NTLM not working on ATS 6.2.1 ( recursive prompt for auth)

I have configured ATS as a web proxy in a NTLM authentication domain; and, have a plugin with a function for TS_HTTP_READ_REQUEST_HDR_HOOK say, func_1. In func_1 which is invoked during the request header, I create a hook for TS_HTTP_SEND_RESPONSE_HDR_HOOK (say, func_2) and then do a TSHttpTxnReenable(txnp, TS_EVENT_HTTP_ERROR); Subsequently, when func_2 is invoked during the SEND_RESPONSE_HEADER event, I frame a “407 Proxy Auth required” response header along with Proxy-Authenticate set to “NTLM” and then do a TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE); Func_1() { …. … TSHttpTxnHookAdd(txnp, TS_HTTP_SEND_RESPONSE_HDR_HOOK, TSContCreate(responseHook, TS_NULL_MUTEX)); } TSHttpTxnReenable(txnp, TS_EVENT_HTTP_ERROR); …. return; } func_2() { … TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE); return; } What happens is that even though the client sends a “Connection: keep-alive”, ATS seems to be returning a “Connection: close” which results in the connection getting closed. > GET http://www.purple.com/ HTTP/1.1 > Host: www.purple.com > Proxy-Authorization: NTLM TlRMTVNTUAABAAAABoIIAAAAAAAAAAAAAAAAAAAAAAA= > User-Agent: curl/7.44.0 > Accept: */* > Proxy-Connection: Keep-Alive > < HTTP/1.1 407 Proxy Authentication Required < Date: Sun, 03 Sep 2017 10:47:24 GMT < Connection: close < Via: http/1.1 POD128-CentOS7-01 (ApacheTrafficServer/6.2.1) < Server: ATS/6.2.1 < Proxy-Authenticate: NTLM < Content-Length: 15 < Content-Type: text/plain < 0 15 0 0 0 0 0 0 --:--:-- 0:00:01 --:--:-- 0 * Closing connection 7 Why is ATS closing the connection?And, how can I control this behavior?