Multicast large data size

[anda_skoa]

As jefftee said, the readyRead() signal indicates availability of data. It could be emitted for one byte only or several datagrams as one.

He is also right that fragmentation on any layer below UDP will not show as such on the UDP layer, i.e. no partial datagrams will ever be delivered by the OS.

Cheers,
_

[jefftee]

OK, I was curious so I wrote a small multi-cast server and multi-cast client to prove out what I suspected would occur for your use case. The attached code exhibits the behavior I described in a prior post and demonstrates the out of sequence and lost UDP datagrams and confirms that you will never receive partial datagrams that may have been fragmented and one or more fragments are lost.

Code isn't pretty, but demonstrates how to receive the datagrams and bust them out into individual payloads. The server multi-casts 10,000 datagrams with rotating through an eye catcher of A,B,C,D. The datagrams are random sizes from 512 bytes to 8K in size. I added the eye catcher, a sequence number generated by the server, the length of the payload, and a payload of all binary zeroes.

You can compare the packet type, sequence, and length from the server to the order received by the client(s).

Start one or more clients first, then start the server. Give this a try and let me know if you have any questions.

The project file is attached to this post.

[bon_scott]

Thx for your input
I tried your application and found that the package loss was huge and the server failed to send a lot of the packages, I then converted the multicastServer to a QThread so I could use usleep and msleep between packages, and as expected one can improve package loss rates by modulating the sending times.

Cheers

[jefftee]

I was intentionally blasting packets as quickly as I could to demonstrate the out of sequence and missing packets that can plague UDP. Real world applications will likely have some processing time between sending packets, so it may not be as big of an issue in a real world application, but as you have seen, adding delay between sending the UDP packets will improve reliability significantly.

Good luck with your application, would love to hear how you make out.

Later