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Advanced Transmit Context Descriptor — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 141
5.6.7.1 Maximum Segment Size (MSS) Control
This field specifies the maximum TCP payload segment sent per frame, less any header. The total
length of each frame (or section) sent by the TCP Segmentation mechanism (excluding Ethernet CRC)
is as follows:
The one exception is the last packet of a TCP segmentation which is (typically) shorter.
Software calculates the MSS which is the amount of TCP data which should be used before markers and
CRC are added. As a result, software reduces the MSS sent down to hardware by the maximum amount
of bytes that can be added for markers/CRC. The actual number of bytes of TCP data sent out on the
wire is greater than this MSS value each time markers and CRC are added by hardware.
Note: L5LEN is computed internally. L5LEN = 16 for tagged buffers and 20 for untagged buffers
according to TUCMD. DDPTYP.
The total length is smaller or equal to 9014 bytes.
MSS is ignored when DCMD.TSE is not set.
Note: The header lengths must meet the following:
MACLEN + IPLEN + L4LEN <= 512
Note: MACLEN is augmented by 4 bytes if VLAN is active.
The context descriptor requires valid data only in the fields used by the specific offload options.
Table 42 lists the required valid fields according to the different offload options.
DTYP Descriptor Type
Always set to 0010b for this type of descriptor.
ADV Bits 7:6 - Reserved.
Bit 5 - DEXT, description extension (1b for advanced mode).
Bits 4:0 - Reserved.
IDX Index into the hardware table where CD is placed.
L4LEN 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 Maximum Segment Size Control
See Section 5.6.7.1.
Field Description