Update to IANA Registration Procedures for Pool 3 Values in the Differentiated Services Field Codepoints (DSCP) Registry
updates
- rfc-2474 — Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers
Extracted elements (10)
Pool 2 codepoints (xxxx11) are retained exclusively for Experimental or Local Use, preserving a space for non-standardized experimentation while Pool 3 is reclaimed for formal standardization. This maintains a clear separation between standardized and experimental DSCP assignments.
Pool 3 was opened to Standards Action because the Lower Effort (LE) PHB requires a codepoint with a zero value in the first three bits to avoid IP Precedence bleaching issues. No unassigned Pool 1 codepoints with these properties were available, necessitating use of a Pool 3 codepoint to prevent priority inversion where bleached traffic would receive unintentional LE treatment.
In deployed networks, bleaching (intentionally setting to zero) of the IP Precedence field continues to be used, causing the first three bits of the DSCP field to become zero. DSCP assignments for PHBs like LE must account for this behavior to avoid unintentional traffic reclassification.
IANA is expected to normally make assignments from Pool 1 until it is exhausted, but MAY make assignments from Pool 3 when the codepoint format has properties needed for a specific PHB.
IANA MUST manage Pool 3 codepoint assignments using the Standards Action policy, as defined in Section 4.9 of RFC 8126, meaning values are assigned only through Standards Track or Best Current Practice RFCs in the IETF Stream.
The required characteristics for choosing a requested DSCP value from Pool 3 MUST be explained in the IANA Considerations section of the document that requests any Pool 3 assignment.
Pool 3 codepoints have the form xxxx01 (16 codepoints). This document changes their registration policy from 'experimental or local use' to 'Standards Action', removing EXP/LU permission for Pool 3 while Pool 2 (xxxx11) remains available for experimental or local use.
The DSCP field uses the six most significant bits of the former IPv4 Type of Service (TOS) octet or the former IPv6 Traffic Class octet, capable of conveying 64 distinct codepoints divided into three pools for assignment management.
IANA maintains the 'Differentiated Services Field Codepoints (DSCP)' registry, divided into three pools: Pool 1 (xxxxx0, 32 codepoints, Standards Action), Pool 2 (xxxx11, 16 codepoints, EXP/LU), and Pool 3 (xxxx01, 16 codepoints, now Standards Action per this document). IANA has created a 'DSCP Pool 3 Codepoints' subregistry, initially empty, with a format identical to Pool 1.
This document does not present new security considerations. Security considerations for the use of DSCP values are described in the RFCs that define their usage.