Toshiba V Series User Manual page 119

(12)With a connection established, if the no response state from the remote node
continues when data is sent and the EN311 internal resend processing times out (the
timeout time is about 1 minute), the NOACK bit in the socket status (see section 6.3,
"Socket Interface Information") will be set to 1. User programs should close such
sockets.
Also note that the EN311 will return an error if send or receive requests are issued in
this state.
(13)User program send requests are seen as completed as soon as they are received by
the EN311. This means that even if the remote node goes to the no response state, a
number of send requests equal to the EN311 internal storage capacity will complete
normally.
When the number of send requests exceed the EN311 internal storage capacity,
errors will be returned for all further send requests.
(14)The window size of the remote socket may become smaller during the transmission
from the EN311. The TCP send request data size which is requested by the S
controller is larger the window size of the remote node, the EN311 will return an error.
Once it enters this state, TCP send requests will repeatedly cause errors unless the
remote node itself notifies the window size expansion.
In this case, it is recommended to reestablish the connection after closing it.
Window size: The technique used to notify the available space in the receive buffer
(15)When the EN311 receives a segment with the reset bit (RST) set to 1 (reset
segment), the RCLOSE bit will be set to 1 and the CONN bit will be set to 0 in the
socket status (see section 6.3, "Socket Interface Information"). If the socket status
shows these bits, such sockets should be closed.
Reset segment: A transmitted segment used to forcibly close a connection from one
(16)ACK transmission timing in the EN311 TCP socket
When the internal reception data in the EN311 is read out by the TCP receive
request, the EN311 sends an ACK (acknowledge response) to the node that sent the
data. Note that the EN311 will send an ACK every second to the source node even if
it does not issue the TCP receive request. However, if it does not issue the TCP
receive request, the data reached to the destination socket will be stored in the TCP
receive buffer. The receive buffer on the TCP socket has the limit of 5840 bytes per
packet. The data should be received by the TCP receive request.
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of the local socket to the remote socket. It is possible that the window
size becomes smaller because the receive data has not been
processed for some reason or other.
of the nodes. Connections are forcibly closed in this manner if an
error that cannot be recovered from by normal means such as
retransmission has occurred, or if a node has recovered after
having gone down.
6.2 EN311 Socket Interface Usage Notes
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