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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 go out on a 10 mbit s ethernet taking 1 2 ms each followed by a third carrying the remaining 1176 after a 197 ms pause because tcp is waiting for a full buffer in the case of telnet each user keystroke is echoed back by the server before the user can see it on the screen this delay would become very annoying setting the socket option tcp_nodelay overrides the default 200 ms send delay application programs use this socket option to force output to be sent after writing a character or line of characters the rfc 793 defines the psh push bit as a message to the receiving tcp stack to send this data immediately up to the receiving application 16 there is no way to indicate or control it in user space using berkeley sockets it is controlled by the protocol stack only 75 vulnerabilities edit tcp may be attacked in a variety of ways the results of a thorough security assessment of tcp along with possible mitigations for the identified issues were published in 2009 76 and was pursued within the ietf through 2012 77 notable vulnerabilities include denial of service connection hijacking tcp veto and tcp reset attack denial of service edit by using a spoofed ip address and repeatedly sending purposely assembled syn packets followed by many ack packets attackers can cause the server to consume large amounts of resources keeping track of the bogus connections this is known as a syn flood attack proposed solutions to this problem include syn cookies and cryptographic puzzles though syn cookies come with their own set of vulnerabilities 78 sockstress is a similar attack that might be mitigated with system resource management 79 an advanced dos attack involving the exploitation of the tcp persist timer was analyzed in phrack no 66 80 push and ack floods are other variants 81 connection hijacking edit main article tcp sequence prediction attack an attacker who is able to eavesdrop on a tcp session and redirect packets can hijack a tcp connection to do so the attacker learns the sequence number from the ongoing communication and forges a false segment that looks like the next segment in the stream a simple hijack can result in one packet being erroneously accepted at one end when the receiving host acknowledges the false segment synchronization is lost 82 hijacking may be combined with arp spoofing or other routing attacks that allow an attacker to take permanent control of the tcp connection impersonating a different ip address was not difficult prior to rfc 1948 when the initial sequence number was easily guessable the earlier implementations allowed an attacker to blindly send a sequence of packets that the receiver would believe came from a different ip address without the need to intercept communication through arp or routing attacks it is enough to ensure that the legitimate host of the impersonated ip address is down or bring it to that condition using denial of service attacks this is why the initial sequence number is now chosen at random tcp veto edit an attacker who can eavesdrop and predict the size of the next packet to be sent can cause the receiver to accept a malicious payload without disrupting the existing connection the attacker injects a malicious packet with the sequence number and a payload size of the next expected packet when the legitimate packet is ultimately received it is found to have the same sequence number and length as a packet already received and is silently dropped as a normal duplicate packet the legitimate packet is vetoed by the malicious packet unlike in connection hijacking the connection is never desynchronized and communication continues as normal after the malicious payload is accepted tcp veto gives the attacker less control over the communication but makes the attack particularly resistant to detection the only evidence to the receiver that something is amiss is a single duplicate packet a normal occurrence in an ip network the sender of the vetoed packet never sees any evidence of an attack 83 tcp ports edit see also list of tcp and udp port numbers a tcp connection is ident...