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Header Splitting and Replication — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 127
5.5 Header Splitting and Replication
This feature consists of splitting or replicating a packet’s header to a different memory space. This helps
the host to fetch headers only for processing: headers are replicated through a regular snoop
transaction, in order to be processed by the host processor. It is recommended to perform this
transaction with the DCA feature enabled.
The packet (header + payload) is stored in memory through an optional non-snoop transaction.
The 82575 supports header splitting in several modes:
• Legacy mode: legacy descriptors are used; headers and payloads are not split.
• Advanced mode, no split: advanced descriptors are in use; header and payload are not split.
• Advanced mode, split: Advanced descriptors are in use; header and payload are split to different
buffers.
• Advanced mode, replication: Advanced descriptors are in use; header is replicated in a separate
buffer and in a payload buffe.r
• Advanced mode, replication, conditioned by packet size: Advanced descriptors are in use;
replication is performed only if the packet is larger than the header buffer size.
• Advanced mode, split: always use header buffer: Advanced descriptors are in use; header and
payload are split to different buffers. If no split is done, the first part of the packet is stored in the
header buffer.
Header splitting and header replication modes are shown in Figure 5.
Figure 5. Header Splitting with Replicated Header
The physical address of each buffer is written in the Buffer Addresses fields. The sizes of these buffers
are statically defined by BSIZEPACKET in the SRRCTL[n] registers.
The Packet Buffer Address includes the address of the buffer assigned to the replicated packet,
including header and data payload portions of the received packet. In case of split header, only the
payload is included.