Re: [bess] Benjamin Kaduk's Discuss on draft-ietf-bess-evpn-df-election-framework-07: (with DISCUSS and COMMENT)

["Rabadan, Jorge (Nokia - US/Mountain View)" <[e-mail]>]

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 <[e-mail]>
Date: Tuesday, January 8, 2019 at 6:52 PM
To: The IESG <[e-mail]>
Cc: "[e-mail]" <[e-mail]>, Stephane Litkowski <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>
Subject: Benjamin Kaduk's Discuss on draft-ietf-bess-evpn-df-election-framework-07: (with DISCUSS and COMMENT)
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    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.