LDP sessions: Subnet-match authentication

Hashed Message Authentication Code (HMAC) and MD5 authentication for LDP sessions is extended from a per-session configuration to a subnet-match (that is, longest-prefix-match) configuration. This feature provides flexibility in configuring authentication for automatically targeted LDP (TLDP) sessions, making the deployment of remote loop-free alternate (LFA) and FEC 129 pseudowires easy. To enable this feature, configure the session-group option at the [edit protocols ldp session] hierarchy level, and then enable the required authentication for the configured session group.