70 Chapter 1: Building a Simple Network
Figure 1-43 TCP Header Format
Table 1-4 TCP Header Field Descriptions
TCP Header Field Description Number of Bits
Source Port Number of the calling port 16 bits
Destination Port Number of the called port 16 bits
Sequence Number Number used to ensure correct
sequencing of the arriving data
32 bits
Acknowledgment Number Next expected TCP octet 32 bits
Header Length Number of 32-bit words in the header 4 bits
Reserved Set to zero 6 bits
Code Bits Control functions such as setup and
termination of a session
6 bits
Window Number of octets that the device is
willing to accept
16 bits
Checksum Calculated checksum of the header and
data ?¬?elds
16 bits
Urgent Indicates the end of the urgent data 16 bits
Options One currently de?¬?ned: maximum TCP
segment size
0 or 32 bits, if any
Data Upper-layer protocol data Varies
Source Port (16)
Sequence Number (32)
Acknowledgment Number (32)
20
Bytes
Bit 31 Bit 16 Bit 15 Bit 0
Options (0 or 32 If Any)
Data (Varies)
Destination Port (16)
Header
Length (4) Window (16) Reserved (6)
Urgent (16) Checksum (16)
Code Bits(6)
Understanding TCP/IP??™s Transport and Application Layers 71
Figure 1-44 shows a data capture of an Ethernet frame with the TCP header ?¬?eld expanded.
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