8.3 BGP Neighbor Negotiation
8.3.7 Withdrawn routes
Withdrawn routes provide a list of routing updates that are no longer reachable and need to be withdrawn (removed) from the BGP routing table; withdrawn routes have the same format as NLRIs.

The length is the total length, in bytes, of the withdrawn routes; an length of 0 indicates that no routes are to be withdrawn. An UPDATE message that has no NLRI or path attribute information is used to advertise only routes to be withdrawn from service.