Network Time Protocol Working J. Burbank (Editor)
Group JHU/APL
Internet-Draft J. Martin (co-Editor)
Expires: January 9, 2006 Netzwert AG
July, 2005
The Network Time Protocol Version 4 Protocol Specification
<draft-ietf-ntp-ntpv4-proto-00>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 3 of RFC 3978. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she becomes aware will be disclosed, in accordance with
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This document is a submission of the IETF NTP WG. Comments should be
directed to the NTP WG mailing list, ntpwg@lists.ntp.isc.org.
This Internet-Draft will expire on January 9, 2006.
Abstract
The Network Time Protocol (NTP) is widely used to synchronize
computer clocks in the Internet. This memorandum describes
Version 4 of the NTP (NTPv4), introducing several changes from
Version 3 of NTP (NTPv3) described in RFC 1305, including the
introduction of a modified protocol header to accomodate Internet
Protocol Version 6. NTPv4 also includes optional extensions to the
NTPv3 protocol,including an anycast mode and an authentication scheme
designed specifically for multicast and anycast modes.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. NTPv4 Protocol Operation . . . . . . . . . . . . . . . . . . . 4
3. NTPv4 Timestamp Format . . . . . . . . . . . . . . . . . . . . 6
4. NTPv4 Message Formats . . . . . . . . . . . . . . . . . . . . 7
5. NTPv4 Client Operations . . . . . . . . . . . . . . . . . . . 13
6. NTPv4 Server Operations . . . . . . . . . . . . . . . . . . . 15
7. NTPv4 Security . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1 Session Keys and Cookies . . . . . . . . . . . . . . . . . 19
7.2 Session Key List Generation . . . . . . . . . . . . . . . 20
7.3 Sending Messages . . . . . . . . . . . . . . . . . . . . . 20
7.4 Receiving Messages . . . . . . . . . . . . . . . . . . . . 20
7.5 Autokey Protocol Exchanges . . . . . . . . . . . . . . . . 20
8. Operation and Management Issues . . . . . . . . . . . . . . . 22
9. Kiss o' Death Message . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
12. Other Considerations . . . . . . . . . . . . . . . . . . . . . 25
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
14.1 Normative References . . . . . . . . . . . . . . . . . . 27
14.2 Informative References . . . . . . . . . . . . . . . . . 27
15. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . 29
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1. Introduction
The Network Time Protocol Version 3 (NTPv3) specified in RFC 1305
[MIL92] has been widely used to synchronize computer clocks in the
global Internet. It provides comprehensive mechanisms to access
national time and frequency dissemination services, organize the NTP
subnet of servers and clients and adjust the system clock in each
participant. In most places of the Internet of today, NTP provides
accuracies of 1-50 ms, depending on the characteristics of the
synchronization source and network paths.
NTP is designed for use by clients and servers with a wide range of
capabilities and over a wide range of network jitter and clock
frequency wander characteristics. Many users of NTP in the Internet
of today use a software distribution available from www.ntp.org. The
distribution, which includes the full suite of NTP options,
mitigation algorithms and security schemes, is a relatively complex,
real-time application. While the software has been ported to a wide
variety of hardware platforms ranging from personal computers to
supercomputers, its sheer size and complexity is not appropriate for
many applications. This facilitated the development of the Simple
Network Time Protocol Version 4 (SNTPv4) as described in RFC 2030
[MIL96].
Since the standardization of NTPv3, there has been significant
development which has led to Version 4 of the Network Time Protocol
(NTPv4). This document describes NTPv4, which introduces new
functionality to NTPv3 as described in RFC 1305, and functionality
expanded from that of SNTPv4 as described in RFC 2030 (SNTPv4 is a
subset of NTPv4).
When operating with current and previous versions of NTP and SNTP,
NTPv4 requires no changes to the protocol or implementations now
running or likely to be implemented specifically for future NTP or
SNTP versions. The NTP and SNTP packet formats are the same and the
arithmetic operations to calculate the client time, clock offset and
roundtrip delay are the same. To a NTP or SNTP server, NTP and SNTP
clients are indistinguishable; to a NTP or SNTP client, NTP and SNTP
servers are indistinguishable.
An important provision in this memo is the interpretation of certain
NTP header fields which provide for IPv6 and OSI addressing. The
only significant difference between the NTPv3 and NTPv4 header
formats is the four-octet Reference Identifier field, which is used
primarily to detect and avoid synchronization loops. In all NTP and
SNTP versions providing IPv4 addressing, primary servers use a four-
character ASCII reference clock identifier in this field, while
secondary servers use the 32-bit IPv4 address of the synchronization
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source. In NTPv4 providing IPv6 and OSI addressing, primary
servers use the same clock identifier, but secondary servers use the
first 32 bits of the MD5 hash of the IPv6 or NSAP address of the
synchronization source. A further use of this field is when the
server sends a kiss-o'-death message documented later in this
document.
In the case of OSI, the Connectionless Transport Service (CLTS) is
used as in [ISO86]. Each NTP packet is transmitted as the TS-
Userdata parameter of a T-UNITDATA Request primitive. Alternately,
the header can be encapsulated in a TPDU which itself is transported
using UDP, as described in RFC-1240 [DOB91]. It is not advised that
NTP be operated at the upper layers of the OSI stack, such as might
be inferred from RFC-1698 [FUR94], as this could seriously degrade
accuracy. With the header formats defined in this memo, it is in
principle possible to interwork between servers and clients of one
protocol family and another, although the practical difficulties may
make this inadvisable.
In the following, indented paragraphs such as this one contain
information not required by the formal protocol specification, but
considered good practice in protocol implementations.
This document is organized as follows. Section 2 describes how the
protocol works, the various modes and how IP addresses and UDP ports
are used. Section 3 describes the NTP timestamp format and Section 4
the NTP message format. Section 5 summarizes NTP client operations
and Section 6 summarizes NTP server operations. Section 7 summarizes
the optional security mechanisms present within the NTPv4 protocol.
Section 8 summarizes operation and management issues. Section 9
describes the kiss-o'-death message, whose functionality is
similar to the ICMP Source Quench and ICMP Destination Unreachable
messages. Section 10 presents NTPv4 security considerations.
Section 11 presents various other considerations when implementing
and/or configuring NTPv4.
NTPv4 is hereafter referred to simply as NTP, unless explicitly
noted.
2. NTP Protocol Operation
Unless excepted in context, reference to broadcast address means IPv4
broadcast address, IPv4 multicast group address or IPv6 site-local
scope address. Further information on the broadcast/multicast model
is in RFC 1112 [DEE89]. Details of address format, scoping rules,
etc., are beyond the scope of this memo. NTPv4 can operate with
either unicast (point to point), broadcast (point to multipoint) or
anycast (multipoint to point) addressing modes. A unicast client
sends a request to a designated server at its unicast address and
expects a reply from which it can determine the time and, optionally,
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the roundtrip delay and clock offset relative to the server. A
broadcast server periodically sends an unsolicited message to a
designated broadcast address. A broadcast client listens on this
address and ordinarily sends no requests.
Anycast is designed for use with a set of cooperating servers whose
addresses are not known beforehand. The anycast client sends an
ordinary NTP client request to a designated broadcast address. One
or more anycast servers listen on that address. Upon receiving a
request, an anycast server sends an ordinary NTP server reply to the
client. The client then binds to the server from which the first
such message was received and continues operation with that unicast
addresses. Subsequent replies from other anycast servers are
ignored.
Broadcast servers should respond to client unicast requests, as
well as send unsolicited broadcast messages. Broadcast clients
may send unicast requests in order to measure the network
propagation delay between the server and client and then continue
operation in listen-only mode. However, broadcast servers may
choose not to respond to unicast requests, so unicast clients
should be prepared to abandon the measurement and assume a default
value for the delay.
The client and server addresses are assigned following the usual
IPv4, IPv6 or OSI conventions. For NTP multicast, the IANA has
reserved the IPv4 group address 224.0.1.1 and the IPv6 group address
ending :101, with prefix determined by scoping rules. The NTP
broadcast address for OSI has yet to be determined. Notwithstanding
the IANA reserved addresses, other multicast addresses can be used
which do not conflict with others assigned in scope. In the case of
IPv4 multicast or IPv6 broadcast addresses, the client must implement
the Internet Group Management Protocol (IGMP) as described in RFC-
3376 [CAIN02], in order that the local router joins the multicast
group and relays messages to the IPv4 or IPv6 multicast group. The
scoping, routing and group membership procedures are determined by
considerations beyond the scope of this memo.
It is important to adjust the time-to-live (TTL) field in the IP
header of multicast messages to a reasonable value in order to
limit the network resources used by this (and any other) multicast
service. Only multicast clients in scope will receive multicast
server messages. Only cooperating anycast servers in scope will
reply to a client request. The engineering principles which
determine the proper values to be used are beyond the scope of
this memo.
While not integral to the NTP specification, it is intended that
IP broadcast addresses will be used primarily in IP subnets and
LAN segments including a fully functional NTP server with a number
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of dependent NTP broadcast clients on the same subnet, while IP
multicast group addresses will be used only in cases where the TTL
is engineered specifically for each service domain.
3. NTP Timestamp
NTPv4 uses the standard NTP timestamp format described in RFC-1305.
NTP data are specified as integer or fixed-point quantities, with
bits numbered in big-endian fashion from 0 starting at the left or
most significant end. Unless specified otherwise, all quantities
are unsigned and may occupy the full field width with an implied 0
preceding bit 0.
NTP timestamps are represented as a 64-bit unsigned fixed-point
number, in seconds relative to 0h on 1 January 1900. The integer
part is in the first 32 bits and the fraction part in the
last 32 bits. In the fraction part, the non-significant low order
bits are not specified and ordinarily set to 0. The NTP timestamp
format is as shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. NTP Timestamp Format
It is advisable to fill the non-significant low order bits of the
timestamp with a random, unbiased bitstring, both to avoid
systematic roundoff errors and as a means of loop detection and
replay detection (see below). It is important that the bitstring
be unpredictable by a intruder. One way of doing this is to
generate a random 128-bit bitstring at startup. After that, each
time the system clock is read the string consisting of the
timestamp and bitstring is hashed with the MD5 algorithm, then the
non-significant bits of the timestamp are copied from the result.
The NTP format allows convenient multiple-precision arithmetic and
conversion to UDP/TIME message (seconds), but does complicate the
conversion to ICMP Timestamp message (milliseconds) and Unix time
values (seconds and microseconds or seconds and nanoseconds). The
maximum number that can be represented is 4,294,967,295 seconds with
a precision of about 232 picoseconds, which should be adequate for
even the most exotic requirements.
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Note that, since some time in 1968 (second 2,147,483,648) the most
significant bit (bit 0 of the integer part) has been set and that the
64-bit field will overflow some time in 2036 (second 4,294,967,296).
There will exist a 232-picosecond interval, henceforth ignored, every
136 years when the 64-bit field will be 0, which by convention is
interpreted as an invalid or unavailable timestamp.
If bit 0 is set, the UTC time is in the range 1968-2036 and UTC time
is reckoned from 0h 0m 0s UTC on 1 January 1900. If bit 0 is not
set, the time is in the range 2036-2104 and UTC time is reckoned from
6h 28m 16s UTC on 7 February 2036. Note that when calculating the
correspondence, 2000 is a leap year and leap seconds are not included
in the reckoning.
4. NTP Message Formats
Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
[POS80], which itself is a client of the Internet Protocol (IP)
[DAR81] [DER98]. The structure of the IP and UDP headers is described
in the cited specification documents and will not be detailed further
here. The UDP port number assigned to NTP is 123, which should be
used in both the Source Port and Destination Port fields in the UDP
header. The remaining UDP header fields should be set as described in
the specification.
Below is a description of the NTPv4 message format, which follows
the IP and UDP headers. This format is identical to that described in
RFC 1305, with the exception of the contents of the reference
identifier field. The header fields are defined in Figure 2.
Leap Indicator (LI):
This is a two-bit field indicating an impending leap second to be
inserted in the NTP timescale. The bits are set before 23:59 on
the day of insertion and reset after 00:00 on the following day.
This causes the number of seconds (rollover interval) in the day
of insertion to be increased or decreased by one. In the case of
primary servers the bits are set by operator intervention, while
in the case of secondary servers the bits are set by the protocol.
The possible values of the LI field, and corresponding meanings,
are as follows:
LI Meaning
---------------------------------------------
0 no warning
1 last minute has 61 seconds
2 last minute has 59 seconds)
3 alarm condition (clock not synchronized)
On startup, servers set this field to 3 (clock not synchronized)
and set this field to some other value when synchronized to the
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primary reference clock. Once set to other than 3, the field is
never set to that value again, even if all synchronization sources
become unreachable or defective.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LI | VN |Mode | Strat | Poll | Prec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Dispersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Reference Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Origin Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receive Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Transmit Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Extension Field 1 (Optional) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Extension Field 2 (Optional) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Authentication .
. (Optional) (160 bits) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 NTP Message Format
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Version (VN):
This is a three-bit integer indicating the NTP/SNTP version
number, currently 4. If necessary to distinguish between IPv4,
IPv6 and OSI, the encapsulating context must be inspected.
Mode:
This is a three-bit number indicating the protocol mode. The
values are defined as follows:
Mode Meaning
------------------------------------
0 reserved
1 symmetric active
2 symmetric passive
3 client
4 server
5 broadcast
6 reserved for NTP control message
7 reserved for private use
In unicast and anycast modes, the client sets this field to 3
(client) in the request and the server sets it to 4 (server) in
the reply. In broadcast mode, the server sets this field to 5
(broadcast).
Stratum (Strat):
This is a eight-bit unsigned integer indicating the stratum.
This field is significant only in SNTP server messages, where the
values are defined as follows:
Stratum Meaning
----------------------------------------------
0 kiss-o'-death message
1 primary reference (e.g., synchronized by radio clock)
2-15 secondary reference (synchronized by NTP or SNTP)
16-255 reserved
Poll Interval (Poll):
This is an eight-bit unsigned integer used as an exponent of two,
where the resulting value is the maximum interval between
successive messages in seconds. This field is significant only in
SNTP server messages, where the values range from 4 (16 s) to 17
(131,072 s - about 36 h).
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Precision (Prec):
This is an eight-bit signed integer used as an exponent of
two, where the resulting value is the precision of the system
clock in seconds. This field is significant only in server
messages, where the values range from -6 for mains-frequency
clocks to -20 for microsecond clocks found in some workstations.
Root Delay:
This is a 32-bit signed fixed-point number indicating the
total roundtrip delay to the primary reference source, in seconds
with fraction point between bits 15 and 16. Note that this
variable can take on both positive and negative values, depending
on the relative time and frequency offsets. This field is
significant only in server messages, where the values range from
negative values of a few milliseconds to positive values of
several hundred milliseconds.
Root Dispersion:
This is a 32-bit unsigned fixed-point number indicating the
nominal error relative to the primary reference source, in seconds
with fraction point between bits 15 and 16. This field is
significant only in server messages, where the values range from
zero to several hundred milliseconds.
Reference Identifier:
This is a 32-bit bitstring identifying the particular reference
source. This field is significant only in server messages, where
for stratum 0 (kiss-o'-death message) and 1 (primary server), the
value is a four-character ASCII string, left justified and zero
padded to 32 bits. For IPv4 secondary servers,the value is the
32-bit IPv4 address of the synchronization source. For IPv6 and
OSI secondary servers, the value is the first 32 bits of the MD5
hash of the IPv6 or NSAP address of the synchronization source.
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Primary (stratum 1) servers set this field to a code identifying
the external reference source according to the below table.
Code External Reference Source
----------------------------------------------------------------
LOCL uncalibrated local clock
CESM calibrated Cesium clock
RBDM calibrated Rubidium clock
PPS calibrated quartz clock or other pulse-per-second
source
IRIG Inter-Range Instrumentation Group
ACTS NIST telephone modem service
USNO USNO telephone modem service
PTB PTB (Germany) telephone modem service
TDF Allouis (France) Radio 164 kHz
DCF Mainflingen (Germany) Radio 77.5 kHz
MSF Rugby (UK) Radio 60 kHz
WWV Ft. Collins (US) Radio 2.5, 5, 10, 15, 20 MHz
WWVB Boulder (US) Radio 60 kHz
WWVH Kaui Hawaii (US) Radio 2.5, 5, 10, 15 MHz
CHU Ottawa (Canada) Radio 3330, 7335, 14670 kHz
LORC LORAN-C radionavigation system
OMEG OMEGA radionavigation system
GPS Global Positioning Service
If the external reference is one of those listed, the associated
code should be used. Codes for sources not listed can be contrived
as appropriate.
In previous NTP and SNTP secondary servers and clients this field
was often used to walk-back the synchronization subnet to the root
(primary server) for management purposes.
Reference Timestamp:
This field is significant only in server messages, where the value
is the time at which the system clock was last set or corrected,
in 64-bit timestamp format.
Originate Timestamp:
This is the time at which the request departed the client for the
server, in 64-bit timestamp format.
Receive Timestamp:
This is the time at which the request arrived at the server or the
reply arrived at the client, in 64-bit timestamp format.
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Transmit Timestamp:
This is the time at which the request departed the client or the
reply departed the server, in 64-bit timestamp format.
NTPv4 Extension Fields:
NTPv4 defines new extension field formats. These fields are
processed in order and may be transmitted with or without value
fields. The last field is padded to a 64-bit boundary, all others
fields are padded to 32-bit boundaries. The field length is for
all payload and padding.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Value .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Signature .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (as needed) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 NTPv4 Extension Field
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Authentication (optional):
The authentication field format is shown in Figure 4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Message Digest +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Optional Authentication Field
When the NTP authentication scheme is implemented, the 16-bit Key
Identifier and 128-bit Message Digest fields contain the Message
Authentication Code (MAC) information which uses an MD5 cryptosum
of NTP header plus extension fields.
5. NTP Client Operations
An NTP client can operate in unicast, broadcast or anycast modes. In
unicast mode the client sends a request (NTP mode 3) to a designated
unicast server and expects a reply (NTP mode 4) from that server. In
broadcast client mode it sends no request and waits for a broadcast
(NTP mode 5) from one or more broadcast servers. In anycast mode,
the client sends a request (NTP mode 3) to a designated broadcast
address and expects a reply (NTP mode 4) from one or more anycast
servers. The client uses the first reply received to establish the
particular server for subsequent unicast operations. Later replies
from this server (duplicates) or any other server are ignored. Other
than the selection of address in the request, the operations of
anycast and unicast clients are identical.
Client requests are normally sent at intervals depending on the
frequency tolerance of the client clock and the required accuracy.
However, under no conditions should requests be sent at less than
one minute intervals. Further discussion on this point is in
Section 9.
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A unicast or anycast client initializes the NTP message header, sends
the request to the server and strips the time of day from the
Transmit Timestamp field of the reply. For this purpose, all of the
NTP header fields shown above are set to 0, except the Mode, VN and
optional Transmit Timestamp fields.
NTP and SNTP clients set the mode field to 3 (client) for unicast and
anycast requests. They set the VN field to any version number
supported by the server selected by configuration or discovery and
can interoperate with all previous version NTP and SNTP servers.
Servers reply with the same version as the request, so the VN field
of the request also specifies the VN field of the reply. An NTP
client can specify the earliest acceptable version on the expectation
that any server of that or later version will respond. NTPv4 servers
are backwards compatible with NTPv3 as defined in RFC 1305, NTPv2
as defined in RFC 1119 [MIL89], and NTPv1 as defined in RFC 1059
[MIL88]. NTPv0 defined in RFC 959 [MIL85] is not supported.
In unicast and anycast modes, the Transmit Timestamp field in the
request should be set to the time of day according to the client
clock in NTP timestamp format. This allows a simple calculation to
determine the propagation delay between the server and client and to
align the system clock generally within a few tens of milliseconds
relative to the server. In addition, this provides a simple method
to verify that the server reply is in fact a legitimate response to
the specific client request and avoid replays. In broadcast mode,
the client has no information to calculate the propagation delay or
determine the validity of the server, unless one of the NTP
authentication schemes is used. The following table summarizes the
required NTP client operations in unicast, anycast and broadcast
modes. The recommended error checks are shown in the Reply and
Broadcast columns in the table. The message should be considered
valid only if all the fields shown contain values in the respective
ranges. Whether to believe the message if one or more of the fields
marked "ignore" contain invalid values is at the discretion of the
implementation.
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Field Name Unicast/Anycast Broadcast
Request Reply
---------------------------------------------------------------
LI 0 0-3 0-3
VN 1-4 copied from 1-4
request
Mode 1 or 3 2 or 4 5
Stratum 0 0-15 0-15
Poll 0 ignore ignore
Precision 0 ignore ignore
Root Delay 0 ignore ignore
Root Dispersion 0 ignore ignore
Reference Identifier 0 ignore ignore
Reference Timestamp 0 ignore ignore
Originate Timestamp 0 (see text) ignore
Receive Timestamp 0 (see text) ignore
Transmit Timestamp (see text) nonzero nonzero
Authenticator optional optional optional
[Need to add a similar table for symmetric modes of operation]
6. NTP Server Operations
An NTP server operating with either an NTP or SNTP client of the same
or previous versions retains no persistent state. Since a SNTP
server ordinarily does not implement the full suite of grooming and
mitigation algorithms intended to support redundant servers and
diverse network paths, a SNTP server should be operated only in
conjunction with a source of external synchronization, such as a
reliable radio clock or telephone modem. In this case it operates as
a primary (stratum 1) server.
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An NTP server can operate with any unicast, anycast or broadcast
address or any combination of these addresses. A unicast or anycast
server receives a request (NTP mode 3), modifies certain fields in
the NTP header, and sends a reply (NTP mode 4), possibly using the
same message buffer as the request. A anycast server listens on the
designated broadcast address, but uses its own unicast IP address in
the source address field of the reply. Other than the selection of
address in the reply, the operations of anycast and unicast servers
are identical. Broadcast messages are normally sent at poll
intervals from 64 s to 1024 s, depending on the expected frequency
tolerance of the client clocks and the required accuracy.
Unicast and anycast servers copy the VN and Poll fields of the
request intact to the reply and set the Stratum field to 1.
Note that SNTP servers normally operate as primary (stratum 1)
servers. While operating at higher strata (up to 15) and at the
same time synchronizing to an external source such as a GPS
receiver is not forbidden, this is strongly discouraged.
If the Mode field of the request is 3 (client), the reply is set to 4
(server). If this field is set to 1 (symmetric active), the reply is
set to 2 (symmetric passive). This allows clients configured in
either client (NTP mode 3) or symmetric active (NTP mode 1) to
interoperate successfully, even if configured in possibly suboptimal
ways. For any other value in the Mode field, the request is
discarded. In broadcast (unsolicited) mode, the VN field is set to
4, the Mode field is set to 5 (broadcast), and the Poll field set to
the nearest integer base-2 logarithm of the poll interval.
Note that it is highly desirable that a broadcast server also
supports unicast clients. This is so a potential broadcast client
can calculate the propagation delay using a client/server exchange
prior to switching to broadcast client (listen-only) mode. A
anycast server by design also is a unicast server. There does not
seem to be a great advantage for a server to operate as both
broadcast and anycast at the same time, although the protocol
specification does not forbid it.
A broadcast or anycast server may or may not respond if not
synchronized to a correctly operating reference source, but the
preferred option is to respond, since this allows reachability to be
determined regardless of synchronization state. If the server has
never synchronized to a reference source, the LI field is set to 3
(unsynchronized). Once synchronized to a reference source, the LI
field is set to one of the other three values and remains at the last
value set even if the reference source becomes unreachable or turns
faulty.
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If synchronized to a reference source the Stratum field is set to 1
and the Reference Identifier field is set to the ASCII source
identifier shown in Figure 2. If not synchronized, the Stratum field
is set to zero and the Reference Identifier field set to an ASCII
error identifier described below. In broadcast mode, the server
sends broadcasts only if synchronized to a correctly operating
reference source.
The Precision field is set to reflect the maximum reading error of
the system clock. For all practical cases it is computed as the
negative base-2 logarithm of the number of significant bits to the
right of the decimal point in the NTP timestamp format. The Root
Delay and Root Dispersion fields are set to 0 for a primary server;
optionally, the Root Dispersion field can be set to a value
corresponding to the maximum expected error of the radio clock
itself.
The timestamp fields in the server message are set as follows. If
the server is unsynchronized or first coming up, all timestamp fields
are set to zero with one exception. If the message is a reply to a
previously received client request, the Transmit Timestamp field of
the request is copied unchanged to the Originate Timestamp field of
the reply. It is important that this field be copied intact, as an
NTP or SNTP client uses it to avoid bogus messages.
If the server is synchronized, the Reference Timestamp is set to the
time the last update was received from the reference source. The
Originate Timestamp field is set as in the unsynchronized case above.
The Transmit Timestamp field are set to the time of day when the
message is sent. In broadcast messages the Receive Timestamp field
is set to zero and copied from the Transmit Timestamp field in other
messages. The following table summarizes these actions.
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Field Name Unicast/Anycast Broadcast
Request Reply
----------------------------------------------------------------
LI ignore as needed as needed
VN 1-4 copied from 4
request
Mode 1 or 3 2 or 4 5
Stratum ignore 1 1
Poll ignore copied from log2 poll
request interval
Precision ignore -log2 server -log2 server
significant significant
bits bits
Root Delay ignore 0 0
Root Dispersion ignore 0 0
Reference Identifier ignore source ident source ident
Reference Timestamp ignore time of last time of last
source update source update
Originate Timestamp ignore copied from 0
transmit
timestamp
Receive Timestamp ignore time of day 0
Transmit Timestamp (see text) time of day time of day
Authenticator optional optional optional
[Need to add a similar table for symmetric modes of operation]
There is some latitude on the part of most clients to forgive invalid
timestamps, such as might occur when first coming up or during
periods when the reference source is inoperative. The most important
indicator of an unhealthy server is the Stratum field, in which a
value of 0 indicates an unsynchronized condition. When this value is
displayed, clients should discard the server message, regardless of
the contents of other fields.
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7. NTPv4 Security
NTPv4 employs the Autokey security protocol, which works
independently for each client, with tentative outcomes confirmed only
after both succeed. Public keys and certificates are obtained and
verified relatively infrequently using X.509 certificates and
certificate trails. Session keys are derived from public keys. Each
NTP message is individually authenticated using the session key and
the message digest (keyed MD5). A proventic trail is a sequence of
NTP servers each synchronized and cryptographically veritifed to the
next lower stratum server and ending on one or more trusted servers.
Proventic trails are constructed from each server to the trusted
servers at decreasing stratum levels. When server time and at least
one proventic trail are verified, the peer is admitted to the
population and used to synchronize the system clock.
7.1 Session Keys and Cookies
NTPv4 session keys have four 32-bit words, as shown in Figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. NTPv4 Session Key Format
The session key value is the 16-octet MD5 message digest of the
session key. Key IDs have pseudo-random values and are used only
once. A special key ID value of zero is used as a NAK reply. In
multicast mode, and in any message including an extension field, the
cookie has a public value (zero). In client/server modes, the cookie
is a hash of the addresses and a private value. In symmetric modes,
the cookie is a random roll. In the event that both peers generate
cookies, the agreed-upon cookie is the exclusive-OR of the two
values.
The server generates a cookie unique to the client and server
addresses and its own private value. It returns the cookie,
signature, and timestampe to the client in an extension field. The
cookie is transmitted from server to client encrypted by the client
public key. The server uses the cookie to validate requests and
construct replies. The client uses the cookie to validate the reply
and checks that the request key ID matches the reply key ID.
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7.2 Session Key List Generation
The server rolls a random 32-bit seed as the initial key ID and
selects the cookie. Messages with a zero cookie contain only public
values. The initial session key is constructed using the given
addressses, cookie and initial key ID. The session key value is
stored in the key cache. The next session key is constructed using
the first four octets of the session key value as the new key ID.
The server continues to generate the full list. The final index
number and last key ID are provided in an extension field with
signature and timestamp.
7.3 Sending Messages
The MAC consists of the MD5 message digest of the NTP header and
extension fields using the session key ID and value stored in the key
cache. The server uses the session key ID list in reverse order and
discards each key value after use. An extension field containing the
last index number and key ID is included in the first packet
transmitted (last on the list). This extension field can be provided
upon request at any time. When all entries in the key list are used,
a new one is generated.
7.4 Receiving Messages
The intent is not to hide the message contents. Rather, the goal is
to verify its source and that it has not been modified in transit.
The MAC message digest is compared with the computed digest of the
NTP header and extension fields using the session key ID in the MAC
and the key value computed from the addresses, key ID and cookie. If
the cookie is zero, the message contains public values. Anybody can
validate the message or make a valid message containing any values.
If the cookie has been determined by secret means, nobody except the
parties to the secret can validate a message or make a valid message.
7.5 Autokey Protocol Exchanges
There are five types of Autokey protocol exchanges:
1. Parameter Exchange (ASSOC message): This message exchanges host
names, agrees on digest/signature and identity schemes. This
protocol exchange is unsigned. Optionally, host name/address can
be verified using reverse-DNS. An initial association request is
sent by the client, sending the host name and status word
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest/Signature NID | Client | Ident | Host |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6 Status Word Format
If the server digest NID and ID scheme agree, the server responds
with an association response message, sending host name and
status word. The client, upon agreeing with digest NID and ID
scheme, then sends a certificate request. The server responds
with an X.509 certificate and signature. The certificate
request/response cycle repeats as needed. A primary (Stratum 1)
certifcate is explicitly trusted and self-signed. Secondary
certificates are signed by the next lower stratum server and
validated with its public key.
2. Certificate Exchange (CERT message): This exchange is used to
obtain and verify certificates on the trail to a trusted root
certificate. Certificate exchanges follow the same process as
parameter exchanges.
3. Identity Exchange (IFF, GW, and MV messages): This exchange is
used to verify server identity using an agreed identity scheme
(TC, IFF, GQ, MV). This exchange is a challenge-response scheme.
The client initiates by sending a challenge request. The server
then provides the challenge response.
4. Values Exchange (COOKIE and AUTO messages): This exchange is used
to obtain and verify the cookie, autokey values, and leapseconds
table, depending on the association mode (client-server,
broadcast, symmetric). For cookie exchanges, the client sends
its public key to the server without signature when not
synchronized. Symmetric active peers send its public key and
signature to passive peer when synchronized. The server cookie
is encrypted from the hash of source/destination addresses, zero
key ID, and server private value. A symmetric passive cookie is a
random value for every exchange. The server private value is
refreshed and protocol restarted once per day. For autokey
exchanges, the server generates a key list and signature is
calculated to last about one hour. A client sends requests to
the server without signature when not synchronized. The server
replies with the last index number and key ID on the list.
Broadcast servers uses AUTO response for the first message after
regenerating the key and ASSOC responses for all other messages.
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5. Signature Exchange (SIGN message): This exchange requests the
server to sign and return a client certificate. The exchange is
valid only when the client has synchronized to a proventic source
and the server identity has been confirmed. This exchange is
used to authenticate clients to servers, with the server acting
as de facto certificate authority using an encrypted credential
scheme. The client sends a certificate to the server with or
without signature. The server extracts the requested data and
signs that data with the server private key. The client then
verifies the certificate and signature. Subsequently, the client
supplies this certificate rather than self-signed certificates,
so clients can verify with the server public key.
8. Configuration and Management
The means used in the configuration and management of NTP servers
and clients is the NTP control and monitoring protocol defined in
RFC 1305.
Unicast clients must be provided with one or more designated server
names or IP addresses. If more than one server is provided, one can
be used for active operation and one of the others for backup should
the active one fail or show an error condition. It is not normally
useful to use more than one server at a time, as with millions of
NTP-enabled devices expected in the near future, such use could
result in unnecessary strain on network and server resources.
Broadcast servers and anycast clients must be provided with the TTL
and local broadcast or multicast group address. Unicast and anycast
servers and broadcast clients may be configured with a list of
address-mask pairs for access control, so that only those clients or
servers known to be trusted will be accepted. Multicast servers and
clients must implement the IGMP protocol and be provided with the
local broadcast or multicast group address as well. The
configuration data for cryptographic authentication is beyond the
scope of this memo.
There are several scenarios which provide automatic server discovery
and selection for NTP clients with no pre-specified server
configuration. For instance a role server with CNAME such as
pool.ntp.org returns a randomized list of volunteer secondary server
addresses and the client can select one or more as candidates. For
an IP subnet or LAN segment including a NTP or SNTP server, NTP
clients can be configured as broadcast clients. The same approach
can be used with multicast servers and clients. In both cases,
provision of an access control list is a good way to insure only
trusted sources can be used to set the system clock.
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In another scenario suitable for an extended network with significant
network propagation delays, clients can be configured for anycast
addresses, both upon initial startup and after some period when the
currently selected unicast source has not been heard. Following the
defined protocol, the client binds to the server from which the first
reply is received and continues operation in unicast mode.
9. The Kiss-o'-Death Packet
In the interest of self-preservation, it is important that NTP
servers have a mechanism to supress or otherwise influence the amount
of queries performed by NTP clients.
According to the NTPv3 specification RFC 1305, if the Stratum field
in the NTP header is 1, indicating a primary server, the Reference
Identifier field contains an ASCII string identifying the particular
reference clock type. However, in RFC 1305 nothing is said about the
Reference Identifier field if the Stratum field is 0, which is called
out as "unspecified". However, if the Stratum field is 0, the
Reference Identifier field can be used to convey messages useful for
status reporting and access control. In NTPv4 and SNTPv4, packets of
this kind are called Kiss-o'-Death (KoD) packets and the ASCII
messages they convey are called kiss codes. The KoD packets got
their name because an early use was to tell clients to stop sending
packets that violate server access controls.
The kiss codes can provide useful information for an intelligent
client. These codes are encoded in four-character ASCII strings left
justified and zero filled. The strings are designed for character
displays and log files. A list of the currently-defined kiss codes
is in the following table.
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Code Meaning
--------------------------------------------------------------
ACST The association belongs to a anycast server
AUTH Server authentication failed
AUTO Autokey sequence failed
BCST The association belongs to a broadcast server
CRYP Cryptographic authentication or identification failed
DENY Access denied by remote server
DROP Lost peer in symmetric mode
RSTR Access denied due to local policy
INIT The association has not yet synchronized for the first
time
MCST The association belongs to a manycast server
NKEY No key found. Either the key was never installed or
is not trusted
RATE Rate exceeded. The server has temporarily denied access
because the client exceeded the rate threshold
RMOT Somebody is tinkering with the association from a remote
host running ntpdc. Not to worry unless some rascal has
stolen your keys
STEP A step change in system time has occurred, but the
association has not yet resynchronized
In general, an NTP client should stop sending to a particular server
if that server returns a reply with a Stratum field of 0, regardless
of kiss code, and an alternate server is available. If no alternate
server is available, the client should retransmit using an
exponential-backoff algorithm described in Section 11.
10. Security Considerations
In the case of NTP as specified herein, there is a very real
vulnerability that NTP broadcast clients can be disrupted by
misbehaving or hostile SNTP or NTP broadcast servers elsewhere in
the Internet. It is strongly recommended that access controls
and/or cryptographic authentication means be provided for
additional security in such cases.
While not required in a conforming NTP client implementation, there
are a variety of recommended checks that an NTP client can perform
that are designed to avoid various types of abuse that might happen
as the result of server implementation errors or malicious attack.
These recommended checks are as follows:
1. When the IP source and destination addresses are available for the
client request, they should match the interchanged addresses in
the server reply.
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2. When the UDP source and destination ports are available for the
client request, they should match the interchanged ports in the
server reply.
3. The Originate Timestamp in the server reply should match the
Transmit Timestamp used in the client request.
4. The server reply should be discarded if any of the LI, Stratum, or
Transmit Timestamp fields are 0 or the Mode field is not 4
(unicast) or 5 (broadcast).
5. A truly paranoid client can check the Root Delay and Root
Dispersion fields are each greater than or equal to 0 and less
than infinity, where infinity is currently a cozy number like 16
seconds. This check avoids using a server whose synchronization
source has expired for a very long time.
11. IANA Considerations
12. Other Considerations
NTP and SNTP clients can consume considerable network and server
resources if not "good network citizens." There are now consumer
Internet commodity devices numbering in the millions that are
potential customers of public and private NTP and SNTP servers.
Recent experience strongly suggests that device designers pay
particular attention to minimizing resource impacts, especially if
large numbers of these devices are deployed. The most important
design consideration is the interval between client requests, called
the poll interval. It is extremely important that the design use the
maximum poll interval consistent with acceptable accuracy.
1. A client MUST NOT use a poll interval less than TBD minutes.
2. A client SHOULD increase the poll interval using exponential
backoff as performance permits and especially if the server does
not respond within a reasonable time.
3. A client SHOULD use local servers whenever available to avoid
unnecessary traffic on backbone networks.
4. A client MUST allow the operator to configure the primary and/or
alternate server names or addresses in addition to or in place of
a firmware default IP address.
5. If a firmware default server IP address is provided, it MUST be a
server operated by the manufacturer or seller of the device or
another server, but only with the operator's permission.
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6. A client SHOULD use the Domain Name System (DNS) to resolve the
server IP addresses, so the operator can do effective load
balancing among a server clique and change IP address binding to
canonical names.
7. A client SHOULD re-resolve the server IP address on a periodic
intervals, but not less than the time-to-live field in the DNS
response.
8. A client SHOULD support the NTP access-refusal mechanism, so that
a server kiss-o'-death reply in response to a client request
causes the client to cease sending requests to that server and to
switch to an alternate, if available.
If the firmware or documentation includes specific server names, the
names should be those the manufacturer or seller operates as a
customer convenience or those for which specific permission has been
obtained from the operator. A DNS request for a generic server name
such as ntp.mytimeserver.com results should result in a random
selection of server IP addresses available for that purpose. Each
time a DNS request is received, a new randomized list is returned.
The client ordinarily uses the first address on the list.
When selecting candidate SNTP or NTP servers, it is imperative to
respect the server operator's conditions of access. Lists of
public servers and their conditions of access are available at
www.ntp.org. A semi-automatic server discovery scheme using DNS
is described at that site. Some ISPs operate public servers,
although finding them via their helpdesks can be difficult.
A well behaved client operates as follows (note that steps 2 - 4
comprise a synchronization loop):
1. Consider the specified frequency tolerance of the system clock
oscillator. Define the required accuracy of the system clock,
then calculate the maximum timeout. For instance, if the
frequency tolerance is 200 parts-per-million (PPM) and the
required accuracy is one minute, the maximum timeout is about 3.5
days. Use the longest maximum timeout possible given the system
constraints to minimize time server aggregate load, but never less
than 15 minutes.
2. When first coming up or after reset, randomize the timeout from
one to five minutes. This is to minimize shock when 3000 PCs are
rebooted at the same time power is restored after a blackout.
Assume at this time the IP address is unknown and the system clock
is unsynchronized. Otherwise use the timeout value as calculated
in previous loop steps. Note that it may be necessary to refrain
from implementing the aforementioned random delay for some classes
of ICSA certification.
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3. When the timer reaches zero, if the IP address is not known, send
a DNS query packet; otherwise send a NTP request packet to that
address. If no reply packet has been heard since the last
timeout, double the timeout, but not greater than the maximum
timeout. If primary and secondary time servers have been
configured, alternate queries between the primary and secondary
servers when no successful response has been received.
4. If a DNS reply packet is received, save the IP address and
continue in step 2. If a KoD packet is received remove that time
server from the list, activate the secondary time server and
continue in step 2. If a received packet fails the sanity checks,
drop that packet and also continue in step 2. If a valid NTP
packet is received, update the system clock, set the timeout to
the maximum, and continue to step 2.
13. Acknowledgements
This document has drawn significant material from the document
<draft-mills-sntp-v4-00.txt>. As a result, the authors would like
to acknowledge D. Plonka of the University of Wisconsin and J.
Montgomery of Netgear, who were significant contributors to that
draft.
14. References
14.1 Normative References
[MIL92] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.
[MIL96] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6, and OSI", RFC 2030, October 1996.
14.2 Informative References
[CAIN02] Cain, B., Deering, S., Kouvalas, I., Fenner, B. and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, Cereva Networks, October 2002.
[DAR81] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[DEE89] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[DER98] Deering, S., Hinden R., "Internet Protocol, Version 6 (IPv6),"
RFC 2460, December 1998.
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[DOB91] Dobbins, K, Haggerty, W. and C. Shue, "OSI connectionless
transport services on top of UDP - Version: 1", RFC 1240,
June 1991.
[ISO86] International Standards 8602 - Information Processing
Systems - OSI: Connectionless Transport Protocol
Specification. International Standards Organization,
December 1986.
[MIL85] Mills, D., "Network Time Protocol (NTP)", RFC 958,
September 1985.
[MIL88] Mills, D., "Network Time Protocol (Version 1) Specification
and Implementation", RFC 1059, July 1988.
[MIL89] Mills, D., "Network Time Protocol (Version 2) Specification
and Implementation," RFC 1119, September 1989.
[POS80] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
15. Authors' Addresses
Jack L. Burbank (Editor)
The Johns Hopkins University Applied Physics Laboratory (JHU/APL)
11100 Johns Hopkins Road
Laurel, MD 20723
Phone: +1 443-778-7127
EMail: jack.burbank@jhuapl.edu
Jim Martin (co-Editor)
Netzwert AG
An den Treptowers 1
D-12435 Berlin
Phone: +49.30/5 900 800-180
EMail: jim@Netzwert.AG
Dr. David L. Mills
The University of Delaware
Electrical Engineering Department
University of Delaware
Newark, DE 19716
Phone: (302) 831-8247
EMail: mills@udel.edu
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