What minute interval does L2TP rekey take place?

Question

Created Oct ’20

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Could you tell me the specifications of the VPN (L2TP) connection for the iphone?
I am using an iphone 8 (iOS version: 13.3.1).

When I make a VPN connection between my iphone and my router, the VPN is disconnected after 1 hour and 48 minutes.
I would like to prevent this from being disconnected.

The router's log says the VPN is disconnected due to the expiration of SA lifetime.
I know that iphone sends the packet for IKE Phase1 and Phase2 that have 3600 seconds lifetime.

I can see from the log that the phase 1 and 2 SAs are created at the start of the VPN connection, and the iphone sends the rekey packet for only phase 2 48 minutes later.
However, the next rekey packet for Phase 2 is not done 48 minutes later, but 1 hour later.
So, just before receiving the rekey packet from iphone, the router has deleted the SA due to lifetime expiration and the VPN is disconnected.

The rekey interval on iphone is sometimes 48 minutes and sometimes 60 minutes, what is the correct movement to rekey at what minute interval?
I would like to know about the specifications of the iphone rekey.

Thank you.

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Answer 1

ta00 OP

Nov ’20

Does anyone know a solution to this problem?
I have reported the following content and logs about this issue to feedback assistant.

FB8916490 (VPN connection(L2TP) will be disconnected in 1 hour and 48 minutes)

Basic Information
Please provide a descriptive title for your feedback:
VPN connection(L2TP) will be disconnected in 1 hour and 48 minutes

Which area are you seeing an issue with?
VPN

What type of feedback are you reporting?
Incorrect/Unexpected Behavior

My environment
Device: iPhone7
iOS: 12.4
(iPhone8 iOS: 13.3 also has the same problem)
VPN Type: L2TP

Discription of the problem
When you are connecting to a VPN with L2TP, the VPN may be disconnected after 1 hour and 48 minutes.
According to the log of the VPN server (VPN router), the lifetime of SA is 60 minutes and the iPhone sends out rekey packet every 48 minutes.
If the problem occurs, the SA has been deleted and the VPN has been disconnected because the iPhone does not send out the rekey packet in 48 minutes and it is sent out after 60 minutes of lifetime.

SA is created (0 minutes)
48 minutes later, iPhone sends out a rekey to the VPN server (the first rekey is always sent out in 48 minutes)(after 48 minutes)
After another 48 minutes iPhone does not send out rekey to the VPN server(after 1 hour 36 minutes)
60 minutes after the first rekey, SA is deleted (VPN disconnected)(after 1 hour 48 minutes)

Instruction to reproduce the problem
1. set the VPN type to L2TP and connect to VPN
2. wait 1 hour and 48 minutes, or 2 hours and 36 minutes

Result I expected
I want rekeys to be sent out from the iPhone in 48 minute cycles.

Date and time the VPN was disconnected
November 24, 2020 at 18:40
November 24, 2020 at 21:23
November 24, 2020 at 23:21
November 25, 2020 at 1:19
November 25, 2020 at 3:17
November 25, 2020 at 5:24
November 25, 2020 at 7:38

Thank you.

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