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

[Eric C Rosen <[e-mail]>]

H Alexey,

In BGP, anything that is announced by a Capability is always optional.  
Per the draft, if one doesn't send the Multiple Labels Capability, one 
sends exactly one label in the NLRI.   So in that sense, the mandatory 
minimum value is 1.  I thought this part was clear from the first bullet 
point in Section 2.

When the Multiple Labels Capability is sent at session establishment 
time, I thought the new text:

"A triple of the form <AFI=x,SAFI=y,Count=0> or <AFI=x,SAFI=y,Count=1>
  MUST NOT be sent.  If such a triple is received, it MUST be ignored."

implied that the mandatory minimum value is 2, as that text precludes 
the use of the only smaller values.

If you think this isn't clear enough, I can some text like "Any 
implementation that sends a Multiple Labels Capability MUST be able to 
support at least two labels in the NLRI.  However, there may be 
deployment scenarios in which a larger number of labels is needed."

Would that be okay?

I wouldn't want to make the mandatory minimum any larger, because (a) 
some deployment scenarios might only require two labels, and (b) we 
don't want to have a larger mandatory minimum because the number of 
labels is not merely a control plane issue, but also affects the data 
plane.  E.g., if some platform can only push two labels at a time, 
there's no value to requiring it to support three labels in the control 
plane.

Eric