Can anyone help me write this query more efficiently?

I have a table that captures TCP traffic, and I'd like to update a column called RowNumForFlow which is simly the sequential number of the IP packet in that flow. The code below works fine, but it is slow.

declare @FlowID int
declare @LastRowNumInFlow int
declare @counter1 int
set @counter1 = 0

while (@counter1 < 1)
BEGIN
set @counter1 = @counter1 + 1   

-- 1)
select  top 1
        @FlowID = t.FlowID
from    Traffic t 
where   t.RowNumInFlow is null

if (@FlowID is null)
    break

-- 2)
set @LastRowNumInFlow = null

select  top 1
        @LastRowNumInFlow = RowNumInFlow
from    Traffic 
where   FlowID=@FlowID and RowNumInFlow is not null
order by ID desc

if @LastRowNumInFlow is null
    set @LastRowNumInFlow = 1
else
    set @LastRowNumInFlow = @LastRowNumInFlow + 1

update Traffic set RowNumInFlow = @LastRowNumInFlow
where ID = (select top 1 ID from Traffic where 
            flowid = @FlowID and RowNumInFlow is null)


END

Example table values after query has run:

    
ID  FlowID  RowNumInFlow
448923  44  1
448924  44  2
448988  44  3
448989  44  4
448990  44  5
448991  44  6
448992  44  7
448993  44  8
448995  44  9
448996  44  10
449065  44  11
449063  45  1
449170  45  2
449171  45  3
449172  45  4
449187  45  5