Packets are just tossed out on the network pointed at the other endpoint and you hope that they get there. UDP on the other hand, makes no guarantees of delivery. However, in poor network conditions, it can be slower due to frequent retransmissions. It’s perfect for transfers of data where you want to ensure it gets there. Data is guaranteed to get there and get there in the right order. If packets aren’t received by the other end, there are requests for retransmission. Setting up a TCP connection involves the “three-way-handshake” and each packet contains information to ensure the packet will get there.
The IP-PBXs I’m familiar with, both commercial and open source, all send RTP over UDP. As part of its range of tricks to get through firewalls, I understand that Skype can use TCP if it is unable to send over UDP, but I’ve never captured it doing so. In all the VoIP systems I’ve worked with, I can’t think offhand of any other systems that send audio over TCP. I mean, the typical mindset to date has been… who in their right mind would send audio streams over TCP? Now to be fair, if Aircell isn’t blocking TCP, this was probably a decent assumption to make. (They probably are by now or will be soon.) The Tringme Flash-based softphone is sending audio over TCP and Aircell is not recognizing and blocking VoIP calls over TCP. Let’s look at the network capture from a call I had today with Phweet CEO Stuart Henshall using the Flash softphone:Īnd there you go… one very possible reason why Phweet may work and Skype, Sightspeed and others were blocked is simply this: The Flash-based softphone is actually from Tringme and is integrated into Phweet’s services. To understand, we need to look at the traffic from the Flash softphone that is being used by Phweet. So why was Phweet’s web-based solution different? Why did it work when Skype, Sightspeed and other services fail? A VoIP call starts up… a stream of typically 100 packets per second start streaming over UDP between two endpoints… the networking monitoring software notices this, watches it for several seconds, decides it’s VoIP, and blocks the stream. So how did Andy Abramson make a call using Phweet on the new Aircell Gogo Internet service on American Airlines planes? Why was the call not blocked like Skype or Sightspeed calls were?Īs I discussed in my last post, VoIP calls have certain network characteristics that make them easy to identify and, in Aircell’s case, block. Update #2: As expected, GigaOm is now reporting that Aircell is blocking the Tringme VoIP client used by Phweet.
Odds are that it works for a similar reason to what I outline for Phweet below (and that eventually it will be blocked). Update #1: Since publishing this post this morning, I’ve learned of David Berlind’s success using AOL’s video chat from the plane.
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