diff --git a/bgpd/bgp_aspath.c b/bgpd/bgp_aspath.c
index a9602d9..5a73eef 100644
--- a/bgpd/bgp_aspath.c
+++ b/bgpd/bgp_aspath.c
@@ -671,58 +671,79 @@
   return aspath;
 }
 
-/* parse as-segment byte stream in struct assegment */
+/* parse as-segment byte stream in struct assegment
+ *
+ * Returns NULL if the AS_PATH or AS4_PATH is not valid.
+ */
 static struct assegment *
-assegments_parse (struct stream *s, size_t length, int use32bit)
+assegments_parse (struct stream *s, size_t length, int use32bit, int as4_path)
 {
   struct assegment_header segh;
   struct assegment *seg, *prev = NULL, *head = NULL;
-  size_t bytes = 0;
   
-  /* empty aspath (ie iBGP or somesuch) */
-  if (length == 0)
-    return NULL;
+  assert (length > 0);  /* does not expect empty AS_PATH or AS4_PATH    */
   
   if (BGP_DEBUG (as4, AS4_SEGMENT))
     zlog_debug ("[AS4SEG] Parse aspath segment: got total byte length %lu",
 		(unsigned long) length);
-  /* basic checks */
-  if ( (STREAM_READABLE(s) < length)
-      || (STREAM_READABLE(s) < AS_HEADER_SIZE) 
-      || (length % AS16_VALUE_SIZE ))
+
+  /* double check that length does not exceed stream    */
+  if (STREAM_READABLE(s) < length)
     return NULL;
   
-  while ( (STREAM_READABLE(s) > AS_HEADER_SIZE)
-         && (bytes < length))
+  /* deal with each segment in turn                             */
+  while (length > 0)
     {
       int i;
-      int seg_size;
+      size_t seg_size;
       
       /* softly softly, get the header first on its own */
+      if (length >= AS_HEADER_SIZE)
+        {
       segh.type = stream_getc (s);
       segh.length = stream_getc (s);
       
       seg_size = ASSEGMENT_SIZE(segh.length, use32bit);
+                                      /* includes the header bytes */
 
       if (BGP_DEBUG (as4, AS4_SEGMENT))
 	zlog_debug ("[AS4SEG] Parse aspath segment: got type %d, length %d",
                     segh.type, segh.length);
       
-      /* check it.. */
-      if ( ((bytes + seg_size) > length)
-          /* 1771bis 4.3b: seg length contains one or more */
-          || (segh.length == 0) 
-          /* Paranoia in case someone changes type of segment length.
-           * Shift both values by 0x10 to make the comparison operate
-           * on more, than 8 bits (otherwise it's a warning, bug #564).
+          switch (segh.type)
+          {
+            case AS_SEQUENCE:
+            case AS_SET:
+              break ;
+
+            case AS_CONFED_SEQUENCE:
+            case AS_CONFED_SET:
+              if (!as4_path)
+                break ;
+              /* RFC4893 3: "invalid for the AS4_PATH attribute"            */
+              /* fall through */
+
+            default:    /* reject unknown or invalid AS_PATH segment types  */
+              seg_size = 0 ;
+          } ;
+        }
+      else
+        seg_size = 0 ;
+
+     /* Stop now if segment is not valid (discarding anything collected to date)
+      *
+      * RFC4271 4.3, Path Attributes, b) AS_PATH:
+      *
+      *   "path segment value field contains one or more AS numbers"
            */
-          || ((sizeof segh.length > 1) && (0x10 + segh.length > 0x10 + AS_SEGMENT_MAX)) )
+      if ((seg_size == 0) || (seg_size > length) || (segh.length == 0))
         {
-          if (head)
             assegment_free_all (head);
           return NULL;
         }
       
+      length -= seg_size ;
+      
       /* now its safe to trust lengths */
       seg = assegment_new (segh.type, segh.length);
       
@@ -734,11 +755,9 @@
       for (i = 0; i < segh.length; i++)
 	seg->as[i] = (use32bit) ? stream_getl (s) : stream_getw (s);
 
-      bytes += seg_size;
-      
       if (BGP_DEBUG (as4, AS4_SEGMENT))
-	zlog_debug ("[AS4SEG] Parse aspath segment: Bytes now: %lu",
-	            (unsigned long) bytes);
+	zlog_debug ("[AS4SEG] Parse aspath segment: length left: %lu",
+	            (unsigned long) length);
       
       prev = seg;
     }
@@ -746,30 +765,42 @@
   return assegment_normalise (head);
 }
 
-/* AS path parse function.  pnt is a pointer to byte stream and length
-   is length of byte stream.  If there is same AS path in the the AS
-   path hash then return it else make new AS path structure. */
+/* AS path parse function -- parses AS_PATH and AS4_PATH attributes
+ *
+ * Requires: s        -- stream, currently positioned before first segment
+ *                       of AS_PATH or AS4_PATH (ie after attribute header)
+ *           length   -- length of the value of the AS_PATH or AS4_PATH
+ *           use32bit -- true <=> 4Byte ASN, otherwise 2Byte ASN
+ *           as4_path -- true <=> AS4_PATH, otherwise AS_PATH
+ *
+ * Returns: if valid: address of struct aspath in the hash of known aspaths,
+ *                    with reference count incremented.
+ *              else: NULL
+ *
+ * NB: empty AS path (length == 0) is valid.  The returned struct aspath will
+ *     have segments == NULL and str == zero length string (unique).
+ */
 struct aspath *
-aspath_parse (struct stream *s, size_t length, int use32bit)
+aspath_parse (struct stream *s, size_t length, int use32bit, int as4_path)
 {
   struct aspath as;
   struct aspath *find;
 
-  /* If length is odd it's malformed AS path. */
-  /* Nit-picking: if (use32bit == 0) it is malformed if odd,
-   * otherwise its malformed when length is larger than 2 and (length-2) 
-   * is not dividable by 4.
-   * But... this time we're lazy
-   */
-  if (length % AS16_VALUE_SIZE )
-    return NULL;
-
+  /* Parse each segment and construct normalised list of struct assegment */
   memset (&as, 0, sizeof (struct aspath));
-  as.segments = assegments_parse (s, length, use32bit);
+  if (length != 0)
+    {
+      as.segments = assegments_parse (s, length, use32bit, as4_path);
+
+      if (as.segments == NULL)
+        return NULL ;   /* Invalid AS_PATH or AS4_PATH  */
+    } ;
   
   /* If already same aspath exist then return it. */
   find = hash_get (ashash, &as, aspath_hash_alloc);
   
+  assert(find) ;        /* valid aspath, so must find or create */
+  
   /* aspath_hash_alloc dupes segments too. that probably could be
    * optimised out.
    */
@@ -777,8 +808,6 @@
   if (as.str)
     XFREE (MTYPE_AS_STR, as.str);
   
-  if (! find)
-    return NULL;
   find->refcnt++;
 
   return find;
@@ -1602,7 +1631,7 @@
 struct aspath *
 aspath_empty (void)
 {
-  return aspath_parse (NULL, 0, 1); /* 32Bit ;-) */
+  return aspath_parse (NULL, 0, 1, 0); /* 32Bit ;-) not AS4_PATH */
 }
 
 struct aspath *
diff --git a/bgpd/bgp_aspath.h b/bgpd/bgp_aspath.h
index b8a5dfa..d63b914 100644
--- a/bgpd/bgp_aspath.h
+++ b/bgpd/bgp_aspath.h
@@ -65,7 +65,7 @@
 /* Prototypes. */
 extern void aspath_init (void);
 extern void aspath_finish (void);
-extern struct aspath *aspath_parse (struct stream *, size_t, int);
+extern struct aspath *aspath_parse (struct stream *, size_t, int, int);
 extern struct aspath *aspath_dup (struct aspath *);
 extern struct aspath *aspath_aggregate (struct aspath *, struct aspath *);
 extern struct aspath *aspath_prepend (struct aspath *, struct aspath *);
diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index 5e7536a..3996afd 100644
--- a/bgpd/bgp_attr.c
+++ b/bgpd/bgp_attr.c
@@ -807,54 +807,78 @@
 
   return 0;
 }
-
-/* Parse AS path information.  This function is wrapper of
-   aspath_parse. */
-static int
+/* Parse AS path information.  This function is wrapper of aspath_parse.
+ *
+ * Parses AS_PATH or AS4_PATH.
+ *
+ * Returns: if valid: address of struct aspath in the hash of known aspaths,
+ *                    with reference count incremented.
+ *              else: NULL
+ *
+ * NB: empty AS path (length == 0) is valid.  The returned struct aspath will
+ *     have segments == NULL and str == zero length string (unique).
+ */
+static struct aspath *
 bgp_attr_aspath (struct peer *peer, bgp_size_t length, 
-		 struct attr *attr, u_char flag, u_char *startp)
+		 struct attr *attr, u_char flag, u_char *startp, int as4_path)
 {
-  bgp_size_t total;
+  u_char require ;
+  struct aspath *asp ;
 
-  total = length + (CHECK_FLAG (flag, BGP_ATTR_FLAG_EXTLEN) ? 4 : 3);
+  /* Check the attribute flags                                          */
+  require = as4_path ? BGP_ATTR_FLAG_OPTIONAL | BGP_ATTR_FLAG_TRANS
+                     :                          BGP_ATTR_FLAG_TRANS ;
 
-  /* Flag check. */
-  if (CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL)
-      || ! CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
+  if ((flag & (BGP_ATTR_FLAG_OPTIONAL | BGP_ATTR_FLAG_TRANS)) != require)
     {
+      const char* path_type ;
+      bgp_size_t total;
+
+      path_type = as4_path ? "AS4_PATH" : "AS_PATH" ;
+
+      if (!CHECK_FLAG(flag, BGP_ATTR_FLAG_TRANS))
       zlog (peer->log, LOG_ERR, 
-	    "As-Path attribute flag isn't transitive %d", flag);
+            "%s attribute flag isn't transitive %d", path_type, flag) ;
+
+      if ((flag & BGP_ATTR_FLAG_OPTIONAL) != (require & BGP_ATTR_FLAG_OPTIONAL))
+        zlog (peer->log, LOG_ERR,
+            "%s attribute flag must %sbe optional %d", path_type,
+            (flag & BGP_ATTR_FLAG_OPTIONAL) ? "not " : "", flag) ;
+
+      total = length + (CHECK_FLAG (flag, BGP_ATTR_FLAG_EXTLEN) ? 4 : 3);
+
       bgp_notify_send_with_data (peer, 
 				 BGP_NOTIFY_UPDATE_ERR, 
 				 BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR,
 				 startp, total);
-      return -1;
-    }
 
-  /*
-   * peer with AS4 => will get 4Byte ASnums
-   * otherwise, will get 16 Bit
+      return NULL ;
+    } ;
+
+  /* Parse the AS_PATH/AS4_PATH body.
+   *
+   * For AS_PATH  peer with AS4 => 4Byte ASN otherwise 2Byte ASN
+   *     AS4_PATH 4Byte ASN
    */
-  attr->aspath = aspath_parse (peer->ibuf, length, 
-                               CHECK_FLAG (peer->cap, PEER_CAP_AS4_RCV));
+  asp = aspath_parse (peer->ibuf, length,
+               as4_path || CHECK_FLAG (peer->cap, PEER_CAP_AS4_RCV), as4_path) ;
 
-  /* In case of IBGP, length will be zero. */
-  if (! attr->aspath)
+  if (asp != NULL)
+    {
+      attr->flag |= ATTR_FLAG_BIT (as4_path ? BGP_ATTR_AS4_PATH
+                                            : BGP_ATTR_AS_PATH) ;
+    }
+  else
     {
       zlog (peer->log, LOG_ERR, "Malformed AS path length is %d", length);
+
+      /* TODO: should BGP_NOTIFY_UPDATE_MAL_AS_PATH be sent for AS4_PATH ??  */
       bgp_notify_send (peer, 
 		       BGP_NOTIFY_UPDATE_ERR, 
 		       BGP_NOTIFY_UPDATE_MAL_AS_PATH);
-      return -1;
-    }
+    } ;
 
-  /* Forward pointer. */
-/*  stream_forward_getp (peer->ibuf, length);*/
-
-  /* Set aspath attribute flag. */
-  attr->flag |= ATTR_FLAG_BIT (BGP_ATTR_AS_PATH);
-
-  return 0;
+  return asp ;
 }
 
 static int bgp_attr_aspath_check( struct peer *peer, 
@@ -912,21 +936,6 @@
 
 }
 
-/* Parse AS4 path information.  This function is another wrapper of
-   aspath_parse. */
-static int
-bgp_attr_as4_path (struct peer *peer, bgp_size_t length, 
-		 struct attr *attr, struct aspath **as4_path)
-{
-  *as4_path = aspath_parse (peer->ibuf, length, 1);
-
-  /* Set aspath attribute flag. */
-  if (as4_path)
-    attr->flag |= ATTR_FLAG_BIT (BGP_ATTR_AS4_PATH);
-
-  return 0;
-}
-
 /* Nexthop attribute. */
 static int
 bgp_attr_nexthop (struct peer *peer, bgp_size_t length, 
@@ -1657,10 +1666,12 @@
 	  ret = bgp_attr_origin (peer, length, attr, flag, startp);
 	  break;
 	case BGP_ATTR_AS_PATH:
-	  ret = bgp_attr_aspath (peer, length, attr, flag, startp);
+          attr->aspath = bgp_attr_aspath (peer, length, attr, flag, startp, 0);
+          ret = attr->aspath ? 0 : -1 ;
 	  break;
 	case BGP_ATTR_AS4_PATH:
-	  ret = bgp_attr_as4_path (peer, length, attr, &as4_path );
+          as4_path = bgp_attr_aspath (peer, length, attr, flag, startp, 1);
+          ret = as4_path  ? 0 : -1 ;
 	  break;
 	case BGP_ATTR_NEXT_HOP:	
 	  ret = bgp_attr_nexthop (peer, length, attr, flag, startp);
diff --git a/tests/aspath_test.c b/tests/aspath_test.c
index fb504f3..9e51e8d 100644
--- a/tests/aspath_test.c
+++ b/tests/aspath_test.c
@@ -649,7 +649,7 @@
       s = stream_new (len);
       stream_put (s, data, len);
     }
-  as = aspath_parse (s, len, use32bit);
+  as = aspath_parse (s, len, use32bit, 0);
   
   if (s)
     stream_free (s);