Subject: Re: [martini] Proposed work split: tackle telephone-number registration first

From: "Bernard Aboba" <[email-masked]>
Date: Sat, 30 Jan 2010 19:12:48 -0800

Paul Kyzivat said:

"But isn't the whole point here that the pbx *can't* be reached at its 

domain name using DNS lookup? This connection is supposed to be in lieu 

of such a direct connection."

[BA] Dynamic DNS providers are widely available and their fees are quite

small compared with those of a typical SIP trunking provider, so I don't

think the bottleneck is really the ability to provision SRV/NAPTR RRs or

dynamically provision A/AAAA RRs. 

Even if such RRs were populated, service providers would prefer to utilize 

"implicit registration" for the purposes of determining PBX reachability.

Most of SSPs don't offer DNS services, and don't have partnerships with 

companies that do.  Given that, they'd prefer to avoid dependencies on 

third parties that they have no control over. 

However, with domain registration the SSP would have a wider set of choices:

a. If you can demonstrate ownership of the domain via the mechanisms

Dean mentioned, the SSP could include your proven domain in the incoming
Request-URI. 

b. If you don't own a domain, then they'll provide you with a 

sub-domain for the purposes of domain registration (but probably

not for DNS uses, since most SSPs don't offer that). 

Choice a) would come in handy in some cases.  For example,

I have set up PBXes that support dynamic registration, purchased

domains, gotten them set up with dynamic DNS providers, etc. only

to find that incoming INVITEs from the SSP still contained 

Request-URIs of the form <phone-number>@sp.com, because

the SSP wasn't set up to handle anything else (at least not for

the fees they were being paid). 

As an aside, no loops resulted from this behavior because the

PBX was operating as a B2BUA, and was provisioned to treat

those incoming Request-URIs as if they were registered by

the PBX acting as a UA, not as though the PBX were acting as a Proxy. 

So from the PBXes point of view, it did not route those requests,

it merely terminated them.