The Generalized TTL Security Mechanism (GTSM) describes a generalized use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6) to verify that the packet was sourced by a node on a connected link, thereby protecting the router\'s IP control plane from CPU utilization-based attacks. This technique improves security and is used by many protocols. This document defines the GTSM use for the Label Distribution Protocol (LDP). This specification uses a bit reserved in RFC 5036 and therefore updates RFC 5036. [STANDARDS-TRACK]