Document shepherd write-up:
A Lower Effort Per-Hop Behavior (LE PHB)
Document Shepherd: David Black
Responsible AD: Spencer Dawkins
This document specifies properties and characteristics of a Lower
Effort (LE) per-hop behavior (PHB). The primary objective of this LE
PHB is to protect best-effort (BE) traffic (packets forwarded with
the default PHB) from LE traffic in congestion situations, i.e., when
resources become scarce, best-effort traffic has precedence over LE
traffic and may preempt it. Alternatively, packets forwarded by the
LE PHB can be associated with a scavenger service class, i.e., they
scavenge otherwise unused resources only. There are numerous uses
for this PHB, e.g., for background traffic of low precedence, such as
bulk data transfers with low priority in time, non time-critical
backups, larger software updates, web search engines while gathering
information from web servers and so on. This document recommends a
standard DSCP value for the LE PHB. This specification obsoletes RFC
3662 and updates the DSCP recommended in RFC 4594 and RFC 8325 to use
the DSCP assigned in this specification.
When Diffserv was originally designed, interest in less-than-best effort
(aka scavenger) forwarding behavior eventually resulted in publication of
RFC 3662 which specified the Diffserv Lower Effort (LE) PHB/PDB.
In 20/20 hindsight, RFC 3662 had a number of drawbacks, as it was not
a full PHB specification and in particular did not recommend a default
DSCP (Diffserv Codepoint) for Lower Effort traffic. The default DSCP
recommendation eventually occurred in practice as a side effect of
publishing RFC 4594 on Diffserv Service Classes. The recommended
DSCP, CS1, has turned out to be problematic in practice - e.g., see the
discussion of CS1 in RFC 7657 on Diffserv interaction with real time
This draft cleans up the LE PHB situation by providing a full PHB
specification of the Lower Effort PHB that obsoletes RFC 3662 and
recommends a newly chosen default DSCP, 000001, which is expected to
avoid the problems encountered with CS1 and provide a solid Diffserv
specification for lower effort/less-than-best-effort/scavenger traffic.
Proposed Standard is appropriate for this document in support of
consistent deployment of the updated LE PHB as part of Diffserv.
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.
Specifying the Lower Effort PHB was relatively straightforward in
the WG. In contrast, determining which DSCP to recommend as the
default for that PHB was not. The underlying problem is that a
non-negligible amount of deployed Internet equipment "bleaches"
the three most significant bits of the DSCP field in IP headers to
zero, even though that violates Diffserv requirements. This made
it problematic to use the initially suggested 000010 value, as that
value can and does result from this three-bit bleaching of DSCP
values for higher priority traffic that should not be forwarded
as lower effort (LE) traffic.
After much discussion and evaluation of measurement results on Internet
traffic in both TSVWG and MAPRG, the TSVWG working group chose 000001
value as the recommended default DSCP. This decision necessitated
publication of RFC 8436 to change the IANA procedures for managing the
DSCP registry so that this DSCP value 000001 could be assigned as the
default DSCP for the LE PHB in this document.
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 and Brian Carpenter who was one of the
original chairs of the Diffserv WG. There is clear consensus in the
TSVWG WG on the need to update the LE PHB specification to replace
and obsolete RFC 3662.
3. Intellectual Property
The draft author has stated his direct, personal knowledge that any
IPR related to this document has already been disclosed, in conformance
with BCPs 78 and 79.
4. Other Points
The shepherd has checked the IANA Considerations.
idnits generated a number of comments that don't reflect actual problems,
plus three warnings about missing references and a Downref complaint.
The three warnings about missing references are all in quoted text
(existing or updated) for other documents - in each case the document
involved contains the necessary reference, so the warnings can be ignored.
This document contains a Downref to RFC 2475 on Diffserv Architecture.
In the shepherd's considered opinion, this Downref is justified because
an understanding of RFC 2475 is necessary to understand this document,
and RFC 2475's security considerations are explicitly referenced by
this document's security considerations. That Downref will need to
be noted in the IETF Last Call announcement.