Causes of Dropped Packets

RAP drops UDP packets if the publisher sends packets too quickly for the network to keep up, or if the subscriber does not load data as quickly as it receives it.

One of the most significant factors that affects the performance and throughput of the message flow activity in the RAP system is the network. RAP is designed to accommodate high-speed message transfers that route messages from a publisher to one or more destinations, using UDP multicasting. Since UDP is not a reliable protocol, reliability has been added through the resend mechanism. The most important factor in maximizing RAP message flow performance and throughput is minimizing or eliminating packet retransmission due to dropped packets.

UDP packets are dropped when:

• The network cannot keep pace with the pace at which the publisher sends the packets. In this case, the publishing socket SO_SNDBUF is too small, or the network is too slow, or the subscriber's socket SO_RCVBUF is too small to keep up with the workload.

  You must reconfigure the operating system to change the maximum values for SO_SNDBUF and SO_RCVBUF. RAP publisher and subscriber software allocate the maximum buffer space allowed by the operating system for SO_SNDBUF and SO_RCVBUF buffering.

• The subscriber does not load data as quickly as it receives it. This is likely due to CPU speed, or the number of CPUs on the subscriber system. This may also be caused by an improperly tuned or configured Adaptive Server or Sybase IQ server, which may prevent data from loading as fast as possible.