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........................................... 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...................................... 6
Appendix A. NTP Remote Operations Service Specification... 6
11. Security Considerations............................... 912. Authors' Addresses.................................... 9
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 in RFC 1119.
The motivation behind the implementation of a Remote Operations
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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, 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.
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-
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.
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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
contacted.
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
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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.
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!)
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].
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.
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
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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.
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 10.0 524.0
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
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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".
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.UK
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