82583V Datasheet, PDF (154/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

82583V GbE Controller—Inline Functions
7.3
Note:
7.3.1
Note:
7.3.2
TCP Segmentation
TCP segmentation is an offloading option of the TCP/IP stack. This is often referred to
as Transmit Segmentation Offloading (TSO). This feature obligates the software device
driver and hardware to carve up TCP messages, larger than the Maximum Transmission
Unit (MTU) of the medium, into MSS sized frames that have appropriate layer 2, 3 (IP),
and 4 (TCP) headers. These headers must have the correct sequence number, IP
identification, checksum fields, options and flag values as required. This is done by
breaking up the data into segments smaller than or equal to the MSS.
Note that some of these values (such as the checksum values) are unique for each
packet of the TCP message, and other fields such as the source IP address are constant
for all frames associated with the TCP message.
The offloading of these mechanisms to the software device driver and the 82583V
saves significant CPU cycles. The software device driver shares the additional tasks to
support these options with the 82583V.
TCP Segmentation Performance Advantages
Performance advantages for a hardware implementation of TCP segmentation offload
include:
• The stack does not need to partition the block to fit the MTU size, saving CPU
cycles.
• The stack only computes one Ethernet, IP, and TCP header per segment (entire
packet), saving CPU cycles.
• The stack interfaces with the software device driver only once per block transfer,
instead of once per frame.
• Interrupts are easily reduced to once per TCP message instead of once per frame.
• Fewer I/O accesses are required to command the the 82583V.
TCP segmentation requires the transmit context descriptor format and the transmit
data descriptor format.
Ethernet Packet Format
A TCP message can be fragmented across multiple pages in host memory. The 82583V
partitions the data packet into standard Ethernet frames prior to transmission. The
82583V supports calculating the Ethernet, IP, TCP, and UDP headers, including
checksum, on a frame-by-frame basis.
L2
L3
L4
Ethernet
IP
TCP
Figure 38. TCP/IP Packet Format
DATA
FCS
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