PLOGI Process
After a successful FLOGI process, the N_Port can communicate in the switched fabric. This means sending FC frames to another FC node. That other FC node (either an initiator or target) is connected to the fabric through an N_Port as well. As you already know, FCP is a protocol that is focused on the secure and reliable transmission of the frames. That’s why, for the N_Port to be allowed to communicate with a destination N_Port, it needs to establish and negotiate a connection to it. This is known as the port login (PLOGI). The PLOGI must be completed successfully before the nodes can perform a ULP operation.
The steps in the PLOGI process are as follows:
The initiator N_Port sends a PLOGI frame that contains its operating parameters in the payload (see Figure 11-15).
Figure 11-15 PLOGI Initiation
The target N_Port responds with an ACC frame that specifies the target N_Port operating parameters (see Figure 11-16).
Figure 11-16 PLOGI ACC
The number of N_Ports an N_Port is logged in to depends on the communication requirements of this N_Port. It can be connected to a single N_Port or multiple N_Ports at the same time.
After the FLOGI and PLOGI processes are complete, the N_Port can use the following ELS commands to query and verify the fabric and port parameters without performing the PLOGI process and thus forcing a logout of the current session:
- ADISC: The address discovery is used to confirm the address of another port or to discover whether the other port has a hard-coded address.
- FDISC: Fabric discovery (discover the Fabric Service parameters).
- PDISC: Port discovery is used to verify the service parameters of another N_Port.
Process Login
After the successful FLOGI and PLOGI processes, the N_Ports have knowledge of their capabilities and operating parameters. The next step is the PRLI process, which is used to establish a session between two FC-4-level logical processes. It is executed by the PRLI command and allows one or more images of the N_Port to be related to another N_Port. Thus, an image pair is created, or a specific communication between ULPs is negotiated.
The steps of the PRLI process are as follows:
The initiator N_Port sends a PRLI frame with the information for its ULP support (see Figure 11-17).
Figure 11-17 PRLI Initiation
The target N_Port responds with an ACC frame (see Figure 11-18). It contains the information for the ULP support at the target. At this point, a channel has been successfully opened and communication takes place. The relationship between the initiator process and the target process is known as an image pair.
Figure 11-18 PRLI ACC
At the end of the data exchange, the initiator sends a PRLO frame.
The target responds with an ACC frame, and the image pair is then terminated. For a new communication to happen, a new image pair needs to be established.