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Versions: 00 01 02 03 04

Internet Engineering Task Force                                 H. Stenn
Internet-Draft                                   Network Time Foundation
Obsoletes: 7822 (if approved)                           December 4, 2017
Intended status: Standards Track
Expires: June 7, 2018


        Network Time Protocol Version 4 (NTPv4) Extension Fields
                  draft-stenn-ntp-extension-fields-04

Abstract

   Network Time Protocol version 4 (NTPv4) defines the optional usage of
   extension fields.  An extension field, as defined in RFC 5905
   [RFC5905] and RFC 5906 [RFC5906], 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).

   This proposal obsoletes RFC 7822 [RFC7822].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 7, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     2.2.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   4
   3.  NTP MAC - RFC 5906 Update . . . . . . . . . . . . . . . . . .   4
     3.1.  RFC5906 Section 4. - Autokey Cryptography . . . . . . . .   4
     3.2.  RFC5906 Section 10. - Autokey Protocol Messages . . . . .   4
     3.3.  RFC5906 Section 11.5. - Error Recovery  . . . . . . . . .   5
     3.4.  RFC5906 Section 13. - IANA Consideration  . . . . . . . .   5
   4.  NTP Extension Fields - RFC 5905 Update  . . . . . . . . . . .   5
     4.1.  OLD: RFC5905 7.5 - NTP Extension Field Format . . . . . .   5
     4.2.  NEW: RFC5905 Section 7.5 - NTP Extension Field Format . .   6
     4.3.  NEW: RFC5905 Section 7.5.1 - Extension Fields and MACs  .   8
       4.3.1.  Legacy MAC/EF Parsing Pseudocode  . . . . . . . . . .  10
     4.4.  OLD: RFC5905 Section 9.2. - Peer Process Operations . . .  14
     4.5.  NEW: RFC5905 Section 9.2. - Peer Process Operations . . .  14
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

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 (EFs) 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.

   Additional information about the content of a MAC is specified in RFC
   5906 [RFC5906], but since that RFC is Informational an implementor
   that was not planning to provide Autokey would likely never read that
   document.  The result of this would be interoperability problems, at
   least.  To address this problem, this proposal also includes copying
   and clarifying some of the content of RFC 5906 and putting it into




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   RFC 5905.  Because there is a reasonable chance RFC 5906 will be
   deprecated, this document does not propose changes to RFC 5906.

   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] has always clearly stated 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.

   RFC 7822 [RFC7822] was an attempt to clarify and change the rules
   around MACs, but in doing so, it completely removed the long-standing
   rule that the presence of an extension field required MAC protection,
   added an express limit to the length of a MAC, and required that all
   EFs be at least 28 octets long.  Pushing the decision about whether
   or not a packet must be authenticated later in the process reduces
   throughput performance and opens NTP up to clogging attacks.
   Expressly limiting the length of a MAC prohibits the use of longer
   MACs, should that ever be needed.  Requiring EFs to be at least 28
   octets long is needlessly wasteful.

   This proposal follows existing and proposed behavior of the NTP
   reference implementation in that it describes a simple and clean way
   to identify the case where an extension field must not have or would
   not require a MAC, allows EFs to be on 4-octet boundaries of any
   acceptable length, and provides methods to disambiguate packet
   parsing in the unexpected case where an implementation would choose
   to send a packet that could be ambiguously parsed.  This proposal
   obsoletes RFC 7822 [RFC7822].

   This document better specifies and clarifies extension fields as well
   as 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 expressly defines the behavior
   of a host that receives an unknown extension field.







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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

   EF - Extension Field

   MAC - Message Authentication Code

   NTPv4 - Network Time Protocol, Version 4 RFC 5905 [RFC5905]

3.  NTP MAC - RFC 5906 Update

   This document copies and updates some information in RFC 5906
   [RFC5906] and puts it in to RFC 5905, as follows:

3.1.  RFC5906 Section 4. - Autokey Cryptography

   This section describes some of the cryptography aspects of Autokey.
   The third paragraph describes the use of 128- and 160-bit message
   digests.  The enumeration of 128- and 160-bit message digests is not
   meant to be limiting - other message digest lengths MAY be
   implemented.  This paragraph also describes some of the recommended
   semantic ranges of the key ID.  This information belongs in RFC 5905.
   The key ID value is particularly significant because it provide
   additional disambiguation protection when deciding if the next data
   portion is either a legacy MAC or an extension field.

3.2.  RFC5906 Section 10. - Autokey Protocol Messages

   This section describes the extension field format, including initial
   flag bits, a Code field, and 8-bit Field Type, and the 16-bit Length.
   This proposal expands and clarifies this information and puts it into
   RFC 5905.

   This section says "The reference implementation discards any packet
   with a field length of more than 1024 characters." but this is no
   longer true.








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3.3.  RFC5906 Section 11.5. - Error Recovery

   This section describes the crypto-NAK, which should be described in
   RFC 5905.

3.4.  RFC5906 Section 13. - IANA Consideration

   This section lists the Autokey-related Extension Field Types,
   including Flag Bits, Codes, and Field Types, which should be
   described in RFC 5905, or perhaps in some other document.

4.  NTP Extension Fields - RFC 5905 Update

   This document updates Section 7.5 of RFC 5905 [RFC5905] as follows:

4.1.  OLD: RFC5905 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.

    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.




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4.2.  NEW: RFC5905 Section 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 SHOULD be
   either a legacy MAC or a MAC-EF.  It MAY be both.  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

   All extension fields are zero-padded to a word (four octet) boundary.
   The Field Type is specific to the defined function and detailed
   information about the Field Type is not elaborated here.  The minimum
   size of an Extension Field is a 32-bit word (4 octets), and while the
   maximum extension field size MUST be 65532 octets or less, an NTP
   packet SHOULD NOT exceed the network MTU.

   The Length field is a 16-bit unsigned integer that indicates the
   length of the entire extension field in octets, including any Padding
   octets.  The bottom two bits of the Field Length SHOULD be zero, and
   the size of the extension field SHOULD end on a 32-bit (4 octet)
   boundary.  [RFC5905 Section 7.5 says "All extension fields are zero-
   padded to a word (four octets) boundary." but does not use 'MUST'
   language.  Is it overkill to reiterate this requirement here?  Should
   we use SHOULD or MUST regarding the bottom two bits or the boundary
   of the EF?  It is possible, down the road, that we might find some
   use for those bottom 2 bits, even if we require a 32-bit boundary on
   the last octet of an EF.]

   The Field Type contains the following sub-elements:






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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
   +---------------+---------------+-------------------------------+
   |R|E|O|I|  Code |       Type    |       (Field Length)          |
   +-------------------------------+-------------------------------+

                             Field Type Format

   Where the following Field Type flags are defined:

      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 'R' flag is currently used by Autokey, and by the proposed I-DO
   extension field.  This flag is used after the packet is accepted.]

   [The 'E' flag is currently used by Autokey.  This flag is used after
   the packet is accepted.]

   [The 'O' flag is used by RFC 7821 [RFC7821], the I-DO, SUGGEST-REFID,
   and the LAST-EF proposal.  This flag is used during the initial
   packet analysis.]

   [The 'I' flag is used by the MAC-EF proposal, and would be used by
   any EF proposal that includes a MAC in its data.  This flag is used
   during the initial packet analysis.]

   [The EF Code subtype is currently used by RFC 5906, Autokey
   [RFC5906].  The EF Code subtype is used by the Extended Information
   EF proposal, and is expected to be used by the NTS Extension Field,
   at least.]

   The Field Type, Value, and Padding fields are specific to the defined
   function and are not elaborated here; appropriate Field Type flags,
   the EF Code, and EF Type values are 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, depending on local policy.

   The Length field is a 16-bit unsigned integer that indicates the
   length of the entire extension field in octets, including any
   Padding.



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   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.  The bottom two bits of the
   Field Length SHOULD be zero and the EF data SHOULD be aligned to a
   32-bit (4 octet) boundary.

4.3.  NEW: RFC5905 Section 7.5.1 - Extension Fields and MACs

   With the inclusion of additional Extension Fields, there is now a
   potential that a poorly-designed implementation would produce an
   ambiguous parsing in the presence 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 from
   an existing implementation.

   The first protection from this ambiguity comes from the fact that
   current conforming implementations only support the Autokey EF, which
   uses EF Type 2 and a legacy MAC.  While the Experimental UDP Checksum
   Complement specified by RFC 7821 [RFC7821] uses EF Type 5, it
   specifically prohibits the use of a MAC, and the 0x2000 bit in its
   assigned EF specification of 0x2005 signifies that a MAC is optional
   when this EF is provided.

   [As a side note, the requirement in RFC 7821 [RFC7821] that the UDP
   Checksum Complement EF must have a 28 octet length is demonstrably
   not needed if this proposal is accepted.  It only needs 8 octets: 4
   octets of EF header, 2 octets of must-be-zero padding, and 2 octets
   of Checksum Complement.]

   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:



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      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 followed 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, and if the packet is valid, the
         data must be a legacy MAC.

   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
   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 5906 [RFC5906], 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.  In this case, it MIGHT be a valid
   extension field.

   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.




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   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.

4.3.1.  Legacy MAC/EF Parsing Pseudocode

   Here are two potential pseudocode implementations showing how data
   after the base NTP packet could be analyzed to identify EFs and a
   possible legacy MAC.

   Example 1: Generate a list of possible parsings:



































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   struct pkt_parse {
    foo * ef_ptr;
    foo * legacy_mac;
    struct pkt_parse * next;
   };

   struct pkt_parse pkt_parse_chain = NULL;

   EOPacket = address of last data in packet;
   here = address of the EOBasePacket;
   more_efs = 1;
   while (1) {
       int candidate = 0;
       int ef_len = 0;

       if (EOPacket > here) {
           p = emalloc(pkt_parse);                 // *p is zeroed
           if (this could be a legacy MAC) {       // we know the keyid
               p->legacy_mac = here;
               candidate = 1;
           }
           if (more_efs && this could be an EF) {  // Length field valid
               p->ef_ptr = here;
               ef_len = (the length of the EF);
               here += ef_len;
               if (this is a LAST_EF) {
                   more_efs = 0;
               }
               candidate = 1;
           } else {
               more_efs = 0;
           }
       }

       if (candidate) {
           p->next = pkt_parse_chain;
           pkt_parse_chain = p;
       } else {
           free(p);
           break;
       }
   }

              Example 1: Generate a list of possible parsings

   and at this point we can scan thru the items in pkt_parse_chain to do
   deeper checks, throwing away the parsings that don't make sense.




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   This opens up more questions if we get multiple parsings and at least
   1 of them is "valid".  It's also perfectly reasonable to decide to
   produce a single parse based on precedence rules: Prefer legacy MAC,
   or prefer EF.

   Example 2: Another possible way to handle EF/legacy-MAC parsing:

   // We're at the end of the base NTP packet.
   // A legacy MAC is allowed:
   // - immediately after the base packet
   // - immediately after one or more Autokey EFs(a non-issue, below)
   // - immediately after a LAST-EF

   ef_ok = 1;                              // An EF is allowed here
   legacy_mac_ok = 1;                      // Legacy MAC allowed here
   req_mac = 0;                            // A MAC is not required
   saw_mac = 0;                            // We haven't seen a MAC yet
   authlen = LEN_PKT_NOMAC;                // Length of a base packet
   leg_mac = rbufp->recv_length - authlen; // # bytes after base

   while (leg_mac > 0) {                   // Data after base packet
           if (leg_mac % 4 != 0 || leg_mac < MIN_MAC_LEN) {
                   return: Bad packet length;
           }

           // If ef_ok, this could be an EF or legacy MAC
           skeyid = ntohl(pkt[authlen / 4]);
           opcode = skeyid >> 16;
           len = skeyid & 0xffff;

           if (ef_ok && GET_EXT_FIELD_TYPE(opcode) == EF_FT_LAST) {
                   if (leg_mac > MAX_MAC_LEN) {
                           return: Too much data after LAST_EF;
                   }
                   // Anything here MUST be a legacy MAC
                   ef_ok = 0;
                   legacy_mac_ok = 1;
           } else {
                   if (4 == leg_mac && 0 == skeyid) {
                           break;// Likely crypto-NAK
                   }

                   if (legacy_mac_ok && leg_mac <= MAX_MAC_LEN) {
                           int ksize;

                           // If we find a keyid, we know its alg/length
                           ksize = auth_findkeysize(skeyid);
                           if (ksize != -1) {



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                                   saw_mac = 1;
                                   break;
                           }
                           // If we didn't find it, it can't be a valid
                           // legacy MAC.  It's still a potential EF.
                   }

                   if (!ef_ok) {
                           break;
                   }

                   // At this point, this SHOULD be an EF

                   if (   len % 4 != 0
                       || len < 4
                       || len + authlen > rbufp-> recv_length) {
                           return: Bad length;
                   }

                   if (opcode & EF_FL_reoI) {      // EF contains MAC
                           saw_mac = 1;            // Just a hint
                   }
                   if (opcode & EF_FL_reOi) {      // MAC optional
                           // Empty
                   } else {
                           req_mac = 1;    // MAC required
                   }
                   switch (GET_EXT_FIELD_TYPE(opcode)) {
                   case EF_FT_AK:          // Autokey
                           // extract calling group name for later
                           break;
                   case EF_FT_LAST:        // LAST-EF
                           legacy_mac_ok = 1;
                           break;
                   default:
                           legacy_mac_ok = 0;
                           break;
                   }
           }

           authlen += len;
           leg_mac -= len;
   }

   if (leg_mac < 0) {
           return: Malformed packet
   }




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          Example 2: Another way to handle EF/legacy-MAC parsing

4.4.  OLD: RFC5905 Section 9.2. - Peer Process Operations

   ...

   FXMIT. ... This message includes the normal NTP header data shown in
   Figure 8, but with a MAC consisting of four octets of zeros. ...

4.5.  NEW: RFC5905 Section 9.2. - Peer Process Operations

   ...

   FXMIT. ... This message includes the normal NTP header data shown in
   Figure 8, but with a MAC consisting of four octets of zeros.  This
   can be a legacy MAC or a MAC-EF.  If it's a MAC-EF, the crypto-NAK
   MUST be the only MAC in the MAC-EF payload.  ...

5.  Acknowledgements

   The author wishes to acknowledge the contributions of Sam Weiler.

6.  IANA Considerations

   This memo requests IANA to allocate the following bits in the NTP
   Extension Field Types table:

      0x8000: R: Response (0: Request, 1: Response)

      0x4000: E: Error (0: OK, 1: Error)

      0x2000: O: MAC Optional (0: MAC required, 1: MAC optional)

      0x1000: I: MAC Included (0: MAC not included, 1: MAC included)

   The following table should be the same as the existing NTP Extension
   Field Table, reformatted to account for the new flag bits.

    0           1
    0123 4567 89012345   What:
   +----+----+--------+
   |REOI|Code|  Type  |
   +----+----+--------+
   |0000|  0 |     0  |  crypto-NAK (with Field Length of 0)
   |    |    |     0  |  RESERVED: Permanently Unassigned
   +----+----+--------+
   |    |    |     1  |  RESERVED: Unassigned
   +----+----+--------+



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   |0000|  0 |     2  |  Autokey: No-Operation Request
   |1000|  0 |     2  |  Autokey: No-Operation Response
   |1100|  0 |     2  |  Autokey: No-Operation Error Response
   +----+----+--------+
   |0000|  1 |     2  |  Autokey: Association Message Request
   |1000|  1 |     2  |  Autokey: Association Message Response
   |1100|  1 |     2  |  Autokey: Association Message Error Response
   +----+----+--------+
   |0000|  2 |     2  |  Autokey: Certificate Message Request
   |1000|  2 |     2  |  Autokey: Certificate Message Response
   |1100|  2 |     2  |  Autokey: Certificate Message Error Response
   +----+----+--------+
   |0000|  3 |     2  |  Autokey: Cookie Message Request
   |1000|  3 |     2  |  Autokey: Cookie Message Response
   |1100|  3 |     2  |  Autokey: Cookie Message Error Response
   +----+----+--------+
   |0000|  4 |     2  |  Autokey: Autokey Message Request
   |1000|  4 |     2  |  Autokey: Autokey Message Response
   |1100|  4 |     2  |  Autokey: Autokey Message Error Response
   +----+----+--------+
   |0000|  5 |     2  |  Autokey: Leapseconds Value Message Request
   |1000|  5 |     2  |  Autokey: Leapseconds Value Message Response
   |1100|  5 |     2  |  Autokey: Leapseconds Value Msg Error Response
   +----+----+--------+
   |0000|  6 |     2  |  Autokey: Sign Message Request
   |1000|  6 |     2  |  Autokey: Sign Message Response
   |1100|  6 |     2  |  Autokey: Sign Message Error Response
   +----+----+--------+
   |0000|  7 |     2  |  Autokey: IFF Identity Message Request
   |1000|  7 |     2  |  Autokey: IFF Identity Message Response
   |1100|  7 |     2  |  Autokey: IFF Identity Message Error Response
   +----+----+--------+
   |0000|  8 |     2  |  Autokey: GQ Identity Message Request
   |1000|  8 |     2  |  Autokey: GQ Identity Message Response
   |1100|  8 |     2  |  Autokey: GQ Identity Message Error Response
   +----+----+--------+
   |0000|  9 |     2  |  Autokey: MV Identity Message Request
   |1000|  9 |     2  |  Autokey: MV Identity Message Response
   |1100|  9 |     2  |  Autokey: MV Identity Message Error Response
   +----+----+--------+
   |0010|  0 |     5  |  Checksum Complement
   +----+----+--------+

                         Current Extension Fields







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7.  Security Considerations

   Additional information TBD

8.  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>.

   [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>.

   [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>.

   [RFC7821]  Mizrahi, T., "UDP Checksum Complement in the Network Time
              Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
              2016, <https://www.rfc-editor.org/info/rfc7821>.

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <https://www.rfc-editor.org/info/rfc7822>.

Author's Address

   Harlan Stenn
   Network Time Foundation
   P.O. Box 918
   Talent, OR  97540
   US

   Email: stenn@nwtime.org













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