Per Hop Behavior Identification Codes
obsoleted by
- rfc-3140 — Per Hop Behavior Identification Codes
Extracted elements (13)
A uniform 16-bit encoding was chosen to uniquely identify PHBs in protocol messages (e.g., bandwidth broker negotiations, MPLS label distribution) because DSCP values are insufficient — there are only 64 DSCP values but no limit on the number of PHBs, and local DSCP mappings may differ across domains.
Bit 15 distinguishes standards-defined PHBs (bit 15 = 0) from experimental/local-use PHBs (bit 15 = 1), allowing unambiguous identification of the encoding case without needing out-of-band context.
Bits 12 and 13 in Case 2 are reserved for future expansion of the PHB identification code or other use, providing forward-compatibility headroom without consuming current encoding space.
PHB sets (e.g., AF11, AF12, AF13 in a single AF class) must be identifiable as a group because traffic within a set may need to follow the same physical path to prevent reordering; a single-bit flag (bit 14) indicates set membership without requiring a separate encoding scheme.
PHB identification codes are intended for use in inter-domain signaling such as MPLS label distribution protocols (RSVP and LDP) and ATM VC call setup signaling to communicate desired forwarding behaviors across technology boundaries.
During the first year of the registry's existence, IANA is requested to refer all assignment requests to the IETF diffserv WG for review; subsequently, requests should be reviewed by the IETF Transport Area Directors or a designated expert.
For standards-defined PHBs, the recommended DSCP value MUST be used in the encoding, even if the network in question has chosen a different local DSCP mapping.
This encoding MUST be used when PHB or PHB set identification is required in protocol messages.
A PHB set encoding is formed by taking the numerically smallest encoding among the PHBs in the set and setting bit 14 to 1. For example, the AF1x set uses the AF11 encoding with bit 14 set.
IANA is requested to create a new 'Per-Hop Behavior Identification Codes' registry with values 0–4095 decimal. Assignments require: assignee identity, PHB description sufficient to distinguish from existing PHBs, and a stable reference document.
The encoding itself raises no security issues, but modifying a PHB identification code may constitute theft or denial of service; protocols using this encoding must be adequately protected.
Case 2 (experimental/local-use PHBs): a 12-bit IANA-assigned PHB identification code is placed left-justified in bits 0–11, bits 12–13 reserved (set to zero), bit 14 is 0 for single PHB or 1 for a set, bit 15 is set to 1 to distinguish from Case 1.
PHBs and sets of PHBs are encoded in an unsigned 16-bit binary field. Case 1 (standards-defined PHBs): the recommended DSCP value is placed left-justified in bits 0–5, bits 6–13 set to zero, bit 14 is 0 for a single PHB or 1 for a set of PHBs, bit 15 is 0.