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s 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 40 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 the connection but the other side can the terminating side should continue reading the data until the other side terminates as well 41 42 resource usage edit 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 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 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 data transfer edit the transmission control protocol differs in several key features compared to the user datagram protocol ordered data transfer the destination host rearranges segments according to a sequence number 16 retransmission of lost packets any cumulative stream not acknowledged is retransmitted 16 error free data transfer corrupted packets are treated as lost and are retransmitted 17 flow control limits the rate a sender transfers data to guarantee reliable delivery the receiver continually hints the sender on how much data can be received when the receiving host s buffer fills the next acknowledgment suspends the transfer and allows the data in the buffer to be processed 16 congestion control lost packets presumed due to congestion trigger a reduction in data delivery rate 16 reliable transmission edit 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 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 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 an 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 43 tcp incurs complexity due to retransmission ambiguity 44 duplicate ack based retransmission edit if 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 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 45 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 the right segments retransmission ambiguity can cause spurious fast retransmissions and congestion avoidance if there is reordering beyond the duplicate acknowledgment threshold 46 in the last two decades more packet reordering has been observed over the internet 47 which led tcp implementations such as the one in the linux kernel to adopt heuristic methods to scale the duplicate acknowledgment threshold 48 recently there have been efforts to completely phase out duplicate ack based fast retransmissions and replace them with timer based ones 49 not to be confused with the classic rto discussed below the time based loss detection algorithm called recent acknowledgment rack 50 has been adopted as the default algorithm in linux and windows 51 timeout based retransmission edit 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 the initial timer value is smoothed rtt max g 4 rtt variation where g is the clock granularity 52 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 a sender to assume an unacknowledged packet to be lost after sufficient time elapses i e determining the rto time 53 retransmission ambiguity can lead a sender s estimate of rtt to be imprecise 53 in an environment with variable rtts spurious timeouts can occur 54 if the rtt is under estimated then the rto fires and triggers a needless retransmit and slow start after a spurious retransmission when 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 55 56 selective acknowledgement can reduce this effect 57 rfc 6298 specifies that implementations must not use retransmitted segments when estimating rtt 58 karn s algorithm ensures that a good rtt estimate will be produced eventually by waiting until there is an unambiguous acknowledgment before adjusting the rto 59 after spurious retransmissions however it may take significant time before such an unambiguous acknowledgment arrives degrading performance in the interim 60 tcp timestamps also resolve the retransmission ambiguity problem in setting the rto 58 though they do not necessarily improve the rtt estimate 61 error detection edit 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 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 62 flow control edit 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 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 16 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 the receiving host tcp sequence numbers and receive windows behave very much like a clock the receive window shifts each time the receiver receives and acknowledges a new segment of data once it runs out of sequence numbers the sequence number loops back to 0 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 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 the relatively large overhead of the tcp header congestion control edit main article tcp congestion control 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 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 63 in addition senders employ 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 64 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 31 these individual rtt samples are then averaged over time to create 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 maximum segment size edit the maximum segment size mss 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 the maximum transmission unit mtu size of the data link layer of the networks to which the sender and receiver are directly attached tcp senders can use path mtu discovery to infer the minimum mtu along the network path between the sender and receiver and use this to dynamically adjust the mss to avoid ip fragmentation within the network mss announcement may also be called mss negotiation but strictly speaking the mss is not negotiated two completely independent values of mss are permitted for the two directions of data flow in a tcp connection 65 30 so there is no need to agree on a common mss configuration for a bidirectional connection selective acknowledgments edit see also sack panic relying purely on the cumulative acknowledgment scheme employed by the original tcp can lead to inefficiencies when packets are lost for example suppose bytes with sequence number 1 000 to 10 999 are sent in 10 different tcp segments of equal size and the second segment sequence numbers 2 000 to 2 999 is lost during transmission in a pure cumulative acknowledgment protocol the receiver can only send a cumulative ack value of 2 000 the sequence number immediately following the last sequence number of the received data and cannot say that it received bytes 3 000 to 10 999 successfully thus the sender may then have to resend all data starting with sequence number 2 000 to alleviate this issue tcp employs the selective acknowledgment sack option defined in 1996 in rfc 2018 which allows the receiver to acknowledge discontinuous blocks of packets that were received correctly in addition to the sequence number immediately following the last sequence number of the last contiguous byte received successively as in the basic tcp acknowledgment the acknowledgment can include a number of sack blocks where each sack block is conveyed by the left edge of block the first sequence number of the block and the right edge of block the sequence number immediately following the last sequence number of the block with a block being a contiguous range that the receiver correctly received in the example above the receiver would send an ack segment with a cumulative ack value of 2 000 and a sack option header with sequence numbers 3 000 and 11 000 the sender would accordingly retransmit only the second segment with sequence numbers 2 000 to 2 999 a tcp sender may interpret an out of order segment delivery as a lost segment if it does so the tcp sender will retransmit the segment previous to the out of order packet and slow its data delivery rate for that connection the duplicate sack option an extension to the sack option that was defined in may 2000 in rfc 2883 solves this problem once the tcp receiver detects a second duplicate...
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