Overview

Programming Protocol-Independent Packet Processors (P4) is an open-source language for programming the data plane on networking devices. P4Runtime is an API for controlling the data plane on devices defined in a P4 program. The P4 language and P4Runtime specification are maintained at p4.org.

The SR Linux eXtensible Data Path (XDP) is not programmed in P4. However, SR Linux is packaged with a fixed P4 program named p4info, installed in /etc/opt/srlinux/p4rt/p4info.pb.txt and also available on GitHub, that provides support for marking packets for trapping to a P4Runtime client via PacketIn messages, and transmitting packets from the P4Runtime client to an interface on the device via PacketOut messages. The following fields can be used to mark frames for extraction:

  • VLAN ID
  • Ethertype
  • TTL

This could for example be used to redirect traceroute packets with TTL=0, TTL=1, or TTL=2 to a P4runtime client, so they can be enriched with information that is not visible to the device for the following ACL rules:

  • TTL=0, IPv4 (ethertype 0x0800)

  • TTL=1, IPv4 (ethertype 0x0800)

  • TTL=2, IPv4 (ethertype 0x0800)

  • TTL=0, IPv6 (ethertype 0x86DD)

  • TTL=1, IPv6 (ethertype 0x86DD)

  • TTL=2, IPv6 (ethertype 0x86DD)

This use case is applicable only to transit traffic. In the SR Linux implementation, packets destined for a local node address (a local interface IP, where the interface can be Ethernet, LAG, or loopback) are never sent to the P4Runtime client:

  • Any packet that has a destination IP that is a local interface IP is not subject to P4 pipeline processing.
  • Any packet that has a remote destination IP (not belonging to a local node) follows the P4 logic (if there is a matching rule), including packets with TTL=1.

Another use case is to use a free ethertype to allow the P4Runtime client to transmit and receive packets on all internal links on all devices in a network as a means of topology discovery.

To accommodate these use cases, SR Linux runs a grpc_server service named p4rt that provides the interface between P4Runtime clients and SR Linux.

SR Linux p4rt service

SR Linux supports packet input/output to P4Runtime clients through a grpc_server service named p4rt. This grpc_server process exposes instances of P4Runtime RPCs that P4Runtime clients can connect to, with mandatory arbitration to elect a single P4Runtime client as the primary (see P4Runtime client arbitration).

Instead of running multiple processes, SR Linux runs a single process with multiple sockets.

See Configuring SR Linux for P4Runtime for information about configuring the p4rt grpc_server service.