SIP interview questions with sample answers
  • In your own words, explain what SIP is and how it’s used. …
  • What is the significance of the Call-ID header field? …
  • Describe the differences between transactions, dialogs and sessions. …
  • Explain the advantages of using SIP URIs and how they compare to IP addresses.

SIP (Session Initiation protocol) Interview Questions | Part – 1

Question 1. What Is Sip?

Answer :

The Session Initiation Protocol (SIP) is a signaling, presence and instant messaging protocol developed to set up, modify, and tear down multimedia sessions, request and deliver presence and instant messages over the Internet. SIP is a text-encoded protocol.

Question 2. What Is Request-uri?

Answer :

The Request-URI is a special form of SIP URI and indicates the resource to which the request is being sent, also known as the request target.

Question 3. What Is The Significance Of The Via Header Field?

Answer :

Each SIP device that originates or forwards a SIP message stamps its own address in a “Via” header field, usually written as a host name that can be resolved into an IP address using a DNS query. The Via header field contains the SIP version number (2.0), a “/”, then UDP for UDP transport, a space, then the hostname or address, a colon, then a port number. And also contain “branch” parameter.

Question 4. Where The Branch Parameter Is Used And What Is Its Significance?

Answer :

A “branch” parameter is used in “Via” header field. It is signified as the transaction identifier used to correlate the reponses to the request.

Question 5. What Is The Significance Of Max-forwards Header Field?

Answer :

Max-Forwards header field is used for simple loop detection. It is initialized to some large integer and decremented by each SIP server which receives and forwards the request.

Question 6. What Are The Significance Of To And From Header Fields?

Answer :

To and From header fields show the originator and destination of the SIP request.

Question 7. Why The Name Labels Will Be Used In To And From Header Fields?

Answer :

When a name label is used, the SIP URI is enclosed in brackets and used for routing the request.The name label could be displayed during alerting, but is not used by the protocol.

Question 8. What Is The Sigificance Of The Call-id Header Field?

Answer :

The Call-ID header field is an identifier used to keep track of a particular SIP session. The originator of the request creates a locally unique string, then usually adds an “@” and its host name to make it globally unique. In addition to the Call-ID, each party in the session also contributes a random identifier, unique for each call. These identifiers, called tags, are included in the To and From header fields as the session is established.

The user agent that generates the initial INVITE to establish the session generates the unique Call-ID and From tag. In the response to the INVITE, the user agent answering the request will generate the To tag. The combination of the local tag (contained in the From header field), remote tag (contained in the To header field), and the Call-ID uniquely identifies the established session, known as a “dialog.” This dialog identifier is used by both parties to identify this call because they could have multiple calls set up between them. Subsequent requests within the established session will use this dialog identifier.

Question 9. What Is Dialog Identifier?

Answer :

The combination of the local tag (contained in the From header field), remote tag (contained in the To header field), and the Call-ID uniquely identifies the established session, known as a “dialog.” This dialog identifier is used by both parties to identify this call because they could have multiple calls set up between them. Subsequent requests within the established session will use this dialog identifier.

Question 10. When The Cseq Header Field Value Will Be Incremented? Or What Is The Significance Of Cseq Header Field?

Answer :

CSeq or Command Sequence contains a number followed by method name.This number is incremented for each new request sent.

Question 11. What Are The Minimum Required Header Fields In Any Sip Message?

Answer :

Via, Max-Forwards, To, From, Call-ID and CSeq are the minimum required header fields.

Question 12. What Is The Significance Of Contact Header Field?

Answer :

Contact header field contains the device-URI where the originator of request or response can be directly reached. Usually contains IP address of the device which generated the request or response.

Question 13. What Is Received Parameter And In Which Header Field It Will Be Appeared?

Answer :

Usually the received parameter can be found in Via header field of the response message. This parameter contains the literal IP address that the request was received from, which typically is the same address that the URI in the Via resolves using DNS.

Question 14. Is The To And From Header Fields Define The Direction Of The Sip Message?

Answer :

No, the To and From header fields in SIP are defined to indicate the direction of the request, not the direction of the message.

Question 15. Is The Branch Parameter In Via Header Field Of The Ack Sent For 200ok Would Be Same As That Of Invite In A Session?

Answer :

No, branch parameter in Via header field of the ACK would be different than the INVITE message of that session because ACK sent to acknowledge the 200OK is considered as a seperate transaction.

Question 16. Is The Branch Parameter In Via Header Field Of The Bye Message Would Be Same As Ack Sent For 200ok And That Of Invite In A Session?

Answer :

No, because BYE message is considered as a new transaction.

Question 17. What Is Dialog?

Answer :

A dialog is a peer-to-peer SIP relationship between two UAs that persists for some time.  A dialog is established by SIP messages, such as a 2xx response to an INVITE request.  A dialog is identified by a call identifier, local tag, and a remote tag.  A dialog was formerly known as a call leg in RFC2543.

Question 18. What Is A Session?

Answer :

A multimedia session is a set of multimedia senders and receivers and the data streams flowing from senders to receivers. A multimedia conference is an example of a multimedia session.

Question 19. What Is The Difference Between Dialog And Session?

Answer :

A dialog is a sip relationship. A session is a media relationship. Dialog mean a “call has already been setup” in the signaling sense, while a session mean the “media has already been setup” in the media sense.

Question 20. What Does “part Of A Dialog” Mean? Are “invite”, “ack” And The Responses Considered “part Of A Dialog”?

Answer :

Dialog represents state, which includes sequence numbers, route sets, and URIs. A request sent as part of a dialog means that it includes route headers from the route set of the dialog, and uses the next highest sequence number in the dialog.

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FAQ

What is SIP protocol?

Answer : The Session Initiation Protocol (SIP) is a signaling, presence and instant messaging protocol developed to set up, modify, and tear down multimedia sessions, request and deliver presence and instant messages over the Internet.

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