中心信令通过sip向摄像头播放广播

Question

我创建了一个SIP中心信令服务器，接了一个大华的摄像头上去，想通过服务器向摄像头播放广播。注册心跳都是正常的，也能收到摄像头的广播INVITE消息请求，如下是请求与我的回复：
摄像头请求：

INVITE sip:34020000001370000001@10.1.7.118:5060 SIP/2.0
Call-ID: 2023050513581111799611532
Contact: <sip:34020000001370000001@10.1.7.77:5060>
Content-Type: application/sdp
CSeq: 1 INVITE
Expires: 120
From: <sip:34020000001370000001@10.1.7.77:5060>;tag=2b81f72d6362bedf5e7188c61b6750f0
Max-Forwards: 70
Subject: 34020000001370000001:03d7a8ef3276a8ef327,34020000001370000001:0552354c54f3354c54f
To: <sip:34020000001370000001@10.1.7.118:5060>
User-Agent: SIP UAS V3.0.0.954271
Via: SIP/2.0/UDP 10.1.7.77:5060;rport=5060;branch=z9hG4bK9384137aed01571c7ab5f8fc2a543eb4;received=10.1.7.77
Content-Length: 233

v=0
o=34020000001370000001 0 0 IN IP4 10.1.7.77
s=Play
i=VCam BroadCast Session
c=IN IP4 10.1.7.77
t=0 0
m=audio 10008 RTP/AVP 8 96
a=recvonly
a=rtpmap:8 PCMA/8000/1
a=rtpmap:96 PS/90000
y=0912379370
f=v/0/0/0/0/0a/1/8/1

我的回复：

SIP/2.0 200 OK
CSeq: 1 INVITE
Call-ID: 2023050513581111799611532
From: <sip:34020000001370000001@10.1.7.77:5060>;tag=2b81f72d6362bedf5e7188c61b6750f0
To: <sip:34020000001370000001@10.1.7.118:5060>
Via: SIP/2.0/UDP 10.1.7.77:5060;rport=5060;branch=z9hG4bK9384137aed01571c7ab5f8fc2a543eb4;received=10.1.7.77
Contact: <sip:34020000001370000001@10.1.7.118:5060>
Content-Type: application/SDP
Content-Length: 176

v=0
o=34020000001370000001 0 0 IN IP4 10.1.7.118
s=Play
c=IN IP4 10.1.7.118
t=0 0
m=audio 8000 RTP/AVP 8
a=rtpmap:8 PCMA/8000
a=sendonly
y=0912379370
f=v/////a/1/8/1

后面就接不上摄像头的ACK请求了，为什么？