Clarified Intra-ISD beaconing

Addresses #6 (Section 2.1)

draft-dekater-scion-controlplane.md

@@ -388,9 +388,9 @@ The RPC messages are transported via the {{Connect}}'s rpc protocol; a gRPC-like
 In SCION, the *control service* of each AS is responsible for the beaconing process. The control service generates, receives, and propagates so-called *path-segment construction beacons (PCBs)* on a regular basis, to iteratively construct path segments. PCBs contain topology and authentication information, and can also include additional metadata that helps with path management and selection. The beaconing process itself is divided into routing processes on two levels, where *inter-ISD* or core beaconing is based on the (selective) sending of PCBs without a defined direction, and *intra-ISD* beaconing on top-to-bottom propagation.
 
 - *Inter-ISD or core beaconing* is the process of constructing path segments between core ASes in the same or in different ISDs. During core beaconing, the control service of a core AS either initiates PCBs or propagates PCBs received from neighboring core ASes to other neighboring core ASes. Core beaconing is periodic; PCBs are sent over policy-compliant paths to discover multiple paths between any pair of core ASes.
-- *Intra-ISD beaconing* creates path segments from core ASes to non-core ASes. For this, the control service of a core AS creates PCBs and sends them to the non-core child ASes (typically customer ASes). The control service of a non-core child AS receives these PCBs and forwards them to its child ASes, and so on. This procedure continues until the PCB reaches an AS without any customer (leaf AS). As a result, all ASes within an ISD receive path segments to reach the core ASes of their ISD.
+- *Intra-ISD beaconing is the process of constructing path segments between ASes in the same ISD. Every core AS originates PCBs at regular intervals and sends these to all egress interfaces that have connections to neighbor ASes. A PCB sent to a neighboring core AS initiates a core segment, whereas a PCB sent to a child AS initiates an up/down segment.
 
-On its way, a PCB accumulates cryptographically protected path- and forwarding information per traversed AS. At every AS, metadata as well as information about the AS's ingress and egress interfaces are added to the PCB.
+On its way, a PCB accumulates cryptographically protected path information and forwarding information per traversed AS. At every AS, metadata as well as information about the AS's ingress and egress interfaces are added to the PCB.
 
 ### Peering Links