip6_prefix, net6_prefix(&msg->net));
+      state->def_ip6_pxlen = tlv->plen;
+    }
   }
 
   put_time16(&tlv->interval, msg->interval);