gtps2m22 | ACF/SNA Data Communications Reference |
When an RTP endpoint sends an NLP, a copy of the data portion of the NLP is kept until the remote RTP endpoint acknowledges receipt of the data. If NLPs are lost in the network, the data needs to be retransmitted by the RTP endpoint.
HPR support uses a selective retransmission mechanism where data is not retransmitted unless instructed to do so by the remote RTP endpoint. Additionally, the remote RTP endpoint indicates which messages must be retransmitted. This sophisticated approach has the following distinct advantages over other algorithms:
When the TPF system sends an NLP that contains data, the data portion of the NLP is saved in the HPRMT. When the remote RTP endpoint acknowledges receipt of the data, the data is removed from the HPRMT.
If the TPF system receives an NLP with a STATUS segment indicating that data was lost, the lost data is retrieved from the HPRMT and retransmitted. If the TPF system is asked to retransmit data that does not exist in the HPRMT, the TPF system will break the RTP connection. This can happen when the HPRMT is not defined or if it is full.
See HPRMT for more information about the HPRMT.
When the TPF system receives an NLP over an RTP connection, the byte sequence number (BSN) of the NLP is checked to see if this is the next expected message, a duplicate message, or a message received out of order. If it is the next expected message, the message is processed. If it is a duplicate message, it is discarded.
When a message is received out of order, it is queued on the RTP input queue and the TPF system asks the remote RTP endpoint to retransmit the missing data. When the missing data is retransmitted and received by the TPF system, the message on the RTP input queue is processed because it is now the next expected message.
The RTP input queue contains all messages received out of order for a given RTP connection. If the RTP input queue becomes too large or if the remote RTP endpoint does not retransmit the missing data after a certain amount of time, the TPF system will break the RTP connection.