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Figure 3.7 A SIP reliability example of an INVITE transaction.
the UAC will reach the UAS if authentication challenges occur by proxies in the path.
3.9 Multicast Support
SIP support for UDP multicast has been mentioned in previous sections. There are two main uses for multicast in SIP.
SIP registration can be done using multicast, by sending the REGISTER message to the well-known “All SIP Servers” URI sip:sip.mcast.net at IP address 224.0.1.75 for IPv4. The ttl parameter is usually set to 1 to indicate that only a single hop should be used.
RFC 2543 defined sending other SIP messages, including INVITE, over multicast. However, this was not included in RFC 3261 and is no longer considered standard SIP.
SIP Clients and Servers |
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The use of a multicast address is indicated by the maddr parameter in a URI or in a SIP message using the maddr parameter in the Via header field.
3.10 Conclusion
This chapter introduced SIP clients and servers, discussing user agents, gateways, proxies, redirect servers, and registrars. SIP URIs, reliability, and retransmissions were also covered.
3.11Questions
Q3.1 Fill in the missing messages in the call flow in Figure 3.8 with two UAs and one proxy.
Q3.2 A UA sends an OPTIONS to another UA, which does not respond. Assume T1 = 500 ms and T2 = 4 seconds. Show the timing of the retransmissions relative to t = 0 when the first OPTIONS is sent. How many messages are sent all together?
Q3.3 Fill in the CSeq header fields (number and method) for each of the messages in Figure 3.9.
Figure 3.8 Call flow for Question Q3.1.
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SIP: Understanding the Session Initiation Protocol |
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Figure 3.9 Call flow for Question Q3.3.
Q3.4 Add the missing SIP messages and responses for the call flow in Figure 3.10. Assume the proxy does not Record-Route (Hint: there are 6 missing messages that will result in just a single media session between Alice and Bob.)
Figure 3.10 Call flow for Question Q3.4.
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Q3.5 Fill in the time intervals for the retransmission example in Figure 3.11.
Q3.6 What are two ways that a proxy knows to fork a request?
Q3.7 When does a proxy generate an ACK to a response, and when does it just forward the response without generating an ACK?
Q3.8 What is the difference between a redirect server and a proxy server?
Q3.9 What is the purpose of a SIP registrar server?
Q3.10 A UA sends a REGISTER. After 2.3 seconds, a 100 Trying response is received. After another 0.7 second, a 200 OK response is received. In total, how many times was the REGISTER request sent?
Figure 3.11 Call flow for Question Q3.6.