PCEP operates over TCP using destination TCP port 4189. The PCC always initiates the connection. When the user configures the PCEP local address and the peer address on the PCC, the PCC initiates a TCP connection to the PCE. When a connection is established, the PCC and PCE exchange OPEN messages, which initializes the PCEP session and exchanges the session parameters to be negotiated.
The PCC always checks first to determine if the remote PCE address is reachable out-of-band through the management port. If the remote address is not reachable, the PCC tries to reach the remote PCE address in-band. When the session comes up out-of-band, the management IP address is always used; the local address configured by the user is ignored and is only used for an in-band session.
A keepalive mechanism is used as an acknowledgment of the acceptance of the session within the negotiated parameters. It is also used as a maintenance function to detect whether the PCEP peer is still alive.
The negotiated parameters include the keepalive timer and the dead-timer, and one or more PCEP capabilities such as support of stateful PCE and the LSP Path type.
The following figure shows the PCEP session initialization steps.
If the session to the PCE times out, the router acting as a PCC keeps the last successfully programmed path provided by the PCE until the session to the PCE is reestablished. Any subsequent change to the LSP state is synchronized at the time the session is reestablished.
When a PCEP session to a peer times out or closes, the rate at which the PCEP speaker attempts to reestablish the session is subject to an exponential back-off mechanism.