Network Time Protocol I-Do Extension Field

The first implementation of NTPv4 was released in 2003. NTPv4 is defined by RFC 5905. It contains a public-key security protocol, autokey, which is defined by RFC 5906. Until very recently, autokey has been the only defined "user" of NTP packet Extension Fields. New proposals for extension fields are being written and there is currently no convenient way to learn if a remote instance of NTP supports any extension fields or not. This proposal contains a method to tell a remote instance of NTP what we support, and ask what they support.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

If an incoming packet contains an unrecognized extension field, one of two things will happen. Either that extension field will be ignored, or the entire packet will be dropped. If an extension field is present there ordinarily SHOULD be a MAC following the extension field. Some extension fields are unable to be "signed" by a MAC, regardless of whether or not that MAC is a traditional MAC or an extension field MAC.

Field Type: TBD (Recommendation for IANA: 0x0007 (I-Do, MAC required), 0x2007 (I-Do, MAC OPTIONAL), 0x8007 (I-Do Response, MAC required), 0xA007 I-Do Response, MAC OPTIONAL)) Field Length: as needed Payload: An enumeration of the suppported base Field Types, followed by any padding, 0x0000, needed to fill the payload to the desired 32-bit boundary. Example: A system that wants to advertise support for Autokey and I-Do, sending to a system that wants to advertise support for I-Do, NTS, and MAC-As-Extension-Field

The sender of any I-Do extension field MUST send an extension field with a Field Type of 0x0007 (I-Do, MAC required) or 0x2007 (I-Do, MAC OPTIONAL) and SHOULD include a paylod with any 0x0000 padding values after enumerating the supported base Extension Field Types. The responding system MUST reply with an extension field with a Field Type of 0x8007 (I-Do Response, MAC required) or 0xA007 (I-Do Response, MAC OPTIONAL), and SHOULD include a paylod with any 0x0000 padding values after enumerating the supported base Extension Field Types. The following information is included here until it is specified in a better location. If the Field Type does not have bit 0x2000 set, there MUST be a MAC included later in the packet for this field to be accepted. If the Field Type has bit 0x2000 set, the presence of a MAC later in the packet is OPTIONAL. Any system that receives an I-Do extension field as either an "offer" or a "response" SHOULD scan the entire payload looking for nonzero values that specify the capabilities of the remote association. Any system that receives an I-Do "offer", 0x0007 or 0x2007, SHOULD reply with an I-Do "response", 0x8007 or 0xA007. Any system that sends an I-Do "offer" or "response" may send as few or as many of its supported Field Types as it chooses. At any subsequent time, either side may re-negotiate the list of supported field types it is prepared to accept from the other system by sending a new I-Do extension field. The most-recently received I-Do list replaces any previous I-Do list.