Re: [Tsv-art] [v6ops] [Last-Call] Tsvart last call review of draft-ietf-v6ops-ipv6-ehs-packet-drops-05

Tom Herbert <tom@herbertland.com> Wed, 10 March 2021 02:03 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Tue, 9 Mar 2021 19:03:13 -0700
Message-ID: <CALx6S349X7fQR=9Dj+n5X7ovXsSjLYibv-C-+bL0nkWsYP5NGA@mail.gmail.com>
To: Fernando Gont <fgont@si6networks.com>
Cc: Fred Baker <fredbaker.ietf@gmail.com>, Mark Smith <markzzzsmith@gmail.com>, Gorry Fairhurst <gorry@erg.abdn.ac.uk>, IPv6 Operations <v6ops@ietf.org>, draft-ietf-v6ops-ipv6-ehs-packet-drops.all@ietf.org, last-call@ietf.org, tsv-art@ietf.org
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Subject: Re: [Tsv-art] [v6ops] [Last-Call] Tsvart last call review of draft-ietf-v6ops-ipv6-ehs-packet-drops-05
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On Tue, Mar 9, 2021 at 4:03 PM Fernando Gont <fgont@si6networks.com> wrote:
>
> On 9/3/21 19:07, Tom Herbert wrote:
> [...]
> >
> > Yes, ACLs on transport layer ports are common requirements, however
> > the problem arises from related requirements that arise due to the
> > limitations of routers to be able to locate the transport layer
> > information in a packet. An example of such an implied requirement
> > from this draft is "don't send packets with IPv6 header chains that
> > are too long because some routers can't parse deep enough into packets
> > to find the transport layer ports due to implementation constraints
> > (like limited size parsing buffer)".
>
> You seem to be reading more from the document than what we actually said
> in the document.
>
> There are no requirements in this document. We simply explain things
> operators need to do, what are the associated limitations in real-world
> devices, and what's the likely outcome.
>
> That's not an implied requirement, but simply a description of facts.
>
It's obvious that the implied or at least inferred requirement is that
if a host wants to increase the probability of packets making it to
the destination then they should not make header chains too long. This
would also be an obvious interoperability requirement, i.e. if I make
my header chains too long then packets will be dropped and my host
stack is not interoperable with some elements in the network.

>
>
> > While the rationale for the
> > requirement may make sense, the problem, at least from the host stack
> > perspective of trying to send packets with low probability they'll be
> > dropped, is that a requirement that "don't IPv6 header chains that are
> > too long" is is useless without any quantification as exactly to what
> > "too long" might be.
>
> "too long" for the processing device(s). You don't know what devices
> will process your packets, hence cannot even guess what "too long" might
> mean.
>
> What you know for sure is that the longer the chain, the lower the
> chances of your packets surviving -- as per RFC7872.
>
That seems to me more like an assumption than a proven fact. To prove
it we'd need the data that correlates the length of the chain with
probability of drop, or alternatively, one could survey common router
implementations' capabilities and similarly extrapolate the
correlation. If we had this data then we could derive a meaningful
quantified requirement for both what routers are expected to process
and what hosts can expect. RFC7872 doesn't really have sufficient data
to make this correlation, and besides that it is not current.

In any case, this draft qualitatively describes why routers are
droppings. Which I suppose is good, but, given that information, I
don't see much that helps host developers that are sending packets in
the network and are trying to go beyond sending packets that conform
to the least common denominator of plain TCP/IP.

Tom

> Thanks,
> --
> Fernando Gont
> SI6 Networks
> e-mail: fgont@si6networks.com
> PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
>
>
>
>