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103-472: Conventionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The receiver of the packet echoes the congestion indication to the sender, which reduces its transmission rate as if it detected a dropped packet. Rather than responding properly or ignoring

206-435: 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

309-422: A web browser or web crawler , initiates communication by making a request for a web page or other resource using HTTP, and the server responds with the content of that resource or an error message . A web server can also accept and store resources sent from the user agent if configured to do so. The hardware used to run a web server can vary according to the volume of requests that it needs to handle. At

412-473: 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

515-514: 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

618-464: 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

721-429: 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

824-456: 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

927-834: 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 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

1030-483: A few cellular carriers in iOS 10 and 100% for iOS 11 The Solaris kernel supports three states of ECN for TCP: As of Solaris 11.4, the default behavior is active . ECN usage can be modified via ipadm set-prop -p ecn=active tcp . Since ECN marking in routers is dependent on some form of active queue management , routers must be configured with a suitable queue discipline in order to perform ECN marking. Cisco IOS routers perform ECN marking if configured with

1133-476: 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

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1236-587: A graceful fallback for transmissions without ECN capability, mitigating issues with ECN-intolerant hosts or firewalls. Mac OS X 10.5 and 10.6 implement ECN support for TCP. It is controlled using the Boolean sysctl variables net.inet.tcp.ecn_negotiate_in and net.inet.tcp.ecn_initiate_out . The first variable enables ECN on incoming connections that already have ECN flags set; the second one tries to initiate outgoing connections with ECN enabled. Both variables default to 0 , but can be set to 1 to enable

1339-449: 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

1442-418: 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

1545-1064: A long time and so Apache suffered, even more, the competition of commercial servers and, above all, of other open-source servers which meanwhile had already achieved far superior performances (mostly when serving static content) since the beginning of their development and at the time of the Apache decline were able to offer also a long enough list of well tested advanced features. In fact, a few years after 2000 started, not only other commercial and highly competitive web servers, e.g. LiteSpeed , but also many other open-source programs, often of excellent quality and very high performances, among which should be noted Hiawatha , Cherokee HTTP server , Lighttpd , Nginx and other derived/related products also available with commercial support, emerged. Around 2007–2008, most popular web browsers increased their previous default limit of 2 persistent connections per host-domain (a limit recommended by RFC-2616) to 4, 6 or 8 persistent connections per host-domain, in order to speed up

1648-514: A physical file system path, to an absolute path under the target website's root directory. Website's root directory may be specified by a configuration file or by some internal rule of the web server by using the name of the website which is the host part of the URL found in HTTP client request. Path translation to file system is done for the following types of web resources: The web server appends

1751-468: A queue that uses random early detection (RED)) that is experiencing congestion and the corresponding router supports ECN, it may change the code point to CE instead of dropping the packet . This act is referred to as "marking" and its purpose is to inform the receiving endpoint of impending congestion . At the receiving endpoint, this congestion indication is handled by the upper layer protocol ( transport layer protocol) and needs to be echoed back to

1854-660: A segment with the CWR bit set. To see affected packets with tcpdump , use the filter predicate (tcp[13] & 0xc0 != 0) . Since the Transmission Control Protocol (TCP) does not perform congestion control on control packets (pure ACKs, SYN, FIN segments), control packets are usually not marked as ECN-capable. A 2009 proposal suggests marking SYN-ACK packets as ECN-capable. This improvement, known as ECN+, has been shown to provide dramatic improvements to performance of short-lived TCP connections. ECN

1957-454: 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

2060-489: 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

2163-481: 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

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2266-406: A single outstanding segment, when it is able to avoid an RTO timeout; this is often the case for interactive connections, such as remote logins, and transactional protocols, such as HTTP requests, the conversational phase of SMTP, or SQL requests. Effects of ECN on bulk throughput are less clear because modern TCP implementations are fairly good at resending dropped segments in a timely manner when

2369-407: 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

2472-424: 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)

2575-428: A strong impetus to the adoption of reverse proxies in front of slower web servers and it gave also one more chance to the emerging new web servers that could show all their speed and their capability to handle very high numbers of concurrent connections without requiring too many hardware resources (expensive computers with lots of CPUs, RAM and fast disks). In 2015, RFCs published new protocol version [HTTP/2], and as

2678-451: A web server and some of the tasks that it may perform in order to have a sufficiently wide scenario about the topic. A web server program plays the role of a server in a client–server model by implementing one or more versions of HTTP protocol, often including the HTTPS secure variant and other features and extensions that are considered useful for its planned usage. The complexity and

2781-427: A web server implements one or more of the above-mentioned advanced features then the path part of a valid URL may not always match an existing file system path under website directory tree (a file or a directory in file system ) because it can refer to a virtual name of an internal or external module processor for dynamic requests. Web server programs are able to translate an URL path (all or part of it), that refers to

2884-550: Is also defined for other transport layer protocols that perform congestion control, notably DCCP and Stream Control Transmission Protocol (SCTP). The general principle is similar to TCP, although the details of the on-the-wire encoding differ. It is possible to use ECN with protocols layered above UDP . However, UDP requires that congestion control be performed by the application, and early UDP based protocols such as DNS did not use ECN. More recent UDP based protocols such as QUIC are using ECN for congestion control. Since ECN

2987-621: Is echoed back by setting the ECE bit. ECN uses the two least significant (right-most) bits of the Traffic Class field in the IPv4 or IPv6 header to encode four different code points: When both endpoints support ECN they mark their packets with ECT(0) or ECT(1). Routers treat the ECT(0) and ECT(1) codepoints as equivalent. If the packet traverses an active queue management (AQM) queue (e.g.,

3090-487: Is enabled by default in Windows Server versions, because Data Center Transmission Control Protocol (DCTCP) is used. In previous Windows versions and non-server versions it is disabled by default. ECN support can be enabled using a shell command such as netsh interface tcp set global ecncapability=enabled . On FreeBSD , ECN for TCP can be configured using the net.inet.tcp.ecn.enable sysctl . By default, it

3193-643: Is enabled only for incoming connections that request it. It can also be enabled for all connections or disabled entirely. NetBSD  4.0 implements ECN support for TCP; it can be activated through the sysctl interface by setting 1 as value for the sysctl net.inet.tcp.ecn.enable parameter. Likewise, the sysctl net.inet.tcp.ecn can be used in OpenBSD . Since version 2.4.20 of the Linux kernel , released in November 2002, Linux supports three working modes of

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3296-562: 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 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

3399-417: Is only effective in combination with an Active Queue Management (AQM) policy, the benefits of ECN depend on the precise AQM being used. A few observations, however, appear to hold across different AQMs. As expected, ECN reduces the number of packets dropped by a TCP connection, which, by avoiding a retransmission, reduces latency and especially jitter. This effect is most drastic when the TCP connection has

3502-481: Is referring to, so that that resource can be returned to the requesting client. This process is performed with every request that is made to a web server, with some of the requests being served with a file, such as an HTML document, or a gif image, others with the results of running a CGI program, and others by some other process, such as a built-in module handler, a PHP document, or a Java servlet." In practice, web server programs that implement advanced features, beyond

3605-456: 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

3708-565: 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

3811-441: Is used in data center networks. Whereas the standard TCP congestion control algorithm is only able to detect the presence of congestion, DCTCP, using ECN, is able to gauge the extent of congestion. DCTCP modifies the TCP receiver to always relay the exact ECN marking of incoming packets at the cost of ignoring a function that is meant to preserve signalling reliability. This makes a DCTCP sender vulnerable to loss of ACKs from

3914-470: The ALTQ queueing implementation for a number of queuing disciplines , notably RED and Blue . FreeBSD 11 included CoDel , PIE, FQ-CoDel and FQ-PIE queuing disciplines implementation in ipfw /dummynet framework with ECN marking capability. Data Center Transmission Control Protocol ( Data Center TCP or DCTCP ) utilizes ECN to enhance the Transmission Control Protocol congestion control algorithm. It

4017-601: The CGI to communicate with external programs. These capabilities, along with the multimedia features of NCSA's Mosaic browser (also able to manage HTML FORMs in order to send data to a web server) highlighted the potential of web technology for publishing and distributed computing applications. In the second half of 1994, the development of NCSA httpd stalled to the point that a group of external software developers, webmasters and other professional figures interested in that server, started to write and collect patches thanks to

4120-492: The Congestion Experienced code point, the TCP receiver echoes back this congestion indication using the ECE flag in the TCP header. When an endpoint receives a TCP segment with the ECE bit it reduces its congestion window as for a packet drop. It then acknowledges the congestion indication by sending a segment with the CWR bit set. A node keeps transmitting TCP segments with the ECE bit set until it receives

4223-634: 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

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4326-515: 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

4429-546: 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

4532-627: The WRED queuing discipline since version 12.2(8)T. Linux routers perform ECN marking if configured with one of the RED or GRED queue disciplines with an explicit ecn parameter, by using the sfb discipline, by using the CoDel Fair Queuing (fq_codel) discipline, or the CAKE queuing discipline. Modern BSD implementations, such as FreeBSD , NetBSD and OpenBSD , have support for ECN marking in

4635-534: 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

4738-564: 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 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

4841-480: The public domain . This statement freed web server developers from any possible legal issue about the development of derivative work based on that source code (a threat that in practice never existed). At the beginning of 1994, the most notable among new web servers was NCSA httpd which ran on a variety of Unix -based OSs and could serve dynamically generated content by implementing the POST HTTP method and

4944-566: The ECN for TCP, as configured through the sysctl interface by setting parameter /proc/sys/net/ipv4/tcp_ecn to one of the following values: Beginning with version 4.1 of the Linux kernel, released in June 2015, the tcp_ecn_fallback mechanism is enabled by default when ECN is enabled (the value of 1). The fallback mechanism attempts ECN connectivity in the initial setup of outgoing connections, with

5047-608: 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,

5150-509: 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

5253-629: The NCSA httpd source code being available to the public domain. At the beginning of 1995 those patches were all applied to the last release of NCSA source code and, after several tests, the Apache HTTP server project was started. At the end of 1994, a new commercial web server, named Netsite , was released with specific features. It was the first one of many other similar products that were developed first by Netscape , then also by Sun Microsystems , and finally by Oracle Corporation . In mid-1995,

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5356-453: 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

5459-548: The SYN and SYN-ACK segments. When ECN has been negotiated on a TCP connection, the sender indicates that IP packets that carry TCP segments of that connection are carrying traffic from an ECN Capable Transport by marking them with an ECT code point. This allows intermediate routers that support ECN to mark those IP packets with the CE code point instead of dropping them in order to signal impending congestion. Upon receiving an IP packet with

5562-415: 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

5665-414: The TCP header. The first, ECN-Echo (ECE) is used to echo back the congestion indication (i.e., signal the sender to reduce the transmission rate). The second, Congestion Window Reduced (CWR), to acknowledge that the congestion-indication echoing was received. Use of ECN on a TCP connection is optional; for ECN to be used, it must be negotiated at connection establishment by including suitable options in

5768-463: 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

5871-404: 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

5974-441: The above-mentioned history articles. In March 1989, Sir Tim Berners-Lee proposed a new project to his employer CERN , with the goal of easing the exchange of information between scientists by using a hypertext system. The proposal titled "HyperText and CERN" , asked for comments and it was read by several people. In October 1990 the proposal was reformulated and enriched (having as co-author Robert Cailliau ), and finally, it

6077-519: 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

6180-399: The adoption and the usage of those programs along with their porting to other operating systems . In December 1991, the first web server outside Europe was installed at SLAC (U.S.A.). This was a very important event because it started trans-continental web communications between web browsers and web servers. In 1991–1993, CERN web server program continued to be actively developed by

6283-436: The application of web servers well beyond their original purpose of serving human-readable pages. This is a very brief history of web server programs , so some information necessarily overlaps with the histories of the web browsers , the World Wide Web and the Internet ; therefore, for the sake of clarity and understandability, some key historical information below reported may be similar to that found also in one or more of

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6386-537: The availability of new protocol , not only because they had the work force and the time to do so, but also because usually their previous implementation of SPDY protocol could be reused as a starting point and because most used web browsers implemented it very quickly for the same reason. Another reason that prompted those developers to act quickly was that webmasters felt the pressure of the ever increasing web traffic and they really wanted to install and to try – as soon as possible – something that could drastically lower

6489-469: The bits, some outdated or faulty network equipment has historically dropped or mangled packets that have ECN bits set. As of 2015, measurements suggested that the fraction of web servers on the public Internet for which setting ECN prevents network connections had been reduced to less than 1%. Passive support has existed in Ubuntu Linux since 12.04 and in Windows Server since 2012. Passive support in

6592-474: 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,

6695-607: 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

6798-407: 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,

6901-870: The efficiency of a web server program may vary a lot depending on (e.g.): Although web server programs differ in how they are implemented, most of them offer the following common features. These are basic features that most web servers usually have. A few other more advanced and popular features ( only a very short selection ) are the following ones. A web server program, when it is running, usually performs several general tasks , (e.g.): Web server programs are able: Once an HTTP request message has been decoded and verified, its values can be used to determine whether that request can be satisfied or not. This requires many other steps, including security checks . Web server programs usually perform some type of URL normalization ( URL found in most HTTP request messages) in order to: The term URL normalization refers to

7004-406: The first version of IIS was released, for Windows NT OS, by Microsoft . This marked the entry, in the field of World Wide Web technologies, of a very important commercial developer and vendor that has played and still is playing a key role on both sides (client and server) of the web. In the second half of 1995, CERN and NCSA web servers started to decline (in global percentage usage) because of

7107-408: The following reasons: Without ECN, congestion indication echo is achieved indirectly by the detection of lost packets. With ECN, the congestion is indicated by setting the ECN field within an IP packet to CE (Congestion Experienced) and is echoed back by the receiver to the transmitter by setting proper bits in the header of the transport protocol. For example, when using TCP, the congestion indication

7210-428: 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

7313-483: The implementation of new specifications was not trivial at all, a dilemma arose among developers of less popular web servers (e.g. with a percentage of usage lower than 1% .. 2%), about adding or not adding support for that new protocol version. In fact supporting HTTP/2 often required radical changes to their internal implementation due to many factors (practically always required encrypted connections, capability to distinguish between HTTP/1.x and HTTP/2 connections on

7416-407: 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

7519-513: 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

7622-464: 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

7725-417: 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

7828-405: The low end of the range are embedded systems , such as a router that runs a small web server as its configuration interface. A high-traffic Internet website might handle requests with hundreds of servers that run on racks of high-speed computers. A resource sent from a web server can be a pre-existing file ( static content ) available to the web server, or it can be generated at the time of

7931-408: The maximum number of concurrent connections allowed and to improve their level of scalability. Between 1996 and 1999, Netscape Enterprise Server and Microsoft's IIS emerged among the leading commercial options whereas among the freely available and open-source programs Apache HTTP Server held the lead as the preferred server (because of its reliability and its many features). In those years there

8034-474: 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,

8137-516: The most popular websites has increased from 8.5% in 2012 to over 70% in May 2017. Adoption across the Internet now requires clients to actively request ECN. In June 2015, Apple announced that ECN will be enabled by default on its supported and future products, to help drive the adoption of ECN signaling industry-wide. ECN requires specific support at both the Internet layer and the transport layer for

8240-442: 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

8343-500: The number of TCP/IP connections and speedup accesses to hosted websites. In 2020–2021 the HTTP/2 dynamics about its implementation (by top web servers and popular web browsers) were partly replicated after the publication of advanced drafts of future RFC about HTTP/3 protocol. The following technical overview should be considered only as an attempt to give a few very limited examples about some features that may be implemented in

8446-407: 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

8549-428: The path found in requested URL (HTTP request message) and appends it to the path of the (Host) website root directory. On an Apache server , this is commonly /home/www/website (on Unix machines, usually it is: /var/www/website ). See the following examples of how it may result. URL path translation for a static file request Transmission Control Protocol The Transmission Control Protocol ( TCP )

8652-431: The process of modifying and standardizing a URL in a consistent manner. There are several types of normalization that may be performed, including the conversion of the scheme and host to lowercase. Among the most important normalizations are the removal of "." and ".." path segments and adding trailing slashes to a non-empty path component. "URL mapping is the process by which a URL is analyzed to figure out what resource it

8755-418: 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 –

8858-446: The receiver, which it has no mechanism to detect or cope with. As of July 2014, algorithms that provide equivalent or better receiver feedback in a more reliable approach are an active research topic. Web server A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content ) or its secure variant HTTPS . A user agent, commonly

8961-407: 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,

9064-504: 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

9167-424: The request ( dynamic content ) by another program that communicates with the server software. The former usually can be served faster and can be more easily cached for repeated requests, while the latter supports a broader range of applications. Technologies such as REST and SOAP , which use HTTP as a basis for general computer-to-computer communication, as well as support for WebDAV extensions, have extended

9270-572: The respective behavior. In June 2015, Apple Inc. announced that OS X 10.11 would have ECN turned on by default, but the OS shipped without that default behavior. In macOS Sierra, ECN is enabled for half of TCP sessions. In June 2015, Apple Inc. announced that iOS 9 , its next version of iOS, would support ECN and have it turned on by default. TCP ECN negotiation is enabled on 5% of randomly selected connections over Wi-Fi / Ethernet in iOS 9 and 50% of randomly selected connections over Wi-Fi / Ethernet and

9373-429: The retrieval of heavy web pages with lots of images, and to mitigate the problem of the shortage of persistent connections dedicated to dynamic objects used for bi-directional notifications of events in web pages. Within a year, these changes, on average, nearly tripled the maximum number of persistent connections that web servers had to manage. This trend (of increasing the number of persistent connections) definitely gave

9476-631: 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

9579-416: The same TCP port, binary representation of HTTP messages, message priority, compression of HTTP headers, use of streams also known as TCP/IP sub-connections and related flow-control, etc.) and so a few developers of those web servers opted for not supporting new HTTP/2 version (at least in the near future) also because of these main reasons: Instead, developers of most popular web servers, rushed to offer

9682-524: The sender's window is large. Use of ECN has been found to be detrimental to performance on highly congested networks when using AQM algorithms that never drop packets. Modern AQM implementations avoid this pitfall by dropping rather than marking packets at very high load. Many modern implementations of the TCP/IP protocol suite have some support for ECN; however, they usually ship with ECN disabled. Windows versions since Windows Server 2008 and Windows Vista support ECN for TCP. Since Windows Server 2012, it

9785-404: 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

9888-638: 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

9991-426: The simple static content serving (e.g. URL rewrite engine, dynamic content serving), usually have to figure out how that URL has to be handled, e.g. as a: One or more configuration files of web server may specify the mapping of parts of URL path (e.g. initial parts of file path , filename extension and other path components) to a specific URL handler (file, directory, external program or internal module). When

10094-404: The transmitting node in order to signal it to reduce its transmission rate. Because the CE indication can only be handled effectively by an upper layer protocol that supports it, ECN is only used in conjunction with upper layer protocols, such as TCP , that support congestion control and have a method for echoing the CE indication to the transmitting endpoint. TCP supports ECN using two flags in

10197-720: The widespread adoption of new web servers which had a much faster development cycle along with more features, more fixes applied, and more performances than the previous ones. At the end of 1996, there were already over fifty known (different) web server software programs that were available to everybody who wanted to own an Internet domain name and/or to host websites. Many of them lived only shortly and were replaced by other web servers. The publication of RFCs about protocol versions HTTP/1.0 (1996) and HTTP/1.1 (1997, 1999), forced most web servers to comply (not always completely) with those standards. The use of TCP/IP persistent connections (HTTP/1.1) required web servers both to increase

10300-418: The www group, meanwhile, thanks to the availability of its source code and the public specifications of the HTTP protocol, many other implementations of web servers started to be developed. In April 1993, CERN issued a public official statement stating that the three components of Web software (the basic line-mode client, the web server and the library of common code), along with their source code , were put in

10403-817: Was also another commercial, highly innovative and thus notable web server called Zeus ( now discontinued ) that was known as one of the fastest and most scalable web servers available on market, at least till the first decade of 2000s, despite its low percentage of usage. Apache resulted in the most used web server from mid-1996 to the end of 2015 when, after a few years of decline, it was surpassed initially by IIS and then by Nginx. Afterward IIS dropped to much lower percentages of usage than Apache (see also market share ). From 2005–2006, Apache started to improve its speed and its scalability level by introducing new performance features (e.g. event MPM and new content cache). As those new performance improvements initially were marked as experimental, they were not enabled by its users for

10506-461: Was approved. Between late 1990 and early 1991 the project resulted in Berners-Lee and his developers writing and testing several software libraries along with three programs, which initially ran on NeXTSTEP OS installed on NeXT workstations: Those early browsers retrieved web pages written in a simple early form of HTML , from web server(s) using a new basic communication protocol that

10609-472: Was named HTTP 0.9 . In August 1991 Tim Berners-Lee announced the birth of WWW technology and encouraged scientists to adopt and develop it. Soon after, those programs, along with their source code , were made available to people interested in their usage. Although the source code was not formally licensed or placed in the public domain, CERN informally allowed users and developers to experiment and further develop on top of them. Berners-Lee started promoting

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