Misplaced Pages

#874125

114-462: Hubs feature a list of clients or users connected to them. Users can search for files and download them from other clients, as well as chat with other users. NeoModus was started as a company funded by the adware "Direct Connect" by Jon Hess in November, 1999 while he was in high school. The first third-party client was called "DClite", which never fully supported the file sharing aspects of

228-462: A hard disk . Hard disks have been the ubiquitous form of non-volatile storage since the early 1960s. Where files contain only temporary information, they may be stored in RAM . Computer files can be also stored on other media in some cases, such as magnetic tapes , compact discs , Digital Versatile Discs , Zip drives , USB flash drives , etc. The use of solid state drives is also beginning to rival

342-428: A link to the file. There can be many links to a file, but when they are all removed, the kernel considers that file's memory space free to be reallocated. This free space is commonly considered a security risk (due to the existence of file recovery software ). Any secure-deletion program uses kernel-space (system) functions to wipe the file's data. File moves within a file system complete almost immediately because

456-435: A retransmission timeout (RTO) that is based on the estimated round-trip time (RTT) between the sender and receiver, as well as the variance in this round-trip time. There are subtleties in the estimation of RTT. For example, senders must be careful when calculating RTT samples for retransmitted packets; typically they use Karn's Algorithm or TCP timestamps. These individual RTT samples are then averaged over time to create

570-454: A FAT extension allowing long file names). File manager programs are utility programs that allow users to manipulate files directly. They allow you to move, create, delete and rename files and folders, although they do not actually allow you to read the contents of a file or store information in it. Every computer system provides at least one file-manager program for its native file system. For example, File Explorer (formerly Windows Explorer)

684-473: A FIN packet, which the other end acknowledges with an ACK. Therefore, a typical tear-down requires a pair of FIN and ACK segments from each TCP endpoint. After the side that sent the first FIN has responded with the final ACK, it waits for a timeout before finally closing the connection, during which time the local port is unavailable for new connections; this state lets the TCP client resend the final acknowledgment to

798-514: A PC sends data to a smartphone that is slowly processing received data, the smartphone must be able to regulate the data flow so as not to be overwhelmed. TCP uses a sliding window flow control protocol. In each TCP segment, the receiver specifies in the receive window field the amount of additionally received data (in bytes) that it is willing to buffer for the connection. The sending host can send only up to that amount of data before it must wait for an acknowledgment and receive window update from

912-464: A TCP header creating a TCP segment. The TCP segment is then encapsulated into an Internet Protocol (IP) datagram, and exchanged with peers. The term TCP packet appears in both informal and formal usage, whereas in more precise terminology segment refers to the TCP protocol data unit (PDU), datagram to the IP PDU, and frame to the data link layer PDU: Processes transmit data by calling on

1026-429: A TCP segment is retransmitted, it retains the same sequence number as the original delivery attempt. This conflation of delivery and logical data ordering means that, when acknowledgment is received after a retransmission, the sender cannot tell whether the original transmission or the retransmission is being acknowledged, the so-called retransmission ambiguity . TCP incurs complexity due to retransmission ambiguity. If

1140-467: A complete replacement of the Direct Connect protocol called ADC, or unofficially, Advanced Direct Connect. ADC uses the same network topology , concepts, and terminology as the original protocol. One example of an added feature to the protocol, in comparison with the original protocol, is the broadcasting of Tiger-Tree Hashing of shared files (TTH). The advantages of this include verifying that

1254-525: A computer file. Files can be shared with and transferred between computers and mobile devices via removable media , networks , or the Internet . Different types of computer files are designed for different purposes. A file may be designed to store a written message, a document, a spreadsheet , an image , a video , a program , or any wide variety of other kinds of data. Certain files can store multiple data types at once. By using computer programs,

SECTION 10

#1732793420875

1368-456: A connection before entering the data transfer phase. After data transfer is completed, the connection termination closes the connection and releases all allocated resources. A TCP connection is managed by an operating system through a resource that represents the local end-point for communications, the Internet socket . During the lifetime of a TCP connection, the local end-point undergoes

1482-427: A device, no space will have been freed up on the source device or partition. The user would need to merge the remaining files from the source, including the incompletely written (truncated) last file. With the individual deletion method, the file moving software also does not need to cumulatively keep track of all files finished transferring for the case that a user manually aborts the file transfer. A file manager using

1596-524: A directory and a file. In environments in which a file is named, a file's name and the path to the file's directory must uniquely identify it among all other files in the computer system—no two files can have the same name and path. Where a file is anonymous, named references to it will exist within a namespace. In most cases, any name within the namespace will refer to exactly zero or one file. However, any file may be represented within any namespace by zero, one or more names. Any string of characters may be

1710-847: A dot (period) at the end of a file name, followed by a few letters to identify the type of file. An extension of .txt identifies a text file; a .doc extension identifies any type of document or documentation, commonly in the Microsoft Word file format ; and so on . Even when extensions are used in a computer system, the degree to which the computer system recognizes and heeds them can vary; in some systems, they are required, while in other systems, they are completely ignored if they are presented. Many modern computer systems provide methods for protecting files against accidental and deliberate damage. Computers that allow for multiple users implement file permissions to control who may or may not modify, delete, or create files and folders. For example,

1824-503: A file is downloaded correctly, and the ability to find files independently of their names. As the protocol allows hubs to redirect users to other hubs , malicious hubs have redirected users to places other than real Direct Connect hubs, effectively causing a Distributed Denial of Service attack. The hubs may alter the IP in client to client connections, pointing to a potential victim. The CTM Exploit surfaced in 2006–2007, during which period

1938-514: A given user may be granted only permission to read a file or folder, but not to modify or delete it; or a user may be given permission to read and modify files or folders, but not to execute them. Permissions may also be used to allow only certain users to see the contents of a file or folder. Permissions protect against unauthorized tampering or destruction of information in files, and keep private information confidential from unauthorized users. Another protection mechanism implemented in many computers

2052-476: A good RTT estimate will be produced—eventually—by waiting until there is an unambiguous acknowledgment before adjusting the RTO. After spurious retransmissions, however, it may take significant time before such an unambiguous acknowledgment arrives, degrading performance in the interim. TCP timestamps also resolve the retransmission ambiguity problem in setting the RTO, though they do not necessarily improve

2166-449: A half-duplex close sequence. If the host actively closes a connection, while still having unread incoming data available, the host sends the signal RST (losing any received data) instead of FIN. This assures that a TCP application is aware there was a data loss. A connection can be in a half-open state, in which case one side has terminated the connection, but the other has not. The side that has terminated can no longer send any data into

2280-418: A header that includes (among other data) the destination IP address . When the client program on the destination computer receives them, the TCP software in the transport layer re-assembles the segments and ensures they are correctly ordered and error-free as it streams the file contents to the receiving application. Transmission Control Protocol accepts data from a data stream, divides it into chunks, and adds

2394-464: A hub-to-hub basis. An incoming request for a client-client connection cannot be linked with an actual connection. A search result cannot be linked with a particular search. The ability to kick or move (redirect) a user to another hub is supported by the protocol. If a user is kicked, the hub is not required to give that user a specific reason, and there is no restriction on where a user can be redirected to. However, if another client in power instructs

SECTION 20

#1732793420875

2508-446: A later point through overwriting. There are many ways by which a file can become corrupted. Most commonly, the issue happens in the process of writing the file to a disk . For example, if an image-editing program unexpectedly crashes while saving an image, that file may be corrupted because the program could not save its entirety. The program itself might warn the user that there was an error, allowing for another attempt at saving

2622-420: A low level, on a file. Only the kernel deals with files, and it handles all user-space interaction with files in a manner that is transparent to the user-space programs. The operating system provides a level of abstraction , which means that interaction with a file from user-space is simply through its filename (instead of its inode ). For example, rm filename will not delete the file itself, but only

2736-406: A name of its own, but also a path, which identifies the folder or folders in which a file or folder resides. In the path, some sort of special character—such as a slash—is used to separate the file and folder names. For example, in the illustration shown in this article, the path /Payroll/Salaries/Managers uniquely identifies a file called Managers in a folder called Salaries , which in turn

2850-494: A new "memory" vacuum tube it had developed, RCA stated: "the results of countless computations can be kept 'on file' and taken out again. Such a 'file' now exists in a 'memory' tube developed at RCA Laboratories. Electronically it retains figures fed into calculating machines, holds them in storage while it memorizes new ones – speeds intelligent solutions through mazes of mathematics." In 1952, "file" denoted, among other things, information stored on punched cards . In early use,

2964-418: A number of bytes , that indicates how much storage is occupied by the file. In most modern operating systems the size can be any non-negative whole number of bytes up to a system limit. Many older operating systems kept track only of the number of blocks or tracks occupied by a file on a physical storage device. In such systems, software employed other methods to track the exact byte count (e.g., CP/M used

3078-443: A payroll file might contain information concerning all the employees in a company and their payroll details; each record in the payroll file concerns just one employee, and all the records have the common trait of being related to payroll—this is very similar to placing all payroll information into a specific filing cabinet in an office that does not have a computer. A text file may contain lines of text, corresponding to printed lines on

3192-648: A person can open, read, change, save, and close a computer file. Computer files may be reopened, modified, and copied an arbitrary number of times. Files are typically organized in a file system , which tracks file locations on the disk and enables user access. The word "file" derives from the Latin filum ("a thread, string"). "File" was used in the context of computer storage as early as January 1940. In Punched Card Methods in Scientific Computation , W. J. Eckert stated, "The first extensive use of

3306-429: A piece of paper. Alternatively, a file may contain an arbitrary binary image (a blob ) or it may contain an executable . The way information is grouped into a file is entirely up to how it is designed. This has led to a plethora of more or less standardized file structures for all imaginable purposes, from the simplest to the most complex. Most computer files are used by computer programs which create, modify or delete

3420-454: A sender to assume an unacknowledged packet to be lost after sufficient time elapses (i.e., determining the RTO time). Retransmission ambiguity can lead a sender's estimate of RTT to be imprecise. In an environment with variable RTTs, spurious timeouts can occur: if the RTT is under-estimated, then the RTO fires and triggers a needless retransmit and slow-start. After a spurious retransmission, when

3534-489: A sequence number by the receiver of data to tell the sender that data has been received to the specified byte. ACKs do not imply that the data has been delivered to the application, they merely signify that it is now the receiver's responsibility to deliver the data. Reliability is achieved by the sender detecting lost data and retransmitting it. TCP uses two primary techniques to identify loss. Retransmission timeout (RTO) and duplicate cumulative acknowledgments (DupAcks). When

Direct Connect (protocol) - Misplaced Pages Continue

3648-481: A series of state changes: Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may establish a connection by initiating an active open using the three-way (or 3-step) handshake: Steps 1 and 2 establish and acknowledge the sequence number for one direction (client to server). Steps 2 and 3 establish and acknowledge

3762-407: A single segment (say segment number 100) in a stream is lost, then the receiver cannot acknowledge packets above that segment number (100) because it uses cumulative ACKs. Hence the receiver acknowledges packet 99 again on the receipt of another data packet. This duplicate acknowledgement is used as a signal for packet loss. That is, if the sender receives three duplicate acknowledgments, it retransmits

3876-424: A smoothed round trip time (SRTT) using Jacobson's algorithm . This SRTT value is what is used as the round-trip time estimate. Enhancing TCP to reliably handle loss, minimize errors, manage congestion and go fast in very high-speed environments are ongoing areas of research and standards development. As a result, there are a number of TCP congestion avoidance algorithm variations. The maximum segment size (MSS)

3990-433: A special control character, Ctrl-Z , to signal the end of text files). The general definition of a file does not require that its size have any real meaning, however, unless the data within the file happens to correspond to data within a pool of persistent storage. A special case is a zero byte file ; these files can be newly created files that have not yet had any data written to them, or may serve as some kind of flag in

4104-438: A tree-like structure in which one "master folder" (or "root folder" — the name varies from one operating system to another) can contain any number of levels of other folders and files. Folders can be named just as files can (except for the root folder, which often does not have a name). The use of folders makes it easier to organize files in a logical way. When a computer allows the use of folders, each file and folder has not only

4218-439: A website. It can, however, add the command "+rules" (where '+' indicates to the hub that it's a command - this may vary) to display the hub's rules. The peer-to-peer part of the protocol is based on a concept of "slots" (similar to number of open positions for a job). These slots denote the number of people that are allowed to download from a user at any given time and are controlled by the client. In client-to-client connections,

4332-419: A well-formed name for a file or a link depending upon the context of application. Whether or not a name is well-formed depends on the type of computer system being used. Early computers permitted only a few letters or digits in the name of a file, but modern computers allow long names (some up to 255 characters) containing almost any combination of Unicode letters or Unicode digits, making it easier to understand

4446-487: Is connection-oriented , meaning that sender and receiver firstly need to establish a connection based on agreed parameters; they do this through three-way handshake procedure. The server must be listening (passive open) for connection requests from clients before a connection is established. Three-way handshake (active open), retransmission , and error detection adds to reliability but lengthens latency . Applications that do not require reliable data stream service may use

4560-436: Is a read-only flag. When this flag is turned on for a file (which can be accomplished by a computer program or by a human user), the file can be examined, but it cannot be modified. This flag is useful for critical information that must not be modified or erased, such as special files that are used only by internal parts of the computer system. Some systems also include a hidden flag to make certain files invisible; this flag

4674-479: Is an area of memory which is manipulated to establish a filename etc. and then passed to the operating system as a parameter; it was used by older IBM operating systems and early PC operating systems including CP/M and early versions of MS-DOS . A file handle is generally either an opaque data type or an integer; it was introduced in around 1961 by the ALGOL-based Burroughs MCP running on

Direct Connect (protocol) - Misplaced Pages Continue

4788-484: Is available as a protocol extension). Clients connect to a central server acting as a "hub". This hub provides content discovery and allows clients to negotiate direct connections between each other for transferring content. Since this central hub only deals with metadata, it does not have anywhere near the same bandwidth requirements as if it also had been serving the content itself; an estimate shows that handling 1000 users would require about 2.5 mbit/s of bandwidth. There

4902-601: Is commonly referred to as TCP/IP . TCP provides reliable , ordered, and error-checked delivery of a stream of octets (bytes) between applications running on hosts communicating via an IP network. Major internet applications such as the World Wide Web , email, remote administration , and file transfer rely on TCP, which is part of the Transport layer of the TCP/IP suite. SSL/TLS often runs on top of TCP. TCP

5016-542: Is commonly used in Microsoft Windows operating systems, and Nautilus is common under several distributions of Linux. Transmission Control Protocol The Transmission Control Protocol ( TCP ) is one of the main protocols of the Internet protocol suite . It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite

5130-420: Is contained in a folder called Payroll . The folder and file names are separated by slashes in this example; the topmost or root folder has no name, and so the path begins with a slash (if the root folder had a name, it would precede this first slash). Many computer systems use extensions in file names to help identify what they contain, also known as the file type. On Windows computers, extensions consist of

5244-464: Is corrupted. There are services that provide on demand file corruption, which essentially fill a given file with random data so that it cannot be opened or read, yet still seems legitimate. One of the most effective countermeasures for unintentional file corruption is backing up important files. In the event of an important file becoming corrupted, the user can simply replace it with the backed up version. When computer files contain information that

5358-613: Is extremely important, a back-up process is used to protect against disasters that might destroy the files. Backing up files simply means making copies of the files in a separate location so that they can be restored if something happens to the computer, or if they are deleted accidentally. There are many ways to back up files. Most computer systems provide utility programs to assist in the back-up process, which can become very time-consuming if there are many files to safeguard. Files are often copied to removable media such as writable CDs or cartridge tapes. Copying files to another hard disk in

5472-405: Is no official specification of the protocol, meaning that every client and hub (besides the original NeoModus client and hub) has been forced to reverse engineer the information. As such, any protocol specification this article may reference is likely inaccurate and/or incomplete. The client-server (as well as client-client, where one client acts as "server") aspect of the protocol stipulates that

5586-468: Is the default port for hubs, and 412 for client-to-client connections. If either of these ports are already in use, the port number is incremented until the number of a free port is found for use. For example, if 411, 412 and 413 are in use, then port 414 will be used. Hub addresses are in the following form: dchub://example.com[:411], where 411 is an optional port. There is no global identification scheme; instead, users are identified with their nickname on

5700-456: Is the largest amount of data, specified in bytes, that TCP is willing to receive in a single segment. For best performance, the MSS should be set small enough to avoid IP fragmentation , which can lead to packet loss and excessive retransmissions. To accomplish this, typically the MSS is announced by each side using the MSS option when the TCP connection is established. The option value is derived from

5814-435: Is used by the computer system to hide essential system files that users should not alter. Any file that has any useful purpose must have some physical manifestation. That is, a file (an abstract concept) in a real computer system must have a real physical analogue if it is to exist at all. In physical terms, most computer files are stored on some type of data storage device. For example, most operating systems store files on

SECTION 50

#1732793420875

5928-565: Is used in PPP or the Ethernet frame. However, introduction of errors in packets between CRC-protected hops is common and the 16-bit TCP checksum catches most of these. TCP uses an end-to-end flow control protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably. Having a mechanism for flow control is essential in an environment where machines of diverse network speeds communicate. For example, if

6042-627: The mv command for instance, the former method is used when selecting files individually, possibly with the use of wildcards (example: mv -n sourcePath/* targetPath , while the latter method is used when selecting entire directories (example: mv -n sourcePath targetPath ). Microsoft Windows Explorer uses the former method for mass storage file moves, but the latter method using Media Transfer Protocol , as described in Media Transfer Protocol § File move behavior . The former method (individual deletion from source) has

6156-468: The Burroughs B5000 but is now ubiquitous. When a file is said to be corrupted, it is because its contents have been saved to the computer in such a way that they cannot be properly read, either by a human or by software. Depending on the extent of the damage, the original file can sometimes be recovered , or at least partially understood. A file may be created corrupt, or it may be corrupted at

6270-634: The Internet Protocol Suite . The following Internet Experiment Note (IEN) documents describe the evolution of TCP into the modern version: TCP was standardized in January 1980 as RFC   761 . In 2004, Vint Cerf and Bob Kahn received the Turing Award for their foundational work on TCP/IP. The Transmission Control Protocol provides a communication service at an intermediate level between an application program and

6384-619: The NTFS file system that is the normal file system for recent versions of Windows. Each system has its own advantages and disadvantages. Standard FAT allows only eight-character file names (plus a three-character extension) with no spaces, for example, whereas NTFS allows much longer names that can contain spaces. You can call a file " Payroll records " in NTFS, but in FAT you would be restricted to something like payroll.dat (unless you were using VFAT ,

6498-589: The Real-time Transport Protocol (RTP) operating over the User Datagram Protocol (UDP) are usually recommended instead. TCP is a reliable byte stream delivery service that guarantees that all bytes received will be identical and in the same order as those sent. Since packet transfer by many networks is not reliable, TCP achieves this using a technique known as positive acknowledgment with re-transmission . This requires

6612-539: The User Datagram Protocol (UDP) instead, which provides a connectionless datagram service that prioritizes time over reliability. TCP employs network congestion avoidance . However, there are vulnerabilities in TCP, including denial of service , connection hijacking , TCP veto, and reset attack . In May 1974, Vint Cerf and Bob Kahn described an internetworking protocol for sharing resources using packet switching among network nodes. The authors had been working with Gérard Le Lann to incorporate concepts from

6726-546: The User Datagram Protocol : TCP uses a sequence number to identify each byte of data. The sequence number identifies the order of the bytes sent from each computer so that the data can be reconstructed in order, regardless of any out-of-order delivery that may occur. The sequence number of the first byte is chosen by the transmitter for the first packet, which is flagged SYN. This number can be arbitrary, and should, in fact, be unpredictable to defend against TCP sequence prediction attacks . Acknowledgments (ACKs) are sent with

6840-471: The World Wide Web (WWW), email, File Transfer Protocol , Secure Shell , peer-to-peer file sharing , and streaming media . TCP is optimized for accurate delivery rather than timely delivery and can incur relatively long delays (on the order of seconds) while waiting for out-of-order messages or re-transmissions of lost messages. Therefore, it is not particularly suitable for real-time applications such as voice over IP . For such applications, protocols like

6954-458: The DC network by improving software, protocols and other services in the network. The DCNF maintains a list of articles, papers and more documentation that relate to DC. Computer file In computing , a computer file is a resource for recording data on a computer storage device , primarily identified by its filename . Just as words can be written on paper, so too can data be written to

SECTION 60

#1732793420875

7068-608: The French CYCLADES project into the new network. The specification of the resulting protocol, RFC 675 ( Specification of Internet Transmission Control Program ), was written by Vint Cerf, Yogen Dalal , and Carl Sunshine, and published in December 1974. It contains the first attested use of the term internet , as a shorthand for internetwork . The Transmission Control Program incorporated both connection-oriented links and datagram services between hosts. In version 4,

7182-509: The Internet Protocol. It provides host-to-host connectivity at the transport layer of the Internet model . An application does not need to know the particular mechanisms for sending data via a link to another host, such as the required IP fragmentation to accommodate the maximum transmission unit of the transmission medium. At the transport layer, TCP handles all handshaking and transmission details and presents an abstraction of

7296-453: The RTT estimate. Sequence numbers allow receivers to discard duplicate packets and properly sequence out-of-order packets. Acknowledgments allow senders to determine when to retransmit lost packets. To assure correctness a checksum field is included; see § Checksum computation for details. The TCP checksum is a weak check by modern standards and is normally paired with a CRC integrity check at layer 2 , below both TCP and IP, such as

7410-415: The TCP and passing buffers of data as arguments. The TCP packages the data from these buffers into segments and calls on the internet module [e.g. IP] to transmit each segment to the destination TCP. A TCP segment consists of a segment header and a data section. The segment header contains 10 mandatory fields, and an optional extension field ( Options , pink background in table). The data section follows

7524-463: The TCP implementation must perform a lookup on this table to find the destination process. Each entry in the table is known as a Transmission Control Block or TCB. It contains information about the endpoints (IP and port), status of the connection, running data about the packets that are being exchanged and buffers for sending and receiving data. The number of sessions in the server side is limited only by memory and can grow as new connections arrive, but

7638-404: The TCP sender attempts recovery by sending a small packet so that the receiver responds by sending another acknowledgment containing the new window size. If a receiver is processing incoming data in small increments, it may repeatedly advertise a small receive window. This is referred to as the silly window syndrome , since it is inefficient to send only a few bytes of data in a TCP segment, given

7752-519: The acknowledgments for the original transmissions arrive, the sender may believe them to be acknowledging the retransmission and conclude, incorrectly, that segments sent between the original transmission and retransmission have been lost, causing further needless retransmissions to the extent that the link truly becomes congested; selective acknowledgement can reduce this effect. RFC 6298 specifies that implementations must not use retransmitted segments when estimating RTT. Karn's algorithm ensures that

7866-410: The archive file are to lower the number of files for easier transfer, to reduce storage usage, or just to organize outdated files. The archive file must often be unpacked before next using. The most basic operations that programs can perform on a file are: Files on a computer can be created, moved, modified, grown, shrunk ( truncated ), and deleted. In most cases, computer programs that are executed on

7980-407: The benefit that space is released from the source device or partition imminently after the transfer has begun, meaning after the first file is finished. With the latter method, space is only freed after the transfer of the entire selection has finished. If an incomplete file transfer with the latter method is aborted unexpectedly, perhaps due to an unexpected power-off, system halt or disconnection of

8094-526: The classic RTO discussed below). The time based loss detection algorithm called Recent Acknowledgment (RACK) has been adopted as the default algorithm in Linux and Windows. When a sender transmits a segment, it initializes a timer with a conservative estimate of the arrival time of the acknowledgment. The segment is retransmitted if the timer expires, with a new timeout threshold of twice the previous value, resulting in exponential backoff behavior. Typically,

8208-607: The client must allocate an ephemeral port before sending the first SYN to the server. This port remains allocated during the whole conversation and effectively limits the number of outgoing connections from each of the client's IP addresses. If an application fails to properly close unrequired connections, a client can run out of resources and become unable to establish new TCP connections, even from other applications. Both endpoints must also allocate space for unacknowledged packets and received (but unread) data. The Transmission Control Protocol differs in several key features compared to

8322-559: The computer handle these operations, but the user of a computer can also manipulate files if necessary. For instance, Microsoft Word files are normally created and modified by the Microsoft Word program in response to user commands, but the user can also move, rename , or delete these files directly by using a file manager program such as Windows Explorer (on Windows computers) or by command lines (CLI). In Unix-like systems, user space programs do not operate directly, at

8436-407: The connection, but the other side can. The terminating side should continue reading the data until the other side terminates as well. Most implementations allocate an entry in a table that maps a session to a running operating system process. Because TCP packets do not include a session identifier, both endpoints identify the session using the client's address and port. Whenever a packet is received,

8550-410: The data content does not need to be rewritten. Only the paths need to be changed. There are two distinct implementations of file moves. When moving files between devices or partitions, some file managing software deletes each selected file from the source directory individually after being transferred, while other software deletes all files at once only after every file has been transferred. With

8664-440: The document file is arranged in a format that the word-processing program understands, the user is able to choose the name and location of the file and provide the bulk of the information (such as words and text) that will be stored in the file. Many applications pack all their data files into a single file called an archive file , using internal markers to discern the different types of information contained within. The benefits of

8778-504: The early Hollerith Tabulator in astronomy was made by Comrie . He used it for building a table from successive differences, and for adding large numbers of harmonic terms". "Tables of functions are constructed from their differences with great efficiency, either as printed tables or as a file of punched cards ." In February 1950, in a Radio Corporation of America (RCA) advertisement in Popular Science magazine describing

8892-403: The existence of directory hierarchies, i.e., directories containing sub-directories. A name that refers to a file within a directory must be typically unique. In other words, there must be no identical names within a directory. However, in some operating systems, a name may include a specification of type that means a directory can contain an identical name for more than one type of object such as

9006-414: The file and the son is the current copy. The way a computer organizes, names, stores and manipulates files is globally referred to as its file system . Most computers have at least one file system. Some computers allow the use of several different file systems. For instance, on newer MS Windows computers, the older FAT-type file systems of MS-DOS and old versions of Windows are supported, in addition to

9120-414: The file system) file-specific data outside of the file format, but linked to the file, for example extended attributes or forks . On other file systems this can be done via sidecar files or software-specific databases. All those methods, however, are more susceptible to loss of metadata than container and archive file formats. At any instant in time, a file has a specific size, normally expressed as

9234-526: The file system, or are accidents (the results of aborted disk operations). For example, the file to which the link /bin/ls points in a typical Unix-like system probably has a defined size that seldom changes. Compare this with /dev/null which is also a file, but as a character special file , its size is not meaningful. Information in a computer file can consist of smaller packets of information (often called " records " or "lines") that are individually different but share some common traits. For example,

9348-423: The file. Some other examples of reasons for which files become corrupted include: Although file corruption usually happens accidentally, it may also be done on purpose as a mean of procrastination , as to fool someone else into thinking an assignment was ready at an earlier date, potentially gaining time to finish said assignment or making experiments, with the purpose of documenting the consequences when such file

9462-402: The files for their own use on an as-needed basis. The programmers who create the programs decide what files are needed, how they are to be used and (often) their names. In some cases, computer programs manipulate files that are made visible to the computer user. For example, in a word-processing program , the user manipulates document files that the user personally names. Although the content of

9576-475: The full file sharing aspects of the protocol, but a port to Java , however, did. Later on, other clients such as DCTC (Direct Connect Text Client) and DC++ became popular. The DCDev archive contains discussions of protocol changes for development of DC in the years 2003–2005. The Direct Connect protocol is a text-based computer protocol, in which commands and their information are sent in clear text, without encryption in original NeoModus software ( encryption

9690-439: The hard disk drive. In Unix-like operating systems, many files have no associated physical storage device. Examples are /dev/null and most files under directories /dev , /proc and /sys . These are virtual files: they exist as objects within the operating system kernel. As seen by a running user program, files are usually represented either by a file control block or by a file handle . A file control block (FCB)

9804-476: The header and is the payload data carried for the application. The length of the data section is not specified in the segment header; it can be calculated by subtracting the combined length of the segment header and IP header from the total IP datagram length specified in the IP header. TCP protocol operations may be divided into three phases. Connection establishment is a multi-step handshake process that establishes

9918-417: The hub to kick, that client may send out a notification message before doing so. Redirecting a user must be accompanied by a reason. There is no HTTP referer equivalent. Hubs may send out user commands to clients. These commands are only raw protocol commands and are used mostly for making a particular task simpler. For example, the hub cannot send a user command that will trigger the default browser to visit

10032-407: The individual units of data transmission that a message is divided into for efficient routing through the network. For example, when an HTML file is sent from a web server, the TCP software layer of that server divides the file into segments and forwards them individually to the internet layer in the network stack . The internet layer software encapsulates each TCP segment into an IP packet by adding

10146-513: The initial timer value is smoothed RTT + max ( G , 4 × RTT variation ) {\displaystyle {\text{smoothed RTT}}+\max(G,4\times {\text{RTT variation}})} , where G {\displaystyle G} is the clock granularity. This guards against excessive transmission traffic due to faulty or malicious actors, such as man-in-the-middle denial of service attackers . Accurate RTT estimates are important for loss recovery, as it allows

10260-464: The lack of acknowledgments, are used by senders to infer network conditions between the TCP sender and receiver. Coupled with timers, TCP senders and receivers can alter the behavior of the flow of data. This is more generally referred to as congestion control or congestion avoidance. Modern implementations of TCP contain four intertwined algorithms: slow start , congestion avoidance , fast retransmit , and fast recovery . In addition, senders employ

10374-417: The last unacknowledged packet. A threshold of three is used because the network may reorder segments causing duplicate acknowledgements. This threshold has been demonstrated to avoid spurious retransmissions due to reordering. Some TCP implementations use selective acknowledgements (SACKs) to provide explicit feedback about the segments that have been received. This greatly improves TCP's ability to retransmit

10488-418: The latter (afterwards deletion) method will have to only delete the files from the source directory that have already finished transferring. In modern computer systems, files are typically accessed using names ( filenames ). In some operating systems, the name is associated with the file itself. In others, the file is anonymous, and is pointed to by links that have names. In the latter case, a user can identify

10602-557: The monolithic Transmission Control Program was divided into a modular architecture consisting of the Transmission Control Protocol and the Internet Protocol . This resulted in a networking model that became known informally as TCP/IP , although formally it was variously referred to as the DoD internet architecture model ( DoD model for short) or DARPA model . Later, it became the part of, and synonymous with,

10716-413: The name of the link with the file itself, but this is a false analogue, especially where there exists more than one link to the same file. Files (or links to files) can be located in directories. However, more generally, a directory can contain either a list of files or a list of links to files. Within this definition, it is of paramount importance that the term "file" includes directories. This permits

10830-442: The network connection to the application typically through a network socket interface. At the lower levels of the protocol stack, due to network congestion , traffic load balancing , or unpredictable network behavior, IP packets may be lost , duplicated, or delivered out of order . TCP detects these problems, requests re-transmission of lost data, rearranges out-of-order data and even helps minimize network congestion to reduce

10944-407: The occurrence of the other problems. If the data still remains undelivered, the source is notified of this failure. Once the TCP receiver has reassembled the sequence of octets originally transmitted, it passes them to the receiving application. Thus, TCP abstracts the application's communication from the underlying networking details. TCP is used extensively by many internet applications, including

11058-562: The parties generate a random number to see who should be allowed to download first, and the client with the greater number wins. Transporting downloads and connecting to the hub requires TCP , while active searches use UDP . There are two kinds of modes a user can be in: either "active" or "passive" mode. Clients using active mode can download from anyone else on the network, while clients using passive mode users can only download from active users. In NeoModus Direct Connect, passive mode users receive other passive mode users' search results, but

11172-533: The protocol. Hess released a new version of Direct Connect, requiring a simple encryption key to initiate a connection, locking out third-party clients. The encryption key was cracked, and the author of DClite released a new version of DClite compatible with the new software from NeoModus. Some time after, DClite was rewritten as Open Direct Connect with the purpose of having an MDI user interface and using plug-ins for file sharing protocols (similar to MLDonkey ). Open Direct Connect also did not have complete support for

11286-518: The purpose of a file at a glance. Some computer systems allow file names to contain spaces; others do not. Case-sensitivity of file names is determined by the file system . Unix file systems are usually case sensitive and allow user-level applications to create files whose names differ only in the case of characters. Microsoft Windows supports multiple file systems, each with different policies regarding case-sensitivity. The common FAT file system can have multiple files whose names differ only in case if

11400-418: The receiver to respond with an acknowledgment message as it receives the data. The sender keeps a record of each packet it sends and maintains a timer from when the packet was sent. The sender re-transmits a packet if the timer expires before receiving the acknowledgment. The timer is needed in case a packet gets lost or corrupted. While IP handles actual delivery of the data, TCP keeps track of segments –

11514-407: The receiving host. When a receiver advertises a window size of 0, the sender stops sending data and starts its persist timer . The persist timer is used to protect TCP from a deadlock situation that could arise if a subsequent window size update from the receiver is lost, and the sender cannot send more data until receiving a new window size update from the receiver. When the persist timer expires,

11628-504: The relatively large overhead of the TCP header. The final main aspect of TCP is congestion control . TCP uses a number of mechanisms to achieve high performance and avoid congestive collapse , a gridlock situation where network performance is severely degraded. These mechanisms control the rate of data entering the network, keeping the data flow below a rate that would trigger collapse. They also yield an approximately max-min fair allocation between flows. Acknowledgments for data sent, or

11742-488: The results directly. Protocol delimiters are "$ ", "|", and U+0020   SPACE . Protocol have for them (and few others) escape sequence and most software use them correctly in login (Lock to Key) sequence. For some reason that escape sequence was ignored by DC++ developers and they use HTML equivalent if these characters are to be viewed by the user. Continued interest exists in features such as ratings and language packs. The authors of DC++ also proposed

11856-687: The right segments. Retransmission ambiguity can cause spurious fast retransmissions and congestion avoidance if there is reordering beyond the duplicate acknowledgment threshold. In the last two decades more packet reordering has been observed over the Internet which led TCP implementations, such as the one in the Linux Kernel to adopt heuristic methods to scale the duplicate acknowledgment threshold. Recently, there have been efforts to completely phase out dupack based fast-retransmissions and replace them with timer based ones. (Not to be confused with

11970-513: The rules for how the bytes must be organized and interpreted meaningfully. For example, the bytes of a plain text file ( .txt in Windows) are associated with either ASCII or UTF-8 characters, while the bytes of image, video, and audio files are interpreted otherwise. Most file types also allocate a few bytes for metadata , which allows a file to carry some basic information about itself. Some file systems can store arbitrary (not interpreted by

12084-402: The same computer protects against failure of one disk, but if it is necessary to protect against failure or destruction of the entire computer, then copies of the files must be made on other media that can be taken away from the computer and stored in a safe, distant location. The grandfather-father-son backup method automatically makes three back-ups; the grandfather file is the oldest copy of

12198-404: The sequence number for the other direction (server to client). Following the completion of these steps, both the client and server have received acknowledgments and a full-duplex communication is established. The connection termination phase uses a four-way handshake, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits

12312-638: The server in case the ACK is lost in transit. The time duration is implementation-dependent, but some common values are 30 seconds, 1 minute, and 2 minutes. After the timeout, the client enters the CLOSED state and the local port becomes available for new connections. It is also possible to terminate the connection by a 3-way handshake, when host A sends a FIN and host B replies with a FIN & ACK (combining two steps into one) and host A replies with an ACK. Some operating systems, such as Linux and HP-UX , implement

12426-460: The server respond first when a connection is being made. For example, when a client connects to a hub's socket , the hub is first to respond to the client. The protocol lacks a specified default character encoding for clients or hubs. The original client and hub use ASCII encoding instead of that of the Operating system . This allows migration to UTF-8 encoding in newer software. Port 411

12540-611: The underlying hardware, rather than the contents stored on it, was denominated a "file". For example, the IBM 350 disk drives were denominated "disk files". The introduction, c.  1961 , by the Burroughs MCP and the MIT Compatible Time-Sharing System of the concept of a " file system " that managed several virtual "files" on one storage device is the origin of the contemporary denotation of

12654-580: The user uses a disk editor to edit the file names in the directory entries . User applications, however, will usually not allow the user to create multiple files with the same name but differing in case. Most computers organize files into hierarchies using folders, directories, or catalogs. The concept is the same irrespective of the terminology used. Each folder can contain an arbitrary number of files, and it can also contain other folders. These other folders are referred to as subfolders. Subfolders can contain still more files and folders and so on, thus building

12768-429: The user will not be able to download anything. In DC++ , users will not receive those search results. In NeoModus Direct Connect, all users will be sent at most five search results per query. If a user has searched, DC++ will respond with ten search results when the user is in active mode and five when the user is in passive mode. Passive clients will be sent search results through the hub, while active clients will receive

12882-452: The whole Direct Connect network suffered from DDoS attacks. The situation prompted developers to take security issues more seriously. As of February 2009, an extension for clients was proposed in order for the attacked party to find out the hub sending the connecting users. The Direct Connect Network Foundation (DCNF) is a non-profit organization registered in Sweden that aims to improve

12996-402: The word. Although the contemporary " register file " demonstrates the early concept of files, its use has greatly decreased. On most modern operating systems , files are organized into one-dimensional arrays of bytes . The format of a file is defined by its content since a file is solely a container for data. On some platforms the format is indicated by its filename extension , specifying

#874125