From: David Laight <David.Laight@ACULAB.COM> To: 'Willy Tarreau' <firstname.lastname@example.org>, "email@example.com" <firstname.lastname@example.org>, "email@example.com" <firstname.lastname@example.org> Cc: David Miller <email@example.com>, Jakub Kicinski <firstname.lastname@example.org>, Alexey Kuznetsov <email@example.com>, Hideaki YOSHIFUJI <firstname.lastname@example.org>, Amit Klein <email@example.com>, Eric Dumazet <firstname.lastname@example.org> Subject: RE: [PATCH net-next] ipv6: use prandom_u32() for ID generation Date: Mon, 31 May 2021 10:41:18 +0000 [thread overview] Message-ID: <e4cc31c1fead46b3aa1132937a720da2@AcuMS.aculab.com> (raw) In-Reply-To: <email@example.com> From: Willy Tarreau > Sent: 29 May 2021 12:08 > > This is a complement to commit aa6dd211e4b1 ("inet: use bigger hash > table for IP ID generation"), but focusing on some specific aspects > of IPv6. > > Contary to IPv4, IPv6 only uses packet IDs with fragments, and with a > minimum MTU of 1280, it's much less easy to force a remote peer to > produce many fragments to explore its ID sequence. In addition packet > IDs are 32-bit in IPv6, which further complicates their analysis. On > the other hand, it is often easier to choose among plenty of possible > source addresses and partially work around the bigger hash table the > commit above permits, which leaves IPv6 partially exposed to some > possibilities of remote analysis at the risk of weakening some > protocols like DNS if some IDs can be predicted with a good enough > probability. > > Given the wide range of permitted IDs, the risk of collision is extremely > low so there's no need to rely on the positive increment algorithm that > is shared with the IPv4 code via ip_idents_reserve(). We have a fast > PRNG, so let's simply call prandom_u32() and be done with it. > > Performance measurements at 10 Gbps couldn't show any difference with > the previous code, even when using a single core, because due to the > large fragments, we're limited to only ~930 kpps at 10 Gbps and the cost > of the random generation is completely offset by other operations and by > the network transfer time. In addition, this change removes the need to > update a shared entry in the idents table so it may even end up being > slightly faster on large scale systems where this matters. > > The risk of at least one collision here is about 1/80 million among > 10 IDs, 1/850k among 100 IDs, and still only 1/8.5k among 1000 IDs, > which remains very low compared to IPv4 where all IDs are reused > every 4 to 80ms on a 10 Gbps flow depending on packet sizes. The problem is that, on average, 1 in 2^32 packets will use the same id as the previous one. If a fragment of such a pair gets lost horrid things are likely to happen. Note that this is different from an ID being reused after a count of packets or after a time delay. So you still need something to ensure IDs aren't reused immediately. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2021-05-31 10:41 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-29 11:07 Willy Tarreau 2021-05-31 10:41 ` David Laight [this message] 2021-05-31 11:19 ` Willy Tarreau 2021-05-31 19:27 ` Eric Dumazet 2021-06-01 5:20 ` patchwork-bot+netdevbpf
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