82598EB Datasheet, PDF (250/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Transmit Functionality
IDX (4)
This field holds the index into the hardware context table where this context descriptor is placed. The
index is pointing to the per-queue descriptors (eight descriptors).
Note: Because the 82598 supports only eight context descriptors per queue, the MSB is reserved
and should be set to 0b.
L4LEN (8)
This field holds the Layer 4 header length. If TSE is set, this field is greater than or equal to 12 and less
than or equal to 255. Otherwise, this field is ignored.
MSS (16)
This field controls the Maximum Segment Size (MSS). This specifies the maximum TCP payload
segment sent per frame, not including any header. The total length of each frame (or section) sent by
the TCP segmentation mechanism (excluding Ethernet CRC) is as follows:
1. If TSE is set: Total length of an outgoing packet is equal to:
MSS + MACLEN + IPLEN + L4LEN +4 (if VLE set)
The one exception is the last packet of a TCP segmentation, which is (typically) shorter.
Software calculates the MSS that is the amount of TCP data that should be used before CRCs are added.
Software reduces the MSS sent down to hardware by the maximum amount of bytes that can be added
for CRC. The actual number of bytes of TCP data sent out on the wire is greater than this MSS value
each time CRCs are added by hardware.
Note: MSS is ignored when DCMD.TSE is not set.
The headers lengths must meet the following:
MACLEN + IPLEN + L4LEN < 512
Note: MACLEN is augmented by four bytes if VLAN is active.
The context descriptor requires valid data only in the fields used by the specific offload options. The
following table describes the required valid fields according to the different offload options.
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