Standard Metadata - 2022.2 English

Vitis Networking P4 User Guide (UG1308)

Document ID
UG1308
Version
2022.2 English
Revision
Standard Metadata is defined by the architecture and contains the following fields:
drop
This field can be used to drop a packet. The drop field defaults to a value of 0 when a new packet arrives. You can update this field in P4 at any stage in the Parser or Match-Action. At the input of the Deparser, the drop field is evaluated and the packet dropped if the drop field has a value of 1. In the case of a dropped packet, the user metadata can still optionally be output from Vitis Networking P4 (see the Metadata section in Chapter 3Vitis Networking P4 Tool Flows for more information).
Note: Because this drop field is evaluated at the input to the Deparser, the corresponding packets are dropped before any headers are emitted into the packet stream and therefore without any potential reduction on packet stream bandwidth.
ingress_timestamp
There is a 64-bit timestamp counter in Vitis Networking P4 that increments every clock cycle from startup. For each new packet that arrives at the Parser input, this standard metadata field is populated with the timestamp counter value. This timestamp value can then be used at any stage in the Parser or Match-Action of the P4 code.
parsed_bytes
This field is automatically updated at the end of the Parser to provide the number of bytes of headers that were successfully extracted during the Parser execution. It is equivalent to the “nextBitIndex” pointer referred to in the P4 Language Specification (see Resource Optimizations), but the value is given in bytes rather than bits. It is also equivalent to the byte offset where the packet payload begins following the packet headers.
parser_error
This field contains a parsing error code. This value is automatically updated by the Parser to be used in the Match-Action of the P4 code. The error codes are identified in the following table.
Table 1. Parsing Error Codes
Error Code Type Description
0 NoError No error.
1 PacketTooShort Not enough bits in packet for 'extract'.
2 NoMatch 'select' expression has no matches.
3 StackOutOfBounds Reference to an invalid element of a header stack.
4 HeaderTooShort Extracting too many bits into a varbit field.
5 ParserTimeout Parser execution time limit exceeded. Note: This error type is not currently used by the Vitis Networking P4 architecture.
6 HeaderDepthLimitExceeded Extracting a header that exceeds the Header Depth Limit packet offset of 8191 bytes. In this case, the header is marked as invalid and the parser execution is terminated for that packet.
7-99 user defined New errors can be defined by the user with the use of the 'error' structure. User-defined errors can only be triggered with verify statements.

Whenever a verify statement is triggered, parsing terminates immediately and the parser_error field is updated with the error ID that was triggered. This field can be accessed in the Match-Action of the P4 code and multiple actions can be taken based on the error. For example, to drop the packet:

if (smeta.parser_error != error.NoError) {
  smeta.drop = 1;
  return;
}
Note: VitisNetP4 treats the accept and reject parsing states in exactly the same way, without any architecture-specific distinction. Packets are not automatically dropped when the parser reaches the reject state. You can optionally drop packets or perform other actions in the Match-Action control block based on the parser_error metadata, as shown above.