SIP PRACK

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 (Provisional Response Acknowledgement) method described in RFC 3262 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).

Configuration

SIP PRACK is configured in the SIP SGP object as displayed below. For information on configuring the SIP Signaling and associated functionality, refer to the Configure SIP (Single SIP IP) topic.

PRACK Support

There is a drop down menu with the following selections. PRACK is disabled by Default.

Disabled (default)- Disable sending Provisional Ack message.

Supported - Provides the ability to handle PRACK transactions during call setup if the SIP peer requires it.

Required - Requires the SIP peer and the IMG 2020 to use PRACK transactions during call setup.

PRACK Timer (sec)

The PRACK Timer is used to request an extension of the transaction at proxies in case the INVITE transaction takes some time to generate a final response (RFC 3262 page 3). When the PRACK Refresh Timer expires the IMG 2020 will send out a 1xx reliable response.

60 (seconds) - Wait 60 seconds before sending the 1xx message.

150 (seconds) (default) - Wait 150 seconds before sending the 1xx message.

Diagram - SIP PRACK Handshake

UAC (User Agent Client) - UAS (User Agent Server) behavior

The following table displays the overall behavior of UAS and UAC with various SGP configuration combinations.

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 2020 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 2020 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 2020.

• The IMG 2020 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 2020 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.

Call Flows

Refer to the SIP PRACK Call Flows topic for call flow information

Troubleshooting

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

• Verify PRACK is enabled in the SIP Profile object.

• Verify sure the correct SIP Profile is assigned to the External Gateway.

• Verify External Gateway IMG 2020 is communicating with supports PRACK.