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[TCP]: secure_tcp_sequence_number() should not use a too fast clock 

TCP V4 sequence numbers are 32bits, and RFC 793 assumed a 250 KHz clock.
In order to follow network speed increase, we can use a faster clock, but
we should limit this clock so that the delay between two rollovers is
greater than MSL (TCP Maximum Segment Lifetime : 2 minutes)

Choosing a 64 nsec clock should be OK, since the rollovers occur every
274 seconds.

Problem spotted by Denys Fedoryshchenko

[ This bug was introduced by f859581519 ]

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2007-10-01 13:58:36 -07:00 committed by David S. Miller
parent 32740ddc10
commit 9b42c336d0
1 changed files with 6 additions and 4 deletions
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10
drivers/char/random.c
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@ -1550,11 +1550,13 @@ __u32 secure_tcp_sequence_number(__be32 saddr, __be32 daddr,
* As close as possible to RFC 793, which * As close as possible to RFC 793, which
* suggests using a 250 kHz clock. * suggests using a 250 kHz clock.
* Further reading shows this assumes 2 Mb/s networks. * Further reading shows this assumes 2 Mb/s networks.
* For 10 Gb/s Ethernet, a 1 GHz clock is appropriate. * For 10 Mb/s Ethernet, a 1 MHz clock is appropriate.
* That's funny, Linux has one built in! Use it! * For 10 Gb/s Ethernet, a 1 GHz clock should be ok, but
* (Networks are faster now - should this be increased?) * we also need to limit the resolution so that the u32 seq
* overlaps less than one time per MSL (2 minutes).
* Choosing a clock of 64 ns period is OK. (period of 274 s)
*/ */
seq += ktime_get_real().tv64; seq += ktime_get_real().tv64 >> 6;
#if 0 #if 0
printk("init_seq(%lx, %lx, %d, %d) = %d\n", printk("init_seq(%lx, %lx, %d, %d) = %d\n",
saddr, daddr, sport, dport, seq); saddr, daddr, sport, dport, seq);