Re: Benjamin Kaduk's No Objection on draft-ietf-6man-grand-05: (with COMMENT)

[Jen Linkova <[e-mail]>]

Hi Ben,

Thanks a lot for your thorough review and text suggestions!

> Section 1
>
>    return traffic flow.  While waiting for the resolution to complete
>    routers only keep a very small number of packets in the queue, as
>    recommended in Section 7.2.2 [RFC4861].  All subsequent packets
>    arriving before the resolution process finishes are likely to be
>    dropped.  [...]
>
> My reading of RFC 4861 is that it suggests for new packets arriving to
> replace the oldest entry in this queue of packets, which doesn't really
> seem consistent with "all subsequent packets [...] are likely to be
> dropped".  "Any additional packets arriving before the resolution
> process finishes are likely to result in dropped packets" might be more
> consistent with the RFC 4861 guidance.

Wow, [facepalm] - thanks a lot for catching this.
I'll update the draft with the text you propose.


> Section 2
>
>                                       As per Section 7.2.2 of [RFC4861]
>        Routers MUST buffer at least one data packet and MAY buffer more,
>        while resolving the packet destination address.  However most
>        router implementations limit the buffer size to a few packets
>        only, so all subsequent packets for the host global address are
>        dropped, until the address resolution process is completed.
>
> [same as above re "subsequent"]

I'll update the text with the following:
"However, most router implementations limit the buffer size to a few
packets only, and some implementations are known to buffer just one
packet.
So any subsequent packets arriving before the address resolution
process is completed are causing packet loss by replacing older
packets in the buffer.
"

>    router's neighbor cache.  However, the same sequence of events happen
>    when the host starts using a new global address previously unseen by
>    the router, such as a new privacy address [RFC4941] or if the
>    router's Neighbor Cache has been flushed.
>
> RFC 4941 has been obsoleted by RFC 8981.

That's what happens when the author is lazy and not moving the draft
forward fast enough...References become obsolete...Thanks, fixed!

> Section 3
>
>    o  If the router does not have a Neighbor Cache entry for the
>       address, a STALE entry needs to be created.
>
> (editorial) The lead-in to this list leaves me unclear about when it's
> desired for this entry to be created.  The conditional nature of this
> statement suggests that creating as a result of receiving the first
> packet for that address might be okay, but I don't think that's actually
> the intent.

How about:
"If the router does not have a Neighbor Cache entry for the address, a
STALE entry needs to be created proactively, prior to arrival of the
first packet intended for that address."

> Section 4.1
>
>    To minimize the potential disruption in case of duplicate addresses
>    the node should not set the Override flag for a preferred address and
>    must not set the Override flag if the address is in Optimistic
>    [RFC4429] state.
>
> I suggest adding "preferred address" to the terminology list (with RFC
> 4862 reference).

Done.

>    It should be noted that the proposed mechanism does not cause any
>    significant increase in multicast traffic.  The additional multicast
>    unsolicited NA would proactively create a STALE cache entry on
>    routers as discussed below.  When the router receives the return
>    traffic flows it does not need to send multicast NSes to the
>    solicited node multicast address but would be sending unicast NSes
>    instead.  Therefore the total amount of multicast traffic should not
>    increase.
>
> Hmm, I might frame this as "this procedure would only produce an
> increase in the overall amount of multicast traffic if no return traffic
> arrives to the node that sent the unsolicited NA".

How about this text:
"It should be noted that the proposed mechanism does not cause any
significant increase in multicast traffic. The additional multicast
unsolicited NA would proactively create a STALE cache entry on routers
as discussed below. When the router receives the return traffic flows
it does not need to send multicast NSes to the solicited node
multicast address but would be sending unicast NSes instead. Therefore
this procedure would only produce an increase in the overall amount of
multicast traffic if no return traffic arrives for the address that
sent the unsolicited NA or if the router does not create a STALE entry
upon receiving such NA. The increase would be negligible as that
additional traffic is a few orders of magnitude less than the usual
level of Neighbor Discovery multicast traffic."

> Section 4.2
>
>    This document updates [RFC4861] so that routers create a new Neighbor
>    Cache entry upon receiving an unsolicited Neighbor Advertisement.
>
> I'd suggest "upon receiving an unsolicited Neighbor Advertisement for an
> address that does not already have a Neighbor Cache entry".

Done.

> Section 5.2
>
>    However most likely those packets would have been dropped anyway:
>    creating the INCOMPLETE entry is usually triggered by traffic, so the
>    router probably has some packets in the buffer already, dropping
>    subsequent packets received before the address resolution is
>    completed.
>
> I *think* the packet-dropping scenario here is the same one that I
> mentioned previously in the context of "subsequent packets" (not) being
> dropped, involving the very small queue on the router of packets to an
> address waiting for resolution.

Yes, you are right.

>  If so, I might suggest a phrasing such
> as "the creation of the INCOMPLETE entry is usually triggered by inbound
> traffic, and due to the limited queue size on the router for packets
> directed to addresses that are waiting for resolution, most packets
> arriving prior to the solicited NA would be dropped anyway".

Actually as a result of other comments that section has been re-written anyway.
The new text:
https://datatracker.ietf.org/doc/html/draft-ietf-6man-grand-06#section-5.2

Please let me know if it does not address your concerns or more work is needed.

> Section 5.3.1
>
> As a thought experiment, I tried comparing this scenario to a similar
> one where the host uses optimistic DAD but does not send an unsolicited
> NA.  In that case, it seems like the optimistic traffic would go out,
> and when the return traffic arrives the router would create a STALE
> entry and do full discovery to find the original owner of the address,
> so some of the responses to the optimistic (probe) traffic might make it
> to the original owner of the address.  That's in contrast to this
> scenario where the return traffic goes to the host that triggered it.
> IIUC in both cases we rely on the host properly doing DAD to stop using
> the optimistic address, and so the scenarios do seem pretty similar
> overall.

Yes. The last paragraph of that section is saying more or less the
same (well, at least that was my intention, not sure if that section
is clear enough).
Do you think the text shall be clarified further?

> Section 6.1.1
>
>    NEW TEXT:
>
>    ------------------------------------------------------------------
>
>    When a valid Neighbor Advertisement is received (either solicited or
>    unsolicited), the Neighbor Cache is searched for the target's entry.
>    If no entry exists, hosts SHOULD silently discard the advertisement.
>    There is no need to create an entry if none exists, since the
>    recipient has apparently not initiated any communication with the
>    target.  Routers SHOULD create a new entry for the target address
>    with the link-layer address set to the Target link-layer address
>    option (if supplied).  The entry's reachability state MUST also be
>    set to STALE.  If the received Neighbor Advertisement does not
>    contain the Target link-layer address option the advertisement SHOULD
>    be silently discarded.
>
> (nit) I'm not sure why we need the word "also" in "MUST also be set to
> STALE".

Removed.

> Section 10
>
> I'd probably reiterate the (small) risk of the 7-second window of loss
> of traffic discussed in §5.3.2 as a known+accepted security
> consideration.

I've added the following text:
"As discussed in Section 5.3.2 the worst case scenario might cause a
disruption for up to 7 seconds. This risk is considered acceptable due
to very low probability of that scenario. More importantly, for all
cases described in Section 5 the rightful owner can prevent disruption
caused by an accidental address duplication just by implementing the
mechanism described in this document. If the rightful owner sends
unsolicited NAs before using the address, the STALE entry would be
created on the router NC and any subsequent unsolicited NAs sent from
the host with an Optimistic address would not override the NC entry."


>    A malicious host could attempt to exhaust the neighbor cache on the
>    router by creating a large number of STALE entries.  However this
>    attack vector is not new and this document does not increase the risk
>    of such an attack: the attacker could do it, for example, by sending
>    a NS or RS packet with SLLAO included.  All recommendations from
>    [RFC6583] still apply.
>
> Would there be any value in having the neighbor cache entries created on
> routers by unsolicited NA be placed in a dedicated storage class, such
> that the NDP prioritization recommended by RFC 6583 would be expanded to
> include "never seen", "unsolicited NA", and "seen before" categories
> with corresponding eviction priorities?

To be honest I'm not sure what kind of attack it would prevent.
Treating "never seen" differently from "seen before" (aka STALE and
REACHABLE) helps protecting from outside scanning.
For the host inside the segment there are multiple ways to create a
STALE entry, as mentioned above. So the router shall probably just
treat all STALE entries the same way.

> Section 12.1
>
> RFC 6775 and 8505 probably don't need to be normative, since the
> registration-based approach to ND is the approach not taken by this
> document.
>
> We also only cite RFC 8305 once, just as an example, so that may not
> need to be normative either.

Yes, good point, fixed.

--
SY, Jen Linkova aka Furry