Figure 5-15 SDP inclusion in SIP messages in Visual Studio .NET

Encoder Data Matrix ECC200 in Visual Studio .NET Figure 5-15 SDP inclusion in SIP messages

Figure 5-15 SDP inclusion in SIP messages
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Daniel<sip:Collins@station1workcom>
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Boss<sip:Manager@station2workcom>
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INVITE sip:manager@station2workcom SIP/20 From: Daniel<sip:Collins@station1workcom>; tag = abcd1234 To: Boss<sip:Manager@station2workcom> CSeq: 1 INVITE Content-Length: 213 Content-Type: application/sdp Content-Disposition: session v=0 o=collins 123456 001 IN IP4 station1workcom s= c=IN IP4 station1workcom t=0 0 m=audio 4444 RTP/AVP 2 a=rtpmap 2 G726-32/8000 m=audio 4666 RTP/AVP 4 a=rtpmap 4 G723/8000 m=audio 4888 RTP/AVP 15 a=rtpmap 15 G728/8000
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SIP/20 200 OK From: Daniel<sip:Collins@station1workcom>; tag = abcd1234 To: Boss<sip:Manager@station2workcom>; tag=xyz789 CSeq: 1 INVITE Content-Length: 163 Content-Type: application/sdp Content-Disposition: session v=0 o=collins 45678 001 IN IP4 station2workcom s= c=IN IP4 station2workcom t=0 0 m=audio 0 RTP/AVP 2 m=audio 0 RTP/AVP 4 m=audio 6666 RTP/AVP 15 a=rtpmap 15 G728/8000
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ACK sip:manager@station2workcom SIP/20 From: Daniel<sip:Collins@station1workcom>; tag = abcd1234 To: Boss<sip:Manager@station2workcom>; tag=xyz789 CSeq: 1 ACK Content-Length: 0
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Negotiation of Media
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Situations can arise where one party offers a number of media types and the responding party can support several of the offered media types In the
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
The Session Initiation Protocol (SIP)
The Session Initiation Protocol (SIP)
example of Figure 5-15, we assume that Boss supports only G728 If Boss were to indicate that both G723 and G728 can be supported, then we have reduced the selection of coder/decoders (codecs) from three to two, but we have still not finalized exactly which codec will be used Ideally, Boss s device should select just a single codec from the offered set Such a selection would eliminate any confusion as to the codec to be used If, however, Boss were to reply with more than one codec, then the initiator of the session should choose which of the two to use and should generate a new offer that includes just a single codec For SIP, this reoffer would mean the issuance of a new INVITE request with a message body that includes just the single selected codec This new INVITE would exist within an already established dialog and, as such, it would use the same dialog identifier (From and To headers, including tag values), Call-ID, and Request-URI as would be used by any other request (such as INFO or BYE) within an established dialog This scenario would appear as shown in Figure 5-16 In Figure 5-16, we note that the initial INVITE specifies the offered media formats, but it uses the inactive attribute to specify that the media formats are set such that the media in question will neither be sent nor received (at least not for the present) As in Figure 5-15, Daniel offers the three coding schemes G726, G723, and G728 In this case, Boss replies with G723 and G728 Collins acknowledges the response and issues a reINVITE with G728 as the only coding choice, and this time the inactive attribute is not present The o line of the re-INVITE indicates the same sender and session number, but the session version is increased by 1 Although an ACK can carry a message body, we cannot use the ACK request in Figure 5-16 to carry an updated version of the offer that was in the initial INVITE In SIP, two mechanisms are used for the answer/offer exchange In the first case, an offer is made within an INVITE and is answered with a 2XX response In the second case, an offer is made within a 2XX response and the ACK carries the answer The second case would apply if the initial INVITE does not contain an offer (a valid situation) If the first INVITE does contain an offer in the form of an SDP description, however, the ACK cannot be used to modify that offer Not only can there be overlap between the supported media formats of the two parties in a session, but it is quite possible that the receiver of an offer (the receiver of an INVITE that contains an SDP description) cannot support any of the media formats proposed in the offer If, for example, in Figure 5-15, Boss could only support voice coded according to G711 m-law (RTP payload type 0), then there would be a mismatch between the capabilities of the called party and those of the calling party In such a situation,
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The Session Initiation Protocol (SIP)
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