XDR is like a four-octet aligned version of PER for a cut-down version of ASN.1. It's really neat.

XDR would not need much work to be a full-blown ER for ASN.1... But XDR is extremely inefficient as to booleans (4 bytes per!) and optional fields (since they are encoded as a 4-byte boolean followed by the value if the field is present).