CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

Active open A client socket initiates a connection by sending a SYN segment which
contains the server’s port number and the client’s Initial Sequence Number
(ISN). The client socket enters into the SYN_SENT state.
Passive open Indicates a server socket is opened and listening or waiting for incoming SYN.
Active close Initiated by the client socket when it finished receiving data. The client socket
sends a FIN segment and enters into the FIN_WAIT_1 or FIN_WAIT_2 state.
The connection is now a half-closed connection, where the client no longer
sends data but still able to receive data from the server.
The server socket enters into the passive close state upon receiving the FIN
segment from the client.
Passive close Occurs when the server socket receives the FIN segment from the client active
close. The server socket enters into the CLOSE_WAIT state. The server socket
is waiting for the server application to close. The server sends its FIN when the
server application is closed.
Simultaneous
open
Occurs when both client and server applications send a SYN to each other to
establish a TCP connection. However, the possibly is small, as the server
required to know the port of the client host that the SYN should destined to.
TCP simultaneous open is normally found in P2P applications.
Simul. close Occurs when both client and server applications perform active close.
- A connection goes through the following states during its lifetime:
State Description
LISTEN The server socket waits for a connection request from any remote client host.
SYN_SENT The client socket waits for an acknowledgment after a connection request is
sent.
SYN_RECEIVED The server socket receives a connection request and pending to reply an
acknowledgment to the client. The SYN Flood DoS attack would typically
cause a lot of TCP connections in this state.
ESTABLISHED Represents an established connection. Data communication is allowed. The
normal state for the data transfer phase of a connection.
FIN_WAIT_1 The client socket has initiated active close and wait for an acknowledgment
from the server after a connection termination request (FIN segment) is sent.
FIN_WAIT_2 The client socket has received an acknowledgment for its connection
termination request but is still waiting for the connection termination request
(FIN segment) from the server.
CLOSE_WAIT The remote client socket is closed. The server socket enters into passive
close state and waits for a connection termination request from the server
application.
CLOSING The client socket waiting for a connection termination acknowledgment from
the remote server socket.
LAST_ACK The server application and socket are closed. Waiting for the final ACK for
the connection termination request from the remote client socket.
TIME_WAIT The client socket waiting for enough time to pass to make sure the stray
packets destined to the closed socket are flushed out from the network.
Note: Application layer protocol (eg: HTTP and Telnet) servers tend to initiates active close than
clients, which reverse the client and server roles in TCP. TCP assumes that clients tend to initiate
active close (by first sending a segment with the FIN bit set).
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