Customer reported that calls randomly dropped when
calling from IP phone to IMS. Customer started the call from IP phone, and call
failed after about 10 seconds. Initially after the voice implementation, this
issue never appeared.
When calling from IP phone to local IMS, IMS used 183 Session
Progress SIP message to indicate the progress of proceeding the call.
In the first 183 message, IMS replied SDP content to finish the
early media negotiation with IP phone, in which there was also ‘Require:
100rel’ content to indicate this was a reliable negotiation:
Corresponding to this message, AR
router correctly forwarded it to IP phone throughly and sent PRACK message from
IP phone to IMS:
During this call, IMS could not finish
locating the callee, so it continued sending 183 to indicate that call was
still in proceeding:
However, AR router could not
correctly give response to more 183 SIP message, not forwarding 183 to IP
phone. IMS would resend such 183 message because it didn’t get PRACK response
any more, and finally released the call due to SIP message interaction failure:
The root cause is that AR router didn’t correctly give response to a
series of 183 Session Progress message from IMS side, which led to the call
failure. These 183 messages included ‘Require: 100rel’ content, which indicated
that the message interaction was reliable. Under this circumstance, client side
must reply PRACK message to server side.
In this case, configuration adjustment was
used to resolve this issue.
[Huawei-voice-sipserver]number-parameter 31 1
SIP server reset succeeds.
After this adjustment, AR router will directly reply PRACK to any
reliable 183 message instead of forwarding it thoroughly to IP phone. Once if
there is any SDP content to request early media negotiation with IP phone, AR
router will remove ‘Require: 100rel’ content and send modified SIP message to
IP phone in order to finish the media negotiation. This will not affect the