What can be concluded?

Refer the exhibit. A CUCM engineer configured SAF between two clusters. Based on the SIP INVITE message that is received, what can be concluded?


A. A SIP INVITE message is sent every 15 minutes to maintain the call
B. the route header dictates the incoming SIP trunk is used
C. All DTMF methods are supported between the clusters
D. A SIP proxy is between the two clusters that are managing signaling
E. the call has been forwarded by extension 4050 in the calling cluster

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6 thoughts on “What can be concluded?

  1. Related to this question is the ‘session-expires’ header. It is 1800 seconds.

    From at the bottom of page 10 of https://www.rfc-editor.org/pdfrfc/rfc4028.txt.pdf:

    “It is RECOMMENDED that this refresh be sent once half the session interval has elapsed’.

    that is 1800 seconds / 2 = 900 seconds = 15 minutes.

    So the correct answer is ‘A. A SIP INVITE message is sent every 15 minutes to maintain the call’

  2. The ‘expires’ header has nothing to do with this question. From RFC 3261 page 79:

    ‘The UAC MAY add an Expires header field (Section 20.19) to limit the validity of the invitation. If the time indicated in the Expires header field is reached and no final answer for the INVITE has been received, the UAC core SHOULD generate a CANCEL request for the INVITE, as per Section 9.’

    20.19 Expires
    The Expires header field gives the relative time after which the message (or content) expires. The precise meaning of this is method dependent. The expiration time in an INVITE does not affect the duration of the actual session that may result from the invitation.

  3. 900 seconds/60 seconds = 15 minutes.
    That is true, but the Re-Invite or Refresh message is send when half of that time expires, not when it expires. So here it will be on 7.5 minutes.

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