Our inbound route control mechanism is based on manipulating NAT tables at the Web proxy to reflect the current choice of best provider. We use the iptables packet filtering facility in the Linux 2.4 kernel to install and update NAT tables at the proxy. The NAT rules associate destination addresses with the best provider link such that the source address on packets directed to a destination in the table are translated to an address that is announced to the chosen provider.
For example, suppose ISP 1 is selected for transfers involving destination 1.2.3.4 and the addresses 10.1.1.1 was announced over the link to ISP 1. Then we insert a NAT rule for the destination 1.2.3.4 that (1) matches packets with a source IP of defaultIP and destination 1.2.3.4, and (2) translates the source IP address on such packets to 10.1.1.1.
Notice that if the NAT rule blindly translates the source IP on all packets destined for 1.2.3.4 to 10.1.1.1, then it will not be possible to measure the performance to 1.2.3.4 via ISP 2, assuming that a different IP address, e.g., 10.1.1.2, was announced over the link to ISP 2. This is because the NAT translates the source address used for probing 1.2.3.4 across ISP 2 (i.e., 10.1.1.2) to 10.1.1.1, since ISP 1 is considered to be the best for destination 1.2.3.4. To get around this problem in our implementation, we simply construct the NAT rule to only translate packets with a specific source IP address (in this case defaultIP). Measurement packets that belong to probes (active measurement) or client connections (passive measurement) are sent with the appropriate source address, corresponding to the ISP to be measured.