FYI
We just posted revision 08 which addresses all the comments given during the
review (thank you all for that). From the authors perspective, the document is
ready to go.
Thanks.
Jorge
-----Original Message-----
From: "Rabadan, Jorge (Nokia - US/Mountain View)" <jorge.raba...@nokia.com>
Date: Thursday, January 17, 2019 at 11:10 PM
To: Benjamin Kaduk <ka...@mit.edu>, The IESG <i...@ietf.org>
Cc: "draft-ietf-bess-evpn-df-election-framew...@ietf.org"
<draft-ietf-bess-evpn-df-election-framew...@ietf.org>, Stephane Litkowski
<stephane.litkow...@orange.com>, "bess-cha...@ietf.org" <bess-cha...@ietf.org>,
"bess@ietf.org" <bess@ietf.org>
Subject: Re: Benjamin Kaduk's Discuss on
draft-ietf-bess-evpn-df-election-framework-07: (with DISCUSS and COMMENT)
Benjamin,
Thank you very much for your review.
Satya and I have looked at your points one by one, please see in-line. Let
us know if you are ok now. We'll publish revision 08 soon.
Thanks.
Jorge
-----Original Message-----
From: Benjamin Kaduk <ka...@mit.edu>
Date: Tuesday, January 8, 2019 at 6:52 PM
To: The IESG <i...@ietf.org>
Cc: "draft-ietf-bess-evpn-df-election-framew...@ietf.org"
<draft-ietf-bess-evpn-df-election-framew...@ietf.org>, Stephane Litkowski
<stephane.litkow...@orange.com>, "bess-cha...@ietf.org" <bess-cha...@ietf.org>,
"stephane.litkow...@orange.com" <stephane.litkow...@orange.com>,
"bess@ietf.org" <bess@ietf.org>
Subject: Benjamin Kaduk's Discuss on
draft-ietf-bess-evpn-df-election-framework-07: (with DISCUSS and COMMENT)
Resent-From: <alias-boun...@ietf.org>
Resent-To: <jorge.raba...@nokia.com>, <satya...@cisco.com>,
<saja...@cisco.com>, <jdr...@juniper.net>, <kiran.naga...@nokia.com>,
<senthil.sathap...@nokia.com>
Resent-Date: Tuesday, January 8, 2019 at 6:51 PM
Benjamin Kaduk has entered the following ballot position for
draft-ietf-bess-evpn-df-election-framework-07: Discuss
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----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------
It's not really clear to me that the question of Updating 7432 has
been
settled by the responses to the directorate reviews; I've noted a
few
places in the text that are problematic in this regard, in the
COMMENT
section.
[JORGE] We finally agreed on making it updating 7432 and explain why in
the intro/abstract. Thanks.
[concerns about combinatoric explosion were overblown; removed]
Section 3.3
Section 7.6 of [RFC7432] describes how the value of the ES-Import
Route Target for ESI types 1, 2, and 3 can be auto-derived by
using
the high-order six bytes of the nine byte ESI value. The same
auto-
derivation procedure can be extended to ESI types 0, 4, and 5 as
long
as it is ensured that the auto-derived values for ES-Import RT
among
different ES types don't overlap.
How do I ensure that the auto-derived values don't overlap?
[JORGE] The autoderivation in RFC7432 for ESI types 1, 2 and 3 can be used
"only if it produces ESIs that satisfy the uniqueness requirement specified
above." RFC7432 does not specify how the overlap is avoided, it is out of
scope, but one may think the operator must manually check that the values
auto-deriving the 9-octet ESI value don't match. We added the following text,
let us know if it is okay:
"As in [RFC7432], the mechanism to guarantee that the auto-derived ESI or
ES-import RT values for different ESIs do not match is out of scope of this
document."
Section 4.2
The ESI value MAY be set to all 0's in the
Weight
function below if the operator so chooses.
I'm not 100% sure I'm interpreting this correctly, but this sounds
like a
piece of device-specific configuration (i.e., configured by the
operator)
that must be the same across all devices for correct operation, but
is not
covered by the advertisement of the DF Election Exctended
Community. This
would decrease the robustness of the system to basically the
"experimental"
level of DF election algorithm 31, which also relies on universal
agreement
of manual configuration. Is this actually something we want to
include?
[Satya] This is to accommodate the case in
https://tools.ietf.org/html/draft-mohanty-bess-evpn-bum-opt-00.
Specifically, if the same set of PES are multi-homed to the same set of
ESes, setting the ES to 0, would result in the "same unique" PE be the DF for a
given EVI for all those ESes.
Use can be made of this property in the optimization in
https://tools.ietf.org/html/draft-mohanty-bess-evpn-bum-opt-00.
Yes, it would need manual configuration to enable this.
Section 5
The AC-DF capability MAY be used with any "DF Alg" algorithm. It
MUST
As written, this suggests that it is true for any current or future
algorithm, which is in conflict with the text in Section 3.2 that
notes
that "for any new DF Alg defined in future, its
applicability/compatibility
to the existing capabilities must be assessed on a case by case
basis." It
seems more prudent to make the assessment after the relevant
technologies
are both extant, so I would suggest this be non-normative text,
perhaps
"the AC-DF capability is expected to be of general applicability
with any
future 'DF Alg' algorithm".
[JORGE] Good point. We added "The AC-DF capability is expected to be of
general applicability with any future DF Algorithm."
----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------
Section 1.2.1
I a little bit wonder if the risk of poor distribution of DFs with
the
default algorithm is being oversold -- any "hash identifiers into
buckets"
scheme will be susceptible to pessimal input, but if the inputs are
not
attacker-controlled and the pessimal inputs are unlikely to occur
randomly,
we may not need to care.
2- Even in the case when the Ethernet Tag distribution is
uniform the
instance of a PE being up or down results in re-computation
((v
mod N-1) or (v mod N+1) as is the case); the resulting modulus
value need not be uniformly distributed because it can be
subject
to the primality of N-1 or N+1 as may be the case.
This is making some assumptions about the (potential) distribution
of the
tag values that could be made more clear, as otherwise the
primality is
not particularly relevant (particularly for an actual uniform
distribution
that covers all possible values). Similarly below, by the CLRS
reference
(CLRS probably has the ability to assume that we're running on
binary
computers and may even be doing things like operating on pointers,
which
tend to have fixed structure in the low-order bits due to alignment
considerations, etc. For these (human-allocated?) integer
identifiers it's
less clear what assumptions should come into play.)
[Satya] As I replied to Adam:
The Ethernet tag that identifies the BD can be as large as 2^24; however,
it is not guaranteed that the tenant BD on the ES will conform to a uniform
distribution. In fact, it up to the customer what BDs they will configure on
the ES. Quoting Knuth [Art of Computer Programming Pg. 516]
" In general, we want to avoid values of M that divide r^k+a or r^k−a,
where k
and a are small numbers and r is the radix of the alphabetic
character set
(usually r=64, 256 or 100), since a remainder modulo such a value of
M tends
to be largely a simple superposition of key digits. Such
considerations
suggest that we choose M to be a prime number such that
r^k!=a(modulo)M or
r^k!=−a(modulo)M for small k & a."
In our case, N is the number of PEs in RFC 7432 which corresponds to M
above.
Since N, N-1 or N+1 need not satisfy the primality properties of the M
above; as per RFC 7432 modulo based DF assignment, whenever a PE goes down or a
new PE boots up (hosting the same Ethernet Segment), the modulo scheme need not
necessarily map BDs to PEs uniformly.
Section 1.3
Section 2.2 describes some of the issues that exist in the
Default DF
There is no section 2.2; presumably this is supposed to be 1.2.
[JORGE] changed, thx.
o HRW and AC-DF mechanisms are independent of each other.
Therefore,
a PE MAY support either HRW or AC-DF independently or MAY
support
both of them together. A PE MAY also support AC-DF capability
along
with the Default DF election algorithm per [RFC7432].
This seems a little confusing since just a couple paragraphs ago
you are
distinguishing between "election algorithms" and "capabilities",
but here
the two new things (one of each type) are lumped together as
"mechanisms".
If election algorithms and capabilities are inherently independent
things,
then maybe there is not a need to reiterate the independence of HRW
and
AC-DF here.
[JORGE] they are indeed independent, but since in the future may be some
capabilities that only make sense for certain DF Algs, we believe it is better
to explicitly state here that the DF Alg and capability defined are compatible.
Let us know if it is not okay.
Section 3
This section describes the BGP extensions required to support
the new
DF Election procedures. In addition, since the EVPN specification
[RFC7432] does leave several questions open as to the precise
final
state machine behavior of the DF election, section 3.1 describes
precisely the intended behavior.
This text sounds like we should be Update:ing 7432.
[JORGE] yes, we finally converged into this too. Rev 08 will update 7432.
Thanks.
Section 3.2
- Otherwise if even a single advertisement for the type-4
route is
not received with the locally configured DF Alg and
capability,
nit: shouldn't this be "received without"?
[JORGE] fixed, thanks.
Section 3.2.1
[RFC7432] implementations (i.e., those that predate this
specification) will not advertise the DF Election Extended
Community.
This wording also suggests that we should be Update:ing 7432.
[JORGE] done. Thx.
Section 4
I note that the state of the art in non-cryptographic fast hashing
has
improved a lot since 1998 and we have things like the Jenkins hash
that are
supposed to be superior to CRC-32 and such.
[HRW1999] provides pseudo-random
functions based on the Unix utilities rand and srand and easily
constructed XOR functions that perform considerably well. This
imparts very good properties in the load balancing context. Also
each
server independently and unambiguously arrives at the primary
server
selection. [...]
It's not really clear to me that this text adds much value -- we go
on
later to say that we explicitly use a Wrand() function from HRW1999.
[Satya] Agreed. We can take it out for brevity. We can edit this a bit as I
don’t see it harming anything.
Section 4.2
1. DF(v) = Si: Weight(v, Es, Si) >= Weight(v, Es, Sj), for all
j. In
case of a tie, choose the PE whose IP address is numerically
the
least. Note 0 <= i,j < Number of PEs in the redundancy group.
I strongly suggest expanding out the notation with more words, e.g.
"DF(v)
is defined to be the address Si such that [...]". We probably
shouldn't
assume much abstract math background from RFC readership.
(Similarly for
BDF(v). The BDF(v) expression doesn't even say what the i, j, and
k are
evaluated over.)
Denote the PEs addresses as S0, S1, .. SN-1.
[Satya] DF(v): is defined to be the address Si (index i) for which
weight(v, Es, Si) is the highest, 0 <= i < N-1
Similarly, BDF(v) is defined as that PE with address Sk for which the
computed weight is the next highest after the weight of the DF.
j is the running index from 0 to N-1, i, k are selected values.
HRW solves the disadvantages pointed out in Section 2.2.1 and
ensures:
Again, this is now Section 1.2.1
[JORGE] fixed, thanks.
o More importantly it avoids the needless disruption case of
Section
2.2.1 (3), that is inherent in the existing Default DF
Election.
and here.
(Also, this bullet point is just describing the same situation as
the
previous one, if I understand correctly.)
[JORGE] fixed, thanks.
Section 5
modify the DF Election procedures by removing from consideration
any
candidate PE in the ES that cannot forward traffic on the AC that
belongs to the BD. [...]
What guarantees that the ACS information is available on all PEs
involved
in the election?
[JORGE] the ACS information is available on all PEs since it is distributed
by the A-D routes as explained later. The withdrawal of an A-D per-EVI route
indicates the AC state goes down. But since this is the behavior in RFC7432, we
don't think there is a need to explain that.
In particular, when used with the Default DF Alg, the AC-DF
capability modifies the Step 3 in the DF Election procedure
described
in [RFC7432] Section 8.5, as follows:
Only a single paragraph follows, but the referenced document has
three
paragraphs in the indicated step. Are the last two paragraphs no
longer
intended to apply? In particular, if we apply this paragraph as a
direct
replacement for the RFC 7432 step 3, then there is no longer a
normative
description of the modulus-based algorithm, which seems incorrect.
Also,
there's a lot of style/editorial changes, that make the difference
in
behavior harder to read from the diff. (Side note: I don't think
this
particular text implies that this document needs an Updates:
relation to
RFC 7432, since it is a behavior change conditional on the use of a
negotiated feature.)
[JORGE] we changed the text to the following. Hopefully it makes it clear:
----------------------
In particular, when used with the Default DF Alg, the AC-DF
capability modifies the Step 3 in the DF Election procedure described
in [RFC7432] Section 8.5, as follows:
3. When the timer expires, each PE builds an ordered "candidate" list
of the IP addresses of all the PE nodes attached to the Ethernet
Segment (including itself), in increasing numeric value. The
candidate list is based on the Originator Router's IP addresses of
the ES routes, but excludes any PE from whom no Ethernet A-D per
ES route has been received, or from whom the route has been
withdrawn. Afterwards, the DF Election algorithm is applied on a
per <ES, Ethernet Tag>, however, the IP address for a PE will not
be considered candidate for a given <ES, Ethernet Tag> until the
corresponding Ethernet A-D per EVI route has been received from
that PE. In other words, the ACS on the ES for a given PE must be
UP so that the PE is considered as candidate for a given BD. If
the Default DF Alg is used, every PE in the resulting candidate
list is then given an ordinal indicating its position in the
ordered list, starting with 0 as the ordinal for the PE with the
numerically lowest IP address. The ordinals are used to determine
which PE node will be the DF for a given Ethernet Tag on the
Ethernet Segment, using the following rule:
Assuming a redundancy group of N PE nodes, for VLAN-based service,
the PE with ordinal i is the DF for an <ES, Ethernet Tag V> when
(V mod N)= i. In the case of VLAN-(aware) bundle service, then the
numerically lowest VLAN value in that bundle on that ES MUST be
used in the modulo function as Ethernet Tag.
It should be noted that using the "Originating Router's IP
address" field in the Ethernet Segment route to get the PE IP
address needed for the ordered list allows for a CE to be
multihomed across different ASes if such a need ever arises.
<snip>
----------------------
a) When PE1 and PE2 discover ES12, they advertise an ES route for
ES12 with the associated ES-import extended community and the
DF
Election Extended Community indicating AC-DF=1; they start a
timer
at the same time. [...]
(nit?) This text implies some synchronization between PE1 and PE2
for
starting the timer, whereas I think the intent is just to note that
they
each start a timer as they advertise the route, independently of
each other.
[JORGE] good catch. Changed to:
"a) When PE1 and PE2 discover ES12, they advertise an ES route for ES12
with the associated ES-import extended community and the DF Election Extended
Community indicating AC-DF=1; they start a DF Wait timer (independently).
Likewise, PE2 and PE3 advertise an ES route for ES23 with AC-DF=1 and start a
DF Wait timer."
In addition to the events defined in the FSM in Section 3.1, the
following events SHALL modify the candidate PE list and trigger
the
DF re-election in a PE for a given <ES,VLAN> or <ES,VLAN
Bundle>. In
the FSM of Figure 3, the events below MUST trigger a transition
from
DF_DONE to DF_CALC:
Then why are they not listed as part of the referenced FSM (or at
least
mentioned with a forward-reference)?
[JORGE] we added the following at the end of section 3.1:
"The above events and transitions are defined for the Default DF
Election Algorithm. As described in Section 5, the use of the AC-DF capability
introduces additional events and transitions."
Section 7
Are there any considerations to discuss about increased resource
consumption (e.g., for storing and transmiting Ethernet A-Ds
per-<ES,VLAN>
vs. per-<ES,VLAN Bundle>) and the risk of DoS due to reaching
resource
caps?
[JORGE] we don't think there are additional security considerations since
there are no additional Ethernet A-D routes suggested by the procedures in this
document. The procedures suggest to change the DF Election and make it per VLAN
as opposed to per VLAN bundle, but the amount of routes needed does not change.
Note that the network will not benefit of the new
procedures if the configuration of one of the PEs in the ES is
changed to the Default [RFC7432] DF Election.
Isn't this the case if there is not unanimity among all PEs in the
ES about
what election algorithm is preferred, which is a broader possible
case than
one being changed to use the default algorithms?
[JORGE] ok, changed to:
"Note that the network will not benefit of the new procedures if the DF
Election Alg is not consistently configured on all the PEs in the ES (if there
is no unanimity among all the PEs, the DF Election Alg falls back to the
Default [RFC7432] DF Election)."
Section 8
o Allocate Sub-Type value 0x06 in the "EVPN Extended Community
Sub-
Types" registry defined in [RFC7153] as follows:
Sometimes we see language about "confirm the existing early
allocation",
but I assume that the RFC Editor and IANA have a standard way of
sorting
this stuff out.
[JORGE] ok, we'll wait for RFC Editor / IANA edits.
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