D&C GLug - Home Page

[ Date Index ] [ Thread Index ] [ <= Previous by date / thread ] [ Next by date / thread => ]

Re: [LUG] OT - SIP providers?

 

On Wed, 7 Oct 2009, tom wrote:

I used to play with small tunneling servers that you could use to map any protocol into port 80 or 8080 to get through port restrictions and the NAT problem goes away when you use them - ok they're not so easy to configure by hand but they were code wise miniscule and could have been used to overcome the oversights in the SIP design (at least I think that's what they were).

SIP was designed around the same time that NAT was - no-one really new in the mid-late 90's what was going to happen. I remember reading the "We're running out of IP addresses" scare stories back then too... We still haven't, but would have if it were not for NAT. (And CIDR)

SIP is actually rather clever - and one of the reasons it encodes the endpoint IP address into the data packets is so that a registrar (PBX if you like) can do what's called a re-invite and have the data stream bypass the PBX and go endpoint to endpoint. This is advantageous for many instances (unless you're recording the call!) but NAT then really gets in the way because the endpoint IP address changes.

IAX addresses most of the NAT issues by only using a single port and a multiplexed protocol, but it has other issues, however I use it a lot for sites behind NAT. It's not popular outside the Asterisk world though.

This may indeed be what Skype has added. Its not a huge leap in internet programming, but there does seem to be a huge amount of amnesia setting in about the internet. Simple point to point VOIP, once resolved, does not need this huge clutter of infrastructure. OK its very useful in an all singing all dancing setup but most of us dont have a full blown PABX just to call grandma.

Indeed - you don't need a PBX to call grandma, but you do need something to translate an easy to use number or name into an IP address or other endpoint. We call these things telephone exchanges. BT has many and I have a few :)

Gordon

--
The Mailing List for the Devon & Cornwall LUG
http://mailman.dclug.org.uk/listinfo/list
FAQ: http://www.dcglug.org.uk/linux_adm/list-faq.html