[Bier] Clarification regarding label-range-size in draft-ietf-bier-ospf-bier-extensions-10

[Senthil Dhanaraj <[e-mail]>]

Dear Authors of draft-ietf-bier-ospf-bier-extensions-10,

A clarification required regarding the ¡°label-range-size¡± field in BIER MPLS Sub TLV.
Kindly check.


RFC 8279  (Section 3)

   A BFR MUST support SI values in the range [0,15] and MAY support SI
   values in the range [0,255].  ("Supporting the values in a given
   range" means, in this context, that any value in the given range is
   legal and will be properly interpreted.)  Note that for a given
   BitStringLength, the total number of BFR-ids that can be represented
   is the product of the BitStringLength and the number of supported
   SIs.  For example, if a deployment uses (in a given sub-domain) a
   BitStringLength of 64 and supports 256 SIs, that deployment can only
   support 16384 BFR-ids in that sub-domain.  Even a deployment that
   supports 256 SIs will not be able to support 65,535 BFR-ids unless it   // [SEN1]
   uses a BitStringLength of at least 256.

[SEN1] In general, the assumption is that,
       Max BFR-Ids supported = [bsl * NumOfSI] (ofcourse in the range [1 ¨C 65535])


draft-ietf-bier-ospf-bier-extensions-10 (Section 2.2)

The BIER MPLS Encapsulation Sub-TLV has the following format:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Lbl Range Size |                Label Range Base               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |BS Len |                     Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      The "label range" is the set of labels beginning with the label

      range base and ending with ((label range base)+(label range size)-   // [SEN2]

      1).  A unique label range is allocated for each BitStream length

      and Sub-domain-ID.  These labels are used for BIER forwarding as

      described in [I-D.ietf-bier-architecture] and

      [I-D.ietf-bier-mpls-encapsulation].



      The size of the label range is determined by the number of Set

      Identifiers (SI) (section 1 of [I-D.ietf-bier-architecture]) that

      are used in the network.  Each SI maps to a single label in the

      label range.  The first label is for SI=0, the second label is for

      SI=1, etc.



[SEN2] Max ¡°label range size¡± = 255

       ¡°label-end¡± = ¡°label range base¡± + 255 ¨C 1

       Which means, we can support only 255 SIs, never 256 SIs ?

       255 (SIs) * 256 (bsl) = 65280 BFR¡¯s.



       How about modifying the rule as below ?

1.  Allow 0 as valid value for ¡°label range size¡±

2.  ¡°label-end¡± = ¡°label range base¡± + ¡°label range size¡±



Thanks,

Senthil
»ªÎª¼¼ÊõÓÐÏÞ¹«Ë¾ Huawei Technologies Co., Ltd.
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