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I have a program which is using the Websocket TCP: The client is an extension in Chrome and the server is an application written in C++.
When I send small data from the client to the server, it works fine. But when I send large amounts of data (e.g. a source html page), it will be slightly delayed.

For Example:

  • Client sends: 1,2,3
    • Server receives: 1,2
  • Client sends: 4
    • Server receives: 3
  • Client sends: 5
    • Server receives: 4

It's seems like it's a delay.

  1. This is my code client:

var m_cWebsocket = new WebSocket("Servername"); 
if (m_cWebsocket == null) { return false; } 
m_cWebsocket.onopen = onWebsocketOpen(m_cWebsocket); m_cWebsocket.onmessage = onWebsocketMessage; 
m_cWebsocket.onerror = onWebsocketError; 
m_cWebsocket.onclose = onWebsocketError; 
I using m_cWebsocket.send(strMsg) to send data.

  1. Server code

while (true) { recv(sSocket, szBufferTmp, 99990, 0); //recv(sSocket, szBufferTmp, 99990, MSG_PEEK); //some process }

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    Hi could you show us some of the code you are using to send and receive data over the websocket? Commented Aug 27, 2018 at 4:57

1 Answer 1

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Since you haven't posted any code to show your implementation of the TCP server or client I can only speculate and try to explain what might be going on here.

That means the potential problems and solutions I outline below may or may not apply to you, but regardless this information should still be helpful to others who might find this question in the future.

TL;DR: (most likely) It's either the server is too slow, the server is not properly waiting for complete 'tcp packets' to be buffered, or the server doesn't know when to properly start and stop and is de-synching while it waits for what it thinks is a 'full packet' as defined by something like a buffer size.

It sounds to me like you are pushing data from the client either faster than the server the server can read, or more likely, the server is buffering a set number of bytes from the current TCP Stream and waiting for the buffer to fill before outputting additional data.

If you are sending this over localhost it's unlikely you are not close to limit of the stream though, and I would expect a server written in C++ would be able to keep up with the javascript client.

So this leads me to believe that the issue is in fact the stream buffer on the C++ side.

Now since the server has no way to know to what data you are sending and or how much of it you are sending, it is common for a TCP stream to utilize a stream buffer that contiguously reads data from the socket until either the buffer has filled to a known size, or until it sees a predefined 'stop character'. This would usually be something like a "line end" or \n character, sometimes \n\r (line feed, carriage feed) depending on your operating system.

Since you haven't specified how you are receiving your data, I'm going to just assume you created either a char or byte buffer of a certain size. I'm a pretty rusty on my C++ socket information so I might be wrong, but I do believe there is a default 'read timeout' on C++ tcp streams as well.

This means you are possibly running into 1 of 2 issues.

Situation 1) You are waiting until that byte/char buffer is filled before outputing it's data. Issue is that will act like a bus that only leaves the station when all seats are filled. If you don't fill all the seats, you server is just sitting and waiting until it gets more data to fill up fully and output your data.

Situation 2) You are running up against the socket read timeout and therefore the function is not getting all the data before outputting the data. This is like a bus that is running by the clock. Every 10 minutes that bus leaves the station, doesn't matter if that bus is full or empty, it's leaving and the next bus will pick up anyone who shows up late. In your case, the TCP stream isn't able to load 1, 2 and 3 onto a bus fast enough, so the bus leaves with just 1, 2 on it because after 20ms of not receiving data, the server is exiting from the function and outputing the data. On the next loop however, there is 3 waiting at the top of the stream buffer ready to get on the next bus out. The Stream will load 3, wait til those 20ms are finished, and then exit before repeating this loop.

I think it's more likely the first situation is occurring though, as I would expect the server to either start catching up, or falling further behind as the 2 servers either begin to sync together, or have internall TPC stream buffer fill up as the server falls further and further behind.

Main point here, you need some way to synchronize the client and the server connections. I would recommend sending a "start byte" and "End byte" to single when a message has begun and finished, so you don't exit the function too early.
Or send a start byte, followed by the packet size in bytes, then filling up the buffer until your buffer has the correct numbers of bytes. Additionally you could include an end byte as well for some basic error checking.

This is a pretty involved topic and hard to really give you a good answer without any code from you, but this should also help anyone in the future who might be having a similar issue.

EDIT I went back and re-read your question and noticed you said it was only with large amounts of data, so I think my original assumption was wrong, and it's more likely situation 2 because the client is sending the data to your server faster than the server can read it, and thus might be bottle necking the connection and the client is only able to send additional data once the server has emptied part of it's TCP stream buffer.

Think of it like a tube of of water. The socket (tube) can only accept (fill up) with so much data (water) before it's full. Once you let some water out the bottom though, you can fill it up a little bit more. The only reason it works for small files is that the file is too small to fill the entire tube.

Additional thoughts: You can see how I was approaching this problem in C# in this question: Continuously reading from serial port asynchronously properly

And another similar question I had previously (again in C#): How to use Task.WhenAny with ReadLineAsync to get data from any TcpClient

It's been awhile since I've played with TCP streams though, so my apologies in that I don't remember all the niche details and caveats of the protocal, but hopefully this information is enough to get you in the ball park for solving your problem.

Full disclaimer, it's been over 2 years since I last touched C++ TCP sockets, and have since worked with sockets/websockets in other languages (such as C# and JavaScript), so I may have some facts wrong about the behavior of C++ TCP sockets specifically, but the core information should still apply. If I got anything wrong, someone in the comments will most likely have the correct information.

Lastly, welcome to stack overflow!

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2 Comments

Yes, my error is "client is sending the data to your server faster than the server can read it" when I use recv(sSocket, szBufferTmp, 99990, MSG_PEEK); I can see data in input queue. But When I use recv(sSocket, szBufferTmp, 99990, 0) to recieve data my function is block here. So how I can fix it.
@Khanh I need more information before I can help you. What does the client sending code look like? What does the server code look like? If you don't want to block, you should look at some of the links I provided which demonstrate reading from a socket asynchronously. Without getting a full picture of your code though I'd just be shooting in the dark.

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