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Versions: 00                                                            
Network Working Group                               J-F. Boudreault
Internet-Draft                                      Marc Blanchet
Expires: May 13, 2002                               Viagenie inc.
                                                    November 13, 2001

                        Reference ID for NTPv6

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on May 13, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   This document proposes a solution to solve the reference ID
issue for IPv6 NTP secondary server.

Table of Contents

   1.  Definition of Reference ID in NTP Protocol
   2.  Problem Statement
   3.  Protocol Change
        3.1 Description
        3.2 Impacts
        3.3 Implementation
   4.  Using Timestamps
        4.1 Description
        4.2 Impacts
        4.3 Implementation
   5.  Ipv6 Address Compression
        5.1 Description
        5.2 Impacts
        5.3 Implementation
   6.  Discussion on solutions
   7.  Acknowledgements

1. Definition of reference ID in NTP protocol

        According to RFC 1305 (Network Time Protocol Version 3),
Reference Clock Identifier "is a 32-bit code identifying the
particular reference clock. In the case of stratum 0 (unspecified)
of stratum 1 (primary reference), this is a four-octet,
left-justified, zero-padded ASCII string. [...] In the case of
stratum 2 and greater (secondary reference) this is the four-octet
Internet address of the primary reference host." Reference ID is
included as a 32-bits field in every NTP packet.

        This reference ID is essential to avoid 'loopback' during the
selection of synchronization source for NTP server. A 'loopback' happens
when a NTP server choose a reference source that is a secondary NTP server
who use itself as a source. This result in false synchronization.
Reference ID being the IPv4 address of the reference source, one can avoid
'loopback' by comparing this field (from NTP server reply packet) with our
IPv4 address, and reject the potential source if they are equal.

2. Problem Statement

        The Reference ID is a 32 bits field in the NTP packet definition.
It is enough for IPv4 32 bits addresses but not enough for IPv6 128 bits
addresses. There is a problem if a secondary server is using a IPv6-only
NTP server as reference source. We need a way to avoid 'loopback' for
synchronization of secondary NTP server with a IPv6-only NTP server.

        In this document we present our understanding of three
possible solutions. The first solution is to change the size of
reference ID field in NTP packet definition from 32 bits to 128 bits.
The second solution is to use timestamps informations to avoid 'loopback'.
The third solution is to apply an algorithm the IPv6 address to compress
it to 32 bits and use these IPv6-compressed address in reference ID.

3. Protocol Change

3.1 Description

        A solution is to change the reference ID field from 32 bits to
        128 bits. This way one could put IPv6 source as reference ID
        and IPv6 embedded IPv4 for IPv4 source. We need to change NTP
        packet definition to modify reference ID field from 32 bits to
        128 bits.

3.2 Impacts

        By using a new protocol definition for ntp packet, this breaks
        the support for old version of ntp. To support them, one needs
        to implement both versions of ntp packet, one with 32 bits
        reference id and one with 128 bits reference id. When a server
        receives a packet, it first puts it in a buffer structure to
        look at the version field, and decide which structure to use
        according to ntp version. When server will reply to request,
        it will need to make sure it respond with the same version
        and use the correct packet definition that the client support.
        If a secondary server using IPv6 reference source receive a
        request from an old version client, it will need to ignore it
        because it will not be able to put his reference id in the old ntp
        packet definition. This will need an important redefinition
        of ntp specifications.

3.3 Implementation

        To change the size of reference ID, one needs to modify
        the definition of NTP packet structure. This will have
        big impact on compatiblity with older versions of NTP.
        To maintain this compatiblity, one will need to support
        old packet format. It means that it needs to implement
        both NTP packet structure definitions, one with a
        32 bits reference ID field and one with a 128 bits
        reference ID field.

        In reception procedure, the server will need to first put the
        packet in a NTP structure buffer, analyze the version
        of the packet and put it in the correct NTP packet

        In transmission procedure, the server will need to first know
        the NTP version of the destination, and put
        informations in correct packet format. In every
        procedure that use reference ID, one will need to support
        both versions and test every time to know which version
        is used.

4. Using Timestamps

4.1 Description

        This was first proposed by David L. Mills in RFC 2030 (Simple Network
        Time Protocol Version 4) to resolve reference id problem for IPv6
        source. By using other informations from ntp packet, one could avoid
        loopback by comparing timestamps of the last transmitted packet from
        reference source and originate timestamp of the reply from ntp server.
        When a reference source is selected, each time ntp server receive a
        packet from this source, it will update his reference id to the low
        32 bits of transmit timestamp field from this ntp packet.

        When a ntp server is looking for synchronization source, it will
        compare the low 32 bits from originate timestamp field of ntp reply
        packet (this will correspond to his transmit timestamp from request)
        with reference id field. If they are equal, that is because this server
        updated his reference id after received the request packet, and
        that means that it is using this server as synchronization source.
        In this case it needs to be discarded.

4.2 Impacts

        It could append that a ntp request packet is send to a
        ntp server at the same time than the reference source of
        this server send it a packet. By this way, the ntp
        server will adjust his reference id to transmit time from
        his ntp source. If this value is the same than the
        transmit timestamp of ntp request packet, the requester
        will reject this server even if it is not using itself
        as a source. This way could reject a good potential
        source of synchronization.

4.3 Implementation

        In clock update procedure, when the stratum of system
        is more than 1 (secondary server), the server will put the low
        32 bits from transmit timestamp of the last received
        NTP packet from reference source.

        In clock selection procedure, the server needs to modifiy
        loopback detection test to compare reference ID
        field with originate timestamp field.

        In reception procedure, every time the server receive a packet
        from our reference source, it will need to update reference
        ID with the low 32 bits of the transmit timestamp field.

        Depending on the exact implementation of NTP, one need
        to be sure we can obtain the informations we need to
        use this solution. That could be a problem in some

5. IPv6 Address Compression

5.1 Description

        By applying a compression algorithm, one could compress
        IPv6 addresses to 32 bits and put the compressed address
        as the Reference Id. The proposed way is to ignore
        some bits because of some assumptions in the use of these
        bits in the IPv6 address.

        In order to avoid to overlap the IPv4 address space in the
        reference ID field, the leftmost 3 bits of the Reference ID
        field is '111'b when an IPv6 address is compressed in the
        reference ID. '111' in the first 3 bits correspond to the
        Class D and E IPv4 addresses. Since this address space is
        not used for Reference ID field, this gives no overlap between
        the two spaces.

        The IPv6 address architecture [RFC2373] defines specific
        boundaries in the address as shown below.

         | 3 |  13 |          32         |   16   |    64 bits      |
         |001| TLA |       NLA ID        | SLA ID | Interface ID    |

        The last 64 bits is the interface ID. If the NTP server uses
        autoconfig with a unique layer2 identifier, then this is identified
        by a "uniqueness" bit in the EUI-64 format used in the Interface ID.
        This identifier is used for the Reference ID. If the identifier is
        more than 32 bits (for example: Ethernet 48 bits), then the
        last rightmost 28 bits (32 - 3 - 1 bits) are used for the Reference ID.
        and one bit is 1 for identifying a unique address.

        If the NTP server is not having a unique interface identifier
        in its address, then the following assumptions are made:
        - we expect that most NTP server sites that are interconnected
         will only have one or a few NTP servers. This means that the
        differentiator is mostly based on the identification of the site
        instead of the host. The algorithm will try to use most of the
        site identification (the first 48 bits) instead of the subnet id
        (the next 16 bits) or interface id (the last 64 bits).
        - because of the compression of 0 in writing IPv6 addresses makes
        easier to put most zeros in a manually assigned address, then
        most of the leftmost bits in the rightmost 64 bits of the
        address will be all zeros. So this compression algorithm will
        only use the rightmost 4 bits of the rightmost 64 bits of the
        - for the same reason of zero compression for writing, the
        SLA ID will usually have many zeros at the left. This algorithm
        only uses the rightmost 4 bits of this field.
        - since the current addressing architecture is only defined for
        the 001 as the first three bits, these bits are redundant and
        not used.
        - this leaves us with 45 bits of TLA/NLA bits and only 32-3-4-4=21
        bits left. For no real reasons except the basis of the current
        allocation policies, this algorithm uses the
        rightmost 4 bits of the TLA field and the rightmost 17 bits of the
        NLA field.

        The chosen bits are identified with the number of bits below
        the following figure.

         | 3 |  13 |          32         |   16   |    64 bits      |
         |001| TLA |       NLA ID        | SLA ID | Interface ID    |
                 |4|         |    16     |      |4|               |4|

5.2 Algorithm

5.2.1 Unique Interface ID of the IPv6 address based on EUI-64

        If the IPv6 address has the "uniqueness" bit on, then
        the reference ID is composed of the following fields:
         | 4  |  4 |       24                |
         |1111| A  |        B                |
         |                 32                |

        The compression algorithm is the following:
        - the first leftmost 4 bits are '1111'b.
        - the next 4 bits, identified as A in the previous figure,
        are the 16-19 bits of the Interface ID
        of the IPv6 address, counting with 0 as the leftmost bit
        of the Interface ID field (of 64 bits).
        - the next 24 bits, identified as B in the previous figure,
        are the rightmost 24 bits of the IPv6 address.

        TBD: discussion on 64 bits EUI-64 identifiers

5.2.2 Non unique Interface ID

        The reference ID is then composed of the following fields:
         | 4  |  4 |       16        | 4  |  4 |
         |1110| A  |        B        | C  | D  |
         |                32                   |

        The compression algorithm is the following:
        - the rightmost 3 bits are '111'b
        - the next field, identified as A, is of 4 bits and the bits
        are the 12-15th bits of the IPv6 address, starting with zero as the
        leftmost bit.
        - the next field, identified as B, is of 16 bits and the bits
        are the 42-47th bits of the IPv6 address, starting with zero as the
        leftmost bit.
        - the next field, identified as C, is of 4 bits and the bits
        are the 60-63th bits of the IPv6 address, starting with zero as
        the leftmost bit.
        - the last field, identified as D, is of 4 bits and the bits
        are the rightmost 4 bits of the IPv6 address.

5.2 Impacts

        This compression algorithm is obviously going to produce some
        false hits. Our current understanding of the previous proposal
        based on Timestamps tells us that that proposal also exhibits
        some false hits.  Which one is more probable than the other
        is difficult to guess. We consider for now essentially equal
        unless the contrary is demonstrated.

        By compressing IPv6 address, it could append that two
        IPv6 addresses give the same 32-bits compressed address.
        If this append, a ntp server could reject a source that
        use a reference server with the same IPv6-compressed
        address. So we could reject a good potential source of
        synchronization due to IPv6-compressed addresses conflict.

        TBD: discussion on other addresses: link-local, site-local,
                ipv4-compatible, 6to4, ...

5.3 Implementation

        Each time one uses IPv6 address to create or compare reference id, we
        just need to apply the algorithm to obtain 32 bits IPv6-compressed
        address that will fit the field. By this way we will be able to
        avoid 'loopback' by comparing reference source IPv6-compressed
        address with our IPv6-compressed address. They will be the same if
        we are the reference source and we apply the same algorithm to
        compress IPv6 address.

        Every time we set reference ID from IP address or
        compare IP address with reference ID, we will apply
        a algorithm to the IP address. This algorithm will
        return a 32 bits address corresponding to the complete
        IPv4 address or compressed IPv6 address.

        The key advantage of this proposal is the fact that it is very
        easy to compute the reference ID. In fact, it has been implemented
        with a small C macro on the current IPv6 branch of the ntpd code.

        It also brings backward compatibility with IPv4 servers already
        in place.

6. Discussion on Solutions

        There is no 'perfect' solution for this problem. Every
        solutions has some consequences. In both case of using
        timestamps or IPv6 compressed address, we could have
        conflicts and reject good potential source of reference.
        And changing of the protocol definition without braking
        compatibility with older version has important implications
        for deployment and version negociation.

        We believe that using IPv6-compressed address is the
        better solution. Being easy to implement, we could
        minimise the possiblity of conflict with a good
        compression algorithm.

7. Acknowledgements

8. References

9. Authors' Addresses

   Jean-Francois Boudreault
   Viagenie inc.
   2875 boul. Laurier, bureau 300
   Sainte-Foy, QC  G1V 2M2

   Phone: +1 418 656 9254
   EMail: Jean-Francois.Boudreault@viagenie.qc.ca
   URI:   http://www.viagenie.qc.ca/

   Marc Blanchet
   Viagenie inc.
   2875 boul. Laurier, bureau 300
   Sainte-Foy, QC  G1V 2M2

   Phone: +1 418 656 9254
   EMail: Marc.Blanchet@viagenie.qc.ca
   URI:   http://www.viagenie.qc.ca/

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