A field of "x-ws.ajax: delta=true" is added after NGINX.

[nexthop]

Hi all:

I met a stranger problem. a field of "x-ws.ajax: delta=true" is added after
NGINX.

1) Topology
     client   ---------[NGINX] -----------------------[IIS 6.0] server
                        ^___Act as reverse proxy

     NGINX( 1.0.5 and keep-alive enabled)
--------------------------------------------------------------------------------------------------
2) Issue
    (1) We can not list a directory from IIS server.
    (2) If we bypass NGINX, everything is good well.
--------------------------------------------------------------------------------------------------

3) we sniff on both sides of NGINX and compare the pcap file, we found an
extra-field is added after NGIX.

---------------------------------------------------------------Before
NGINX-------------------------------------------------------------
POST /server/WorkFlowProcess/WaittingProcessWorkFlow.aspx?_dc=1313745754796
HTTP/1.1
Connection: keep-alive
Host: www.abc.com
Accept: */*
Accept-Language: en
x-requested-with: XMLHttpRequest
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer:
http://www.abc.com/server/WorkFlowProcess/WaittingProcessWorkFlow.aspx
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
Content-Length: 1172
Cache-Control: no-cache
Cookie: ASP.NET_SessionId=2lscnq45aaihogaw0igsoi45

----------------------------------------------After
NGINX---------------------------------------------------------------------------------
 POST /server/WorkFlowProcess/WaittingProcessWorkFlow.aspx?_dc=1313745754796
HTTP/1.1
Accept: */*
Accept-Language: en
x-ws.ajax: delta=true
        *  <============Here, the filed is added*
x-requested-with: XMLHttpRequest
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer:
http://www.abc.com/server/WorkFlowProcess/WaittingProcessWorkFlow.aspx
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
Host: www.abc.com
Content-Length: 1172
Connection: Keep-Alive
Cache-Control: no-cache
Cookie: ASP.NET_SessionId=2lscnq45aaihogaw0igsoi45
============================================================================

We suspect that the extra field is not recongized by server.

My question is :
1) what does it means? I can not find any usful information from google,
there is no such source code in nginx code tree.
2) what does it impact on?

Appreciate your help at advice.

thanks
George
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