Before we can discuss very much about the operation of the TCP we need to introduce some detailed terminology. The maintenance of a TCP connection requires the remembering of several variables. We conceive of these variables being stored in a connection record called a Transmission Control Block or TCB. Among the variables stored in the TCB are the local and remote socket numbers, the security and precedence of the connection, pointers to the user's send and receive buffers, pointers to the retransmit queue and to the current segment. In addition several variables relating to the send and receive sequence numbers are stored in the TCB.
Send Sequence Variables SND.UNA - send unacknowledged SND.NXT - send next SND.WND - send window SND.UP - send urgent pointer SND.WL1 - segment sequence number used for last window update SND.WL2 - segment acknowledgment number used for last window update ISS - initial send sequence number Receive Sequence Variables RCV.NXT - receive next RCV.WND - receive window RCV.UP - receive urgent pointer IRS - initial receive sequence number
The following diagrams may help to relate some of these variables to the sequence space.
Send Sequence Space
1 2 3 4 ----------|----------|----------|---------- SND.UNA SND.NXT SND.UNA +SND.WND 1 - old sequence numbers which have been acknowledged 2 - sequence numbers of unacknowledged data 3 - sequence numbers allowed for new data transmission 4 - future sequence numbers which are not yet allowed Send Sequence Space Figure 4.
The send window is the portion of the sequence space labeled 3 in figure 4.
Receive Sequence Space
1 2 3 ----------|----------|---------- RCV.NXT RCV.NXT +RCV.WND 1 - old sequence numbers which have been acknowledged 2 - sequence numbers allowed for new reception 3 - future sequence numbers which are not yet allowed Receive Sequence Space Figure 5.
The receive window is the portion of the sequence space labeled 2 in figure 5.
There are also some variables used frequently in the discussion that take their values from the fields of the current segment.
Current Segment Variables SEG.SEQ - segment sequence number SEG.ACK - segment acknowledgment number SEG.LEN - segment length SEG.WND - segment window SEG.UP - segment urgent pointer SEG.PRC - segment precedence value
A connection progresses through a series of states during its lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional because it represents the state when there is no TCB, and therefore, no connection. Briefly the meanings of the states are:
LISTEN - represents waiting for a connection request from any remote TCP and port. SYN-SENT - represents waiting for a matching connection request after having sent a connection request. SYN-RECEIVED - represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request. ESTABLISHED - represents 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 - represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent. FIN-WAIT-2 - represents waiting for a connection termination request from the remote TCP. CLOSE-WAIT - represents waiting for a connection termination request from the local user. CLOSING - represents waiting for a connection termination request acknowledgment from the remote TCP. LAST-ACK - represents 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 - represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, ABORT, and STATUS; the incoming segments, particularly those containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in figure 6 illustrates only state changes, together with the causing events and resulting actions, but addresses neither error conditions nor actions which are not connected with state changes. In a later section, more detail is offered with respect to the reaction of the TCP to events.
NOTE BENE: this diagram is only a summary and must not be taken as the total specification.
+---------+ ---------\ active OPEN | CLOSED | \ ----------- +---------+<---------\ \ create TCB | ^ \ \ snd SYN passive OPEN | | CLOSE \ \ ------------ | | ---------- \ \ create TCB | | delete TCB \ \ V | \ \ +---------+ CLOSE | \ | LISTEN | ---------- | | +---------+ delete TCB | | rcv SYN | | SEND | | ----------- | | ------- | V +---------+ snd SYN,ACK / \ snd SYN +---------+ | |<----------------- ------------------>| | | SYN | rcv SYN | SYN | | RCVD |<-----------------------------------------------| SENT | | | snd ACK | | | |------------------ -------------------| | +---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+ | -------------- | | ----------- | x | | snd ACK | V V | CLOSE +---------+ | ------- | ESTAB | | snd FIN +---------+ | CLOSE | | rcv FIN V ------- | | ------- +---------+ snd FIN / \ snd ACK +---------+ | FIN |<----------------- ------------------>| CLOSE | | WAIT-1 |------------------ | WAIT | +---------+ rcv FIN \ +---------+ | rcv ACK of FIN ------- | CLOSE | | -------------- snd ACK | ------- | V x V snd FIN V +---------+ +---------+ +---------+ |FINWAIT-2| | CLOSING | | LAST-ACK| +---------+ +---------+ +---------+ | rcv ACK of FIN | rcv ACK of FIN | | rcv FIN -------------- | Timeout=2MSL -------------- | | ------- x V ------------ x V \ snd ACK +---------+delete TCB +---------+ ------------------------>|TIME WAIT|------------------>| CLOSED | +---------+ +---------+ TCP Connection State Diagram Figure 6.