Intel PCI-X Destination Port Sequence Number, TCP Header, Length Checksum Urgent Pointer Options

Receive and Transmit Description

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.

											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											U	A		P	R	S	F
							Reserved				R	C		S	S	Y	I										Window
	Length
											G	K		H	T	N	N
							Checksum																			Urgent Pointer
																Options
	Figure 3-11. TCP Header (Traditional Representation)
			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
																		R E		U		A	P		R	S	FI		TCP				Reserved
								Window												R		C	S		S	Y	N		Header
																		S
																				G		K	H	T		N			Length
							Urgent Pointer																			Checksum
																Options

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 Figure 3-13and Figure 3-14). 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.

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:

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