Meaning of "Expires" column in sip register status command

[pandiarajan.it21]

Hi All,

Upon running "show sip-ua register status", it displays the list of trunks configured in the gateway. In the result, "expires" column refers to "The amount of time, in seconds, until registration expires."
So, does this mean the SIP trunk registration has an expiry date? Can't we make a  SIP registration forever? What is preffered in general?

Line                             peer       expires(sec) registered P-Associ-URI
================================ ========== ============ ========== ============
1000                             20001      54           no
1001                             20004      54           no
154001                           -1         56           yes
4175209020                       20002      54           no

[Vivek Batra · Accepted solution]

So, does this mean the SIP trunk registration has an expiry date?

Yes, SIP trunk registered for limited time which is conveyed by UAC in SIP REGISTER message. For example as follows, bold RED shows that UAC wants to register for 3600 seconds.

REGISTER sip:192.168.15.10:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.15.32:5060;branch=z9hG4bK7B198F
From: <sip:8888@192.168.15.10>;tag=1AC5DD4-1269
To: <sip:8888@192.168.15.10>
Date: Sat, 04 Jul 2015 12:12:08 GMT
Call-ID: E5DDA33-217C11E5-80DCFFDD-1D51949D
User-Agent: Cisco-SIPGateway/IOS-12.x
Max-Forwards: 70
Timestamp: 1436011928
CSeq: 8 REGISTER
Contact: <sip:8888@192.168.15.32:5060>
Expires:  3600
Supported: path
Content-Length: 0

Can't we make a  SIP registration forever? 

No, expiration value should be defined under expire header which generally ranges from 60 to 65535 seconds. If not present, server applies it's local policy and as per RFC, consider it to be registered for 3600 seconds by default.

What is preferred in general?

As per RFC 3261, default expiration interval is 3600 seconds. However UAC should send REGISTER request periodically after half of the duration accepted by server. For example if received following 200 OK with expire interval as 120 seconds, UAC should send refresh REGISTER request after 60 seconds.

SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.15.32:5060;branch=z9hG4bK9413FC
From: <sip:8888@192.168.15.10>;tag=1B52888-C2C
To: <sip:8888@192.168.15.10>;tag=911023673
Date: Sat, 04 Jul 2015 12:21:44 GMT
Call-ID: E5DDA33-217C11E5-80DCFFDD-1D51949D
CSeq: 21 REGISTER
Expires: 120
Contact: <sip:8888@192.168.15.32:5060>
Supported: X-cisco-sis-6.0.0
Content-Length: 0

[pandiarajan.it21]

Thanks a lot vivek. Good to know that it re-registers automatically.