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IP and TCP/UDP Headers — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 153
The TCP header is first shown in the traditional (RFC 793) representation. Because byte and bit
ordering is confusing in that representation, the TCP header is also shown in little-endian format. The
actual data is fetched from memory in little-endian format.
Figure 14. TCP Header (Traditional Representation)
Figure 15. TCP Header (Little Endian)
The TCP header is always a multiple of 32 bit words. TCP options may occupy space at the end of the
TCP header and are a multiple of 8 bits in length. All options are included in the checksum.
The checksum also covers a 96-bit pseudo header conceptually prefixed to the TCP Header. The IPv4
pseudo header contains the IPv4 Source Address, the IPv4 Destination Address, the IPv4 Protocol field,
and TCP Length. The IPv6 pseudo header contains the IPv6 Source Address, the IPv6 Destination
Address, the IPv6 Payload Length, and the IPv6 Next Header field. Software pre-calculates the PARTIAL
pseudo header sum, which includes IPv4 SA, DA and protocol types, but NOT the TCP length, and
stores this value into the TCP checksum field of the packet. For both IPv4 and IPv6, hardware needs to
factor in the TCP length to the software supplied pseudo header partial checksum.
Note: The Protocol ID field should always be added the least significant byte (LSB) of the 16 bit
pseudo header sum, where the most significant byte (MSB) of the 16 bit sum is the byte
that corresponds to the first checksum byte out on the wire.
The TCP Length field is the TCP Header Length including option fields plus the data length in bytes,
which is calculated by hardware on a frame by frame basis. The TCP Length does not count the 12 bytes
of the pseudo header. The TCP length of the packet is determined by hardware as:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Source Port Destination Port
Sequence Number
Acknowledgement Number
TCP Header
Length
Reserved U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Window
Checksum Urgent Pointer
Options
Byte3 Byte2 Byte1 Byte0
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
Destination Port Source Port
LSB Sequence Number MSB
Acknowledgement Number
Window R E
S
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
FI
N
TCP Header
Length
Reserved
Urgent Pointer
Checksum
Options