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

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Message-ID: <84a4b04f-492e-4004-8fb8-25464aea56e3@kernel.org>
Date: Tue, 22 Apr 2025 09:41:23 -0700
From: David Ahern <dsahern@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com,
 pabeni@...hat.com, horms@...nel.org, idosch@...dia.com, kuniyu@...zon.com,
 Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 2/3] ip: load balance tcp connections to single
 dst addr and port

On 4/20/25 12:04 PM, Willem de Bruijn 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.
> 

can the call to inet_hash_connect be moved up? Get an actual sport
assignment and then use it for routing lookups.