Subject: Re: [Int-area] I-D Action: draft-ietf-intarea-tunnels-05.txt

From: "Templin, Fred L" <[email-masked]>
Date: Wed, 3 May 2017 17:48:46 +0000

Hi Joe,

Sorry for the extended delay - see below for responses:

Thanks - Fred

> -----Original Message-----
> From: Joe Touch [mailto:[email-masked]]
> Sent: Wednesday, March 29, 2017 3:06 PM
> To: Templin, Fred L <[email-masked]>
> Cc: [email-masked]
> Subject: Re: [Int-area] I-D Action: draft-ietf-intarea-tunnels-05.txt
> 
> Hi, Fred,
> 
> I think we're agreed except for PMTUD and PLMTUD. See below about the
> latter (AFAICT, if it black holes, then the PLMTUD would detect a
> failure to make forward progress). Black-holing in PMTUD is known and
> not something this (or any other doc) is fixing, but also a known standard.
> 
> Joe
> 
> 
> On 3/29/2017 2:18 PM, Templin, Fred L wrote:
> > ...
> >>> 10) Section 4.2.1, bulleted list toward bottom of section, tunnel
> >>>    "atom" is a very strange word to me. Tunnel "cell"?
> >> The concept of an atomic packet was defined in RFC 6864. This is derived
> >> from that. Cell would be introducing a new term, one that is overloaded
> >> with ATM, which we want to avoid.
> > OK, but then please find a way to call it an "atomic packet".
> 
> Agreed.

OK.

> > ...
> >>> 13) Section 4.2.2, final sentence is incorrect. RFC4821-style MTU
> >>>     probing cannot be used by tunnels due to ECMP because the probe
> >>>     packets may take a different path than the data packets. That
> >>>     is why AERO no longer uses RFC4821 probing.
> >> Regarding ECMP issues, I think we need to wrap this issue up. Here's
> >> what I propose:
> >>
> >> - point out that ECMP causes problems with PMTUD (and can cause problems
> >> with PLMTUD).
> >> - an interface has two choices:
> >>     - keep track of PMTU based on other packet context (flowID or next
> >> header info)
> >>     - merge PMTU feedback, taking the MIN of reported values
> > The problem is that some paths in the multipath may fail to deliver
> > the ICMPs. So there is no way to know whether the MIN has been
> > determined.
> 
> That's no different from the multicast case, though.

In terms of PMTUD, you are right that in a certain sense unicast destinations
with ECMP are like multicast. In other words, a single destination with multiple
paths - some of which may not deliver ICMPs.

But (unlike unicast) the Internet community seems to be OK with the fact
that some multicast group members might not receive multicasts due to
an MTU black hole. For unicast destinations, it can be a real problem  if
there is an MTU black hole along one or more paths of the multipath. 

> >  Also the ingress may be handling transit packets sourced
> > by a very large number of original sources each of which produce
> > a very large number of distinct flows. So, there is no way for the
> > ingress to cache all of the flow information it handles.
> Min requires maintaining the same information as any interface would keep.

I was referring to PLPMTUD here. For PLPMTUD, the probes sent by the
tunnel ingress may take a very different path of the multipath than most
data packets will take. So, there is no way for the ingress to definitely
determine the Min MTU.

> >> There's no magic here. It's a lot like multicast - either keep track in
> >> a way that you *think* correlates to the different PMTU feedback or take
> >> the MIN.
> > It only works if all paths on the mutlipath can be counted on to deliver
> > the ICMPs. If any paths in the multipath fail to deliver the ICMPs, it
> > black holes. And, this is a known problem.
> Again, same as multicast, and frankly also the same as unicast when the
> ICMPs are blocked. That's a known problem with PMTUD.

As above, in some sense it is like multicast. But, the Internet community
has deemed it OK for multicast to black hole for some group members.
For unicast, the behavior has to be deterministic and neither PMTUD
nor PLPMTUD can guarantee that for tunnels.

> >> The current doc does need a scrub to make this point clearly and
> >> consistently.
> > It doesn't work, regardless of the amount of scrubbing.
> 
> If your point is that PMTUD doesn't work and should never be used,
> that's clearly not accurate and unlikely to get WG consensus. You're
> welcome to try, though. However, 1981bis is on its way to increased
> standards maturity as we speak.

Multipath really is problematic for both PMTUD and PLPMTUD *for
tunnels* - that is aside from any considerations for promoting
RFC1981bis to standards-track in the more general sense.

> > ...
> >>> 17) Section 4.2.3 cites RFC4821, but PLPMTUD cannot be used by
> >>>     tunnels due to ECMP.
> >> I disagree; it can, but the system needs to either take the MIN or have
> >> a way to decouple discovered PMTUs in way that can be trusted to
> >> reasonably correspond to the ECMP splitting.
> > It doesn't work in the generalized case. The ECMP might split into a
> > multitude of distinct paths, and there is no way for the ingress to
> > known which of the paths have been tested. And, all it takes is
> > one un-tested path in the multipath and there is potential for a
> > black hole.
> If PLPMTUD thinks the protocol is making forward progress, then it is
> not a black hole.

"Making forward progress" over some paths of the multipath does not
guarantee progress over all paths - some paths might black hole.

> >
> >>> 20) Section 4.3.3, fourth paragraph, "A multipoint tunnel MUST
> >>>     have support for broadcast and multicast" - I think this
> >>>     would be better as a "SHOULD". RFC2529 and AERO support
> >>>     multicast, but RFC5214 does not yet it is widely deployed.
> >> Multicast or its equivalent. Otherwise, you can't support IPv6
> >> multicast, which is a required capability of IPv6.
> > Large NBMA links can connect many nodes - thousands or more.
> > So, for link-scoped multicast, serialized multicast (i.e., multicast
> > via iterative unicast) would not scale.
> 
> Serial multicast is not the only equivalent. LANE pushed broadcast ARPs
> to a unicast ARP server.

Yes, and NBMA had the MARS proposal.

> And yes, that won't scale to millions of links, but then if/when it
> doesn't scale, then you cease to be able to claim this is a valid IP
> link. Multicast is not an optional protocol for IPv6 in particular.

RFC5214 and AERO work fine with IPv6.

> > That is why some large NBMA links (e.g., RFC5214, AERO) use unicast
> > NS/NA/RS/RA instead of link-scoped multicast, as permitted by RFC4861.
> > Link-scoped multicast service discovery (e.g., DHCPv6 discovery) is
> > supported via multicast mapping to a unicast link-layer address.
> Essentially like LANE.

Unlike LANE, AERO links are link-local-only (i.e., there are no on-link
subnets). AERO links connect only routers and/or hosts that act like
routers.

> >>> 23) Section 5.1, first sub-bullet under "Tunnels must obey core IP
> >>>     requirements", Are you meaning to talk about IPv4 DF=1?
> >> Yes, and that should be made more explicit. Also honoring the EMTU_R
> >> limits until told otherwise.
> > OK.
> >
> > One other comment. I agree with figures 12 and 13 but (and I think this is
> > a crucial point) I think they need a supporting sentence or two explaining
> > why the procedure is "fragment then encapsulate" and not "encapsulate
> > then fragment".
> Agreed.

Good.

> > This is the difference between tunnel fragmentation
> > and ordinary outer fragmentation, where your document is correctly
> > advocating tunnel fragmentation. To the best of my knowledge, this was
> > first documented in Section 3.1.7 of RFC2764 and should be cited as such.
> > At least, that is what Bob B. suggested to me about 10yrs ago.
> I'll check that...

Please see the final paragraph of Section 3.12 of the AERO spec for example
text.

Thanks - Fred
[email-masked]

> ----

---

From: Joe Touch <[email-masked]>
Date: Wed, 3 May 2017 11:47:13 -0700

Hi, Fred,

Your response keeps raising the same issues that I think we agree upon:

    - PMTUD always has the possibility of creating black holes (whether
in the presence of multipath or not)

    - PLPMTUD can generate cases where the MIN isn't actually detected

and some claims with which I disagree:

    - that PMPMTUD probes travel different paths than the data

However, PLPMTUD probes are generated by transport or higher layers,
using the same transport protocol and port pairs as the data (they ARE
the data). Those probes should be used to determine an MTU only for a
given flow; the potential hazards of sharing that information across
flows is already discussed in that RFC. The entire point of PMPMTUD is
that the probes travel the same path as the data - and yes, any
multipath system could give false information, but should eventually
converge on a useful minimum as long as data packets get through.

    - that tunnels behave differently than unicast links in this regard

I don't yet see any explanation as to why this would be true.

So I'm left with the following, which I propose as the way forward:

    - the text will be clear about the potential issues for multipath
potentially taking a long time to converge

    - the text will be clear about the potential issues for black-holing

However, in both cases, I don't see a good reason to flag how a tunnel
behaves as unique.

Does that work?

If not, why not?

Joe

---

From: "Templin, Fred L" <[email-masked]>
Date: Wed, 3 May 2017 20:07:44 +0000

Hi Joe,

> Sent: Wednesday, May 03, 2017 11:47 AM

I think I have already said this many times, but let me try again. The
tunnel ingress is not the source of the transit packet; it is only the
source of the tunnel packet. And, the ingress may have to tunnel
a wide variety of transit packets sourced by many original sources.
So, if ECMP somewhere within the tunnel peeks at the transit
packet, then the packet could take a very different path within
the tunnel than the ingress' PLPMTUD probes took. 

> the data).

Maybe we are talking past each other - I thought the document was
asking the tunnel ingress to act as a packetization layer and send
PLMTUD probes as if it were a transport or higher layer?

> Those probes should be used to determine an MTU only for a
> flows is already discussed in that RFC.

I don't think RFC4821 does enough to take ECMP into account when it
talks about storing and sharing discovered MTUs.
 
> The entire point of PMPMTUD is

If the probes look exactly like the data, then the probes will travel the
same ECMP path as the data and will discover a valid MTU yes. The
trouble with tunnels is that there is no way for the ingress to send a
probe for each and every transit packet flow - and, if a flow arrives
that has not been probed it can black hole.

>     - that tunnels behave differently than unicast links in this regard

You won't get me to make this argument - I am convinced that tunnels
are links.

> potentially taking a long time to converge

It's not that it could take a long time to converge - it is that it might
never converge if some paths of the multipath block ICMPs.

> However, in both cases, I don't see a good reason to flag how a tunnel

The tunnel ingress is only the source of the tunnel packets; it is not the
source of the transit packets. The only way for PLPMTUD to function
properly in the presence of ECMP would be for the source of the transit
packets to do the RFC4821 probing - not the tunnel ingress.

Thanks - Fred

---

From: Joe Touch <[email-masked]>
Date: Wed, 3 May 2017 13:38:26 -0700

HI, Fred,

> the tunnel than the ingress' PLPMTUD probes took. 
OK, so we have to separate things out.

PLPMTUD E2E will still work fine - or not - and isn't affected by the
presence of a tunnel per se.

The same is true for PMTUD E2E.

The issue is how the  tunnel itself figures out MTU values larger than
the required minimums for transit to the egress. There are two cases:
    - for egress reassembly, a tunnel protocol can communicate that
reassembly MTU explicitly (and the MIN taken for multipoint tunnels, as
with multicast)
    - for transit between ingress and egress, the tunnel would need to
either use PMTUD (and be susceptible to black holes) or PLPMTUD
        - if it uses PLPMTUD, then steps need to be taken to either
establish the ingress-egress as a single flow (so that ECMP would treat
it as such) or to somehow try to manage subsets of arriving flows as a
single flow
            and any such method needs to be careful to ensure that the
probe packets are not distinguishable from non-probes, or could create
false information

Would that be sufficient?
...
> talks about storing and sharing discovered MTUs.
While I agree, this document is not intended to update that nor to
account for its deficiencies, so I'm not sure what to add other than as
stated above.

...
> never converge if some paths of the multipath block ICMPs.
If the path can't differentiate data and probes (see above), I can't see
how they can be prevented from converging.

...
> packets to do the RFC4821 probing - not the tunnel ingress.
Agreed - the only reason the ingress would want to do PLPMTUD probing is
to increase the size of the chunks sent to the egress.

Joe

---

From: "Templin, Fred L" <[email-masked]>
Date: Wed, 3 May 2017 21:24:38 +0000

Hi Joe,

> Sent: Wednesday, May 03, 2017 1:38 PM

Agreed - E2E from transit packet source to transit packet destination is
not affected by the presence of the tunnel.

OK.

> reassembly MTU explicitly (and the MIN taken for multipoint tunnels, as

I thought for multipoint tunnels all egress' had to have the same reassembly
MTU? Or, if taking the minimum, that minimum value could be conveyed in
the MTU option in a Router Advertisement message.

> either use PMTUD (and be susceptible to black holes)

Yes.

> or PLPMTUD

The "pipe model", yes. The only problem is that an ECMP device could still
do deep-packet inspection and make multipath decisions based on the
transit packet.

> Would that be sufficient?

I think some would still say that an ECMP device that does deep packet
inspection could still trigger multipath based on the transit packet.

> account for its deficiencies, so I'm not sure what to add other than as

Agreed, this is an RFC4821 issue and potentially subject for an erratum
for that document (not this one).

> how they can be prevented from converging.

If the transit packet is visible in-the-clear, then some deep-packet
inspecting ECMP device could still invoke multipath. But, it just occurred
to me that if the transit packet is encrypted then ECMP devices would
only see the tunnel packet headers and not the transit. So, maybe the
tunnel can implement the RFC4821 pipe model for IPsec/TLS?

> to increase the size of the chunks sent to the egress.

I think it still would have problems. For instance, if it discovered that a
1400byte chunk could fit over one path in the multipath, another path
could be constrained to only 1399 bytes and ECMP could bite you again.

Thanks - Fred

---

From: Joe Touch <[email-masked]>
Date: Wed, 3 May 2017 14:38:24 -0700

Winnowing down... I think we're converging. Let me know if not...

Joe

> the MTU option in a Router Advertisement message.
The egresses could theoretically convey different reassembly MTUs, but
the ingress would take the min. If you have  a control protocol that has
all the information, you can push that operation there.

> do deep-packet inspection and make multipath decisions based on the
See next response...
> inspection could still trigger multipath based on the transit packet.

I think I already cover that in saying "not distinguishable" (e.g.,
either by encryption or mimicry)

> tunnel can implement the RFC4821 pipe model for IPsec/TLS?
Encryption is one approach. Some sort of mimicry is another - e.g., if
the ingress encapsulation includes both net and transport, with ports
assigned per "flow group" (such that a single incoming flow belongs to
only one tunneled flow).

> could be constrained to only 1399 bytes and ECMP could bite you again.
If that happens, PLPMTUD would learn it and adapt. Either enough packets
would get through or the ingress would adapt.

Joe

---

From: "Templin, Fred L" <[email-masked]>
Date: Wed, 3 May 2017 21:59:25 +0000

Hi Joe,

> Sent: Wednesday, May 03, 2017 2:38 PM

Sounds OK.

> either by encryption or mimicry)

Encryption for sure, yes. But, I'm not sure about mimicry.

> Encryption is one approach.

Right.

> Some sort of mimicry is another - e.g., if

I think this depends on how deep the ECMP device could inspect. If it
goes past all of the tunnel headers and inspects the transit packet then
ECMP problems can occur.

> would get through or the ingress would adapt.

The problem is that if there are N paths in the multipath the ingress has
no way of knowing that it has probed all N of them. And, if a transit
packet arrives that would be tunneled over a path that has not been
probed, it could black hole if the MTU is too small.

Thanks - Fred

---

From: Joe Touch <[email-masked]>
Date: Wed, 3 May 2017 15:04:12 -0700

Winding down to the last part:

(I agree that encryption or mimicry is useful only when it works, but
not much more can be said than that)

> probed, it could black hole if the MTU is too small.

That's correct - PLPMTUD can fail at any time if the PMTU changes and
becomes smaller (for any reason, including link reconfiguration, path
changes, multipath selection).

That's why it keeps retrying. Again, this isn't new or unique to tunnels.

Joe

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 4 May 2017 17:28:24 +0000

Hi Joe,

> Sent: Wednesday, May 03, 2017 3:04 PM

What is different about tunnels is that they are the sources of the tunnel
packets but are not the sources of the transit packets. And, except for
some tunnel-inserted obfuscation such as encryption, there is no way
for the tunnel to act like it is the source of the transit packets.

That said, it sounds like you may have some ideas on how to re-formulate
some of the text. If you want to do that and then have me re-review it
I could do that.

Thanks - Fred

---

From: Joe Touch <[email-masked]>
Date: Tue, 16 May 2017 16:13:15 -0700

HI Fred,

I'm in the process of the next update, and wanted to clarify the following:

On 3/28/2017 9:36 AM, Templin, Fred L wrote:
> 18) Section 4.3.3, For Multipoint Tunnels, please cite AERO, as
>     well as ISATAP [RFC5214] and 6over4 [RFC2529]. They define
>     an NBMA multipoint tunnel framework that has been around a
>     lot longer than the other examples cited.
That section doesn't list examples of multipoint tunnels per se. It
lists systems that use multipoint tunnels (LISP, TRILL) and discusses
mechanisms that support multipoint tunnels (LANE, etc.).

I'm not sure there's a clearly good, original ref for multipoint tunnels
- the concept has been around in some form since basically the notion of
a tunnel.

Joe

---

From: Joe Touch <[email-masked]>
Date: Tue, 16 May 2017 16:16:43 -0700

Fred,

Regarding the following point:

On 3/28/2017 9:36 AM, Templin, Fred L wrote:
> 19) Section 4.3.3, third paragraph, I thought it was said earlier
>     that all ingress/egress pairs must support the same MTU. I
>     thought we agreed earlier on that that multi-MTU subnets
>     don't work. So, I think it should say that all ingress pairs
>     MUST support a uniform MTU.
The cited section allows for ingress/egress pairs that don't support the
same MTU, but then says that the MTU used for that set is the minimum of
those MTUs.

There's no way (short of configuration management) to ensure that
ingress/egress pairs MUST support the same MTU, but no real value to
requiring that - AFAICT, we just use the min across that set (which is
what that section says to do).

So yes, we do not *use* different MTUs per-destination, but we don't
need to require it AFAICT.

Joe

---

From: "Templin, Fred L" <[email-masked]>
Date: Tue, 16 May 2017 23:24:31 +0000

Interesting timing on this message, but see below:

> Sent: Tuesday, May 16, 2017 4:13 PM

AERO embraces the tunnel as a link in the spirit of your document while
LISP does not. AERO defines "AERO interface" and "AERO link" - LISP goes
out of its way to avoid saying anything about interfaces or links.

> - the concept has been around in some form since basically the notion of

RFC2529 is the first I am aware of.

Fred

---

From: "Templin, Fred L" <[email-masked]>
Date: Tue, 16 May 2017 23:31:05 +0000

Joe,

> Sent: Tuesday, May 16, 2017 4:17 PM

What I am trying to get at is the need for an ingress to determine the
minimum before it sends a packet that may be too big for one of the
multipoint link egresses. On IPv6 links, this could be through receipt
of an MTU option in a Router Advertisement message. Other ways
are possible (e.g., static configuration) but there needs to be some
way for all ingresses to determine the minimum.

Thanks - Fred

---

From: Joe Touch <[email-masked]>
Date: Tue, 16 May 2017 16:36:30 -0700

HI, Fred,

AOK - I can add that. The point is that *IF* you get multiple answers
(regardless of how), you use the min.

And the ingress needs to determine that value before sending for sure
and there's always the possibility of PMTUD doing odd things if ICMPs
are blackholed here - but partial delivery in that case isn't unique to
tunnels.

I'll mention it, though.

joe

---

From: Joe Touch <[email-masked]>
Date: Tue, 16 May 2017 16:54:34 -0700

> RFC2529 is the first I am aware of.
AOK - thanks.

Joe

---

From: Joe Touch <[email-masked]>
Date: Wed, 17 May 2017 11:52:15 -0700

Hi, Fred,

Circling back to this item:

> why the procedure is "fragment then encapsulate" and not "encapsulate
Will do.

> advocating tunnel fragmentation.
Yeah, but I was unable to find definitive and RFC citations for those
terms ("inner fragmentation" and "outer fragmentation"). 4459 mentions
fragmentation of 'inner' and 'outer', but those terms go back to 2003
and before, and I'm not sure warrant a citation.

> To the best of my knowledge, this was
The idea of differentiating inner and outer fragmentation goes back to
RFC2003 at least, AFAICT.

That section of RFC2764 mentions that outer fragmentation avoids
fragmentation inside the tunnel, but doesn't recommend it (it just says
"alternative"). Further, it claims that none of the existing tunneling
protocols support this (even though RFC2003 does).

I'm not convinced this is worth tracking down for its origins. Let me
know if you feel otherwise, but we'd need stronger evidence AFAICT.

Joe

---

From: Joe Touch <[email-masked]>
Date: Wed, 17 May 2017 13:56:37 -0700

Hi, Fred (et al.),

> why the procedure is "fragment then encapsulate" and not "encapsulate
It's actually "on-path fragment (during transit processing, if that
occurs), then encapsulate then (source) fragment.". Figs 12/13 represent
the last two steps; fig 11 describes the first.

I've added a paragraph and clarified the pseudocode to make that clear.

Joe

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 18 May 2017 02:10:11 +0000

Hi Joe,

Can't get to it just now, but I promise I will respond to these as early as possible.
About this particular question, you may know that I hold Bob B. in the highest
esteem and took his word on this subject as I would a voice from the heavens.

I know Bob probably pulls his pants on one leg at a time in the morning just
like we all do, but his long tenure as RFC Editor and authorship of bedrock
docs like RFC1122 have held me in awe of him. That is not to say that I do
not have high regards for you (I do) but to me Bob has always been "the
man". I know - I need to get over that and move on...

Thanks - Fred 

> Sent: Wednesday, May 17, 2017 11:52 AM