ATech Machine LIBRA-02 M Informações Técnicas descarregar pdf (Página 274)

100

101

handled through numerical IP addresses, it can be very helpful to associate a name

(such as www.google.com) with what may in fact be multiple IP addresses.

In the case of VoIP, the use of a domain name can take something like

[email protected] (extension@server) and make it available as [email protected]

(which looks so much sexier on a business card).

The SIP URI

A SIP URI generally looks like sip:endpoint@domain.tld. Depending on your SIP client,

you may be able to dial a SIP URI as endpoint@domain.tld, or even just as endpoint (if

you have a proxy server and the endpoint you are calling is part of your domain).

For a SIP telephone, which often only has a numerical dialpad, it can be problematic

to dial a SIP URI by name,

†

so it has become common to use numerical dialing to reach

external resources. We are also used to making “phone calls” using “phone numbers.”

The SIP protocol itself, however, only understands resource@address, so whatever you

dial must ultimately be converted to this format before SIP can do anything with it.

Usually the only reason you can dial something by “phone number” from your SIP

phone is because you are registered to a resource that understands how to convert the

numerical strings you dial into SIP URIs.

In Asterisk, the resource part of the URI (the part before the @) must match an extension

in the dialplan.

‡

The address portion will be the address (or hostname) of the Asterisk

server itself. So, a URI of sip:[email protected] will end up at an extension called

100, somewhere in the dialplan of the server that provides SIP service for

shifteight.org.

What is dialed (100) may not in any way relate to the actual identifier of the endpoint

being connected to. For example, we might have a user named Leif whose phone may

be a device that registers itself by its MAC address, and therefore could be something

like [email protected].

Much of the purpose of the Asterisk dialplan is to

simplify addressing for users and to handle the complexities of the various protocols

that Asterisk supports.

SRV Records

A Service Record (SRV) is a somewhat new type of DNS record that provides informa-

tion about available services. Defined in RFC 2782, it is often used by newer protocols

† Do you know where the @ symbol is on your dialpad?

‡ Bear in mind that an extension in Asterisk can be any alphanumeric string, such as leif or 100.

§ You could actually dial this URI directly from your phone and bypass the Asterisk server, but you can see

how dialing 100 is going to be a lot more popular than trying to figure out how to type

[email protected] into your phone using just the numeric dialpad (it can be done, by the way).

238 | Chapter 12: Internet Call Routing

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