INVITE Retransmission in VoLTE

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Hello VoLTE experts,

If a UE sends an INVITE message but does not receive a 100 Trying response, how many times will it retransmit the INVITE message before stopping?

Thanks in advance!

INVITE Retransmission in VoLTE
INVITE Retransmission in VoLTE1024×1024 91.3 KB

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When a UE sends an INVITE message and does not receive a 100 Trying response, it typically retransmits the INVITE a few times before stopping. The exact number of retries depends on the network configuration and UE implementation.

Generally, the UE will attempt 3 to 5 retransmissions before stopping and may then trigger an error or failure notification.

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3

Theoretically, the UE retransmits the INVITE message following an exponential backoff mechanism, doubling the T1 timer until it reaches Timer B (default: 64 × T1).

Assuming a default T1 value of 500ms, the retransmissions occur as follows:

  • The first INVITE is sent at 0 ms.
  • The second is sent 500 ms later.
  • The third follows 1 second after the second.
  • The fourth occurs 2 seconds after the third.
  • The fifth at 4 seconds, sixth at 8 seconds, seventh at 16 seconds, and the eighth and final retransmission at 32 seconds, reaching Timer B.

In practical scenarios, retransmissions may occur more frequently due to network-specific configurations.

4

Thank you!

Could you please clarify which network entity is responsible for configuring these timers?

Additionally, what is the maximum number of retries that can be achieved? Since retransmissions should be limited to avoid excessive consumption of network resources, there must be a defined upper limit.

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There is no explicitly defined maximum, or at least I am not aware of one.

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Actually that retranamission frequency is not dependent on SIP it depends on TCP.Remember SIP can run on UDP as well.So in that case there wont be any retransmission.Rather than retransmission what sip does is SIP user agent wait till X amount of time until he receives a response from the other end.That value is called sip session timer.So invite carries the session timer where How long A party is willing to wait for response from other end.Once it expires it A party drops the session and no longer accepting any response.Therefore once again SIP protocol dont do retransmitions that is goverened by TCP.SIP on UDP has no retransmisstions what so ever.
Thank you