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Network Working Group                                       J. CrowcroftRequest for Comments:  1165                                          UCL                                                               J. Onions                                                   Nottingham University                                                               June 1990Network Time Protocol (NTP) over the OSIRemote Operations ServiceStatus 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 Contents1. Introduction...........................................11.1 Motivation............................................12. Protocol Overview......................................23. Operation of the Protocol..............................34. Network Considerations.................................45. Implementation Model...................................46. Constructing NTP Data Fields...........................47. Discussion.............................................48. Prototype Experience...................................59. References.............................................510. Acknowledgements......................................6Appendix A. NTP Remote Operations Service Specification...611. Security Considerations...............................912. Authors' Addresses....................................91.  Introduction   This document describes the Remote Operations and Abstract Syntax for   the operation of the Network Time Protocol (NTP) over an ISO OSI   stack.   NTP itself is documented in great detail inRFC 1119.1.1  Motivation   The motivation behind the implementation of a Remote OperationsCrowcroft & 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          environment.      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          architecture.   The need for time synchronisation is clear, andRFC 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 inRFC 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-   ASSOCIATE.REQUEST, and receive an A-ASSOCIATE.INDICATION.   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 fromRFC 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      contacted.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 dataCrowcroft & 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 inAppendix 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   heresy!)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 inAppendixA is unchanged fromRFC 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 furtherCrowcroft & 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   operations.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 126.0.0.1), and three US sites.      Address            Strat Poll Reach    Delay   Offset    Disp      =============================================================      +126.0.0.1            3   64  377     718.0      0.0      3.0      +umd1.umd.edu         1 1024  177     535.0     13.0     13.0      *128.4.0.5            1   64  167     545.0     10.0    524.09.  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 TimeCrowcroft & 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       -- NTP definitions for ROS specification       --       -- Julian Onions, Nottingham University, UK.       --       -- Mon Jun  5 10:07:07 1989       --       NTP DEFINITIONS ::=       BEGIN       update OPERATION        ARGUMENT Packet        ::= 0       query OPERATION        ARGUMENT NULL        RESULT ClockInfoList        ::= 1       -- Data Structures       BindArgument ::=        fullbind SEQUENCE {                psap[0] IA5String OPTIONAL,                version[1] BITSTRING {                        version-0(0),                        version-1(1),                        version-2(2)                } 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 {                        refused(0),                        validation(1),                        version(2),     -- version not supported                        badarg(3),      -- bad bind argument                        congested(4)    -- catch all!                },                supplementary[1] IA5String OPTIONAL        }                                        -- 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 {                        configured(0),                        authentable(1),                        sane(2),                        candidate(3),                        sync(4),                        broadcast(5),                        referenceClock(6),                        selected(7),                        inactive(8)        },        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       }       Leap ::= [APPLICATION 0] ENUMERATED {                nowarning(0),                plussecond(1),                minussecond(2),                alarm(3)        }       SmallFixed ::= [APPLICATION 1] IMPLICIT SEQUENCE {                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       TimeStamp ::= [APPLICATION 2] IMPLICIT SEQUENCE {                integer INTEGER,                fraction INTEGER        }       KeyId ::= [APPLICATION 4] INTEGER       Mode ::= [APPLICATION 4] ENUMERATED {                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       END11. 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   EMail:  JON@CS.UCL.AC.UK   Julian P. Onions   Computer Science Department   Nottingham University   University Park   Nottingham, NG7 2RD UK   EMail:  JPO@CS.NOTT.AC.UKCrowcroft & Onions                                              [Page 9]

RFC 1165                      NTP over OSI                     June 1990Crowcroft & Onions                                             [Page 10]