JUNOS software for SRX-series services gateways is a distributed parallel processing high throughput and high performance system. This section explains how a session is created and the process a packet undergoes as it transits the services gateway.
Here is an overview of the main components involved in setting up a session for a packet and processing the packets both discretely and as part of a flow as they transit the SRX 5600 and SRX 5800 services gateways:
The NPU session table contains an entry for a session if the session is established on an SPU for a packet that had previously entered the device via the interface and was processed by this NPU. The SPU installs the session in the NPU table when it creates the session.
An NPU determines if a session exists for a packet by checking the packet information against its session table. If the packet matches an existing session, the NPU sends the packet and the metadata for it to the SPU. If there is no session, the NPU sends the packet to the central point for SPU assignment.
The SPU maintains a session table with entries for all sessions that it established and whose packets it processes. When an SPU receives a packet from an NPU, it checks its session table to ensure that the packet belongs to it. It also checks its session table when it receives a packet from the central point (CP) and a message to establish a session for that packet to verify that there is not an existing session for the packet.
The central point’s main function is to delegate session processing to one of the SPUs. If the session has not yet been established, the central point selects an SPU to establish the session for the flow, based on load- balancing criteria. If the session already exists, the central point forwards packets for that flow to the SPU hosting it. It also redirects packets to the correct SPU in the event that the NPU fails to do so.
The central point maintains a global session table with information about the owner SPU of a particular session. It functions as a central repository and resource manager for the whole system.