Loss deduction algorithm

  To measure the packet loss rate along a particular path, it is necessary to know how many packets were sent from the source and how many were received at the destination. From these values the one-way loss rate can be derived as:

1 - (packetsreceived/packetssent)

Unfortunately, from the standpoint of a single endpoint, we cannot observe both of these variables directly. The source host can measure how many packets it has sent to the target host, but it cannot know how many of those packets are successfully received. Similarly, the source host can observe the number of packets it has received from the target, but it cannot know how many more packets were originally sent. In the remainder of this section we will explain how TCP's error control mechanisms can be used to derive the unknown variable, and hence the loss rate, in each direction.