A method comprising: generating, with a first Application-Layer Traffic Optimization, ALTO, server that executes a first ALTO client that communicates with a second ALTO server, a first ALTO network map that includes PID entries to describe each of a first set of one or more PIDs, wherein each of the first set of PIDs is associated with a subset of one or more endpoints of a first network; generating, with the first ALTO server, a first ALTO cost map that comprises, for each combination of the first set of PIDs, a cost entry that includes a cost value to specify a cost to traverse a network path through the first network that connects the respective combination of the first set of PIDs; receiving, with the first ALTO client, a second ALTO network map that includes PID entries to describe each of a second set of one or more PIDs, wherein each of the second set of PIDs is associated with a subset of one or more endpoints of a second network; receiving, with the first ALTO client, a second ALTO cost map that comprises, for each combination of the second set of PIDs, a cost entry that includes a cost value to specify a cost to traverse a network path through the second network that connects the respective combination of the second set of PIDs; generating a master cost map comprising one or more master cost entries that each include a PID selected from the first set of PIDs and a PID selected from the second set of PIDs and that each include a cost value that represents a cost to traverse a network path from the PID selected from the first set of PIDs and the PID selected from the second set of PIDs, wherein generating the master cost map comprises determining the one or more master cost entries based on the cost values of the cost entries of the first ALTO cost map and the cost values of the cost entries of the second ALTO cost map; and outputting the master cost map from the first ALTO server to a second ALTO client. generating, with a first Application-Layer Traffic Optimization, ALTO, server that executes a first ALTO client that communicates with a second ALTO server, a first ALTO network map that includes PID entries to describe each of a first set of one or more PIDs, wherein each of the first set of PIDs is associated with a subset of one or more endpoints of a first network; generating, with the first ALTO server, a first ALTO cost map that comprises, for each combination of the first set of PIDs, a cost entry that includes a cost value to specify a cost to traverse a network path through the first network that connects the respective combination of the first set of PIDs; receiving, with the first ALTO client, a second ALTO network map that includes PID entries to describe each of a second set of one or more PIDs, wherein each of the second set of PIDs is associated with a subset of one or more endpoints of a second network; receiving, with the first ALTO client, a second ALTO cost map that comprises, for each combination of the second set of PIDs, a cost entry that includes a cost value to specify a cost to traverse a network path through the second network that connects the respective combination of the second set of PIDs; generating a master cost map comprising one or more master cost entries that each include a PID selected from the first set of PIDs and a PID selected from the second set of PIDs and that each include a cost value that represents a cost to traverse a network path from the PID selected from the first set of PIDs and the PID selected from the second set of PIDs, wherein generating the master cost map comprises determining the one or more master cost entries based on the cost values of the cost entries of the first ALTO cost map and the cost values of the cost entries of the second ALTO cost map; and outputting the master cost map from the first ALTO server to a second ALTO client.