SIP is a protocol used to initiate and manage calls in a communications network. Sip trunk is based on the Session Initiation Protocol. It was developed in the late 1990s, and it has since been widely deployed. In this article, we will cover the Refer-To request header and the SIP INVITE request to the SIP Server 122. We will also discuss the actions that SIP devices take during the first and second dialogs and the MSML conferencing object.
Refer-To request header
The Refer-To request header for session initiation protocols (SIP) is a standardized request header field that specifies the URL of a third party. Refer-To requests are accepted for both inbound and outbound calls. The Refer-To header can be as long as 400 symbols. Refer-To request headers are used in many different applications, including call transfers.
Refer-To headers can indicate who is to be replaced by an existing dialog. This request header may also be used to specify whether the dialog being replaced belongs to the same party. If the REFER request has a Replaces header, it must be matched with the original dialog. In addition, it must reference a valid Refererred-By Authenticated Identity Body. The UA or UAS may then apply a local policy to authorize the rest of the request.
SIP INVITE request to SIP Server 122
When making a call over SIP, it is necessary to follow some rules that ensure a successful connection. A SIP INVITE request must include the following headers. The To header should contain the callee’s SIP URI. The SIP server then looks up this information and then forwards the Invite to the callee. The SIP server then sends back a SIP Response 100 to the callee.
The SIP INVITE request message contains several fields. The header fields are used to configure the message. The values of the caller’s URI, callee’s URI, globally unique identifier, and sequence number are also included in the SIP request.
SIP device actions during the existence of the first and second dialogs
The first and second dialogs represent different states in the session initiation protocol call control process. The state of the first dialog refers to the state of the communication method, whereas the second dialog refers to the state of the media. The first dialog starts with the user dialing a phone number, which rings at the far end and then answers the call. At step 308, the first dialog is established. The next step is to negotiate the first media between the initiating and receiving devices. Depending on the capabilities of the initiating and receiving devices, this negotiation may be done automatically. However, if the initiating and receiving devices are capable of supporting multiple media types, they exchange information to negotiate the type.
When the first dialog fails, the second dialog is initiated. This step is called connection preservation. It involves preserving the connection state for each media channel. When both call legs become healthy, the state is called “normal”. When any call leg fails to maintain the connection, the state is called “preserved.” In this case, the connection is preserved and the media channels are re-established after the SIP refresh interval expires.
SIP device actions during the existence of the MSML conferencing object
The MSML conferencing object specifies how an audio/video conference should be set up. It also provides a means to use advanced conferencing services. The object contains media resources and state information for each participant, and a single logical output per type of media. For example, an audio conference mix consists of the audio of the participants, while a video conference mix is the audio and video of the other participant.
During the existence of an MSML conferencing object, SIP dialogs are established between an AS and a MS. The user responds to prompts by making DTMF selections. The AS sends a 200 OK response to the MS. SIP device actions during the existence of the MSML conferencing object may be terminated by the AS or MS.
Cisco CallManager actions during the existence of the first and second dialogs
When a call is originating on a Cisco CallManager system, actions should be taken to ensure that the call is answered. These actions include configuring the attendant console and assigning a user ID and password. These actions can be done using the Attendant Console User Configuration window.
The Attendant Console supports dial rules and directory lookup rules. Using these rules, attendants can transform directory numbers and caller IDs into numbers. These rules are configurable in the Attendant Console using the GlobalUI.properties file located in the C:Program FilesCiscoCallManager Attendant Console directory.
Cisco CallManager processes INVITE messages using standard protocol rules. The “Replaces” message header specifies that the first service will send a new dialog when a previous one fails. This message is sent to the second service receives it. The second service receives it and sends it an ACK message. This message completes the session establishment.