Zhang,
On 5/4/13 6:31 AM, SIP Learner wrote:
> Hi, everyone!
>
>
> I am a newcomer to SIP and I have a problem with Section 8.1.1.2 of RFC3261.
>
>
> Following is a short qoute from Section 8.1.1.2 of RFC 3261 (I enclosed the
> key sentence within asterisks to make them stand out):
>
>
> "A UAC may learn how to populate the To header field for a particular request
> in a number of ways... Frequently, the user will not enter a complete URI,
> but rather a string of digits or letters (for example, "bob"). ...*** The RHS
> will frequently be the home domain of the requestor, which allows for the
> home domain to process the outgoing request."***
>
>
> As far as I know, when Alice (whose SIP URI is al...@atlanta.com) is trying
> to call Bob (whose SIP URI is b...@biloxi.com), her UA will (at least
> initially) populate the To header field of the outgong INVITE with
> b...@biloxi.com. But according to the above quote enclosed in asterisks, when
> sending outgoing request to Bob, Alice (the requestor) will FREQUENTLY inclue
> b...@atlanta.com in the To header field, where atlanta.com is Alice's home
> domain. What does this mean? I am really confused :-(
>
>
> Anyone please help me out with this, thank you very much!
This is all about user interface, and isn't standardized.
In common practice there is a big difference between numeric addresses
(phone numbers) and email-style addresses (b...@biloxi.com). For better
or worse, most of the focus with sip has been on phone numbers.
For email-style addresses there isn't enough information in "bob" to
infer that it should be sip:b...@biloxi.com. So either the user must type
in more. If he just types "bob" then you *might* decide this means the
bob in the same domain as the caller, or else just consider it an error.
Or if you have access to an address book you could look up "bob" there
and translate it to a url.
For numbers, things are handled differently depending upon the system,
and often revised while flowing through the network. The generally
preferred way to represent phone numbers in SIP is via a "tel:" URI (RFC
3966), or else a sip URI with the user part having the same format as a
tel URI. E.g.,
tel:+12125551212
sip:+12125551...@example.com;user=phone
In many respects the two URIs above are equivalent. The syntax beginning
with "+" denotes an E.164 number, including a country code.
Some systems don't support the tel: uri itself, and expect the sip form.
In principle, the domain portion of the sip URI *should* denote the
domain which hosts that particular number. But often these are
constructed using whatever domain is convenient, usually that of the
caller's service provider. Further explanation of that is a longer story
than I want to type here.
Of course callers will typically not want to type that full sip URI, or
the tel URI, or even the E.164 number. They will usually type something
shorter that is unambiguous in the context of the caller. In that case
*something* will probably need to expand it into the E.164 form.
Sometimes the calling device can do that.
Other times a server acting on behalf of the caller may do it. In that
case the string of digits that were dialed need to be conveyed to the
server that will do the translation, by encoding them as a URI. For
instance, perhaps the caller dialed "2125551212". The best sanctioned
way of representing that is:
sip:2125551...@example.com;user=dialstring
But often it may simply be sent as:
sip:2125551...@example.com
or
sip:2125551...@example.com;user=phone
(The last, with user=phone, is *incorrect* but still sometimes used.)
In all these cases, the call will presumably be routed to a server for
example.com. It then may handle it directly if it is responsible for
that number. Or it may translate to an E.164 number and route it to
another server in a different domain. The detailed routing of phone
numbers is complex, so I won't discuss it here.
Business systems, with internal extensions may use variations on these
approaches.
Good Luck,
Paul
_______________________________________________
Sip-implementors mailing list
Sip-implementors@lists.cs.columbia.edu
https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
