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Jumbo Frame Support — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 27
2.5.1 Jumbo Frame Support
The 82575 supports jumbo frames to increase performance and decrease CPU utilization. By default,
the 82575 might receive packets with a maximum size of 1522 bytes. If large frame reception is
enabled by the RCTL register, the 82575 supports jumbo packet reception of up to 9018 bytes
(including CRC and headers). On the transmit size, jumbo packets are always supported by the 82575.
It is the responsibility of the software device driver to initiate jumbo packets only when it is configured
to do so.
2.5.2 Receive and Transmit Checksum Offloading
The 82575 provides the ability to offload the IP, TCP, and UDP checksum requirements from the
software device driver. For common frame types, the hardware automatically calculates, inserts, and
checks the appropriate checksum values normally handled by software.
For transmits where the 82575 is doing non-TCP segmentation, every transmitted Ethernet packet can
have two checksums calculated and inserted by the 82575. Typically these would be the IPv4 and either
TCP or UDP checksums. The software device driver specifies which portions of the packet are included
in the checksum calculations, and where the calculated values are inserted, via descriptor(s).
For receives, the hardware recognizes the packet type and performs the checksum calculations as well
as error checking automatically. Checksum and error information is provided to software via the receive
descriptor(s). Refer to Section 5.5.1 for details.
2.5.3 TCP Segmentation
The 82575 implements a TCP segmentation capability for transmits that enables the software device
driver to offload packet segmentation and encapsulation to the hardware. The software device driver
can send the 82575 the entire IP (IPv6 or IPv6), TCP or UDP message sent down by the Network
Operating System (NOS) for transmission. The 82575 segments the packet into legal Ethernet frames
and transmit them on the wire. By handling the segmentation tasks, the hardware alleviates the
software from handling some of the framing responsibilities. This reduces the overhead on the CPU for
the transmission process thus reducing overall CPU utilization.
2.5.4 Receive Fragmented UDP Checksum Offloading
The 82575 provides the ability to offload inbound fragmented UDP packet reassembly. The 82575
provides the partial checksum calculation for each incoming UDP fragment so that the software device
driver is required to sum the partial checksum words for each fragment to produce the complete
checksum. The fragmented UDP checksum offload is provided to IPv4 packets.
2.6 Buffer and Descriptor Structure
Software allocates the transmit and receive buffers, and also forms the descriptors that contain
pointers to, and the status of, those buffers. A conceptual ownership boundary exists between the
driver software and the hardware of the buffers and descriptors.