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117-607: NTP is intended to synchronize participating computers to within a few milliseconds of Coordinated Universal Time (UTC). It uses the intersection algorithm , a modified version of Marzullo's algorithm , to select accurate time servers and is designed to mitigate the effects of variable network latency . NTP can usually maintain time to within tens of milliseconds over the public Internet , and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more. The protocol

234-550: A Windows domain identity, known as MS-SNTP. This system is implemented in the reference ntpd and chrony, using samba for the domain connection. Synchronize Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or in time . Systems that operate with all parts in synchrony are said to be synchronous or in sync —and those that are not are asynchronous . Today, time synchronization can occur between systems around

351-454: A configuration file , an attached reference clock, or a remote server. Although the NTP clock is actually halted during the event, because of the requirement that time must appear to be strictly increasing , any processes that query the system time cause it to increase by a tiny amount, preserving the order of events. If a negative leap second should ever become necessary, it would be deleted with

468-672: A content-addressable , peer-to-peer method of storing and sharing hypermedia distribution protocol, with nodes in the IPFS network forming a distributed file system . Jami is a peer-to-peer chat and SIP app. JXTA is a peer-to-peer protocol designed for the Java platform . Netsukuku is a Wireless community network designed to be independent from the Internet. Open Garden is a connection-sharing application that shares Internet access with other devices using Wi-Fi or Bluetooth. Resilio Sync

585-614: A server experiences an increase in load it redirects incoming peers to other peers who have agreed to mirror the content, thus off-loading balance from the server. All of the information is retained at the server. This system makes use of the fact that the bottleneck is most likely in the outgoing bandwidth than the CPU , hence its server-centric design. It assigns peers to other peers who are 'close in IP ' to its neighbors [same prefix range] in an attempt to use locality. If multiple peers are found with

702-471: A symmetric key mode, which is not useful against MITM. The public key system known as "autokey" in NTPv4 adapted from IPSec offers useful authentication, but is not practical for a busy server. Autokey was also later found to suffer from several design flaws, with no correction published, save for a change in the message authentication code . Autokey should no longer be used. Network Time Security (NTS)

819-431: A synchronous circuit requires a clock signal . A clock signal simply signals the start or end of some time period, often measured in microseconds or nanoseconds, that has an arbitrary relationship to any other system of measurement of the passage of minutes, hours, and days. In a different sense, electronic systems are sometimes synchronized to make events at points far apart appear simultaneous or near-simultaneous from

936-498: A P2P network are more stable and available than files on central networks. In a centralized network, a simple loss of connection between the server and clients can cause a failure, but in P2P networks, the connections between every node must be lost to cause a data-sharing failure. In a centralized system, the administrators are responsible for all data recovery and backups, while in P2P systems, each node requires its backup system. Because of

1053-534: A P2P network requires that at least one node in the network has the requested data, and that node must be able to connect to the node requesting the data. This requirement is occasionally hard to meet because users may delete or stop sharing data at any point. In a P2P network, the community of users is entirely responsible for deciding which content is available. Unpopular files eventually disappear and become unavailable as fewer people share them. Popular files, however, are highly and easily distributed. Popular files on

1170-409: A centralized functionality but benefit from the decentralized aggregation of nodes provided by unstructured networks. CoopNet (Cooperative Networking) was a proposed system for off-loading serving to peers who have recently downloaded content, proposed by computer scientists Venkata N. Padmanabhan and Kunwadee Sripanidkulchai, working at Microsoft Research and Carnegie Mellon University . When

1287-505: A certain perspective. Timekeeping technologies such as the GPS satellites and Network Time Protocol (NTP) provide real-time access to a close approximation to the UTC timescale and are used for many terrestrial synchronization applications of this kind. In computer science (especially parallel computing ), synchronization is the coordination of simultaneous threads or processes to complete

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1404-602: A combination of peer-to-peer and client–server models. A common hybrid model is to have a central server that helps peers find each other. Spotify was an example of a hybrid model [until 2014]. There are a variety of hybrid models, all of which make trade-offs between the centralized functionality provided by a structured server/client network and the node equality afforded by the pure peer-to-peer unstructured networks. Currently, hybrid models have better performance than either pure unstructured networks or pure structured networks because certain functions, such as searching, do require

1521-430: A few incidents. It is able to achieve improved precision on LAN connections, using hardware timestamping on the network adapter. Support for Network Time Security (NTS) was added on version 4.0. chrony is available under GNU General Public License version 2 , was created by Richard Curnow in 1997 and is currently maintained by Miroslav Lichvar . On the day of a leap second event, ntpd receives notification from either

1638-458: A focus on security and encompassing a privilege separated design. Whilst it is aimed more closely at the simpler generic needs of OpenBSD users, it also includes some protocol security improvements while still being compatible with existing NTP servers. The simpler code base sacrifices accuracy, deemed unnecessary in this use case. A portable version is available in Linux package repositories. NTPsec

1755-675: A given time. Trustworthiness of sources is a potential security threat that can be seen with peer-to-peer systems. A study ordered by the European Union found that illegal downloading may lead to an increase in overall video game sales because newer games charge for extra features or levels. The paper concluded that piracy had a negative financial impact on movies, music, and literature. The study relied on self-reported data about game purchases and use of illegal download sites. Pains were taken to remove effects of false and misremembered responses. Peer-to-peer applications present one of

1872-443: A hierarchical, semi-layered system of time sources. Each level of this hierarchy is termed a stratum and is assigned a number starting with zero for the reference clock at the top. A server synchronized to a stratum n server runs at stratum n + 1. The number represents the distance from the reference clock and is used to prevent cyclical dependencies in the hierarchy. Stratum is not always an indication of quality or reliability; it

1989-563: A hierarchy of servers similar to the NTP stratum model. In 1985, NTP version 0 (NTPv0) was implemented in both Fuzzball and Unix, and the NTP packet header and round-trip delay and offset calculations, which have persisted into NTPv4, were documented in RFC   958 . Despite the relatively slow computers and networks available at the time, accuracy of better than 100 milliseconds was usually obtained on Atlantic spanning links, with accuracy of tens of milliseconds on Ethernet networks. In 1988,

2106-460: A hybrid between the two). Unstructured peer-to-peer networks do not impose a particular structure on the overlay network by design, but rather are formed by nodes that randomly form connections to each other. ( Gnutella , Gossip , and Kazaa are examples of unstructured P2P protocols). Because there is no structure globally imposed upon them, unstructured networks are easy to build and allow for localized optimizations to different regions of

2223-445: A man-in-the-middle attack can be used to alter clocks on client computers and allow a number of attacks based on bypassing of cryptographic key expiration. Some of the services affected by fake NTP messages identified are TLS , DNSSEC , various caching schemes (such as DNS cache), Border Gateway Protocol (BGP), Bitcoin and a number of persistent login schemes. NTP has been used in distributed denial of service attacks . A small query

2340-559: A meme refers to the egalitarian social networking that has emerged throughout society, enabled by Internet technologies in general. While P2P systems had previously been used in many application domains, the concept was popularized by file sharing systems such as the music-sharing application Napster . The peer-to-peer movement allowed millions of Internet users to connect "directly, forming groups and collaborating to become user-created search engines, virtual supercomputers, and filesystems". The basic concept of peer-to-peer computing

2457-807: A more psychological and information-processing direction. Some peer-to-peer networks (e.g. Freenet ) place a heavy emphasis on privacy and anonymity —that is, ensuring that the contents of communications are hidden from eavesdroppers, and that the identities/locations of the participants are concealed. Public key cryptography can be used to provide encryption , data validation , authorization, and authentication for data/messages. Onion routing and other mix network protocols (e.g. Tarzan) can be used to provide anonymity. Perpetrators of live streaming sexual abuse and other cybercrimes have used peer-to-peer platforms to carry out activities with anonymity. Although peer-to-peer networks can be used for legitimate purposes, rights holders have targeted peer-to-peer over

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2574-482: A much more complete specification of the NTPv1 protocol, with associated algorithms, was published in RFC   1059 . It drew on the experimental results and clock filter algorithm documented in RFC   956 and was the first version to describe the client–server and peer-to-peer modes. In 1991, the NTPv1 architecture, protocol and algorithms were brought to the attention of a wider engineering community with

2691-477: A peer-to-peer network in conjunction with streaming servers to stream audio and video to their clients. Peercasting is employed for multicasting streams. Additionally, a project called LionShare , undertaken by Pennsylvania State University , MIT, and Simon Fraser University , aims to facilitate file sharing among educational institutions globally. Another notable program, Osiris , enables users to create anonymous and autonomous web portals that are distributed via

2808-464: A peer-to-peer network. Dat is a distributed version-controlled publishing platform. I2P , is an overlay network used to browse the Internet anonymously . Unlike the related I2P, the Tor network is not itself peer-to-peer ; however, it can enable peer-to-peer applications to be built on top of it via onion services . The InterPlanetary File System (IPFS) is a protocol and network designed to create

2925-459: A portion of their resources, such as processing power, disk storage, or network bandwidth , directly available to other network participants, without the need for central coordination by servers or stable hosts. Peers are both suppliers and consumers of resources, in contrast to the traditional client–server model in which the consumption and supply of resources are divided. While P2P systems had previously been used in many application domains ,

3042-449: A proprietary peer-to-peer technology called "Delivery Optimization" to deploy operating system updates using end-users' PCs either on the local network or other PCs. According to Microsoft's Channel 9, this led to a 30%-50% reduction in Internet bandwidth usage. Artisoft's LANtastic was built as a peer-to-peer operating system where machines can function as both servers and workstations simultaneously. Hotline Communications Hotline Client

3159-466: A simpler codebase allowing for better security and lower resource consumption. It does not however compromise on accuracy, instead syncing faster and better than the reference ntpd in many circumstances. It is versatile enough for ordinary computers, which are unstable, go into sleep mode or have intermittent connection to the Internet. It is also designed for virtual machines, a more unstable environment. Chrony has been evaluated as "trustworthy", with only

3276-443: A task with correct runtime order and no unexpected race conditions ; see synchronization (computer science) for details. Synchronization is also an important concept in the following fields: Synchronization of multiple interacting dynamical systems can occur when the systems are autonomous oscillators . Poincaré phase oscillators are model systems that can interact and partially synchronize within random or regular networks. In

3393-419: A warning of any impending leap second adjustment, but no information about local time zones or daylight saving time is transmitted. The current protocol is version 4 (NTPv4), which is backward compatible with version 3. In 1979, network time synchronization technology was used in what was possibly the first public demonstration of Internet services running over a trans-Atlantic satellite network, at

3510-511: Is a fork of the reference implementation that has been systematically security-hardened . The fork point was in June 2015 and was in response to a series of compromises in 2014. The first production release shipped in October 2017. Between removal of unsafe features, removal of support for obsolete hardware, and removal of support for obsolete Unix variants, NTPsec has been able to pare away 75% of

3627-693: Is a directory-syncing app. Research includes projects such as the Chord project , the PAST storage utility , the P-Grid , and the CoopNet content distribution system . Secure Scuttlebutt is a peer-to-peer gossip protocol capable of supporting many different types of applications, primarily social networking . Syncthing is also a directory-syncing app. Tradepal l and M-commerce applications are designed to power real-time marketplaces. The U.S. Department of Defense

Network Time Protocol - Misplaced Pages Continue

3744-504: Is a secure version of NTPv4 with TLS and AEAD . The main improvement over previous attempts is that a separate "key establishment" server handles the heavy asymmetric cryptography, which needs to be done only once. If the server goes down, previous users would still be able to fetch time without fear of MITM. NTS is currently supported by several time servers, including Cloudflare . It is supported by NTPSec and chrony. Microsoft also has an approach to authenticate NTPv3/SNTPv4 packets using

3861-552: Is also known to have uploaded fake music and movies to P2P networks in order to deter illegal file sharing. Consequently, the P2P networks of today have seen an enormous increase of their security and file verification mechanisms. Modern hashing , chunk verification and different encryption methods have made most networks resistant to almost any type of attack, even when major parts of the respective network have been replaced by faked or nonfunctional hosts. The decentralized nature of P2P networks increases robustness because it removes

3978-417: Is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources. A brief description of strata 0, 1, 2 and 3 is provided below. The upper limit for stratum is 15; stratum 16 is used to indicate that a device is unsynchronized. The NTP algorithms on each computer interact to construct a Bellman–Ford shortest-path spanning tree , to minimize the accumulated round-trip delay to

4095-697: Is conducting research on P2P networks as part of its modern network warfare strategy. In May 2003, Anthony Tether , then director of DARPA , testified that the United States military uses P2P networks. WebTorrent is a P2P streaming torrent client in JavaScript for use in web browsers , as well as in the WebTorrent Desktop standalone version that bridges WebTorrent and BitTorrent serverless networks. Microsoft , in Windows 10 , uses

4212-701: Is defined as similar movements between two or more people who are temporally aligned. This is different from mimicry, which occurs after a short delay. Line dance and military step are examples. Muscular bonding is the idea that moving in time evokes particular emotions. This sparked some of the first research into movement synchronization and its effects on human emotion. In groups, synchronization of movement has been shown to increase conformity, cooperation and trust. In dyads , groups of two people, synchronization has been demonstrated to increase affiliation, self-esteem, compassion and altruistic behaviour and increase rapport. During arguments, synchrony between

4329-420: Is derived from the best three remaining candidates. The clock frequency is then adjusted to reduce the offset gradually ("discipline"), creating a feedback loop . Accurate synchronization is achieved when both the incoming and outgoing routes between the client and the server have symmetrical nominal delay. If the routes do not have a common nominal delay, a systematic bias exists of half the difference between

4446-482: Is designed around the notion of equal peer nodes simultaneously functioning as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client–server model where communication is usually to and from a central server. A typical example of a file transfer that uses the client-server model is the File Transfer Protocol (FTP) service in which

4563-409: Is high. Therefore, the community should work together to get these features in open-source software. This would reduce the need for custom simulators, and hence increase repeatability and reputability of experiments." Popular simulators that were widely used in the past are NS2, OMNeT++, SimPy, NetLogo, PlanetLab, ProtoPeer, QTM, PeerSim, ONE, P2PStrmSim, PlanetSim, GNUSim, and Bharambe. Besides all

4680-710: Is necessary to synchronize video frames from multiple cameras. In addition to enabling basic editing, synchronization can also be used for 3D reconstruction In electric power systems, alternator synchronization is required when multiple generators are connected to an electrical grid. Arbiters are needed in digital electronic systems such as microprocessors to deal with asynchronous inputs. There are also electronic digital circuits called synchronizers that attempt to perform arbitration in one clock cycle. Synchronizers, unlike arbiters, are prone to failure. (See metastability in electronics ). Encryption systems usually require some synchronization mechanism to ensure that

4797-562: Is not to be mixed with a public NTP pool as leap smear is non-standard and will throw off client calculation in a mix. Because adjusting system time is generally a privileged operation, part or all of NTP code has to be run with some privileges in order to support its core functionality. Only a few other security problems have been identified in the reference implementation of the NTP codebase, but those that appeared in 2009 were cause for significant concern. The protocol has been undergoing revision and review throughout its history. The codebase for

Network Time Protocol - Misplaced Pages Continue

4914-453: Is required to separate the effect of intentionality from the beneficial effect of synchrony. Synchronization is important in digital telephony , video and digital audio where streams of sampled data are manipulated. Synchronization of image and sound was an important technical problem in sound film . More sophisticated film, video, and audio applications use time code to synchronize audio and video. In movie and television production it

5031-532: Is sent to an NTP server with the return IP address spoofed to be the target address. Similar to the DNS amplification attack , the server responds with a much larger reply that allows an attacker to substantially increase the amount of data being sent to the target. To avoid participating in an attack, NTP server software can be upgraded or servers can be configured to ignore external queries. NTP itself includes support for authenticating servers to clients. NTPv3 supports

5148-460: Is strictly a client-server relationship. In May 1999, with millions more people on the Internet, Shawn Fanning introduced the music and file-sharing application called Napster. Napster was the beginning of peer-to-peer networks, as we know them today, where "participating users establish a virtual network, entirely independent from the physical network, without having to obey any administrative authorities or restrictions". A peer-to-peer network

5265-514: Is that peer-to-peer applications act as servers as well as clients, meaning that they can be more vulnerable to remote exploits . Since each node plays a role in routing traffic through the network, malicious users can perform a variety of "routing attacks", or denial of service attacks. Examples of common routing attacks include "incorrect lookup routing" whereby malicious nodes deliberately forward requests incorrectly or return false results, "incorrect routing updates" where malicious nodes corrupt

5382-416: Is that results must be reproducible so that other researchers can replicate, validate, and extend existing work." If the research cannot be reproduced, then the opportunity for further research is hindered. "Even though new simulators continue to be released, the research community tends towards only a handful of open-source simulators. The demand for features in simulators, as shown by our criteria and survey,

5499-527: Is usually described in terms of a client–server model , but can as easily be used in peer-to-peer relationships where both peers consider the other to be a potential time source. Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123. They can also use broadcasting or multicasting , where clients passively listen to time updates after an initial round-trip calibrating exchange. NTP supplies

5616-703: The Daytime and Time protocols for recording the time of events, as well as the ICMP Timestamp messages and IP Timestamp option ( RFC   781 ). More complete synchronization systems, although lacking NTP's data analysis and clock disciplining algorithms, include the Unix daemon timed , which uses an election algorithm to appoint a server for all the clients; and the Digital Time Synchronization Service (DTSS), which uses

5733-769: The National Computer Conference in New York. The technology was later described in the 1981 Internet Engineering Note (IEN) 173 and a public protocol was developed from it that was documented in RFC   778 . The technology was first deployed in a local area network as part of the Hello routing protocol and implemented in the Fuzzball router , an experimental operating system used in network prototyping, where it ran for many years. Other related network tools were available both then and now. They include

5850-509: The RIAA , MPAA , and entertainment industry to filter out copyrighted content. Although server-client networks are able to monitor and manage content availability, they can have more stability in the availability of the content they choose to host. A client should not have trouble accessing obscure content that is being shared on a stable centralized network. P2P networks, however, are more unreliable in sharing unpopular files because sharing files in

5967-475: The YaCy . Some prominent research projects include the Chord project , Kademlia , PAST storage utility , P-Grid , a self-organized and emerging overlay network, and CoopNet content distribution system . DHT-based networks have also been widely utilized for accomplishing efficient resource discovery for grid computing systems, as it aids in resource management and scheduling of applications. Hybrid models are

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6084-488: The application layer peers can communicate with each other directly, via the logical overlay links (each of which corresponds to a path through the underlying physical network). Overlays are used for indexing and peer discovery, and make the P2P system independent from the physical network topology. Based on how the nodes are linked to each other within the overlay network, and how resources are indexed and located, we can classify networks as unstructured or structured (or as

6201-454: The binding problem of cognitive neuroscience in perceptual cognition ("feature binding") and in language cognition ("variable binding"). There is a concept that the synchronization of biochemical reactions determines biological homeostasis . According to this theory, all reactions occurring in a living cell are synchronized in terms of quantities and timescales to maintain biological network functional. Synchronization of movement

6318-475: The order of the event is important. Peer-to-peer Peer-to-peer ( P2P ) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network, forming a peer-to-peer network of nodes . In addition, a personal area network (PAN) is also in nature a type of decentralized peer-to-peer network typically between two devices. Peers make

6435-422: The single point of failure that can be inherent in a client–server based system. As nodes arrive and demand on the system increases, the total capacity of the system also increases, and the likelihood of failure decreases. If one peer on the network fails to function properly, the whole network is not compromised or damaged. In contrast, in a typical client–server architecture, clients share only their demands with

6552-598: The "freeloader problem"). Freeloading can have a profound impact on the network and in some cases can cause the community to collapse. In these types of networks "users have natural disincentives to cooperate because cooperation consumes their own resources and may degrade their own performance". Studying the social attributes of P2P networks is challenging due to large populations of turnover, asymmetry of interest and zero-cost identity. A variety of incentive mechanisms have been implemented to encourage or even force nodes to contribute resources. Some researchers have explored

6669-531: The 365 different computer viruses that were tested for. Corrupted data can also be distributed on P2P networks by modifying files that are already being shared on the network. For example, on the FastTrack network, the RIAA managed to introduce faked chunks into downloads and downloaded files (mostly MP3 files). Files infected with the RIAA virus were unusable afterwards and contained malicious code. The RIAA

6786-521: The IANA side, a ntp (network time protocols ) work group is in charge of reviewing proposed drafts. The protocol has significantly progressed since NTPv4. As of 2022, three RFC documents describing updates to the protocol have been published, not counting the numerous peripheral standards such as NTS ( RFC   8915 ). Mills had mentioned plans for a "NTPv5" on his page, but one was never published. An unrelated draft termed "NTPv5" by M. Lichvar of chrony

6903-642: The Internet, and direct people towards a client–server -based application architecture. The client–server model provides financial barriers-to-entry to small publishers and individuals, and can be less efficient for sharing large files. As a reaction to this bandwidth throttling , several P2P applications started implementing protocol obfuscation, such as the BitTorrent protocol encryption . Techniques for achieving "protocol obfuscation" involves removing otherwise easily identifiable properties of protocols, such as deterministic byte sequences and packet sizes, by making

7020-502: The Linux Foundation's Core Infrastructure Initiative, suggested that both NTP and NTPsec were more problematic than Chrony from a security standpoint. NTP servers can be susceptible to man-in-the-middle attacks unless packets are cryptographically signed for authentication. The computational overhead involved can make this impractical on busy servers, particularly during denial of service attacks. NTP message spoofing from

7137-591: The NTP provider for W32Time became compatible with a significant subset of NTPv3. Microsoft states that W32Time cannot reliably maintain time synchronization with one second accuracy. If higher accuracy is desired, Microsoft recommends using a newer version of Windows or different NTP implementation. Beginning with Windows 10 version 1607 and Windows Server 2016 , W32Time can be configured to reach time accuracy of 1 s, 50 ms or 1 ms under certain specified operating conditions. In 2004, Henning Brauer of OpenBSD presented OpenNTPD , an NTPv3/SNTPv4 implementation with

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7254-500: The United States, over conflicts with copyright law. Two major cases are Grokster vs RIAA and MGM Studios, Inc. v. Grokster, Ltd. . In the last case, the Court unanimously held that defendant peer-to-peer file sharing companies Grokster and Streamcast could be sued for inducing copyright infringement. The P2PTV and PDTP protocols are used in various peer-to-peer applications. Some proprietary multimedia applications leverage

7371-527: The answered download requests on the gnutella network contained some form of malware, whereas only 3% of the content on OpenFT contained malware. In both cases, the top three most common types of malware accounted for the large majority of cases (99% in gnutella, and 65% in OpenFT). Another study analyzing traffic on the Kazaa network found that 15% of the 500,000 file sample taken were infected by one or more of

7488-462: The architecture was popularized by the Internet file sharing system Napster , originally released in 1999. P2P is used in many protocols such as BitTorrent file sharing over the Internet and in personal networks like Miracast displaying and Bluetooth radio. The concept has inspired new structures and philosophies in many areas of human interaction. In such social contexts, peer-to-peer as

7605-451: The arguing pair has been noted to decrease; however, it is not clear whether this is due to the change in emotion or other factors. There is evidence to show that movement synchronization requires other people to cause its beneficial effects, as the effect on affiliation does not occur when one of the dyad is synchronizing their movements to something outside the dyad. This is known as interpersonal synchrony. There has been dispute regarding

7722-447: The availability of files being shared. If the administrators decide to no longer distribute a file, they simply have to remove it from their servers, and it will no longer be available to users. Along with leaving the users powerless in deciding what is distributed throughout the community, this makes the entire system vulnerable to threats and requests from the government and other large forces. For example, YouTube has been pressured by

7839-445: The benefits of enabling virtual communities to self-organize and introduce incentives for resource sharing and cooperation, arguing that the social aspect missing from today's P2P systems should be seen both as a goal and a means for self-organized virtual communities to be built and fostered. Ongoing research efforts for designing effective incentive mechanisms in P2P systems, based on principles from game theory, are beginning to take on

7956-587: The case of global synchronization of phase oscillators, an abrupt transition from unsynchronized to full synchronization takes place when the coupling strength exceeds a critical threshold. This is known as the Kuramoto model phase transition . Synchronization is an emergent property that occurs in a broad range of dynamical systems, including neural signaling, the beating of the heart and the synchronization of fire-fly light waves. A unified approach that quantifies synchronization in chaotic systems can be derived from

8073-503: The cases the file sharing technology was ruled to be legal as long as the developers had no ability to prevent the sharing of the copyrighted material. To establish criminal liability for the copyright infringement on peer-to-peer systems, the government must prove that the defendant infringed a copyright willingly for the purpose of personal financial gain or commercial advantage. Fair use exceptions allow limited use of copyrighted material to be downloaded without acquiring permission from

8190-407: The client and server programs are distinct: the clients initiate the transfer, and the servers satisfy these requests. Peer-to-peer networks generally implement some form of virtual overlay network on top of the physical network topology, where the nodes in the overlay form a subset of the nodes in the physical network. Data is still exchanged directly over the underlying TCP/IP network, but at

8307-446: The clock, that can misbehave when synchronized to servers that use different algorithms. The software has been ported to almost every computing platform, including personal computers. It runs as a daemon called ntpd under Unix or as a service under Windows. Reference clocks are supported and their offsets are filtered and analysed in the same way as remote servers, although they are usually polled more frequently. This implementation

8424-511: The coherent activity of subpopulations of neurons emerges. Moreover, this synchronization mechanism circumvents the superposition problem by more effectively identifying the signature of synchronous neuronal signals as belonging together for subsequent (sub-)cortical information processing areas. In cognitive science, integrative (phase) synchronization mechanisms in cognitive neuroarchitectures of modern connectionism that include coupled oscillators (e.g."Oscillatory Networks" ) are used to solve

8541-473: The content (especially with protocols such as Bittorrent that require users to share, refer a performance measurement study ). This property is one of the major advantages of using P2P networks because it makes the setup and running costs very small for the original content distributor. Peer-to-peer file sharing networks such as Gnutella , G2 , and the eDonkey network have been useful in popularizing peer-to-peer technologies. These advancements have paved

8658-405: The content managed by it, there is no guarantee that flooding will find a peer that has the desired data. Popular content is likely to be available at several peers and any peer searching for it is likely to find the same thing. But if a peer is looking for rare data shared by only a few other peers, then it is highly unlikely that the search will be successful. In structured peer-to-peer networks

8775-504: The core issues in the network neutrality controversy. Internet service providers ( ISPs ) have been known to throttle P2P file-sharing traffic due to its high- bandwidth usage. Compared to Web browsing, e-mail or many other uses of the internet, where data is only transferred in short intervals and relative small quantities, P2P file-sharing often consists of relatively heavy bandwidth usage due to ongoing file transfers and swarm/network coordination packets. In October 2007, Comcast , one of

8892-474: The data look as if it were random. The ISP's solution to the high bandwidth is P2P caching , where an ISP stores the part of files most accessed by P2P clients in order to save access to the Internet. Researchers have used computer simulations to aid in understanding and evaluating the complex behaviors of individuals within the network. "Networking research often relies on simulation in order to test and evaluate new ideas. An important requirement of this process

9009-427: The end of the 19th century, important ports provided time signals in the form of a signal gun, flag, or dropping time ball so that mariners could check and correct their chronometers for error. Synchronization was important in the operation of 19th-century railways, these being the first major means of transport fast enough for differences in local mean time between nearby towns to be noticeable. Each line handled

9126-457: The first rollover occurs on February 7, 2036. NTPv4 introduces a 128-bit date format: 64 bits for the second and 64 bits for the fractional-second. The most-significant 32 bits of this format is the Era Number which resolves rollover ambiguity in most cases. According to Mills, "The 64-bit value for the fraction is enough to resolve the amount of time it takes a photon to pass an electron at

9243-436: The forward and backward travel times. A number of approaches have been proposed to measure asymmetry, but among practical implementations only chrony seems to have one included. The NTP reference implementation , along with the protocol, has been continuously developed for over 20 years. Backwards compatibility has been maintained as new features have been added. It contains several sensitive algorithms, especially to discipline

9360-423: The involvement with sharing copyrighted material. Peer-to-peer networking involves data transfer from one user to another without using an intermediate server. Companies developing P2P applications have been involved in numerous legal cases, primarily in the United States, primarily over issues surrounding copyright law. Two major cases are Grokster vs RIAA and MGM Studios, Inc. v. Grokster, Ltd. In both of

9477-402: The lack of central authority in P2P networks, forces such as the recording industry, RIAA , MPAA , and the government are unable to delete or stop the sharing of content on P2P systems. In P2P networks, clients both provide and use resources. This means that unlike client–server systems, the content-serving capacity of peer-to-peer networks can actually increase as more users begin to access

9594-427: The largest broadband Internet providers in the United States, started blocking P2P applications such as BitTorrent . Their rationale was that P2P is mostly used to share illegal content, and their infrastructure is not designed for continuous, high-bandwidth traffic. Critics point out that P2P networking has legitimate legal uses, and that this is another way that large providers are trying to control use and content on

9711-529: The network using a hash table : that is, ( key , value ) pairs are stored in the DHT, and any participating node can efficiently retrieve the value associated with a given key. However, in order to route traffic efficiently through the network, nodes in a structured overlay must maintain lists of neighbors that satisfy specific criteria. This makes them less robust in networks with a high rate of churn (i.e. with large numbers of nodes frequently joining and leaving

9828-561: The network). More recent evaluation of P2P resource discovery solutions under real workloads have pointed out several issues in DHT-based solutions such as high cost of advertising/discovering resources and static and dynamic load imbalance. Notable distributed networks that use DHTs include Tixati , an alternative to BitTorrent's distributed tracker, the Kad network , the Storm botnet , and

9945-399: The network, the search query must be flooded through the network to find as many peers as possible that share the data. Flooding causes a very high amount of signaling traffic in the network, uses more CPU /memory (by requiring every peer to process all search queries), and does not ensure that search queries will always be resolved. Furthermore, since there is no correlation between a peer and

10062-474: The original codebase, making the remainder easier to audit . A 2017 audit of the code showed eight security issues, including two that were not present in the original reference implementation, but NTPsec did not suffer from eight other issues that remained in the reference implementation. chrony is an independent NTP implementation mainly sponsored by Red Hat , who uses it as the default time program in their distributions. Being written from scratch, chrony has

10179-437: The overlay is organized into a specific topology, and the protocol ensures that any node can efficiently search the network for a file/resource, even if the resource is extremely rare. The most common type of structured P2P networks implement a distributed hash table (DHT), in which a variant of consistent hashing is used to assign ownership of each file to a particular peer. This enables peers to search for resources on

10296-409: The overlay. Also, because the role of all peers in the network is the same, unstructured networks are highly robust in the face of high rates of "churn"—that is, when large numbers of peers are frequently joining and leaving the network. However, the primary limitations of unstructured networks also arise from this lack of structure. In particular, when a peer wants to find a desired piece of data in

10413-501: The packet to indicate error, such as XFAC to indicate a network disconnection. The IANA maintains a registry for refid source names and KoD codes. Informal assignments can still appear. The 64-bit binary fixed-point timestamps used by NTP consist of a 32-bit part for seconds and a 32-bit part for fractional second, giving a time scale that rolls over every 2 seconds (136 years) and a theoretical resolution of 2 seconds (233 picoseconds). NTP uses an epoch of January 1, 1900. Therefore,

10530-421: The problem by synchronizing all its stations to headquarters as a standard railway time . In some territories, companies shared a single railroad track and needed to avoid collisions. The need for strict timekeeping led the companies to settle on one standard, and civil authorities eventually abandoned local mean time in favor of railway time. In electrical engineering terms, for digital logic and data transfer,

10647-481: The publication of an article by David L. Mills in the IEEE Transactions on Communications . In 1989, RFC   1119 was published defining NTPv2 by means of a state machine , with pseudocode to describe its operation. It introduced a management protocol and cryptographic authentication scheme which have both survived into NTPv4, along with the bulk of the algorithm. However the design of NTPv2

10764-484: The receiving cipher is decoding the right bits at the right time. Automotive transmissions contain synchronizers that bring the toothed rotating parts (gears and splined shaft) to the same rotational velocity before engaging the teeth. Flash synchronization synchronizes the flash with the shutter . Some systems may be only approximately synchronized, or plesiochronous . Some applications require that relative offsets between events be determined. For others, only

10881-655: The reference implementation has undergone security audits from several sources for several years. A stack buffer overflow exploit was discovered and patched in 2014. Apple was concerned enough about this vulnerability that it used its auto-update capability for the first time. On systems using the reference implementation, which is running with root user's credential, this could allow unlimited access. Some other implementations, such as OpenNTPD , have smaller code base and adopted other mitigation measures like privilege separation, are not subject to this flaw. A 2017 security audit of three NTP implementations, conducted on behalf of

10998-411: The response packet, t 3 + θ − δ / 2 = t 2 {\displaystyle t_{3}+\theta -\delta /2=t_{2}} Solving for θ yields the definition of the time offset. The values for θ and δ are passed through filters and subjected to statistical analysis ("mitigation"). Outliers are discarded and an estimate of time offset

11115-401: The rights holders. These documents are usually news reporting or under the lines of research and scholarly work. Controversies have developed over the concern of illegitimate use of peer-to-peer networks regarding public safety and national security. When a file is downloaded through a peer-to-peer network, it is impossible to know who created the file or what users are connected to the network at

11232-512: The round-trip delay δ by δ = ( t 3 − t 0 ) − ( t 2 − t 1 ) , {\displaystyle \delta ={(t_{3}-t_{0})-(t_{2}-t_{1})},} where To derive the expression for the offset, note that for the request packet, t 0 + θ + δ / 2 = t 1 {\displaystyle t_{0}+\theta +\delta /2=t_{1}} and for

11349-455: The routing tables of neighboring nodes by sending them false information, and "incorrect routing network partition" where when new nodes are joining they bootstrap via a malicious node, which places the new node in a partition of the network that is populated by other malicious nodes. The prevalence of malware varies between different peer-to-peer protocols. Studies analyzing the spread of malware on P2P networks found, for example, that 63% of

11466-441: The same file it designates that the node choose the fastest of its neighbors. Streaming media is transmitted by having clients cache the previous stream, and then transmit it piece-wise to new nodes. Peer-to-peer systems pose unique challenges from a computer security perspective. Like any other form of software , P2P applications can contain vulnerabilities . What makes this particularly dangerous for P2P software, however,

11583-509: The same as with the Time Protocol, as only one server is used. In 1996, SNTP was updated to SNTPv4 with some features of the then-in-development NTPv4. The current version of SNTPv4 was merged into the main NTPv4 standard in 2010. SNTP is fully interoperable with NTP since it does not define a new protocol. However, the simple algorithms provide times of reduced accuracy and thus it is inadvisable to sync time from an SNTP source. NTP uses

11700-477: The sequence 23:59:58, 00:00:00, skipping 23:59:59. An alternative implementation, called leap smearing, consists in introducing the leap second incrementally during a period of 24 hours, from noon to noon in UTC time. This implementation is used by Google (both internally and on their public NTP servers), Amazon AWS, and Facebook. Chrony supports leap smear in smoothtime and leapsecmode configurations, but such use

11817-424: The source address. Refids serve to detect and prevent timing loops to the first degree. The refid field is filled with status words in the case of kiss-o'-death (KoD) packets, which tell the client to stop sending requests so that the server can rest. Some examples are INIT (initialization), STEP (step time change), and RATE (client requesting too fast). The program output may additionally use codes not transmitted in

11934-654: The speed of light. The 64-bit second value is enough to provide unambiguous time representation until the universe goes dim." A typical NTP client regularly polls one or more NTP servers. The client must compute its time offset and round-trip delay . Time offset θ is positive or negative (client time > server time) difference in absolute time between the two clocks. It is defined by θ = ( t 1 − t 0 ) + ( t 2 − t 3 ) 2 , {\displaystyle \theta ={\frac {(t_{1}-t_{0})+(t_{2}-t_{3})}{2}},} and

12051-419: The statistical analysis of measured data. In cognitive neuroscience, (stimulus-dependent) (phase-)synchronous oscillations of neuron populations serve to solve the general binding problem . According to the so-called Binding-By-Synchrony (BBS) Hypothesis a precise temporal correlation between the impulses of neurons ("cross-correlation analysis" ) and thus a stimulus-dependent temporal synchronization of

12168-450: The stratum 1 servers for all the clients. In addition to stratum, the protocol is able to identify the synchronization source for each server in terms of a reference identifier (refid). For servers on stratum 2 and below, the refid is an encoded form of the upstream time server's IP address. For IPv4, this is simply the 32-bit address; for IPv6, it would be the first 32 bits of the MD5 hash of

12285-415: The system, but not their resources. In this case, as more clients join the system, fewer resources are available to serve each client, and if the central server fails, the entire network is taken down. There are both advantages and disadvantages in P2P networks related to the topic of data backup , recovery, and availability. In a centralized network, the system administrators are the only forces controlling

12402-509: The true effect of synchrony in these studies. Research in this area detailing the positive effects of synchrony, have attributed this to synchrony alone; however, many of the experiments incorporate a shared intention to achieve synchrony. Indeed, the Reinforcement of Cooperation Model suggests that perception of synchrony leads to reinforcement that cooperation is occurring, which leads to the pro-social effects of synchrony. More research

12519-450: The user or client perspective that offers a self-organizing approach to newsgroup servers. However, news servers communicate with one another as peers to propagate Usenet news articles over the entire group of network servers. The same consideration applies to SMTP email in the sense that the core email-relaying network of mail transfer agents has a peer-to-peer character, while the periphery of Email clients and their direct connections

12636-554: The way for Peer-to-peer content delivery networks and services, including distributed caching systems like Correli Caches to enhance performance. Furthermore, peer-to-peer networks have made possible the software publication and distribution, enabling efficient sharing of Linux distribution and various games though file sharing networks. Peer-to-peer networking involves data transfer from one user to another without using an intermediate server. Companies developing P2P applications have been involved in numerous legal cases, primarily in

12753-491: The world through satellite navigation signals and other time and frequency transfer techniques. Time-keeping and synchronization of clocks is a critical problem in long-distance ocean navigation. Before radio navigation and satellite-based navigation , navigators required accurate time in conjunction with astronomical observations to determine how far east or west their vessel traveled. The invention of an accurate marine chronometer revolutionized marine navigation. By

12870-845: Was audited in 2017, finding 14 potential security issues. All Microsoft Windows versions since Windows 2000 include the Windows Time service (W32Time), which has the ability to synchronize the computer clock to an NTP server. W32Time was originally implemented for the purpose of the Kerberos version 5 authentication protocol, which required time to be within 5 minutes of the correct value to prevent replay attacks . The network time server in Windows 2000 Server (and Windows XP) does not implement NTP disciplined synchronization, only locally disciplined synchronization with NTP/SNTP correction. Beginning with Windows Server 2003 and Windows Vista ,

12987-601: Was built with decentralized servers and tracker software dedicated to any type of files and continues to operate today. Cryptocurrencies are peer-to-peer-based digital currencies that use blockchains Cooperation among a community of participants is key to the continued success of P2P systems aimed at casual human users; these reach their full potential only when large numbers of nodes contribute resources. But in current practice, P2P networks often contain large numbers of users who utilize resources shared by other nodes, but who do not share anything themselves (often referred to as

13104-517: Was criticized for lacking formal correctness by the DTSS community, and the clock selection procedure was modified to incorporate Marzullo's algorithm for NTPv3 onwards. In 1992, RFC   1305 defined NTPv3. The RFC included an analysis of all sources of error, from the reference clock down to the final client, which enabled the calculation of a metric that helps choose the best server where several candidates appear to disagree. Broadcast mode

13221-464: Was envisioned in earlier software systems and networking discussions, reaching back to principles stated in the first Request for Comments , RFC 1. Tim Berners-Lee 's vision for the World Wide Web was close to a P2P network in that it assumed each user of the web would be an active editor and contributor, creating and linking content to form an interlinked "web" of links. The early Internet

13338-429: Was initiated in 2020 and includes security, accuracy, and scaling changes. As NTP replaced the use of the old Time Protocol , some use cases nevertheless found the full protocol too complicated. In 1992, Simple Network Time Protocol ( SNTP ) was defined to fill this niche. The SNTPv3 standard describes a way to use NTPv3, such that no storage of state over an extended period is needed. The topology becomes essentially

13455-487: Was introduced. In subsequent years, as new features were added and algorithm improvements were made, it became apparent that a new protocol version was required. In 2010, RFC   5905 was published containing a proposed specification for NTPv4. Following the retirement of Mills from the University of Delaware , the reference implementation is currently maintained as an open source project led by Harlan Stenn. On

13572-430: Was more open than the present day, where two machines connected to the Internet could send packets to each other without firewalls and other security measures. This contrasts with the broadcasting -like structure of the web as it has developed over the years. As a precursor to the Internet, ARPANET was a successful peer-to-peer network where "every participating node could request and serve content". However, ARPANET

13689-407: Was not self-organized, and it could not "provide any means for context or content-based routing beyond 'simple' address-based routing." Therefore, Usenet , a distributed messaging system that is often described as an early peer-to-peer architecture, was established. It was developed in 1979 as a system that enforces a decentralized model of control. The basic model is a client–server model from

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