Query to update rowNum - tsql

Question

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

Answer 1

Something like this:
update
T
set
RowNumInFlow = #TheNumber
FROM
(
SELECT
ID,
ROW_NUMBER() OVER (PARTITION BY FlowID ORDER BY ID) AS TheNumber
FROM
Traffic
) T
After comments:
update
T1
set
RowNumInFlow = TR.TheNumber
FROM
Traffic T1
JOIN
(
SELECT
ID,
ROW_NUMBER() OVER (PARTITION BY FlowID ORDER BY ID) AS TheNumber
FROM
Traffic
) TR ON T1.ID = TR.ID
WHERE
T1.RowNumInFlow IS NULL --like this?