Re: [mpls] Alexey Melnikov's Discuss on draft-ietf-mpls-rfc3107bis-02: (with DISCUSS and COMMENT)

Eric C Rosen <erosen@juniper.net> Tue, 01 August 2017 21:00 UTC

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To: Alexey Melnikov <aamelnikov@fastmail.fm>, The IESG <iesg@ietf.org>
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Subject: Re: [mpls] Alexey Melnikov's Discuss on draft-ietf-mpls-rfc3107bis-02: (with DISCUSS and COMMENT)
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> DISCUSS:
> ----------------------------------------------------------------------
>
> I would like to discuss one issue before recommending approval of this document:
>
> In Section 2.1:
>
>     The value field of the Multiple Labels Capability (shown in Figure 1)
>     consists of one or more triples, where each triple consists of four
>     octets.  The first two octets of a triple specify an AFI value, the
>     third octet specifies a SAFI value, and the fourth specifies a Count.
>     If one of the triples is <AFI,SAFI,Count>, the Count is the maximum
>     number of labels that the BGP speaker sending the Capability can
>     process in a received UPDATE of the specified AFI/SAFI.
>
> I think lack of recommendations on the minimal supported Count value will
> result in lack of interoperability. What are the common Count values used by
> implementations?

An implementation is not required to support more than one label in the 
NLRI.  In that case, of course the Multiple Labels Capability is not  
supposed to be used.  So if the Capability is used, the minimum value is 
2.   It looks like the document does not state that explicitly, but it 
certainly should.  I will fix that.

Thus the literal answer to your question is that the Multiple Labels 
Capability MUST NOT contain a count less than 2.

Typically when SAFI-4 or SAFI-128 routes are used, only one label is 
included.  If an operator has a scenario in which multiple labels are 
needed, it is necessary for the operator to ensure that all his vendors 
can support however many labels he needs.  That's not really something 
that the standard can address.  If the operator needs 3 labels in the 
NLRI, it won't help much if some of his boxes can only support 2.

---------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> In Section 2.3:
>
>        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
>       +-+-+-+-+-+-+-+-+
>       |    Length     |
>       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>       |                 Label                 |Rsrv |S~
>       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>       ~                 Label                 |Rsrv |S|
>       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>       |                          Prefix                               ~
>       ~                                                               |
>       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>
>                      Figure 3: NLRI With Multiple Labels
>
>     - Length:
>
>        The Length field consists of a single octet.  It specifies the
>        length in bits of the remainder of the NLRI field.
>
> I would like to double check that my math is correct. With SAFI=128 and AFI=2,
> assuming the prefix length of 192 bits, this will leave space for:
>
>   (255-192)/24 = 2.625. So this configuration only allows for 2 labels to be included, right?
>
Your arithmetic is accurate ;-)

If you are implying that this is not necessarily the best mechanism for 
associating an arbitrarily long label stack with a prefix, I wouldn't 
disagree.