Pls recommend Library / API for Web Sockets based Chat App

Hi,

Has anyone developed a Web Sockets based Chat app?

I'm wondering what Libraries / APIs to use.
Some of my requirements are:
- Async send/recv of messages
- Send-Recv files (images, pdf etc)
- Maintain chat sessions when switching between Mobile data - Wifi - No Connectivity
- Prevent message loss during low or no connectivity
- Concurrent messaging from multiple devices

Please share your choices & reasons. I'd also like to know how your experience was?
Suggestions & advices welcome!

Thanks in advance
Sans