Out-of-order delivery

Sometimes ping reply packets can be received out of order. In such cases using a buffering and queueing model to interpret the patterns can be difficult and misleading. In this subsection we look at an example in which ping reply packets were received out of order.

Figure 5.10 shows the WRTT plot of an experiment in which packets were sent from atmmon to 203.29.167.243, a Web server in New Zealand. A traceroute record for the destination host is on page . The packet loss rate was 52.6%. A significant portion of the reply packets were received out of order.

Figure 5.10: WRTT plot of out-of-order delivery

From the plot we see while the minimum WRTT is about 50 milliseconds the maximum WRTT exceeds 5000 milliseconds! We also see that the plot seems to be a combination of two individual WRTT plots: in one of the plots the WRTT climbs to below 2000 milliseconds in about 800 milliseconds then decreases and flutters around 1000 millisecond, in the other plot the WRTT climbs up at a slower speed but to a higher level of about 5000 milliseconds and then flutters between 4000 to 5200 milliseconds.

The measurement data file reveals that some packets have been received out of order. If we had not known that the packets have been received out of order, we might have misinterpreted this atypical pattern as being caused by buffering and queueing factors. This is just an example that shows that the buffering and queueing theory is not always applicable in interpreting traffic patterns.

One interpretation for the pattern is that the ping packets have taken two different paths during the transmission. That might be because a router in the connection had a load-splitting mechanism that decided to split the ping traffic into two different links with different bandwidths. Thus packets going through them had different WRTTs. The packets that had taken the lower bandwidth link returned later than those that had been sent after them but had taken the higher bandwidth link.

Out-of-order delivery can be identified by comparing the order the reply packets were received to the order their request packets were sent.