[SIPForum-discussion] Why does SIP Registration do not create a dialog?

[nitin kapoor]

Hello Raj

Basically  dialog  represents a peer-to-peer SIP relationship between
two user agents that persists for some time.  The dialog facilitates
sequencing of
messages between the user agents and proper routing of requests
between both of them.  The dialog represents a context in which to
interpret SIP messages.  Section 8 discussed method independent UA
processing for requests and responses outside of a dialog.  This
section discusses how those requests and responses are used to
construct a dialog, and then how subsequent requests and responses
are sent within a dialog. A dialog is identified at each UA with a
dialog ID, which consists of a Call-ID value, a local tag and a remote
tag.  The dialog ID at each
UA involved in the dialog is not the same.  Specifically, the local
tag at one UA is identical to the remote tag at the peer UA.  The tags
are opaque tokens that facilitate the generation of unique
dialog IDs.A dialog ID is also associated with all responses and with
any request that contains a tag in the To field.  The rules for
computing the dialog ID of a message depend on whether the SIP element
is a UAC
or UAS.  For a UAC, the Call-ID value of the dialog ID is set to the
Call-ID of the message, the remote tag is set to the tag in the To
field of the message, and the local tag is set to the tag in the From
field of the message (these rules apply to both requests and
responses).  As one would expect for a UAS, the Call-ID value of the
dialog ID is set to the Call-ID of the message, the remote tag is set
to the tag in the From field of the message, and the local tag is set
to the tag in the To field of the message.

A dialog contains certain pieces of state needed for further message
transmissions within the dialog.  This state consists of the dialog
ID, a local sequence number (used to order requests from the UA to
its peer), a remote sequence number (used to order requests from its
peer to the UA), a local URI, a remote URI, remote target, a boolean
flag called "secure", and a route set, which is an ordered list of
URIs.  The route set is the list of servers that need to be traversed
to send a request to the peer.  A dialog can also be in the "early"
state, which occurs when it is created with a provisional response,
and then transition to the "confirmed" state when a 2xx final response
arrives.  For other responses, or if no response arrives at all on
that dialog, the early dialog terminates.

*"Dialogs are created through the generation of non-failure responses
to requests with specific methods.  Within this specification, only
2xx and 101-199 responses with a To tag, where the request was
INVITE, will establish a dialog.  A dialog established by a non-final
response to a request is in the "early" state and it is called an
early dialog.  Extensions MAY define other means for creating
dialogs.  Section 13 gives more details that are specific to the
INVITE method.  Here, we describe the process for creation of dialog
state that is not dependent on the method."*

Hope its clear to you now, or for more info you can go for RFC's.

Thanks
Nitin Kapoor



2008/12/26 Raj Jain <rj2807 at gmail.com>

> > On Thu, Dec 25, 2008 at 12:01 PM, Elango <elango_tn at yahoo.co.in> wrote:
> >> Please help me understand why SIP registration does not create a dialog.
>
> Actually, there is some history here and I think that's what causes
> this question to come up again and again.I've been told that there was
> once an intent by some of the authors of SIP to have registrations
> create dialogs (it seems that they wanted to track the UAC that
> created the registration dialog or have a registration context as
> opposed to just indexing on the AOR).
>
> That said, this philosophy obviously didn't work out and as SIP stands
> today -- registrations do not create dialogs.
>
> --
> Raj Jain
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