16
Call Flow Examples
In this chapter, many of the concepts and details presented in the preceding chapters will be illustrated with examples. Each example includes a call flow diagram and a discussion of the example, followed by the message details. Each message is labeled in the figure with a message number for easy reference. For more examples of the protocol, refer to the SIP specification [1], the SIP call flows [2, 3] documents, and the SIP service examples document [4].
The purpose of the examples in this chapter is to illustrate aspects of the SIP protocol. The interoperation scenarios with the PSTN are not intended to fully define the interworking or show a complete parameter mapping between the protocols. Likewise, simplifications such as minimal authentication and direct client-to-gateway messaging are used to make the examples more clear.
16.1 SIP Call with Authentication, Proxies, and Record-Route
Figure 16.1 shows a basic SIP call between two SIP UAs involving two proxy servers. Rather than perform a DNS query on the SIP URI of the called party, the calling SIP phone sends the INVITE request to a proxy server for address resolution. The proxy server requires authentication to perform this service and replies with a 407 Proxy Authorization Required response. Using the nonce from the challenge, the caller resends the INVITE with the caller’s username and password credentials. The proxy checks the credentials, and finding them to be correct, performs the DNS lookup on the Request-URI. The INVITE is then forwarded to the proxy server listed in the DNS SRV record that handles the language.org domain. That proxy then looks up the Request-URI and locates a registration for the called party. The INVITE is forwarded to the destination UAS, a Record-Route header having been inserted to ensure that the proxy is present in
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344 |
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SIP: Understanding the Session Initiation Protocol |
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Figure 16.1 SIP-to-SIP call with authentication, proxies, and Record-Route.
all future requests by either party. This is because a directly routed SIP message to Ada would be blocked by the firewall.
The called party receives the INVITE request and sends 180 Ringing and 200 OK responses, which are routed back to the caller using the Via header chain from the initial INVITE. The ACK sent by the caller includes a Route header built from the Record-Route header field in the 200 OK response. This routing skips the first proxy but includes the firewall proxy. The media session begins with the UAs exchanging RTP and RTCP packets.
The call terminates when the called party, Ada, sends a BYE, which includes a Route header generated from the Record-Route header field in the INVITE. Note that the CSeq for the called user agent is initialized to 1000. The acknowledgment of the BYE with a 200 OK response causes both sides to stop sending media packets.
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Call Flow Examples |
345 |
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M1 |
INVITE sip:ada@language.org SIP/2.0 |
Request-URI |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK454 |
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Max-Forwards: 70 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: sip:ada@language.org |
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Call-ID: f6329a3491e7 |
CSeq initialized to 1 |
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CSeq: 1 INVITE |
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Contact: <sip:babbage@client.analyticalsoc.org> |
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Subject: RE: Software |
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User-Agent: DifferenceEngine/1 |
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Content-Type: application/sdp |
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Content-Length: 137 |
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v=0 |
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o=Babbage 2890844534 2890844534 IN IP4 12.26.17.91 |
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s=- |
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t=0 0 |
Babbage’s IP address |
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c=IN IP4 12.26.17.91 |
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m=audio 49170 RTP/AVP 0 |
Port number |
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a=rtpmap:0 PCMU/8000 |
Codec info |
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M2 SIP/2.0 407 Proxy Authentication Required |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK454 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=34q4356g |
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Call-ID: f6329a3491e7 |
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CSeq: 1 INVITE |
Authentication challenge |
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Proxy-Authenticate: Digest |
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realm=”language.org”, |
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nonce=”9c8e88df84f1cec4341ae6e5a359”, |
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opaque=””, stale=FALSE, algorithm=MD5 |
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M3 |
ACK sip:ada@language.org SIP/2.0 |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK454 |
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Max-Forwards: 70 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=34q4356g |
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Call-ID: f6329a3491e7 |
CSeq not incremented Method set to ACK |
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CSeq: 1 ACK |
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M4 |
INVITE sip:ada@language.org SIP/2.0 |
INVITE resent with credentials |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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Max-Forwards: 70 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: sip:ada@language.org |
Call-ID unchanged |
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Call-ID:f6329a3491e7 |
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CSeq: 2 INVITE |
CSeq incremented |
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Proxy-Authorization: Digest |
Credentials |
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username=”Babbage”, |
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realm=”language.org”, |
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nonce=”9c8e88df84f1cec4341ae6e5a359”, opaque=””, response=”e56131d19580cd833064787ecc”
Contact: <sip:babbage@client.analyticalsoc.org> Subject: RE: Software
User-Agent: DifferenceEngine/1 Content-Type: application/sdp Content-Length: 137
v=0
346 |
SIP: Understanding the Session Initiation Protocol |
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o=Babbage 2890844534 2890844534 IN IP4 12.26.17.91 |
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s=- |
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t=0 0 |
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c=IN IP4 12.26.17.91 |
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m=audio 49170 RTP/AVP 0 |
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a=rtpmap:0 PCMU/8000 |
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M5 |
SIP/2.0 100 Trying |
Credentials accepted |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: sip:ada@language.org |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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M6 |
DNS Query: |
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SRV lookup on _udp._sip.language.org |
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M7 |
DNS Response: |
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_sip._udp.language.org. 300 IN SRV 0 100 5060 proxy.languagel.org. |
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proxy.language.org. 3600 IN A 10.14.92.1 |
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M8 |
INVITE sip:ada@language.org SIP/2.0 |
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Via: SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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Max-Forwards: 69 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: sip:ada@language.org |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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Contact: <sip:babbage@client.analyticalsoc.org> |
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Subject: RE: Software |
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User-Agent: DifferenceEngine/1 |
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Content-Type: application/sdp |
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Content-Length: 137 |
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v=0 |
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o=Babbage 2890844534 2890844534 IN IP4 12.26.17.91 |
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s=- |
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t=0 0 |
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c=IN IP4 12.26.17.91 |
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m=audio 49170 RTP/AVP 0 |
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a=rtpmap:0 PCMU/8000 |
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M9 |
SIP/2.0 100 Trying |
Not Forwarded |
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Via:SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: sip:ada@language.org |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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M10 |
Location Service Query: ada? |
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M11 |
Location Service Response: sip:ada@1.2.3.4 |
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M12 INVITE sip:ada@1.2.3.4 SIP/2.0
|
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Call Flow Examples |
347 |
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Via: SIP/2.0/UDP proxy.language.org:5060;branch=z9hG4bK24105.1 |
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Via: SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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Max-Forwards: 68 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: sip:ada@language.org |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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Contact: <sip:babbage@client.analyticalsoc.org> |
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Subject: RE: Software |
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User-Agent: DifferenceEngine/1 |
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Record-Route: <sip:10.14.92.1;lr> |
Record-Route added by proxy |
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Content-Type: application/sdp |
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Content-Length: 137 |
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v=0 |
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o=Babbage 2890844534 2890844534 IN IP4 12.26.17.91 |
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s=- |
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t=0 0 |
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c=IN IP4 12.26.17.91 |
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m=audio 49170 RTP/AVP 0 |
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a=rtpmap:0 PCMU/8000 |
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M13 |
SIP/2.0 180 Ringing |
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Via : SIP/2.0/UDP proxy.language.org:5060;branch=z9hG4bK24105.1 |
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|
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;received=10.14.92.1 |
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|
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Via: SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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|
Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=65a3547e3 |
Tag added by called party |
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|
Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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Contact: sip:ada@drawingroom.language.org |
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Record-Route: <sip:10.14.92.1;lr> |
|
M14 |
SIP/2.0 180 Ringing |
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Via: SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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|
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=65a3547e3 |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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Contact: sip:ada@drawingroom.language.org |
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Record-Route: <sip:10.14.92.1;lr> |
|
M15 |
SIP/2.0 180 Ringing |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=65a3547e3 |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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M16 |
SIP/2.0 200 OK |
Call accepted |
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Via: SIP/2.0/UDP 10.14.92.1:5060;branch=z9hG4bK24105.1 |
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Via: SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221
From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 To: <sip:ada@language.org>;tag=65a3547e3
Call-ID: f6329a3491e7
348 |
SIP: Understanding the Session Initiation Protocol |
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CSeq: 2 INVITE |
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Contact: sip:ada@drawingroom.language.org |
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Record-Route: <sip:10.14.92.1;lr> |
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Content-Type: application/sdp |
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Content-Length: 126 |
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v=0 |
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o=Ada 2890844536 2890844536 IN IP4 1.2.3.4 |
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s=- |
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t=0 0 |
Ada’s IP address |
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c=IN IP4 1.2.3.4 |
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m=audio 52310 RTP/AVP 0 |
Port number |
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a=rtpmap:0 PCMU/8000 |
Codec information |
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M17 |
SIP/2.0 200 OK |
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Via: SIP/2.0/UDP 15.16.17.18:5060;branch=z9hG4bK3f31049.1 |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=65a3547e3 |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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Contact: sip:ada@drawingroom.language.org |
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Record-Route: <sip:10.14.92.1;lr> |
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Content-Type: application/sdp |
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Content-Length: 126 |
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v=0 |
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o=Ada 2890844536 2890844536 IN IP4 1.2.3.4 |
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s=- |
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t=0 0 |
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c=IN IP4 1.2.3.4 |
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m=audio 52310 RTP/AVP 0 |
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a=rtpmap:0 PCMU/8000 |
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M18 |
SIP/2.0 200 OK |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK221 |
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From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382 |
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To: <sip:ada@language.org>;tag=65a3547e3 |
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Call-ID: f6329a3491e7 |
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CSeq: 2 INVITE |
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Contact: sip:ada@drawingroom.language.org |
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Record-Route: <sip:10.14.92.1;lr> |
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Content-Type: application/sdp |
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Content-Length: 126 |
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v=0 |
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o=Ada 2890844536 2890844536 IN IP4 1.2.3.4 |
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s=- |
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t=0 0 |
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c=IN IP4 1.2.3.4 |
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m=audio 52310 RTP/AVP 0 |
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a=rtpmap:0 PCMU/8000 |
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M19 |
ACK sip:ada@drawingroom.language.org SIP/2.0 |
Sent to ALG |
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Via: SIP/2.0/UDP 12.26.17.91:5060;branch=z9hG4bK789 |
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Max-Forwards: 70
From: Charles Babbage <sip:babbage@analyticalsoc.org>;tag=9382
To: <sip:ada@language.org>;tag=65a3547e3
Call-ID: f6329a3491e7