Layer 4 Models

How devices process UDP segments

This procedure explains how a device sends and receives UDP segments.

• When the device receives a segment:
  • It de-encapsulates it and examines the UDP header for port information.
  • It then maps the local port information and sends the payload up to a higher layer (the application layer) for processing.
    • If it cannot find the upper process based on the port information, it drops the segment.
• When the device wants to send a segment:
  • It encapsulates the payload with a UDP header.
  • It sends the segment to the lower layer for processing.

How TCP Connection handles outgoing data

When TCP Connection is ready to send data in the out buffer: (flowchart here):

• It checks if the TCP connection State is Established:
• If no, it drops the data.
• If yes,
• It adds the new data onto the out buffer.
• It checks if the Nagle service is turned on:
• If yes, it checks if the size of the out buffer is greater than the MSS:
• If yes, it starts the initial data sending step.
• If no, it checks if there is any unacknowledged data:
• If no, it starts the initial data sending step.
• If yes, it continues to wait for more incoming data.
• If no, it starts the initial data sending step.
• The initial data sending step:
• TCP checks if there are any segments to be retransmitted:
• If yes, it stops and waits.
• If no, it checks if the usable window is smaller than the MSS, and there is more data in the out buffer than usable window size:
• If yes, it stops and waits.
• If no, it starts the new segment sending step.
• The new segment sending step:
• It checks if there are any segments to be retransmitted:
• If yes, it stops and waits.
• If no, it goes through the following steps:
• It updates and checks the unacknowledged data buffer for any segments needed to be retransmitted and transmits those first.
• Otherwise, it prepares the data in the out buffer into TCP segments, and it will eventually send all segments out.

How TCP Connection handles incoming TCP segments

When TCP connection receives a TCP segment (flowchart here):

• It updates the Receive-Window variable from the TCP header.
• If the connection is not in the LISTEN state:
• It resets the inactivity timer.
• If the sequence number is equal to the ReceiveNext variable:
• The TCP segment is expected.
• Proceeds to the next step.
• Else
• If the sequence number in the received TCP header is less than the Receive-Next variable.
• If connection is not ESTABLISHED, the segment is a duplicate.
• If the header is not a RST.
• Clears the ACK timer.
• Resets the received ACK counter.
• Sends an ACK out.
• Else
• This is an unexpected segment.
• If the header is a RST, processes RESET.
• Drops this segment.
• If the connection is in the LISTEN state:
• If this is a SYN segment, the server port accepts the connection.
• Else the server port sends back a RST and drops the segment.
• If the connection is in the SYN_SENT state:
• If the TCP header is a SYN or ACK:
• Clears the timer for retransmitting control data.
• Calculates the Receive-MSS and Send-MSS variables from the advertised MSS in the TCP header.
• Updates the local IP.
• Updates the Receive-Next with the ACK number in the TCP header.
• Sets the connection's state to ESTABLISHED.
• If the TCP header is a FIN:
• The TCP connection is refused.
• Processes FIN.
• If the TCP header is a RST:
• The TCP connection was refused.
• Processes RESET.
• If the connection is in the SYN_RECEIVED state:
• If the header is a SYN or RST
• The connection was reset.
• Processes RESET.
• If the TCP header is FIN
  • Processes FIN.
• If the TCP header is an ACK
• Clears the timer for retransmitting control data
• Sets the connection's state to ESTABLISHED.
• If the connection is in the ESTABLISHED state:
• If the header is a SYN or RST
• The connection was reset.
• Processes RESET.
• If the header is a FIN:
• The TCP connection was disconnected.
• Increments the Receive-Next variable.
• Processes FIN.
• If it is an ACK:
• Processes ACK.
• Clears the timer for retransmitting data.
• If the header is an ACK and it's not a RST:
• Checks if it is a duplicate segment.
• If it's not duplicate and not an empty ACK:
• Increments the Receive-Next variable with the size of the received data segment.
• Processes ACK.
• If it's duplicate and not an empty ACK:
• Drops the duplicate segment.
• Clears the timer for sending ACK.
• Sends an ACK.
• Else
• If it is not an empty ACK:
• Processes the data.
• If the connection is ESTABLISHED:
• If acked some packets, and nothing is being retransmitted, and there is buffer data,
• Sends buffer data.
• If it is not an empty ACK:
• Checks if an ACK needs to be sent out.
• If the connection is in the FIN_WAIT_1 state:
• If the header is a SYN or RST
• Processes RESET.
• If the TCP header is a FIN:
• Clears the timer for retransmitting control data.
• Sets the connection's state to CLOSING.
• Sends an ACK.
• If the TCP header is an ACK:
• Clears the timer for retransmitting control data.
• Sets the connection's state to FIN_WAIT_2.
• If the connection is in the FIN_WAIT_2 state:
• If the header is a SYN or RST
• Processes RESET.
• If the TCP header is a FIN
• Sets the connection's state to TIMED_WAIT.
• If the connection is in the LAST_ACK state:
• If the header is a SYN or RST
• Processes RESET.
• If the TCP header is an ACK or FIN:
• Processes ACK.
• Clears the timer for retransmitting control data.
• Sets the connection's state to CLOSED.
• If the connection is in the CLOSING state:
• If the header is a SYN or RST
• Processes RESET.
• If the TCP header is an ACK
• Sets the connection's state to TIMED_WAIT.
• If the connection is in the CLOSED state:
• Clears the timer for retransmitting control data.