TCP packet sticking problem

TCP Stick package problem

TCP Sticky packet means that the data of several packets sent by the sender arrive at the receiver and become a packet , From the receive buffer , The head of the next packet of data is next to the tail of the previous one , There are many reasons for sticking , It could be from the sender , It could be from the receiver .

cause TCP The reason for sticking the bag

Sender reason

The sending end sends multiple packets to the receiving end , More efficient sending to the receiver , So the optimization algorithm is used （Nagle Algorithm ）, Will be multiple with smaller intervals 、 Data with a small amount of data , Merge into a large block of data , Then the packet is sealed .

TCP By default Nagle Algorithm （ The main role ： Reduce the number of newspaper segments in the network ）, and Nagle Algorithms do two things ：

• Only the last group is confirmed , Will send the next packet
• Collect multiple subgroups , Send together when a confirmation arrives

Nagle The algorithm may cause packet sticking problem on the sender .

The receiver's reason

TCP When a packet is received , It will not be handed over to the application layer immediately for processing , In other words, the application layer does not immediately process . actually ,TCP Save the received packet in the receiving cache , The application then actively reads the received packets from the cache . thus , If TCP The speed of receiving packets to the cache is faster than that of applications reading packets from the cache , Multiple packages will be cached , It is possible for an application to read multiple packages that are glued together .

How to deal with it TCP Sticky package

If multiple groups of data sent by the sender are originally different parts of the same piece of data , For example, a file is divided into multiple parts and sent , At this time, of course, there is no need to deal with the sticking phenomenon . If multiple packets are irrelevant , Even juxtaposition , Then we must deal with the sticking phenomenon at this time

The sender

For the packet sticking problem caused by the sender , Can be closed by Nagle Algorithm to solve , Use TCP_NODELAY Option to turn off the algorithm .

The receiving party

The receiver has no way to deal with packet sticking , Only the application layer can handle the problem .

application layer

The solution of application layer is simple and feasible , It can not only solve the sticking problem of the receiver , It can also solve the packet sticking problem of the sender .

terms of settlement ： Loop processing , When an application reads a packet from the receive cache , After reading a piece of data , You should cycle through the next piece of data , Until all the data is processed , But how to judge the length of each data ？

• Format data ： Each data has a fixed format （ Start Rune , Terminator ）, This method is simple and easy , However, when selecting the start and end characters, make sure that the beginning and ending characters are not included inside each data .
• Sending length ： When sending each piece of data , Send the length of the data together , For example, before prescribing data 4 Bits are the length of the data , The application layer can judge the start and end position of each group according to its length .

Why? UDP No sticking problem

TCP In order to ensure reliable transmission and reduce additional overhead （ Every time the contract is awarded, it has to be verified ）, Stream based transmission is adopted , Stream based transport does not consider messages to be one by one , Is unprotected message boundary （ Protect message boundaries ： Transmission protocol refers to the transmission of data on the Internet as an independent message , The receiver can only accept one independent message at a time ）.

UDP Is message transmission oriented , There are protected message boundaries , The receiving party accepts only one piece of information at a time , So there is no sticking problem .

for instance ： There are three packets , The sizes are 2k、4k、6k, If the UDP Send it , No matter how big the receiver's receive cache is , We must send at least three times before we can send the data packet , But use TCP If the agreement is sent , We only need the receiver's receive cache to have 12k Size , You can put this... At one time 3 All packets have been sent .