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Nathan Freitas suggests using Amazon Simple Queue Service for exchanging rendezvous messages. "It supports programmatic sending of messages via web service applications as a way to communicate over the Internet."

It looks like messages are relayed through URLs like https://queue.amazonaws.com/ https://sqs.us-east-1.amazonaws.com/ https://sqs.us-east-2.amazonaws.com/ etc.

Here is an example of a SendMessage call: !https://sqs.us-east-2.amazonaws.com/123456789012/MyQueue?Action=SendMessage&MessageBody=Your%20message%20text&Version=2012-11-05&AUTHPARAMS

There's a command-line interface.

I'm not sure how you would send a message back to the client, and have it match up with the message the client sent initially. Maybe a separate queue per client?