[Serusers] Why does added headers show up in the CANCEL?

[Henrik Gustafsson]

Hello! I'm playing around with my own module which is adding or 
replacing a header in proxied INVITE requests.

I'm using the "parse_headers ... anchor_lump ... insert_new_lump_before" 
method on the proxied request and the outgoing INVITE looks 
great with the new header.

But the request is forwarded by using the tm module and when the 
transaction receives a CANCEL the outgoing CANCEL gets the 
header added to the initial INVITE. That would not be a problem 
for a normal SIP UA, since unknown headers should be ignored, 
but this UA from a big  telecom vendor throws a 400 Bad request.

Why is the added headers added to the CANCEL? Shouldn't just 
the required and transaction identifying headers be enough in a 
CANCEL?  Could I do anything different to prevent this from 
happening?

/Henrik Gustafsson