SIP电话设计思路

1. caller调用方法Call createCall(String callee)，过程如下：
    1）把String callee转换为javax.sip.Address；
    2）创建请求，Request invite = protocolProvider.getMessageFactory    ().createRequest(
                toHeader.getAddress().getURI()
                , Request.INVITE
                , callIdHeader
                , cSeqHeader
                , fromHeader
                , toHeader
                , viaHeaders
                , maxForwards);
   3）创建sdp的ContentTypeHeader，contentTypeHeader=headerFactory.createContentTypeHeader(
                    "application", "sdp");
   4）创建客户端事务ClientTransaction，clientTransaction=sipProvider.getNewClientTransaction(invite);
   5）create the call，and creates a new call participant with participantAddress，
CallParticipantSipImpl callParticipant = new CallParticipantSipImpl(
            containingTransaction.getDialog().getRemoteParty(), call)
，设置callParticipant状态为INITIATING_CALL，添加该新创建的Call;
   6）创建双方具体的多媒体会话约定，设置请求消息的内容为该多媒体数据，然后发送出去。
    invite.setContent(callSession.createSdpOffe (intendedDestination),    contentTypeHeader);
2. callee端process inviteRequest，过程如下：
   1）creates a new call participant，设置participant的sdp描述，
   CallParticipantSipImpl callParticipant
            = createCallParticipantFor(serverTransaction,    sourceProvider);
   callParticipant.setSdpDescription(
                                        new String(invite.getRawContent()));
  2）判断本地uri是否是远端caller要呼叫的uri