Internet Engineering Task Force T. Mizrahi
Internet-Draft Marvell
Intended status: Standards Track D. Mayer
Expires: May 17, 2017 H. Stenn
Network Time Foundation
November 13, 2016
Network Time Protocol Version 4 (NTPv4) Extension Fields
draft-stenn-ntp-extension-fields-00
Abstract
Network Time Protocol version 4 (NTPv4) defines the optional usage of
extension fields. An extension field, as defined in RFC 5905
[RFC5905], resides after the end of the NTP header, and supplies
optional capabilities or information that is not conveyed in the
standard NTP header. This document updates RFC 5905 [RFC5905] by
clarifying some points regarding NTP extension fields and their usage
with legacy Message Authentication Codes (MACs).
With the adoption of this update, the authors recommend rescinding
[Err3627].
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 17, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Terms and Abbreviations . . . . . . . . . . . . . . . . . 3
3. NTP Extension Fields - RFC 5905 Update . . . . . . . . . . . 3
3.1. OLD: 7.5 NTP Extension Field Format . . . . . . . . . . . 3
3.2. NEW: 7.5 NTP Extension Field Format . . . . . . . . . . . 4
3.3. NEW: 7.5.1 Extension Fields and MACs . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The NTP header format consists of a set of fixed fields that may be
followed by optional fields. Two types of optional fields are
defined: extension fields as defined in Section 7.5 of RFC 5905
[RFC5905], and legacy Message Authentication Codes (legacy MACs).
If a legacy MAC is used, it resides at the end of the packet. This
field can be either a 4-octet crypto-NAK or data that is usually 20
or 24 octets long.
NTP extension fields are defined in RFC 5905 [RFC5905] as a generic
mechanism that allows the addition of future extensions and features
without modifying the NTP header format (Section 16 of RFC 5905
[RFC5905]).
Section 7.5 of RFC 5905 [RFC5905] clearly states that "one or more
extension fields can be inserted after the header and before the MAC,
which is always present when an extension field is present."
However, the experimental Checksum Complement RFC 7821 [RFC7821]
cannot be used if the NTP packet contains a MAC.
To allow for extension fields that do not require a MAC, changes to
the NTPv4 specification must be made. [Err3627] was an attempt to
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clarify the rules around MACs, but with the adoption of this proposal
the authors recommend rescinding [Err3627].
This document better specifies and clarifies both Extention Fields
and the requirements and parsing of a legacy MAC, with changes to
address errors found after the publication of RFC 5905 [RFC5905] with
respect to extension fields. Specifically, this document updates
Section 7.5 of RFC 5905 [RFC5905], clarifying the relationship
between extension fields and MACs, and defines the behavior of a host
that receives an unknown extension field.
2. Conventions Used in This Document
2.1. 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].
2.2. Terms and Abbreviations
MAC - Message Authentication Code
NTPv4 - Network Time Protocol, Version 4 RFC 5905 [RFC5905]
3. NTP Extension Fields - RFC 5905 Update
This document updates Section 7.5 of RFC 5905 [RFC5905] as follows:
3.1. OLD: 7.5 NTP Extension Field Format
In NTPv4, one or more extension fields can be inserted after the
header and before the MAC, which is always present when an extension
field is present. Other than defining the field format, this
document makes no use of the field contents. An extension field
contains a request or response message in the format shown in
Figure 14.
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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 |
+-------------------------------+-------------------------------+
. .
. Value .
. .
+-------------------------------+-------------------------------+
| Padding (as needed) |
+---------------------------------------------------------------+
Figure 14: Extension Field Format
All extension fields are zero-padded to a word (four octets)
boundary. The Field Type field is specific to the defined function
and is not elaborated here. While the minimum field length
containing required fields is four words (16 octets), a maximum field
length remains to be established.
The Length field is a 16-bit unsigned integer that indicates the
length of the entire extension field in octets, including the Padding
field.
3.2. NEW: 7.5 NTP Extension Field Format
In NTPv4, one or more extension fields can be inserted after the
header and before the possibly optional legacy MAC. A MAC SHOULD be
present when an extension field is present. A MAC is always present
in some form when NTP packets are authenticated. This MAC can be
either a legacy MAC or a MAC-EF. Other than defining the field
format, this document makes no use of the field contents. An
extension field contains a request or response message in the format
shown in Figure 14.
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 |
+-------------------------------+-------------------------------+
. .
. Value .
. .
+-------------------------------+-------------------------------+
| Padding (as needed) |
+---------------------------------------------------------------+
Figure 14: Extension Field Format
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All extension fields are zero-padded to a word (four octets)
boundary. The Field Type field is specific to the defined function
and is not elaborated here. While the minimum field length
containing required fields is four words (16 octets), a maximum field
length remains to be established.
The Length field is a 16-bit unsigned integer that indicates the
length of the entire extension field in octets, including the Padding
field.
The Field Type contains the following sub-elements:
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
+---------------+---------------+-------------------------------+
|R|E|O|I| Code Field Type | (Field Length) |
+-------------------------------+-------------------------------+
Field Type Format
Where:
R: 0 for a "Query", 1 for a "Response"
E: 0 for "OK", 1 for an "Error"
O: 0 for "MAC Required", 1 for "MAC Optional"
I: 0 for "MAC Not Included", 1 for "MAC Included"
The Code subtype is currently only used by RFC 5906, Autokey
[RFC5906].
The Field Type, Value, and Padding fields are specific to the defined
function and are not elaborated here; the Field Type value is defined
in an IANA registry, and the Length, Value, and Padding values are
defined by the document referred to by the registry. If a host
receives an extension field with an unknown Field Type, the host
SHOULD ignore the extension field and MAY drop the packet altogether
if policy requires it.
While the minimum field length containing required fields is four
words (16 octets), the maximum field length MUST NOT be longer than
65532 octets due to the maximum size of the data represented by the
Length field, and SHOULD be small enough that the size of the NTP
packet received by the client does not exceed the smallest MTU
between the sender and the recipient.
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The Length field is a 16-bit unsigned integer that indicates the
length of the entire extension field in octets, including any Padding
.
3.3. NEW: 7.5.1 Extension Fields and MACs
With the inclusion of additional Extension Fields, there is now a
possibility of an ambiguous parsing of a legacy MAC. If an
implementation offers even a modicum of care, there will be no
ambiguity when parsing an NTP packet that contains a legacy MAC.
If an implementation uses the LAST-EF extension field, the presence
of this field means "I am the last EF in this NTP Packet. Any
subsequent packet data MUST be a legacy MAC." In this case, there is
no parsing ambiguity.
If a system sends its MAC as a MAC-EF and does not send a legacy MAC,
there is no parsing ambiguity.
The only time there is a potential for a parsing ambiguity is when a
legacy MAC is provided and neither of the previous two cases are
present. Even in this case, there is minimal risk.
An Extension Field contains a 2-octet Field Type, a 2-octet Field
Length, and any payload (data and/or padding). If the NTP Packet
parsing is at a point where it is evaluating data after the base
packet, one of the following situations exists:
If the Field Length is not an even multiple of 4, we are not
looking at an extension field. In this case, the only possibility
of having a valid packet is if the data is part of a legacy MAC.
If the Field Length is valid, i.e., an even multiple of 4 octets,
one of the following three cases must be present:
First, the Field Length will be less than the remaining data.
This means subsequent data must parse as some number of
Extension Fields, optionally follwed by a legacy MAC.
Second, the Field Length will exactly match the remaining data.
The third case is where the Field Length is longer than the
remaining packet data. In this case, the current parse cannot
be a valid extension field.
Semantic checking may also be done to validate a potential legacy
MAC. A legacy MAC is a four-octet Key Identifier followed by a
message digest. The usual message digest is 16 octets long but may
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be another size, depending on the digest algorithm. In the Reference
Implementation, a Key Identifier between 1 and 65535, inclusive, is a
symmetric key, while a Key Identifier that is > 65535 is an Autokey
RFC 5905 [RFC5905], or similar. If the receiving system does not
recognize the Key Identifier, the data CANNOT be a valid legacy MAC.
If the receiving system recognizes the Key Identifier, then it also
has knowledge of the digest algorithm and can make sure the digest
payload is the proper length. If this is not the case, then the data
CANNOT be a valid legacy MAC.
It is trivial to parse the data after the base NTP packet and come up
with a list of potential parsings. A local policy choice can specify
the precedence of the parsing options in this case.
If none of the parsings validate, the packet fails authentication.
An implementation has three local policy choices available if LAST-EF
is not used and a legacy MAC may be provided. First, the
implementation may specify EF-precedence. Second, the implementation
may specify legacy-MAC-precedence. Finally, the implementation may
specify "best fit" precedence. In this last case, the packet will
meet one of the three following criteria: First, none of the parsings
will match. Again, this is a case of failed authentication. Second,
exactly one parsing will match and that parsing will be accepted.
Third, multiple parsings will match, in which case the implementation
may choose its behavior.
Additionally, most EFs will require a MAC. If there is a
syntactically-valid parsing that does not include a MAC but
previously scanned EFs require a MAC, then in a multiple-choice
parsing scenario where one of the choices does not include a MAC the
"no MAC provided" choice SHOULD be eliminated.
Note well that this rare situation can be completely avoided by using
LAST-EF, or by indicating that no legacy MAC will be used.
4. IANA Considerations
This memo requests IANA to allocate NTP Extension Field Types 0x0007
(I-Do), 0x2007 (I-Do, MAC OPTIONAL), 0x8007 (I-Do Response), and
0xA007 (I-Do Response, MAC OPTIONAL)for this proposal.
5. Security Considerations
Additional information TBD
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6. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol
Version 4: Autokey Specification", RFC 5906,
DOI 10.17487/RFC5906, June 2010,
<http://www.rfc-editor.org/info/rfc5906>.
[RFC7821] Mizrahi, T., "UDP Checksum Complement in the Network Time
Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
2016, <http://www.rfc-editor.org/info/rfc7821>.
Authors' Addresses
Tal Mizrahi
Marvell
6 Hamda St.
Yokneam, 20692
Israel
Email: talmi@marvell.com
Danny Mayer
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: mayer@ntp.org
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
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