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

i have table represents packets traffic capture called packetsbydirection (showing relevant fields):

    framenumber frametimeepoch         flowid    direction     288        1430221042.150789000     29      direction     289        1430221042.150922000     29      direction b 

now, table has around 2 million rows (packets) , need calculate is, each packet, difference of time between him , previous packet with same direction , same flowid

i have done this query, plus adding indexes previous table make query faster.

select t1.framenumber, flowid, direction,     frametimeepoch - ifnull((                         select max(frametimeepoch)                         packetsbydirection                          flowid = t1.flowid                         , direction t1.direction                         , framenumber < t1.framenumber)                     ,frametimeepoch) timefromlastpacketfromsamedirection packetsbydirection t1 

and result

framenumber        flowid    direction    timefromlastpacketfromsamedirection         288          29     direction                   0         289          29     direction b                   0         290          29     direction                   5.422         291          29     direction b                   4.356         292          30     direction                   0         293          30     direction                   1.302 

and on. now, query takes around 1 hour 600k rows, , i'm working millions of rows don't want try it. here "explain" output of query right (that's lot of iterations):

so question is, there more efficient way this?

thanks

edit: here definition of 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 

not sure if work @ all, maybe running numbers faster?

    select       framenumber,       case when flowid <> @currflow or direction <> @currdir @diff := 0 else @diff := frametimeepoch - @epoch end timefromlastpacketfromsamedirection       , @currflow := flowid, @currdir := direction, @diff, @epoch := frametimeepoch           packetsbydirection, (select @epoch := 0, @currflow :="", @currdir := "", @diff := 0) tmp     order flowid, direction, frametimeepoch