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PRACK and reliable provisionals

A provisional response with Require: 100rel is retransmitted until PRACK, and RAck names that exact response.

Learn in 5 minutes

  1. Offer 100rel and answer with RSeq
  2. Build RAck from RSeq and the INVITE CSeq
  3. Give PRACK its own CSeq number

Visual

183, then PRACK · study drawing · cyphermonkeylabs.com UAPeer INVITE 183 PRACK 200

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The first number is the provisional. The next two name the INVITE, not the PRACK.

Worked example

Check RAck Dissect the PRACK. Separate RAck from CSeq.

  1. Dissect the PRACK.
  2. Read RAck.
  3. Read CSeq and confirm the method token is PRACK.

Fault

The UAS keeps retransmitting 183. The PRACK is present and the UAS answers 481.

RAck's method and number identify the request that carried RSeq, which is the INVITE.

Quiz

100rel is defined in


RAck: 1 1 INVITE means


PRACK's own CSeq on this ladder is


The UAS retransmits the 183 until


100 Trying and 100rel


The 200 OK to PRACK


Ecosystem

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

E911 / emergency calling Location has to ride with the call. RFC 6881 is the BCP, RFC 6442 carries location in SIP, and the FCC MLTS page states Kari’s Law and RAY BAUM’s Act obligations for multi-line systems.

Glossary · Ecosystem map · Ports · Troubleshooting

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