Communications of the ACM

PCBs are flooded through multiple paths over policy-compliant paths to discover multiple paths between any pair of core ASes. SCION’s beacon servers can be configured to implement current BGP policies, as well as additional properties (such as control of upstream ASes) BGP is unable to express.

Path servers store mappings from AS identifiers to sets of such announced path segments and are organized as a hierarchical caching system similar to today’s DNS. ASes, through the master beacon servers, select the set of path segments through which they want to be reached, uploading them to a path server in the ISD core.

Certificate servers store cached
copies of TRCs retrieved from the ISD
core, store cached copies of other ASes’
certificates, and manage keys and cer-
tificates for securing intra-AS com-
munication. Beacon servers require
certificate servers when validating the
authenticity of PCBs.

Border routers forward packets between ASes supporting SCION. In the case of a control packet, the border router forwards it to the appropriate server, and, in the case of a data packet, forwards it either to a host inside the AS or toward the next border router.

Since SCION can operate using any communication fabric inside an AS, the internal routers do not need to be changed.

Path exploration and registration.

Through inter-domain beaconing, core ASes discover paths to other core ASes.

Through intra-domain beaconing,
inside ASes. We did not include name
servers in Figure 2b, as their operation
is similar to today’s DNS.

Beacon servers are responsible for disseminating PCBs, as in Figure 2a. Beacon servers in a core AS generate intra-ISD PCBs that are sent to non-core ASes of the ISD. Non-core AS beacon servers receive these PCBs and re-send them to their customer ASes, resulting in AS-level path segments. Figure 3 outlines PCBs propagated from the ISD core down to customer ASes. At every AS, information about the AS’s interfaces is added to the PCB. The beacon servers generates a set of PCBs it forwards to its customer ASes. In the case of inter-ISD communication, the beaconing process is similar to BGP’s route-advertising process, although it is periodic and

Figure 2. SCION components at different scales: (a) SCION ISD with PCBs propagated from the ISD core down to customer ASes, and path segments for ASes A, B, C, D, and E to the ISD core; and (b) magnified view of an AS with its routers and servers. The path from AS C to the ISD core traverses two internal routers.