question: void verifyUniqueXids ( List < OFMessage > msgs ) { if ( seenXids == null ) seenXids = new HashSet < Long > ( ) ; for ( OFMessage m : msgs ) { long xid = m . getXid ( ) ; assertTrue ( "Xid in messags is 0" , xid != 0 ) ; assertFalse ( "Xid " + xid + " has already been used" , seenXids . contains ( xid ) ) ; seenXids . add ( xid ) ; } }
target:  make sure that the transaction ids in the given messages are not 0 and differ between each other. While it's not a defect per se if the xids are we want to ensure we use different ones for each message we send.