TCP Header Miscellany
Thus far, this chapter has reviewed many of the features of TCP, including many of the TCP Header fields. Table 6-3 lists the Header fields not mentioned elsewhere in this chapter, with a brief explanation of each one.
6-3 Remaining TCP Header Fields
|
Field Name |
Function |
|
PSH |
Meaning "Push," this code bit (represented by Code bits in the TCP header diagram in Figure 6-3) is set by a TCP sender to cause the TCP receiver to immediately pass that segment's data to the receiver's application socket, along with all other in-order data that the receiver has yet to give to the application. |
|
URG |
Meaning "Urgent," this code bit (represented by Code bits in the TCP header diagram in Figure 6-3) is set by the TCP sender to cause the TCP receiver to immediately pass this segment's data to the receiving application process. Any other data that had been received earlier on this connection, but not yet passed to the application, does not have to be passed to the receiving application at this time. |
|
Urgent Pointer |
Used only when the URG bit is set, this 16-bit field defines the offset into this one segment's Data field at which the urgent data ends. |
|
Offset (HLEN) |
Defines the length of the header (HLEN), alternately called the offset to reach the end of the header. The value in the field, multiplied by 4, is the actual header length in bytes. |
|
Checksum |
Checksum computed against the TCP header, not including the TCP data. |
Continue reading here: Passive and Active Mode FTP
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