Intel PCI User Manual
Receive and Transmit Description
62
Software Developer’s Manual
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.
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 3-11. TCP Header (Traditional Representation)
Figure 3-12. 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 (see
). 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,
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.
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
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:
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
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
R
G
A
C
K
C
K
P
S
H
S
H
R
S
T
S
T
S
Y
N
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
R
G
A
C
K
C
K
P
S
H
S
H
R
S
S
T
S
Y
N
Y
N
FI
N
TCP
Header
Length
Length
Reserved
Urgent Pointer
Checksum
Options