| Network Working Group | D. L. Mills |
| Internet-Draft | University of Delaware |
| Intended status: Informational | K.F. O'Donoghue, Ed. |
| Expires: April 24, 2014 | Internet Society |
| D. L. Hart | |
| H. M. Stenn | |
| Network Time Foundation, Inc. | |
| P.F. Chimento, Ed. | |
| Johns Hopkins University/APL | |
| October 21, 2013 |
Control Messages Protocol for Use with Network Time Protocol Version 4
draft-odonoghue-ntpv4-control-02
This document describes the structure of the control messages used with the Network Time Protocol. These control messages can be used to monitor and control the Network Time Protocol application running on any IP network attached computer. The information in this informational RFC was originally described in Appendix B of RFC 1305.
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Editor's Note (to be removed prior to publication): The text below is taken directly from RFC 1305. Input is requested to update the text to reflect current practice. This is required to fully obsolete RFC 1305.
In a comprehensive network-management environment, facilities are presumed available to perform routine NTP control and monitoring functions, such as setting the leap-indicator bits at the primary servers, adjusting the various system parameters and monitoring regular operations. Ordinarily, these functions can be implemented using a network-management protocol such as SNMP and suitable extensions to the MIB database. However, in those cases where such facilities are not available, these functions can be implemented using special NTP control messages described herein. These messages are intended for use only in systems where no other management facilities are available or appropriate, such as in dedicated-function bus peripherals. Support for these messages is not required in order to conform to this specification.
The NTP Control Message has the value 6 specified in the mode field of the first octet of the NTP header and is formatted as shown below. The format of the data field is specific to each command or response; however, in most cases the format is designed to be constructed and viewed by humans and so is coded in free-form ASCII. This facilitates the specification and implementation of simple management tools in the absence of fully evolved network-management facilities. As in ordinary NTP messages, the authenticator field follows the data field. If the authenticator is used the data field is zero-padded to a 32-bit boundary, but the padding bits are not considered part of the data field and are not included in the field count.
IP hosts are not required to reassemble datagrams larger than 576 octets; however, some commands or responses may involve more data than will fit into a single datagram. Accordingly, a simple reassembly feature is included in which each octet of the message data is numbered starting with zero. As each fragment is transmitted the number of its first octet is inserted in the offset field and the number of octets is inserted in the count field. The more-data (M) bit is set in all fragments except the last.
Most control functions involve sending a command and receiving a response, perhaps involving several fragments. The sender chooses a distinct, nonzero sequence number and sets the status field and R and E bits to zero. The responder interprets the opcode and additional information in the data field, updates the status field, sets the R bit to one and returns the three 32-bit words of the header along with additional information in the data field. In case of invalid message format or contents the responder inserts a code in the status field, sets the R and E bits to one and, optionally, inserts a diagnostic message in the data field.
Some commands read or write system variables and peer variables for an association identified in the command. Others read or write variables associated with a radio clock or other device directly connected to a source of primary synchronization information. To identify which type of variable and association a 16-bit association identifier is used. System variables are indicated by the identifier zero. As each association is mobilized a unique, nonzero identifier is created for it. These identifiers are used in a cyclic fashion, so that the chance of using an old identifier which matches a newly created association is remote. A management entity can request a list of current identifiers and subsequently use them to read and write variables for each association. An attempt to use an expired identifier results in an exception response, following which the list can be requested again.
Some exception events, such as when a peer becomes reachable or unreachable, occur spontaneously and are not necessarily associated with a command. An implementation may elect to save the event information for later retrieval or to send an asynchronous response (called a trap) or both. In case of a trap the IP address and port number is determined by a previous command and the sequence field is set as described below. Current status and summary information for the latest exception event is returned in all normal responses. Bits in the status field indicate whether an exception has occurred since the last response and whether more than one exception has occurred.
Commands need not necessarily be sent by an NTP peer, so ordinary access-control procedures may not apply; however, the optional mask/match mechanism suggested in [RFC5905] provides the capability to limit mode 6 processing to selected address ranges.
The Network Time Protocol reference implementation maintained by the University of Delaware and ntp.org provides a utility program, ntpq which enables management and configuration of the ntpd daemon using NTP Control Messages (mode 6). A related utility program, ntpdc, uses an earlier, deprecated implementation-specific binary management protocol using NTP mode 7 datagrams. Due to the implementation complexity of the earlier protocol, the reference implementation has added support for all operations that previously were exposed only via mode 7 to the preferred mode 6 interface. Support for mode 7 requests is likely to be disabled by default in the reference implementation's daemon.
The format of the NTP Control Message header, which immediately follows the UDP header, is shown below. Following is a description of its fields. Bit positions marked as zero are reserved and should always be transmitted as zero.
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 |R E M| Op | Sequence | +---+-----+-----+-----+---------+-------------------------------+ | Status | Association ID | +-------------------------------+-------------------------------+ | Offset | Count | +-------------------------------+-------------------------------+ | | | Data | | (468 octets or less) | | | | +-----------------------------------------------+ | | Padding as needed to next multiple of 32 bits | +---------------+-----------------------------------------------+ | Authenticator (optional, 96 octets or less) | +---------------------------------------------------------------+
LI: This is a two-bit integer that must be zero for control message requests and responses. The Leap Indicator value used at this position in most NTP modes is in the System Status Word provided in some control message responses.
Version Number (VN): This is a three-bit integer indicating a minimum NTP version number. NTP servers should not respond to control messages with an unrecognized version number. Requests may intentionally use a lower version number to enable interoperability with earlier versions. The reference implementation utility ntpq uses version 2 by default. Responses must carry the same version as the corresponding request.
Mode: This is a three-bit integer indicating the mode. It must have the value 6, indicating an NTP control message.
Response Bit (R): Set to zero for commands, one for responses.
Error Bit (E): Set to zero for normal response, one for error response.
More Bit (M): Set to zero for last fragment, one for all others.
Operation Code (Op): This is a five-bit integer specifying the command function. The values are:
+-------+--------------------------------------------------+ | Code | Meaning | +-------+--------------------------------------------------+ | 0 | reserved | | 1 | read status command/response | | 2 | read variables command/response | | 3 | write variables command/response | | 4 | read clock variables command/response | | 5 | write clock variables command/response | | 6 | set trap address/port command/response | | 7 | trap response | | 8 | runtime configuration command/response | | 9 | export configuration to file command/response | | 10 | retrieve remote address stats command/response | | 11 | retrieve ordered list | | 12 | request client-specific nonce command/response | | 13-30 | reserved for future use | | 31 | unset trap address/port command/response | +-------+--------------------------------------------------+
Sequence: This is a 16-bit integer indicating the sequence number. Each request should use a different sequence number. Each response carries the same sequence number as its corresponding request. For asynchronous trap responses, the responder increments the sequence number by one each response, allowing trap receivers to detect missing trap responses. Note the sequence number of each fragment in a multiple-datagram response carries the same sequence number, copied from the request.
Status: This is a 16-bit code indicating the current status of the system, peer or clock, with values coded as described in following sections.
Association ID: This is a 16-bit unsigned integer identifying a valid association, or zero for the system clock.
Offset: This is a 16-bit unsigned integer indicating the offset, in octets, of the first octet in the data area. The offset must be zero in requests. Responses spanning multiple datagrams use a positive offset in all but the first datagram.
Count: This is a 16-bit unsigned integer indicating the length of the data, in octets
Data: This contains the message data for the command or response. The maximum number of data octets is 468.
Padding: Contains zero to three octets with value zero, as needed to ensure the overall control message size is a multiple of 4 octets.
Authenticator (optional): When an NTP authentication mechanism is used, this contains the message authenticator information defined in section 7.3 of [RFC5905].
Status words indicate the present status of the system, associations and clock. They are designed to be interpreted by network-monitoring programs and are in one of four 16-bit formats shown in Figure 6 and described in this section. System and peer status words are associated with responses for all commands except the read clock variables, write clock variables and set trap address/port commands. The association identifier zero specifies the system status word, while a nonzero identifier specifies a particular peer association. The status word returned in response to read clock variables and write clock variables commands indicates the state of the clock hardware and decoding software. A special error status word is used to report malformed command fields or invalid values.
The system status word appears in the status field of the response to a read status or read variables command with a zero association identifier. The format of the system status word is as follows:
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---+-----------+-------+-------+ |LI | ClockSrc | Count | Code | +---+-----------+-------+-------+
Leap Indicator (LI): This is a two-bit code warning of an impending leap second to be inserted/deleted in the last minute of the current day, with bit 0 and bit 1, respectively, coded as follows: (EDITOR: this could refer to RFC 5905 section 7.3 figure 9 instead.)
+------+------------------------------------------------------------+ | LI | Meaning | +------+------------------------------------------------------------+ | 00 | no warning | | 01 | insert second after 23:59:59 of the current day | | 10 | delete second 23:59:59 of the current day | | 11 | unsynchronized | +------+------------------------------------------------------------+
ClockSrc: This is a six-bit integer indicating the current synchronization source, with values coded as follows:
+-------+-----------------------------------------------------------+ | Code | Meaning | +-------+-----------------------------------------------------------+ | 0 | unspecified or unknown | | 1 | Calibrated atomic clock (e.g., PPS, HP 5061) | | 2 | VLF (band 4) or LF (band 5) radio (e.g., OMEGA, WWVB) | | 3 | HF (band 7) radio (e.g., CHU, MSF, WWV/H) | | 4 | UHF (band 9) satellite (e.g., GOES, GPS) | | 5 | local net (e.g., DCN, TSP, DTS) | | 6 | UDP/NTP | | 7 | UDP/TIME | | 8 | eyeball-and-wristwatch | | 9 | telephone modem (e.g., NIST) | | 10-63 | reserved | +-------+-----------------------------------------------------------+
System Event Counter: This is a four-bit integer indicating the number of system events occurring since the last time the System Event Code changed. Upon reaching 15, subsequent events with the same code are not counted.
System Event Code: This is a four-bit integer identifying the latest system exception event, with new values overwriting previous values, and coded as follows:
+-------+---------------------------------------------------------------+ | Code | Meaning | +-------+---------------------------------------------------------------+ | 0 | unspecified | | 1 | frequency correction (drift) file not available | | 2 | frequency correction started (frequency stepped) | | 3 | spike detected and ignored, starting stepout timer | | 4 | frequency training started | | 5 | clock synchronized | | 6 | system restart | | 7 | panic stop (required step greater than panic threshold) | | 8 | no system peer | | 9 | leap second insertion/deletion armed for end of current month | | 10 | leap second disarmed | | 11 | leap second inserted or deleted | | 12 | clock stepped (stepout timer expired) | | 13 | kernel loop discipline status changed | | 14 | leapseconds table loaded from file | | 15 | leapseconds table outdated, updated file needed | +-------+---------------------------------------------------------------+
A peer status word is returned in the status field of a response to a read status, read variables or write variables command and appears also in the list of association identifiers and status words returned by a read status command with a zero association identifier. The format of a peer status word is as follows:
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---------+-----+-------+-------+ | Flags | Sel | Count | Code | +---------+-----+-------+-------+
Peer Status Flags: This is a set of five bits indicating the status of the peer determined by the packet procedure, with bits assigned as follows:
+--------+--------+--------------------------------------------------------+ | Peer | | | | Status | | | | Flag | | | | Bit | Value | Meaning | +--------+--------+--------------------------------------------------------+ | 0 | 0x8000 | configured (peer.config) | | 1 | 0x4000 | authentication enabled (peer.authenable) | | 2 | 0x2000 | authentication okay (peer.authentic) | | 3 | 0x1000 | reachable (peer.reach != 0) | | 4 | 0x0800 | broadcast association | +--------+--------+--------------------------------------------------------+
Peer Selection (Sel): This is a three-bit integer indicating the status of the peer determined by the clock-selection procedure, with values coded as follows:
+-------+-----------------------------------------------------------------+ | Peer | | | Sel | Meaning | +-------+-----------------------------------------------------------------+ | 0 | rejected | | 1 | discarded by intersection algorithm | | 2 | discarded by table overflow (not currently used) | | 3 | discarded by the cluster algorithm | | 4 | included by the combine algorithm | | 5 | backup source (with more than sys.maxclock survivors) | | 6 | system peer (synchronization source) | | 7 | PPS (pulse per second) peer | +-------+-----------------------------------------------------------------+
Peer Event Counter: This is a four-bit integer indicating the number of peer events that occurred since the last time the peer event code changed. Upon reaching 15, subsequent events with the same code are not counted.
Peer Event Code: This is a four-bit integer identifying the latest peer exception event, with new values overwriting previous values, and coded as follows:
+-------+--------------------------------------------------------------------+ | Peer | | | Event | Meaning | | Code | | +-------+--------------------------------------------------------------------+ | 0 | unspecified | | 1 | association mobilized | | 2 | association demobilized | | 3 | peer unreachable | | 4 | peer reachable | | 5 | association restarted or timed out | | 6 | no reply (used only with one-shot ntpd -q, known as ntpdate mode) | | 7 | peer rate limit exceeded (kiss code RATE received) | | 8 | access denied (kiss code DENY received), not currently implemented | | 9 | leap second insertion/deletion at month's end armed by peer vote | | 10 | became system peer (sys.peer) | | 11 | reference clock event (see clock status word) | | 12 | authentication failed | | 13 | popcorn spike suppressed by peer clock filter register | | 14 | entering interleaved mode | | 15 | recovered from interleave error | +-------+--------------------------------------------------------------------+
There are two ways a reference clock can be attached to a NTP service host, as an dedicated device managed by the operating system and as a synthetic peer managed by NTP. As in the read status command, the association identifier is used to identify which one, zero for the system clock and nonzero for a peer clock. Only one system clock is supported by the protocol, although many peer clocks can be supported. A system or peer clock status word appears in the status field of the response to a read clock variables or write clock variables command. This word can be considered an extension of the system status word or the peer status word as appropriate. The format of the clock status word is as follows:
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---------------+-------+-------+ | Reserved | Count | Code | +---------------+-------+-------+
Reserved: An eight-bit integer that should be ignored by requesters and zeroed by responders.
Clock Event Counter: This is a four-bit integer indicating the number of clock events that occurred since the last time the clock event code changed. Upon reaching 15, subsequent events with the same code are not counted.
Clock Event Code: This is a four-bit integer indicating the current clock status, with values coded as follows:
+--------------+----------------------------------------------------------+ | Clock Status | Meaning | +--------------+----------------------------------------------------------+ | 0 | clock operating within nominals | | 1 | reply timeout | | 2 | bad reply format | | 3 | hardware or software fault | | 4 | propagation failure (loss of signal) | | 5 | bad date format or value | | 6 | bad time format or value | | 7-15 | reserved | +--------------+----------------------------------------------------------+
An error status word is returned in the status field of an error response as the result of invalid message format or contents. Its presence is indicated when the E (error) bit is set along with the response (R) bit in the response. The format of the Error Status Word is:
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---------------+---------------+ | Error Code | Reserved | +---------------+---------------+
Error code: an eight-bit integer coded as follows:
+--------------+----------------------------------------------------------+ | Error Status | Meaning | +--------------+----------------------------------------------------------+ | 0 | unspecified | | 1 | authentication failure | | 2 | invalid message length or format | | 3 | invalid opcode | | 4 | unknown association identifier | | 5 | unknown variable name | | 6 | invalid variable value | | 7 | administratively prohibited | | 8-255 | reserved | +--------------+----------------------------------------------------------+
Reserved: Responders should use zero. Requesters should ignore the Reserved value to preserve the possibility of future use.
Commands consist of the header and optional data field shown in Section 3. When present, the data field contains a list of identifiers or assignments in the form <<identifier>>[=<<value>>],<<identifier>>[=<<value>>],... where <<identifier>> is the ASCII name of a system or peer variable specified in Sections 9.1 and 11.1 of RFC 5905 and <<value>> is expressed as a decimal, hexadecimal or string constant in the syntax of the C programming language. Where no ambiguity exists, the "s." or "p." prefixes shown in Figure 5 of Section 7.1 of RFC 5905 [RFC5905] can be suppressed. Whitespace (ASCII nonprinting format effectors) can be added to improve readability for simple monitoring programs that do not reformat the data field. Internet Protocol version 4 addresses are represented as four decimal octets without leading zeros, separated by dots. Internet Protocol version 6 addresses are represented as mandated by [RFC5952], without surrounding square brackets unless a port specification is combined with the address. Timestamps, including reference, originate, receive and transmit values, as well as the logical clock, are represented in units of seconds and fractions, preferably in hexadecimal notation, while delay, offset, dispersion and distance values are represented in units of milliseconds and fractions, preferably in decimal notation. All other values are represented as-is, preferably in decimal notation.
Implementations may define variables other than those listed in Figures 6, 7, 16, 17, 18, 19, 27 and 29 of RFC 5905. Called extramural variables, these are distinguished by the inclusion of some character type other than alphanumeric or "." in the name. For those commands that return a list of assignments in the response data field, if the command data field is empty, it is expected that all available variables defined in Figures 6, 7 and 17 of RFC 5905 will be included in the response. For the read commands, if the command data field is nonempty, an implementation may choose to process this field to individually select which variables are to be returned.
Commands are interpreted as follows:
Read Status (1): The command data field is empty or contains a list of identifiers separated by commas. The command operates in two ways depending on the value of the association identifier. If this identifier is nonzero, the response includes the peer identifier and status word. Optionally, the response data field may contain other information, such as described in the Read Variables command. If the association identifier is zero, the response includes the system identifier (0) and status word, while the data field contains a list of binary-coded pairs <<association identifier>> <<status word>>, one for each currently defined association.
Read Variables (2): The command data field is empty or contains a list of identifiers separated by commas. If the association identifier is nonzero, the response includes the requested peer identifier and status word, while the data field contains a list of peer variables and values as described above. If the association identifier is zero, the data field contains a list of system variables and values. If a peer has been selected as the synchronization source, the response includes the peer identifier and status word; otherwise, the response includes the system identifier (0) and status word.
Write Variables (3): The command data field contains a list of assignments as described above. The variables are updated as indicated. The response is as described for the Read Variables command.
Read Clock Variables (4): The command data field is empty or contains a list of identifiers separated by commas. The association identifier selects the system clock variables or peer clock variables in the same way as in the Read Variables command. The response includes the requested clock identifier and status word and the data field contains a list of clock variables and values, including the last timecode message received from the clock.
Write Clock Variables (5): The command data field contains a list of assignments as described above. The clock variables are updated as indicated. The response is as described for the Read Clock Variables command. The reference implementation daemon requires authentication for this command.
Set Trap Address/Port (6): The command association identifier, status and data fields are ignored. The address and port number for subsequent trap messages are taken from the source address and port of the control message itself. The initial trap counter for trap response messages is taken from the sequence field of the command. The response association identifier, status and data fields are not significant. Implementations should include sanity timeouts which prevent trap transmissions if the monitoring program does not renew this information after a lengthy interval.
Trap Response (7): This command differs from the others described here, which are initiated by a management agent (such as ntpq) and responded to by a NTP daemon. Trap Response is sent by a NTP daemon to any registered trap receivers when a system, peer or clock exception event occurs. The opcode field is 7 and the R bit is set. The trap counter is incremented by one for each trap sent and the sequence field set to that value. The trap message is sent using the IP address and port fields established by the set trap address/port command. If a system trap the association identifier field is set to zero and the status field contains the system status word. If a peer trap the association identifier field is set to that peer and the status field contains the peer status word. Optional ASCII-coded information can be included in the data field.
Configure (8): The command data is parsed and applied as if supplied in the daemon configuration file. The reference implementation daemon requires authentication for this command.
Save Configuration (9): Write a snapshot of the current configuration to the file name supplied as the command data. The reference implementation daemon requires authentication for this command. Further, the command is refused unless a directory in which to store the resulting files has been explicitly configured by the operator.
Read MRU (10): Retrieves records of recently seen remote addresses and associated statistics. Command data consists of name=value pairs controlling the selection of records, as well as a requestor-specific nonce previously retrieved using this command or opcode 12, Request Nonce. The response consists of name=value pairs where some names can appear multiple times using a dot followed by a zero-based index to distinguish them, and to associate elements of the same record with the same index. A new nonce is provided with each successful response.
Read ordered list (11): Retrieves an ordered list. If the command data is empty or the seven characters "ifstats" the associated statistics, status and counters for each local address are returned. If the command data is the characters "addr_restrictions" then the set of IPv4 remote address restrictions followed by the set of IPv6 remote address restrictions (access control lists) are returned. Other command data returns error code 5 (unknown variable name). Similar to Read MRU, response information uses zero-based indexes as part of the variable name preceding the equals sign and value, where each index relates information for a single address or network. This opcode requires authentication.
Request Nonce (12): Retrieves a 96-bit nonce specific to the requesting remote address, which is valid for a limited period. Command data is not used in the request. The nonce consists of a 64-bit NTP timestamp and 32 bits of hash derived from that timestamp, the remote address, and salt known only to the server which varies between daemon runs. The reference implementation honors nonces which were issued less than 16 seconds prior. Regurgitation of the nonce by a managment agent demonstrates to the server that the agent can receive datagrams sent to the source address of the request, making source address "spoofing" more difficult in a similar way as TCP's three-way handshake.
Unset Trap (31): Removes the requesting remote address and port from the list of trap receivers. Command data is not used in the request. If the address and port are not in the list of trap receivers, the error code is 4, bad association.
Editor's Note: To be reviewed by the working group prior to completion.
Editor's Note: To be supplied by the working group prior to completion.
| [RFC5905] | Mills, D., Martin, J., Burbank, J. and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. |
| [RFC5952] | Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 Address Text Representation", RFC 5952, August 2010. |
| [RFC5906] | Haberman, B. and D. Mills, "Network Time Protocol Version 4: Autokey Specification", RFC 5906, June 2010. |
| [RFC5907] | Gerstung, H., Elliott, C. and B. Haberman, "Definitions of Managed Objects for Network Time Protocol Version 4 (NTPv4)", RFC 5907, June 2010. |
| [RFC5908] | Gayraud, R. and B. Lourdelet, "Network Time Protocol (NTP) Server Option for DHCPv6", RFC 5908, June 2010. |