RFC 958

RFC 958. Published in September 1985, this document is the original specification for the Network Time Protocol (NTP), authored by David L. Mills. It established the foundational architecture and algorithms for synchronizing clocks across packet-switched networks with variable latency. Although long since superseded by later versions, this Request for Comments represents a seminal milestone in the history of Internet engineering.

Overview

The primary objective of this specification was to define a protocol capable of synchronizing the clocks of host computers and routing devices across large, interconnected networks like the ARPANET. It introduced a hierarchical, client-server architecture designed to distribute time from primary reference sources, such as those connected to radio clocks or atomic clocks, throughout the system. The protocol was engineered to provide reliable synchronization even in the presence of significant network jitter, packet loss, and failure of individual time servers. This work laid the essential groundwork for the coordinated operation of distributed systems, which would become critical for applications ranging from financial transaction logging to scientific experiment coordination.

Technical details

The protocol operates using a User Datagram Protocol (UDP) transport mechanism, with a designated port number of 123 for time service exchanges. Its core algorithm involves the exchange of timestamped packets between a client and one or more servers, allowing the client to compute estimates of both the time offset and the round-trip delay to each server. Key packet fields defined include the Leap Indicator, Version Number, Mode, Stratum, Poll Interval, Precision, and the critical Reference Timestamp, Originate Timestamp, Receive Timestamp, and Transmit Timestamp. The system employs a filtering and selection process to combine observations from multiple servers, mitigating the effects of unreliable networks and identifying potentially faulty time sources. The original specification also described a simple clock discipline algorithm to adjust the local system clock gradually, avoiding abrupt changes that could disrupt applications.

Historical context and impact

This document emerged during a period of rapid expansion for the ARPANET and the nascent National Science Foundation Network (NSFNET), where the lack of synchronized time was becoming a significant operational hindrance. Prior methods for time distribution, such as the Time Protocol (RFC 868) and the Daytime Protocol (RFC 867), were too simplistic for the precision required by evolving network applications and management tools. The publication of this RFC directly addressed this need, providing a robust, scalable solution. Its immediate impact was to enable precise event ordering in network monitoring systems like those used by the Internet Engineering Task Force (IETF) and to facilitate debugging in complex, distributed environments. The concepts it introduced became the bedrock for all subsequent versions of NTP, which is now one of the oldest continuously operating Internet protocols and is fundamental to the operation of the modern Internet.

Related RFCs and standards

This initial specification was quickly refined and replaced; it was obsoleted by RFC 1059 (NTP Version 1), then by RFC 1119 (NTP Version 2), and ultimately by RFC 1305 (NTP Version 3). The current standard is defined in RFC 5905 (NTP Version 4). The protocol family also includes the simpler Simple Network Time Protocol (SNTP), specified in RFC 4330 and later RFC 5905. Related time distribution protocols developed in the same era include the aforementioned Time Protocol and Internet Control Message Protocol (ICMP) timestamp messages. The ongoing development of NTP is managed within the IETF's Network Time Foundation (NTF) and is closely aligned with standards for time formats, such as those defined in RFC 3339. Its operation often relies on time sources traceable to global standards maintained by institutions like the National Institute of Standards and Technology (NIST) and the United States Naval Observatory (USNO).

Category:Internet standards Category:Network protocols Category:1985 documents