NTP over OSI: Network Time Protocol (NTP) over the OSI Remote Operations Service
Running NTP over the OSI Remote Operations Service instead of UDP.
Crowcroft, J. and J.P. Onions, "Network Time Protocol (NTP) over the OSI Remote Operations Service", RFC 1165, DOI 10.17487/RFC1165, June 1990, <https://www.rfc-editor.org/info/rfc1165>.
@misc{rfc1165,
series = {Request for Comments},
number = 1165,
howpublished = {RFC 1165},
publisher = {RFC Editor},
doi = {10.17487/RFC1165},
url = {https://www.rfc-editor.org/info/rfc1165},
author = {J. Crowcroft and J.P. Onions},
title = {{Network Time Protocol (NTP) over the OSI Remote Operations Service}},
pagetotal = 10,
year = 1990,
month = jun,
} RFC text
Contents
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
stack.
NTP itself is documented in great detail in RFC 1119.
1.1 Motivation
The motivation behind the implementation of a Remote Operations
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.
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-
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.
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.
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
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
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 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
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 10.0 524.0
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
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
}
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,
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
}
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
END
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
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
AI ELI5
On this page
§1Running NTP Over OSI
This RFC describes how to implement the Network Time Protocol over an ISO OSI stack, specifically using the Remote Operations Service (ROS) rather than UDP. NTP itself is documented in RFC 1119.
While NTP was originally designed for the Internet using UDP, it is also needed by OSI applications. The challenge is that OSI typically uses connection-oriented services like X.25 rather than connectionless UDP.
Computers that speak OSI need to tell time to each other, just like Internet computers do. This explains how to make NTP work in that world.
§2Why Use Remote Operations Service
There are four main reasons to implement NTP over ROS. First, it provides a useful time synchronization service to the OSI community. Second, ROS and ASN.1 specifications can be automatically checked and have code generated from them, making implementation easier. Third, NTP can run over connection-oriented services, and connection success or failure can directly determine reachability instead of relying on shift registers.
Fourth, using ROS makes NTP independent of the underlying communications architecture. Time synchronization is critical for OSI applications: for example, X.511 authentication relies on encrypted packets with time stamps. If two hosts have unsynchronized clocks, the host with the faster clock becomes vulnerable to cryptographic attacks.
Connecting like a phone call instead of sending a postcard has advantages: you know right away if the other computer is there, and you do not have to keep resending the same information.
§3Protocol Overview and Connection States
The NTP application functions exactly as RFC 1119 defines it. NTP daemons peer with one another by sending an A-ASSOCIATE.REQUEST and receiving an A-ASSOCIATE.INDICATION. On successful association, they periodically invoke the appropriate Remote Operation with the correct parameters at the appropriate frequency. On failure, they mark the peer as unreachable.
The ROS-based implementation adds new states for each peer: Connected (the host is connected and exchanging synchronization data), Connecting (a connection is in progress), Accepting (a connection is being accepted but negotiation is not yet complete), and Disconnected (the remote host cannot be contacted).
When using a phone call, you dial the other computer to connect. While connected, you can check the time whenever you want. If the call does not go through, you know the computer is not available.
§4Simplified Data Format
With a connection-oriented service, the NTP packet can be simplified. In standard NTP over UDP, every packet must include all necessary information: synchronization data, authentication data, and protocol data such as version and clock precision. But on an established connection, some of this information does not change across multiple exchanges.
Authentication (if used) needs to be done at connection establishment but not necessarily for every packet. Protocol version and local clock precision are unlikely to vary over the lifetime of the connection. The ROS-based NTP therefore removes this redundant information from the basic packet, keeping only the synchronization data (timestamps, stratum, and clock offsets).
On a phone call with the same person, you do not have to introduce yourself every time you speak. NTP over ROS works the same way.
§5Network Properties and Prototype Results
An important insight for time synchronization is that high network speed is less important than the consistency of packet transit times. A low-speed network with little variance in transit times produces better results than a high-speed network with large variations. This suggests that connection-oriented networks with resource allocation at connection time might actually provide better accuracy than connectionless networks, which can suffer large swings in transit time under high load.
Prototype implementations from UK sites show promising results. Sites running NTP over ROS over X.25 achieved similar reliability to the UDP model. These prototypes successfully peered both over ROS with UK sites on X.25 and over UDP with US Internet sites, demonstrating interoperability across different transport mechanisms.
A steady slow road is better for telling time than a fast road with traffic jams. Connection-oriented networks are more steady.