P4 Actions

Hello World Reference Pipeline User Guide (UG1669)

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1.1 English

This section describes the P4 actions in the Hello World Reference Pipeline using an example NACL table and rules set in it. Each row in the table represents a packet flow and each packet flow is described using a packet flow diagram.

Table 1. NACL Table With Example Rules
0 00:00:00:00:10:02 345 456 Redirect
1 00:00:00:00:10:04 486 524 Permit
2 00:50:00:00:10:23 989 835 Drop
* * * * * * Drop 1
  1. * indicates that this example pipeline drops all packets that do not match any rule.

The Pipeline highlights some of the basic capabilities of P4 and implements the following actions when a packet matches all the attributes:

  • Redirect – Rule Match
    • Packet is sent to Arm® Data Path Application (DPA) Software for further processing.
      Note: This reference pipeline does not have accompanying DPA code and so packets sent to the Arm CPU complex for processing using the pipeline Redirect action are dropped.
  • Permit – Rule Match
    • If a packet is received on the DPU port Eth 1/1, it is sent out on port Eth 1/2.
    • If a packet is received on the DPU port Eth 1/2, it is sent out on port Eth 1/1.
  • Drop – Rule Match
    • Packet is dropped.
  • Drop - No Rule Match
    • Packet is dropped

The packet header and payload is not sent through the stages; instead, the parser (a fully P4 programmable parser) creates a Packet Header Vector (PHV) from the packet header. The PHV is also stored in the Packet Buffer (PB). Only the reference to the PHV is used for processing by Ingress/Egress stages. This is done prior to stage 0 in Ingress. The payload stays in PB but is accessible from all stages. Only when a payload needs to be processed by an Arm CPU it is copied from/to PB to/from the Arm memory by a separate DMA engine. This transfer is initiated by a stage in the P4 RxDMA, or P4 TxDMA.