CODEWITHSUNDEEP
node.jstcperlangerlang-otpgen-tcp
Dale Sparrow
Dale Sparrow

Reputation: 121

Strange behavior of OTP gen_tcp with settings {packet,4} and using NodeJS "frame-stream'' for TCP communication

I've been struggling for a while to get my messages framed correctly between my NodeJS server and my erlang gen_tcp server. I was using {packet,line} successfully until I had to send large data messages and needed to switch to message size framing.

I set gen_tcp to {packet,2}

and I'm using the library from: https://github.com/davedoesdev/frame-stream for the NodeJS tcp decode side. It is ALSO set to packet size option 2 and I have tried packet size option 4.

I saw for any messages with a length under 127 characters this setup works well, but any messages longer than this has a problem.

I ran a test by sending longer and longer messages from gen_tcp and then reading out the first four bytes received on the NodeJS side:

on message 127: HEADER: 0 0 0 127 Frame length 127

on message 128: HEADER: 0 0 0 239 <----- This should be 128 Frame length 239 <----- This should be 128

Theories:

Data from wireshark shows the following:

The header bytes are encoded properly by gen_tcp past 128 characters since the hex values proceed as follows:

[00][7e][...]  (126 length)
[00][7f][...]  (127 length)
[00][80][...]  (128 length)
[00][81][...]  (129 length)

So it must be that the error lies when the library on the NodeJS side calls the Node readUInt16BE(0) or readUInt32BE(0) functions. But I checked the endieness and both are big-endian.

If the header bytes are [A,B] then, in binary, this error occurs after [00000000 01111111]

In other words, readUInt16BE(0) reads [000000000 10000000] as 0xef ? which is not even an endian option...?

Thank you for any help in how to solve this.

Kind Regards

Dale

Upvotes: 0

Views: 78

Answers (1)

Dale Sparrow
Dale Sparrow

Reputation: 121

I figured it out, the problem was caused by setting the socket to receive on UTF-8 encoding which supports ascii up to 127.

Dont do this: socket.setEncoding('utf8').

It seems obvious now but that one line of code is hard to spot.

Upvotes: 1

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