[ovs-dev] OVN: IPv6 Periodic Router Advertisement Plan

[Brian Haley]

Sorry for the slow review, was on PTO.  Comments below.

On 08/22/2017 05:43 PM, Mark Michelson wrote:
> Hi folks,
> 
> For a while now, in a "learn the OVN project from scratch" project, I've
> been investigating what it would take in order to allow for OVN to send
> periodic router advertisements (RAs) as part of a greater goal of rounding
> out IPv6 support. I've had some discussions about individual parts of this
> with various people, and now I want to come forward with an actual proposal
> of how I envision accomplishing this.
> 
> First, let's start with what an RA is. It is an ICMPv6 message that a
> router sends out in order to say "I am a router" to any other nodes on a
> link. Beyond that, an RA may contain information such as MTU, supported
> IPv6 prefixes, hop count, and other useful information about the router
> interface. The RA can be sent for one of two reasons:
> 
> 1) A host on the link has sent a router solicitation (RS) message, and the
> router is responding directly to that.
> 2) The router can be configured to periodically send RAs to all nodes on
> the link.
> 
> For (1), Numan Siddique currently has a set of patches up for review that
> adds this support. I am attempting to add (2).
> 
> My primary goal here is determining how to configure ovn-controllers to
> know to periodically send RAs, and to ensure they have the necessary
> information to do so. The rest of this e-mail details how I propose to do
> that.
> 
> There is a patch[1] currently up for consideration that adds a new column
> to the northbound Logical_Router_Port table called ipv6_ra_configs. This
> currently only has two configurable options:
> * address_mode

I'm assuming that based on the mode that certain RA flags will be 
enabled?  Maybe this was covered in Numan's change, haven't taken a look 
there.  For example, with stateful the M flag is set.

> * mtu
> In order to allow periodic router advertisements, I propose the following
> options to be added to the ipv6_ra_configs
> * send_periodic - boolean indicating if periodic RAs should be sent.
> Defaults to false
> * max_interval - integer indicating the maximum number of seconds to wait
> between sending periodic RAs. Defaults to 600 seconds.
> * max_interval - integer indicating the minimum number of seconds to wait
> between sending periodic RAs. Defaults to 0.33 * max_interval (200 seconds
> if max_interval is set to its default).

For min_interval (typo), I would just follow what the RFC says and not 
only do 0.33 if max_interval is at its default:

     Default: 0.33 * MaxRtrAdvInterval If
     MaxRtrAdvInterval >= 9 seconds; otherwise, the
     Default is MaxRtrAdvInterval.

Looks good otherwise.

-Brian

> This is the minimum requirement. RFC 4861 defines many other configuration
> options that will alter the contents of RAs, but those can be added in a
> future update if necessary.
> 
> If someone wishes to periodically send RAs, they can set the send_periodic
> northbound value true and adjust the interval to their liking. ovn-northd,
> upon seeing that the send_periodic is set true, will copy the entirety of
> the ipv6_ra_configs column into the options column of the corresponding
> Port_Binding entry of the southbound database. To scope these as IPv6 RA
> options, the keys in the southbound database will be prepended with
> ipv6_ra_. So for instance, when copying the "address_mode" from the
> northbound database, it would become "ipv6_ra_address_mode" in the
> southbound database. In addition to the contents of the northbound
> Logical_Router_Port ipv6_ra_configs, ovn-northd will insert data about the
> RAs themselves, such as the network prefixes to be advertised (learned from
> the logical router port's networks) and the source ethernet and IP
> addresses to use in RAs.
> 
> Example:
> I create a logical router port as follows:
>      ovn-nbctl lrp-add lr0 lr0-p0 00:11:22:33:44:55 2001:db8::1/64
> I then modify the logical router port's ipv6_ra_configs as follows:
>      ovn-nbctl set Logical_Router_Port lr0-p0
> ipv6_ra_config:send_periodic=true
> When northd runs, it will populate the southbound database's Port_Binding
> for lr0-p0 as follows:
>      options: {ipv6_ra_send_periodic=true,
> ipv6_ra_src_mac=00:11:22:33:44:55, ipv6_ra_src_addr=2001:db8::1,
> ipv6_ra_prefixes=2001:db8::/64}
> 
> ovn-controller already has code that determines which datapaths are local
> to the chassis. With these datapaths found, we can add code to make
> ovn-controller iterate over the port bindings that pertain to that
> datapath. If any port bindings have ipv6_ra_send_periodic=true in their
> options, then the ovn-controller needs to send out periodic RAs.
> 
> The sending of RAs will closely mirror the method by which ovn-controller
> currently sends gratuitous ARP packets. That is, a structure will be
> allocated that contains all the necessary information about what to send in
> the outgoing RA, plus information on when the next RA should be sent out.
> Handling of when to send out RAs will be handled similarly to gratuitous
> ARPs as well, except that instead of having a backoff timer, the interval
> will be recalculated each time based on the configuration.
> 
> If you've made it this far in the e-mail, then thank you! If you have any
> suggestions for other methods of doing this or have any questions about any
> part of the proposed process, please let me know.
> 
> Mark Michelson
> 
> [1] https://patchwork.ozlabs.org/patch/804391/
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