Internet Engineering Task Force H. Stenn
Internet-Draft Network Time Foundation
Intended status: Standards Track March 25, 2019
Expires: September 26, 2019
Network Time Protocol I-Do Extension Field
draft-stenn-ntp-i-do-06
Abstract
This proposal defines and describes a mechanism by which cooperating
NTP instances may communicate any optional features they are willing
to admit they support.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source code and issues list for this draft can be found in
https://github.com/hstenn/ietf-ntp-i-do
Status of This Memo
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This Internet-Draft will expire on September 26, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. The I-Do Extension Field . . . . . . . . . . . . . . . . . . 2
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2. I-DO Packet Format . . . . . . . . . . . . . . . . . . . 4
2.3. Behavior . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The first implementation of NTPv4 was released in 2003, and was
defined by RFC 5905 [RFC5905]. It contains an optional and now
obsolete public-key security protocol, Autokey, which is defined by
RFC 5906 [RFC5906]. Until very recently, Autokey has been the only
implemented use 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 (are willing to admit we) support, and
ask what they (are willing to admit they) support.
1.1. Requirements Language
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 [RFC2119].
2. The I-Do Extension Field
2.1. Overview
The purpose of the I-DO EF is to provide information to the remote
side about our capabilities.
If an incoming packet contains an unrecognized extension field, one
of several things will happen. While that unrecognized extension
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field SHOULD be ignored, an implementation MAY choose to drop the
entire packet.
If any extension field is present there ordinarily SHOULD be a MAC
following the extension field. However, an older conforming NTP
implementation will require that any EF MUST be followed by a MAC.
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.
In the previous two cases, a conforming legacy system that receives
these types of packets will interpret the unrecognized EF as a
missing or legacy MAC, and return a crypto-NAK.
If the remote system replies with a crypto-NAK, that is a good
indication that it is running older software that does not recognize
EFs and thinks we have sent an invalid MAC. In this case, we SHOULD
NOT send that system newer EFs.
If the remote system replies without including an I-DO-RESPONSE EF,
we at least know they can handle EFs, but they either don't
understand I-DO or are not willing to tell us anything. In this
case, we SHOULD NOT send any newer EFs.
If the remote system replies with a packet that includes an I-DO-
RESPONSE EF, then we SHOULD remember what they told us, and use that
information appropriately. In other words, we can exchange packets
containing any new EFs that we agree on, and we should not exchange
packets containing any new EFs that we have not agreed on.
In client/server mode, it makes sense for the client to send an I-DO
to the server, and notice how the server responds. While the server
SHOULD respond with an I-DO-RESPONSE EF, it likely does not make
sense for the server to send an I-DO EF in response to a client
request.
In symmetric mode, either side may initiate sending an I-DO EF, and
the receiving side SHOULD reply with an I-DO-RESPONSE EF.
In broadcast mode, the broadcast server MAY send broadcast packets
that include an I-DO EF, but note that if, counter to recommended
practice, these packets are unauthenticated they MAY cause client
machines to misinterpret the packet as having invalid authentication.
In this situation, the broadcast server SHOULD alternate sending
broadcast server packets with and without an I-DO EF, to insure that
all clients receive time packets they will accept. Note that if, as
recommended, broadcast packets are authenticated, a conforming client
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SHOULD have no difficulty in receiving a broadcast (mode 5) packet
from a server that includes an I-DO EF.
2.2. I-DO Packet Format
The content of the I-DO extension field is an ordinary four octet
Extension Field header followed by a payload consisting of an
appropriate number of two octet I-DO values that use nonzero values
to indicate a supported feature. An I-DO value of zero is ignored.
The payload section must end on a four-octet boundary.
There are two types of nonzero I-DO values that may be used. They
are both defined in the IANA NTP Extension Field Table (Section 4).
These values are either Extension Field Types, where only the low-
order values (0x01 thru 0xFE) are used, or I-DO Types, where all 16
bits are used and the bottom octet is currently always 0xFF.
The examples below are built using information from the following
Standards and proposals:
RFC 5906 [RFC5906]
NTP-EXTENSION-FIELDS [NTP-EXTENSION-FIELD]
MAC-LAST-EF [DRAFT-MAC-LAST-EF]
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Field Type | Field Length |
+-------------------------------+-------------------------------+
| I-Do 1 | ... |
+-------------------------------+-------------------------------+
| I-Do N | Padding |
+---------------------------------------------------------------+
NTP Extension Field: I-DO - Overview
Field Type: TBD (Recommendation for IANA: 0x0007 (I-Do), 0x8007 (I-Do
Response))
Field Length: as needed
Payload: An enumeration of the supported base Field Types, followed
by any zero padding (0x0000) needed to fill the payload to the
desired 32-bit boundary.
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Example: A system that wants to advertise support for Autokey and
I-DO, sending to a system that responds with support for I-DO, NTS,
MAC-EF, and LAST-EF.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Field Type (0x0007) | Field Length (0x0008) |
+-------------------------------+-------------------------------+
| 0x0007 | 0x0002 |
+-------------------------------+-------------------------------+
NTP Extension Field: I-Do - Example
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Field Type (0x8007) | Field Length (0x000a) |
+-------------------------------+-------------------------------+
| 0x0003 | 0x0004 |
+-------------------------------+-------------------------------+
| 0x0007 | 0x0008 |
+-------------------------------+-------------------------------+
NTP Extension Field: I-Do Response - Example
2.3. Behavior
The sender of any I-Do extension field MUST send an extension field
with a Field Type of 0x0007 (I-Do) and SHOULD include a payload with
any 0x0000 padding values after enumerating the supported base
Extension Field Types. If the responding system recognizes the I-Do
extension field, its response MUST include an extension field with a
Field Type of 0x8007 (I-Do Response), and SHOULD include a payload
with any 0x0000 padding values after enumerating the supported base
Extension Field Types.
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, SHOULD reply with
an I-Do "response", 0x8007.
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
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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.
3. Acknowledgements
The author wishes to acknowledge the contributions of Sam Weiler.
4. IANA Considerations
This memo requests IANA to allocate NTP Extension Field Types:
0x0007 (I-DO)
0x8007 (I-DO Response)
and NTP Extension Field I-DO types:
0x00FF through
0xFDFF Reserved for future I-DO types
0xFEFF (I-DO Leap Smear REFIDs)
0xFFFF (I-DO IPv6 REFID hash)
for this proposal.
5. Security Considerations
No addtional or unusual security considerations are expected if this
proposal is adopted.
No feedback has been received suggesting this proposal creates any
new security considerations.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
6.2. Informative References
[DRAFT-MAC-LAST-EF]
Stenn, H., "draft-stenn-ntp-mac-last-ef", 2018.
[NTP-EXTENSION-FIELD]
Stenn, H., "draft-stenn-ntp-extension-fields", 2018.
[RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol
Version 4: Autokey Specification", RFC 5906,
DOI 10.17487/RFC5906, June 2010,
<https://www.rfc-editor.org/info/rfc5906>.
Author's Address
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
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