how to generate barcode in vb.net 2010 SIP Status Codes in Software

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SIP Status Codes
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Status codes communicate the status of a request, or of a session in progress. They are communicated by either entity, the client or the server agents, as well as any network entities such as proxies. The use of status codes allows the various entities to communicate with one another when there are failures or when a request is being processed. Status codes are just that: they are responses that communicate status. To make status codes more useful and easier to manage (as well as to understand), they have been numbered and put into classifications. There are six classifications for status codes:
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Provisional Successful Redirection Client failures Server failures Global failures
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Each of these classifications is defined in the following sections along with their respective codes. Since these are responses, they are sent when a request has been received. For example, if a user agent client (UAC) sends a request to a user agent server (UAS), then the UAS will send a response communicating the proper status of the request. The format therefore is different than for a request. In a request, the first line of the request identifies the method, or message type. In this example you see an INVITE request that includes the destination of the request (or at least the next hop in the network, depending on the type of routing being used):
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INVITE SIP:travis.russell@tekelec.com SIP/2.0
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The response to this request will look like the next example. The content of the response is dependent on the status and the method of the request. This will be explained in more detail in the sections that follow describing each of the responses.
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SIP/2.0 200 OK
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As you can see, the first line of a request and the first line of the response are very different. I also remind you that these are displayed in clear text. In other words, bitoriented protocols must be decoded to understand what they are, while SIP is displayed as plain English. This means that any device capable of capturing SIP messages and displaying them will be capable of displaying all of the information in the SIP message. This could be a security or a privacy concern for some, so keep this in mind. We will discuss more about SIP security in 7. A typical call flow in a SIP network would look something like the example given in Figure 3.1.
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Deby s UAC 126.18.27.0 deby.russell@aol.com Travis s UAS Proxy 2 Proxy 1 135.18.10.10 139.45.38.10 128.10.10.1 raleigh.bellhead.com Losangeles.hometel.com travis.russell@tcg.com
INVITE INVITE INVITE 200 OK 200 OK 200 OK ACK ACK ACK CONVERSATION BYE BYE BYE 200 OK 200 OK 200 OK
Figure 3.1 SIP simple call flow
SIP Status Codes
1xx Provisional Codes
Provisional codes are sent to alert the client (UAC) that the client s request has been received and is being processed. Note that this may indicate that the request was received by a proxy, but not necessarily the end point (UAS). The purpose of a provisional response is to prevent retransmission of the request. When a UAC initiates a request, it starts a timer. When this timer expires, the UAC will resend the request again, assuming that the original request was lost in the network and never received. The UAC will continue retransmission until a response is received. The UAC retransmits every 200 ms until a response is received. The provisional response can also be used to begin a dialog between two entities. A dialog is not the same as a session. A dialog simply establishes a logical conversation (or dialog) between two entities. This dialog is then used to communicate information about a specific session. We will discuss dialogs in more detail in 5. Since provisional responses are used by proxies to stop retransmission, the UAS (the actual destination of the request) has no knowledge of the provisional response. In fact, the UAS may send its own provisional response if it believes it will take longer than 200 ms to process the request. When a proxy sends a provisional request, it is typically 100 TRYING, while the UAC would send 180 RINGING. The following codes have been defined in various RFCs as provisional responses:
100 Trying The 100 TRYING response is typically sent by a proxy to prevent retransmission. This is illustrated in Figure 3.1. Note that each proxy in the path can send this provisional response, to prevent retransmission from the downstream proxy. In other words, a proxy would send the response while processing the request (processing consists of determining the routing for the request) and then sends the request upstream to the next hop. The next (or upstream) proxy would then start its own timer, and if 200 ms is about to elapse, it will send its own provisional response. Each of the proxies within the path maintain its own retransmission timers, and each is capable of sending its own 100 TRYING response. A typical response would look like:
SIP/2.0 100 TRYING VIA: SIP/2.0/UDP pchome101@aol.com:5060;branch=z9hG4bk74gh5 FROM: Travis Russell <sip:travis.russell@tekelec.com> TO: Deby Russell <sip:deby.russell@aol.com> CALL-ID: 82167534@126.18.27.0 CSEQ: 1 INVITE CONTENT LENGTH: 0
180 Ringing The 180 RINGING response is sent only by the UAS. It indicates that the UAS is attempting to alert the end user of an incoming session request. This could also be used by the UAC for triggering the transmission of ringback tones to the caller. Ringback tones are different than alerts, because they are heard by the calling party. The 180 RINGING provisional response would then be used to cause the ringback tones to be sent by the appropriate application server.
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