According to the RFC3262:
The UAS MUST send any non-100 provisional response reliably if the initial request contained a Require header field with the option tag 100rel. If the UAS is unwilling to do so, it MUST reject the initial request with a 420 (Bad Extension) and include an Unsupported header field containing the option tag 100rel.
Lab – FreeSwitch, Obihai Phone
I’m going to demonstrate how to enable PRACK using FreeSwitch and a MicroSIP phone. On FreeSwitch you have to add the line <param name=”enable-100rel” value=”true”/> in the SIP profile for the phones. Then make a call to a phone who supports 100-rel such as the Obihai and the magic will appear. The SIP ladder of the exchange is shown below.
When I tried this lab with MicroSIP it didn’t work. MicroSIP includes in the 180 ringing response the header Supported: 100rel. For Obihai when you configure the phone for 100 rel support it adds the 100rel as Required. Once one of the sides have it as required, it starts.
2023/01/28 10:15:15.632273 192.168.15.128:5060 -> 192.168.15.52:5060
SIP/2.0 180 Ringing
CSeq: 62905665 INVITE
From: “Extension 1000” <sip:firstname.lastname@example.org>;tag=BSat99UpmZH5H
Via: SIP/2.0/UDP 192.168.15.52;branch=z9hG4bK3yccZUX0X27jm;received=192.168.15.52;rport=5060
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, PRACK, NOTIFY
You should use PRACK with care. These method is not often used, so it is easier to find bugs and incomplete implementations. The example is merely illustrative,I’m not suggesting you enable PRACK.
In conclusion, PRACK is a SIP method that provides reliability for provisional responses. It allows clients to acknowledge receipt of provisional responses and request retransmission if necessary.