Re: [PATCH net-next v2] net: ipv4: add support for ECMP hash policy choice

On 08/03/17 14:05, Jakub Sitnicki wrote:

Hi Nikolay,

On Tue, Mar 07, 2017 at 11:01 AM GMT, Nikolay Aleksandrov wrote:

quoted

This patch adds support for ECMP hash policy choice via a new sysctl
called fib_multipath_hash_policy and also adds support for L4 hashes.
The current values for fib_multipath_hash_policy are:
 0 - layer 3 (default)
 1 - layer 4
If there's an skb hash already set and it matches the chosen policy then it
will be used instead of being calculated. The ICMP inner IP addresses use
is removed.

Signed-off-by: Nikolay Aleksandrov <redacted>
---
v2:
 - removed the output_key_hash as it's not needed anymore
 - reverted to my original/internal patch with L3 as default hash

What about ICMP PTB (Fragmentation Needed) forwarding that makes PMTUD
work with ECMP in setups like described in RFC7690 [1]?

  ptb -> router ecmp -> next hop L4/L7 load balancer -> destination

       router --> load balancer 1 --->
            \\--> load balancer 2 ---> load-balanced service
             \--> load balancer N --->

Removing special treatment of ICMP errors will break it, won't it?

Yes, I am aware and this decision was made with that in mind.
We'd like to use the HW hash when available and IIRC that doesn't play well with
special-casing ICMP errors for anycast as it may not match also. Another thing,
again if I remember correctly, was that this behaviour is closer to how hardware
handles ECMP.

One thing we can do is leave the current L3 behaviour with ICMP error handling
and add a new L3 mode that tries to use the skb hash when available and doesn't
care about the packet type.

What do you think ?

FWIW, I gave a run to your patch (default settings, L3 hash) with a test
script [2] that simulates such a setup and it confirmed my worries - PTB
errors don't travel back to the source host any more.

Thanks for testing.

-Jakub

[1] https://tools.ietf.org/html/rfc7690#section-2
[2] https://github.com/jsitnicki/tools/commit/ccb85e68421df4ffd8b7abf00f6f5fe1c6a5af76