LDP Message Processing

The simulation follows message processing rules specified in RFC3036 (LDP Specification). A summary of the algorithm used in the RFC is presented below.

Label Request Message processing

An LSR may transmit a Request message under any of the conditions below:

The LSR recognizes a new FEC via the forwarding tale, and the next hop is its LDP peer. The LIB of this LSR does not have a mapping from the next hop for the given FEC.
Network topology changes, the next hop to the FEC is no longer valid and new mapping is not available.
The LSR receives a Label Request for a FEC from an upstream LDP and it does not have label binding information for this FEC. The FEC next hop is an LDP peer.

Upon receiving a Label Request message, the following procedures will be performed:

Step 1: Extract the FEC from the message and locate the incoming interface
        of the message.
Step 2: Check whether the FEC is an outstanding FEC.
    Alternative 1: This FEC is outstanding
        Step 1: Return
    Alternative 2: This FEC is not outstanding
        Step 1: Continue
Step 3: Check if there is an exact match of the FEC in the routing table.
    Alternative 1: There is an exact match
        Step 1: Continue
    Alternative 2: There is no match
        Step 1: Construct a Notification message of No route and
                send this message back to the sender.
Step 4: Make query to local LIB to find out the corresponding label.
    Alternative 1: The label found
        Step 1: Construct a Label Mapping message and send over
                the incoming interface.
    Alternative 2: The label cannot be found for this FEC
        Step 1: Construct a new Label Request message and send
                the message out using L3 routing.
        Step 2: Construct a Notification message indicating that the
                label cannot be found.

Label Mapping Message processing

Upon receiving a Label Mapping message, the following procedures will be performed:

Step 1: Extract the FEC and the label from the message.
Step 2: Check whether this is an outstanding FEC
    Alternative 1: This FEC is outstanding
        Step 1: Continue
    Alternative 2: This FEC is not outstanding
        Step 1: Send back the server an Notification of Error message.
Step 3: Install the new label to the local LIB using the extracted label,
        FEC and the message incoming interface.