Basic Call Flow(Two-Party session):



In this situation, User A calls User B, then User A places the approach hold. Client A then accepts the cancel hold, then  User A hangs up the call. Note  that hold is unidirectional in nature. On the other hand, a UA that places the  other gathering on hold will for the most part additionally quit sending media, coming about  in no media trade between the UAs. More established UAs may set the association location to when starting hold. Then again, this  conduct has been deplored in support or utilizing the a=inactive SDP  quality if no media is sent, or the a=sendOnly trait if media  is still sent.

Additionally take note of the utilization of the rendering highlight tag in Re-Invite (Hold) to show that User B's UA is no more rendering media to B, i.e., that User B has put the approach hold.

Taste Rendering:

sip.rendering", which decidedly depicts whether the client  specialists is rendering any of the media it is getting. Ex: conferencing. It MUST just be utilized as a part of a Contact header field in a dialog made utilizing the INVITE ask. This parameter has three legitimate qualities: "yes", "no", and "obscure".

The worth "yes" demonstrates positive learning that the client specialists is rendering no less than one of the surges of media that it is accepting.

The worth "no" demonstrates positive learning that the client specialists is  rendering none of the media that it is accepting.

The worth  "obscure" demonstrates that the client operators does not know whether the media connected with the session is being rendered (which may be the  case if the client specialists is going about as a 3pcc (Third Party Call Control

Point to be recollected (Hold):

1) A sends re-welcome to B, with new SDP Offer  that incorporates Attribute(a) = SendOnly/Inactive.

2) B acknowledges the Hold ask for with SDP answer that incorporates Attribute(a) = RecvOnly

3) The media session is unidirectional,but B UA doesn't send information.

Point to be recalled (UnHold):

1) A sends re-welcome to B, with new SDP Offer  that does excludes Attribute(a) = SendOnly/Inactive.

2) B acknowledges the UnHold ask for with SDP answer that does excludes Attribute(a) = RecvOnly

3) RTP stream is re-built between A & B.



B needs all calls sent to the Public Switched Telephone  Network (PSTN) (which is only one more URI to the intermediary server). A calls B. The intermediary server reworks the Request URI, and advances the INVITE to a Gateway. Subtle elements of informing behind the  Gateway are not indicated.

Note that the 181 Call is Being Forwarded reaction is demonstrated as sent  by the intermediary. Entirely talking, the intermediary is acting as a client operators for this situation as an intermediary can't create non-100 temporary reactions.

Note likewise that sending could be finished utilizing a divert (302  Moved Temporarily reaction).


B needs calls to B sent to C if B is occupied (this data  is known to the intermediary). A calls B, B is occupied, the intermediary server  spots call to C.


B needs calls to B sent to C if B is not replied (data is known to the intermediary server). A calls B and nobody answers. The intermediary server then places the call to C.

Cancel Request:

    Cancel is best for INVITE requests, which can take a long time to generate a response. In that usage, a UAS that receives a CANCEL request for an INVITE, but has not yet sent a final response, would " stop ringing " , and then respond to the INVITE with a specific error response (a 487).
    Cancel has no effect on a request to which a UAS has already given a final response.

Client Behaviour:

1) A CANCEL request should not be sent to cancel a request other than INVITE.

2) Once the CANCEL is constructed, the client should check whether it has received any response ( provisional or final ) for the request being cancelled.

3) If no provisional response has been received, the CANCEL request must not be sent ; rather, the client must wait for the arrival of a provisional response before sending the request.

Server Behaviour:
1) The processing of a CANCEL request at a server depends on the type of server. A stateless proxy will forward it, a stateful proxy might respond to it and generate some CANCEL request of its own, and a UAS will respond to it.
2) If the UAS did not find a matching transaction for the CANCEL according to the procedure above, it should respond to the CANCEL with a 481 ( Call Leg/Transaction Does Not Exist.
3) If the original request was an INVITE, the UAS should immediately respond to the INVITE with a 487.


In this situation, Bob calls Alice. Alice then exchanges Bob to  Carol, then Alice disengages with Bob. Weave creates the session to Carol then reports the accomplishment back to Alice in the NOTIFY in F15.

On the off chance that the Transfer comes up short, Bob can send another INVITE back to Alice to re- secure the session.

Notwithstanding the BYE sent by Alice in F9, the dialog in the middle of Alice Bob still exists until the membership made by the REFER has  ended (either because of a NOTIFY containing a

Membership State:  terminated;reason=noresource header field, as in F15, or a 481-Call Leg/Transaction doesn't exits  reaction to a NOTIFY).


In this situation, Alice calls Bob. Bounce puts Alice on hold then calls  Carol to report exchange, then places Carol on hold. Sway exchanges  Alice to Carol, which replaces the session in the middle of Bob and Carol.

Tune then separates session with Bob. Alice reports achievement of  exchange to Bob, who then detaches with Alice. In this case, the Replaces header field  is embedded into the Refer-To URI  by Bob. Note that the Refer-To URI is the Contact URI returned by  Carol in the 200 OK reaction F10. This guarantees that just the right occasion of Carol is come to. The vicinity of the gr URI parameter  in the Contact URI in message F10 shows that the Contact URI is a GRUU  and will be comprehensively routable outside of the dialog. Without knowing the Contact URI is a gruu, Bob must be arranged, if  the activated INVITE had fizzled, to retry the REFER with a Refer-To  URI of the URI used to achieve Carol however with a Require: replaces  header got away in the Refer-To header field, as examined in the  exchange record

Allude: The REFER technique demonstrates that the beneficiary ought to contact an outsider utilizing the contact data gave as a part of the appeal.

A REFER ask for MUST contain precisely one Refer-To header field esteem.

A REFER ask for verifiably creates a membership to the allude


A UA tolerating a very much framed REFER appeal SHOULD ask for endorsement

from the client to continue.

On the off chance that approbation is in all actuality, the UA MUST contact the asset distinguished by

the URI in the Refer-To header field.

On the off chance that the approbation is denied,

- An operators reacting to a REFER technique MUST give back a 400 (Bad Request)

on the off chance that the appeal contained zero or more than one Refer-To header field values.

- An operators (counting intermediaries producing nearby responses)MAY return a

100 (Trying) or any proper 4xx-6xx class reaction.

In the event that no last reaction has been produced by tenets above,

the UA MUST give back a 202 Accepted reaction before the REFER exchange terminates.

Allude is the main component that can make a membership to occasion

allude. In the event that a SUBSCRIBE ask for occasion allude is gotten for a

membership that does not as of now exist, it MUST be dismisses with a


Advise Request:

The NOTIFY instrument ,MUST be utilized to advise the operators sending the

Allude of the status of the reference.

Advise Body:

- Each NOTIFY MUST contain an Event header field with an estimation of allude and potentially an id parameter.

- Each NOTIFY MUST contain a collection of sort "message/sipfrag"

in the event that the membership is pending, the body:

Taste/2.0 100 Trying

2. in the event that the reference was fruitful, the body:

Taste/2.0 200 OK

3. in the event that the reference fizzled, or the body:

Taste/2.0 503 Service Unavailable

4. in the event that the REFER solicitation was acknowledged before approbation to take after the

reference could be gotten and that support was accordingly denies

Taste/2.0 603 Declined

In this scenario, Alice and Bob are in a 2-party call (session) when Bob wishes to add Carol into the conversation.  Bob is capable of   media mixing in a 3-party call.  Bob first sends a re-INVITE to Alice, changing Contact URIs to one that indicates Bob's mixer and  acts like a focus.  As a result, 
Bob includes the "isfocus" feature tag .  Bob then INVITEs Carol  using the same Contact URI.  Note 
that Bob could wait to re-INVITE.  Alice until after Carol has answered.  Bob could also put Alice on  
hold before calling Carol.
In this situation, Alice and Bob are in a 2-gathering call and Carol wishes to join, bringing about a 3-gathering call. Tune could have taken in Bob's dialog identifier utilizing some non-SIP implies, or perhaps from a NOTIFY with the dialog bundle sent by Bob. Hymn sends an INVITE to Bob containing a Join header distinguishing the dialog in the middle of Alice and Bob. Weave re-INVITEs Alice to change to center mode and incorporates the "isfocus" highlight tag . Weave then acknowledges the INVITE from Carol, bringing about the 3-way call.