The Linux Programming Interface

Sockets: Advanced Topics 1271

(labeled M in the diagram). This information is necessary because sequence

numbers don’t begin at 0, as noted in Section 58.6.3.

2. The server TCP must both acknowledge the client TCP’s SYN segment and
   inform the client TCP of its own initial sequence number (labeled N in the
   diagram). (Two sequence numbers are required because a stream socket is
   bidirectional.) The server TCP can perform both operations by returning a single
   segment with both the SYN and the ACK control bits set. (We say that the ACK
   is piggybacked on the SYN.)


3. The client TCP sends an ACK segment to acknowledge the server TCP’s SYN
   segment.

Figure 61-4: TCP state transition diagram

close^

send: FIN (4)

close

send: FIN (6)

active open

; send: SYN (1)

recv: FIN

send: ACK

send: <nil>recv: ACK

recv: SYN, ACK (2)

send: ACK (3)

recv: ACK (3)

send: <nil>

send: <nil>recv: ACK (5)

recv: FIN (6)

send: ACK (7)

recv: FIN, ACK

send: ACK

close

(or timeout)

2MSL

timeout

recv: ACK (7)

send: <nil>

passive close

active close

data-transfer

state

passive open

active open

usual path for client boldface: action by local application

usual path for server

Key recv: segment from peer that caused transition

SYN_RECV

CLOSE_WAIT

LAST_ACK

CLOSED

SYN_SENT

FIN_WAIT1

FIN_WAIT2 TIME_WAIT

send: segment sent to peer during transition

CLOSING

recv: FIN (4)

send: ACK (5)

ESTABLISHED

openpassive

close

recv: SYN (1)

send: SYN, ACK (2)

LISTEN

CLOSED