Yealink SIP-T2 SERIES Administrator's Manual page 378

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
The default value is 5.
5. Click Confirm to accept the change.
Session timer allows a periodic refresh of SIP sessions through an UPDATE request, to
determine whether a SIP session is still active. Session timer is specified in
phones support two refresher modes: UAC and UAS. Whether the endpoint functions as
a UAC or a UAS depends on the UA that initiates the SIP request. If the initiator is
configured as UAC, the other client or the SIP server will function as a UAS. If the initiator
is configured as UAS, the other client or the SIP server will function as a UAC. The session
expiration is negotiated via the Session-Expires header in the INVITE message. The
negotiated refresher is always the UAC and it will send an UPDATE or request at the
negotiated session expiration. The value ―refresher=uac‖ included in the UPDATE
message means that the UAC performs the refresh.
Example of UPDATE message (UAC mode):
UPDATE sip:1058@10.10.20.34:5060 SIP/2.0
Via: SIP/2.0/UDP 10.10.20.32:5060;branch=z9hG4bK2104991394
From: "10111" <sip:10111@10.2.1.48:5060>;tag=2170397024
To: <sip:1058@10.2.1.48:5060>;tag=200382096
Call-ID: 4_1556494084@10.10.20.32
CSeq: 2 UPDATE
Contact: <sip:10111@10.10.20.32:5060>
Max-Forwards: 70
User-Agent: Yealink SIP-T23G 44.80.0.60
Session-Expires: 90;refresher=uac
Supported: timer
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RFC 4028. IP