SIP Dialogs

Dialogs in SIP are not separate elements, but are inherently built on transactional processes. They play a crucial role in organizing ongoing conversations between two or more participants. A SIP conversation is essentially a series of back-and-forth messages that together constitute a session, such as a call or multimedia conference. Understanding the concept of SIP conversations is essential for developers working with real-time communications applications.

Dialog Interface

A dialog can be likened to a continuous conversation you have with someone over time. Just as you don't reintroduce yourself with every sentence, dialogs allow SIP entities to exchange messages within a session without having to renegotiate terms each time.

Key Features of SIP Dialogs

• Session Management: Dialogs manage sessions, ensuring that the communication flows smoothly from start to finish. This includes keeping track of session states and participant roles.
• Context Preservation: They maintain the context of a conversation. This includes who is involved, the sequence of messages, and any specific settings or parameters agreed upon.
• Efficient Message Routing: Dialogs handle the routing of subsequent messages once the session has been established, making sure messages are sent to the right participant at the right time.

A SIP dialog consists of three main components that uniquely identify and manage the session:

• Call-ID: A unique identifier for the dialog, much like a unique ticket number for an event. It remains constant throughout the session.
• Local tag and Remote tag: Tags assigned by each participant. These act like personal identifiers, distinguishing the messages from each participant within the dialog.
• Dialog State: Tracks the current status of the dialog (e.g., "Early", "Confirmed", "Terminated"), similar to tracking the stages of an event from planning through completion.

Key Methods:

createRequest(String method): Generates a new request message based on the dialog, like BYE or INVITE.

createReliableProvisionalResponse(int statusCode): Creates a reliable provisional response for an ongoing dialog.

getLocalParty() and getRemoteParty(): Return the addresses of the local and remote parties, respectively.

getDialogId(): Returns the dialog identifier, crucial for tracking and managing sessions.

sendRequest(ClientTransaction clientTransaction): Sends a request message through this dialog.

terminateOnBye(boolean terminateFlag): Specifies whether the dialog should terminate upon receiving a BYE message.

Let’s figure out how Dialogs work in SIP

1. Establishing a Dialog

A dialog is typically established when a SIP INVITE request is sent and a 200 OK response is received. This marks the start of a dialog.

In this example user sends an INVITE to start a call, and the recipient accepts by sending back a 200 OK, establishing a dialog for this call session:

Request inviteRequest = messageFactory.createRequest(...); // details omitted
ClientTransaction inviteTransaction = sipProvider.getNewClientTransaction(inviteRequest);
inviteTransaction.sendRequest();

2. Maintaining and modifying the Dialog

Once established, subsequent messages (like re-INVITE or UPDATE requests) can be sent within the dialog to adjust session parameters (e.g., change in media).

These messages use the same Call-ID, local tag, and remote tag to maintain continuity.

Request reInviteRequest = dialog.createRequest("INVITE");
ClientTransaction reInviteTransaction = sipProvider.getNewClientTransaction(reInviteRequest);
reInviteTransaction.sendRequest();

3. Terminating a Dialog

Dialogs are terminated when a BYE message is sent by one of the participants, effectively ending the session.

In this example, at the end of a call, one user sends a BYE request, which is acknowledged by a 200 OK response from the other participant, closing the dialog.

Request byeRequest = dialog.createRequest("BYE");
ClientTransaction byeTransaction = sipProvider.getNewClientTransaction(byeRequest);
byeTransaction.sendRequest();

Dialogs are fundamental in managing long-lived communications such as phone calls or multimedia sessions. They help ensure that messages are associated with the correct session, maintain the state of the session, and facilitate orderly communication between all participants. For developers, manipulating dialogs involves creating requests, managing state transitions, and ultimately terminating the dialog when the session ends. This mechanism is essential for building communication applications using SIP.