[reSIProcate] How to differentiate "invite" or "holdoff"?

["常铮" <changzheng@xxxxxxxxxxxxxxxxxx>]

Hi all
I can not differentiate two situations as follows.
(Terminal A and Terminal B as example for my description)

Situation 1:
When initially establish Call,
A send INVITE with Sdp to B, 
then B answer it and return 200ok with Sdp to A, 
and then A recevive 200ok and call virtual function 
  virtual void
  InviteSessionHandler::onAnswer(InviteSessionHandle is, const SipMessage& msg,
const SdpContents& sdp)

Situation 2:
When HoldOff,
A send same INVITE with same Sdp to B, 
then B answer it and return 200ok with Sdp to A, 
and then A recevive 200ok and call virtual function 
  virtual void
  InviteSessionHandler::onAnswer(InviteSessionHandle is, const SipMessage& msg,
const SdpContents& sdp)

I want to know in Terminal A,
How to differentiate this two situations in function onAnswer()?
I think the two "msg" and the two "sdp" are all same, 
maybe there are some difference in "is".

Thank you very much.

ZhengChang , 
Beijing , China