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

SIP Profile

IP Network Element

SIP Prack Call Flows

SIP SGP

 

Diagram - SIP PRACK Handshake

dg_sip_prack.png

 

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:

 

Troubleshooting

If you are experiencing problems with this feature, check the following: