The ICP traffic is added to compute the total traffic flow from sibling to proxy. If cost of traffic from sibling is low, the ICP traffic may be neglected. ICP traffic from sibling is response to ICP requests. Traffic needs not be symmetric, the traffic volume is calculated only flowing into the web cache. Similar calculations may be done for traffic flowing to sibling cache, this is left out to simplify calculations.
Most Internet connections have a loop sided traffic profile, importing more than they export.
Normally connections are full duplex, and the dimensioning factor is traffic flowing into the network. In this case, outflowing traffic needs not be considered. Cost Cost factor for an ISP may be cost of international connection as opposed to local cost of bandwidth, or it may be cost per byte transferred out of cache. For a local sysadmin the cost factor may be the access line cost compared with local traffic exchange cost. Finding this factor is not necessarily easy, as some costs are not paid at the same organizational level as the cache servers are run.