Specifications

SLAA137A

MSP430 Internet Connectivity 17

To prevent the receive buffer from corruption while receiving data, a handshake mechanism is

implemented. This mechanism uses the SOCK_DATA_AVAILABLE flag of the SocketStatus

is clear, and only in this case is an ACK-segment sent back to the other TCP. If the

RxTCPBuffer is empty and the stack has written new data to it, this is indicated to the application

by setting the SOCK_DATA_AVAILABLE flag. If the receive buffer is not empty, the incoming

data segment is discarded and not acknowledged. But due to the retransmission timeout of the

other TCP, no data is lost, because the other TCP retransmits its data until the local TCP

receive buffer is empty and can acknowledge the data. To avoid permanent retransmissions, the

local application should release the receive buffer frequently by calling the API-function

TCPReleaseRxBuffer() (see Section 4.3).

4.2.4 Opening a Connection

A connection can be opened in either passive or active mode by calling the proper API function

TCPPassiveOpen() or TCPActiveOpen(). The TCPPassiveOpen() function places the stack in a

mode in which it is attempting to detect an incoming connection. The TCPLocalPort variable

must have been set to indicate which port the stack must monitor.

Before performing an active open, the IP address of the remote TCP as well as the local and

remote port numbers must be set. After calling the TCPActiveOpen() function, the stack first tries

to determine the MAC address of the opponent by sending an ARP request. The stack checks

the destination IP to determine whether the other TCP is a member of the subnet (see symbols

SUBMASK_1…4), and if so, addresses that TCP directly. If the other TCP is not a member of

the subnet, the ARP request is performed for the default gateway (symbols GWIP_1…4) and

data transfer occurs over the gateway as a router. For further information about subnets and

gateways, see Reference [1].

After finding the MAC address to communicate with, the stack sends out a TCP segment

carrying a SYN flag and the TCP option maximum segment size (MSS). Along with this

segment, the initial sequence number required for opening a connection and also the size of the

receive buffer is sent. This ensures that no arriving segment exceeds the size of the receive

buffer. This frame is also prepared using the function PrepareTCPFrame().

4.2.5 Data Transfer

Once a connection is established (TCPStateMachine changes to ESTABLISHED), data transfer

can begin. The actual state of the connection can be read out by using the API flag register

SocketStatus (see Section 4.3). The receiving and transferring of data into the receive buffer is

done by the function ProcessTCPFrame(). The count of received bytes is stored in the global

variable TCPRxDataCount. New data can only be received when the receive buffer is released.

Use the function TCPReleaseRxBuffer() to do this.

To send data to the other TCP, the application has to write its data into the data area of

TxFrame1 buffer. For direct access to this area, the pointer TCP_TX_BUF should be used.

Sending is initiated by a user call of the function TCPTransmitTxBuffer(). This function checks

whether sending is allowed and then sets the flag SEND_FRAME1 in the TransmitControl

acknowledge segment is received, the transmit buffer is released automatically. The application

then can fill the buffer with new data and send it out.