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Shepherd writeup

Document shepherd write-up:

IANA Assignment of DSCP Pool 3 (xxxx01) Values to require Publication of
             a Standards Track or Best Current Practice RFC

1. Summary

Document Shepherd: David Black
Responsible AD: Spencer Dawkins

The Transport Area WG (tsvwg) is in the process of updating the RFC 3662
specification of the Diffserv Lower Effort (LE) PHB/PDB, which provides
a less than best effort or scavenger forwarding behavior.  This update
includes assignment of a new default Diffserv Codepoint (DSCP) to
replace use of the CS1 DSCP (001000) for that purpose.  The IANA
registration policy for part of the IANA DSCP registry has to be changed
in order to make that assignment; this draft makes that necessary IANA
registration policy change.

The number of possible DSCPs for the LE PHB is surprisingly limited:
  - The three most significant bits have to be '000' to accommodate
     legacy (non-Diffserv) routers whose forwarding behavior is based
     upon only those three bits, the former IP Precedence field
  - The least significant bit must be '1' in order to avoid priority
     inversion that could result from legacy router zeroing of the former
     IP Precedence field in a DSCP that is intended to provide better
     than best effort forwarding.  That IP Precedence zeroing behavior
     has been observed in the Internet.  This requirement excludes
     DSCP Pool 1 (xxxxx0) values.
  - DSCP values whose two least significant bits are '11' are not
     available because DSCP Pool 2 (xxxx11) values have been reserved
     for Experimental or Local Use.
The only remaining DSCPs that could be assigned to the LE PHB are
000001 and 000101, both of which are in DSCP Pool 3 (xxxx01).

DSCP Pool 3 was originally specified for experimental or local use with
a note (in the IANA registry) indicating that they may be utilized for
future Standards Action allocations as necessary.  That has now become
necessary for the LE PHB.  The tsvwg WG's consensus is that it is
better to reclassify the entire DSCP Pool 3 all at once for assignment
by Standards Action rather than incrementally remove individual
DSCPs from that Pool as needed, as a one-time change to the entire
Pool is more likely to have consistent implementation results.

The tsvwg WG intends to assign DSCP 000001 to the updated LE PHB.
That assignment will be made by draft-ietf-tsvwg-le-phb, which specifies
the updated LE PHB.  The IANA registry policy change made by this
draft is a necessary prerequisite to that assignment.

The WG has requested Proposed Standard status because this draft
modifies IANA assignment policy for part of the DSCP registry.

2. Review and Consensus

The Transport Area WG (tsvwg) is a collection of people with varied
interests that don't individually justify their own working groups.

This draft is supported by the portion of the tsvwg working group that
is familiar with and interested in Diffserv.  The draft has received
significant review and critique from a number of Diffserv experts,
including the draft shepherd.  There is clear tsvwg WG consensus on
the need to assign a pool 3 DSCP to the LE PHB, and that consensus
is supported by Internet path measurements that indicate that zeroing
of the former IP Precedence field, while rare, occurs often enough to
be an important design consideration in assignment of a default DSCP
to the updated LE PHB.

3. Intellectual Property

Each draft author has stated his/her direct, personal knowledge that any
IPR related to this document has already been disclosed, in conformance
with BCPs 78 and 79.

4. Other Points

There is a downref to RFC 3260, an Informational RFC that contains new
Diffserv terminology and clarifications.  This downref needs to be
normative, as this draft requests IANA to add that as a reference to
both the new registry table that is created and an existing registry table.
That downref needs to be noted at IETF Last Call.

The shepherd has checked the IANA Considerations, which include updates
to the references for the Pool 1 DSCP table (sub-registry) in addition to
creating a new Pool 3 table (sub-registry).