在VOIP 中的SIP协定中，如何预防呼叫被无穷次数的在网路上被转送?

INVITE sip:bob@biloxi.com SIP/2.0

    Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds

    Max-Forwards: 70

    To: Bob <sip:bob@biloxi.com>

    From: Alice <sip:alice@atlanta.com>;tag=1928301774

    Call-ID: a84b4c76e66710@pc33.atlanta.com

    CSeq: 314159 INVITE

    Contact: <sip:alice@pc33.atlanta.com>

    Content-Type: application/sdp

    Content-Length: 142