header retransmitted   duplicate    matching response
From        same          same               same

 To         same          same    same, but tag may have been added

Call-ID     same          same               same

request URI same          same               n/a

CSeq        same          same               same

Via           -             -      must be local host; check for branch parameter to identify which branch

Looped request are recognized by one of the following:

• The server finds itself in the request's Via list, including any branch parameter. (The server should compute the branch parameter so that it depends on the request URI.) Take a look at sections 6.47.5 and 12.13.1 of RFC2543bis for details on computing and matching branch parameter for loop detection.
• The Via header added by the server prior to proxying the request is the same as one of the Via's in the Via list.