For each conversation allocated, LU 6.2 API establishes a
send buffer. On MPE V, the send buffer is always 2044 bytes.
On MPE XL, the send buffer is the same size as the maximum
RU size for the session. The maximum RU size is a configured value
from 256 through 2048 associated with the session type. For more
information on configuring RU sizes, see the APPC Subsystem
on MPE XL Node Manager's Guide.
Whenever the MCSendData intrinsic is called, data is transferred to the send
buffer from the Data parameter specified in the intrinsic call. When the
send buffer is full, LU 6.2 API flushes the buffer
and transmits the data to the remote TP. No data is transmitted
until the send buffer is full or until an intrinsic is called that
flushes the buffer.
Example 1: Sending Small Data Records |
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Figure 6-1 “Send and Receive Buffers” shows how data flows from the
send buffer of the local TP to the receive buffer of the remote
TP in a one-way file transfer application. This example application
sends 16-byte data records. The send buffer in this example holds
2044 bytes.
Figure 6-1 Send and Receive Buffers
Because each data record in figure 6-1 is only 16 bytes long,
LU 6.2 API can store 127 data records in the 2044-byte
send buffer (127 records of 16 bytes = 2032 bytes). When MCSendData is called for the 128th time, LU 6.2 API
checks to see if 16 more bytes will fit in the send buffer. Only
12 more bytes will fit, so LU 6.2 API transmits
the 127 records in the send buffer and then stores the 128th record
in the send buffer.
Example 2: An Allocation Error |
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In example 2, a local TP receives an allocation error from
the remote TP. After executing the MCAllocate intrinsic, LU 6.2 API does not
wait for a response from the remote TP before it starts executing
calls to MCSendData. If the conversation could not be allocated on
the remote side, the allocation error could be received on any of
the calls to MCSendData. In Figure 6-2 “The Local TP Receives an Allocation
Error”, the allocation
error does not reach the local TP until the local TP has made three
calls to MCSendData. The TP is informed of the error through the
Status parameter in the fourth call to MCSendData.
Figure 6-2 The Local TP Receives an Allocation
Error
To verify that a conversation is allocated successfully, call MCAllocate with the SyncLevel parameter set to 0 (CONFIRM), and then call MCConfirm. MCConfirm flushes the send buffer and requests that the remote
TP send a reply, confirming that the conversation was allocated successfully.
