ACK and BYE methods

By Dimitri Osler
August 9, 2018

ACK method confirms that an entity has received a final response to an INVITE request. In its turn, BYE method signals termination of a dialog and ends a call.

Here is the example:

ACK sip:001234567890@10.135.0.1;transport=UDP SIP/2.0
Via: SIP/2.0/UDP 10.135.0.12:5060;branch=z9hG4bKkio8kgkgmh02yv4m0ok
Max-Forwards: 70
From: "Calling User" <sip:151@10.135.0.1:5060>;tag=m3l2hbp
To: <sip:001234567890@10.135.0.1:5060;user=phone>;tag=as27b249ee
Call-ID: ud04chatv9q@10.135.0.1
CSeq: 10692 ACK
Proxy-Authorization: Digest username="151", realm="10.135.0.1", nonce="WWdAiVlnPjECocBoyYtBDshUs+RxMS+3", uri="sip:001234567890@10.135.0.1;transport=UDP", response="e710cace9e0bc857724f26f4106ea726"
User-Agent: Wildix W-AIR 03.55.00.24 9c7514340722 02:76:f3:d6:64
Content-Length: 0

The ACK from the client (a phone in this case) closes the first Transaction.

BYE sip:151@10.135.0.12;line=12071 SIP/2.0
Via: SIP/2.0/UDP 10.135.0.1;branch=z9hG4bK6cf.a70a01139192f1a06836622bda9535f3.0;rport
Max-Forwards: 69
From: <sip:001234567890@10.135.0.1:5060;user=phone>;tag=as27b249ee
To: "Calling User" <sip:151@10.135.0.1:5060>;tag=m3l2hbp
Call-ID: ud04chatv9q@10.135.0.1
CSeq: 102 BYE
User-Agent: WildixGW 35935
Reason: Q.850;cause=16;text="Normal Clearing"
Content-Length: 0

BYE is the message used to close an ongoing dialog and it opens the second transaction contained in this call as shown in the diagram above. Please note that the CSeq is completely different.

Q.850 provides the hang up reason for this call, including the cause number and the text “Normal Clearing”, which indicates that the disconnection has been generated by user.

As you see the User-Agent reports the server so the hang up is coming from the Server exactly as in the diagram.

SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.135.0.1;branch=z9hG4bK6cf.a70a01139192f1a06836622bda9535f3.0;rport=5060
Max-Forwards: 69
From: <sip:001234567890@10.135.0.1:5060;user=phone>;tag=as27b249ee
To: "Calling User" <sip:151@10.135.0.1:5060>;tag=m3l2hbp
Call-ID: ud04chatv9q@10.135.0.1
CSeq: 102 BYE
User-Agent: Wildix W-AIR 03.55.00.24 9c7514340722
Reason: Q.850;cause=16;text="Normal Clearing"
Content-Length: 0

With this last message OK which definitely closes the transaction, the phone confirms receiving the BYE.