Standard Metadata - 2025.1 English - UG1308

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 Metadata 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 throughout the Parser so that it can be assigned to user metadata field(s) at various points in the Parser, to get several different offset values. The field at the end of the Parser provides the number of header bytes 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.

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;
}