94-469: This list of DNS record types is an overview of resource records (RRs) permissible in zone files of the Domain Name System (DNS). It also contains pseudo-RRs. Other types of records simply provide some types of information (for example, an HINFO record gives a description of the type of computer/OS a host uses), or others return data used in experimental features. The "type" field is also used in
188-446: A label and zero or more resource records (RR), which hold information associated with the domain name. The domain name itself consists of the label, concatenated with the name of its parent node on the right, separated by a dot. The tree sub-divides into zones beginning at the root zone . A DNS zone may consist of as many domains and subdomains as the zone manager chooses. DNS can also be partitioned according to class where
282-435: 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
376-480: A "com" server, and finally an "example.com" server. Name servers in delegations are identified by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. If the name given in the delegation is a subdomain of the domain for which the delegation is being provided, there is a circular dependency . In this case,
470-470: 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
564-514: 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
658-460: 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
752-429: 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
846-408: A cache of data. An authoritative name server can either be a primary server or a secondary server. Historically the terms master/slave and primary/secondary were sometimes used interchangeably but the current practice is to use the latter form. A primary server is a server that stores the original copies of all zone records. A secondary server uses a special automatic updating mechanism in
940-416: A combination of these methods. In a non-recursive query , a DNS resolver queries a DNS server that provides a record either for which the server is authoritative, or it provides a partial result without querying other servers. In case of a caching DNS resolver , the non-recursive query of its local DNS cache delivers a result and reduces the load on upstream DNS servers by caching DNS resource records for
1034-728: A compromise between five competing proposals of solutions to Paul Mockapetris . Mockapetris instead created the Domain Name System in 1983 while at the University of Southern California . The Internet Engineering Task Force published the original specifications in RFC 882 and RFC 883 in November 1983. These were updated in RFC 973 in January 1986. In 1984, four UC Berkeley students, Douglas Terry, Mark Painter, David Riggle, and Songnian Zhou, wrote
SECTION 10
#17328517372531128-451: 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
1222-399: A dataset from a reliable source. Assuming the resolver has no cached records to accelerate the process, the resolution process starts with a query to one of the root servers. In typical operation, the root servers do not answer directly, but respond with a referral to more authoritative servers, e.g., a query for "www.wikipedia.org" is referred to the org servers. The resolver now queries
1316-580: A general purpose database, the DNS has also been used in combating unsolicited email (spam) by storing a real-time blackhole list (RBL). The DNS database is traditionally stored in a structured text file, the zone file , but other database systems are common. The Domain Name System originally used the User Datagram Protocol (UDP) as transport over IP. Reliability, security, and privacy concerns spawned
1410-473: 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
1504-449: 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
1598-418: 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
1692-407: A period of time after an initial response from upstream DNS servers. In a recursive query , a DNS resolver queries a single DNS server, which may in turn query other DNS servers on behalf of the requester. For example, a simple stub resolver running on a home router typically makes a recursive query to the DNS server run by the user's ISP . A recursive query is one for which the DNS server answers
1786-451: 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
1880-489: 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
1974-481: 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
SECTION 20
#17328517372532068-469: A service's location on the network to change without affecting the end users, who continue to use the same hostname. Users take advantage of this when they use meaningful Uniform Resource Locators ( URLs ) and e-mail addresses without having to know how the computer actually locates the services. An important and ubiquitous function of the DNS is its central role in distributed Internet services such as cloud services and content delivery networks . When
2162-407: 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
2256-424: 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)
2350-480: A time to live (TTL), which indicates how long the information remains valid before it needs to be discarded or refreshed. This TTL is determined by the administrator of the authoritative DNS server and can range from a few seconds to several days or even weeks. Transmission Control Protocol The Transmission Control Protocol ( TCP ) is one of the main protocols of the Internet protocol suite . It originated in
2444-477: A type of error called a "lame delegation" or "lame response". Domain name resolvers determine the domain name servers responsible for the domain name in question by a sequence of queries starting with the right-most (top-level) domain label. For proper operation of its domain name resolver, a network host is configured with an initial cache ( hints ) of the known addresses of the root name servers. The hints are updated periodically by an administrator by retrieving
2538-455: A user accesses a distributed Internet service using a URL, the domain name of the URL is translated to the IP address of a server that is proximal to the user. The key functionality of the DNS exploited here is that different users can simultaneously receive different translations for the same domain name, a key point of divergence from a traditional phone-book view of the DNS. This process of using
2632-489: Is known as the LDH rule (letters, digits, hyphen). Domain names are interpreted in a case-independent manner. Labels may not start or end with a hyphen. An additional rule requires that top-level domain names should not be all-numeric. The limited set of ASCII characters permitted in the DNS prevented the representation of names and words of many languages in their native alphabets or scripts. To make this possible, ICANN approved
2726-484: Is only achieved with at least 6 labels (counting the last null label). Although no technical limitation exists to prevent domain name labels from using any character that is representable by an octet, hostnames use a preferred format and character set. The characters allowed in labels are a subset of the ASCII character set, consisting of characters a through z , A through Z , digits 0 through 9 , and hyphen. This rule
2820-683: Is part of the Transport layer of the TCP/IP suite. SSL/TLS often runs on top of TCP. TCP 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
2914-399: Is served by the root name servers , the servers to query when looking up ( resolving ) a TLD . An authoritative name server is a name server that only gives answers to DNS queries from data that have been configured by an original source, for example, the domain administrator or by dynamic DNS methods, in contrast to answers obtained via a query to another name server that only maintains
List of DNS record types - Misplaced Pages Continue
3008-453: 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
3102-557: 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
3196-532: The Internationalizing Domain Names in Applications (IDNA) system, by which user applications, such as web browsers, map Unicode strings into the valid DNS character set using Punycode . In 2009, ICANN approved the installation of internationalized domain name country code top-level domains ( ccTLD s) . In addition, many registries of the existing top-level domain names ( TLD s ) have adopted
3290-563: 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
3384-503: 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
3478-539: 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
3572-546: 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
3666-527: 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
3760-478: The top-level domain ; for example, the domain name www.example.com belongs to the top-level domain com . The hierarchy of domains descends from right to left; each label to the left specifies a subdivision, or subdomain of the domain to the right. For example, the label example specifies a subdomain of the com domain, and www is a subdomain of example.com. This tree of subdivisions may have up to 127 levels. A label may contain zero to 63 characters, because
3854-404: The " Authoritative Answer " ( AA ) bit in its responses. This flag is usually reproduced prominently in the output of DNS administration query tools, such as dig , to indicate that the responding name server is an authority for the domain name in question. When a name server is designated as the authoritative server for a domain name for which it does not have authoritative data, it presents
List of DNS record types - Misplaced Pages Continue
3948-526: The ARPANET. Elizabeth Feinler developed and maintained the first ARPANET directory. Maintenance of numerical addresses, called the Assigned Numbers List, was handled by Jon Postel at the University of Southern California 's Information Sciences Institute (ISI), whose team worked closely with SRI. Addresses were assigned manually. Computers, including their hostnames and addresses, were added to
4042-462: The DNS database are for start of authority ( SOA ), IP addresses ( A and AAAA ), SMTP mail exchangers (MX), name servers (NS), pointers for reverse DNS lookups (PTR), and domain name aliases (CNAME). Although not intended to be a general purpose database, DNS has been expanded over time to store records for other types of data for either automatic lookups, such as DNSSEC records, or for human queries such as responsible person (RP) records. As
4136-401: The DNS protocol in communication with its primary to maintain an identical copy of the primary records. Every DNS zone must be assigned a set of authoritative name servers. This set of servers is stored in the parent domain zone with name server (NS) records. An authoritative server indicates its status of supplying definitive answers, deemed authoritative , by setting a protocol flag, called
4230-468: The DNS to assign proximal servers to users is key to providing faster and more reliable responses on the Internet and is widely used by most major Internet services. The DNS reflects the structure of administrative responsibility on the Internet. Each subdomain is a zone of administrative autonomy delegated to a manager. For zones operated by a registry , administrative information is often complemented by
4324-596: 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,
4418-456: The IDNA system, guided by RFC 5890, RFC 5891, RFC 5892, RFC 5893. The Domain Name System is maintained by a distributed database system, which uses the client–server model . The nodes of this database are the name servers . Each domain has at least one authoritative DNS server that publishes information about that domain and the name servers of any domains subordinate to it. The top of the hierarchy
4512-430: The IP address spaces . The Domain Name System maintains the domain name hierarchy and provides translation services between it and the address spaces. Internet name servers and a communication protocol implement the Domain Name System. A DNS name server is a server that stores the DNS records for a domain; a DNS name server responds with answers to queries against its database. The most common types of records stored in
4606-501: 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
4700-402: The Internet, and increase performance in end-user applications, the Domain Name System supports DNS cache servers which store DNS query results for a period of time determined in the configuration ( time-to-live ) of the domain name record in question. Typically, such caching DNS servers also implement the recursive algorithm necessary to resolve a given name starting with the DNS root through to
4794-453: 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
SECTION 50
#17328517372534888-415: 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
4982-463: 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
5076-404: 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
5170-519: 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
5264-704: The associated entities. Most prominently, it translates readily memorized domain names to the numerical IP addresses needed for locating and identifying computer services and devices with the underlying network protocols . The Domain Name System has been an essential component of the functionality of the Internet since 1985. The Domain Name System delegates the responsibility of assigning domain names and mapping those names to Internet resources by designating authoritative name servers for each domain. Network administrators may delegate authority over subdomains of their allocated name space to other name servers. This mechanism provides distributed and fault-tolerant service and
5358-543: The authoritative name servers of the queried domain. With this function implemented in the name server, user applications gain efficiency in design and operation. The combination of DNS caching and recursive functions in a name server is not mandatory; the functions can be implemented independently in servers for special purposes. Internet service providers typically provide recursive and caching name servers for their customers. In addition, many home networking routers implement DNS caches and recursion to improve efficiency in
5452-522: 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,
5546-607: 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
5640-466: The computer. Computers at educational institutions would have the domain edu , for example. She and her team managed the Host Naming Registry from 1972 to 1989. By the early 1980s, maintaining a single, centralized host table had become slow and unwieldy and the emerging network required an automated naming system to address technical and personnel issues. Postel directed the task of forging
5734-407: 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,
SECTION 60
#17328517372535828-438: The delegation for example.org. The glue records are address records that provide IP addresses for ns1.example.org. The resolver uses one or more of these IP addresses to query one of the domain's authoritative servers, which allows it to complete the DNS query. A common approach to reduce the burden on DNS servers is to cache the results of name resolution locally or on intermediary resolver hosts. Each DNS query result comes with
5922-738: The first Unix name server implementation for the Berkeley Internet Name Domain, commonly referred to as BIND . In 1985, Kevin Dunlap of DEC substantially revised the DNS implementation. Mike Karels , Phil Almquist, and Paul Vixie then took over BIND maintenance. Internet Systems Consortium was founded in 1994 by Rick Adams , Paul Vixie , and Carl Malamud , expressly to provide a home for BIND development and maintenance. BIND versions from 4.9.3 onward were developed and maintained by ISC, with support provided by ISC's sponsors. As co-architects/programmers, Bob Halley and Paul Vixie released
6016-453: The first production-ready version of BIND version 8 in May 1997. Since 2000, over 43 different core developers have worked on BIND. In November 1987, RFC 1034 and RFC 1035 superseded the 1983 DNS specifications. Several additional Request for Comments have proposed extensions to the core DNS protocols. The domain name space consists of a tree data structure . Each node or leaf in the tree has
6110-428: 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
6204-407: 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
6298-510: The initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite 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
6392-513: 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
6486-464: 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
6580-417: 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
6674-433: The length is only allowed to take 6 bits. The null label of length zero is reserved for the root zone. The full domain name may not exceed the length of 253 characters in its textual representation (or 254 with the trailing dot). In the internal binary representation of the DNS this maximum length of 253 requires 255 octets of storage, as it also stores the length of the first of many labels and adds last null byte. 255 length
6768-422: The local network. The client side of the DNS is called a DNS resolver. A resolver is responsible for initiating and sequencing the queries that ultimately lead to a full resolution (translation) of the resource sought, e.g., translation of a domain name into an IP address. DNS resolvers are classified by a variety of query methods, such as recursive , non-recursive , and iterative . A resolution process may use
6862-542: 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,
6956-408: The name server and IP address. For example, if the authoritative name server for example.org is ns1.example.org, a computer trying to resolve www.example.org first resolves ns1.example.org. As ns1 is contained in example.org, this requires resolving example.org first, which presents a circular dependency. To break the dependency, the name server for the top level domain org includes glue along with
7050-409: The name server providing the delegation must also provide one or more IP addresses for the authoritative name server mentioned in the delegation. This information is called glue . The delegating name server provides this glue in the form of records in the additional section of the DNS response, and provides the delegation in the authority section of the response. A glue record is a combination of
7144-442: 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
7238-559: The networks and creating the Internet: Commercialization, privatization, broader access leads to the modern Internet: Examples of Internet services: The Domain Name System ( DNS ) is a hierarchical and distributed name service that provides a naming system for computers , services, and other resources on the Internet or other Internet Protocol (IP) networks. It associates various information with domain names ( identification strings ) assigned to each of
7332-407: 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
7426-532: The primary file by contacting the SRI Network Information Center (NIC), directed by Feinler, via telephone during business hours. Later, Feinler set up a WHOIS directory on a server in the NIC for retrieval of information about resources, contacts, and entities. She and her team developed the concept of domains. Feinler suggested that domains should be based on the location of the physical address of
7520-437: The protocol for various operations. Progress has rendered some of the originally defined record-types obsolete. Of the records listed at IANA, some have limited use, for various reasons. Some are marked obsolete in the list, some are for very obscure services, some are for older versions of services, and some have special notes saying they are "not right". Resource record Early research and development: Merging
7614-402: The query completely by querying other name servers as needed. In typical operation, a client issues a recursive query to a caching recursive DNS server, which subsequently issues non-recursive queries to determine the answer and send a single answer back to the client. The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in
7708-404: The query headers. DNS servers are not required to support recursive queries. The iterative query procedure is a process in which a DNS resolver queries a chain of one or more DNS servers. Each server refers the client to the next server in the chain, until the current server can fully resolve the request. For example, a possible resolution of www.example.com would query a global root server, then
7802-418: 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 –
7896-407: 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,
7990-472: The registry's RDAP and WHOIS services. That data can be used to gain insight on, and track responsibility for, a given host on the Internet. Using a simpler, more memorable name in place of a host's numerical address dates back to the ARPANET era. The Stanford Research Institute (now SRI International ) maintained a text file named HOSTS.TXT that mapped host names to the numerical addresses of computers on
8084-504: 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
8178-625: 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
8272-516: The root servers, and as a result, root name servers actually are involved in only a relatively small fraction of all requests. In theory, authoritative name servers are sufficient for the operation of the Internet. However, with only authoritative name servers operating, every DNS query must start with recursive queries at the root zone of the Domain Name System and each user system would have to implement resolver software capable of recursive operation. To improve efficiency, reduce DNS traffic across
8366-629: The separate classes can be thought of as an array of parallel namespace trees. Administrative responsibility for any zone may be divided by creating additional zones. Authority over the new zone is said to be delegated to a designated name server. The parent zone ceases to be authoritative for the new zone. The definitive descriptions of the rules for forming domain names appear in RFC 1035, RFC 1123, RFC 2181, and RFC 5892. A domain name consists of one or more parts, technically called labels , that are conventionally concatenated , and delimited by dots, such as example.com. The right-most label conveys
8460-404: 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
8554-631: 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
8648-423: The servers referred to, and iteratively repeats this process until it receives an authoritative answer. The diagram illustrates this process for the host that is named by the fully qualified domain name "www.wikipedia.org". This mechanism would place a large traffic burden on the root servers, if every resolution on the Internet required starting at the root. In practice caching is used in DNS servers to off-load
8742-521: The use of the Transmission Control Protocol (TCP) as well as numerous other protocol developments. An often-used analogy to explain the DNS is that it serves as the phone book for the Internet by translating human-friendly computer hostnames into IP addresses. For example, the hostname www.example.com within the domain name example.com translates to the addresses 93.184.216.34 ( IPv4 ) and 2606:2800:220:1:248:1893:25c8:1946 ( IPv6 ). The DNS can be quickly and transparently updated, allowing
8836-412: Was designed to avoid a single large central database. In addition, the DNS specifies the technical functionality of the database service that is at its core. It defines the DNS protocol, a detailed specification of the data structures and data communication exchanges used in the DNS, as part of the Internet protocol suite . The Internet maintains two principal namespaces , the domain name hierarchy and
#252747