[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZyJo1561ADF_e2GO@shredder.mtl.com>
Date: Wed, 30 Oct 2024 19:11:51 +0200
From: Ido Schimmel <idosch@...sch.org>
To: David Ahern <dsahern@...nel.org>
Cc: Vladimir Vdovin <deliran@...dict.gg>, netdev@...r.kernel.org,
davem@...emloft.net
Subject: Re: [PATCH] net: ipv4: Cache pmtu for all packet paths if multipath
enabled
On Tue, Oct 29, 2024 at 05:22:23PM -0600, David Ahern wrote:
> On 10/29/24 9:21 AM, Vladimir Vdovin wrote:
> > Check number of paths by fib_info_num_path(),
> > and update_or_create_fnhe() for every path.
> > Problem is that pmtu is cached only for the oif
> > that has received icmp message "need to frag",
> > other oifs will still try to use "default" iface mtu.
> >
> > An example topology showing the problem:
> >
> > | host1
> > +---------+
> > | dummy0 | 10.179.20.18/32 mtu9000
> > +---------+
> > +-----------+----------------+
> > +---------+ +---------+
> > | ens17f0 | 10.179.2.141/31 | ens17f1 | 10.179.2.13/31
> > +---------+ +---------+
> > | (all here have mtu 9000) |
> > +------+ +------+
> > | ro1 | 10.179.2.140/31 | ro2 | 10.179.2.12/31
> > +------+ +------+
> > | |
> > ---------+------------+-------------------+------
> > |
> > +-----+
> > | ro3 | 10.10.10.10 mtu1500
> > +-----+
> > |
> > ========================================
> > some networks
> > ========================================
> > |
> > +-----+
> > | eth0| 10.10.30.30 mtu9000
> > +-----+
> > | host2
> >
> > host1 have enabled multipath and
> > sysctl net.ipv4.fib_multipath_hash_policy = 1:
> >
> > default proto static src 10.179.20.18
> > nexthop via 10.179.2.12 dev ens17f1 weight 1
> > nexthop via 10.179.2.140 dev ens17f0 weight 1
> >
> > When host1 tries to do pmtud from 10.179.20.18/32 to host2,
> > host1 receives at ens17f1 iface an icmp packet from ro3 that ro3 mtu=1500.
> > And host1 caches it in nexthop exceptions cache.
> >
> > Problem is that it is cached only for the iface that has received icmp,
> > and there is no way that ro3 will send icmp msg to host1 via another path.
> >
> > Host1 now have this routes to host2:
> >
> > ip r g 10.10.30.30 sport 30000 dport 443
> > 10.10.30.30 via 10.179.2.12 dev ens17f1 src 10.179.20.18 uid 0
> > cache expires 521sec mtu 1500
> >
> > ip r g 10.10.30.30 sport 30033 dport 443
> > 10.10.30.30 via 10.179.2.140 dev ens17f0 src 10.179.20.18 uid 0
> > cache
> >
>
> well known problem, and years ago I meant to send a similar patch.
Doesn't IPv6 suffer from a similar problem?
>
> Can you add a test case under selftests; you will see many pmtu,
> redirect and multipath tests.
>
> > So when host1 tries again to reach host2 with mtu>1500,
> > if packet flow is lucky enough to be hashed with oif=ens17f1 its ok,
> > if oif=ens17f0 it blackholes and still gets icmp msgs from ro3 to ens17f1,
> > until lucky day when ro3 will send it through another flow to ens17f0.
> >
> > Signed-off-by: Vladimir Vdovin <deliran@...dict.gg>
Thanks for the detailed commit message
Powered by blists - more mailing lists