Re: [bess] Adam Roach's No Objection on draft-ietf-bess-evpn-df-election-framework-07: (with COMMENT)

["Satya Mohanty (satyamoh)" <[e-mail]>]

Adam,



Thanks to you and Benjamin for this review and feedback on the hashing. Please see inline [Satya].

I will add my response to other queries in the forthcoming Jorge's reply to  Benjamin (as there are other comments to be addressed).



Best,

--Satya



On 1/9/19, 10:31 PM, "Adam Roach" <[e-mail]> wrote:



    Adam Roach has entered the following ballot position for

    draft-ietf-bess-evpn-df-election-framework-07: No Objection



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    COMMENT:

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    I have one minor comment that the authors may wish to consider.



    §1.2.1:



    >  It is well-known that for good

    >  hash distribution using the modulus operation, the modulus N should

    >  be a prime-number not too close to a power of 2 [CLRS2009].



    I suppose this refers to the explanation in [CLRS2009] §11.3.1. The

[Satya] Yes, it is from [CLRS2009] §11.3.1

    description there is pretty hand-wavy and not completely accurate except under

    certain (admittedly common) conditions -- in which this case is not included.

    You may wish to consider instead citing "The Art of Computer Programming (Vol.

    3)" by Knuth, as it captures a lot more of the nuance behind why this rule of

    thumb is frequently true, and covers the general case. There is probably a set

    of considerations to take into account for Ethernet Tags with an even

    distribution, but only because you have a relatively small set of potential

    inputs -- not for any of the reasons cited in [CLRS2009]. Quoting Knuth:



      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.



    I see that Benjamin has made a related comment. I share his objection to

    the way point #2 is phrased, and think it needs to be reworded to properly

    capture the subtleties implied by the preceding passage.



[Satya] Yes, noted. I will change point #2.  Here is a suggested description of Point #2



Th 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.