Calculate difference between 2 epoch times, large table, optimize having 3 indexes

Question

I have this table that represents packets from a traffic capture called PacketsByDirection (showing only relevant fields):

    FrameNumber FrameTimeEpoch         FlowID    Direction
    288        1430221042.150789000     29      Direction A
    289        1430221042.150922000     29      Direction B

Now, this table has around 2 million rows (packets) and what I need to calculate is, for each packet, the difference of time between him and the previous packet with the same Direction and the same FlowID

I have done this with this query, plus adding indexes to the previous table to make the query faster.

SELECT t1.FrameNumber, flowid, direction,
    FrameTimeEpoch - IFNULL((
                        SELECT MAX(FrameTimeEpoch)
                        FROM PacketsByDirection 
                        WHERE flowid = t1.flowid
                        AND Direction LIKE t1.Direction
                        AND FrameNumber < t1.FrameNumber)
                    ,FrameTimeEpoch) AS TimeFromLastPacketFromSameDirection
FROM PacketsByDirection AS t1

And the result is something like

FrameNumber        FlowID    Direction    TimeFromLastPacketFromSameDirection
        288          29     Direction A                   0
        289          29     Direction B                   0
        290          29     Direction A                   5.422
        291          29     Direction B                   4.356
        292          30     Direction A                   0
        293          30     Direction A                   1.302

And so on. Now, this query takes around 1 hour for 600k rows, and now I'm working with millions of rows so I don't even want to try it. Here is the "explain" output of the query right now (that's a lot of iterations):

So my question is, is there another more efficient way to do this?

Thanks

EDIT: Here is the definition of the table

CREATE TABLE `packetsbydirection` (
  `FrameNumber` int(11) NOT NULL DEFAULT '0',
  `FrameTimeEpoch` varchar(45) NOT NULL,
  `IPSrc` varchar(45) NOT NULL,
  `TCPSrcPort` varchar(45) DEFAULT NULL,
  `UDPSrcport` varchar(45) DEFAULT NULL,
  `IPDst` varchar(45) NOT NULL,
  `TCPDstport` varchar(45) DEFAULT NULL,
  `UDPDstport` varchar(45) DEFAULT NULL,
  `IPLength` varchar(45) NOT NULL,
  `FlowID` int(11) NOT NULL,
  `Direction` varchar(11) CHARACTER SET utf8 DEFAULT NULL,
  KEY `Index2` (`Direction`),
  KEY `Index3` (`FlowID`),
  KEY `Index4` (`FrameNumber`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Answer 1

Not sure if this could work at all, but maybe running numbers could be faster?

    SELECT
      FrameNumber,
      case when FlowID <> @currflow or Direction <> @currdir then @diff := 0 else @diff := FrameTimeEpoch - @epoch end as TimeFromLastPacketFromSameDirection
      , @currflow := FlowID, @currdir := Direction, @diff, @epoch := FrameTimeEpoch
    FROM
      packetsbydirection, (select @epoch := 0, @currflow :="", @currdir := "", @diff := 0) as tmp
    ORDER BY FlowID, Direction, FrameTimeEpoch