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he 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 opening of successive tcp connections between two endpoints it works by skipping the three way handshake using a cryptographic cookie it is similar to an earlier proposal called t tcp which was not widely adopted due to security issues 94 tcp fast open was published as rfc 7413 in 2014 95 proposed in may 2013 proportional rate reduction prr is a tcp extension developed by google engineers prr ensures that the tcp window size after recovery is as close to the slow start threshold as possible 96 the algorithm is designed to improve the speed of recovery and is the default congestion control algorithm in linux 3 2 kernels 97 deprecated proposals edit tcp cookie transactions tcpct is an extension proposed in december 2009 98 to secure servers against denial of service attacks unlike syn cookies tcpct does not conflict with other tcp extensions such as window scaling tcpct was designed due to the necessities of dnssec where servers have to handle large numbers of short lived tcp connections in 2016 tcpct was deprecated in favor of tcp fast open the status of the original rfc was changed to historic 99 hardware implementations edit one way to overcome the processing power requirements of tcp is to build hardware implementations of it widely known as tcp offload engines toes the main problem of toes is that they are hard to integrate into computing systems requiring extensive changes in the operating system of the computer or device tcp low power tcplp has been demonstrated to work in resource constrained environments where otherwise udp based coap is preferred 100 101 wire image and ossification edit the wire data of tcp provides significant information gathering and modification opportunities to on path observers as the protocol metadata is transmitted in cleartext 102 103 while this transparency is useful to network operators 104 and researchers 105 information gathered from protocol metadata may reduce the end user s privacy 106 this visibility and malleability of metadata has led to tcp being difficult to extend a case of protocol ossification as any intermediate node a middlebox can make decisions based on that metadata or even modify it 107 108 breaking the end to end principle 109 one measurement found that a third of paths across the internet encounter at least one intermediary that modifies tcp metadata and 6 5 of paths encounter harmful ossifying effects from intermediaries 110 avoiding extensibility hazards from intermediaries placed significant constraints on the design of mptcp 111 112 and difficulties caused by intermediaries have hindered the deployment of tcp fast open in web browsers 113 another source of ossification is the difficulty of modification of tcp functions at the endpoints typically in the operating system kernel 114 or in hardware with a tcp offload engine 115 performance edit as tcp provides applications with the abstraction of a reliable byte stream it can suffer from head of line blocking if packets are reordered or lost and need to be retransmitted and thus are reordered data from sequentially later parts of the stream may be received before sequentially earlier parts of the stream however the later data cannot typically be used until the earlier data has been received incurring network latency if multiple independent higher level messages are encapsulated and multiplexed onto a single tcp connection then head of line blocking can cause processing of a fully received message that was sent later to wait for delivery of a message that was sent earlier 116 web browsers attempt to mitigate head of line blocking by opening multiple parallel connections this incurs the cost of connection establishment repeatedly as well as multiplying the resources needed to track those connections at the endpoints 117 parallel connections also have congestion control operating independently of each other rather than being able to pool information together and respond more promptly to observed network conditions 118 tcp s aggressive initial sending patterns can cause congestion if multiple parallel connections are opened and the per connection fairness model leads to a monopolization of resources by applications that take this approach 119 connection establishment is a major contributor to latency as experienced by web users 120 121 tcp s three way handshake introduces one rtt of latency during connection establishment before data can be sent 121 for short flows these delays are very significant 122 transport layer security tls requires a handshake of its own for key exchange at connection establishment because of the layered design the tcp handshake and the tls handshake proceed serially the tls handshake cannot begin until the tcp handshake has concluded 123 two rtts are required for connection establishment with tls 1 2 over tcp 124 tls 1 3 allows for zero rtt connection resumption in some circumstances but when layered over tcp one rtt is still required for the tcp handshake and this cannot assist the initial connection zero rtt handshakes also present cryptographic challenges as efficient replay safe and forward secure non interactive key exchange is an open research topic 125 tcp fast open allows the transmission of data in the initial i e syn and syn ack packets removing one rtt of latency during connection establishment 126 however tcp fast open has been difficult to deploy due to protocol ossification as of 2020 update no web browsers used it by default 113 tcp throughput is affected by packet reordering reordered packets can cause duplicate acknowledgments to be sent which if they cross a threshold will then trigger a spurious retransmission and congestion control transmission behavior can also become bursty as large ranges are acknowledged all at once when a reordered packet at the range s start is received in a manner similar to how head of line blocking affects applications 127 blanton allman 2002 found that throughput was inversely related to the amount of reordering up to a threshold where all reordering triggers spurious retransmission 128 mitigating reordering depends on a sender s ability to determine that it has sent a spurious retransmission and hence on resolving retransmission ambiguity 129 reducing reordering induced spurious retransmissions may slow recovery from genuine loss 130 selective acknowledgment can provide a significant benefit to throughput bruyeron hemon zhang 1998 measured gains of up to 45 131 an important factor in the improvement is that selective acknowledgment can more often avoid going into slow start after a loss and can hence better use available bandwidth 132 however tcp can only selectively acknowledge a maximum of three blocks of sequence numbers this can limit the retransmission rate and hence loss recovery or cause needless retransmissions especially in high loss environments 133 134 tcp was originally designed for wired networks where packet loss is considered to be the result of network congestion and the congestion window size is reduced dramatically as a precaution however wireless links are known to experience sporadic and usually temporary losses due to fading shadowing handover interference and other radio effects that are not strictly congestion after the erroneous back off of the congestion window size due to wireless packet loss there may be a congestion avoidance phase with a conservative decrease in window size this causes the radio link to be underused extensive research on combating these harmful effects has been conducted suggested solutions can be categorized as end to end solutions which require modifications at the client or server 135 link layer solutions such as radio link protocol in cellular networks or proxy based solutions which require some changes in the network without modifying end nodes 135 136 a number of alternative congestion control algorithms such as vegas westwood veno and santa cruz have been proposed to help solve the wireless problem citation needed acceleration edit not to be confused with tcp offload engine the idea of a tcp accelerator is to terminate tcp connections inside the network processor and then relay the data to a second connection toward the end system the data packets that originate from the sender are buffered at the accelerator node which is responsible for performing local retransmissions in the event of packet loss thus in case of losses the feedback loop between the sender and the receiver is shortened to the one between the acceleration node and the receiver which guarantees a faster delivery of data to the receiver 137 since tcp is a rate adaptive protocol the rate at which the tcp sender injects packets into the network is directly proportional to the prevailing load condition within the network as well as the processing capacity of the receiver the prevailing conditions within the network are judged by the sender on the basis of the acknowledgments received by it the acceleration node splits the feedback loop between the sender and the receiver and thus guarantees a shorter round trip time rtt per packet a shorter rtt is beneficial as it ensures a quicker response time to any changes in the network and a faster adaptation by the sender to combat these changes disadvantages of the method include the fact that the tcp session has to be directed through the accelerator this means that if routing changes so that the accelerator is no longer in the path the connection will be broken it also destroys the end to end property of the tcp ack mechanism when the ack is received by the sender the packet has been stored by the accelerator not delivered to the receiver debugging edit a packet sniffer which taps tcp traffic on a network link can be useful in debugging networks network stacks and applications that use tcp by showing an engineer what packets are passing through a link some networking stacks support the so_debug socket option which can be enabled on the socket using setsockopt that option dumps all the packets tcp states and events on that socket which is helpful in debugging netstat is another utility that can be used for debugging alternatives edit for many applications tcp is not appropriate the application cannot normally access the packets coming after a lost packet until the retransmitted copy of the lost packet is received this causes problems for real time applications such as streaming media real time multiplayer games and voice over ip voip where it is generally more useful to get most of the dat...
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