What Does UDP Timeout Mean?

Learn what UDP timeout is and how it can affect communication on a network.

Written by Adrian Angwenyi

Updated at April 4th, 2023

What does UDP Timeout mean?

UDP Timeout refers to the amount of time a UDP Pinhole stays open on a Firewall or Router. Depending on the equipment this timeout can range from a few seconds to many minutes. Most devices fall under the minute(s) range.

Quick Tip: We recommend UDP Timeout for Stratus phones to be set at 30 or 60 seconds.

Warning: Leaving a UDP Port open for an extended period of time can be a security risk for some network devices.


Brief Explanation of UDP Timeouts

UDP Ports must be opened from the Inside of your Network (explanation below), that port will timeout within a set timeframe after the last message was transmitted.

  1. Your Phone Registers using local port 5060 sending it's registration to the Switch.
  2. As registration passes through your Firewall, the Firewall assigns a random UDP Port for the external connection (sometimes will use the same internal port).
  3. The Switch keeps this as Registration data and knows it can contact the phone at this Port, as long the port doesn't close or change.
  4. Re-registration or Keep-Alive can be sent to keep this port open. Typically the port doesn't change often.
  5. If the UDP Timeout is too short, the firewall will close the port before Re-registration occurs. In time period; SIP messaging from the Switch would be sent to a port that has already been closed by the Firewall, and the packets will be dropped. Picture Example Below.

What are the Symptoms of UDP Timeouts?

  • Incoming Call Goes straight to Voicemail, or doesn't ring certain phones in a ring group.
  • Calls drop at exactly 17 minutes, 32 minutes, 47 minutes..
  • Calls Randomly Drop.
  • Firewall Port (registration port) for SIP Traffic changes frequently.

What are NOT Symptoms of UDP Timeouts?

  • Phone appears to have lost registration. The phone and the Switch are unaware of the Firewalls UDP Timeout Length, both sides will think Registration is Active.
  • Not able to place Outbound calls. The phone will always open the firewall port itself because it's inside the Network, the switch will always respond to the same port.
  • Call Quality issues.

How do I recognize UDP Timeouts on my Network?

Outside of the normal symptoms mentioned above, it is easily recognizable on a SIP Trace of a failed or dropped call. You will see 5 duplicated SIP Messages such as an INVITE, all sent quickly with no response. Followed by a BYE from the Switch either right away or two minutes later.