Technical Summary
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
communication.
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.
Working Group Summary
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.
Document Quality
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, David Black, and Brian Carpenter, 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.
Personnel
Document Shepherd: David Black
Responsible AD: Spencer Dawkins
RFC Editor Note
RFC Editor Note
Because this draft formally updates draft-ietf-tsvwg-rtcweb-qos,
which is approved but waiting in the MISSREF state, we ask the RFC Editor
to take the following actions:
Please make the changes described in Section 12 of this draft to
draft-ietf-tsvwg-rtcweb-qos.
Please replace [RFCXXXX] in the updated text with the RFC number
assigned to this draft.
Please add [RFCXXXX] (with the RFC number assigned to this draft) as
a normative reference in draft-ietf-tsvwg-rtcweb-qos.
Please remove Section 12 from this draft (because all those changes
have already been applied).