SIP PRACK (Provisional Response Acknowledgement)
Overview
There are two types of responses defined by SIP. They are provisional and final. Final responses (2xx-6xx) convey the result of the request processing and are sent reliably. SIP Provisional responses do not have an acknowledgement system so they are not reliable. There are certain scenarios in which the provisional SIP responses (1xx) must be delivered reliably. For example in a SIP/ PSTN inter-working scenario it is crucial that the 180 and 183 messages are not missed. To solve this problem the SIP PRACK method guarantees a reliable and ordered delivery of provisional responses in SIP. PRACK Improves network reliability by adding an acknowledgement system to the provisional Responses (1xx)
RFC
3262 - Reliability of Provisional Responses in the Session Initiation Protocol (SIP)
Configuration
IMG 1010 - SIP Profile - 10.5.3
IMG 1010 - SIP PRACK Call Flows
Diagram - SIP PRACK Handshake
UAC (User Agent Client) - UAS (User Agent Server) Behavior
The following table shows the overall behavior of UAS and UAC with various SGP configuration combinations.
UAC | UAS | Call Processing |
|---|---|---|
SGP: PRACK Disabled | SGP: PRACK Disabled | Normal Call |
SGP: PRACK Disabled | SGP: PRACK Supported | Normal Call |
SGP: PRACK Disabled | SGP: PRACK Require | Call Rejected |
SGP: PRACK Supported | SGP: PRACK Disabled | Normal Call |
SGP: PRACK Supported | SGP: PRACK Supported | PRACK Call |
SGP: PRACK Supported | SGP: PRACK Require | PRACK Call |
SGP: PRACK Require | SGP: PRACK Disabled | Call Rejected |
SGP: PRACK Require | SGP: PRACK Supported | PRACK Call |
SGP: PRACK Require | SGP: PRACK Require | PRACK Call |
Call Tracing:
Success
16:32:35.762 CALL(SIP) (00:0004:00) SENT 183 Session Progress Reliable (100rel) to 10.129.45.102:8000 UDP
16:32:35.782 CALL(SIP) (00:0004:00) RCVD PRACK from 10.129.45.102:8000 Cseq:2 with Via sent-by: 10.129.45.102 UDP
16:32:35.782 CALL(SIP) (00:0004:00) SENT 200 OK PRACK to 10.129.45.102:8000 UDP
Failure
21:16:47.845 CALL(SIP) (01:00004:00) SENT 421 Extension Required [PRACK support is required] to 10.129.45.104:5060 Cseq:1
21:18:09.286 CALL(SIP) (01:00005:00) SENT 420 Bad Extension [Unsupported SIP request arrived at L3UA-TUC] to 10.129.45.104:5060 Cseq:1
SDP SUPPORT in PRACK
Overview of SDP support in SIP PRACK
The IMG now supports embedding the SDP (Session Description Protocol) information within the PRACK message.
PRACK sip:222@10.129.45.104:5060 SIP/2.0 Via: SIP/2.0/UDP 10.129.47.146:8000;branch=z9hG4bK3b1bce 0-22330 Max-Forwards: 70 From: sip:111@10.129.47.146:8000;tag=48346074 To: sip:222@10.129.45.104;tag=a94c095b773be1dd6e8d668a785a9c84904eaa2e Call-ID: 11408@10.129.47.146 CSeq: 2 PRACK RAck: 1 1 INVITE Content-Type: application/sdp Content-Length: 174 v=0 o=_ 2890844527 2890844527 IN IP4 10.129.47.146 s=- c=IN IP4 10.129.47.146 t=0 0 m=audio 9000 RTP/AVP 0 101 a=rtpmap:0 PCMU/8000 a=rtpmap:101 telephone-event/8000 |
Above is a sample PRACK message that contains the SDP information in it. The IMG supports incoming PRACK with SDP support and all currently implemented rules of SDP decoding/overlap failure still apply. This means the following:
An invalid SDP message will be ignored by the IMG
The IMG will accept a valid PRACK message which has invalid SDP information embedded. It will discard only the SDP portion of the PRACK message. (Media parameters will not change.)
The IMG does not support sending out a PRACK message with the SDP information embedded. It will only send out a PRACK message without the SDP information embedded.
Troubleshooting
If you are experiencing problems with this feature, check the following:
Verify PRACK is enabled in the SIP SGP (see IMG 1010 - SIP Profile - 10.5.3)
Verify sure the correct SIP SGP is assigned to the External Gateway
Verify External Gateway must support PRACK.