Packet Descriptors

The main mechanism for holding information about the frames captured in the Napatech Network Adapters is the Packet Descriptor. Each captured frame gets a Packet Descriptor attached to it before being transferred to the host server.

Napatech Network Adapters support 4 different types of Packet Descriptors:

PCAP Descriptor
The standard PCAP Descriptor ensures compatibility with many industry applications, as well as open-source tools.

Standard Napatech Descriptor
The Standard Napatech Descriptor has the same format as the PCAP descriptor, but utilizes the 4 unused bytes of the PCAP descriptor to provide some additional information, such as Ethernet CRC errors, Code Violation errors, slicing of the frame and information about optional extended descriptors.

Extended Descriptors
Extended Descriptors are optional, and each Napatech Network Adapter model has one or more Extended Descriptors to choose from, that can be selected based on the needs of the server application. The use of Extended Descriptors has the potential to accelerate many types of network applications, as the descriptor information is always located in the same position for the application to find it. This information can include:

• Frame type: IPv4, IPv6, UDP, TCP, ...
• Encapsulation: Number of VLAN tags, number of MPLS labels, ...
• Hash key: 2-, 3- or 5-tuple hash key, GTP hash key, GRE hash key,...
• Hash key type: For dynamic hash key selection
• Offsets: Start of IP header, start of UDP/TCP header and start of UDP/TCP payload
• Coloring/Tagging:  Coloring/tagging information

User-defined Descriptor
A User-defined Descriptor is a special Extended Descriptor (also optional) that creates a series of empty bytes prepended to each captured frame, for the user application to define and utilize (e.g. to attach additional frame-related information before storage). The User-defined Descriptor can be configured to be between 8 and 48 bytes long.