Recommendation ITU-T G.873.1 defines the automatic protection switching (APS) protocol and protection switching operation for the linear protection schemes for the optical transport network at the optical data unit k (ODUk) level. 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 text in clause 7.1 of G.873.1 (2017).