3.10. Well-Known Numbers

3.10.1. UDP and TCP Ports

The UDP port for LDP Hello messages is 646.

The TCP port for establishing LDP session connections is 646.

3.10.2. Implicit NULL Label

The Implicit NULL label is defined in [RFC3031] as follows:

"The Implicit NULL label is a label with special semantics which an LSR can bind to an address prefix. If LSR Ru, by consulting its ILM (Incoming Label Map) sees that labeled packet P must be forwarded next to Rd, but that Rd has distributed a binding of Implicit NULL to the corresponding address prefix, then instead of replacing the value of the label on top of the label stack, Ru pops the label stack, and then forwards the resulting packet to Rd."

The implicit NULL label is represented in LDP as a Generic Label TLV with a Label field value of 3, as defined in [RFC3032].