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s and calls on the internet module to transmit each segment to the destination tcp 19 a tcp segment consists of a segment header and a data section the segment header contains 10 mandatory fields and an optional extension field options octets 20 through 56 in table the data section follows the header and is the payload data carried for the application 20 the length of the data section is not specified in the segment header it can be calculated by subtracting the combined length of the segment header and ip header from the total ip datagram length specified in the ip header citation needed tcp header format 20 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 port destination port 4 32 sequence number 8 64 acknowledgement number meaningful when ack bit set 12 96 data offset reserved cwr ece urg ack psh rst syn fin window 16 128 checksum urgent pointer meaningful when urg bit set 21 20 160 options if present data offset will be greater than 5 padded with zeroes to a multiple of 32 bits since data offset counts words of 4 octets 56 448 60 480 data 64 512 source port 16 bits identifies the sending port destination port 16 bits identifies the receiving port sequence number 32 bits has a dual role if the syn flag is set 1 then this is the initial sequence number the sequence number of the actual first data byte and the acknowledged number in the corresponding ack are then this sequence number plus 1 if the syn flag is unset 0 then this is the accumulated sequence number of the first data byte of this segment for the current session acknowledgment number 32 bits if the ack flag is set then the value of this field is the next sequence number that the sender of the ack is expecting this acknowledges receipt of all prior bytes if any 22 the first ack sent by each end acknowledges the other end s initial sequence number itself but no data 23 data offset doffset 4 bits specifies the size of the tcp header in 32 bit words the minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes allowing for up to 40 bytes of options in the header this field gets its name from the fact that it is also the offset from the start of the tcp segment to the actual data citation needed reserved rsrvd 4 bits for future use and should be set to zero senders should not set these and receivers should ignore them if set in the absence of further specification and implementation from 2003 to 2017 the last bit bit 103 of the header was defined as the ns nonce sum flag by the experimental rfc 3540 ecn nonce ecn nonce never gained widespread use and the rfc was moved to historic status 24 a rfc draft 25 proposes a new use for this bit the bit is now used for negotiating the use of accurate ecn flags 8 bits contains 8 1 bit flags control bits as follows when using tcpdump a set flag is indicated with the character in parentheses cwr w 1 bit congestion window reduced cwr flag is set by the sending host to indicate that it received a tcp segment with the ece flag set and had responded in congestion control mechanism 26 a ece e 1 bit ecn echo has a dual role depending on the value of the syn flag it indicates if the syn flag is set 1 the tcp peer is ecn capable 27 if the syn flag is unset 0 a packet with the congestion experienced flag set ecn 11 in its ip header was received during normal transmission a this serves as an indication of network congestion or impending congestion to the tcp sender 28 urg u 1 bit indicates that the urgent pointer field is significant ack 1 bit indicates that the acknowledgment field is significant all packets after the initial syn packet sent by the client should have this flag set 29 psh p 1 bit push function asks to push the buffered data onto the wire i e send on the sender end and to the receiving application at the receiver end 20 rst r 1 bit reset the connection syn s 1 bit synchronize sequence numbers only the first packet sent from each end should have this flag set some other flags and fields change meaning based on this flag and some are only valid when it is set and others when it is clear fin f 1 bit last packet from sender window 16 bits the size of the receive window which specifies the number of window size units b that the sender of this segment is currently willing to receive c see flow control and window scaling checksum 16 bits the 16 bit checksum field is used for error checking of the tcp header the payload and an ip pseudo header the pseudo header consists of the source ip address the destination ip address the protocol number for the tcp protocol 6 and the length of the tcp headers and payload in bytes urgent pointer 16 bits if the urg flag is set then this 16 bit field is an offset from the sequence number indicating the last urgent data byte options tcp option variable length up to 40 bytes 320 bits options length bytes data offset 5 4 equivalent bit formula per rfc 9293 data offset 5 32 the length of this field is determined by the data offset field the tcp header padding is used to ensure that the tcp header ends and data begins on a 32 bit boundary the padding is composed of zeros 30 options have up to three fields option kind 1 byte option length 1 byte option data variable the option kind field indicates the type of option and is the only field that is not optional depending on option kind value the next two fields may be set option length indicates the total length of the option and option data contains data associated with the option if applicable for example an option kind byte of 1 indicates that this is a no operation option used only for padding and does not have an option length or option data fields following it an option kind byte of 0 marks the end of options and is also only one byte an option kind byte of 2 is used to indicate maximum segment size option and will be followed by an option length byte specifying the length of the mss field option length is the total length of the given options field including option kind and option length fields so while the mss value is typically expressed in two bytes option length will be 4 as an example an mss option field with a value of 0x05b4 is coded as 0x02 0x04 0x05b4 in the tcp options section some options may only be sent when syn is set they are indicated below as syn option kind and standard lengths given as option kind option length option kind option length option data purpose notes 0 n a n a end of options list 1 n a n a no operation this may be used to align option fields on 32 bit boundaries for better performance 2 4 ss maximum segment size see maximum segment size for details syn 3 3 s window scale see window scaling for details 31 syn 4 2 n a selective acknowledgement permitted see selective acknowledgments for details 32 syn 5 n 10 18 26 or 34 bbbb eeee selective acknowledgement sack 33 these first two bytes are followed by a list of 1 4 blocks being selectively acknowledged specified as 32 bit begin end pointers 8 10 tttt eeee timestamp and echo of previous timestamp see tcp timestamps for details 31 28 4 n a user timeout option see rfc 5482 29 n n a tcp authentication option tcp ao for message authentication replacing md5 authentication option 19 originally designed to protect bgp sessions 34 see rfc 5925 30 n n a multipath tcp mptcp see multipath tcp for details the remaining option kind values are historical obsolete experimental not yet standardized or unassigned option number assignments are maintained by the internet assigned numbers authority iana 35 data variable the payload of the tcp packet protocol operation edit a simplified tcp state diagram tcp protocol operations may be divided into three phases connection establishment is a multi step handshake process that establishes a connection before entering the data transfer phase after data transfer is completed the connection termination closes the connection and releases all allocated resources a tcp connection is managed by an operating system through a resource that represents the local endpoint for communications the internet socket during the lifetime of a tcp connection the local endpoint undergoes a series of state changes 36 tcp socket states state endpoint description listen server waiting for a connection request from any remote tcp endpoint syn sent client waiting for a matching connection request after having sent a connection request syn received server waiting for a confirming connection request acknowledgment after having both received and sent a connection request established server and client an open connection data received can be delivered to the user the normal state for the data transfer phase of the connection fin wait 1 server and client waiting for a connection termination request from the remote tcp or an acknowledgment of the connection termination request previously sent fin wait 2 server and client waiting for a connection termination request from the remote tcp close wait server and client waiting for a connection termination request from the local user closing server and client waiting for a connection termination request acknowledgment from the remote tcp last ack server and client waiting for an acknowledgment of the connection termination request previously sent to the remote tcp which includes an acknowledgment of its connection termination request time wait server or client waiting for enough time to pass to be sure that all remaining packets on the connection have expired closed server and client no connection state at all connection establishment edit connection establishment before a client attempts to connect with a server the server must first bind to and listen at a port to open it up for connections this is called a passive open once the passive open is established a client may establish a connection by initiating an active open using the three way or 3 step handshake syn the active open is performed by the client sending a syn to the server the client sets the segment s sequence number to a random value x syn ack in response the server replies with a syn ack the acknowledgment number is set to one more than the received sequence number i e x 1 and the sequence number that the server chooses for the packet is another random number y ack finally the client sends an ack back to the server the sequence number is set to the received acknowledgment value i e x 1 and the acknowledgment number is set to one more than the received sequence number i e y 1 steps 1 and 2 establish and acknowledge the sequence number for one direction client to server steps 2 and 3 establish and acknowledge the sequence number for the other direction server to client following the completion of these steps both the client and server have received acknowledgments and a full duplex communication is established connection termination edit connection termination detailed tcp close sequence diagram the connection termination phase uses a four way handshake with each side of the connection terminating independently when an endpoint wishes to stop its half of the connection it transmits a fin packet which the other end acknowledges with an ack therefore a typical tear down requires a pair of fin and ack segments from each tcp endpoint after the side that sent the first fin has responded with the final ack it waits for a timeout before finally closing the connection during which time the local port is unavailable for new connections this state lets the tcp client resend the final acknowledgment to the server in case the ack is lost in transit the time duration is implementation dependent but some common values are 30 seconds 1 minute and 2 minutes after the timeout the client enters the closed state and the local port becomes available for new connections 37 it is also possible to terminate the connection by a 3 way handshake when host a sends a fin and host b replies with a fin ack combining two steps into one and host a replies with an ack 38 some operating systems such as linux 39 implement a half duplex close sequence if the host actively closes a connection while still having unread incoming data available the host sends the signal rst losing any received data instead of fin this assures that a tcp application is aware there was a data loss 40 a connection can be in a half open state in which case one side has terminated the connection but the other has not the side that has terminated can no longer send any data into the connection but the other side can the terminating side should continue reading the data until the other side terminates as well 41 42 resource usage edit most implementations allocate an entry in a table that maps a session to a running operating system process because tcp packets do not include a session identifier both endpoints identify the session using the client s address and port whenever a packet is received the tcp implementation must perform a lookup on this table to find the destination process each entry in the table is known as a transmission control block or tcb it contains information about the endpoints ip and port status of the connection running data about the packets that are being exchanged and buffers for sending and receiving data the number of sessions in the server side is limited only by memory and can grow as new connections arrive but the client must allocate an ephemeral port before sending the first syn to the server this port remains allocated during the whole conversation and effectively limits the number of outgoing connections from each of the client s ip addresses if an application fails to properly close unrequired connections a client can run out of resources and become unable to establish new tcp connections even from other applications both endpoints must also allocate space for unacknowledged packets and received but unread data data transfer edit the transmission control protocol differs in several key features compared to the user datagram protocol ordered data transfer the destination host rearranges segments according to a sequence number 16 retransmission of lost packets any cumulative stream not acknowledged is retransmitted 16 error free data transfer corrupted packets are treated as lost and are retransmitted 17 flow control limits the rate a sender transfers data to guarantee reliable delivery the receiver continually hints the sender on how much data can be received when the receiving host s buffer fills the next acknowledgment suspends the transfer and allows the data in the buffer to be processed 16 congestion control lost packets presumed due to congestion trigger a reduction in data delivery rate 16 reliable transmission edit tcp uses a sequence number to identify each byte of data the sequence number identifies the order of the bytes sent from each computer so that the data can be reconstructed in order regardless of any out of order delivery that m...
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