Python Socket Flush

Question

I am trying to make sure that every time I call the socket.send function my buffer is sent (flushed) to my server (which is in C using unix socket).

From my understanding (and from what I see on this board) just disabling the naggle algo. should do it but my server still receive my data in chunk of 4096 bytes (default set)...

Im using the following code in Python v2.5.4:

 self.sck = socket( AF_INET, SOCK_STREAM )

 self.sck.setsockopt( IPPROTO_TCP, TCP_NODELAY, 1 ) # That doesn't seems to work...

 self.sck.connect( ( "127.0.0.1", "12345" ) )

 while( 1 ):
      self.sck.send( "test\n" )
      self.sck.send( "" ) # Still trying to flush...

Enabling/Disabling TCP_NODELAY seems that have no effect whatsoever... Is this a bug or I am missing something?

TIA

Answer 1

This is a common question about TCP protocol. TCP itself has no way to send data in specific chunks. It's designed only for sending stream of data. If you need such functionality, you should implement it yourself. For example, send your chunks in separate lines or first send chunk size and then the chunk itself.

In most cases, you don't need to care about the Naggle algorithm. This algorithm is better described by the name TCP_NODELAY. If you disable it, you may achieve smaller delays for small chunks but lower speed for large chunks at the same time.

Answer 2

TCP does not provide any kind of guaranteed "packet" sending to the other end. You are sending data as fast as you can, and TCP is helpfully batching up the data into as much as it can send at once. Your server is receiving data 4096 bytes at a time, probably because that's what it asked for (in a recv() call).

TCP is a stream protocol and therefore you will have to implement some kind of framing yourself. There are no built-in message boundaries.