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Lecture 12

EE 284 Lecture 12: sip notes 2
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Department
Electrical Engineering
Course Code
EE 284
Professor
balaji

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The invite method can also be used for establishing a multiparty call. Once it has received a
final response to an INVITE, the client that initi- ated the INVITE sends an ACK. The
ACK method is used as a confirmation that the final response has been received. For
example, if the response to an INVITE indicates that the called user is busy and the call
cannot be com- pleted at that time, then the calling client will send an ACK. On the other
hand, if the response to the INVITE indicates that the called user is being alerted or that
the call is being forwarded, then the client does not send ACK, because such responses
are not considered final.
The BYE method terminates a session. The method can be issued by either the calling or
called party and is used when the party in question hangs up.
The OPTIONS method queries a server as to its capabilities. This method could be used,
for example, to determine whether a called user agent can support a particular type of
media or how a called user agent would respond if sent an INVITE. In such a case, the
response might indi- cate that the user can support certain types of media or perhaps that
the user is currently unavailable.
The CANCEL method is used to terminate a pending request. For exam- ple, CANCEL
could be used to terminate a session where an INVITE has been sent, but a final response
has not yet been received.
The REGISTER method is used by a user-agent client to log in and reg- ister its address
with a SIP server, thereby letting the registrar know the address at which the user can be
reached. The user-agent client might reg- ister with a local SIP server at startup, with a
known registrar server whose address is configured within the user agent, or by
multicasting to the “all-SIP-servers” multicast address.
A client can register with multiple servers and a given client can have multiple
registrations at a single server. Such a situation would occur in the event that a user has
logged in at several terminals or devices. If a user has multiple active registrations, then
calls to the user may be sent to all regis- tered destinations. This capability can enable a
“one-number” service where a user publishes just a single number, but when that number
is called, the user’s office phone, home office phone, and wireless phone all ring. Fig- ure
5-7 shows an example of how this would work and also shows the usage of the CANCEL
message in such a scenario.
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The SIP INFO method is specified in RFC 2976. This method is a means for transferring
information during an ongoing session, that is, in the mid- dle of a call. Examples of its use could
include
The transfer of Dual-Tone Multifrequency (DTMF) digits
The transfer of account balance information
The transfer of midcall signaling information generated in another net- work (such as
the PSTN) and passed to the IP network via a gateway
SIP RESPONSE
The start line of a SIP response is a status line. This contains a status code, which is a
three-digit number indicating the outcome of the request. The start line will also
contain a reason phrase, which provides a textual description of the outcome. The
reason code will be interpreted and acted upon by the client software, while the
reason-phrase could be presented to the human user to aid in understanding the
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Description
The invite method can also be used for establishing a multiparty call. Once it has received a final response to an INVITE, the client that initi ated the INVITE sends an ACK. The ACK method is used as a confirmation that the final response has been received. For example, if the response to an INVITE indicates that the called user is busy and the call cannot be com pleted at that time, then the calling client will send an ACK. On the other hand, if the response to the INVITE indicates that the called user is being alerted or that the call is being forwarded, then the client does not send ACK, because such responses are not considered final. The BYE method terminates a session. The method can be issued by either the calling or called party and is used when the party in question hangs up. The OPTIONS method queries a server as to its capabilities. This method could be used, for example, to determine whether a called user agent can support a particular type of media or how a called user agent would respond if sent an INVITE. In such a case, the response might indi cate that the user can support certain types of media or perhaps that the user is currently unavailable. The CANCEL method is used to terminate a pending request. For exam ple, CANCEL could be used to terminate a session where an INVITE has been sent, but a final response has not yet been received. The REGISTER method is used by a useragent client to log in and reg ister its address with a SIP server, thereby letting the registrar know the address at which the user can be reached. The useragent client might reg ister with a local SIP server at startup, with a known registrar server whose address is configured within the user agent, or by multicasting to the allSIPservers multicast address. A client can register with multiple servers and a given client can have multiple registrations at a single server. Such a situation would occur in the event that a user has logged in at several terminals or devices. If a user has multiple active registrations, then calls to the user may be sent to all regis tered destinations. This capability can enable a onenumber service where a user publishes just a single number, but when that number is called, the users office phone, home office phone, and wireless phone all ring. Fig ure 57 shows an example of how this would work and also shows the usage of the CANCEL message in such a scenario.
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