| The Label Distribution Protocol (LDP)
defined in this document is a new protocol designed for distributing labels.
It is the method by which LSRs establish Label Switched Paths
(LSPs) through a network. |
| |
| LDP associates a Forwarding
Equivalence Class (FEC) with each LSP it creates. The
FEC associated with an LSP specifies which packets are "mapped"
to that LSP. |
| |
| Two LSRs which use LDP to
exchange label/FEC mapping information are known as "LDP Peers"
with respect to that information, and we say that exist an "LDP Session"
between them. The protocol is bidirectional, i.e., a single session allows
each peer to learn the other's label mapping. |
| |
| LDP Messages |
| |
| There are four categories of LDP
messages: |
| |
- Discovery messages, used to announce and maintain the presence
of an LSR in a network.
- Session messages, used to establish, maintain, and terminate
sessions between LDP peers.
- Advertisement messages, used to create, change, and delete
mapping for FECs.
- Notification messages, used to provide advisory information and
to signal error information.
|
| Discovery messages are used by LSRs
to indicate their presence in the network by sending Hello messages
periodically. A Hello message is a UDP packet transmitted to
the LDP port at the group multicast address "all routers on this
subnet". When two LSRs discover themselve an initialization
procedure is executed over TCP protocol. Upon completion of this
procedure, the two LSRs are LDP Peers, and may exchange
advertisement messages. LDP uses the UDP protocol for
Discovery messages and the TCP protocol for Session,
Advertisement and Notification messages. |
