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45-500: The Kad network uses a UDP -based protocol to: Note that the Kad network is not used to actually transfer files across the P2P network. Instead, when a file transfer is initiated, clients connect directly to each other (using the standard public IP network). This traffic is susceptible to blocking/shaping/tracking by an ISP or any other opportunistic middle-man. As with all decentralized networks,

90-647: A data section (the payload data for the application). The UDP datagram header consists of 4 fields, each of which is 2 bytes (16 bits): The use of the Checksum and Source Port fields is optional in IPv4 (light purple background in table). In IPv6 only the Source Port field is optional. If not used, these fields should be set to zero. The method used to compute the checksum is defined in RFC ; 768 , and efficient calculation

135-497: A carry-out (17th bit) is produced, swing that 17th carry bit around and add it to the least significant bit of the running total. Finally, the sum is then ones' complemented to yield the value of the UDP checksum field. If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as the ones' complement (all 1s) as a zero-value checksum indicates no checksum has been calculated. In this case, any specific processing

180-411: A combination of TCP and TLS to ensure reliability and security respectively. This means that HTTP/3 uses a single handshake to set up a connection, rather than having two separate handshakes for TCP and TLS, meaning the overall time to establish a connection is reduced. Transmission Control Protocol is a connection-oriented protocol and requires handshaking to set up end-to-end communications. Once

225-450: A connection and that UDP doesn't keep track of what it has sent. UDP provides checksums for data integrity , and port numbers for addressing different functions at the source and destination of the datagram. It has no handshaking dialogues and thus exposes the user's program to any unreliability of the underlying network; there is no guarantee of delivery, ordering, or duplicate protection. If error-correction facilities are needed at

270-437: A connection is set up, user data may be sent bi-directionally over the connection. User Datagram Protocol is a simpler message-based connectionless protocol . Connectionless protocols do not set up a dedicated end-to-end connection. Communication is achieved by transmitting information in one direction from source to destination without verifying the readiness or state of the receiver. Datagram socket A network socket

315-477: A different header layout, the method used to compute the checksum is changed accordingly: Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses. When computing the checksum, again a pseudo header is used that mimics the real IPv6 header : The checksum

360-528: A high degree of reliability, a protocol such as the Transmission Control Protocol may be used instead. Most often, UDP applications do not employ reliability mechanisms and may even be hindered by them. Streaming media , real-time multiplayer games and voice over IP (VoIP) are examples of applications that often use UDP. In these particular applications, loss of packets is not usually a fatal problem. In VoIP, for example, latency and jitter are

405-422: A network using the protocols that the stack implements. The operating system forwards the payload of incoming IP packets to the corresponding application by extracting the socket address information from the IP and transport protocol headers and stripping the headers from the application data. The application programming interface (API) that programs use to communicate with the protocol stack, using network sockets,

450-404: A node or device. The application programming interface (API) for the network protocol stack creates a handle for each socket created by an application, commonly referred to as a socket descriptor . In Unix-like operating systems , this descriptor is a type of file descriptor . It is stored by the application process for use with every read and write operation on the communication channel. At

495-585: A number of secondary characteristics such as size, extension, bit-rate, and more. Features vary based on client used. Only a few major clients currently support the Kad network implementation. However, they comprise over 80% of the user base and are probably closer to 95% of ed2k installations. There are a number of minor variants, or forks, of eMule which support the same basic features as eMule itself. They include: aMule (A Linux client similar to eMule) and eMule Mods (not eMule Plus ), possibly others. User Datagram Protocol In computer networking ,

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540-724: A socket to its endpoint of data transmission, which is a combination of an IP address and a port . In this way, UDP provides application multiplexing . A port is a software structure that is identified by the port number , a 16-bit integer value, allowing for port numbers between 0 and 65535. Port 0 is reserved but is a permissible source port value if the sending process does not expect messages in response. The Internet Assigned Numbers Authority (IANA) has divided port numbers into three ranges. Port numbers 0 through 1023 are used for common, well-known services. On Unix -like operating systems , using one of these ports requires superuser operating permission. Port numbers 1024 through 49151 are

585-406: A source address, for example, for only sending data, but if a program binds a socket to a source address, the socket can be used to receive data sent to that address. Based on this address, Internet sockets deliver incoming data packets to the appropriate application process . Socket often refers specifically to an internet socket or TCP socket. An internet socket is minimally characterized by

630-420: Is a software structure within a network node of a computer network that serves as an endpoint for sending and receiving data across the network. The structure and properties of a socket are defined by an application programming interface (API) for the networking architecture. Sockets are created only during the lifetime of a process of an application running in the node. Because of the standardization of

675-420: Is also used for the software endpoint of node-internal inter-process communication (IPC), which often uses the same API as a network socket. The use of the term socket in software is analogous to the function of an electrical female connector , a device in hardware for communication between nodes interconnected with an electrical cable . Similarly, the term port is used for external physical endpoints at

720-608: Is called a socket API . Development of application programs that utilize this API is called socket programming or network programming . Internet socket APIs are usually based on the Berkeley sockets standard. In the Berkeley sockets standard, sockets are a form of file descriptor , due to the Unix philosophy that "everything is a file", and the analogies between sockets and files. Both have functions to read, write, open, and close. In practice,

765-431: Is computed over the following fields: Lacking reliability, UDP applications may encounter some packet loss, reordering, errors or duplication. If using UDP, the end-user applications must provide any necessary handshaking such as real-time confirmation that the message has been received. Applications, such as TFTP , may add rudimentary reliability mechanisms into the application layer as needed. If an application requires

810-454: Is different; i.e. since they have different socket pair tuples. UDP sockets do not have an established state , because the protocol is connectionless . A UDP server process handles incoming datagrams from all remote clients sequentially through the same socket. UDP sockets are not identified by the remote address, but only by the local address, although each message has an associated remote address that can be retrieved from each datagram with

855-403: Is discussed in RFC  1071 : Checksum is the 16-bit ones' complement of the ones' complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets. In other words, all 16-bit words are summed using ones' complement arithmetic. Add the 16-bit values up. On each addition, if

900-403: Is established with the remote socket, providing a duplex byte stream . A server may create several concurrently established TCP sockets with the same local port number and local IP address, each mapped to its own server-child process, serving its own client process. They are treated as different sockets by the operating system since the remote socket address (the client IP address or port number)

945-414: Is generally transmitted using UDP. Real-time video and audio streaming protocols are designed to handle occasional lost packets, so only slight degradation in quality occurs, rather than large delays if lost packets were retransmitted. Because both TCP and UDP run over the same network, in the mid-2000s a few businesses found that an increase in UDP traffic from these real-time applications slightly hindered

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990-459: Is not required at the receiver, because all 0s and all 1s are equal to zero in 1's complement arithmetic. The differences between IPv4 and IPv6 are in the pseudo header used to compute the checksum, and that the checksum is not optional in IPv6. Under specific conditions, a UDP application using IPv6 is allowed to use a zero UDP zero-checksum mode with a tunnel protocol. When UDP runs over IPv4,

1035-486: Is preferable to waiting for packets delayed due to retransmission , which may not be an option in a real-time system . The protocol was designed by David P. Reed in 1980 and formally defined in RFC   768 . UDP is a simple message-oriented transport layer protocol that is documented in RFC  768 . Although UDP provides integrity verification (via checksum ) of the header and payload, it provides no guarantees to

1080-448: Is the network-facing access handle to the network socket. The remote process establishes a network socket in its own instance of the protocol stack and uses the networking API to connect to the application, presenting its own socket address for use by the application. A protocol stack , usually provided by the operating system (rather than as a separate library, for instance), is a set of services that allows processes to communicate over

1125-579: Is uniquely identified by the socket number. In other textbooks, the term socket refers to a local socket address, i.e. a "combination of an IP address and a port number". In the original definition of socket given in RFC 147, as it was related to the ARPA network in 1971, "the socket is specified as a 32-bit number with even sockets identifying receiving sockets and odd sockets identifying sending sockets." Today, however, socket communications are bidirectional. Within

1170-510: The OSI model . Networking equipment such as routers , which operate at the internet layer , and switches , which operate at the link layer , do not require implementations of the transport layer. However, stateful network firewalls , network address translators , and proxy servers keep track of active socket pairs. In multilayer switches and quality of service (QoS) support in routers, packet flows may be identified by extracting information about

1215-553: The TCP/IP protocols in the development of the Internet , the term network socket is most commonly used in the context of the Internet protocol suite, and is therefore often also referred to as Internet socket . In this context, a socket is externally identified to other hosts by its socket address , which is the triad of transport protocol , IP address , and port number . The term socket

1260-498: The User Datagram Protocol ( UDP ) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets ) to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths. UDP is a connectionless protocol meaning that messages are sent without negotiating

1305-406: The listening state . These sockets are waiting for initiatives from client programs. A TCP server may serve several clients concurrently by creating a unique dedicated socket for each client connection in a new child process or processing thread for each client. These are in the established state when a socket-to-socket virtual connection or virtual circuit (VC), also known as a TCP session ,

1350-402: The registered ports used for IANA-registered services. Ports 49152 through 65535 are dynamic ports that are not officially designated for any specific service and may be used for any purpose. These may also be used as ephemeral ports , which software running on the host may use to dynamically create communications endpoints as needed. A UDP datagram consists of a datagram header followed by

1395-471: The upper layer protocol for message delivery and the UDP layer retains no state of UDP messages once sent. For this reason, UDP sometimes is referred to as Unreliable Datagram Protocol . If transmission reliability is desired, it must be implemented in the user's application. A number of UDP's attributes make it especially suited for certain applications. Applications can use datagram sockets to establish host-to-host communications. An application binds

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1440-544: The Kad network requires no official or common servers. As such, it cannot be disabled by shutting down a given subset of key nodes . While the decentralization of the network prevents a simple shut-down, traffic analysis and deep packet inspection will more readily identify the traffic as P2P due to the high variable-destination packet throughput. The large packet volume typically causes a reduction in available CPU and/or network resources usually associated with P2P traffic. The Kad network supports searching of files by name and

1485-445: The checksum is computed using a pseudo header that contains some of the same information from the real IPv4 header . The pseudo header is not the real IPv4 header used to send an IP packet, it is used only for the checksum calculation. UDP checksum computation is optional for IPv4. If a checksum is not used it should be set to the value zero. The checksum is calculated over the following fields: As IPv6 has larger addresses and

1530-435: The differences strain the analogy, and different interfaces (send and receive) are used on a socket. In inter-process communication , each end generally has its own socket. In the standard Internet protocols TCP and UDP, a socket address is the combination of an IP address and a port number , much like one end of a telephone connection is the combination of a phone number and a particular extension . Sockets need not have

1575-438: The following: The distinctions between a socket (internal representation), socket descriptor (abstract identifier), and socket address (public address) are subtle, and these are not always distinguished in everyday usage. Further, specific definitions of a socket differ between authors. In IETF Request for Comments , Internet Standards , in many textbooks, as well as in this article, the term socket refers to an entity that

1620-527: The host with address 203.0.113.0. It illustrates the creation of a socket (getSocket), connecting it to the remote host, sending the string, and finally closing the socket: Several types of Internet socket are available: Other socket types are implemented over other transport protocols, such as Systems Network Architecture and Unix domain sockets for internal inter-process communication. Computer processes that provide application services are referred to as servers , and create sockets on startup that are in

1665-550: The licensing constraints of AT&T 's copyright-protected Unix . In c. 1987, AT&T introduced the STREAMS -based Transport Layer Interface (TLI) in UNIX System V Release 3 (SVR3). and continued into Release 4 (SVR4). Other early implementations were written for TOPS-20 , MVS , VM , IBM-DOS (PCIP). The socket is primarily a concept used in the transport layer of the Internet protocol suite or session layer of

1710-447: The network interface level, an application may instead use Transmission Control Protocol (TCP) or Stream Control Transmission Protocol (SCTP) which are designed for this purpose. UDP is suitable for purposes where error checking and correction are either not necessary or are performed in the application; UDP avoids the overhead of such processing in the protocol stack . Time-sensitive applications often use UDP because dropping packets

1755-481: The networking application programming interface (API). Communicating local and remote sockets are called socket pairs . Each socket pair is described by a unique 4-tuple consisting of source and destination IP addresses and port numbers, i.e. of local and remote socket addresses. As discussed above, in the TCP case, a socket pair is associated on each end of the connection with a unique 4-tuple. The term socket dates to

1800-432: The operating system and the application that created a socket, a socket is referred to by a unique integer value called a socket descriptor . On Unix-like operating systems and Microsoft Windows , the command-line tools netstat or ss are used to list established sockets and related information. This example, modeled according to the Berkeley socket interface, sends the string "Hello, world!" via TCP to port 80 of

1845-496: The performance of applications using TCP such as point of sale , accounting , and database systems (when TCP detects packet loss, it will throttle back its data rate usage). Some VPN systems such as OpenVPN may use UDP and perform error checking at the application level while implementing reliable connections. QUIC is a transport protocol built on top of UDP. QUIC provides a reliable and secure connection. HTTP/3 uses QUIC as opposed to earlier versions of HTTPS which use

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1890-737: The primary concerns. The use of TCP would cause jitter if any packets were lost as TCP does not provide subsequent data to the application while it is requesting a re-send of the missing data. Numerous key Internet applications use UDP, including: the Domain Name System (DNS), the Simple Network Management Protocol (SNMP), the Routing Information Protocol (RIP) and the Dynamic Host Configuration Protocol (DHCP). Voice and video traffic

1935-527: The proliferation of the TCP/IP protocols in use on the Internet, the term network socket usually refers to use with the Internet Protocol (IP). It is therefore often also called Internet socket . An application can communicate with a remote process by exchanging data with TCP/IP by knowing the combination of protocol type, IP address, and port number. This combination is often known as a socket address . It

1980-566: The publication of RFC 147 in 1971, when it was used in the ARPANET. Most modern implementations of sockets are based on Berkeley sockets (1983), and other stacks such as Winsock (1991). The Berkeley sockets API in the Berkeley Software Distribution (BSD), originated with the 4.2BSD Unix operating system as an API. Only in 1989, however, could UC Berkeley release versions of its operating system and networking library free from

2025-711: The time of creation with the API, a network socket is bound to the combination of a type of network protocol to be used for transmissions, a network address of the host, and a port number . Ports are numbered resources that represent another type of software structure of the node. They are used as service types, and, once created by a process, serve as an externally (from the network) addressable location component, so that other hosts may establish connections. Network sockets may be dedicated for persistent connections for communication between two nodes, or they may participate in connectionless and multicast communications. In practice, due to

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