checking for empty AS_PATH

[Yakov Rekhter <[e-mail]>]

Folks,

In 5.1.2:
 
   When a BGP speaker originates a route then:
 
     a) the originating speaker shall include its own AS number in
        the AS_PATH attribute of all UPDATE messages sent to an external
        peer.  (In this case, the AS number of the originating speaker's
        autonomous system will be the only entry in the AS_PATH attribute).

However, in 6.3 (Update message error handling):
 
    The AS_PATH attribute is checked for syntactic correctness.  If the
    path is syntactically incorrect, then the Error Subcode is set to
    Malformed AS_PATH.
 
Since an empty AS_PATH is *syntactically* correct, it seems that
a receiver would *never* check whether the AS_PATH is empty or not.

So, let me suggest that we'll add the following to 6.3:

   If an UPDATE was received from an external peer, and the UPDATE
   contains non-empty NLRI, the local system checks whether the
   AS_PATH attribute is empty. If that is the case, then the Error
   Subcode is set to Malformed AS_PATH.

Yakov.

P.S. In fact, we should probably make the check even stronger by checking
whether the most recent AS in the AS_PATH is the same as the AS of the
external peer.