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

[Ido Schimmel <idosch@nvidia.com>]

On Thu, Apr 24, 2025 at 10:35:19AM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.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@google.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>