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ceiver 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 identified by a four tuple of the source address source port destination address and destination port d 84 85 port numbers are used to identify different services and to allow multiple connections between hosts 17 tcp uses 16 bit port numbers providing 65 536 possible values for each of the source and destination ports 20 the dependency of connection identity on addresses means that tcp connections are bound to a single network path tcp cannot use other routes that multihomed hosts have available and connections break if an endpoint s address changes 86 port numbers are categorized into three basic categories well known registered and dynamic or private the well known ports are assigned by the internet assigned numbers authority iana and are typically used by system level processes well known applications running as servers and passively listening for connections typically use these ports some examples include ftp 20 and 21 ssh 22 telnet 23 smtp 25 http over ssl tls 443 and http 80 e registered ports 1024 49151 may be assigned to specific services by third party developers but some operating systems also allocate ephemeral client ports from this range dynamic or private ports 49152 65535 are not associated with any registered services and are commonly used exclusively as ephemeral ports for temporary client connections network address translation nat typically uses dynamic port numbers on the public facing side to disambiguate the flow of traffic that is passing between a public network and a private subnetwork thereby allowing many ip addresses and their ports on the subnet to be serviced by a single public facing address development edit tcp is a complex protocol however while significant enhancements have been made and proposed over the years its most basic operation has not changed significantly since its first specification rfc 675 in 1974 and the v4 specification rfc 793 published in september 1981 rfc 1122 published in october 1989 clarified a number of tcp protocol implementation requirements a list of the 8 required specifications and over 20 strongly encouraged enhancements is available in rfc 7414 among this list is rfc 2581 tcp congestion control one of the most important tcp related rfcs in recent years describes updated algorithms that avoid undue congestion in 2001 rfc 3168 was written to describe explicit congestion notification ecn a congestion avoidance signaling mechanism the original tcp congestion avoidance algorithm was known as tcp tahoe but many alternative algorithms have since been proposed including tcp reno tcp vegas fast tcp tcp new reno and tcp hybla multipath tcp mptcp 87 88 is an ongoing effort within the ietf that aims at allowing a tcp connection to use multiple paths to maximize resource usage and increase redundancy the redundancy offered by multipath tcp in the context of wireless networks enables the simultaneous use of different networks which brings higher throughput and better handover capabilities multipath tcp also brings performance benefits in datacenter environments 89 the reference implementation 90 of multipath tcp was developed in the linux kernel 91 multipath tcp is used to support the siri voice recognition application on iphones ipads and macs 92 tcpcrypt is an extension proposed in july 2010 to provide transport level encryption directly in tcp itself it is designed to work transparently and not require any configuration unlike tls ssl tcpcrypt itself does not provide authentication but provides simple primitives down to the application to do that the tcpcrypt rfc was published by the ietf in may 2019 93 tcp fast open is an extension to speed up the openin...
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