Troubleshooting missing ACK in SIP

Publisher

OpenSIPS Project

URL

https://blog.opensips.org/2017/02/22/troubleshooting-missing-ack-in-sip/

Published

2017-02-22

Updated

unknown

Accessed

2026-09-25

HTTP status

200

License

all rights reserved (blog); no-redistribution; short excerpts and locators only

Source notes citing this source

• Sources disagree on how quickly a UAS tears down a call after a missing ACK. The OpenSIPS blog says 5 to 10 seconds, but 64*T1 with T1 = 500 ms gives about 32 seconds. disputed in context
• Applying NAT fix-ups too broadly can also cause a missing ACK. Rewriting the Contact of a reply that came from another proxy, not from the endpoint, removes the routing information needed to reach the callee. field report in context
• The first diagnostic step for a suspected missing ACK is a SIP capture showing whether the ACK actually reaches the callee device. Then check that the Contact in the 200 OK carries an address the caller side can reach. field report in context
• A common cause of a missing ACK is a proxy that does not handle a NATed callee: the callee's private IP stays in the Contact of the 200 OK, so the ACK is sent to an address it cannot reach. field report in context
• For OpenSIPS, the blog advises calling fix_nated_contact() in onreply routes only for replies that come directly from endpoints, not from intermediate proxies. in context

Cite this source record

APA

WarmTransfer. (2017, February 22). Troubleshooting missing ACK in SIP. WarmTransfer. https://warmtransfer.net/knowledge/sources/opensips-blog-troubleshooting-missing-ack

BibTeX

@misc{warmtransfer-opensips-blog-troubleshooting-missing-ack,
  title  = {Troubleshooting missing ACK in SIP},
  author = {{WarmTransfer}},
  year   = {2017},
  url    = {https://warmtransfer.net/knowledge/sources/opensips-blog-troubleshooting-missing-ack},
  note   = {OpenSIPS Project, accessed 2026-09-25}
}