A Lower-Effort Per-Hop Behavior (LE PHB) for Differentiated Services
Extracted elements (21)
CS1 ('001000') was the earlier recommended codepoint for LE/Low-Priority Data traffic (per RFC 3662 and RFC 4594), but this is problematic: CS1 was defined in RFC 2474 to have higher precedence than CS0 (the default PHB), so Diffserv domains implementing CS1 per RFC 2474 cause a priority inversion that forwards LE packets with higher precedence than BE packets — the opposite of the intended behavior.
DSCP '000001' was chosen from Pool 3 (codepoint space xxxx01) rather than the Standards Action Pool 1 (xxxxx0), because some network domains set the former IP Precedence bits to zero, which could accidentally re-mark other standardized DSCPs to the LE PHB DSCP if it were taken from Pool 1. Measurements of observable DSCP re-marking behavior in the Internet informed this choice.
The LE PHB possesses no parameters that need to be configured, requires no admission or policing, and needs no traffic meters, droppers, or shapers. Because LE traffic causes no harm to other PHBs, providers can trust LE markings from end systems and need not police or re-mark incoming LE traffic, simplifying deployment.
The LE PHB's 'no harm' property — that it does not consume resources from other PHBs and may be preempted completely — enables use as a security tool: untrusted external traffic or multicast from untrusted sources can be marked LE so that attackers cannot preempt internal (non-LE) traffic by flooding, and such traffic cannot harm non-LE behavior aggregates.
Applications sending LE-marked flows must tolerate short or very long interruptions due to starvation during severe congestion. When LE traffic is mapped to the default PHB in domains that do not support LE, it competes with BE traffic; in such cases applications SHOULD additionally use a lower-than-BE transport protocol such as LEDBAT (RFC 6817) to enforce lower-than-BE precedence.
RFC 4594 is updated to replace every occurrence of the CS1 DSCP ('001000') for the Low-Priority Data service class with the LE DSCP ('000001'), and the reference for this service class is updated from RFC 3662 to RFC 8622. The prior Section 2.3 re-marking recommendation for IP Precedence network segments is also removed as unnecessary with the new LE DSCP.
RFC 8325 (Mapping Diffserv to IEEE 802.11) is updated to add LE DSCP as the primary mapping for Low-Priority Data to UP 1 / AC_BK (Background), retaining CS1 as a legacy mapping. The recommendation to map Low-Priority Data is updated to include both LE DSCP and legacy CS1 DSCP mapped to UP 1.
A packet forwarded with the LE PHB SHOULD have lower precedence than packets forwarded with the default PHB; in the case of congestion, LE-marked traffic SHOULD be dropped prior to dropping any default PHB traffic.
'DSCP bleaching' (setting the DSCP to '000000', the default PHB) is NOT RECOMMENDED for LE-marked packets, as it may cause effects contrary to the original intent of protecting BE traffic from LE traffic. DS domains without LE support SHOULD map LE DSCP to the default PHB without re-marking to '000000' so that downstream domains can still treat packets as LE.
ECN SHOULD be used for LE packets; an LE implementation SHOULD also apply Congestion Experienced (CE) marking for ECT-marked packets, and transport protocols used for LE SHOULD support and employ ECN.
Existing implementations using CS1 SHOULD transition to use the unambiguous LE codepoint '000001' whenever possible. A DS domain still using CS1 for LE traffic SHOULD re-mark traffic to LE DSCP '000001' at egress to the next DS domain.
LE traffic SHOULD be congestion controlled, using a congestion-controlled transport or an appropriate congestion control method. Transport protocols or applications SHOULD be able to detect and react to starvation situations caused by prolonged absence of LE forwarding resources.
Non-LE traffic (e.g., BE traffic) SHOULD NOT be re-marked to LE. Re-marking traffic from another PHB results in that traffic being downgraded, which may violate the operational objectives of the original PHB.
Operators of DS domains that cannot implement the LE PHB SHOULD NOT drop packets marked with the LE DSCP; they SHOULD map such packets to the default PHB and SHOULD preserve the LE DSCP marking.
Packets SHOULD be pre-marked in DS-aware end systems by applications due to their specific knowledge of packet precedence. There is no incentive for DS domains to distrust this initial LE marking because letting LE traffic enter a domain causes no harm, and thus policing (e.g., rate limiting) is not necessary at the DS boundary.
The LE PHB SHOULD NOT be used for a customer's normal Internet traffic, and packets SHOULD NOT be downgraded to the LE PHB instead of being dropped, particularly when the packets are unauthorized traffic.
The LE DSCP can reveal to an observer (e.g., a state-level actor) that transferred content is considered low priority or low urgency, potentially allowing surveillance resources to be focused on non-LE (i.e., higher-priority) traffic while ignoring LE background traffic such as software downloads or telemetry data. The disclosed information has limited utility without correlation to user identity metadata.
The Lower-Effort Per-Hop Behavior (LE PHB) is a Differentiated Services PHB where packets have lower forwarding precedence than all other PHBs, including the default (Best-Effort) PHB. In congestion, LE-marked traffic SHOULD be dropped before any default PHB traffic and may be starved completely. It is analogous to a background process priority in an operating system.
The RECOMMENDED DSCP codepoint for the LE PHB is '000001' (binary), decimal value 1, assigned from DSCP Pool 3 (codepoint space xxxx01) via Standards Action. This replaces the earlier informal practice of using CS1 ('001000') for Lower-Effort traffic.
IANA assigned DSCP codepoint '000001' (binary), decimal value 1, with name 'LE' in the 'Differentiated Services Field Codepoints (DSCP)' registry under DSCP Pool 3 Codepoints (codepoint space xxxx01, Standards Action procedure). Reference is RFC 8622.
An attacker able to modify the DSCP field of a packet to LE may carry out a downgrade attack, causing QoS degradation of the re-marked traffic. There are no other specific security exposures for the LE PHB; general Diffserv security considerations from RFC 2474 and RFC 2475 apply.