An entity identifier (used only for "Originator-ID-Symmetric:" and "Recipient-ID-Symmetric:" fields) is constructed as an IKsubfld. More restrictively, an entity identifier subfield assumes the following form:
<user>@<domain-qualified-host>
In order to support universal interoperability, it is necessary to assume a universal form for the naming information. For the case of installations which transform local host names before transmission into the broader Internet, it is strongly recommended that the host name as presented to the Internet be employed.