[1] draft-cai-ssdp-v1-03 - Simple Service Discovery Protocol/1.0 ( (2011-03-21 00:41:36 +09:00 版)) http://tools.ietf.org/html/draft-cai-ssdp-v1-03#section-4.2.1.1

[2] RFC 7505 - A "Null MX" No Service Resource Record for Domains That Accept No Mail (2015-07-01 10:19:40 +09:00 版) https://tools.ietf.org/html/rfc7505