OPEN Call ... An initial send sequence number (ISS) is selected. Send a SYN segment of the form: <SEQ=ISS><CTL=SYN><TSval=my.TSclock><WSopt=Rcv.Wind.Scale> ... SEND Call CLOSED STATE (i.e., TCB does not exist) ... LISTEN STATE If the foreign socket is specified, then change the connection from passive to active, select an ISS. Send a SYN segment containing the options: <TSval=my.TSclock> and <WSopt=Rcv.Wind.Scale>. Set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. ... SYN-SENT STATE SYN-RECEIVED STATE ... ESTABLISHED STATE CLOSE-WAIT STATE Segmentize the buffer and send it with a piggybacked acknowledgment (acknowledgment value = RCV.NXT). ... If the urgent flag is set ... If the Snd.TS.OK flag is set, then include the TCP Timestamps option <TSval=my.TSclock,TSecr=TS.Recent> in each data segment. Scale the receive window for transmission in the segment header: SEG.WND = (SND.WND >> Rcv.Wind.Scale). SEGMENT ARRIVES ... If the state is LISTEN then first check for an RST ... second check for an ACK ... third check for a SYN if the SYN bit is set, check the security. If the ... ... If the SEG.PRC is less than the TCB.PRC then continue. Check for a Window Scale option (WSopt); if one is found, save SEG.WSopt in Snd.Wind.Scale and set Snd.WS.OK flag on. Otherwise, set both Snd.Wind.Scale and Rcv.Wind.Scale to zero and clear Snd.WS.OK flag. Check for a TSopt option; if one is found, save SEG.TSval in the variable TS.Recent and turn on the Snd.TS.OK bit. Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other control or text should be queued for processing later. ISS should be selected and a SYN segment sent of the form: <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> If the Snd.WS.OK bit is on, include a WSopt option <WSopt=Rcv.Wind.Scale> in this segment. If the Snd.TS.OK bit is on, include a TSopt <TSval=my.TSclock,TSecr=TS.Recent> in this segment. Last.ACK.sent is set to RCV.NXT. SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection state should be changed to SYN-RECEIVED. Note that any other incoming control or data (combined with SYN) will be processed in the SYN-RECEIVED state, but processing of SYN and ACK should not be repeated. If the listen was not fully specified (i.e., the foreign socket was not fully specified), then the unspecified fields should be filled in now. fourth other text or control ... If the state is SYN-SENT then first check the ACK bit ... fourth check the SYN bit ... If the SYN bit is on and the security/compartment and precedence are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ, and any acknowledgements on the retransmission queue which are thereby acknowledged should be removed. Check for a Window Scale option (WSopt); if is found, save SEG.WSopt in Snd.Wind.Scale; otherwise, set both Snd.Wind.Scale and Rcv.Wind.Scale to zero. Check for a TSopt option; if one is found, save SEG.TSval in variable TS.Recent and turn on the Snd.TS.OK bit in the connection control block. If the ACK bit is set, use my.TSclock - SEG.TSecr as the initial RTT estimate. If SND.UNA > ISS (our SYN has been ACKed), change the connection state to ESTABLISHED, form an ACK segment: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> and send it. If the Snd.Echo.OK bit is on, include a TSopt option <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment. Last.ACK.sent is set to RCV.NXT. Data or controls which were queued for transmission may be included. If there are other controls or text in the segment then continue processing at the sixth step below where the URG bit is checked, otherwise return. Otherwise enter SYN-RECEIVED, form a SYN,ACK segment: <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> and send it. If the Snd.Echo.OK bit is on, include a TSopt option <TSval=my.TSclock,TSecr=TS.Recent> in this segment. If the Snd.WS.OK bit is on, include a WSopt option <WSopt=Rcv.Wind.Scale> in this segment. Last.ACK.sent is set to RCV.NXT. If there are other controls or text in the segment, queue them for processing after the ESTABLISHED state has been reached, return. fifth, if neither of the SYN or RST bits is set then drop the segment and return. Otherwise, First, check sequence number SYN-RECEIVED STATE ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE CLOSE-WAIT STATE CLOSING STATE LAST-ACK STATE TIME-WAIT STATE Segments are processed in sequence. Initial tests on arrival are used to discard old duplicates, but further processing is done in SEG.SEQ order. If a segment's contents straddle the boundary between old and new, only the new parts should be processed. Rescale the received window field: TrueWindow = SEG.WND << Snd.Wind.Scale, and use "TrueWindow" in place of SEG.WND in the following steps. Check whether the segment contains a Timestamps option and bit Snd.TS.OK is on. If so: If SEG.TSval < TS.Recent, then test whether connection has been idle less than 24 days; if both are true, then the segment is not acceptable; follow steps below for an unacceptable segment. If SEG.SEQ is equal to Last.ACK.sent, then save SEG.ECopt in variable TS.Recent. There are four cases for the acceptability test for an incoming segment: ... If an incoming segment is not acceptable, an acknowledgment should be sent in reply (unless the RST bit is set, if so drop the segment and return): <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> Last.ACK.sent is set to SEG.ACK of the acknowledgment. If the Snd.Echo.OK bit is on, include the Timestamps option <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment. Set Last.ACK.sent to SEG.ACK and send the ACK segment. After sending the acknowledgment, drop the unacceptable segment and return. ... fifth check the ACK field. if the ACK bit is off drop the segment and return. if the ACK bit is on ... ESTABLISHED STATE If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK. Also compute a new estimate of round-trip time. If Snd.TS.OK bit is on, use my.TSclock - SEG.TSecr; otherwise use the elapsed time since the first segment in the retransmission queue was sent. Any segments on the retransmission queue which are thereby entirely acknowledged... ... Seventh, process the segment text. ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE ... Send an acknowledgment of the form: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> If the Snd.TS.OK bit is on, include Timestamps option <TSval=my.TSclock,TSecr=TS.Recent> in this ACK segment. Set Last.ACK.sent to SEG.ACK of the acknowledgment, and send it. This acknowledgment should be piggy-backed on a segment being transmitted if possible without incurring undue delay. ...