UDP File Transfer - Yes, UDP

Question

I have a requirement to create a UDP file transfer system. I know TCP is guaranteed and much more reliable, but I need to transfer huge files between locations and I think the speed advantage in this project outweighs the benefits using TCP. I’m just starting this project, but would like some guidance if anyone has done this before. I will be writing both sides (client and server) so I don’t need to worry about feature limitations in other products.

In a nutshell I need to:

• Take large files and send them in chunks
• Be able to throttle bandwidth from the client
• Create some kind of packet numbering system for errors, retransmitions and assembling files by chunk on server (yes, all the stuff we get from TCP for free :-)
• Configurable datagram size – I think some firewalls complain if they get too big?
• Anything else I may be missing

I’m starting this journey using UdpClient and would like to write this app in C#. Any words of wisdom (other than to use TCP)?

---

It’s been done with huge success. We used to use RocketStream.com, but they sold their product to another company for internal use only. We typically get speeds that are 30X faster than FTP or raw TCP byte transfers.

Answer 1

in regards to

Configurable datagram size – I think some firewalls complain if they get too big?

one datagram could be up to 65,536 bytes. cosidering all the ip header information you'll end up with 65,507 bytes for payload. but you have to consider how all devices are configured along you network path. typically most devices have set an MTU-size of 1500 bytes so this will be typically your limit "on the internet". if you set up a dedicated network between your locations you can increase your MTU an all devices.

further in regards to

Create some kind of packet numbering system for errors, retransmitions and assembling files by chunk on server (yes, all the stuff we get from TCP for free :-)

i think the best thing in your case would be to implement a application level protocol. like

32 byte sequence number 8 byte crc32 checksum (correct me on the bytesize) any bytes left can be used for data

hope this gives you some bit of a direction

::edit::

from experience i can tell you UDP is about 10-15% faster than TCP on dedicated and UDP-tuned networks.

Answer 2

I'm not convinced the speed gain will be tremendous, but an interesting experiment. Such a protocol will look and behave more like one of the traditional modem based protocols, and probably ZModem is one of the better examples to get some inspiration from (implements an ack window, adaptive block size, etc).

There are already some people who tried this, check out this site.