TCP

TCP Message Structure

• Source port, Destination port: Identify the remote and local port numbers.
• Serial number (seq)
• Confirmation number (ack): The seq of the next datagram you want to receive, which indicates that all data up to seq N-1 has been received correctly.
• ACK: The initial seq of the expected data to be received.
• SYN: Used to establish a connection.
• FIN: Used to release the connection.
• Window size (WIN): The number of bytes can receive(the size of the receiving window) for flow control.
• Checksum: It was set up to ensure high reliability. It checks the sum of head, data and pseudo-TCP head.

TCP Three Handshakes

• Explanation:
  1. Client sends SYN = 1and seq = x to Server.
  2. Server receives the package and returns ack = x + 1and seq = y.
  3. Client receives the package and returns ack = y + 1and seq = x + 1 .

TCP Four Handwaves

• Explanation:
  1. Client requests to disconnect from the Server and sends FIN = 1and seq = u. Now Client has been disconnected.
  2. Server accepts the disconnection from Client and returns ack = u + 1and seq = v.
  3. Server requests to disconnect from the Client and sends FIN = 1and seq = wand ack = u + 1(next seq of datagram will receive). Now Server has been disconnected.
  4. Client accepts the disconnection from Server and returnsseq = u + 1and ack = w + 1.

Reliable Transfer

• Duplicate Cumulative Acknowledgements

  If the sender receives 3 confirmations of the same package, it will retransfer the last unconfirmed package.

• Retransmission Timeout

  The sender uses an estimated time as the maximum timeout for the confirmation. If the confirmation package is not received after this upper limit, the sender will retransfer the packet.

• Example

  1. The sender sends the first TCP segment with seq = 1 and 1460 bytes. The receiver responds with ack = 1461 to indicate that it has fully received and requested the next message segment.
  2. The sender then sends the second TCP message segment with seq = 1461 . The receiver responds with a TCP message segment with ack = 2921 (1461+1460) to indicate that it has fully received and requested the next message segment.
  3. The sender did not receive a response after sending the third message, so when the timer was out, sender re-send the third message.
  4. The third message has been successfully received, and the receiver can directly confirm the fifth message, because the forth and fifth have been received.

Slide Window

Both Sender and Receiver have a window. The Receiver tells the Sender its own window size through the window field in the TCP message and the Sender sets its own window size according to this value.

• If the left byte of the sending window has been sent and received confirmation, swipe to the right.
• If the left byte of the receiving window has sent confirmation to the host, swipe to the right.

The receiving window only confirm the last sequentially(按序到达) byte. For example, the byte received is {31, 34, 35}, where {31} arrives in order and {34, 35} are not, so only 31 is confirmed. After the sender received confirmation by a byte, it knows that all bytes before this byte have been received.

Flow Control

Receiver’s Advertised Window: The message sender(can be client or server) indicates the number of bytes can be received in the WIN. The receiver sends up to the number of bytes allowed by WIN .

• Example:

  1. Message2: The window size provided is 6144 bytes.
  2. Message4-9: The sender sent 6144 bytes data ***and stopped.
  3. Message10: Confirmed 6145 data (ack 6145) and the window size is 2048.
  4. Message11-12: Sent 2048 bytes data and complete the customer’s data transfer(FIN).
  5. Message13: The same ack as message10, but announced a 4096 window size.
  6. Message14: Confirmed the last 2048 bytes of data and FIN.
  7. Message15-16: Only used to announce a larger window size.
  8. Message17-18: Complete the usual closure process(FIN).

Congestion Control

Slow Start

Suppose cwnd is 1, swnd = cwnd, so the sender can only send one message. After the receiver receives the message, it will send a confirmation to the sender. After receiving the confirmation message, the sender will add cwnd to 2. (2 ⇒ 4, 4 ⇒ 8…cwnd increases exponentially)

Avoid Congestion

cwnd can only be added 1 linearly. Assuming that 4 of the 24 messages are lost and after a period of time, the retransmission timer of the 4 lost messages timed out, and the sender found the congestion. ssthresh reduced to half of cwnd and cwnd is placed at 1.

FRR (Fast Retransmit and Recovery)

The sender sends message1. After receiving the message1, the receiver will confirm message1. Before the confirmation of message1 reaches the sender, the sender can also send message2. If the sender receives 3 repeated confirmations, it immediately retransmit the message.

SACK (Selective Acknowledgment)

• The TCP protocol was inefficient when packets were lost.

  For example, 10,000 bytes of data are sent through 10 groups. If the first group is lost, the receiver cannot say that it has successfully received 1,000 to 9,999 bytes, but has not received the first group containing 0 to 999 bytes. Therefore, the sender may have to retransit all 10,000 bytes.