Simple Network Time Protocol (SNTP) V4

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Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6, and OSI

The Simple Network Time Protocol (SNTP) Version 4 is an adaptation of the Network Time Protocol (NTP) used to synchronize computer clocks in the Internet. SNTP can be used when the ultimate performance of the full NTP implementation described in RFC-1305 is not needed or justified. When operating with current and previous NTP and SNTP versions, SNTP Version 4 involves no changes to the NTP specification or known implementations, but rather a clarification of certain design features of NTP which allow operation in a simple, stateless remote-procedure call (RPC) mode with accuracy and reliability expectations similar to the UDP/TIME protocol described in RFC-868.

The only significant protocol change in SNTP Version 4 over previous versions of NTP and SNTP is a modified header interpretation to accommodate Internet Protocol Version 6 (IPv6) and OSI addressing. However, SNTP Version 4 includes certain optional extensions to the basic Version 3 model, including an anycast mode and an authentication scheme designed specifically for multicast and anycast modes. While the anycast mode extension is described in this document, the authentication scheme extension will be described in another document to be published later. Until such time that a definitive specification is published, these extensions should be considered provisional.


The Network Time Protocol (NTP) Version 3 specified in RFC-1305 is widely used to synchronize computer clocks in the global Internet. It provides comprehensive mechanisms to access national time and frequency dissemination services, organize the time-synchronization subnet and adjust the local clock in each participating subnet peer. 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.

RFC-1305 specifies the NTP Version 3 protocol machine in terms of events, states, transition functions and actions and, in addition, engineered algorithms to improve the timekeeping quality and mitigate among several synchronization sources, some of which may be faulty. To achieve accuracies in the low milliseconds over paths spanning major portions of the Internet of today, these intricate algorithms, or their functional equivalents, are necessary. However, in many cases accuracies in the order of significant fractions of a second are acceptable. In such cases, simpler protocols such as the Time Protocol, have been used for this purpose. These protocols usually involve an RPC exchange where the client requests the time of day and the server returns it in seconds past some known reference epoch.

NTP is designed for use by clients and servers with a wide range of capabilities and over a wide range of network delays and jitter characteristics. Most users of the Internet NTP synchronization subnet of today use a software package including the full suite of NTP options and algorithms, which are relatively complex, real-time applications. 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. Accordingly, it is useful to explore alternative access strategies using simpler software appropriate for less stringent accuracy expectations.

The Simple Network Time Protocol (SNTP) Version 4, which is a simplified access strategy for servers and clients using NTP Version 3 as now specified and deployed on the Internet, as well as NTP Version 4 now under development. The access paradigm is identical to the UDP/TIME Protocol and, in fact, it should be easily possible to adapt a UDP/TIME client implementation, say for a personal computer, to operate using SNTP. Moreover, SNTP is also designed to operate in a dedicated server configuration including an integrated radio clock. With careful design and control of the various latencies in the system, which is practical in a dedicated design, it is possible to deliver time accurate to the order of microseconds.

SNTP Version 4 is designed to coexist with existing NTP and SNTP Version 3 clients and servers, as well as proposed Version 4 clients and servers. When operating with current and previous versions of NTP and SNTP, SNTP Version 4 requires no changes to the protocol or implementations now running or likely to be implemented specifically for NTP is SNTP Version 4. To an NTP or SNTP server, NTP and SNTP clients are undistinguishable; to an NTP or SNTP client, NTP and SNTP servers are undistinguishable. Like NTP servers operating in non-symmetric modes, SNTP servers are stateless and can support large numbers of clients; however, unlike most NTP clients, SNTP clients normally operate with only a single server. NTP and SNTP Version 3 servers can operate in unicast and multicast modes. In addition, SNTP Version 4 clients and servers can implement extensions to operate in anycast mode.

It is strongly recommended that SNTP is used only at the extremities of the synchronization subnet. SNTP clients should operate only at the leaves (highest stratum) of the subnet and in configurations where no NTP or SNTP client is dependent on another SNTP client for synchronization. SNTP servers should operate only at the root (stratum 1) of the subnet and then only in configurations where no other source of synchronization other than a reliable radio or modem time service is available. The full degree of reliability ordinarily expected of primary servers is possible only using the redundant sources, diverse subnet paths and crafted algorithms of a full NTP implementation. This extends to the primary source of synchronization itself in the form of multiple radio or modem sources and backup paths to other primary servers should all sources fail or the majority deliver incorrect time. Therefore, the use of SNTP rather than NTP in primary servers should be carefully considered.

An important provision in this document is the reinterpretation of certain NTP Version 4 header fields which provide for IPv6 and OSI addressing and optional anycast extensions designed specifically for multicast service. These additions are in conjunction with the proposed NTP Version 4 specification, which will appear as a separate document. The only difference between the current NTP Version 3 and proposed NTP Version 4 header formats is the interpretation of the four-octet Reference Identifier field, which is used primarily to detect and avoid synchronization loops. In Version 3 and Version 4 primary (stratum-1) servers, this field contains the four-character ASCII reference identifier defined later in this document. In Version 3 secondary servers and clients, it contains the 32-bit IPv4 address of the synchronization source. In Version 4 secondary servers and clients, it contains the low order 32 bits of the last transmit timestamp received from the synchronization source.

In the case of OSI, the Connectionless Transport Service (CLTS) is used. Each SNTP packet is transmitted as that TS-User data parameter of a T-UNITDATA Request primitive. Alternately, the header can be encapsulated in a TPDU which itself is transported using UDP. It is not advised that NTP is operated at the upper layers of the OSI stack, such as might be inferred from, as this could seriously degrade accuracy. With the header formats defined in this document, 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.

Operating Modes and Addressing

SNTP Version 4 can operate in either unicast (point to point), multicast (point to multipoint) or anycast (multipoint to point) modes. A unicast client sends a request to a designated server at its unicast address and expects a reply from which it can determine the time and, optionally, the roundtrip delay and local clock offset relative to the server. A multicast server periodically sends an unsolicited message to a designated IPv4 or IPv6 local broadcast address or multicast group address and ordinarily expects no requests from clients. A multicast client listens on this address and ordinarily sends no requests. An anycast client sends a request to a designated IPv4 or IPv6 local broadcast address or multicast group address. One or more anycast servers reply with their individual unicast addresses. The client binds to the first one received, then continues operation in unicast mode.

Multicast servers should respond to client unicast requests, as well as send unsolicited multicast messages. Multicast clients may send unicast requests in order to determine the network propagation delay between the server and client and then continue operation in multicast mode.

In unicast mode, the client and server end-system addresses are assigned following the usual IPv4, IPv6 or OSI conventions. In multicast mode, the server uses a designated local broadcast address or multicast group address. An IP local broadcast address has a scope limited to a single IP subnet since routers do not propagate IP broadcast datagrams. On the other hand, an IP multicast group address has scope extending to potentially the entire Internet. The scoping, routing and group membership procedures are determined by considerations beyond the scope of this document. For IPv4, the IANA has assigned the multicast group address for NTP, which is used both by multicast servers and anycast clients. NTP multicast addresses for IPv6 and OSI have yet to be determined.

Multicast clients listen on the designated local broadcast address or multicast group address. In the case of local broadcast addresses, no further provisions are necessary. In the case of IP multicast addresses, the multicast client and anycast server must implement the Internet Group Management Protocol (IGMP), in order that the local router joins the multicast group and relays messages to the IPv4 or IPv6 multicast group addresses assigned by the IANA. Other than the IP addressing conventions and IGMP, there is no difference in server or client operations with either the local broadcast address or multicast group address.

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 co-operating anycast servers in scope will reply to a client request. The engineering principles which determine the proper value to be used are beyond the scope of this document.

Anycast mode is designed for use with a set of cooperating servers whose addresses are not known beforehand by the client. An anycast client sends a request to the designated local broadcast or multicast group address as described below. For this purpose, the NTP multicast group address assigned by the IANA is used. One or more anycast servers listen on the designated local broadcast address or multicast group address. Each anycast server, upon receiving a request, sends a unicast reply message to the originating client. The client then binds to the first such message received and continues operation in unicast mode. Subsequent replies from other anycast servers are ignored.

In the case of SNTP, as specified herein, there is a very real vulnerability that SNTP multicast clients can be disrupted by misbehaving or hostile SNTP or NTP multicast servers elsewhere on the Internet since at present all such servers use the same IPv4 multicast group address assigned by the IANA. Where necessary, access control based on the server source address can be used to select only the designated server known to and trusted by the client. The use of cryptographic authentication scheme defined in RFC-1305 is optional; however, implementors should be advised that extensions to this scheme are planned specifically for NTP multicast and anycast modes.

While not integral to the SNTP 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 SNTP multicast 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.

In NTP Version 3, the reference identifier was often used to walk-back the synchronization subnet to the root (primary server) for management purposes. In NTP Version 4, this feature is not available, since the addresses are longer than 32 bits. However, the intent in the protocol design was to provide a way to detect and avoid loops. A peer could determine that a loop was possible by comparing the contents of this field with the IPv4 destination address in the same packet. An NTP Version 4 server can accomplish the same thing by comparing the contents of this field with the low order 32 bits of the originate timestamp in the same packet. There is a small possibility of false alarm in this scheme, but the false alarm rate can be minimized by randomizing the low order unused bits of the transmit timestamp.


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