Shulou(Shulou.com)06/03 Report--

TCPTCP, which provides connection-oriented services, must establish a connection before transmitting data, and release the connection after the data transfer is completed. Therefore, TCP is a reliable transportation service, but because of this, it inevitably increases a lot of overhead, such as confirmation, flow control and so on. The corresponding application layer protocols are SMTP,Telnet,HTTP,FTP and so on. TCP message header

Source port and destination port

Processes on    computers communicate with other processes through the computer port, and a computer port can only be occupied by one process at a time, so you can know which two processes need to communicate by specifying the source port and the destination port. The source port and destination port are represented by 16 bits, and the number of ports of the computer can be calculated to be 2 ^ 16.

Serial number

   occupies 4 bytes, which represents the number of the first byte of the data sent in this paragraph. Each byte of the byte stream transmitted in the TCP connection is numbered sequentially. Because the sequence number is represented by 32 bits, every 2 ^ 32 bytes, the sequence number is wound back, starting at 0 again.

   for example, the serial number field value of a message is 301, and the data carried has a total of 100 fields. obviously, the data sequence number of the next message field (if any) should start at 401.

Confirmation number

   occupies 4 bytes, which is the sequence number of the first data byte expected to receive the next message.

   for example, B received a message sent by A with a sequence number field of 501 and a data length of 200 bytes, indicating that B correctly received the data sent by A up to the serial number 700. Therefore, B expects the next data sequence number of A to be 701, so B sets the confirmation number to 701 in the confirmation message sent to A.

Data offset

   occupies 4 places, indicating how far the data of the TCP message is from the beginning of the TCP segment.

Keep

   occupies 6 places and is reserved for future use, but it should all be 0 at present

Emergency pointer URG

   when URG=1 indicates that the emergency pointer field is valid. Tell the system that there is emergency data in this message.

Confirm ACK

   the confirmation number field is valid only when ACK=1. TCP stipulates that ACK must be set to 1 for all messages after the connection is established.

Push PSH

   when two application processes communicate interactively, sometimes the application process on one side wants to receive a response from the other immediately after typing a command, so PSH=1

Reset RST

   when RST=1 indicates that there is a serious error in the TCP connection, you must release the connection and then re-establish the connection

Synchronous SYN

   is used to synchronize sequence numbers when a connection is established. When SYN=1,ACK=0 indicates that it is a connection request message, if you agree to connect, you should make SYN=1,ACK=1 in the response message

Terminating FIN

   is used to release connections. When FIN=1, indicates that the data of the sender of this message has been sent and requests that it be released

Window

   occupies 2 bytes, indicating the amount of data that the other party is now allowed to send, that is, telling the other party that the amount of data allowed to be sent by the other party from the confirmation number of this paragraph reaches this value, and it needs to be confirmed by ACK before the subsequent data can be transmitted.

Check and

   occupies 2 bytes. Check the header and data to provide additional reliability.

Emergency pointer

   occupies 2 bytes, indicating the number of bytes of emergency data in this paragraph

Option

The maximum length of    can be calculated according to the length of the head of TCP. The length of the TCP header is represented by 4 bits, and the maximum length of the option section is: (2 ^ 4-1) * 4-20mm 40 bytes

TCP three-way handshake

At the beginning, both client An and server B are in the CLOSED state. The client opens the connection actively, and the server passively opens the connection.

The TCP server B process first creates the transmission control block TCB, and is always ready to accept the connection request from the client A process. At this time, server B enters the LISTEN state. The TCP client A process also first creates the transmission control block TCB, and then sends a connection request message to server B, which is the same part of the SYN=1 in the header of the message, and selects an initial sequence number seq=x. At this time, the TCP client A process enters the SYN-SENT state. TCP stipulates that SYN message segments (SYN=1 message segments) cannot carry data, but need to consume a serial number. After receiving the request message, TCP server B sends a confirmation message if it agrees to connect. The confirmation message should be ACK=1,SYN=1, and the confirmation number is ack=x+1. At the same time, you should initialize a sequence number seq=y for yourself. At this point, the TCP server B process enters the SYN-RCVD state. This message cannot carry data either, but it also consumes a serial number. After the TCP client A process receives the confirmation, it also gives the confirmation to server B. Confirm the ACK=1,ack=y+1 of the message and its own serial number seq=x+1. At this time, the TCP connection is established and client An enters the ESTABLISHED (established connection) state. Server B also enters the ESTABLISHED state after receiving the confirmation from client A, after which the two parties can start to communicate. Why does TCP client A send an acknowledgement for the last time?

The main purpose of    is to prevent invalid link request messages from being suddenly transmitted to server B, resulting in errors.

Why three handshakes instead of two handshakes?

If    uses two handshakes to establish a connection, client A sends the first request connection and does not lose it, just because it has been stuck in the network node for too long. Because client An of TCP did not receive the acknowledgement message this time, it thought that server B did not receive it. At this time, client An and server B re-sent the message to server B. after that, client An and server B completed the connection through two handshakes and transmitted data. Then close the connection. At this time, the stranded request connection, the network unblocked to server B, this message should have been invalid, but the two-time handshake mechanism will allow client An and server B to establish a connection again, which will lead to unnecessary errors and waste of resources.

If    uses a three-way handshake, even if the invalid message is sent, server B receives the invalid message and replies to the acknowledgement message, but client A will not send an acknowledgement again. Since server B does not receive an acknowledgement, it is known that client A did not request a connection.

TCP four-way handshake

After the data transmission is complete, both parties can release the connection. At the beginning, both client An and server B are in the ESTABLISHED state, and then client A shuts down actively, and server B shuts down passively

The client A process sends a connection release message and stops sending data. Release the header of the data message, FIN=1, whose sequence number is seq=u (equal to the sequence number of the last byte of the previously transmitted data plus 1), and client An enters the FIN-WAIT-1 (termination waiting 1) state. TCP stipulates that FIN message segments consume a serial number even if they do not carry data. Server B receives the connection release message, sends out a confirmation message, ACK=1,ack=u+1, and takes its own serial number seq=v. At this time, server B enters the CLOSE-WAIT (shutdown waiting) state. TCP server B notifies the high-level application process that client An is released in the direction of server B. at this time, client An is in a semi-closed state, that is, client A has no data to send, but if server B sends data, client A still has to accept it. This state will continue for a while, that is, the duration of the entire CLOSE-WAIT state. After client A receives the confirmation request from server B, client An enters the FIN-WAIT-2 state and waits for server B to send the connection release message (before that, it needs to accept the last data sent by server B). After server B sends the final data, it sends a connection release message to client A, FIN=1,ack=u+1. Since server B is in a semi-closed state, server B probably sends some more data. Assuming that the sequence number at this time is seq=w, server B enters the LAST-ACK state and waits for client A to confirm. After receiving the connection release message from server B, client A must issue an acknowledgement, ACK=1,ack=w+1, and its serial number is seq=u+1. At this time, client An enters the TIME-WAIT state. Note that the TCP connection has not been released at this time. The time of 2 ∗ * ∗ MSL (maximum message segment life) has elapsed, and client A cancels the corresponding TCB before entering CLOSED state. As soon as server B receives the confirmation from client A, it immediately enters the CLOSED state. Similarly, after revoking the TCB, the TCP connection is terminated. As you can see, server B ends the TCP connection earlier than client A. Why does client An end up waiting for 2MSL? Ensure that the last ACK message sent by client A can reach server B, because this ACK message may be lost. From the point of view of server B, I have sent the FIN+ACK message request disconnected, client A has not responded to me, it should be that the request disconnection message I sent has not been received, so server B will send it again. Client A can receive the retransmitted message within this 2MSL period, then give a response message, and restart the 2MSL timer. Prevent "invalid connection request message segments" like those mentioned in the three-way handshake from appearing in this connection. After client A sends the last acknowledgement message, during this 2MSL time, all the message segments generated during the duration of this connection can disappear from the network. In this way, the request message for the old connection will not appear in the new connection. Why does it take three handshakes to establish a connection and four waves to close the connection?

When    establishes a connection, server B receives the SYN message of the connection request in LISTEN state, and sends ACK and SYN to client An in one message.

When    closes the connection, when server B receives the other party's FIN message, it only means that the other party no longer sends data but can still receive data, and it may not send all the data to the other party, so its own side can immediately close it, or send some data to the other party, and then send a FIN message to the other party to indicate that it agrees to close the connection now. Therefore, their own ACK and FIN will generally be sent separately. Which leads to one more time.

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