[mpls] comments on draft-kompella-mpls-special-purpose-labels-02

[Eric Rosen <[e-mail]>]

A few comments:

- The extended special purpose label space has about a million possible
  values (16 - 1048559), with an allocation policy of "standards action".
  This encourages squatting on "unofficial" codepoints while discouraging
  the publication of non-standards-track drafts describing the use of those
  codepoints.

  I think it would be better to have a much smaller piece of the label space
  to be allocated under the "standards action" policy.

- With a large extended special purpose label space, why do we need a
  process for reusing deprecated (non-extended) special purpose label
  values?  Especially when we already know that "IETF-wide surveys" don't
  necessarily determine correctly whether some feature is actually in use
  somewhere.

- "an arbitrary string of consecutive extension labels is legal, and
  semantically equivalent to a single extension label"

  I don't see the sense of this.  It will only lead to interoperability
  issues when some implementation puts in lots of extraneous labels and some
  other implementation can't handle it properly (as mentioned in the
  Security Considerations).

- Section 3, point 4, on the possible use of codepoint 3, doesn't seem to
  make any change to existing process or usage.  I'm not sure why it is even
  in the document.

- The fact that every special purpose label has a corresponding extended
  special purpose label seems problematic to me.  Suppose some
  implementation decides that when it needs to push "IPv4 explicit null" on
  a packet, it is always going to use the extended special purpose label.
  This seems perfectly legal, but will not interoperate with other
  implementations that understand "IPv4 explicit null" but that don't
  support the extended special purpose label space.

- From section 3.1:

       An extension label MUST be followed by another label L (and thus
       MUST have the bottom-of-stack bit clear).  L MUST be interpreted
       as an "extended special purpose label" from a new registry
       created by this document (see Section 5).  Whether or not L has
       the bottom-of-stack bit set depends on whether other labels
       follow L. Only L is interpreted as an extended special purpose
       label; labels following L are interpreted as normal.

  If all labels following a special purpose label "are interpreted as
  normal", it follows that there can only be one special purpose label in
  the stack.  I don't think that's the intention.

  Probably the final sentence should be "an extension label only affects the
  interpretation of the label immediately below it".