Hi, im trying to build a UAC and I'm coming up with some trouble whenever I 
receive a SIP 491 Request Pending Response. This happens because I try to place 
a call on hold using an Invite request rigth before Asterisk sends me a 
Re-Invite for the same call. I respond to the 491 response with an ACK however 
for some strange reason Asterisk doesn't accept the ACK and insists on 
retransmitting the 491 Response. Asterisk replies with the following 491 
response: 

SIP/2.0 491 Request Pending 
Via: SIP/2.0/UDP 
10.110.7.89:5070;branch=z9hG4bK5a668c33f196837c3602266b23b389e0;received=10.110.7.89
 
From: <sip:30...@10.110.7.20:5070>;tag=SIPTester 
To: <sip:30...@10.110.7.20>;tag=as2ea72122 
Call-ID: 0dd43bb5a64eb5a2fb0114193821f...@10.110.7.89 
CSeq: 5 INVITE 
User-Agent: Asterisk PBX 
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY 
Supported: replaces 
Content-Length: 0 
X-Asterisk-HangupCause: Normal 



And I send the following ACK: 



ACK sip:30...@10.110.7.20 SIP/2.0 
Call-ID: 0dd43bb5a64eb5a2fb0114193821f...@10.110.7.89 
Max-Forwards: 70 
From: <sip:30...@10.110.7.20:5070>;tag=SIPTester 
To: <sip:30...@10.110.7.20>;tag=as2ea72122 
Via: SIP/2.0/UDP 
10.110.7.89:5070;branch=z9hG4bK5a668c33f196837c3602266b23b389e0 
CSeq: 5 ACK 
Content-Length: 0 

However this doesn't seem to be valid because Asterisk insists be resending the 
same 491 Response until it sends 6 messages and the decides to destroy the 
dialog: 

Aug 21 11:21:06 <http://www.voip-info.org/boards/Aug%2021%2011:21:06>  
WARNING9686 <http://www.voip-info.org/boards/9686> : chan_sip.c:1967 
retrans_pkt: Maximum retries exceeded on transmission 
0dd43bb5a64eb5a2fb0114193821f...@10.110.7.89 for seqno 5 (Critical Response) - 
See doc/sip-retransmit.txt. 
Aug 21 11:21:06 <http://www.voip-info.org/boards/Aug%2021%2011:21:06>  
WARNING9686 <http://www.voip-info.org/boards/9686> : chan_sip.c:1989 
retrans_pkt: Hanging up call 0dd43bb5a64eb5a2fb0114193821f...@10.110.7.89 - no 
reply to our critical packet (see doc/sip-retransmit.txt). 

Does anyone have a clue of what it is I'm doing wrong? Do I have to send a 
CANCEL request of the hold's INVITE? 

Thanks in advance!

 

Joaquín 

 

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