RE: [PATCH net-next] ipv6: use prandom_u32() for ID generation

[David Laight]

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

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