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ut 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 packet it sends a d ack to indicate that no segments were lost allowing the tcp sender to reinstate the higher transmission rate the sack option is not mandatory and comes into operation only if both parties support it this is negotiated when a connection is established sack uses a tcp header option see tcp segment structure for details the use of sack has become widespread all popular tcp stacks support it selective acknowledgment is also used in stream control transmission protocol sctp selective acknowledgements can be reneged where the receiver unilaterally discards the selectively acknowledged data rfc 2018 discouraged such behavior but did not prohibit it to allow receivers the option of reneging if they for example ran out of buffer space 66 the possibility of reneging leads to implementation complexity for both senders and receivers and also imposes memory costs on the sender 67 window scaling edit main article tcp window scale option for more efficient use of high bandwidth networks a larger tcp window size may be used a 16 bit tcp window size field controls the flow of data and its value is limited to 65 535 bytes since the size field cannot be expanded beyond this limit a scaling factor is used the tcp window scale option as defined in rfc 1323 is an option used to increase the maximum window size to 1 gigabyte scaling up to these larger window sizes is necessary for tcp tuning the window scale option is used only during the tcp 3 way handshake the window scale value represents the number of bits to left shift the 16 bit window size field when interpreting it the window scale value can be set from 0 no shift to 14 for each direction independently both sides must send the option in their syn segments to enable window scaling in either direction some routers and packet firewalls rewrite the window scaling factor during a transmission this causes sending and receiving sides to assume different tcp window sizes the result is non stable traffic that may be very slow the problem is visible on some sites behind a defective router 68 tcp timestamps edit tcp timestamps defined in rfc 1323 in 1992 can help tcp determine in which order packets were sent tcp timestamps are not normally aligned to the system clock and start at some random value many operating systems will increment the timestamp for every elapsed millisecond however the rfc only states that the ticks should be proportional there are two timestamp fields a 4 byte sender timestamp value my timestamp a 4 byte echo reply timestamp value the most recent timestamp received from you tcp timestamps are used in an algorithm known as protection against wrapped sequence numbers or paws paws is used when the receive window crosses the sequence number wraparound boundary in the case where a packet was potentially retransmitted it answers the question is this sequence number in the first 4 gb or the second and the timestamp is used to break the tie also the eifel detection algorithm uses tcp timestamps to determine if retransmissions are occurring because packets are lost or simply out of order 69 tcp timestamps are enabled by default in linux 70 and disabled by default in windows server 2008 2012 and 2016 71 recent statistics show that the level of tcp timestamp adoption has stagnated at 40 owing to windows server dropping support since windows server 2008 72 out of band data edit it is possible to interrupt or abort the queued stream instead of waiting for the stream to finish this is done by specifying the data as urgent this marks the transmission as out of band data oob and tells the receiving program to process it immediately when finished tcp informs the application and resumes the stream queue an example is when tcp is used for a remote login session where the user can send a keyboard sequence that interrupts or aborts the remotely running program without waiting for the program to finish its current transfer 16 the urgent pointer only alters the processing on the remote host and doesn t expedite any processing on the network itself the capability is implemented differently or poorly on different systems or may not be supported where it is available it is prudent to assume only single bytes of oob data will be reliably handled 73 74 since the feature is not frequently used it is not well tested on some platforms and has been associated with vulnerabilities winnuke for instance forcing data delivery edit normally tcp waits for 200 ms for a full packet of data to send nagle s algorithm tries to group small messages into a single packet this wait creates small but potentially serious delays if repeated constantly during a file transfer for example a typical send block would be 4 kb a typical mss is 1460 so 2 packets ...
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