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The SIP ALG problem

A SIP ALG rewrites SIP and SDP in transit and often corrupts the call. It cannot rewrite TLS.

Learn in 5 minutes

  1. Recognize an in-path rewrite of Via, Contact, or c=
  2. Treat a broken Content-Length after a rewrite as corruption
  3. Prefer TLS when you need the ALG to be unable to edit the body

Visual

UDP rewritten, TLS not rewritten · study drawing · cyphermonkeylabs.com PhoneALGCarrier INVITE INVITE 400 INVITE

SVG HTML PNG

The ALG is not an SBC you configured. It is an unexpected editor on the path.

Worked example

See a stale length Dissect the forwarded INVITE. Content-Length does not match the body.

  1. Press Dissect.
  2. Read the Content-Length warning.
  3. Find the note that the length was left stale.

Fault

UDP calls fail with 400. TLS calls to the same carrier succeed. The UDP body length does not match.

A SIP ALG rewrites SIP and SDP in transit and often corrupts calls. It cannot rewrite TLS.

Quiz

A SIP ALG


The ALG and TLS


A stale Content-Length after a path rewrite means


The defensive actions are


An ALG is not


400 Bad Request in this ladder is


Ecosystem

Glossary, sister systems, and the integration seam for this room.

AI omni-channel agents A voice bot still needs a session. SIP creates, modifies, and ends sessions. Omni-channel, in the contact-center sense, means the same contact can also live on chat or another digital channel. Those channels do not replace the voice session. On Amazon Connect the platform owns the managed PSTN voice channel. Bridging an outside carrier into Connect with a Chime Voice Connector is not a supported pattern unless AWS reviews it. When an SBC forks the voice leg to a recorder, that fork is SIPREC, a different plane from the customer dialog.

AI agents A voice bot still needs a session. On Amazon Connect the platform owns the managed PSTN voice channel. Bridging an outside carrier into Connect with a Chime Voice Connector is not a supported pattern unless AWS reviews it. The FreeSWITCH manual page also describes SignalWire, founded by the FreeSWITCH team, as a hosted platform with AI agents behind REST APIs and SWML.

Glossary · Ecosystem map · Ports · Troubleshooting

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