Re: [PATCH net-next 5/5] ipv6: Compute multipath hash for forwarded ICMP errors from offending packet

[Tom Herbert]

On Fri, Oct 28, 2016 at 1:32 AM, Jakub Sitnicki <jkbs@xxxxxxxxxx> wrote:
> On Thu, Oct 27, 2016 at 10:35 PM GMT, Tom Herbert wrote:
>> On Mon, Oct 24, 2016 at 2:28 AM, Jakub Sitnicki <jkbs@xxxxxxxxxx> wrote:
>>> Same as for the transmit path, let's do our best to ensure that received
>>> ICMP errors that may be subject to forwarding will be routed the same
>>> path as flow that triggered the error, if it was going in the opposite
>>> direction.
>>>
>> Unfortunately our ability to do this is generally quite limited. This
>> patch will select the route for multipath, but I don't believe sets
>> the same link in LAG and definitely can't help switches doing ECMP to
>> route the ICMP packet in the same way as the flow would be. Did you
>> see a problem that warrants solving this case?
>
> The motivation here is to bring IPv6 ECMP routing on par with IPv4 to
> enable its wider use, targeting anycast services. Forwarding ICMP errors
> back to the source host, at the L3 layer, is what we thought would be a
> step forward.
>
> Similar to change in IPv4 routing introduced in commit 79a131592dbb
> ("ipv4: ICMP packet inspection for multipath", [1]) we do our best at
> L3, leaving any potential problems with LAG at lower layer (L2)
> unaddressed.
>
ICMP will almost certainly take a different path in the network than
TCP or UDP due to ECMP. If we ever get proper flow label support for
ECMP then that could solve the problem if all the devices do a hash
just on <srcIP, destIP, FlowLabel>.

If this patch is being done to be compatible with IPv4 I guess that's
okay, but it would be false advertisement to say this makes ICMP
follow the same path as the flow being targeted in an error.
Fortunately, I doubt anyone can have a dependency on this for ICMP.

In the realm of OAM with UDP encapsulation this requirement does come
up (that OAM messages can follow the same path as a particular flow).
That case is solvable by always using a UDP encapsulation with same
addresses, ports, and flow label. Unfortunately for that we still have
a few devices that insist on looking into the UDP payload to do
ECMP...

Tom

>>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>>> index 1184c2b..c0f38ea 100644
>>> --- a/net/ipv6/route.c
>>> +++ b/net/ipv6/route.c
>
> [...]
>
>>> @@ -1168,6 +1192,8 @@ void ip6_route_input(struct sk_buff *skb)
>>>         tun_info = skb_tunnel_info(skb);
>>>         if (tun_info && !(tun_info->mode & IP_TUNNEL_INFO_TX))
>>>                 fl6.flowi6_tun_key.tun_id = tun_info->key.tun_id;
>>> +       if (unlikely(fl6.flowi6_proto == IPPROTO_ICMPV6))
>>> +               fl6.mp_hash = ip6_multipath_icmp_hash(skb);
>>
>> I will point out that this is only
>
> Sorry, looks like part of your reply got cut short. Could you repost?
>
> -Jakub
>
> [1] https://git.kernel.org/torvalds/c/79a131592dbb81a2dba208622a2ffbfc53f28bc0