netdev - Re: [PATCH net-next v2 2/3] ip: load balance tcp connections to single dst addr and port

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Message-ID: <CANn89iLD0bGKdFzSJDiiFhT=Np3LKHM86WE8daFO1zdEzWA+fg@mail.gmail.com>
Date: Thu, 24 Apr 2025 09:51:15 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, horms@...nel.org, idosch@...dia.com, 
	kuniyu@...zon.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 2/3] ip: load balance tcp connections to
 single dst addr and port

On Thu, Apr 24, 2025 at 7:35 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> Load balance new TCP connections across nexthops also when they
> connect to the same service at a single remote address and port.
>
> This affects only port-based multipath hashing:
> fib_multipath_hash_policy 1 or 3.
>
> Local connections must choose both a source address and port when
> connecting to a remote service, in ip_route_connect. This
> "chicken-and-egg problem" (commit 2d7192d6cbab ("ipv4: Sanitize and
> simplify ip_route_{connect,newports}()")) is resolved by first
> selecting a source address, by looking up a route using the zero
> wildcard source port and address.
>
> As a result multiple connections to the same destination address and
> port have no entropy in fib_multipath_hash.
>
> This is not a problem when forwarding, as skb-based hashing has a
> 4-tuple. Nor when establishing UDP connections, as autobind there
> selects a port before reaching ip_route_connect.
>
> Load balance also TCP, by using a random port in fib_multipath_hash.
> Port assignment in inet_hash_connect is not atomic with
> ip_route_connect. Thus ports are unpredictable, effectively random.
>
> Implementation details:
>
> Do not actually pass a random fl4_sport, as that affects not only
> hashing, but routing more broadly, and can match a source port based
> policy route, which existing wildcard port 0 will not. Instead,
> define a new wildcard flowi flag that is used only for hashing.
>
> Selecting a random source is equivalent to just selecting a random
> hash entirely. But for code clarity, follow the normal 4-tuple hash
> process and only update this field.
>
> fib_multipath_hash can be reached with zero sport from other code
> paths, so explicitly pass this flowi flag, rather than trying to infer
> this case in the function itself.
>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>