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Network Working Group J. Crowcroft Request for Comments: 1165 UCL

                                                             J. Onions
                                                 Nottingham University
                                                             June 1990
              Network Time Protocol (NTP) over the OSI
                     Remote Operations Service

Status of this Memo

 This memo suggests an Experimental Protocol for the OSI and Internet
 communities.  Hosts in either community, and in particular those on
 both are encouraged to experiment with this mechanism.  Please refer
 to the current edition of the "IAB Official Protocol Standards" for
 the standardization state and status of this protocol.  Distribution
 of this memo is unlimited.

Table of Contents

 1. Introduction...........................................    1
 1.1 Motivation............................................    1
 2. Protocol Overview......................................    2
 3. Operation of the Protocol..............................    3
 4. Network Considerations.................................    4
 5. Implementation Model...................................    4
 6. Constructing NTP Data Fields...........................    4
 7. Discussion.............................................    4
 8. Prototype Experience...................................    5
 9. References.............................................    5
 10. Acknowledgements......................................    6
 Appendix A. NTP Remote Operations Service Specification...    6
 11. Security Considerations...............................    9
 12. Authors' Addresses....................................    9

1. Introduction

 This document describes the Remote Operations and Abstract Syntax for
 the operation of the Network Time Protocol (NTP) over an ISO OSI
 NTP itself is documented in great detail in RFC 1119.

1.1 Motivation

 The motivation behind the implementation of a Remote Operations

Crowcroft & Onions [Page 1] RFC 1165 NTP over OSI June 1990

 Service implementation of NTP is fourfold.
    1.  The inclusion of a useful service to an OSI
    2.  The feasibility of automatically checking a ROS/ASN.1
        specification, and automatically generating code to
        implement the protocol.
    3.  The feasibility of running NTP on connection oriented
        network services (CONS or X.25), and consequentially,
        the ability to use connection success or failure to
        optimise reachability discovery.
    4.  The generalisation of the last point: the use of ROS
        makes NTP independent of the underlying communications
 The need for time synchronisation is clear, and RFC 1119 indicates a
 few of the necessary uses of this service.  However, it is becoming
 clear that OSI applications are very much in need of this service
 too.  Not just in the local context but across the wide area.  For
 example much of the strong authentication outlined in X.511 is based
 on encrypted packets with time stamps to indicate how long the packet
 is valid for.  If two hosts have clocks that are not closely
 synchronised, the host with the faster clock will be more prone to
 cryptographic attacks from the slower, and the slower host will
 possibly find it is unauthentable.
 A similar problem occurs with the X.500 directory and the service
 control limiting the time allowed for the search.
 Authentication between NTP peers and between clients and servers is
 not addressed here, as the choice of mechanism is still the subject
 of some debate.

2. Protocol Overview

 The NTP application functions exactly as in RFC 1119.  The use of
 remote operations and the underlying Application support means that
 for NTP daemons to peer with one another, they send an A-
 On successful association, they subsequently periodically invoke the
 appropriate Remote Operation with the appropriate parameters at the
 appropriate frequency.
 On failure, they mark the peer as unreachable.

Crowcroft & Onions [Page 2] RFC 1165 NTP over OSI June 1990

 The states that an ntp daemon records for each peer are enhanced from
 RFC 1119 to include:
    Connected: this indicates the host is connected with its peer and
    synchronisation data is being exchanged.
    Connecting: this state indicates that a connection is in progress.
    Hosts at large distances may take several seconds to connect, and
    such blocking can perturb the exchange of data with other hosts.
    Therefore, the connection is made asynchronously.
    Accepting: this state indicates that a connection is being
    accepted from another host, but the necessary negotiation of
    transport session etc has not been fulfilled yet.  This is another
    asynchronous part.
    Disconnected: this state is reached if the remote host cannot be

3. Operation of the Protocol

 The use of a connection oriented service means that the operation of
 the NTP algorithm is slightly different.  This stems firstly from
 some necessary adjustments made to the protocol and secondly from
 some optimisations that are possible through the use of connections.
 Firstly, the reachability of the host can be directly determined.
 The NTP protocol maintains a shift register to determine if it is
 likely that a peer is still responding and exchanging data.  This
 works by recording over the last eight transfers how many responses
 have been received.  If there have been no responses to the last
 eight packets, then the host is deemed unreachable.
 Naturally, with a connection to the remote host, the reachability is
 immediately determinable.  Either a connection is established or the
 connection is broken or not yet made.  For this reason it is not
 necessary to rely on the shift register to determine reachability.
 Secondly, there are a large number of optimisations that can be made
 by use of the connection oriented mode.  The NTP packet format can be
 broken into several categories.
    a) Synchronisation data
    b) Authentication data
    c) Protocol data

Crowcroft & Onions [Page 3] RFC 1165 NTP over OSI June 1990

 Of these classes of data, only the first (a) is necessary to maintain
 the synchronisation between hosts.  Information such as protocol
 version and the precision of the local clock are not likely to vary
 over the lifetime of the connection.  Likewise the authentication if
 in use need only be done at connection establishment and is not
 necessarily required for every packet.
 For these reason, the NTP protocol can be simplified slightly to
 remove this information.  This can be seen in the specification for
 the Packet in Appendix A.

4. Network Considerations

 Although on first inspection it might be thought that a high speed
 network is necessary for accurate synchronisation, this is not the
 case.  What is more important is the dispersion of the packet
 traversal times.  It is normally the case that a low speed network
 with little variance in packet transit times will give better results
 than a high speed network with large differences in individual packet
 transit times.  This would lead us to think that connection oriented
 networks with resource allocation done at connection time might lead
 to higher accuracies than connectionless networks which can suffer
 large swings in packet transit time under high loading.  (This is

5. Implementation Model

 Ideally, the implementor will provide interoperability between the
 existing UDP based NTP service, and a ROS based service.
 To this end, the internal records that hold NTP state information,
 can be kept the same as existing implementations, and for
 optimisation reasons, the internal representations of NTP packets can
 be the same.  Translation between these and appropriate ROS/ASN
 concrete encodings can be provided by automatic translators such as
 Rosy [ISODE].

6. Constructing NTP Data Fields

 The way in which the data fields in the Packet described in Appendix
 A is unchanged from RFC 1119.  This simplifies implementations based
 on existing ones, and encourages interworking.

7. Discussion

 From the limited testing of this model so far done, the results would
 seem to indicate that the ROS based model running over an X.25
 service is of similar reliability as the UDP model.  Until further

Crowcroft & Onions [Page 4] RFC 1165 NTP over OSI June 1990

 experimentation can be performed, specific data can not be given.
 However, in the UK where the most common method of time
 synchronisation is the system administrators watch and typing in the
 time to the nearest minute, this method is clearly far superior.
 Connection management is transparent to NTP since it is implemented
 beneath the Remote Operations Service.  However, an NTP
 implementation must have access to the status of connections, and
 uses this not only for reachability information but also to find the
 information gleaned at connect time and no longer exchanged in NTP

8. Prototype Experience

 There are a number of UK sites running NTP over ROS over X.25 with an
 earlier ROS specification, with at least one site peering both over
 ROS with UK sites on X.25, and over UDP with US Internet sites.
 Initial experience is promising.  The table below shows the
 reachabilities, delays, offsets and dispersions for the central UK
 site peering with 2 JANET sites (IP addresses not meaningful, but
 shown as, and three US sites.
    Address            Strat Poll Reach    Delay   Offset    Disp
    +            3   64  377     718.0      0.0      3.0         1 1024  177     535.0     13.0     13.0
    *            1   64  167     545.0     10.0    524.0

9. References

 1.  Mills, D., "Network Time Protocol (Version 2) Specification and
     Implementation", RFC-1119, UDEL, September 1989.
 2.  Mills, D., "Algorithms for Synchronizing Network Clocks", RFC-
     956, M/A-COM Linkabit, September 1985.
 3.  Postel, J. "User Datagram Protocol", RFC-768, USC Information
     Sciences Institute, August 1980.
 4.  ISO TC97, "Specification of Abstract Syntax Notation One
     (ASN.1)", Draft International Standard ISO/DIS 8824, 6 June 1985.
 5.  CCITT, "Remote Operations: Model, Notation and Service
     Definition", CCITT X.ros0 or ISO/DP 9072/1, Geneva, October 1986.
 6.  Mills, D., "Internet Time Synchronization: The Network Time

Crowcroft & Onions [Page 5] RFC 1165 NTP over OSI June 1990

     Protocol (NTP)", RFC 1129, UDEL, October 1989.
 7.  Mills, D., "Measured Performance of the Network Time Protocol in
     the Internet System", RFC 1128, October 1989.
 8.  Rose M., et al, "The ISO Development Environment: User's Manual".

10. Acknowledgements

     The Authors would like to thank Dave Mills for his valuable
     comments on an earlier version of this document.

Appendix A. ROS "Header" Format

  1. - NTP definitions for ROS specification
  2. -
  3. - Julian Onions, Nottingham University, UK.
  4. -
  5. - Mon Jun 5 10:07:07 1989
  6. -
     update OPERATION
      ARGUMENT Packet
      ::= 0
     query OPERATION
      RESULT ClockInfoList
      ::= 1
  1. - Data Structures
     BindArgument ::=
      fullbind SEQUENCE {
              psap[0] IA5String OPTIONAL,
              version[1] BITSTRING {
              } DEFAULT version-2,
              authentication[2] Authentication OPTIONAL,
              mode[3] BindMode

Crowcroft & Onions [Page 6] RFC 1165 NTP over OSI June 1990

     Authentication ::= ANY
     BindMode ::= ENUMERATED {
              normal(0),      -- standard NTP
              query(1)        -- queries only
     BindResult ::=
      SEQUENCE {
              version[1] INTEGER DEFAULT 2,
              authentication[2] Authentication OPTIONAL,
              mode[3] BindMode
     BindError ::=
      SEQUENCE {
              reason[0] INTEGER {
                      version(2),     -- version not supported
                      badarg(3),      -- bad bind argument
                      congested(4)    -- catch all!
              supplementary[1] IA5String OPTIONAL
  1. - basic exchange packet
     Packet ::= SEQUENCE {
      leap                    Leap,
      mode                    Mode,
      stratum[1]              INTEGER,
      pollInterval[2]         INTEGER,
      precision[3]            INTEGER,
      synchDistance           SmallFixed,
      synchDispersion         SmallFixed,
      referenceClockIdentifier ClockIdentifier,
      referenceTimestamp      TimeStamp,
      originateTimestamp      TimeStamp,
      receiveTimestamp        TimeStamp,
      transmitTimestamp       TimeStamp
     ClockInfoList ::= SET OF ClockInfo
     ClockInfo ::= SEQUENCE {
      remoteAddress           Address,

Crowcroft & Onions [Page 7] RFC 1165 NTP over OSI June 1990

      localAddress            Address,
      flags[0]                BIT STRING {
      packetsSent[1]          INTEGER,
      packetsReceived[2]      INTEGER,
      packetsDropped[3]       INTEGER,
      timer[4]                INTEGER,
      leap                    Leap,
      stratum[5]              INTEGER,
      ppoll[6]                INTEGER,
      hpoll[7]                INTEGER,
      precision[8]            INTEGER,
      reachability[9]         INTEGER,
      estdisp[10]             INTEGER,
      estdelay[11]            INTEGER,
      estoffset[12]           INTEGER,
      reference[13]           ClockIdentifier OPTIONAL,
      reftime                 TimeStamp,
      filters                 SEQUENCE OF Filter
              integer INTEGER,
              fraction INTEGER
     ClockIdentifier ::= CHOICE {
                      referenceClock[0] PrintableString,
                      inetaddr[1] OCTET STRING,
                      psapaddr[2] OCTET STRING

Crowcroft & Onions [Page 8] RFC 1165 NTP over OSI June 1990

              integer INTEGER,
              fraction INTEGER
              unspecified (0),
              symmetricActive (1),
              symmetricPassive (2),
              client (3),
              server (4),
              broadcast (5),
              reservered (6),
              private (7)
     Filter ::= SEQUENCE {
              offset INTEGER,
              delay INTEGER
     Address ::= OCTET STRING -- for now

11. Security Considerations

 Security issues are not discussed in this memo.

12. Authors' Addresses

 Jon Crowcroft
 Computer Science Department
 University College London
 Gower Street
 London WC1E 6BT UK
 Julian P. Onions
 Computer Science Department
 Nottingham University
 University Park
 Nottingham, NG7 2RD UK

Crowcroft & Onions [Page 9] RFC 1165 NTP over OSI June 1990

Crowcroft & Onions [Page 10]

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