Detailed explanation of TCP (3)

9. Byte stream oriented

In this case of byte stream oriented, there is an important problem to pay attention to: the sticky packet problem (referring to the application side datagram)

For TCP, the data is transmitted as shown below

But for UDP, since UDP transmits datagrams, it is transmitted one by one

Then the question is, how to solve the sticky package problem?

Solve by designing a reasonable application layer protocol!!!

1. Set "terminator"/"delimiter" for application layer data

2. Set "length" for application layer data

Method 1:

Method 2:

10.Some exceptions in TCP

Common exceptions:

(1) Process termination

No matter how the process is terminated, the corresponding PCB will be released in essence, and the corresponding file descriptor will also be released, which will also trigger the wave four times.

"Process termination" does not mean that the connection is terminated. Process termination is actually equivalent to calling socket.close().

(2) Machine restart

When the machine restarts, it actually kills the process first, still waving four times.

(3) The machine is powered off/the network cable is disconnected

Unexpected!!The machine is too late to do anything

​ <1>It is the receiver who loses power

At this time, the other side is still sending data. Obviously, the sender will no longer have ACK at this time!

So the timeout is retransmitted. After several retransmissions, it will try to reset the connection and reset the segment.Then the sender will give up the connection, and the resources corresponding to the connection will be recycled.

​ <2>It is the sender who loses power

At this time, the other party is trying to receive data, but no data is received at this time

How does the receiver know that the sender is dead?Or is it that the sender hasn't sent it yet?

The strategy adopted by the receiver at this time is the "heartbeat packet" mechanism (also called "keep alive")

Every once in a while, send a PING packet to the other party and expect the other party to return a PONG packet.

If the PING packet has been sent, your bureau has no PONG after crossing the river, and you can't retry several times, it is considered that the other party has hung up.

Classic Interview Questions

How to achieve reliable transmission based on UDP protocol?

This question seems to be asking about UDP, but it is actually a TCP test!!!

1. Implement the confirmation response mechanism. After each data is received, Daou will feedback an ACK (this time this is not returned by the kernel, but the application defines an ACK packet and sends it back).

2. Realize serial number/confirm serial number, and realize deduplication.

3. Realize timeout retransmission.

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4. Implement connection management.

5. To improve efficiency, implement a sliding window.

6. To limit the sliding window, implement flow control/congestion control.

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7. Realize delayed response, piggyback response, heartbeat mechanism...

Which scenario is suitable to use TCP and which scenario is suitable to use UDP?

1. If reliability is required, TCP is preferred.

2. If the single data table of the number of beds is long (more than 64K), TCP is still used.

3. If you pay special attention to efficiency, give priority to UDP.

​ Typical scenario: communication between hosts in a computer room.

4. If broadcasting is required, UDP is preferred.

​ A piece of data is sent to multiple hosts at the same time. UDP itself supports broadcasting, but TCP itself does not support it. It can only send data to each host through multiple connections and polling in the application layer program.(pseudo broadcast).