Forward loss

Deriving the loss rate in the forward direction, from source to target, is straightforward. The source host can observe how many data packets it has sent, and then can use TCP's error control mechanisms to query the target host about which packets were received. Accordingly, we divide our algorithm into two phases:

• Data-seeding. During this phase, the source host sends a series of in-sequence TCP data packets to the target. Each packet sent represents a binary sample of the loss rate, although the value of each sample is not known at this point. At the end of the data-seeding phase, the measurement period is concluded and any packets lost after this point are not counted in the loss measurement.
• Hole-filling. The hole-filling phase is about discovering which of the packets sent in the previous phase have been lost. This phase starts by sending a TCP data packet with a sequence number one greater than the last packet sent in the data-seeding phase. If the target responds by acknowledging this packet, then no packets have been lost. However, if any packets have been lost there there will be a ``hole'' in the sequence space and the target will respond with an acknowledgment indicating exactly where the hole is. For each such acknowledgment, the source host retransmits the corresponding packet, thereby ``filling the hole'', and records that a packet was lost. We repeat this procedure until the last packet sent in the data-seeding phase has been acknowledged. Unlike data-seeding, hole-filling must be reliable and so the implementation must timeout and retransmit its packets when expected acknowledgments do not arrive.