I was remembering my mother in the 60s when she would wet her hair, curl small areas and hair pin in place. When it dried and she combed it out, she looked like an angel. But hairpinning is a term in voice over Internet (IP communications). It is the process of returning a message from a beginning endpoint back in the direction it came from. Why? It is a way to get to its original destination endpoint.
Traditional telephone systems, analog and digital, actually use hairpinning connections. In voice over Internet, hairpinning can be avoided. The two endpoints can be connected immediately after the call is initiated. The form of direct communication following call setup is called shuffling. In VoIP networks, hairpin connections are considered inefficient and high-end systems support shuffling.
Anyone who has attempted to start their "Vonage, 8x8, Lingo, Nettalk, or Sunrocket (kaput) type" voice over Internet service, knows the frustration of working with and troubleshooting the effective setup and use of devices on various networks around the world. What does hairpinning have to do with this?
Sometimes a beginning endpoint and its router in the subnetwork doesn't recognize that a certain message that is intended for a certain destination endpoint in the same subnetwork because it only knows its public IP address. The Internet Network Address Translation (NAT) server must have the capability to recognize this potential issue, so that it can "hairpin" the message back to the subnetwork so that it can reach its final and intended destination.
In voice over Internet protocol, "hairpinning" is a sequence of events where a device (ATA adapter or IP phone or nowadays) connects to a private branch exchange, also known as a business phone system and then to another endpoint phone in order to get the call going between the two.
What is tough about it is that it often takes place over a path that is oddly enough longer than it needs to be and more than the shortest possible circuit between the two endpoints. In other words, it's like Jack Nicholson's family from the Shining feeling pushed to go here and there in a maze toward an endpoint, to put up with the inconvenience of such, in order to get there because in their fear, they couldn't think of an alternative.
Remember in The Shining, Wendy is concerned about the phone lines being out due to the heavy snowfall? Just a coincidence that I threw this analogy into the post. I was doing this thing called Word Association that I learned from Ray Bradbury. I'm reading more into this than I need to, right? Don't forget to call Mom and Dad this week.
And consider some signing your SIP team up for the study of such terms, even your salespeople need to be familiar. Consider TheSIPSchool.com, Kamailio Training, Illinois Institute of Technology's VoIP Lab, FreeSWITCH Training, or Digium's Asterisk Boot Camps. If you choose The SIP School, tell them DIDX sent you so you can get your well-earned discount.