SIP/2.0 200 OK
Allow: INVITE,BYE,ACK,OPTIONS,CANCEL,INFO,PRACK,SUBSCRIBE,NOTIFY,REFER
Call-ID: 50000
CSeq: 42 OPTIONS
From: <sip:nm@nm>;tag=root
To: <sip:nm2@nm2>;tag=D-A4D68D01-040b57cd086efbed6b3757823e1d2760ead7d8c8
Via: SIP/2.0/UDP nm;received=224.186.237.184;rport=26810;branch=foo
Content-Length: 0

HTTP/1.1 401 Unauthorized
Content-Length: 0
Connection: close
WWW-Authenticate: Digest realm="realm@easycwmp",qop="auth",nonce="9ddfe8381808ec4a8baefb3646501341000f9a5f",opaque="328458fab28345ae87ab3210a8513b14eff452a2"