NTP WG J. Burbank, Ed. Internet-Draft JHU APL Expires: April 26, 2006 J. Martin, Ed. Netzwert AG D. Mills U. Del. October 23, 2005 The Network Time Protocol Version 4 Protocol Specification draft-ietf-ntp-ntpv4-proto-01 Status of this Memo 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 Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 26, 2006. Copyright Notice Copyright (C) The Internet Society (2005). 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. Burbank, et al. Expires April 26, 2006 [Page 1] Internet-Draft NTPv4 Protocol Specification October 2005 NTPv4 also includes optional extensions to the NTPv3 protocol,including a dynamic server discovery mechanism and an authentication scheme designed specifically for multicast and dynamically discovered servers. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. NTP Timestamp . . . . . . . . . . . . . . . . . . . . . . . 5 3. NTP Message Formats . . . . . . . . . . . . . . . . . . . . 7 3.1 Leap Indicator (LI) . . . . . . . . . . . . . . . . . . . 8 3.2 Version (VN) . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.4 Stratum (Strat) . . . . . . . . . . . . . . . . . . . . . 10 3.5 Poll Interval (Poll) . . . . . . . . . . . . . . . . . . . 10 3.6 Precision (Prec) . . . . . . . . . . . . . . . . . . . . . 10 3.7 Root Delay . . . . . . . . . . . . . . . . . . . . . . . . 10 3.8 Root Dispersion . . . . . . . . . . . . . . . . . . . . . 10 3.9 Reference Identifier . . . . . . . . . . . . . . . . . . . 11 3.10 Reference Timestamp . . . . . . . . . . . . . . . . . . 12 3.11 Originate Timestamp . . . . . . . . . . . . . . . . . . 12 3.12 Receive Timestamp . . . . . . . . . . . . . . . . . . . 12 3.13 Transmit Timestamp . . . . . . . . . . . . . . . . . . . 12 3.14 NTPv4 Extension Fields . . . . . . . . . . . . . . . . . 12 3.15 Authentication (optional) . . . . . . . . . . . . . . . 13 4. NTP Protocol Operation . . . . . . . . . . . . . . . . . . . 14 5. SNTP Protocol Operation . . . . . . . . . . . . . . . . . . 14 6. NTP Server Operations . . . . . . . . . . . . . . . . . . . 15 7. NTP Client Operations . . . . . . . . . . . . . . . . . . . 17 8. NTP Symmetric Peer Operations . . . . . . . . . . . . . . . 20 9. NTPv4 Security . . . . . . . . . . . . . . . . . . . . . . . 20 9.1 Session Keys and Cookies . . . . . . . . . . . . . . . . . 20 9.2 Session Key List Generation . . . . . . . . . . . . . . . 21 9.3 Sending Messages . . . . . . . . . . . . . . . . . . . . . 21 9.4 Receiving Messages . . . . . . . . . . . . . . . . . . . . 21 9.5 Autokey Protocol Exchanges . . . . . . . . . . . . . . . . 22 10. Dynamic Server Discovery . . . . . . . . . . . . . . . . . . 23 11. NTP Control Messages . . . . . . . . . . . . . . . . . . . . 24 11.1 NTP Control Message Format . . . . . . . . . . . . . . . 26 11.2 Status Words . . . . . . . . . . . . . . . . . . . . . . 27 11.2.1 System Status Word . . . . . . . . . . . . . . . . . 28 11.2.2 Peer Status Word . . . . . . . . . . . . . . . . . . 29 11.2.3 Clock Status Word . . . . . . . . . . . . . . . . . 31 11.2.4 Error Status Word . . . . . . . . . . . . . . . . . 32 11.3 Commands . . . . . . . . . . . . . . . . . . . . . . . . 33 12. The Kiss-o'-Death Packet . . . . . . . . . . . . . . . . . . 35 13. Security Considerations . . . . . . . . . . . . . . . . . . 36 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 37 Burbank, et al. Expires April 26, 2006 [Page 2] Internet-Draft NTPv4 Protocol Specification October 2005 15. Other Considerations . . . . . . . . . . . . . . . . . . . . 37 16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 39 17.1 Normative References . . . . . . . . . . . . . . . . . . 39 17.2 Informative References . . . . . . . . . . . . . . . . . 39 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 40 Intellectual Property and Copyright Statements . . . . . . . 42 Burbank, et al. Expires April 26, 2006 [Page 3] Internet-Draft NTPv4 Protocol Specification October 2005 1. Introduction The Network Time Protocol Version 3 (NTPv3) specified in [1] 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 [2]. 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 round trip 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 source. In NTPv4 providing IPv6 and OSI addressing, primary servers Burbank, et al. Expires April 26, 2006 [Page 4] Internet-Draft NTPv4 Protocol Specification October 2005 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 [3]. 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 [4]. It is not advised that NTP be operated at the upper layers of the OSI stack, such as might be inferred from [5], 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 the NTP timestamp format and Section 3 the NTP message format. Section 4 provides general NTP protocol details, with the subset SNTP described in Section 5. This is followed by specific sections on Server (Section 6), Client(Section 7), and Symmetric Peer(Section 8) modes of operation.Section 9 summarizes the optional security mechanisms present within the NTPv4 protocol. Section 10 defines the new mechanism for server discovery. Section 11 describes the control and management mechanism for NTP. Section 12 describes the kiss-o'-death message, whose functionality is similar to the ICMP Source Quench and ICMP Destination Unreachable messages. Section 13 presents NTPv4 security considerations and Section 14 discusses IANA Considerations. Finally, Section 15 presents various other considerations when implementing and/or configuring NTPv4. NTPv4 is hereafter referred to simply as NTP, unless explicitly noted. 2. 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. Burbank, et al. Expires April 26, 2006 [Page 5] Internet-Draft NTPv4 Protocol Specification October 2005 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 defined as: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. 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. Burbank, et al. Expires April 26, 2006 [Page 6] Internet-Draft NTPv4 Protocol Specification October 2005 3. NTP Message Formats Both NTP and SNTP are layered above of the User Datagram Protocol (UDP) [6], which itself is layered on the Internet Protocol (IP) [7] [8]. 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 as: Burbank, et al. Expires April 26, 2006 [Page 7] Internet-Draft NTPv4 Protocol Specification October 2005 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) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3.1 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 Burbank, et al. Expires April 26, 2006 [Page 8] Internet-Draft NTPv4 Protocol Specification October 2005 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 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. 3.2 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. 3.3 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 mode or discovery mode, 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 Burbank, et al. Expires April 26, 2006 [Page 9] Internet-Draft NTPv4 Protocol Specification October 2005 (broadcast). 3.4 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 | +---------+-------------------------------------------------------+ 3.5 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). 3.6 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. 3.7 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. 3.8 Root Dispersion This is a 32-bit unsigned fixed-point number indicating the nominal error relative to the primary reference source, in seconds with Burbank, et al. Expires April 26, 2006 [Page 10] Internet-Draft NTPv4 Protocol Specification October 2005 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. 3.9 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. 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. Burbank, et al. Expires April 26, 2006 [Page 11] Internet-Draft NTPv4 Protocol Specification October 2005 3.10 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. 3.11 Originate Timestamp This is the time at which the request departed the client for the server, in 64-bit timestamp format. 3.12 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. 3.13 Transmit Timestamp This is the time at which the request departed the client or the reply departed the server, in 64-bit timestamp format. 3.14 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. Burbank, et al. Expires April 26, 2006 [Page 12] Internet-Draft NTPv4 Protocol Specification October 2005 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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3.15 Authentication (optional) The authentication field format is defined as: 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 + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 Burbank, et al. Expires April 26, 2006 [Page 13] Internet-Draft NTPv4 Protocol Specification October 2005 NTP header plus extension fields. 4. NTP Protocol Operation The NTP protocol defines three operational roles, Client, Server, and Symmetric Peer. Clients request or receive time unsolicited from Servers. Servers respond to requests or send periodic time updates to Clients. Symmetric Peers exchange time data bidirectionally. A given NTP implementation can operate in any or all of these modes. NTP messages make use of two different communication modes, one to one and one to many, commonly referred to as unicast and broadcast. For the purposes of this document, the term broadcast is interpreted to mean any available one to many mechanism. For IPv4 this equates to either IPv4 broadcast or IPv4 multicast. For IPv6 this equates to IPv6 multicast. For OSI this XXXXX. For this purpose, IANA has allocated the IPv4 multicast address 224.0.1.1 and the IPv6 multicast address ending :101, with prefix determined by scoping rules. The NTP broadcast address for OSI has yet to be determined. 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 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. NTP messages are layered on top of UDP. All messages MUST be sent with a destination port of 123, and SHOULD be sent with a source port of 123. 5. SNTP Protocol Operation SNTP operates using the same message formats, addresses, and ports as NTP. However, it is stateless, operating only in the Client or Server roles. Thus it is compatible with, and a subset of, NTP. Without the complexity and state required by the Symmetric Peer, SNTP is apropriate for devices which require a significantly smaller Burbank, et al. Expires April 26, 2006 [Page 14] Internet-Draft NTPv4 Protocol Specification October 2005 resource footprint. 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. 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. 6. NTP Server Operations Fundamentally, the NTP Server role consists of listening for client requests, and providing time and associated details as a response. Additionally, a Server can provide time and associated details periodically via a broadcast mechanism. An NTP server operating with either an NTP or SNTP client of the same or previous versions retains no persistent state. An NTP server can communicate via unicast, broadcast, or both. A server receiving a unicast request (NTP mode 3), modifies fields in the NTP header as described below, and sends a reply (NTP mode 4), possibly using the same message buffer as the request. When operating in a broadcast mode, unsolicited messages (NTP mode 5) with field values as described below are normally sent at intervals ranging from 64 s to 1024 s, depending on the expected frequency tolerance of the client clocks and the required accuracy. A broadcast server may or may not send messages if not synchronized to a correctly operating source, but the preferred option is to transmit, since this allows reachability to be determined regardless of synchronization state. The Leap Indicator (LI) is set to 3 (unsynchronized) if the server has never synchronized to a reference source. Once synchronized, 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. The Version (VN) is copied from the request packet, if responding to a unicast request. For broadcast, this is set to 4. The Mode is set to Server (4) if in response to a unicast request. For broadcast, this is set to Broadcast (5). Burbank, et al. Expires April 26, 2006 [Page 15] Internet-Draft NTPv4 Protocol Specification October 2005 The Stratum field to the server's current stratum, if synchronized. If synchronized to a reference source the Stratum field is set to 1. If unsynchronized this field is set to 0. The Poll field is coppied from the request, if responding to a unicast request. For broadcast, this is set to the nearest integer base-2 logarithm of the poll interval. 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. If the server is synchronized to a reference source, the value of the Reference ID is set to a four-character ASCII string identifying the source, left justified and zero padded to 32bits. 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. If unsynchronized, it is set to an ASCII error identifier. 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 server is synchronized and up, 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: Burbank, et al. Expires April 26, 2006 [Page 16] Internet-Draft NTPv4 Protocol Specification October 2005 +----------------+----------------+----------------+----------------+ | Field Name | Unicast | Unicast Reply | Broadcast | | | Request | | | +----------------+----------------+----------------+----------------+ | 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 | ignore | 0 | 0 | | Dispersion | | | | | Reference | ignore | source ident | source ident | | Identifier | | | | | Reference | ignore | time of last | time of last | | Timestamp | | src. update | src. update | | Originate | ignore | copied from | 0 | | Timestamp | | xmit timestamp | | | Receive | ignore | time of day | 0 | | Timestamp | | | | | Transmit | (see text) | time of day | time of day | | Timestamp | | | | | Authenticator | optional | optional | optional | +----------------+----------------+----------------+----------------+ 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 7. NTP Client Operations The role of an NTP client is to determine the current time (and associated information) from an NTP server. This can be done actively, by sending a unicast request to a configured server, or passively by listening on a known address for periodic server messages. Burbank, et al. Expires April 26, 2006 [Page 17] Internet-Draft NTPv4 Protocol Specification October 2005 An NTP client can operate in unicast or broadcast 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. 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. 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 12. A unicast 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 in Section 3 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 [9], and NTPv1 as defined in [10]. NTPv0 defined in [11] is not supported. In unicast mode, 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 Burbank, et al. Expires April 26, 2006 [Page 18] Internet-Draft NTPv4 Protocol Specification October 2005 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. 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. The following table summarizes the proper setting of these fields: +----------------+----------------+----------------+----------------+ | Field Name | Unicast | Unicast Reply | Broadcast | | | Request | | | +----------------+----------------+----------------+----------------+ | 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 | 0 | ignore | ignore | | Dispersion | | | | | Reference | 0 | ignore | ignore | | Identifier | | | | | Reference | 0 | ignore | ignore | | Timestamp | | | | | Originate | 0 | (see text) | ignore | | Timestamp | | | | | Receive | 0 | (see text) | ignore | | Timestamp | | | | | Transmit | (see text) | nonzero | nonzero | | Timestamp | | | | | Authenticator | optional | optional | optional | +----------------+----------------+----------------+----------------+ Burbank, et al. Expires April 26, 2006 [Page 19] Internet-Draft NTPv4 Protocol Specification October 2005 8. NTP Symmetric Peer Operations NTP Symmetric Peer mode is intended for configurations where a set of low-stratum peers operate as mutual backups for each other. Each peer normally operates with oner or more sources, such as a reference clock, or a subset of primary or secondry servers known to be reliable or authentic. Symmetric Peer mode is exclusive to the NTP protocol and is specifically excluded from SNTP operation. 9. 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. 9.1 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 Burbank, et al. Expires April 26, 2006 [Page 20] Internet-Draft NTPv4 Protocol Specification October 2005 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. 9.2 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. 9.3 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. 9.4 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 Burbank, et al. Expires April 26, 2006 [Page 21] Internet-Draft NTPv4 Protocol Specification October 2005 parties to the secret can validate a message or make a valid message. 9.5 Autokey Protocol Exchanges There are five types of Autokey protocol exchanges: 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 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. 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. 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. 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 Burbank, et al. Expires April 26, 2006 [Page 22] Internet-Draft NTPv4 Protocol Specification October 2005 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. 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. 10. Dynamic Server Discovery NTPv4 provides a mechanism, commonly known as "Manycast", for a client to dynamically discover the existance of one or more servers with no a-priori knowledge. Once servers are discovered, they are then treated as any other unicast server. A client employing server discovery is configured with MinServers, the minimum number of desired servers and MaxServers, the maximum number of desired servers. The discovery mechanism is a simple expanding ring search, using IP multicast with increasing TTLs or Hop Counts. The multicast address used MUST be scoped to the local site, as defined by [12]. The client initiates the discovery process by sending an NTP message to the configured multicast address with an IP TTL or Hop Count of 1. This message has all of the NTP header fields set to 0, except the Mode, VN and optional Transmit Timestamp fields. The Mode is set to 3. It then starts a retry timer (Default: 64 seconds) and listens for unicast responses from servers. The source address of any server responses are treated as newly configured unicast servers, up to a limit of MaxServers. If the number of discovered servers is less than MinServers when the retry timer expires, an identical NTP Burbank, et al. Expires April 26, 2006 [Page 23] Internet-Draft NTPv4 Protocol Specification October 2005 message is sent with an increased TTL/Hop Count, and the retry timer is restarted. This continues until either MinServers servers have been discovered or a configured maximum TTL/Hop Count is reached. If the configured maximum TTL/Hop Count is reached, packets continue to be periodically sent at the maximum TTL/Hop Count. If at some subsequent time, the number of valid servers drops below MinServers, the process restarts at the initial state. A server configured to provide server discovery will listen on the specified multicast address for discovery messages from clients. If the server is in scope of the current TTL and is itself synchronized to a valid source it replies to the discovery message from the client with an ordinary unicast server message as described in Section 6 11. NTP Control Messages 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 Burbank, et al. Expires April 26, 2006 [Page 24] Internet-Draft NTPv4 Protocol Specification October 2005 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 elsewhere in this document provides the capability to control access by mode number, so this could be used to limit access for control messages (mode 6) to selected address ranges. Burbank, et al. Expires April 26, 2006 [Page 25] Internet-Draft NTPv4 Protocol Specification October 2005 11.1 NTP Control Message Format The format of the NTP Control Message header, which immediately follows the UDP header, is shown in Figure X. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |00 |VN | 6 | REM | Op | Sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status | Association ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Offset | Count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Data (468 Octets Max) . . . | | Padding (zeros) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authenticator (optional)(96) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NTP Control Message Format Version Number (VN): This is a three-bit integer indicating the NTP version number, currently four (4) 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. Values currently defined include the following: Burbank, et al. Expires April 26, 2006 [Page 26] Internet-Draft NTPv4 Protocol Specification October 2005 +-------+----------------------------------------+ | Value | 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 | | 8-31 | reserved | +-------+----------------------------------------+ Sequence: This is a 16-bit integer indicating the sequence number of the command or response. 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 integer identifying a valid association. Offset: This is a 16-bit integer indicating the offset, in octets, of the first octet in the data area. Count: This is a 16-bit integer indicating the length of the data field, in octets. Data: This contains the message data for the command or response. The maximum number of data octets is 468. Authenticator (optional): When the NTP authentication mechanism is implemented, this contains the authenticator information. 11.2 Status Words 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<$&fig6> 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 Burbank, et al. Expires April 26, 2006 [Page 27] Internet-Draft NTPv4 Protocol Specification October 2005 the clock hardware and decoding software. A special error status word is used to report malformed command fields or invalid values. 11.2.1 System Status Word 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 | Clock Source | Count | Code | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ System Status Word 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: +-------+------------------------------------------+ | Value | Meaning | +-------+------------------------------------------+ | 00 | no warning | | 01 | last minute has 61 seconds | | 10 | last minute has 59 seconds | | 11 | alarm condition (clock not synchronized) | +-------+------------------------------------------+ Clock Source: This is a six-bit integer indicating the current synchronization source, with values coded as follows: +-------+---------------------------------------------------------+ | Value | Meaning | +-------+---------------------------------------------------------+ | 0 | unspecified or unknown | | 1 | Calibrated atomic clock (e.g.,, 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 Burbank, et al. Expires April 26, 2006 [Page 28] Internet-Draft NTPv4 Protocol Specification October 2005 number of system exception events occurring since the last time the system status word was returned in a response or included in a trap message. The counter is cleared when returned in the status field of a response and freezes when it reaches the value 15. 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: +---------------------------------+---------------------------------+ | Value | Meaning | +---------------------------------+---------------------------------+ | 0 | unspecified | | 1 | system restart | | 2 | system or hardware fault | | 3 | system new status word (leap | | | bits or synchronization change) | | 4 | system new synchronization | | | source or stratum (sys.peer or | | | sys.stratum change | | 5 | system clock reset (offset | | | correction exceeds CLOCK.MAX) | | 6 | system invalid time or date | | | (see NTP specification) | | 7 | system clock exception (see | | | system clock status word) | | 8-15 | reserved | +---------------------------------+---------------------------------+ 11.2.2 Peer Status Word 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 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Peer Status | Sel | Count | Code | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Peer Status Word Peer Status: This is a five-bit code indicating the status of the peer determined by the packet procedure, with bits assigned as follows: Burbank, et al. Expires April 26, 2006 [Page 29] Internet-Draft NTPv4 Protocol Specification October 2005 +-------+------------------------------------------+ | Value | Meaning | +-------+------------------------------------------+ | 0 | configured (peer.config) | | 1 | authentication enabled (peer.authenable) | | 2 | authentication okay (peer.authentic) | | 3 | reachability okay (peer.reach) | | 4 | reserved | +-------+------------------------------------------+ 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: +---------------------------------+---------------------------------+ | Value | Meaning | +---------------------------------+---------------------------------+ | 0 | rejected | | 1 | passed sanity checks | | 2 | passed correctness checks | | 3 | passed candidate checks (if | | | limit check implemented) | | 4 | passed outlyer checks | | 5 | current synchronization source; | | | max distance exceeded (if limit | | | check implemented) | | 6 | current synchronization source; | | | max distance okay | | 7 | reserved | +---------------------------------+---------------------------------+ Peer Event Counter: This is a four-bit integer indicating the number of peer exception events that occurred since the last time the peer status word was returned in a response or included in a trap message. The counter is cleared when returned in the status field of a response and freezes when it reaches the value 15. 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: Burbank, et al. Expires April 26, 2006 [Page 30] Internet-Draft NTPv4 Protocol Specification October 2005 +---------------------------------+---------------------------------+ | Value | Meaning | +---------------------------------+---------------------------------+ | 0 | unspecified | | 1 | peer IP error | | 2 | peer authentication failure | | | (peer.authentic bit was one now | | | zero) | | 3 | peer unreachable (peer.reach | | | was nonzero now zero) | | 4 | peer reachable (peer.reach was | | | zero now nonzero) | | 5 | peer clock exception (see peer | | | clock status word) | | 6-15 | reserved | +---------------------------------+---------------------------------+ 11.2.3 Clock Status Word 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 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Clock Status | Code | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Clock Status Word Clock Status: This is an eight-bit integer indicating the current clock status, with values coded as follows: Burbank, et al. Expires April 26, 2006 [Page 31] Internet-Draft NTPv4 Protocol Specification October 2005 +-------+---------------------------------+ | Value | Meaning | +-------+---------------------------------+ | 0 | clock operating within nominals | | 1 | reply timeout | | 2 | bad reply format | | 3 | hardware or software fault | | 4 | propagation failure | | 5 | bad date format or value | | 6 | bad time format or value | | 7-255 | reserved | +-------+---------------------------------+ Clock Event Code: This is an eight-bit integer identifying the latest clock exception event, with new values overwriting previous values. When a change to any nonzero value occurs in the radio status field, the radio status field is copied to the clock event code field and a system or peer clock exception event is declared as appropriate. 11.2.4 Error Status Word 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. It consists of an eight-bit integer coded as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Error Code | Reserved | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Error Status Word +-------+----------------------------------+ | Value | 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 | +-------+----------------------------------+ Burbank, et al. Expires April 26, 2006 [Page 32] Internet-Draft NTPv4 Protocol Specification October 2005 11.3 Commands Commands consist of the header and optional data field shown in Figure 6. When present, the data field contains a list of identifiers or assignments in the form <>[=<>],<>[=<>],... where <> is the ASCII name of a system or peer variable specified in Table 2 or Table 3 and <> is expressed as a decimal, hexadecimal or string constant in the syntax of the C programming language. Where no ambiguity exists, the <169>sys.<170> or <169>peer.<170> prefixes shown in Table 2 or Table 4 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 addresses are represented as four octets in the form [n.n.n.n], where n is in decimal notation and the brackets are optional. 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 Table 2 or Table 3. Called extramural variables, these are distinguished by the inclusion of some character type other than alphanumeric or <169>.<170> 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 Table 3 or Table 4 of the NTP specification 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 Burbank, et al. Expires April 26, 2006 [Page 33] Internet-Draft NTPv4 Protocol Specification October 2005 <> <>, 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. 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 message is sent 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 Burbank, et al. Expires April 26, 2006 [Page 34] Internet-Draft NTPv4 Protocol Specification October 2005 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. 12. 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. +---------------------------------+---------------------------------+ | Code | Meaning | +---------------------------------+---------------------------------+ | ACST | The association belongs to an | | | 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 | Burbank, et al. Expires April 26, 2006 [Page 35] Internet-Draft NTPv4 Protocol Specification October 2005 | MCST | The association belongs to a | | | dynamically discovered 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 15. 13. 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: When the IP source and destination addresses are available for the client request, they should match the interchanged addresses in the server reply. When the UDP source and destination ports are available for the client request, they should match the interchanged ports in the server reply. Burbank, et al. Expires April 26, 2006 [Page 36] Internet-Draft NTPv4 Protocol Specification October 2005 The Originate Timestamp in the server reply should match the Transmit Timestamp used in the client request. 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). 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. 14. IANA Considerations 15. 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. A client MUST NOT use a poll interval less than TBD minutes. 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. A client SHOULD use local servers whenever available to avoid unnecessary traffic on backbone networks. 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. 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. A client SHOULD use the Domain Name System (DNS) to resolve the server IP addresses, so the operator can do effective load Burbank, et al. Expires April 26, 2006 [Page 37] Internet-Draft NTPv4 Protocol Specification October 2005 balancing among a server clique and change IP address binding to canonical names. 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. 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): 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. 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. Burbank, et al. Expires April 26, 2006 [Page 38] Internet-Draft NTPv4 Protocol Specification October 2005 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. 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. 16. 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. 17. References 17.1 Normative References [1] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992. [2] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI", RFC 2030, October 1996. 17.2 Informative References [3] International Standards Organization, "International Standards 8602 - Information Processing Systems - OSI: Connectionless Transport Protocol Specification.", ISO 8602, December 1986. [4] Shue, C., Haggerty, W., and K. Dobbins, "OSI connectionless transport services on top of UDP: Version 1", RFC 1240, June 1991. [5] Furniss, P., "Octet Sequences for Upper-Layer OSI to Support Basic Communications Applications", RFC 1698, October 1994. Burbank, et al. Expires April 26, 2006 [Page 39] Internet-Draft NTPv4 Protocol Specification October 2005 [6] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [7] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [8] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [9] Mills, D., "Network Time Protocol (version 2) specification and implementation", STD 12, RFC 1119, September 1989. [10] Mills, D., "Network Time Protocol (version 1) specification and implementation", RFC 1059, July 1988. [11] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985. [12] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC 2365, July 1998. [13] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [14] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989. Authors' Addresses Jack Burbank (editor) Johns Hopkins University Applied Physics Lab 11100 Johns Hopkins Road Laurel, MD 20723-6099 US Phone: +1 443 778 7127 Email: jack.burbank@jhuapl.edu Burbank, et al. Expires April 26, 2006 [Page 40] Internet-Draft NTPv4 Protocol Specification October 2005 Jim Martin (editor) Netzwert AG An den Treptowers 1 Berlin 12435 Germany Phone: +49.30/5 900 80-1180 Email: jim@netzwert.ag Dr. David L. Mills University of Delaware Newark, DE 19716 US Phone: +1 302 831 8247 Email: mills@udel.edu Burbank, et al. Expires April 26, 2006 [Page 41] Internet-Draft NTPv4 Protocol Specification October 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Burbank, et al. Expires April 26, 2006 [Page 42]