Mitsubishi Electric FX3U-ENET-ADP User Manual page 21

FX -ENET-ADP User's Manual
3U
6) Flag bit of TCP/IP (SYN, ACK, PSH, FIN, RST, and URG)
In communication using TCP, these flag bits indicate segments where connection/disconnection or
response confirmation is executed or emergency data is included.
a) SYN (Synchronized Flag)
When this bit is ON (1), it indicates that the initial sequence number value is set in the sequence
number field.
This bit is used when the connection is newly opened.
b) ACK (Acknowledgment Flag)
When this bit is ON (1), it indicates that ACK (confirmation response number) field is valid.
It also indicates that this segment includes the information on response confirmation.
When this bit is OFF (0), it indicates that ACK (confirmation response number) field is invalid.
c) PSH (Push Flag)
When this bit is ON (1), the host that has received this segment sends the data to the upper
application with high priority.
This bit is to be turned ON when the data should be sent to an external device as soon as possible.
When this bit is OFF (0), the timing when the received data is sent to the upper application depends
on the TCP layer of the receiving side.
d) FIN (Fin Flag)
When this bit is ON (1), it indicates that there is no more data to be sent from the segment source and
that the send source wants to disconnect.
However, data still can be received from the external device.
The connection is on until the segment whose FIN bit is ON is received from the external device.
e) RST (Reset Flag)
When this bit is ON (1), the host from which the segment has sent disconnects unilaterally (forcibly).
Disconnection by this method is used when an unrecoverable error with the normal method has
occurred or when the host has been restored after being down.
f) URG (Urgent Flag)
When this bit is ON (1), it indicates that this data segment includes the emergency data flag.
1 Introduction
1.1 Outline
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