netdev - Re: [PATCH v6 0/4] Add support for IPV6 RLB to balance-alb mode

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Date:   Wed, 23 Mar 2022 09:38:45 -0700
From:   Jay Vosburgh <jay.vosburgh@...onical.com>
To:     Jakub Kicinski <kuba@...nel.org>
cc:     David Ahern <dsahern@...nel.org>,
        Sun Shouxin <sunshouxin@...natelecom.cn>, vfalico@...il.com,
        andy@...yhouse.net, davem@...emloft.net, pabeni@...hat.com,
        yoshfuji@...ux-ipv6.org, oliver@...kum.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, huyd12@...natelecom.cn
Subject: Re: [PATCH v6 0/4] Add support for IPV6 RLB to balance-alb mode

Jakub Kicinski <kuba@...nel.org> wrote:

>On Wed, 23 Mar 2022 08:35:03 -0600 David Ahern wrote:
>> On 3/23/22 6:09 AM, Sun Shouxin wrote:
>> > This patch is implementing IPV6 RLB for balance-alb mode.
>>
>> net-next is closed, so this set needs to be delayed until it re-opens.
>
>What I'm not sure of is why this gets reposted after Jiri nacked
>it. A conclusion needs to be reached on whether we want this
>functionality in the first place. Or someone needs to explain 
>to me what the conclusion is if I'm not reading the room right :)

	The summary (from my perspective) is that the alb/rlb technology
more or less predates LACP, and is a complicated method to implement
load balancing across a set of local network peers.  The existing
implementation for IPv4 uses per-peer tailored ARP messages to "assign"
particular peers on the subnet to particular bonding interfaces.  I do
encounter users employing the alb/rlb mode, but it is uncommon; LACP is
by far the most common mode that I see, with active-backup a distant
second.

	The only real advantage alb/rlb has over LACP is that the
balance is done via traffic monitoring (i.e., assigning new peers to the
least busy bond interface, with periodic rebalances) instead of a hash
as with LACP.  In principle, some traffic patterns may end up with hash
collisions on LACP, but will be more evenly balanced via the alb/rlb
logic (but this hasn't been mentioned by the submitter that I recall).
The alb/rlb logic also balances all traffic that will transit through a
given router together (because it works via magic ARPs), so the scope of
its utility is limited.

	As much as I'm all in favor of IPv6 being a first class citizen,
I haven't seen a compelling use case or significant advantage over LACP
stated for alb/rlb over IPv6 that justifies the complexity of the
implementation that will need to be maintained going forward.

	-J

---
	-Jay Vosburgh, jay.vosburgh@...onical.com