Recommendation ITU-T G.873.1 defines the automatic protection switching (APS) protocol and protection switching operation for linear protection schemes of the optical transport network at the optical data unit k (ODUk) level. The protection schemes considered in this Recommendation are: – ODUk subnetwork connection protection with inherent monitoring (1+1, 1:n); – ODUk subnetwork connection protection with non-intrusive monitoring (1+1); – ODUk subnetwork connection protection with sublayer monitoring (1+1, 1:n). – ODUk compound link subnetwork connection group protection with inherent monitoring (1+1, 1:1). In addition, client-related protection architectures are described. Corrigendum 1 provides an editorial correction to the text in clause 7.1. Amendment 1 adds ODUCn as a server layer of protected entities in Table 8-1 and provides minor editorial changes.