TCP kernel hash function
From: Soohyun Cho (s0c6496_at_cs.tamu.edu)
Date: 05/30/04
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Date: Sun, 30 May 2004 03:05:44 -0500
Hi all,
I want to find established TCP connections between same source-destination
hosts regardless of their ports.
While I was looking at Linux TCP kernel source codes I found Linux use hash
tables to find owner socket id of received packets.
This may be good to improve TCP session lookup speed than searching TCP
sockets using doubly linked lists as in BSD.
However if I want to find TCP connections going to the same destination host
I found it is very inefficient because Linux does not maintain linked lists
of socks.
I found sock->pprev or sock->next of struct sock are nothing but itself or
empty.
I needed to search whole possible hash value space to find small set of TCP
connections.
In Linux, this value is tcp_ehash_size and it is 131072. It's huge enough to
degrade TCP performance.
It took too much time to find some TCP socks going to the same hosts.
I couldn't narrow down the lookup speed because tcp_hashfn() generates so
randomly distributed hash numbers rather than grouping them according to
destination ip address.
The hash value could be anything from 0 to 131072..
I think this happens because source port number space is so big.
Is there any way to retrieve this kind of information fast in Linux kernel?
Do I have to insert additional structure to struct sock for this?
I would really appreciate who answer my question.
Thanks,
Soohyun Cho.
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