SIP Interview Questions & Answers

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SIP Interview Questions & Answers

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SIP Interview Questions

SIP Interview Questions
    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.

    2. 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.

    3. 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.

    4. 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.

    5. 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.

    6. 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.

    7. 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.

    8. 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.

    9. 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.

    10. 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.

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

      Answer :

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

    12. 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.

    13. 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.

    14. 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.

    15. 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.

    16. 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.

    17. 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.

    18. 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.

    19. 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.

    20. 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.'

    21. Question 21. What Does "outside The Dialog" Mean Then? Is A Request Is Outside The Dialog, Does It Still Affect Or Change Anything "inside The Dialog" Then?

      Answer :

      Outside the dialog means that it is not within the context of a dialog. It doesnt use the callid, cseq, or route sets of any existing dialogs, and when such a reuqest is set, it has no impact on the state of an existing dialog.

    22. Question 22. What Is The Difference Between Transaction, Dialog And Session?

      Answer :

      A transaction refers to a fundamental unit of message exchange. It basically includes a request-response cycle. 

      A Dialog represents a peer-to-peer relationship between two user agents.It is usually created through generation of "non-failure" responses to request. 

      A session refers to the exchange of media between two or more Endpoints. Also, there can be dialogs without SIP sessions.

    23. Question 23. What Is The Advantage Of Using Sip Uris? Over Ip Address?

      Answer :

      A request routed using only IP addresses will reach only one end point—only one device. Since communication is typically user-to-user instead of device-to-device, a more useful addressing scheme would allow a particular user to call another particular user, which would result in the request reaching the target user regardless of which he is currently using, or if he has multiple devices.SIP URI is a name that is resolved to an IP address by using SIP proxy server and DNS lookups at the time of the call.

    24. Question 24. What Is Sip Uri?

      Answer :

      SIP URI is a name that is resolved to an IP address by using SIP proxy server and DNS lookups at the time of the call.

    25. Question 25. What Is Sip Proxy Server?

      Answer :

      A SIP proxy server sits in the middle of SIP message exchange, receives messages and forwards them, but it does not set up or terminate sessions. A proxy server typically has access to a database or a location service to aid it in processing the request (determining the next hop). Databases could contain SIP registrations, presence information, or any other type of information about where a user is located.

    26. Question 26. What Is Stateful Proxy?

      Answer :

      A stateful proxy server keeps track of requests and responses received in the past and uses that information in processing future requests and responses. For example, a stateful proxy server starts a timer when a request is forwarded. If no response to the request is received within the timer period, the proxy will retransmit the request, relieving the user agent of this task. A stateful proxy usually sends a 100 Trying response when it receives an INVITE.

    27. Question 27. What Is Stateless Proxy?

      Answer :

      A stateless proxy server processes each SIP request or response only based on the message contents. Once the message has been parsed, processed, and forwarded or responded to, no information about the message is stored—no dialog information is stored. A stateless proxy never retransmits a message, and does not use any SIP timers.A stateless proxy never sends a 100 Trying response.

    28. Question 28. What Is Transaction Stateful Proxy?

      Answer :

      A transaction stateful proxy keeps state about a transaction but only for the duration that the request is pending. For example, a transaction stateful proxy would keep state when it receives an INVITE request until it received a 200 OK or a final failure response (e.g., 404 Not Found). After that, it would destroy the state information. This allows a proxy to perform useful search services but minimize the amount of state storage required.

    29. Question 29. How The Proxy Server Is Different From A User Agent Or Gateway?

      Answer :

      A proxy server is different from a gateway or user agent in three key ways:

      1. A proxy does not issue request, it only responds to requests from user agents. (A CANCEL request is an exception to this rule)
      2. A proxy server has no media capabilities.
      3. A proxy server does not parse message bodies, it relies exclusively on message header fields.

    30. Question 30. What Is Outbound Proxy Server?

      Answer :

      The proxy server to which the user agent sends the SIP request is called the outbound proxy server. This proxy server will authenticate the user agent and may pull up a profile of the user and apply outbound routing services.

    31. Question 31. What Is Inbound Proxy Server?

      Answer :

      The proxy server from which the user agent recieves the SIP request is called the inbound proxy server.

    32. Question 32. What Is Forking Proxy Server?

      Answer :

      a proxy server that receives an INVITE request, then forwards it to a number of locations at the same time. This “forking” proxy server keeps track of each of the outstanding requests and the response to each. This is useful if the location service or database lookup returns multiple possible locations for the called party that need to be tried. For example forking proxy F1 is received from UA1 which forks to two user agents UA2 & UA3. Each user agent begins alerting and sending back two provisional resposes(180 Ringing) to F1. The two responses are identical except for having different To tags and Contact URIs. Finally, one of the two UAs answers and sends a 200 OK response. The forking proxy server sends a CANCEL to the second UA to stop that phone alerting. If both UAs had answered, the forking proxy would have forwarded both 200 OK responses back to the caller who then would have had to choose which one, probably accepting one and sending a BYE to the other one.

    33. Question 33. Explain About Session-expires Header Field?

      Answer :

      A SIP session timer has been proposed to limit the time period over which a stateful proxy must maintain state information. In the initial INVITE request, a Session-Expires header field indicates a timer interval after which stateful proxies may discard state information about the session. User agents must tear down the call after the expiration of the timer. The caller can send re-INVITEs to refresh the timer, enabling a “keep alive” mechanism for SIP. This solves the problem of how long to store state information in cases where a BYE request is lost or misdirected.

    34. Question 34. What Is Redirect Server?

      Answer :

      A redirect server is a type of SIP server that responds the requests but does not forwards the requests. Like proxy server, a redirect server uses a database or location service to look up a user. The location information, however, is sent back to the caller in a redirection class response (3xx), which, after the ACK, concludes the transaction.

    35. Question 35. Is The Ack Sent To Non-2xx Final Response Is Considered To Be Part Of The Same Transaction As Invite?

      Answer :

      Yes, the ACK sent to non-2xx final response is considered to be part of the same transaction as INVITE. ACK request reuses the same branch ID as INVITE. Only an ACK sent in response to a 200 OK is considered a separate transaction with a unique branch ID. Also an ACK to a non-2xx final response is a hop-by-hop response, not an end-to-end response.

    36. Question 36. How Would You Describe Its Potential?

      Answer :

      Henderson -IMS, with its universal appeal to both wireline and wireless carriers, will form the basis for achieving the ultimate goal of telecommunication carrier operations: a single enhanced services architecture for delivering any service, using any media, to reach any customer, regardless of how they connect to the network.

      Longhenry -It is very likely that IMS will be deployed, in some fashion, by most major carriers over the next five years. It is the only standards-based method of addressing the fundamental challenge with which carriers struggle when considering application roadmaps. That is, how to minimize the cost of new application deployments so that they can profitably address the opportunities available in micro-segments of their subscriber base.

      McCracken -The potential for IMS is one of simplification for the service provider and end customer. Simplification for the service provider lies in the fact that IMS will establish a horizontal set of capabilities that can be applied to all the networks they run and to all the services they want to deploy. For well established service providers, reaching this potential will not happen overnight because it may not make business sense to take existing services and migrate them to IMS. But IMS will help new services tremendously. Customers will enjoy a simplified communications experience because they will have a single profile to manage and they will enjoy the same services across any network and device, so they can focus on the task at hand, as opposed to figuring out the technology.

      Hluchyj -The potential for IMS is enormous, as it fundamentally changes the communications paradigm. Voice, data, and video can all be converged in an IMS landscape. From a service provider perspective, the delivery model is far more efficient and dynamic. From a subscriber perspective, they have access to an experience that is simply not available with today's legacy technology. 

      Burger -IP will continue to be a disruptive technology to traditional telecom operators for years to come and help to transform operator business models from voice-driven to service-driven businesses. Operators require more applications to meet competitive threats and will need a better environment for creating and deploying high value multimedia services. IMS addresses this need.

      Scott Erickson, President, IMS Service Delivery Solutions, Telcordia -IMS has the potential to change communications as we know it. Carriers now have the opportunity to cast themselves free of the costly underlying network hardware and exert direct and flexible control over their services through software. IP-based communications provide carriers with the flexibility and openness to quickly offer new revenue generating services, while enabling carriers to lower their total cost of doing business, increase their revenue potential, realize higher productivity and reduce churn.

      Marinho -Lucent’s primary market research indicated a pent up demand for the type of blended lifestyle services that IMS can enable. For example, U.S. service providers have a potential market for converged services worth an estimated $10 billon dollars, with that figure being potentially reached five years after such services are introduced to the consumer and enterprise markets.

      Gray -Ubiquity believes that IMS will have significant potential based on the enhanced services it will enable. It has potential to enrich the personal lives of consumers and potential to enrich the productivity of business and enterprise users, changing the world of communications as we know it. This change has the potential to drive new economic growth in our industry.

    37. Question 37. What Are Some Potential “killer Apps” That Ims Might Enable?

      Answer :

      Henderson -IMS will facilitate new multimedia services, accelerate the migration of legacy TDM services to IP, and make any telecom service more seamless and ubiquitous using any type of access device. Services involving multimedia, presence, and follow-me features will no longer have the technical barriers between access networks that existed before IMS.

      Longhenry -There are no true “killer apps” for IMS, as any application that can be delivered over IMS can also be delivered via a stand alone proprietary application server. Rather, the “killer ROI case” is that IMS can enable carriers to deploy ten applications from a common standardized (i.e., cost competitive) platform, instead of from ten distinct platforms requiring ten separate integration efforts.

      McCracken -Some of the most compelling applications will be ones that capitalize on the ability of IMS networks to bring voice, video, and data streams together to deliver so called “rich media services” across multiple access networks and devices. These services will bring these traditionally separate capabilities together and enable more compelling person-to-person communications than were ever possible on separate networks. IMS provides a common framework that can be leveraged across all the access methods and devices that people want to use for their personal and business communications. It breaks down the proprietary “service silos” that have existed for years.

      Hluchyj -It doesn't really matter what the next “killer app” is -that is something that is always going to change. What’s great about IMS is that it employs a distributed model where application creation mirrors Web application development. In the old telecom world, introducing new services was expensive and risky. In an IMS world, the cost of experimentation is driven way down, and when the cost of experimentation is lowered, the opportunity for innovation expands exponentially. The next killer app will always be right around the corner.

      Burger -IMS is all about SIP-based applications. Potential applications include audio and video conferencing, video messaging, hosted services, multi-player gaming, and ubiquitous services/personalized content. By providing a standard communications infrastructure based on IP, IMS allows multimedia application convergence

      Scott Erickson, President, IMS Service Delivery Solutions, Telcordia -For carriers, the real ‘killer app’ is the ability to provide a myriad of exciting IP-enabled next generation services quickly, such as push-to-video, mobile TV, multi-party gaming, presence and location-based services, TV Caller ID, and converged VPNs. IMS provides carriers with the agility, flexibility and speed-to-delivery that is critical for them to drive increased revenue from these new offers and secure their future growth and viability.

      Marinho -The search for the “killer app” may not be applicable to a broad market, as we traditionally might think. The key is identifying the “right” combination of services that addresses demand in specific market segments. IMS-based services, for example, can enable friends to plan a night out in real-time, including discussing the possibilities, viewing a movie trailer and purchasing tickets, picking a restaurant and making the reservation, and receiving custom directions to the restaurant.

      Gray -IMS not only enables the killer app, but it enables the “Killer Capability”, meaning that multiple applications or blended applications are enabled by IMS. Given Ubiquity’s role in enabling application development and rapid service creation, we see “Instant Multimedia Sharing” applications such as music, video, and voice being widely adaopted.

    38. Question 38. Who Stands To Benefit The Most From Ims? Carriers? Equipment Manufacturers? End Users?

      Answer :

      Henderson -IMS is a positive for the entire telecommunications industry. Carriers, and eventually enterprise organizations, will benefit from unified service architecture. Equipment manufacturers will become more efficient, focusing resources on a single IMS product line, rather then different products for different markets. End users, particularly business users, will benefit from seamless services, regardless of access or device.

      Longhenry -End users will ultimately benefit most from IMS, as they will gain access to exponentially more applications could be commercially viable in a non-IMS world. The best analogy is probably the user experience available from DoCoMo and iMode, as third party applications can be quickly developed, integrated, and delivered to a fad-oriented subscriber base. IMS enables this in a standardized fashion. 

      McCracken -The ones who will benefit most are the service providers and the end customers. Service providers will have a core network that finally separates session control from the bearer path and the services. This creates an architecture more suited to bringing new applications to market more quickly, and reduces a lot of traditional duplication. End users will benefit because they will have simplified communications that will be personalized across any network and device. They will be able to enjoy a much richer multimedia communications experience that will no longer be dependent on a specified access method tied to a specific device. Equipment manufacturers will face greater competition as proprietary architectures are broken apart with IMS, opening up new opportunities for nimble companies to shine with innovative services.

      Hluchyj -The benefits really extend across the industry. By standardizing the different network components, service providers can pick the pieces we want and manufacturers can focus on their core competency areas. The result is lower cost infrastructure and a much greater variety of services that can be rapidly deployed, which benefits the consumers.

      Burger -If everyone can agree to standardize on IMS, then the whole industry benefits. Carriers will be able take advantage of all of the benefits of open systems equipment, such improved ROI, faster time to deployment, and best of breed solutions. End users will benefit from new and innovative personalized services, accessible anytime and anywhere. Equipment manufacturers will benefit from supplying products that enable the deployment of new services

      Scott Erickson, President, IMS Service Delivery Solutions, Telcordia -Carriers. Wireless carriers will speed new services to market. Wireline carriers will be able to increase customer satisfaction through bundled offers. MVNOs will launch themselves more rapidly into niche markets while simplifying service creation. MSOs will evolve by using IMS to compete with both wireline and wireless carriers. All will benefit from the added revenue from new services and increased customer satisfaction.

      Marinho -End users benefit from an improved communications experience geared to their personal needs and requirements. Carriers benefit from the pent up demand, market opportunity, and “stickiness” of IMS services. Manufacturers benefit from the adoption of the technology and increased network capacity demand that is driven by IMS based services.

      Gray -Many stakeholders will benefit from IMS. First, and foremost, End Users will benefit by having new services, more services, and more choice in services. Moreover, End Users will benefit by the personal and professional productivity provided by enhanced services. Once successful business models are established, the pull through effect will benefit carriers, equipment providers, and application developers.

    39. Question 39. What Is The Timeline For Real Commercial Deployments Of Ims?

      Answer :

      Henderson -Carriers are already purchasing IMS-compatible components and systems, and realizing early benefits from IMS architecture principles in small pilot deployments and specific services. However, fully interoperating, standards-based, multi-vendor IMS solutions are probably still a few years out.

      Longhenry -IMS deployments would be happening across the telecommunications environment today if IMS client architecture was on par with IMS server infrastructure. Unfortunately, the concept of an IMS client has not yet gained sufficient momentum in the standardization process and, therefore, advanced client-server IMS applications cannot yet be seamlessly delivered to subscribers. As this is addressed in the next 24 months, IMS deployments will rapidly accelerate.

      McCracken -Technically, there are commercial deployments today for some initial IMS components, like a barebones CSCF (Call Session Control Function) in support of push-to-talk services, but these deployments are only starting to whet the appetite for what’s to come later in 2006 and into 2007. Once SIP-based session control is used for more services than PTT, that’s when other elements, like a Home Subscriber Server (HSS), will start to show their value. IMS will co-exist with today’s networks for many years to come, and services will be migrated as the business evolves. Ironically, IMS may see quicker deployment by fixed service providers because they can leverage their broadband access networks and more powerful devices to deliver some of the multimedia capabilities sooner than wireless providers.

      Hluchyj -Some solutions are IMS-ready today, and network operators are already laying the groundwork of their IMS deployments. That said, we anticipate that the delivery of enhanced applications built on IMS will be a 2007/2008 event.

      Burger -Early commercial deployments are actually happening now. Brooktrout/Excel worked with TMN, the largest provider in Portugal, to deliver one of the first live 3G video messaging services. The goal was to deliver real-time video services in time for the European soccer championships. For TMN, this was an opportunity for a new revenue-generating service, since voice is rapidly becoming a commodity.

      Scott Erickson, President, IMS Service Delivery Solutions, Telcordia -IMS is already seeing significant global momentum. Some carriers have already begun to make inroads in IMS-ready deployments. The beauty of IMS is that carriers can proceed at the pace that is right for them, their business, and their customers, based on their existing network investments and customers’ needs.

      Marinho -We expect to see commercial services based on IMS solutions begin to roll out later this year, and the momentum will pick up in 2007.

      Gray -Based on evaluations and lab trials, we would expect to see real deployments in the next 12 – 24 months. Ubiquity is involved now in helping carriers plan, develop and deploy new services, even pre-IMS, to realize revenues and services today that will be IMS-ready when IMS networks are deployed, regardless of the timeframe.

    40. Question 40. What Is The Significance Of Update Method?

      Answer :

      The UPDATE method is used to modify the state of a session without changing the state of the dialog. A session is established in SIP using an INVITE request in an offer/answer manner. Typically, a session offer is made in the INVITE and an answer made in a response to the INVITE. In an established session, a re-INVITE is used to update session parameters. However in pending session (INVITE sent but no final response received) UPDATE is used to update session parameters. Basically UPDATE is used to perform QoS and negotiate end-to-end attributes prior to session establishment.

    41. Question 41. What Is The Significance Of Prack Method?

      Answer :

      The PRACK method is used to acknowledge receipt of reliably transported provisional responses (1xx). The reliability of 2xx, 3xx, 4xx, 5xx, and 6xx responses to INVITEs is achieved using the ACK method. However, in cases where a provisional response, such as 180 Ringing, is critical in determining the call state, it may be necessary for the receipt of a provisional response to be confirmed. The PRACK method applies to all provisional responses except the 100 Trying response, which is never reliably transported. A PRACK is generated by a UAC when a provisional response has been received containing a RSeq reliable sequence number and a Supported: 100rel header. The PRACK copy the number in the RSeq and the CSeq of the response in a RAck header.

    42. Question 42. What Is The Significance Of Info Method?

      Answer :

      The INFO method is used by a user agent to send call signaling information to another user agent with which it has an established media session. This is different from a re-INVITE since it does not change the media characteristics of the call. The request is end-to-end, and is never initiated by proxies. A proxy will always forward an INFO request—it is up to the UAS to check to see if the dialog is valid. INFO requests for unknown dialogs receive a 481 Transaction/Dialog Does Not Exist response.

    43. Question 43. What Is The Significance Of Message Method?

      Answer :

      The MESSAGE method is used to transport instant messages (IM) using SIP. IM usually consists of short message exchanged in near-real time by participants engaged in a "conversation." MESSAGEs may be sent within a dialog or outside a dialog, but they do not establish a dialog by themselves.

    44. Question 44. What Is The Significance Of Notify Method?

      Answer :

      The NOTIFY method is used by a user agent to convey information about the occurrence of a particular event. A NOTIFY is always sent within a dialog when a subscription exists between the subscriber and the notifier. A NOTIFY request normally receives a 200 OK response to indicate that it has been received. If a 481 Dialog/Transaction Does Not Exist response is received, the subscription is automatically terminated and no more NOTIFYs are sent. NOTIFY requests contain an Event header field indicating the package and a Subscription-State header field indicating the current state of the subscription.

    45. Question 45. What Is The Significance Of Subscribe Method?

      Answer :

      The SUBSCRIBE method is used by a user agent to establish a subscription for the purpose of receiving notifications (via the NOTIFY method) about a particular event. A successful subscription establishes a dialog between the UAC and the UAS. The subscription request contains an Expires header field, which indicates the desired duration of the existence of the subscription. After this time period passes, the subscription is automatically terminated. The subscription can be refreshed by sending another SUBSCRIBE within the dialog before the expiration time.

      A server accepting a subscription returns a 200 OK response also containing an Expires header field. The expiration timer can be the same as the request, or the server may shorten the interval, but it may not lengthen the interval. There is no "UNSUBSCRIBE" method used in SIP—instead a SUBSCRIBE with Expires:0 requests the termination of a subscription and hence the dialog. A terminated subscription (either due to timeout out or a termination request) will result in a final NOTIFY indicating that the subscription has been terminated.

    46. Question 46. What Is The Significance Of Refer Method?

      Answer :

      The REFER method is used by a user agent to request another user agent to access a URI or URL resource. The resource is identified by a URI or URL in the required Refer-To header field. A REFER request can be sent either inside or outside an existing dialog.

    47. Question 47. What Is The Significance Of Options Method?

      Answer :

      The OPTIONS method is used to query a user agent or server about its capabilities and discover its current availability.

    48. Question 48. What Is The Significance Of Cancel Method?

      Answer :

      The CANCEL method is used to terminate pending searches or call attempts. It can be generated by either user agents or proxy servers provided that a 1xx response containing a tag has been received, but no final response has been received. The branch ID for a CANCEL matches the INVITE that it is canceling. A CANCEL only has meaning for an INVITE since only an INVITE may take several seconds (or minutes) to complete. All other SIP requests complete immediately. A user agent confirms the cancellation with a 200 OK response to the CANCEL and replies to the INVITE with a 487 Request Terminated response.

    49. Question 49. What Is The Significance Of Ack Message?

      Answer :

      The ACK method is used to acknowledge final responses to INVITE requests. Final responses to all other requests are never acknowledged.An ACK may contain an application/sdp message body. This is permitted if the initial INVITE did not contain a SDP message body. If the INVITE contained a message body, the ACK may not contain a message body. For 2xx responses, the ACK is end-to-end, but for all other final responses it is done on a hop-by-hop basis when stateful proxies are involved. A hop-by-hop ACK reuses the same branch ID as the INVITE since it is considered part of the same transaction. An end-to-end ACK uses a different branch ID as it is considered a new transaction.

    50. Question 50. What Is The Significance Of Bye Message?

      Answer :

      The BYE method is used to terminate an established media session. A BYE is sent only by user agents participating in the session, never by proxies or other third parties. It is an end-to-end method, so responses are only generated by the other user agent.

    51. Question 51. How To Cancel All The Registrations Using A Single Register Message?

      Answer :

      The contact header field is equal to * and Expires: 0

    52. Question 52. What Is Third Party Registration?

      Answer :

      If the UA sending the registration request on behalf of other UA it is called third party registration. In this case, the From header will contain the URI of the party submitting the registration on behalf of the party identified in the To header.

    53. Question 53. What Is The Significance Of The Expires Header In Register Message?

      Answer :

      The presence of an Expires header sets the expiration of Contacts with no expires parameter. If an expires parameter is present, it sets the expiration time for that Contact only.

    54. Question 54. What Is The Registration And Why It Is Required?

      Answer :

      The registration creates a temporary binding between the Address of Record (AOR) URI in the To header and the device URI in the Contact header.The binding of  Registration is not required to enable a user agent to use a proxy server for outgoing calls. It is necessary, however, for a user agent to register to receive incoming calls from proxies that serve that domain unless some non-SIP mechanism is used by the location service to populate the SIP URIs and Contacts of end-points.

    55. Question 55. What Is The Significance Of The Register Message?

      Answer :

      The REGISTER method is used by a user agent to notify a SIP network of its current Contact URI (IP address).

    56. Question 56. What Is The Significance Of Invite Message?

      Answer :

      The INVITE method is used to establish media sessions between user agents. An INVITE usually has a message body containing the media information of the caller. If an INVITE does not contain media information, the ACK contains the media information of the UAC. If the media information contained in the ACK is not acceptable, then the called party must send a BYE to cancel the session—a CANCEL cannot be sent because the session is already established.

    57. Question 57. What Is The Significance Of The Expires Header In An Invite Message?

      Answer :

      An Expires header in an INVITE indicates to the UAS how long the call request is valid. For example, the UAS could leave an unanswered INVITE request displayed on a screen for the duration of specified in the Expires header. Once a session is established, the Expires header has no meaning—the expiration of the time does not terminate the media session. Instead, a Session-Expires header can be used to place a time limit on an established session.

    58. Question 58. If The Re-invite Is Failed Or Refused, Will The Call/session Discontinued?

      Answer :

      No, If a re-INVITE is refused or fails in any way, the session continues as if the INVITE had never been sent.

    59. Question 59. Can You List Out The Sip Methods?

      Answer :

      The INVITE, REGISTER, BYE, ACK, CANCEL, and OPTIONS methods are the original six methods in SIP. The REFER, SUBSCRIBE, NOTIFY,MESSAGE, UPDATE, INFO, and PRACK methods are described in separate RFCs.

    60. Question 60. What Is Turn Protocol?

      Answer :

      Traversal Using Relay NAT (TURN) protocol allows a client to obtain a transport IP address and port that it can receive packets sent from a single IP address in the public Internet. For some NAT topologies such as a client behind a symmetric NAT, using a relay located in the public Internet is the only approach that allows communication to take place.

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