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work 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 data in a timely fashion than it is to get all of the data in order for historical and performance reasons most storage area networks sans use fibre channel protocol fcp over fibre channel connections for embedded systems network booting and servers that serve simple requests from huge numbers of clients e g dns servers the complexity of tcp can be a problem tricks such as transmitting data between two hosts that are both behind nat using stun or similar systems are far simpler without a relatively complex protocol like tcp in the way generally where tcp is unsuitable the user datagram protocol udp is used this provides the same application multiplexing and checksums that tcp does but does not handle streams or retransmission giving the application developer the ability to code them in a way suitable for the situation or to replace them with other methods such as forward error correction or error concealment stream control transmission protocol sctp is another protocol that provides reliable stream oriented services similar to tcp it is newer and considerably more complex than tcp and has not yet seen widespread deployment however it is especially designed to be used in situations where reliability and near real time considerations are important venturi transport protocol vtp is a patented proprietary protocol that is designed to replace tcp transparently to overcome perceived inefficiencies related to wireless data transport especially in mobile communications and wireless networks data transmission may become unstable due to high latency and signal interference the tcp congestion avoidance algorithm works very well for ad hoc environments where the data sender is not known in advance if the environment is predictable a timing based protocol such as asynchronous transfer mode atm can avoid tcp s retransmission overhead udp based data transfer protocol udt is designed for high bandwidth high latency network environments particularly for large scale data transfers such as file or media streaming in networks with a high bandwidth delay product 138 udt offers advantages over tcp providing better efficiency and fairness multipurpose transaction protocol mtp ip is patented proprietary software that is designed to adaptively achieve high throughput and transaction performance in a wide variety of network conditions particularly those where tcp is perceived to be inefficient checksum computation edit further information internet checksum tcp checksum for ipv4 edit when tcp runs over ipv4 the method used to compute the checksum is defined as follows 30 the checksum field is the 16 bit ones complement of the ones complement sum of all 16 bit words in the header and text the checksum computation needs to ensure the 16 bit alignment of the data being summed if a segment contains an odd number of header and text octets alignment can be achieved by padding the last octet with zeros on its right to form a 16 bit word for checksum purposes the pad is not transmitted as part of the segment while computing the checksum the checksum field itself is replaced with zeros in other words after appropriate padding all 16 bit words are added using ones complement arithmetic the sum is then bitwise complemented and inserted as the checksum field a pseudo header that mimics the ipv4 packet header used in the checksum computation is as follows tcp pseudo header for checksum computation ipv4 offset octet 0 1 2 3 octet bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0 source address 4 32 destination address 8 64 zeroes protocol 6 tcp length 12 96 source port destination port 16 128 sequence number 20 160 acknowledgement number 24 192 data offset reserved flags window 28 224 checksum urgent pointer 32 256 options 36 288 data 40 320 the checksum is computed over the following fields source address 32 bits the source address in the ipv4 header destination address 32 bits the destination address in the ipv4 header zeroes 8 bits all zeroes protocol 8 bits the protocol value for tcp 6 tcp length 16 bits the length of the tcp header and data measured in octets for example let s say we have ipv4 packet with total length of 200 bytes and ihl value of 5 which indicates a length of 5 bits 32 bits 160 bits 20 bytes we can compute the tcp length as total length ipv4 header length i e 200 20 which results in 180 bytes tcp checksum for ipv6 edit when tcp runs over ipv6 the method used to compute the checksum is changed 139 any transport or other upper layer protocol that includes the addresses from the ip header in its checksum computation must be modified for use over ipv6 to include the 128 bit ipv6 addresses instead of 32 bit ipv4 addresses a pseudo header that mimics the ipv6 header for computation of the checksum is shown below tcp pseudo header for checksum computation ipv6 offset octet 0 1 2 3 octet bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0 source address 4 32 8 64 12 96 16 128 destination address 20 160 24 192 28 224 32 256 tcp length 36 288 zeroes next header 6 40 320 source port destination port 44 352 sequence number 48 384 acknowledgement number 52 416 data offset reserved flags window 56 448 checksum urgent pointer 60 480 options 64 512 data 68 544 the checksum is computed over the following fields source address 128 bits the address in the ipv6 header destination address 128 bits the final destination if the ipv6 packet doesn t contain a routing header tcp uses the destination address in the ipv6 header otherwise at the originating node it uses the address in the last element of the routing header and at the receiving node it uses the destination address in the ipv6 header tcp length 32 bits the length of the tcp header and data measured in octets zeroes 24 bits zeroes 0 all zeroes next header 8 bits the protocol value for tcp 6 checksum offload edit many tcp ip software stack implementations provide options to use hardware assistance to automatically compute the checksum in the network adapter prior to transmission onto the network or upon reception from the network for validation this may reduce cpu load associated with calculating the checksum potentially increasing overall network performance this feature may cause packet analyzers that are unaware or uncertain about the use of checksum offload to report invalid checksums in outbound packets that have not yet reached the network adapter 140 this will only occur for packets that are intercepted before being transmitted by the network adapter all packets transmitted by the network adaptor on the wire will have valid checksums 141 this issue can also occur when monitoring packets being transmitted between virtual machines on the same host where a virtual device driver may omit the checksum calculation as an optimization knowing that the checksum will be calculated later by the vm host kernel or its physical hardware see also edit micro bursting networking tcp global synchronization tcp fusion tcp pacing tcp stealth transport layer comparison of transport layer protocols wtcp a proxy based modification of tcp for wireless networks notes edit 1 2 added to header by rfc 3168 windows size units are by default bytes window size is relative to the segment identified by the sequence number in the acknowledgment field equivalently a pair of network sockets for the source and destination each of which is made up of an address and a port as of the latest standard http 3 quic is used as a transport instead of tcp references edit comer d e 2021 internetworking with tcp ip 6th ed pearson labrador miguel a perez alfredo j wightman pedro m 2010 location based information systems developing real time trac...
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