netdev - Re: [PATCH net-next v3 0/4] net: ipv6: Improve user experience with multipath routes

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Message-ID: <71e661bd-e26d-2629-06bb-888f6a09b06d@cumulusnetworks.com>
Date:   Sun, 29 Jan 2017 18:29:07 -0700
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Roopa Prabhu <roopa@...ulusnetworks.com>
Cc:     netdev@...r.kernel.org, nicolas.dichtel@...nd.com
Subject: Re: [PATCH net-next v3 0/4] net: ipv6: Improve user experience with
 multipath routes

On 1/29/17 4:55 PM, Roopa Prabhu wrote:
> On 1/29/17, 10:02 AM, David Ahern wrote:
>> On 1/28/17 6:00 PM, Roopa Prabhu wrote:
>>>> 4. Route Appends
>>>>    - IPv6 allows nexthops to be appended to an existing route. In this
>>>>      case one notification is sent per nexthop added
>>> thanks for listing all of these...I think you mentioned this case to me..
>>> but I don't remember now why this notification is
>>> sent per nexthop added. This is an update to an existing multipath route.
>>> so seems like the notification should be a RTM_NEWROUTE with the full RTA_MULTIPATH route
>>> (similar to route add)
>> It could be; it's a question of what should userspace get -- the full route or the change? Append to me suggests the latter - userspace is told what changed. It is simpler kernel code wise to send the full new route. The append changes were done after our conversation. ;-)
> ok, yeah. you listing all the cases here made it more simpler to understand in the context of other notifications :). I would prefer all
> RTM_NEWLINK notifications (ie new add or update to an existing route..replace/append), contain the full route via RTA_MULTIPATH.

Ok. I will send a v4 in a day or 2 (wait for more comments) with APPEND only generating 1 notification. Below response is based on this change.

> 
>>
>>> Same holds for replace, I know the code might be tricky here...but the route replace
>>> is also an update to an existing multipath route and hence should be a RTM_NEWROUTE
>>> with the full multipath route (RTA_MULTIPATH) that changed (from userspace semantics POV)
>> It is. The only change on a replace is the encoding of all routes in RTA_MULTIPATH which is the point of this patch set. On successful replace, only 1 notification is sent with the full route.
> ok, good.
>>
>>> I don't have a better solution, but with the above still being different, wondering
>>> if its worth the risk changing the api for just a few notifications.
>> Data centers are moving to L3, and multipath is a big part of that. Anyone who looks at ip -6 route enough knows it gets painful mentally pulling the individual routes into a single one. In addition, using RTA_MULTIPATH is more efficient at scale.
>>
>> Furthermore, I have a follow on patch set that returns the route matched on an ip route get lookup. Returning the full multipath route is an important part to understanding the full context of the route to an address.
> 
> ok, sure.  If we are moving to RTA_MULTIPATH, I just want to make sure all the notifications consistently move to multipath.
> 
> so, to summarize and to get on the same page as you,
> - all RTM_NEWLINK notifications will contain RTA_MULTIPATH when the route in the kernel is a multipath route:
>             - since ipv6 allows add/append of a single nexthop, the notification for the first nexthop may go out without a RTA_MULTIPATH

No. I notification for all cases - new, replace, append.

> - RTM_DELETE will also contain RTA_MULTIPATH when the user tries to delete the full route with RTA_MULTIPATH
>             - since ipv6 allows deleting of a single nexthop, RTM_DELETE may contain a single nexthop delete ie without RTA_MULTIPATH (this is just to continue
>                 supporting the single nexthop delete support that ipv6 has)

Let's give an example for each case:

1. Add - full multipath route

This route command:
ip -6 ro add vrf red 2001:db8:200::/120 nexthop via 2001:db8:1::2 nexthop via 2001:db8:2::2

generates this notification (ip -t mon ro):

2001:db8:200::/120 table red metric 1024
        nexthop via 2001:db8:1::2  dev eth1 weight 1
        nexthop via 2001:db8:2::2  dev eth2 weight 1


2. Delete - 1 notification for each hop for all combinations of delete commands


3. Replace -  full multipath route

This route command (with the one from Add case already in place):
ip -6 ro replace vrf red 2001:db8:200::/120 nexthop via 2001:db8:1::16 nexthop via 2001:db8:2::16

generates a single notification:

Replaced 2001:db8:200::/120 table red metric 1024
        nexthop via 2001:db8:1::16  dev eth1 weight 1
        nexthop via 2001:db8:2::16  dev eth2 weight 1


4. Append - 1 route after all hops are appended

This append command (starting with the one installed from the Replace case):
ip -6 ro append vrf red 2001:db8:200::/120 nexthop via 2001:db8:2::20 nexthop via 2001:db8:1::20

generates a single notification:

Append 2001:db8:200::/120 table red metric 1024
        nexthop via 2001:db8:2::20  dev eth2 weight 1
        nexthop via 2001:db8:1::20  dev eth1 weight 1
        nexthop via 2001:db8:1::16  dev eth1 weight 1
        nexthop via 2001:db8:2::16  dev eth2 weight 1

(The Replaced and Append annotations are due to a local iproute2 patch; iproute2 does
not currently distinguish NEWROUTE cases.)