[OpenSER-Users] Insert a Header into ACK message

Dear All: I've tried many methods to add Max-Forwarsd header into ACK message (message 8), all methods I tried were fail. According to RFC 3261 8.1.1.6, Max-Forwards must exist in this ACK message. Can anyone tell me how to add a header into ACK message? Thank you very much. UA11 : NUT(Proxy 1) : UA12 | : | : | |-------:------->| : | 1. INVITE |<------:--------| : | 2. 407 |-------:------->| : | 3. ACK | : | : | |-------:------->| : | 4. INVITE | : |-------:------->| 5. INVITE |<------:--------| : | 6. 100 | : | : | | : |<------:--------| 7. 486 (Busy Here) | : | : | | : |-------:------->| 8. ACK | : | : | |<------:--------| : | 9. 486 | : | : | |-------:------->| : |10. ACK | : | : | Best Regards, Fangyu Ling