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Date:	Wed, 22 Oct 2014 20:15:09 +0900
From:	Erik Kline <ek@...gle.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev <netdev@...r.kernel.org>,
	Ben Hutchings <ben@...adent.org.uk>,
	Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: [RFC PATCH v2 net-next] net: ipv6: Add a sysctl to make
 optimistic addresses useful candidates

On Wed, Oct 22, 2014 at 7:12 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Mi, 2014-10-22 at 14:25 +0900, Erik Kline wrote:
>> Hello.
>>
>> On Wed, Oct 22, 2014 at 4:45 AM, Hannes Frederic Sowa
>> <hannes@...essinduktion.org> wrote:
>> > Hi,
>> >
>> > On Di, 2014-10-21 at 13:05 +0900, Erik Kline wrote:
>> >> Add a sysctl that causes an interface's optimistic addresses
>> >> to be considered equivalent to other non-deprecated addresses
>> >> for source address selection purposes.  Preferred addresses
>> >> will still take precedence over optimistic addresses, subject
>> >> to other ranking in the source address selection algorithm.
>> >>
>> >> This is useful where different interfaces are connected to
>> >> different networks from different ISPs (e.g., a cell network
>> >> and a home wifi network).
>> >>
>> >> The current behaviour complies with RFC 3484/6724, and it
>> >> makes sense if the host has only one interface, or has
>> >> multiple interfaces on the same network (same or cooperating
>> >> administrative domain(s), but not in the multiple distinct
>> >> networks case.
>> >>
>> >> For example, if a mobile device has an IPv6 address on an LTE
>> >> network and then connects to IPv6-enabled wifi, while the wifi
>> >> IPv6 address is undergoing DAD, IPv6 connections will try use
>> >> the wifi default route with the LTE IPv6 address, and will get
>> >> stuck until they time out.
>> >>
>> >> Also, because optimistic addresses can actually be used, issue
>> >> an RTM_NEWADDR as soon as DAD starts.  If DAD fails an separate
>> >> RTM_DELADDR will be sent.
>> >>
>> >> Also: add an entry in ip-sysctl.txt for optimistic_dad.
>> >>
>> >> Signed-off-by: Erik Kline <ek@...gle.com>
>> >> ---
>> >>  Documentation/networking/ip-sysctl.txt | 13 ++++++++++++
>> >>  include/linux/ipv6.h                   |  1 +
>> >>  include/uapi/linux/ipv6.h              |  1 +
>> >>  net/ipv6/addrconf.c                    | 36 ++++++++++++++++++++++++++++++++--
>> >>  4 files changed, 49 insertions(+), 2 deletions(-)
>> >>
>> >> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
>> >> index 0307e28..e03cf49 100644
>> >> --- a/Documentation/networking/ip-sysctl.txt
>> >> +++ b/Documentation/networking/ip-sysctl.txt
>> >> @@ -1452,6 +1452,19 @@ suppress_frag_ndisc - INTEGER
>> >>       1 - (default) discard fragmented neighbor discovery packets
>> >>       0 - allow fragmented neighbor discovery packets
>> >>
>> >> +optimistic_dad - BOOLEAN
>> >> +     Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
>> >> +             0: disabled (default)
>> >> +             1: enabled
>> >> +
>> >> +use_optimistic - BOOLEAN
>> >> +     If enabled, do not classify optimistic addresses as deprecated during
>> >> +     source address selection.  Preferred addresses will still be chosen
>> >> +     before optimistic addresses, subject to other ranking in the source
>> >> +     address selection algorithm.
>> >> +             0: disabled (default)
>> >> +             1: enabled
>> >> +
>> >>  icmp/*:
>> >>  ratelimit - INTEGER
>> >>       Limit the maximal rates for sending ICMPv6 packets.
>> >> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
>> >> index ff56053..7121a2e 100644
>> >> --- a/include/linux/ipv6.h
>> >> +++ b/include/linux/ipv6.h
>> >> @@ -42,6 +42,7 @@ struct ipv6_devconf {
>> >>       __s32           accept_ra_from_local;
>> >>  #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
>> >>       __s32           optimistic_dad;
>> >> +     __s32           use_optimistic;
>> >>  #endif
>> >>  #ifdef CONFIG_IPV6_MROUTE
>> >>       __s32           mc_forwarding;
>> >> diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
>> >> index efa2666..e863d08 100644
>> >> --- a/include/uapi/linux/ipv6.h
>> >> +++ b/include/uapi/linux/ipv6.h
>> >> @@ -164,6 +164,7 @@ enum {
>> >>       DEVCONF_MLDV2_UNSOLICITED_REPORT_INTERVAL,
>> >>       DEVCONF_SUPPRESS_FRAG_NDISC,
>> >>       DEVCONF_ACCEPT_RA_FROM_LOCAL,
>> >> +     DEVCONF_USE_OPTIMISTIC,
>> >>       DEVCONF_MAX
>> >>  };
>> >>
>> >> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
>> >> index 725c763..c2fddb7 100644
>> >> --- a/net/ipv6/addrconf.c
>> >> +++ b/net/ipv6/addrconf.c
>> >> @@ -1169,6 +1169,9 @@ enum {
>> >>       IPV6_SADDR_RULE_LABEL,
>> >>       IPV6_SADDR_RULE_PRIVACY,
>> >>       IPV6_SADDR_RULE_ORCHID,
>> >> +#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
>> >> +     IPV6_SADDR_RULE_NOT_OPTIMISTIC,
>> >> +#endif
>> >>       IPV6_SADDR_RULE_PREFIX,
>> >>       IPV6_SADDR_RULE_MAX
>> >>  };
>> >> @@ -1257,10 +1260,17 @@ static int ipv6_get_saddr_eval(struct net *net,
>> >>               score->scopedist = ret;
>> >>               break;
>> >>       case IPV6_SADDR_RULE_PREFERRED:
>> >> +         {
>> >>               /* Rule 3: Avoid deprecated and optimistic addresses */
>> >> +             u8 avoid = IFA_F_DEPRECATED;
>> >> +#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
>> >> +             if (!score->ifa->idev->cnf.use_optimistic)
>> >> +                     avoid |= IFA_F_OPTIMISTIC;
>> >> +#endif
>> >>               ret = ipv6_saddr_preferred(score->addr_type) ||
>> >> -                   !(score->ifa->flags & (IFA_F_DEPRECATED|IFA_F_OPTIMISTIC));
>> >> +                   !(score->ifa->flags & avoid);
>> >>               break;
>> >> +         }
>> >>  #ifdef CONFIG_IPV6_MIP6
>> >>       case IPV6_SADDR_RULE_HOA:
>> >>           {
>> >> @@ -1299,6 +1309,14 @@ static int ipv6_get_saddr_eval(struct net *net,
>> >>               ret = !(ipv6_addr_orchid(&score->ifa->addr) ^
>> >>                       ipv6_addr_orchid(dst->addr));
>> >>               break;
>> >> +#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
>> >> +     case IPV6_SADDR_RULE_NOT_OPTIMISTIC:
>> >> +             /* Optimistic addresses still have lower precedence than other
>> >> +              * preferred addresses.
>> >> +              */
>> >> +             ret = !(score->ifa->flags & IFA_F_OPTIMISTIC);
>> >> +             break;
>> >> +#endif
>> >
>> > I wonder a bit why this rule is not directly ordered after
>> > IPV6_SADDR_RULE_PREFERRED? This would e.g. matter for privacy addresses.
>>
>> Privacy addresses ("tempaddrs") that win in earlier checks are
>> preferred before optimistic in this code (i.e. a tempaddr on the same
>> outgoing interface is preferred before an optimistic address).
>>
>> Similarly, a non-tentative non-privacy address (same outgoing
>> interface, same label, ...) will match before an optimistic address,
>> but only until DAD completes and the address is no longer optimistic.
>> I think this is in keeping with the spirit of the RFC 3484/6724 rules.
>
> Oh yes, I see. I had the evaluation order messed up. ;)
>
> So the question I should be asking would be. AFAIR optimistic addresses
> should be handled like deprecated ones, so I am a bit concerned adding a
> non-conditional rule before the RULE_PREFIX check.
>
> Shouldn't we only break the tie *after* longest prefix match then? If
> you do that before longest prefix match I would prefer ret being masked
> by use_optimisitic flag.

There was some text suggesting that the longest prefix could be
ignored (like for DNS load-balancing reasons).  But I think in this
particular case it doesn't matter, so I'll move it after.  Ack.

>> After there's an RFC 7217 implementation, EUI-64-based SLAAC could be
>> disabled by folks.
>
> Ack.
>
>>
>> >>       case IPV6_SADDR_RULE_PREFIX:
>> >>               /* Rule 8: Use longest matching prefix */
>> >>               ret = ipv6_addr_diff(&score->ifa->addr, dst->addr);
>> >> @@ -3222,8 +3240,13 @@ static void addrconf_dad_begin(struct inet6_ifaddr *ifp)
>> >>        * Optimistic nodes can start receiving
>> >>        * Frames right away
>> >>        */
>> >> -     if (ifp->flags & IFA_F_OPTIMISTIC)
>> >> +     if (ifp->flags & IFA_F_OPTIMISTIC) {
>> >>               ip6_ins_rt(ifp->rt);
>> >> +             /* Because optimistic nodes can receive frames, notify
>> >> +              * listeners. If DAD fails, RTM_DELADDR is sent.
>> >> +              */
>> >> +             ipv6_ifa_notify(RTM_NEWADDR, ifp);
>> >> +     }
>> >
>> > I wonder if we can now delete the ipv6_ifa_notify(RTM_NEWADDR, ifp) in
>> > addrconf_dad_completed.
>>
>> I don't know what everyone's general preference would be, but mine
>> would be to err on the side of notifying on state changes.  It seems
>> harmless to me to keep it in, and something in userspace might want to
>> know if/when DAD completes.
>
> Userspace expects to communicate with an address which gets announced
> via RTM_NEWADDR, so I would make the RTM_NEWADDR notifications
> conditional on use_optimistic flag in both, the completed and the
> dad_begin function. This sounds like the best option to me, no?

That's easy enough to do in addrconf_dad_begin().  Unfortunately, by
the time we get to addrconf_dad_completed() the IFA_F_OPTIMISTIC flag
has already been cleared.

I have a version that attempts to take its best guess as to whether an
RTM_NEWADDR _should_ have already been sent--something like:

#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
        // We probably already sent a notification in addrconf_dad_begin().
        if (!ifp->idev->cnf.optimistic_dad || !ifp->idev->cnf.use_optimistic)
#endif
        ipv6_ifa_notify(RTM_NEWADDR, ifp);

but that doesn't seem that clean to me (apart from the ifdef-y
messiness of it).  Do you think this "best guess" approach is
reasonable?

With just the "use_optimistic" check in addrconf_dad_begin(),
userspace can still communicate with addresses it gets via
RTM_NEWADDR, it will just get /two/ such notifications: one when it
can probably use it and one when it definitely can.

Thoughts?
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